Coordination Chemistry Reviews 336 (2017) 54–77

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Coordination Chemistry Reviews

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Review

Transition metal catalyzed element–element0 additions to alkynes

http://dx.doi.org/10.1016/j.ccr.2017.01.0030010-8545/Crown Copyright � 2017 Published by Elsevier B.V. All rights reserved.

⇑ Corresponding authors.E-mail addresses: ma462@sussex.ac.uk (M.B. Ansell), navarrof@ohsu.edu (O. Navarro), j.spencer@sussex.ac.uk (J. Spencer).

Melvyn B. Ansell a,⇑, Oscar Navarro b,⇑, John Spencer a,⇑aDepartment of Chemistry, University of Sussex, Brighton BN1 9QJ, UKbDivision of Biomaterials and Biomechanics, Department of Restorative Dentistry, School of Dentistry, Oregon Health & Science University, 2730 SW Moody Ave., Portland, OR97239, USA

a r t i c l e i n f o

Article history:Received 7 December 2016Accepted 5 January 2017Available online 7 January 2017

Keywords:AlkynesAlkenesElement–element bondsCatalysisTransition metal

a b s t r a c t

The efficient and stereoselective synthesis of, or precursors to, multi-substituted alkenes has attractedsubstantial interest due to their existence in various industrially and biologically important compounds.One of the most atom economical routes to such alkenes is the transition metal catalyzed hetero ele-ment–element0 p-insertion into alkynes. This article provides a thorough up-to-date review on this areaof chemistry, including discussions on the mechanism, range of EAE0 bonds accessible and the stoichio-metric/catalytic transition metal mediators employed.

Crown Copyright � 2017 Published by Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 552. Transition metal mediated EAE0 additions to alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

2.1. Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 552.2. Silicon–Silicon (SiASi) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

2.2.1. Palladium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 562.2.2. Platinum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 592.2.3. Gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 602.2.4. Iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 602.2.5. Nickel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 602.2.6. Rhodium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

2.3. Boron–Boron (BAB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

2.3.1. Platinum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 612.3.2. Palladium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 642.3.3. Cobalt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 642.3.4. Iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 642.3.5. Iridium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 642.3.6. Copper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 642.3.7. Gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

2.4. Silicon–Boron (SiAB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

2.4.1. Palladium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 652.4.2. Nickel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 682.4.3. Gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

2.5. Tin–Tin (SnASn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

2.5.1. Palladium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 692.5.2. Platinum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 702.5.3. Copper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

Schemalkynes

=

=

zy

Fig.

M.B. Ansell et al. / Coordination Chemistry Reviews 336 (2017) 54–77 55

2.6. Tin–Silicon (SnASi) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

2.6.1. Palladium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 702.6.2. Copper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

2.7. Tin–Boron (SnAB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

2.7.1. Palladium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 712.7.2. Copper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

2.8. Sulfur–Sulfur and Selenium–Selenium (SAS and SeASe) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

2.8.1. Palladium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 722.8.2. Iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 732.8.3. Copper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 732.8.4. Nickel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 732.8.5. Rhenium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 742.8.6. Rhodium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

2.9. Sulfur–Silicon (SASi) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

2.9.1. Gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

2.10. Sulfur–Boron (SAB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

2.10.1. Palladium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

2.11. Germanium–Germanium (GeAGe). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

2.11.1. Palladium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 742.11.2. Platinum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

2.12. Germanium–Tin (GeASn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

2.12.1. Palladium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

2.13. Germanium–Boron (GeAB). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

2.13.1. Nickel, palladium and platinum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

3. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

1. Introduction

Alkenes are defined as either branched or unbranched hydro-carbons that possess at least one carbon–carbon double bond(C@C) and have a general formula of CnH2n [1]. Each carbon atomin a C@C bond is sp2-hybridized, forming r-bonds to three otheratoms (Scheme 1) [2]. If the alkene has more than one substituentaround the C@C bond then two geometric configurations are pos-sible, E or Z (often termed trans or cis, respectively originating fromthe German words ‘entgegen’ meaning opposite and ‘zusammen’meaning together) (Fig. 1).

The importance of alkene stereochemistry is reflected in biolog-ically relevant molecules and is often the difference between anactive or inactive compound [3]. Furthermore, highly functional-ized and stereodefined multi-substituted alkenes are found inmany industrially important compounds including pharmaceuti-cals (Fig. 2) [4–8], dipeptide mimetics [9], and polymeric materials[10]. The stereoselective synthesis of, or precursors to, these

R1 R2transition metal catalyst

E E'+R1

E E'

R2

E E' = Si-Si, Si-B, B-B, Sn-Sn, Sn-Si, Ge-Ge...

e 1. General scheme for transition metal mediated EAE0 additions to.

C CH

H

H

H

sp2 hybridized orbital

2pz orbital

x

121ο

C CH H

HH

R

R'

R R'

E(trans)

Z(cis)

117ο

1. Alkene orbitals, shape and configurations for simple alkenes (olefins).

alkenes has therefore attracted substantial attention from bothacademia and industry. A range of stereoselective syntheses arereported in the literature, methods include Peterson olefination[11,12], the Ramberg-Bäcklund reaction [13], the Wittig reaction(as well as the Wittig-Horner variation) [14,15], olefin metathesis[16], Julia-Lythgoe olefination [17,18], and the McMurry reaction(Fig. 2) [19,20].

Arguably, one of the most atom economical routes (maximumnumber of atoms of the reactants appearing in the product/s)[21] in the synthesis of multi-substituted alkenes is alkyne reduc-tion by its p-insertion into hetero element–element0 (EAE0, whereE and E0 – H) bonds (albeit the newly formed CAE and CAE0 bondsare then further functionalized) [22]. These p-insertions result inthe regio- and stereoselective formation of cis-1-element-2-element0-alkenes in a single step (Scheme 1). A range of EAE0 bondsare accessible including SiASi, SiAB, BAB, SnASn, SnASi, GeAGeetc, and are mediated either stoichiometrically or catalytically bya variety of low-valent transition metal complexes.

Despite the chemical importance of this reaction and the indus-trial relevance of the 1-element-2-element0-alkenes as precursorsto highly functionalized multi-substituted alkenes, the last thor-ough review on this topic was reported by Moberg and Beletskayain 1999 (with a revised edition in 2006) [22,23]. A substantial vol-ume of papers has since been published, and we believe there is astrong argument for an updated review that includes these morerecent developments.

2. Transition metal mediated EE0 additions to alkynes

2.1. Mechanism

The main mediators in the EAE0 additions to alkynes are low-valent platinum group transition metal complexes coordinatedby either phosphine or isocyanide ligand sets. The mechanism iswell established, both computationally and experimentally, andconsists of three major steps: oxidative addition, insertion andreductive elimination [24,25]. The first step in this catalytic cycleis the oxidative cleavage of a EAE0 bond by a M(0)L2 (M = platinum

R1R4

R2Me3Si

R3 OH

R1

R4

R3

R2

acid

R1

R2 R3

R4

base

Peterson olefination

R1 S R2O O

KOH

R1

R2

Ramberg-Backlund reaction

:

R1 H

O

R2 H

P+R3 R3

R3

-+

R2 = arylR1

R2

R1 R2

stabil ized ylide

non-stabi lized ylide

R2 = alkyl

Wittig reaction

R2

R3

R4+

catalystR1 R3

R2 R4

+

Olefin metathesis

R1 SO O

1) nBuLi2) R2CHO

3) Ac2O R1 SO O

AcO R2 Na/Hg

R1

R2

Julia-Lythgoe olefination

TiCl3

K, THF

O

R2R1

O

R3R3+

R2

R1 R3

R3

McMurry reaction

Pharma Relevant Alkenes

tamoxif en

ON

O

OH

H HOH

O O

(-)-ratjadone

O

HO

O

N H

H

OH

HO

nileprost

R1

Fig. 2. Selected examples of multi-substituted alkene synthesis and pharma-relevant alkenes.

L2ME

E'

E'

R'R

E

ME

RE'

R'

R R'

ML2

M E'E

L

R

R'

L

L

oxidative addition

acetylene coordination

migratory insertion

reductive elimination

E E'

MI1

MI2

MI3

Scheme 2. General mechanism for platinum group transition metal mediatedaddition of EAE0 bonds to alkynes.

56 M.B. Ansell et al. / Coordination Chemistry Reviews 336 (2017) 54–77

groupmetal, L = phosphine/isocyanide) species to form cis-(E)(E0)M(II)L2 (MI1). MI1 is often kinetically stable and is isolated experi-mentally for many of the EAE0 bonds discussed above (the relevantEAE0 oxidative additions will be discussed in the appropriate chap-ters) [26–30]. A ligand exchange then occurs with decoordination

of a single L ligand and coordination of the alkyne in its place toyieldMI2. This is swiftly followed by an insertion of the alkyne intoa MAE or MAE0 bond (MI3) [31,32]. The regioselectivity of the EAE0

addition is usually defined by this step and dictating factorsinclude: the energetics of the bonds broken vs. the bonds formed,the sterics of the system and electronic stabilization effects withinthe resulting intermediates [33,34]. Experimental studies suggestthat the insertion is often the rate-limiting step in these reactionpathways [31]. An isomerization and re-coordination of the Lligand results in the E and (E0) vinyl groups adjacent to oneanother. This positioning is then ideally suited for stereoselectivereductive elimination to yield the corresponding Z-1,2-disubstituted alkenes and consequent reformation of M(0)L2(Scheme 2).

This subsequent sections will overview the history, state-of-the-art and scope of this field, arranged by interelement bondsactivated and metals used.

2.2. Silicon–Silicon (SiASi)

2.2.1. PalladiumThe p-insertion of unsaturated moieties into SiASi bonds is

often called bis(silylation). Palladium mediated bis(silylation) ofalkynes is one of the most investigated reactions within this areaof chemistry [35]. The first examples were reported by Kumada

XmMe3-mSi SiMe3-nXn R1 R2+

1 mol% [Cl2Pd(PR3)2]or

[Pd(PR'3)4]

80-100 οC R1

XmMe3-nSi SiMe3-nXn

R2

X = H, F, Cl or OMe; m = 1-2, n = 1-21a : R1 = R2 = Ph or Et1b : R1 = Ph, R2 = H1c : R1 = R2 = CO2Me

R = Et or PhR' = Ph

SiMe2SiMe2 1 mol% [Cl2Pd(PPh3)2]

C6H6, refluxR3 R4+

R3 R4

SiMe2Me2Si

2a : R3 = R4 = CO2Me or H2b : R3 = Ph, R4 = H

1

2

Scheme 3. Kumada’s activated and Sakurai’s strained disilane bis(silylations) [36–38].

Me3Si SiMe3 Si2ClxMe6-x+1) 3 mol% [Cl2Pd(PR3)2]

2) MeMgBr3) H2O

. .Me3Si

Me3Si SiMe3

SiMe3

3

Me3Si

Me3Si SiMe3

SiMe3

+

4

Scheme 4. Bis(silylation) of diynes using chlorinated disilanes [41].

(MeO)Me2Si SiMe2(OMe)+[Pd(PPh3)4]

110 οC

(MeO)Me2Si SiMe2(OMe)

H

SiMe2(OMe)(MeO)Me2Si

H

5

Scheme 5. Bis(silylation) of 1,4-diethynylbenzene towards crack free sol-gels [43].

SiEt2

SiEt2

= carbon atom. = boron atom

Cr

SiMe2

SiMe2

7 8

Fig. 3. Ko’s o-carborane disilane 7 and Braunschweig’s [2]silachromoarenophanes 8[47,48]

Fe R1 R2+2 mol% [Pd(PPh3)4]

toluene, 110 οC

SiMe2

SiMe2

Fe

Me2Si

SiMe2

R1

R2

6a : R1 = R2 = H6b : R1 = Ph, R2 = H

6

Scheme 6. Palladium catalyzed ferrocenyldisilane insertion into alkynes [45].

M.B. Ansell et al. / Coordination Chemistry Reviews 336 (2017) 54–77 57

and Sakurai utilizing activated and strained disilanes, respectively.Kumada and co-workers demonstrated that activated disilanes, ofthe form X3-mMemSiSiMenX3-n (X = H, F, Cl or OMe; m = 1–2,n = 1–2), added to various alkynes when catalytic quantities of

[Cl2Pd (PR3)2] or [Pd(PR03)4] (R = Et or Ph and R0 = Ph) were

employed (Scheme 3) [36,37]. The extension of this protocol tonon-activated disilanes, such as hexamethyldisilane (Me3SiSiMe3),was unsuccessful. Elsewhere, Sakurai showed that the extent ofalkyne bis(silylation) using the strained cyclic disilane, 1,1,2,2-tetramethyl-1,2-disilacyclopentane, was dependent upon the choiceof alkyne [38]. Dimethyl acetylenedicarboxylate, phenylacetyleneand ethylene all underwent bis(silylation) (Scheme 3). However,no reaction was observed with the internal alkynes dipheny-lacetylene and bis(trimethylsilyl)acetylene.

Watanabe performed the bis(silylation) of acetylene using chlo-rinated disilanes, MenSi2Cl6-n (n = 2–5) [39]. The formation of theZ-1,2-disilylated alkenes was favored, although significantquantities of the E-isomers were noted. It was observed that uponheating, Z to E isomerization occurred in the presence of the Pd(0)complex. This work was extended to other activated disilanessuch as methoxymethyldisilanes, (MeO)mMe3-mSiSiMe3-n(OMe)n,as well as the acetylenes, 1-hexyne and trimethylsilylacetylene[40]. Bis(silylation) with Me3SiSiMe3 was extremely sluggish evenat temperatures of 140 �C.

Hiyama and co-workers utilized these chlorinated disilanes inthe palladium catalyzed bis(silylation) of bis(trimethylsilyl)bu-tadiyne [41]. Subsequent treatment of the reaction mixture withMeMgBr resulted in the formation of 1,1,4,4-tetrakis(trimethylsilyl)butatriene (3) and/or 1,1,2,4-tetrakis(trimethylsilyl)-1-buten-3-yne (4) (Scheme 4).

(PhSH2C)Me2Si

H

SiMe2(CH2SPh)

+

4 mol% [Pd(OAc)2]20 mol% ter t-octylisocyanide

toluene, 110 οC

9(PhSCH2)2Me4Si2

Scheme 7. Bis(silylation) of ethynyl[2.2]paracyclophanes [56].

Pd SiMe3SiMe3

NN

N

N

R1 R2 Me3Si SiMe3+1 mol% 10

C6D6, 100 οC, 24 h R1 R2

SiMe3Me3Si

11

11a : R1 = R2 = Ph11b : R1 = Ph, R2 = 4-MeC(O)C6H411c : R1 = Ph, R2 = 4-MeC6H411d : R1 = Ph, R2 = 1-naphthyl11e : R1 = Ph, R2 = SiMe311f : R1 = Ph, R2 = H

10

+

Me3Si SiMe3

1 mol% 10

C6D6, 100 οC, 24 h

Me3Si SiMe3

Me3Si SiMe3

11g

Scheme 8. Bis(silylation) of internal alkynes with Me3SiSiMe3 [57].

58 M.B. Ansell et al. / Coordination Chemistry Reviews 336 (2017) 54–77

The bis(silylation) of a number of internal and terminal alkynesusing the activated disilane Me3SiSiF2Ph was achieved by Ozawa[42]. The catalyst was generated in situ from a the mixture of1 mol% [Pd(g3-allyl)Cl] and 2 mol% PMe2Ph. Reactions were com-pleted within several hours at room temperature giving the corre-sponding Z-alkenes. The choice of disilane was essential with noreactivity arising from the use of Me3SiSiMe3 or PhF2SiSiF2Ph.

Loy and co-workers employed the activated disilane, 1,2-dimethoxy-1,1,2,2-tetramethyldisilane in the bis(silylation) of 1,4-diethynylbenzene to form 5 (Scheme 5). 5 then ring closed at eachalkenyl unit to form the corresponding disilaoxacyclopentenes.Subsequent hydrogenation with hydrogen gas using Pd on carbonresulted in a saturated monomer that underwent ring-openingpolymerization in tetrahydrofuran (THF) or in the presence of cat-alytic quantities of tert-butylammonium hydroxide (TBAH) givingrise to a crack free sol-gel in a matter of seconds [43].

SiiPr2

SiiPr2R+

4 mol% [(η2-ethylene)Pt(

200 οC

SiiPr2

SiiPr2R+

4 mol% [(η2-ethylene)Pt(

200 οC

Scheme 9. Bis(silylation) of alkynes with 3,4-benzo-1,1

Seyferth and co-workers demonstrated that the very reactiveand strained SiASi r bond in octamethyl-1,2-disilacyclobutanewas capable of bis(silylating) a number of alkynes including acet-ylene, phenylacetylene and dimethyl acetylenedicarboxylate whenusing catalytic quantities of [Cl2Pd(PPh3)2]. However, extension ofthis protocol to other internal alkynes was unsuccessful, even attemperatures of 140 �C [44].

Manners showed that the ferrocenyldisilane, [Fe(g5-C5H4)2-(SiMe3)2] inserted into acetylene or phenylacetylene to form theorganometallic rings 6a and 6b, respectively (Scheme 6) [45]. Thereaction of alkynes such as dimethyl acetylenedicarboxylateresulted in a mixture of mono- and di-insertion products withsignificant quantities of the alkyne cyclotrimerization product, acommon occurrence with alkynes such as dimethyl acetylenedi-carboxylate and acetylene [46]. Other palladium mediatedbis(silylations) of alkynes using strained disilanes have appearedin the literature: Ko’s ‘super-aromatic’ o-carborane disilane 7[47], and Braunschweig’s [2]silachromoarenophane 8 [48],bis(silylated) terminal and internal alkynes, respectively (Fig. 3).The cyclic nature of these disilanes pre-conditioned the formationof the Z-configured 1,2-disilylated alkene products.

In 1991, a communication from Ito and co-workers revolu-tionised the field of alkyne bis(silylation) by the introduction ofthe pre-catalytic combination of [Pd(OAc)2] and isocyanide ligands[49]. As a result, the bis(silylation) of alkynes was no longer limitedto activated or strained disilanes. A combination of 2 mol% [Pd(OAc)2]/30 mol% tert-octyl isocyanide was enough to catalyze thebis(silylation) of terminal alkynes such as 1-phenylpropyne,1-phenylhexyne, 1-nonyne and phenylacetylene using thenon-activated disilane, Me3SiSiMe3. Reactions proceeded at110 �C and resulted in unprecedented high stereoselectivities. Itoand co-workers extended this protocol to a range of bis(silylations)including the intramolecular bis(silylation) of alkynes in the stere-oselective synthesis of 1,2,4-triols [50], cyclic tetrakis(organosilyl)ethenes as organic chromophores [51], chiral allenylsilanes [52],and enantioenriched propargyl silanes [53].

Many authors since have utilized the [Pd(OAc)2]/isocyanidecombination within their own work. For example, Strohmann

PPh3)2]

SiiPr2

iPr2Si R

H SiiPr2

iPr2Si R

H+

12a : R = Ph12b : R = nBu

13a : R = Ph13b : R = nBu

PPh3)2]

SiiPr2

iPr2Si R

H

14a : R = mesityl14b : R = SiMe2Ph

,2,2,-tetra(isopropyl)-1,2-disilacyclobut-3-ene [58].

R + HMe2Si SiMe2H1 mol% Au/TiO2

benzene, 65 οCMe2Si SiMe2

R

R

SiMe2Me2Si

R

R

+

15 16

R

HMe2Si SiMe2H

HR H

SiMe2Au

Me2Si

R HAu/TiO2

Au/TiO2

double Si-H activation

Scheme 10. Gold catalyzed bis(silylation)-dehydrogenative addition [66].

Fe FeSiMe2H

SiMe2H+

arene, 65 oC

N2 atmFe

R

Me2SiSiMe2

Fe

R

SiMe2Me2Si

N N

17a : R = H17b : R = Me

R1 R2C6D6, 40 οC

Me2Si

SiMe2

R1

R2

18a : R1 = R2 = Me18b : R1 = H, R2 = Ph

17

18

Scheme 11. Formation of a bis(silyl)Fe(II) complex and resulting reactivity with alkynes [67].

N

NNiSiCl3

SiCl3Ph Ph+

19

N

NNi

Ph

SiCl3Ph

Cl3Si xs. MeMgBr Me3Si

Ph SiMe3

Ph

20 21

Scheme 12. Stoichiometric reaction of a bis(silyl)Ni(II) complex with diphenylacetylene [69].

SiEt2

SiEt2Ph Ph+

5 mol% [Ni(PEt3)4]Et2Si

SiEt2

Ph

Ph SiEt2

Et2Si

PhPh

+

25 26

Scheme 14. Ni(0) catalyzed bis(silyl)ation and phenylene-Si insertion [71].

SiF2

Ni(CO)2

F2Si

tBu

H

tBu H+SiF2

F2Si

SiF2

F2SitBu

H

tBu

H+

tBu

H tBu

H

22 23 24

Scheme 13. Nickel mediated bis(silylation) using a strained cyclic disilane [70].

M.B. Ansell et al. / Coordination Chemistry Reviews 336 (2017) 54–77 59

and co-workers used the pre-catalytic combination above in anumber of alkyne bis(silylations) using 1,1,2,2-tetramethyl-1,2-bis(phenylthiomethyl)disilane as the disilane source [54]. In particu-lar, the bis(silylation) of ethynyl[2.2]paracyclophanes resulted in

the formation of 9 (Scheme 7), which have potential applicationsin chiral catalysis and optoelectronic materials [55].

The addition of Me3SiSiMe3 to internal alkynes is consideredone of the most challenging reactions in alkyne bis(silylation)chemistry. Even Ito’s [Pd(OAc)2]/isocyanide combination was cat-alytically inactive [49]. Navarro, Spencer and co-workers reportedthe synthesis of the novel compound cis-[Pd(ITMe)2(SiMe3)2] (10,ITMe = 1,3,4,5-tetramethylimidazol-2-ylidene), via the oxidativeaddition of Me3SiSiMe3 to [Pd(0)(ITMe)2] under mild conditions.10 was subsequently employed as a pre-catalyst in the bis(silyla-tion) of a range of 1,2-diarylalkynes using Me3SiSiMe3 (Scheme 8)[57]. The resulting stilbenes 11 were synthesized with 100%Z-stereoselectivity and were either novel or previously only syn-thesized in a stoichiometric manner. This unprecedented protocolwas also extended to the bis(silylations) of two alkynes separatedby a phenyl linker, to 1-silyl-2-aryl alkynes and to terminalalkynes.

2.2.2. PlatinumIn contrast, the platinum catalyzed bis(silylation) of alkynes has

been investigated to a lesser extent. The most common

[(Me3P)3RhSi(SiMe3)3]Rh

SiMe3

Me3PMe3P

Me2SiSi

Me

Me

Me3SiMe

Me Me+

(Me3P)3RhSiMe3

Si(SiMe3)2

(Me3P)3RhSiMe2

SiMe(SiMe3)2 (Me3P)3Rh SiMe2

Me Me

SiMe(SiMe3)2

(Me3P)3Rh

Me2Si Me

Me(Me3Si)2MeSi

1,2-mig

1,3-mig

Me Me

red. elim.

oxid. addn.-PMe3

2728

I1

I2I3

I4

Scheme 15. Stoichiometric bis(silylation) of 2-butyne mediated by Rh(I) species[73].

O

Ph

SiMe2

SiMe3 O SiMe2

SiMe3

Ph

5 mol% [RhCl(PPh3)3]

toluene, 110 οC, 4 h

R

SiMe2

SiMe3

5 mol% [RhCl(CO)2]2

toluene, 110 οC, 3-32 h SiMe2

SiMe3

R

30

29

30a : R = Ph30b : R = 3,5-Me2C6H330c : R = 2-MeC6H430d : R = 4-MeOC6H430e : R = 4-O2NC6H4

30f : R = 3-AcC6H430g : R = 5-Me-2-thienyl30h : R = Me30i : R = SiMe330j : R = H

Scheme 16. Rh(I)-catalyzed intramolecular trans-bis(silylation) of alkynes [74].

HnHex B BO

O O

O+

3 mol% [Pt(PPh3)4]

DMF, 80 οC, 24 hnHex

B B

H

OO O

O

31

Scheme 17. The first diboration of alkynes catalyzed by [Pt(PPh3)4] [80].

60 M.B. Ansell et al. / Coordination Chemistry Reviews 336 (2017) 54–77

bis(silylation) mediator is [(g2-ethylene)Pt(PPh3)2]. Ishikawadetailed the bis(silylation) of a number of alkynes using 3,4-benzo-1,1,2,2,-tetra(isopropyl)-1,2-disilacyclobut-3-ene [58]. The reac-tivity and product selectivity using this platinum catalyst differedfrom the palladium analogues and depended on the alkyne used,notably employing extreme temperatures. Reactions with1-hexyne and phenylacetylene resulted in a mixture of 12 and13. The bulky mono-substituted alkynes mesitylacetylene and (phenyldimethylsilyl)acetylene formed 13 as the sole product, whereasdiphenylacetylene resulted in only the 1,2-disilylated alkene 14(Scheme 9).

Investigations into the reactivity of 1,2-bis(dimethylsilyl)carborane by Ko and co-workers were extended to the platinum cat-alyzed bis(silylation) of alkynes. Normal 1,2-bis(silylation) wasobserved in the reaction with phenylacetylene, diphenylacetylene,3-hexyne, 2-butyne and dimethyl acetylenedicarboxylate. How-ever, the use of 1-hexyne resulted in geminal or 1,1-bis(silylation) and the formation of a five-membered disilyl ring[59]. A later report by Ishikawa described the bis(silylation) ofrange of terminal and internal alkynes using cis- and trans-1,2-dimethyl-1,2-diphenyl-disilacyclopentane. The reactions proceededwith high stereospecificity and translation of the cis or trans natureof disilane in all cases [60].

2.2.3. GoldThe redox chemistry between gold(I)/(III) is similar to that of

palladium(0)/(II), given that they are isolobal. This has triggeredsubstantial research into the development of gold catalysts thatare as active as their palladium analogues [61–63]. Despite thiseffort, gold catalysis is very much in its infancy with the onlyreports of alkyne bis(silylation) in the literature being mediatedby gold nanoparticles supported on titanium oxide (Au/TiO2)[64]. Stratakis and co-workers showed that the bis(silylation) of arange of terminal alkynes using hexamethyldisilane and 1,2-diphenyl-1,1,2,2-tetramethyldisilane was possible [65]. In all cases, theZ-alkenes were favored with a small percentage of the E-isomersformed. The heterogeneous catalyst gave comparable activitiesupon recycling. Stratakis extended the protocol to 1,1,2,2-tetramethydisilane (HMe2SiSiMe2H). However, two isomers (15,major, and 16, minor) were isolated (Scheme 10). Mechanistically,this observation was explained by an initial bis(silylation) followedby a dehydrogenative addition to a second alkyne [66].

2.2.4. IronSunada and co-workers reacted 1,2-bis(dimethylsilyl)benzene

with [Fe(mesityl)2]2 (mesityl = 2,4,6-Me3C6H2) in aromatic sol-vents under a nitrogen (N2) atmosphere to form 17 (Scheme 11).Subsequent addition of 2-butyne or phenylacetylene resulted inthe quantitative formation of the disilacarbocycles 18a and 18b,respectively (Scheme 11) [67]. This process was made catalyticupon addition of 1,2-bis(dimethylsilyl)benzene to phenylacetyleneand 20 mol% of Fe [68]. Although this is not a bis(silylation) in thetraditional sense (it lacks a SiASi r bond and it proceeds through adehydrogenative double silylation) it is still a very rare example ofan iron mediated bis(silylation) of alkynes.

2.2.5. NickelKumada and co-workers accomplished the first examples of

alkyne bis(silylation) using a nickel mediator. Bis(silyl)bipyridyl-nickel(II) complex 19 reacted with diphenylacetylene to form 20.Treatment of the latter with MeMgBr, followed by an acidic workup, resulted in the isolation of E-1,2-bis(trimethylsilyl)stilbene(21) (Scheme 12) [69]. Extension to other alkynes yielded mixturesof Z- and E-alkene products.

At the same time, Liu showed that a tetrafluorodisilacy-clobutene underwent oxidative addition to [Ni(CO)4]. The

corresponding bis(silyl)Ni(II) complex 22 was reacted with tert-butylacetylene to form the two new 1,4-disilacyclohexadienes 23and 24, where the tBu groups are syn and anti, respectively(Scheme 13) [70].

The first catalytic bis(silylation) of alkynes employing nickelwas reported by Naka and co-workers. The reaction of 3,4-benzo-1,1,2,2-tetraethyl-1,2-disilacyclobutene with diphenylacetylene inthe presence of catalytic amounts of [Ni(PEt3)4] formed theZ-alkene 25 [71]. As well as bis(silylation), an alkyne insertion intoone of the phenylene-Si bonds occurred, with 26 isolated as aminor product (Scheme 14). This type of insertion was consistentlyobserved on applying the methodology to other alkynes [72].

2.2.6. RhodiumExamples of rhodium mediated alkyne bis(silylations) are rare.

Tilley and co-workers carried out the stoichiometric reaction of[(Me3P)3RhSi(SiMe3)3] (27) with 2-butyne, resulting in the

PtPh3P

Ph3PnPr

nPr

B BO

O O

O+ Pt

BPh3P

Ph3P B OO

OO

nPr

B B

nPr

OO O

O+

32 33

Scheme 18. Platinum-mediated stoichiometric diboration of an alkyne [87].

R1 R2 +

3 mol% [Pt(PPh3)2(η2-C2H4)]or

3 mol% cis-[Pt(PPh3)2(B{4-tBucat})2](34)

toluene, 80 οC R1 R235

35a : R1 = C6H13, R2 = H35b : R1 = Ph, R2 = H35c : R1 = 4-MeOC6H4, R2 = H35d : R1 = 4-NCC6H4, R2 = H

35e : R1 = R2 = Ph35f : R1 = R2 = 4-MeC6H435g : R1 = R2 = 4-MeOC6H4

B BO

OO

O

BB OOO

O

Scheme 19. Pt(0) and Pt(II) catalyzed diboration of internal and terminal alkynes [88].

R P(OEt)2O

B BO

O O

O+

3 mol% [Pt(PPh3)4]

toluene, 80 οCB

R P(OEt)2

B

O

OO O

O

R B BO

O O

O+

3 mol% [Pt(PPh3)4]

toluene, 80 οC

B

R

BO

O OO

BO

O

BO

O

37a/38a : R = nBu 37a/38b : R = Ph

37

38

Scheme 21. Diboration of 1-alkynylphosphonates and 1-alkynylboronates [91].

B BCl

NMe2Cl

Me2NBNBMe2N

R1 R2

ClCl

MeMe

R2R1 +5 mol% [Pt(PPh3)2(η2-C2H4)]

toluene, 95 οC

3636a : R1 = Ph, R2 = Me 36d : R1 = R2 = 4-MeOC6H436b : R1 = R2 = Ph 36e : R1 = nOctyl, R2 = H36c : R1 = R2 = 4-MeC6H4 36f : R1 = Ph, R2 = H

Scheme 20. Diboration-rearrangement of alkynes using 1,2-B2Cl2(NMe2)2 [89].

M.B. Ansell et al. / Coordination Chemistry Reviews 336 (2017) 54–77 61

isolation of the Rh(III) complex 28 [73]. The authors proposed that27 undergoes a facile silyl 1,2- and 1,3-migration (I1 and I2,respectively) process in the presence of alkyne resulting in a[2+2] cycloaddition and the formation of a transient metallasilacy-clobutene I3. The reductive elimination of a SiAC bond in I3 gives aRh(I)-silyl intermediate I4, which then loses one PMe3 ligand. Thisinduces an oxidative addition of a SiASi bond in the tetheredtrisilyl group and subsequent formation of 28 (Scheme 15).

Matsuda and co-workers studied the rhodium(I)-catalyzedintramolecular bis(silylations) of alkynes. Initial testing and opti-mization were executed on the disilanyl ether of a propargylicalcohol [74]. It was observed that 4-silyl-2,5-dihydro-1,2-oxasilole (29) was formed as the sole product (Scheme 16). Thistrans-bis(silylation) proceeded with the complete oppositestereoselectivity to the analogous palladium-catalyzed reaction.

The protocol was then extended to a variety of (2-alkynylphenyl)disilanes affording the corresponding 3-silyl-1-benzosiloles (30)(Scheme 16).

2.3. Boron–Boron (BAB)

2.3.1. PlatinumDue to their low toxicity, high stability under atmospheric con-

ditions and versatile reactivity, the synthesis of organoboronreagents has attracted significant interest. In particular, there issubstantial focus towards Z-1,2-diborylated alkenes as the prod-ucts of alkyne diboration [75]. The resulting newly formed BACbonds are able to participate in Suzuki-Miyaura cross-couplingreactions [76], to build more complex and useful tri- and tetra-substituted alkenes. The first source of BAB bonds investigatedwas diboron tetrahalides. These contain the most reactive BABbond available (the lack of p-donating substituents destabilizesthe boron based p-orbital, this increases the Lewis acidity of theboron atoms and therefore their susceptibility towards nucle-ophilic attack) and often react with unsaturated organic substrateswithout the need for a transition metal mediator or catalyst [77].However, the preparations of diboron tetrahalides are difficultand this therefore limits their synthetic utility [78,79].Tetraalkoxy- and tetraaryloxydiborons are air stable, easily han-dled and, despite their relatively high BAB bond strengths, arenow widely utilized in the stoichiometric and catalytic additionof BAB bonds to alkynes. These diboron reagents will be the mainfocus of this section.

Platinum is by far the most effective and widely studied medi-ator of alkyne diboration. Seminal results by Suzuki and Miyaura[80] indicated that 1-octyne inserted into the BAB bond of bis(pinacolato)diboron (B2pin2) using catalytic quantities of [Pt(PPh3)4] to form 31 (Scheme 17). The protocol was then extendedto a range of internal and terminal alkynes with similarly highstereoselectivities obtained. The rate of diboration was drasticallyaffected by the polarity of the solvent, with the more polar solvents(e.g. DMF) accelerating the rate. However, the authors latershowed that hexane also accelerated the reaction rate and to agreater extent than most polar solvents [81]. Other transitionmetal complexes proved ineffective within this study, e.g. [Pd(PPh3)4] and [Pd(OAc)2]/isocyanide (the best catalysts in the bis(silylation) of alkynes). Suzuki-Miyaura’s coupling protocols have

R1 R2 B BO

O O

O+

1) 2.5 mol% [Pt(PPh3)4], DMF, 80 οC

2) Selectfluor, MeCN, r.t. R1

O F

R2F

39a : R1 = Ph, R2 = H39b : R1 = R2 = Ph

39

N+N+

Cl

F

2 BF4-Selectfluor =

Ph

BB

Ph

OO O

O

39b

Selectfluor

'wet' MeCN

HO

PhH

F

PhF

40

Scheme 22. Stepwise diboration/fluorodeboronation of alkynes [93].

XY N

NH

nBu

nBu

+

I-1) Ag2O, DCM

2) Kardstedt's catalystPt

NXY N

nBu

nBuSiO

Si

4141a : X = Y = C-Cl41b : X = N, Y = C-H41c : X = Y = C-H

Me2SiO

Me2SiPt

SiMe2OSiMe2

Pt

Me2SiO

Me2Si

Kardstedt's catalyst

Scheme 23. Synthesis of NHC-platinum-based catalysts in diboration of alkynes[98].

MBB

NMe2

NMe2n()

n()

Mn()

n() B

B

Me

Me

NMe2

NMe2

[Pt(PEt3)3]

Me Me

42

MBB

NMe2

NMe2n()

n()

Mn()

n() B

B

Me

Me

NMe2

NMe2

Me Me

42

+

5 mol% [Pt(PEt3)3]or

6 mol% Pt sponge

80-100 οC, 2-8 days

n =1 or 2, M = Fe or Cr

Scheme 24. Stoichiometric and catalytic diboration of alkynes using [2]boramet-allarenophanes [99,100].

62 M.B. Ansell et al. / Coordination Chemistry Reviews 336 (2017) 54–77

since been widely employed en route to, for example, enantiomer-ically enriched 1,2-diols [82], 5-benzylidenylbenzopyridyloxepineanalogues as nuclear hormone receptors [83], 1H-phosphindolesas chiral helicenes [84], 10-mesitylborylsubstututed-dibenzoborepin as a photoresponsive material [85], and the pentacyclicalkaloid tylophorine [86].

B BO

O HN

HNR H +

2 m2 mol%

tolu

(pinB-Bdan)

43a : R = Ph'

43b : R = 4-MeC6H443c : R = 2-MeC6H443d : R = 4-MeOC6H443e : R = 4-AcC6H4

43f : R = 4-EtO43g : R = 4-Br43h : R = 2-Br43i : R = 2-thi43j : R = nHex

Scheme 25. Diboration of alkynes usi

Smith and co-workers carried out a stoichiometric diborationby reacting commercially available bis(catecolato)diboron(B2cat2) with [(g2-4-octyne)Pt(PPh3)2]. This resulted in theoxidative addition bis(boryl)Pt(II) complex 32, and theZ-1,2-diborylated alkene 33 (Scheme 18) [87].

Marder and Norman extended the synthesis of bis(boryl)plat-inum(II) complexes to the use of other diborons including B2pin2

and B2(4-tBucat)2 (4-tBucat = 1,2-O2-tBuC6H3). [(g2-ethylene)Pt(PPh3)2] and 34 were then used as catalysts in the diboration ofterminal and internal alkynes employing B2pin2 and B2cat2 asBAB bond sources (Scheme 19). These catalysts were more effi-cient in the stereoselective formation of Z-1,2-diborylated alkenesthan [Pt(PPh3)4], with reactions proceeding smoothly using 3 mol%of either catalyst at 80 �C. The rate and conversions were signifi-cantly affected by the choice of substituents on the alkyne andthe diboron reagent. The presence of p-donating moieties on thealkyne resulted in faster reactions than p-withdrawingsubstituents and the fastest conversions proceeded in the orderof B2cat2 > B2pin2 > B2(4-Butcat)2 [88].

The platinum catalyzed diboration of internal and terminalalkynes using diboron 1,2-B2Cl2(NMe2)2, affords cyclic 1-azonia-2-borata-5-boroles (36) (Scheme 20). The key feature within thesestructures was that the boron and nitrogen atoms exhibited both athree and four-coordinated center. Although the mechanism forforming 36 was unclear, the authors proposed an initial diborationfollowed by a rearrangement of the BACl and BANMe2 bonds [89].

Baker and co-workers developed a phosphine-free platinumcatalyzed diboration of 1-octyne and di-p-methylphenylacetyleneusing B2cat2 [90]. The reactions proceeded using 5 mol% of thecommercially available [Cl2Pt(cod)] (cod = 1,5-cyclooctadiene) at55 �C. This protocol was highly dependent on the choice of diboronsource, no catalytic activity was observed using B2pin2, as well asthe choice of halide and diene on the platinum metal. [Br2Pt(cod)] required pre-stirring for 24 h before a homogeneous cat-alytic mixture was obtained and even then reaction yields werelower. The use of dicyclopentadiene instead of cod as a ligand also

ol% [Pt(dba)2]P[3,5-(CF3)2-C6H3]3

ene, 80 οC, 24 h B B

R H

OO NH

HN

43

2CC6H4C6H4C6H4ophenylyl

ng the diboron, pinB-Bdan [101].

Pd

NN

NN

Ph

Ph

R1 R2 + B BO

OO

O

0.5 - 2 mol% 44

C6D6, r.t. - 100 οCR1

B B

R2

OO O

O

39/45

39a : R1 = Ph, R2 = H39b : R1 = R2 = Ph45a : R1 = 2-MeC6H4, R2 = H45b : R1 = 4-FC6H4, R2 = H45c : R1 = 4-MeOC6H4, R2 = H45d : R1 = 4-F3CC6H4, R2 = H45e : R1 = 3,5-(F3C)2-C6H3, R2 = H

45f : R1 = Mes, R2 = H45g : R1 = cyclohexen-1-yl, R2 = H45h : R1 = 4-MeOC6H4, R2 = 4-EtC6H445i : R1 = Ph, R2 = 1-naphthyl45j : R1 = Ph, R2 = SiMe3

44

Scheme 26. Palladium catalyzed diboration of internal and terminal alkynes [105].

Ir

N

SiiPr2

R H + H BO

O5

1) 1 mol% 49C6H5F, r.t., 10 min

2) 1 atm CO55 οC, 18 h

B

R B

B

OO

OOO

O

51

51a : R = 4-MeC6H451b : R = 4-MeOC6H451c : R = (CH2)2Ph51d : R = (CH2)3Cl51e : R = 4-CF3'C6H4

51f : R = Ph51g : R = nBu51h : R = SiMe351i : R = (CH2)2OSiMe3

HIr

N

NSiiPr2

H

CO1 atm CO

r.t., 10 min

49 50

Scheme 29. Iridium catalyzed dehydrogenative borylation/diboration of terminalalkynes [108].

nPr nPr B BO

O O

O+

10 mol% FeBr210 mol% LiOMe1.5 eq. MeOBpin

THF, 80 οCnPr

B B

nPr

OO O

O

47

nPr nPr B BO

O O

O+

10 mol% FeBr210 mol% LiOMe1.5 eq. MeOBnep

THF, 80 οC

MeOBnep = MeO BO

O

B

nPr nPr

BO

O OO

48

Scheme 28. Iron(II) catalyzed diboration of alkynes in a symmetric and unsym-metric manner [107].

F3C CF3

B BO

O O

O

+5 mol% [Co(PMe3)4]

THF, 80 οC, 24 h

B BO

O OO

F3C CF346

Scheme 27. Cobalt(0) catalyzed diboration of an internal alkynes [106].

M.B. Ansell et al. / Coordination Chemistry Reviews 336 (2017) 54–77 63

resulted in the formation of the 1,2-diborylated alkenes in loweryields.

In a study into new routes for the preparation of 1,1-geminalsp2-organobismetallic derivatives, Srebnik and co-workersdemonstrated the platinum catalyzed diboration of1-alkynylphosphonates and 1-alkynylboronates afforded theZ-1,2-diborylated vinylphosphonates and trisboronated alkeneproducts 37 and 38, respectively (Scheme 21) [91]. The reactionwith alkynylboronates was extremely sensitive to the moisturecontent of the solvent with ‘wet’ solvents resulting in BAC bondcleavage via a hydrodeboronation. Elsewhere, Nishihara reportedthe platinum catalyzed diboration of phenylethynyl MIDA(MIDA = N-methylimidiacetic acid) boronate with B2pin2 to form1,1,2-triboryl-2-phenylethene [92].

Fernandez and co-workers reported the preparation ofa,a0-difluorinated carbonyl compounds. The reactions proceededby an initial platinum(0) catalyzed diboration of internal andterminal alkynes to form Z-1,2-diborylated alkenes. Subsequentwork-up with the electrophilic fluoro-deboronation agent1-(chloromethyl)-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane dite-trafluoroborate (or Selectfluor) resulted in formation of thea,a0-difluorinated carbonyl compounds 39 (Scheme 22) The stere-ochemistry of the diborylated alkene remained in the fluorinatedcarbonyls. If trace amounts of water were found within the solventor reaction mixture the difluoromethyl alcohols 40 were isolated[93]. The authors later optimized this protocol to a one-potdiboration/fluoro-deboronation microwave procedure. Thisresulted in the shortening of reaction times to several minutesand the lowering of catalyst loadings to as little as 0.05 mol% [94].

Fernandez and co-workers extended their investigations intothe use of N-heterocyclic carbene (NHC) [95–97] platinum com-plexes as catalysts. The platinum species 41 were formed by thetransmetallation reaction between the corresponding NHC-silvercompound and Karstedt’s catalyst (Scheme 23). Initial assessmentof 410s catalytic activity in the diboration of alkynes found that41b, with triazoylidene carbene, was the most active and suitablemediator for this reaction. A range of internal and terminal alkyneswere then diborylated using B2cat2 and 5 mol% of 41b [98].

Braunschweig and co-workers demonstrated that alkynes couldinsert into the BAB bond of [2]borametallarenophanes. These BABbonds were deemed moderately strained, but thermally stable. Thediboration was achieved stoichiometrically using [Pt(PPh3)4] and10 equivalents of 2-butyne to yield the ansa-bis(boryl)alkenes 42(Scheme 24) [99]. The diboration was also completed catalyticallyunder both homogeneous and heterogeneous conditions overseveral days [100].

The stereoselective, stepwise reactions of two non-equivalentboryl groups is highly desirable in the catalytic diboration ofalkynes, as it may enable sequential couplings. Suginome andco-workers developed an unsymmetrical diboron, pinB-Bdan

64 M.B. Ansell et al. / Coordination Chemistry Reviews 336 (2017) 54–77

(pin = pinacolato; dan = naphthalene-1,8-diaminato) (Scheme 25)[101]. In the presence of phosphine-platinum catalysts, the dibora-tion of terminal alkynes resulted in the regioselective formation of43 with Bdan, a boryl protecting group, in the terminal position.The palladium-catalyzed Suzuki-Miyaura cross-coupling occurredchemoselectively on the more reactive internal Bpin. This was insharp contrast to the B2pin2 based diborations, where the couplingselectively proceeds initially at the more reactive terminal Bpingroup.

Escribano and co-workers showed that titania-supported plat-inum nanoparticles were efficient catalysts for the diboration ofalkynes under solvent and ligand free conditions in air. Terminaland internal alkynes were accessible at 70 �C using 0.2 mol% ofPt/TiO2. A range of electron-donating and withdrawing aromaticor alkyl, branched and cycloalkyl substituents were accessible.Exclusively Z-1,2-diborylated alkenes were observed in all cases[102]. In contrast, when the support was magnesia (MgO), higherloadings and the use of solvent and elevated temperatures of130 �C were required [103].

2.3.2. PalladiumPalladium-catalyzed diborations of alkynes are rare. Examples

employing [2]borametalloarenophanes were reported byBraunschweig and co-workers. Palladium on carbon was utilizedas a heterogeneous catalytic source, however the diborationsrequired higher temperatures and much longer conversion timesthan their platinum analogues [99,100]. The rarity of palladiummediated alkyne diborations can be attributed to the energetics ofthe BAB bond oxidative addition at the Pd(0) center. Theoretical cal-culations suggest that this is both a kinetically and thermodynami-cally unfavorable process [104]. Despite this, Spencer, Navarro andco-workers recently accomplished the facile diboration ofterminal and internal alkynes using catalytic amounts of[Pd(ITMe)2(PhC„CPh)] (44) [105]. This represented the firstexamples of alkyne diborations utilizing a homogeneous palladiumcatalyst. Both terminal and internal alkynes were accessible with100% syn-stereoselective formation of the corresponding1,2-diborylalkenes 45 (Scheme 26). These diborations proceededwith higher yields and/or under milder reaction conditions thantheir platinum analogues.

A computational study to determine the mechanistic route sug-gested that Pd(0) catalyzed alkyne diborations using NHC ligandsfollowed the same catalytic cycle as phosphines (as detailed inthe introduction of Section 2) [105]. An integral process in thispathway was, as in the case of phosphines, the reversible dissocia-tion of an NHC ligand. These NHC ligands were also key to a

SiMe3

OTfR B B

O

O O

O+

2 mo

1.

1,2-dietho

52a : R = H52b : R = 4-Me52c : R = 4-tBu52d : R = 4-Ph52e : R = 4,5- −(C

MeO OMeB B

O

O O

O+

2 mo7 m

tolue

Scheme 30. Copper-catalyzed diboration of benzyne

successful oxidative addition of the BAB bond at the Pd(0) center.They destabilize the (diboron)Pd(0)L2 (L = ITMe) adduct resultingin a sufficient lowering of the free energy for oxidative addition.

2.3.3. CobaltIn their investigations into the diboration of alkynes, Marder

and co-workers described the diboration of 1,2-bis(4-(trifluoromethyl)phenyl)ethyne with B2cat2 (Scheme 27) using a [Co(PMe3)4]catalyst. Compound 46 was isolated as the major product of thisreaction, with small quantities of the E-isomer detected [106].

2.3.4. IronThe only example of iron catalyzed diboration of alkynes was

detailed by Nakamura in 2015. Initial optimizations focused onthe diboration of 4-octyne using B2pin2. Catalytic quantities ofFeBr2 and LiOMe with 1.5 equivalents of MeOBpin were enoughto afford 47 in high yields [107]. On extending to other Fe(II) andFe(III) catalysts, yields dramatically decreased. The diboration ofa variety of internal alkynes was possible: those with alkyl sub-stituents proceeded in high yields, whereas aryl or bulky alkylgroups retarded the diboration. The role of the additional boratingagent was also assessed. In the absence of MeOBpin the reactionsstill proceeded, but with lower conversions. When using MeOBnep(MeOBnep = 2-methoxy-5,5-dimethyl-1,3,2-dioxaborinane) theunsymmetrical diborylalkene 48was isolated as the major product(Scheme 28), suggesting that the incorporation of the second borylunit was introduced by an electrophilic substitution reaction withMeOBnep or MeOBpin.

2.3.5. IridiumOzerov and co-workers devised a two-step reaction to convert

alkynes into trisborylalkenes. The first step transformed terminalalkynes into alkynylboronates using pinacolborane (HBpin) andiridium complex 49 as a catalyst. Degassing this reaction mixturefollowed by the introduction of a CO atmosphere generated thenew catalyst 50, which mediated the dehydrogenative diborationof the newly formed alkynylboronate with HBpin to form 51(Scheme 29) [108]. This reaction was extended to a range of alkyland aryl terminal alkynes. The authors proposed the reaction toproceed via a hydroboration intermediate or via B2pin2.

2.3.6. CopperExamples of group 11 transition metal catalyzed diboration of

alkynes are very rare, with only one example of copper and oneof gold described in the literature [109]. The first diboration ofalkynes employing a copper catalyst was performed by Yoshida.

l% [(Ph3P)3CuOAc]3 eq. KF

5 eq. [18]crown-6

xyethane, 100 οC, 7-60 hR

B

B

O

O

O

O

52

H2)3−

52f : R = 4,5- −(CH2)4−52g : R = 4,5-Me252h : R = 4,5-(nHex)252i : R = 4-SiMe3

l% [Cu(OAc)2]ol% P(tBu)3

ne, 80 οC, 9h

B B

B BO

O O

OO

OOO

53

s and tetraborylation of propargyl ethers [110].

R1 R2 Si BO

O+

2 mol% [Pd(OAc)2]30 mol% tOcNC

toluene, 110 οC

Si

R1 R2

B OO

54

54a : R1 = nHex, R2 = H54b : R1 = Ph, R2 = H54c : R1 = THPO, R2 = H54d : R1 = MEMO, R2 = H

54e : R1 = cyclohexyl, R2 = H54f : R1 = R2 = Ph54h : R1 = R2 = nBu

Si BO

O+

2 mol% [Pd(OAc)2]30 mol% tOcNC

toluene, 110 οC

BSi

HB

H

Si

O

OO

O

55

Scheme 31. Silaboration of alkynes employing a palladium/isocyanide catalyst [117].

Si

nHex H

B OO

54a

+

I

Me

3 mol% [PdCl2(ddpf)]

KOHdioxane, H2O90 οC, 3 h

Si

nHex H

Me

56

ddpf = 1,1'-bis(diphenylphosphino)ferrocene dppb = 1,4-bis(diphenylphosphino)butane

Si

nHex H

B OO

54a

+O

3 mol% [Rh(CO)2(acac)]3 mol% dppb

MeOH, H2O50 οC, 24 h

Si

nHex H O

57

Scheme 32. Suzuki-Miyaura cross-coupling and conjugate additions at a C-boryl group [118].

Si BN

N+

Me

Me

2 mol% [Pd2(dba)3]5 mol% epto

C6D6, 110 οC, 2 h

SiMe2Ph

BN

NMe

59

nHex H Si BN

NMe

Me

2 mol% [Pd2(dba)3]5 mol% epto

C6D6, 110 οC, 2 h+

nHex

Si B

H

NN

Me

Me

58

Scheme 33. Silaboration and silaboryl carbocyclixation of terminal alkynes [122].

M.B. Ansell et al. / Coordination Chemistry Reviews 336 (2017) 54–77 65

The diboration of alkyl and aryl internal alkynes using B2pin2 in thepresence of [Cu(OAc)2] and PCy3 resulted in high yields of the cor-responding Z-1,2-diborylated alkenes [110]. The authors alsoextended this to the diboration of benzynes to form the resulting1,2-diborylated benzenes (52) (Scheme 30). Changing the phos-phine to P(tBu)3, P(nOc)3 or PPh3 resulted in either prolonged reac-tion times or lower yields. A striking feature of this copper catalysiswas the diboration of propargyl ethers. In all cases the tetrabory-lated product 53 was exclusively isolated (Scheme 30).

2.3.7. GoldNanopourous gold (AuNPore), prepared by dealloying the

monolithic Au30Ag70 alloy in a 70% nitric acid electrolyte, is a

highly active catalyst in the diboration of alkynes. Jin and co-workers optimized a system with phenylacetylene and B2pin2 uti-lizing 2 mol% of AuNPore at 100 �C [111]. The AuNPore catalystwas recyclable with no notable decrease in catalytic activity overmultiple cycles. The protocol was extended to a variety of terminaland internal alkynes, however other diborons were ineffective.Mechanistically, the authors proposed absorption of the B2pin2

onto the AuNPore surface. The BAB bond is then cleaved at thelow coordinate Au atoms to give an Au-Bpin species. The alkynethen adsorbs and reacts rapidly with two Au-Bpin species eitherthrough a simultaneous addition path to form the correspondingZ-adduct or in a stepwise manner.

2.4. Silicon–Boron (SiAB)

2.4.1. PalladiumSilylboranes are attractive precursors in the element–element

additions to unsaturated substrates such as alkynes. According tothe Pauling scale, the electronegativity difference between the Si(2.12) and B (1.88) atoms [112], is such that 1-boryl-2-silyl alkenesare synthesized with chemo-, regio- and stereoselective control ina single transformation [113,114]. The boron and silicon function-alities in these alkene adducts can subsequently undergo chemos-elective stepwise reactivity towards the preparation of morecomplex and unsymmetrical tri- and tetra substituted alkenes[115,116]. The most widely used catalysts for the silaboration of

R H

Si BClO

O+

1) 1 mol% [(η3-C3H5)PdCl(PPh3)]toluene, r.t., 1-3 h

2) iPrOH, pyridine, 50 οC, 24 h R

Si B

H

OOiPrO

+R

Si

B

H

OO

iPrO

60 61

excess alkyneexcess silaborane

>99 : <18-11 : 89-92

60a/61a : R = nHex60b/61b : R = nBu60c/61c : R = NC(CH2)360d/61d : R = TBSO(CH2)2

TBS = ter t-butyldimethylsilyl

60e/61e : R = TBSO(CH2)360f /61f : R = Cl(CH2)360g/61g : R = nOct

Scheme 34. Reagent dependent stereoselective silaboration of terminal alkynes [124].

R H

Si BClO

O+

1) 1 mol% [(η3-C3H5)PdCl(L)]toluene, r.t., 2 h

2) iPrOH, pyridine, r.t., 1 h R

B Si

H

OiPrOO

62

62a : R = nBu62b : R = nOct62c : R = tBuMe2SiO(CH2)262d : R = tBuMe2SiO(CH2)362e : R = AcO(CH2)3

62f : R = Cl(CH2)362g : R = NC(CH2)362h : R = cyclohexyl62i : R = tBuMe2SiOCH(Me)

P(tBu)2L =

Scheme 35. Ligand controlled, stereoselective ‘abnormal’ regioselective silabora-tion [125].

Si BEt2NO

O

R H

+

1 mol% [Pd(dba)2]1.2 mol% PPh3

toluene, r.t. SiMe2

SiMe2

R

R R R

+

major66

minor67

66a/67a : R = Ph66b/67b : R = 4-MeC6H466c/67c : R = 4-MeOC6H466d/67d : R = 4-Me2NC6H4

66e/67e : R = 4-CF3C6H466f /67f : R = 2-MeC6H466g/67g : R = 2,4,6-Me3C6H266h/67h : R = 1-naphthyl

Scheme 37. Regioselective synthesis of disubstituted siloles [127].

66 M.B. Ansell et al. / Coordination Chemistry Reviews 336 (2017) 54–77

alkynes are group 10 transition metal complexes, specificallypalladium-containing complexes.

Ito’s palladium/tert-alkyl isocyanide catalyst combination,previously detailed in alkyne bis(silylations), was effective in thesilaboration of both terminal and internal alkynes to formsyn-1-boryl-2-silyl alkenes (54) with high regio- and stereoselec-tivities (Scheme 31) [117]. The silylborane of choice was(dimethylphenylsilyl)boronic acid pinacol ester (PhMe2SiBpin);

R1

Si B

R2

ClO

OMOH

toluene, 50

63a : R1 = H, R2 = nHex, M = K63b : R1 = H, R2 = nHex, M = Na63c : R1 = H, R2 = nHex, M = Cs63d : R1 = H, R2 = tBuMe2SiO(CH2)3, M = K63e : R1 = H, R2 = Cl(CH2)3, M = K63f : R1 = H, R2 = NC(CH2)3, M = K

666666

64a

H nHex

B-O

SiO

OK+ 1.5 eq. 4-iodoa

5 mol% Pd[P(t

H2ODMSO, 80 οC

63h

nHex H

B-O

SiO

OK+ 1.5 eq. 4-iodo

5 mol% Pd[P

H2ODMSO, 80

Scheme 36. Silylborate formation and resultin

this SiAB compound is thermally stable under inert conditionsand the Bpin functionality improves the stability of the subsequentorgano-compounds towards hydrolysis during purification. In thecase of terminal alkynes, the silaboration proceeded with theaddition of the boryl group at the terminal position. Attempts atsilaboration employing other metal complexes resulted in eitherlower yields and mixtures of regioisomers (e.g. [Pt(PPh3)4]) or noactivity (e.g. [RhCl(PPh3)3]).

The authors later extended this protocol to other silylboranes(i.e. PhMe2SiB(NEt2)2 and PhMe2SiBcat) and to a larger array of

οC R1 R2

B-O

SiO

OM+

63

3g : R1 = H, R2 = cyclohexyl, M = K3h : R1 = nHex, R2 = H, M = K3i : R1 = tBuMe2SiO(CH2)3, R2 = H, M = Na3j : R1 = Cl(CH2)3, R2 = H, M = Na3k : R1 = NC(CH2)3, R2 = H, M = Na3l : R1 = cyclohexyl, R2 = H, M = Na

nisoleBu)3]2

, 2h

SipinBO

nHexH

OMe

64

anisole(tBu)3]2

οC, 2h

Si

nHex H

pinBO

OMe

65

g external-base free cross-coupling [126].

R1R2

Si BO

O+

10 mol% [Pd(acac)2]12 mol% L

DIBALHtoluene, 80 οC, 44 h

B

R1

Si

R2

O

O

L = PEt3, P(OEt)3, PPhMe2 68

68a : R1 = R2 = nBu68b : R1 = nOct, R2 = H68c : R1 = H, R2 = nOct

R Si BO

O+ .

R

Si

BO

O

5 mol% [Pt(acac)2]6 mol% P(OCy)3

DIBALHtoluene, 80 οC, 20 h 69

Scheme 38. Substrate controlled silaborations of 1,3-enynes [128].

R1 R2 Si BO

O+

0.5 - 4 mol% 44

C6D6, r.t. - 100 οCR1

Si B

R2

OO

54/70

54a : R1 = nHex, R2 = H54b : R1 = Ph, R2 = H54f : R1 = R2 = Ph70a : R1 = Me3Si, R2 = H70b : R1 = Mesityl, R2 = H70c : R1 = R2 = CO2Me

70d : R1 = R2 = '4-MeOC6H470e : R1 = R2 = 4-pinBC6H470f : R1 = R2 = 3,5-Me2Pyrryl70g : R1 = Ph, R2 = SiMe370h : R1 = Ph, R2 = Me

Scheme 39. NHC-Pd catalyzed silaboration of alkynes [130].

Si BO

O+R H

1 mol% Au/TiO2

DCM, 25 οCB

R H

SiO

O

73

73a : R = cyclopropyl73b : R = cyclohexyl73c : R = cyclohex-1-enyl73d : R = PhOCH273e : R = 4-MeOC6H4OC(Me)273f : R = tBuPh2SiOCH2

73g : R = AcO(CH2)473h : R = C(O)OEt73i : R = 4-MeC6H473j : R = 2-MeOC6H473k : R = 4-F3CC6H4

Scheme 41. Gold catalyzed ‘abnormal’ silaboration of terminal alkynes [132].

nBu H

Si BO

O+

5 mol% [Ni(acac)2]6 mol% DIBALH

toluene, 80 οC

nBu

nBuH

Bpin

H

SiMe2Ph +H

nBu

SiMe2Ph

nBuH

Bpin

3 : 171 72

Scheme 40. Silaborative dimerization of alkynes [131].

M.B. Ansell et al. / Coordination Chemistry Reviews 336 (2017) 54–77 67

terminal and internal alkynes, including 1,7-octadiyne to afford thedouble silaboration product 55 (Scheme 31). The reactivity of thesyn-1-boryl-2-silyl alkenes was also assessed. Suzuki-Miyauracross-coupling and conjugate additions to methyl vinyl ketonesat the alkenyl boryl group were possible, leading to 56 and 57,respectively (Scheme 32) [118]. Many authors have since utilized[Pd(OAc)2]/isocyanide as a mediator in the silaboration of alkynesincluding in the synthesis of syn-homoallylic alcohols [119],multi-arylated olefins [120], and enamides [121].

Tanaka and co-workers described the silaboration of 1-octyneemploying the silylborane, 1,3-dimethyl-2-dimethylphenysilyl-2-bora-1,3-diazacyclopentane. The corresponding Z-1-boryl-2-silylalkene 58 was isolated by utilizing the pre-catalytic combinationof [Pd2(dba)3] and epto (epto = 4-ethyl-2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane, P(OCH2)3CEt). Pre-heating the pre-catalytic com-bination at 80 �C for 5 min was necessary in order to generatethe active catalytic species, proposed to be [Pd(epto)2]. As observedin Ito’s report, the silaboration of terminal alkynes proceeded in aregioselective manner with the boryl group inserting at the termi-nal position. Low to no yields were observed on applying otherphosphorus containing ligands such as PMe3 and PPh3. The proto-col was also expanded to the silaboryl carbocylization of hepta-1,6-diyne to form 59 (Scheme 33) [122].

Pillot and co-workers synthesized stable organosilylboranespossessing mesityl groups on the boryl atom, (diphenylmethylsilyl)dimesitylborane (PhMe2SiBMes2) and (diphenyl-tert-butylsilyl)dimesitylborane (Ph2

tBuSiBMes2). These silylboranesare not stabilized by electronegative groups on the boron atome.g. oxygen or nitrogen, but instead through the steric bulk of themesityl functionality. They were employed in the silaboration ofterminal alkynes such as phenylacetylene, using the [Pd2(dba)3]/epto catalytic combination. Steric clashing between the sub-stituents of the alkyne and the boryl moiety precluded the silabo-ration of internal alkynes [123].

In their investigations into the silaboration of terminal alkynes,Suginome and co-workers showed that it was possible to tune thestereoselective preference of the reaction by altering the reagentstoichiometry. The reaction parameters were assessed on treating(chlorodimethylsilyl)pinacolborane (ClMe2SiBpin) with 1-octynein the presence of 1 mol% [(g3-C3H5)Pd(PPh3)Cl], followed by sub-sequent addition of isopropyl alcohol (IPA) and pyridine. Whenexcess 1-octyne was used the Z-isomer 60 was isolated as the soleproduct. However, excess ClMe2SiBpin results in the formation ofthe E-isomer 61 as themajor product (Scheme 34). This observationwas applicable to a range of terminal alkynes, although stericallyhindered substituents on the alkyne restricted E-silaboration [124].

It was also possible to tune the regioselectivity in thesilboration of terminal alkynes. The silaboration proceeds with‘normal’ regioselectivity in the presence of catalytic quantities of[(g3-C3H5)Pd(PPh3)Cl]. However, using the more sterically hin-dered phosphine P(tBu)2(biphenyl-2-yl) an inverse or ‘abnormal’regioselectivity was observed, with Z-2-boryl-1-silyl-1-alkenes 62isolated as the major product (Scheme 35) [125].

Suginome and co-workers also hydrolysed the ‘normal’ and‘abnormal’ silaborated alkenes with metal hydroxides MOH(M = Na or K) instead of the IPA/pyridine mixture. This resultedin the formation of a five-membered cyclic borate 63 viaintramolecular attack of the resulting silanol oxygen with thetricoordinated boron atom. The potassium borates 63a and 63h

Me3Sn HMe2Sn SnMe2

Me2Sn SnMe2

RR+

3 mol% [Pd(dba)2]6 mol% P(OEt)3

toluene, 75 οC, 30 h2

Me3Sn H

SnMe2Me2Sn

R

75

75a : R = H 75b : R = Me

2

Scheme 43. Distannation of trimethylstannylethyne in the synthesis of trisstany-lalkenes [138].

R1O

R2Me3Sn SnMe3+

1-2 mol% [Pd(PPh3)4]

THF, r.t. to 60 οC

Me3Sn

R1

SnMe3

R2O

76

76a : R1 = Me, R2 = OEt76b : R1 = Et, R2 = OMe76c : R1 = iPr, R2 = OMe76d : R1 = cyclopropyl, R2 = OMe76e : R1 = H2C=CH(CH2)2, R2 = OMe76f : R1 = THPO(CH2)3, R2 = OMe76g : R1 = tBuMe2SiOCH2, R2 = OMe76h : R1 = Cl(CH2)2, R2 = OEt76i : R1 = Br(CH2)2, R2 = OEt

THP = tetrahydropyranyl

76j : R1 = Cl(CH2)3, R2 = OMe76k : R1 = Br(CH2)3, R2 = OMe76l : R1 = I(CH2)3, R2 = OMe76m : R1 = Br(CH2)4', R2 = OMe76n : R1 = Br(CH2)5, R2 = OMe76o : R1 = I(CH2)5, R2 = OMe76p : R1 = H2C=C(Br)(CH2)4, R2 = OMe76q : R1 = Me, R2 NMe2

Scheme 44. Distannation of alkynyloates and alkynylamides [139].

Sn

nBu

nBu n

5 mol% [Pd(PPh3)4]

H H

dioxane, 85 οC, 4 d

Ph H

neat, 85 οC, 5 d

Sn Sn

nBu

nBu

nBu

nBun

Sn

nBu

nBuSn

nBu

nBuSn

nBu

nBun m

78

79

Scheme 46. Distannation employing the backbone of poly[di(nbutyl)]stannane[142].

Me3Sn

XH2CH2CH2C

SnMe3

RO

MeLi

THF, −98 οC

Me3Sn RO

77

77a : X = Cl, R = OMe77b : X = Br, R = NMe2

Scheme 45. Transmetallation-cyclization of distannylated x-halogeno-alkenoates[140].

R H Me3Sn SnMe3+1 mol% [Pd(PPh3)4]

toluene, 25-85 οC

Me3Sn

R H

SnMe3

74

74a : R = H74b : R = nBu74c : R = Ph

74d : R = PhCH274e : R = MeOCH2'

74f : R = PhOCH2

Scheme 42. The first palladium catalyzed distannation of terminal alkynes [135].

68 M.B. Ansell et al. / Coordination Chemistry Reviews 336 (2017) 54–77

were then subjected to external-base free Suzuki-Miyaura cross-coupling with 4-iodoanisole to form 64 and 65, respectively(Scheme 36) [126].

The authors achieved a different mode of reactivity by substi-tuting one of the substituents of the silicon atom of a silylboranefor an amino group. The reaction of (Et2N)Me2SiBpin with aliphaticor aryl terminal alkynes resulted in the formation of 2,4- and3,4-disubstituted siloles, 66 and 67 respectively (Scheme 37).Isomer 66 was favored in most cases and this was attributed tosteric clashing within intermediates in the catalytic cycle.Deviations in the electronic and steric properties of the alkynesubstituents had little influence on the regioisomer formed.However, altering the phosphine ligand to the more stericallyhindered P(tBu)2(2-biphenyl), resulted in a higher ratio of 66 vs.67. The synthesis of siloles was also accompanied by the formationof the corresponding aminopinacolborane, and was extended toother silylboranes including (Me2N)Me2SiBpin and (pyrrolidino)Me2SiBpin [127].

Moberg and co-workers subjected a number of 1,3-enynes topalladium catalyzed silaborations using PhMe2SiBpin. The reac-tions required relatively high loadings of palladium and phosphineligand, as well as catalytic quantities of diisobutylaluminiumhydride (DIBALH). 1,2-Silaboration led to dienes 68 in all cases.Alternatively, changing the transition metal catalyst to a platinumanalogue and the 1,3-enynes substituent to a sterically hinderingfunctionality resulted in 1,4-silaboration and isolation of the corre-sponding allene 69 (Scheme 38) [128]. Later, Moberg altered thesilaborane to ClMe2SiBpin and, in the presence of isopropanoland pyridine, this resulted in formation of the correspondinghighly substituted 1,3-dienyl-2-silanols. Subsequent Suzuki-Miyaura and Hiyama-Denmark cross-coupling reactions yieldedtri- and tetra-substituted 1,3- or 1,2-dienes in a chemo-, regio-and diastereoselective manner [129].

Most of the reports to date utilize either phosphine or iso-cyanide ligand sets. In their investigations into NHC-palladiumcatalysis, Spencer, Navarro and co-worker observed that [Pd(ITMe)2(PhC„CPh)] (44) catalyzed the silaborations of terminaland internal alkynes with PhMe2SiBpin to afford a number ofknown and novel 1-silyl-2-boryl alkenes 54 and 70 (Scheme 39)[130]. This protocol represented the first example of alkyne silab-orations employing NHC ligands. All reactions proceeded with100% syn-stereoselectivity and, in the case of terminal alkynes100% ‘normal’ regioselectivity. High regioselectivites were alsonoted when using unsymmetrical internal alkynes, with the silylmoiety favoring a position that is geminal to the aryl ring. Unprece-dented mild reaction temperatures for terminal alkynes, shortreactions times, and low catalyst loadings were reported.

2.4.2. NickelIto reported the double insertion of terminal alkynes into the

SiAB bond of PhMe2SiBpin to afford Z,Z-1-silyl-4-boyl-1,3-butadiene derivatives in a regio- and stereoselective manner. Thereactions proceeded using catalytic quantities of [Ni(acac)2] andthe reductant DIBALH to afford a 3:1 mixture of 71 and 72(Scheme 40). The major product 71 was a result of head-to-headdimerization of the alkyne, whereas head-to-tail dimerization gave72. Dimerization yields were increased by using a large excess ofalkyne and were retarded by the introduction of a phosphine. Thisprotocol was also extended to internal alkynes with the exceptionof diphenylacetylene, whichwas inert under the reaction conditions.The application to diynes resulted in intramolecular cyclization andthe formation of the dimethylenecyclohexane derivatives [131].

2.4.3. GoldThe only other metal-mediated alkyne silaboration in the liter-

ature utilized gold nanoparticles supported on titania (Au/TiO2).

Stratakis and co-workers used 1 mol% Au/TiO2 to catalyze thesilaboration of terminal alkynes at room temperature to formsyn-2-boryl-1-silyl-1-alkenes 73 (Scheme 41). These alkenes were

Fe R H+

SnMe2

SnMe2

4 mol% [Pt(PPh3)2(η2-C2H4)]

THF, 65 οCFe

Me2Sn

SnMe2

R

H

8080a' : R = H80b : R = Me

80c : R = nBu80d : R = Ph

Scheme 47. 1,2-Distanna-[2]ferrocenophane distannation of terminal alkynes[143].

R H Me3Si SnMe3+1 mol% Pd(PPh3)4

neat, 60-70 οC, 48 h R

SiMe3Me3Sn

H82

82a : R = nBu82b : R = Ph82c : R = PhCH282d : R = Me2NCH2

82e : R = HOCH282f : R = HOCHMe82g : R = HOCMe282h : R = HO(CH2)2'

Scheme 49. Palladium(0) catalyzed silylstannation of terminal alkynes [149].

M.B. Ansell et al. / Coordination Chemistry Reviews 336 (2017) 54–77 69

formed with opposite or ‘abnormal’ regioselectivities with respectto the analogous palladium examples, which was attributed to thesteric factors imposed by the Au nanoparticles during the 1,2-addition of the silylborane to the alkynes. Side products in thisreaction were the ‘normal’ regioselective silaborated alkenes, thebis(silylated) alkenes and B2pin2. The presence of bis(silylated)alkene and B2pin2 was explained by separately stirring PhMe2-SiBpin under the catalytic conditions in the absence of alkyne.The authors observed the formation of PhMe2SiSiMe2Ph andB2pin2 as a result of metal-catalyzed silylboranemetathesis, a com-peting reaction in this silaboration protocol. Extension to internalalkynes resulted in mixtures of regioisomers or no yield at all[132].

2.5. Tin–Tin (SnASn)

2.5.1. PalladiumOrganostannanes are often utilized in the chemoselective for-

mation of CAC bonds through Migita-Kosugi-Stille reactions[133]. The development of new methodologies in the constructionof CASn bonds is therefore of high interest [134]. A particularlyattractive example is the insertion of alkynes into SnASn bonds,distannation. The resulting alkenes have two new CASn bondsand are frequently formed with high stereoselectivities. Low-valent palladium complexes are regularly used to catalyze the dis-tannation of alkynes.

Some of the first investigations into distannation of alkyneswere carried out by Mitchell and co-workers. Hexamethyldistan-nane (Me3SnSnMe3) and terminal alkynes were mixed in the pres-ence of catalytic quantities of [Pd(PPh3)4] to form Z-1,2-distannylalkenes 74 (Scheme 42). Aryl, alkyl and propargyl ether sub-stituents were tolerated. Distannation of acetylene at elevatedtemperatures initially led to the Z-isomer, which quickly isomer-ized to the thermally stable E-isomer. The Z to E isomerization

nHex H Me3Sn SnMe3+

2 mol2

Cu Sn

Sn

R H

Sn

R H

Cu

Sn OR'

Sn

R

I5

I6

Scheme 48. Copper catalyzed distannation of

was also observed in the absence of catalyst under photolysingconditions [135].

Mitchell later expanded this protocol to a wider variety of ter-minal alkynes including functionalities such as alcohols, amides,esters and silyl groups. The SnASn bond presursor was alsoextended to other hexaalkyldistannes (hexaethyl and hexabutylditin) [136], and to 1,2,4,5-tetrastannacyclohexanes [137]. The lat-ter were further employed in the distannation of trimethylstan-nylethyne to synthesise the first 1,1,2-trisstannylalkenederivatives 75 (Scheme 43) [138].

Piers and co-workers reported the distannation of alkyl-2-alkynyloates using Me3SnSnMe3 and a [Pd(PPh3)4] catalyst in THFat room temperature (or reflux) to form Z-2,3-bis(trimethylstannyl)-2-alkenoates 76 (Scheme 44). A vast array of functionalitywas tolerated including alkenyls, ethers, silyl ethers and primaryhalides [139]. Alkenoates with an x-halogeno-alkyl group weretreated with MeLi which resulted in a transmetallation-cyclization reaction to afford 2-trimethylstannylcycloalk-1-enes77 (Scheme 45) [140]. The distannation protocol was also extendedto N,N-dimethyl-2-alkynylamides and the formation of Z-N,N-dimethyl-2,3-bis(trimethylstannyl)-2-alkenamide 76q. Compounds 76were thermally labile and transferred upon heating or at roomtemperature to the thermodynamically stable E-isomers [139].

The weakness of the CASn bond meant that it was possibleto use vinyltins in electrophilic substitution reactions. Mitchelland co-workers detailed the reactivity potential of theZ-1,2-bis(trimethylstannyl)-1-alkenes with the electrophilesp-tolylsulphonylisocyanate (TSI), dichloromethylmethylether(DCME), trimethylsilyl chlorosulphonates and sulfur oxides[141].

Recently, Foucher and co-workers depicted the insertion ofacetylene and phenylacetylene into the backbone of poly[di(nbu-tyl)]stannane. This resulted in the formation of new alkene-tinpolymers 78 and 79, respectively (Scheme 46) [142].

% [Cu(OAc)(PPh3)3]0 mol% Cs2CO3

DMF, 60 οCnHex

Me3Sn SnMe3

H

Sn Sn

MOR'

Sn Sn OR' M+-

Cu OR'

OR' + MOR'

H

Sn

81

1-octyne and proposed mechanism [145].

Me3Sn

Me2SiSiMe3R1 R2 +

3 mol% [Pd(PPh3)4]

toluene, 40-60 οC R1

Me3Sn SiMe2

R2

Me3Si

83 84

R1

Me3Sn SiMe2

R2

Me3Si

84

Ph H+3 mol% [Pd(PPh3)4]

toluene, 60-80 οCSiMe2Me2Sn

R1 R2

Ph H

8584a/85a : R1 = EtO2C, R2 = H84b/85b : R1 = R2 = MeO2C84c/85c : R1 = Ph, R2 = H

Scheme 50. Palladium(0)-catalyzed silylstannation followed by regioselectivecyclization [150].

Me3Sn SiMe2tBuR OEt +

2 mol% [Pd(OAc)2]12 mol% tOcNC

toluene, r.t. R

Me3Sn SiMe2tBu

OEt86

86a : R = H86b : R = Me

Scheme 51. Silylstannation of 1-alkoxyalkynes [151].

Bu3Sn SiMe2PhR +H1 mol% [Pd(PPh3)4]

[bmim]PF6/Et2O, 70 οC R

Bu3Sn SiMe2Ph

H87

87a : R = Ph87b : R = (CH2)4OH87c : R = nOctyl

Scheme 52. Alkyne silylstannation using ionic liquid immobilised palladium(0)[152].

EtO2C CO2Et

Bu3Sn SiMe3+3 mol% [Pd(PPh3)4]

THF, 50 οCEtO2C CO2Et

H

SiMe3Bu3Sn

88

X Bu3Sn SiMe3+

3 mol% [Pd2(dba)3.CHCl3]or

10 mol% [Pd(OH)2/C]

THF, r.t. X

Bu3Sn

SiMe389

89a : X = C(CH2OBz)289b : X = C(CH2OBn)289c : X = C(CH2O{CMe2}OCH2)89d : X = NTs

TsN Bu3Sn SiMe3+

3 mol% [Pd2(dba)3.CHCl3]6 mol% ItBu.HCl12 mol% Cs2CO3

ClCH2CH2Cl, 40 oCTsN

Bu3Sn

SiMe389d

ItBu.HCl = N+ N

H Cl-

Scheme 53. Silylstannation and bismetallitive cyclization of 1,3-enynes [154].

R CF3 Bu3Sn SiMe3+R

Bu3Sn SiMe3

CF3

2.5 mol% [Cl2Pd(PPh3)2]

THF, reflux, 6 h90

R CF3 Bu3Sn SiMe3+R

Me3Si SnBu3

CF3

2.5 mol% [Cl2Pd(tBuNC)2]

THF, r.t., 6 h91

90a/91a : R = 4-MeOPh90b/91b : R = 3-MeOPh90c/91c : R = 2-MeOPh90d/91d : R = 4-ClPh90e/91e : R = 4-EtO2CPh

90f /91f : R = CO2Et90g/91g : R = P(O)(OEt)290h/91h : R = CH(OH)(4-ClPh)90i /91i : R = CH(OH)(CH=CHPh)90j /91j : R = CH(OH)({CH2}5CH3)

Scheme 54. Palladium catalyst-dependent regioselective silylstannation [155].

R H PhMe2Si Bpin+

1.2 eq. Bu3SnOtBu2 mol% CuCl2 mol% PtBu3

MeCN, r.t. R

SnBu3

H

PhMe2Si

9292a : R = nBu92b : R = nOct92c : R = cyclopentyl92d : R = iBu92e : R = NC(CH2)3

92f : R = iAmyl92g : R = Br(CH2)292h : R = HO(CH2)292i : R = Et2NCH2

Scheme 55. Cu(I) catalyzed ‘abnormal’ silylstannation of terminal alkynes [156].

70 M.B. Ansell et al. / Coordination Chemistry Reviews 336 (2017) 54–77

2.5.2. PlatinumThe only examples of distannation of alkynes employing a plat-

inum catalyst were carried out by Wrackmeyer and co-workers.The distannane, 1,2-distanna-[2]ferrocenophane reacted sequen-tially or in one pot with [Pt(PPh3)2(g2-C2H4)] and a range of

terminal alkynes to form the corresponding 1,4-distanna-[4]ferroceophanes 80 (Scheme 47) [143]. Both terminal and internalalkynes were accessible. However, dimethyl acetylenedicarboxy-late gave the distannation product in a side reaction while favoringcyclotrimerization to form hexamethylbenzene hexacarboxylate.Extension to analogous palladium catalysts such as [Pd(PPh3)4]and [Pd(dba)2] were unsuccessful [144].

2.5.3. CopperIn 2013, Yoshida carried out the first catalytic distannation of

alkynes using a copper catalyst. [Cu(OAc)(PPh3)3] in the presenceof Cs2CO3 was used to optimize the reaction between Me3SnSnMe3and 1-octyne affording 81. The authors then managed to distanny-late 1-hexyne, 1-decyne and branched aliphatic terminal alkynesbearing isoamyl, isobutyl and cyclopentyl, as well as chloro, aminoand cyano functionalities. Alkynes that were sterically congestedresulted in sluggish reactions and low yields. It was proposed thatthe reaction proceeded through a CuASn bonded intermediate I5derived from a CuOR0 complex and a base-activated distannane.Subsequent addition of I5 to a CAC triple bond affordb-stannylalkenyl copper species I6, which is then recaptured withMe3SnOR0 to give the 1,2-distannylated alkene with regenerationof the CuOR0 complex (Scheme 48) [145].

2.6. Tin–Silicon (SnASi)

2.6.1. PalladiumSnASi bond (silylstannation) addition to alkynes results in the

formation of alkenes with a new CASn and CASi bond, often in aregio- and stereoselective manner. Palladium catalyzed silylstan-nation of alkynes are by far the most reported examples withinthe literature and have found application in the synthesis of natu-ral products [146,147], and pharmaceuticals [148]. The first palla-dium catalyzed examples of alkyne silylstannations were shown byMitchell and co-workers. The authors reacted a range of terminalalkynes with (trimethylsilyl)trimethylstannane (Me3SiSnMe3) inthe presence of [Pd(PPh3)4] under solvent-free conditions to yield

R1 R2 + Me3Sn BNMe

MeN 1 mol% [Pd(PPh3)4]

benzene, r.t. or 80 οC R1

Me3Sn B

R2

NMeMeN

9393a : R1 = nHex, R2 = H93b : R1 = Ph, R2 = H93c : R1 = R2 = Ph

93e : R1 = R2 = CO2Me93f : R1 = Ph, R2 = Me'

Scheme 56. Palladium(0)-catalyzed borylstannation of alkynes [157,158].

R1H

R2Me3Sn B

NMe

MeN 5 mol% [Cl2Pd(PPh3)2]

benzene, r.t.

SnMe3

BH

R1

R2O

OPinacol, H+

benzene, r.t.

+Me3Sn B

HR1R2

MeNNMe

94

95

95a : R1 = Me, R2 = H95b : R1 = CH2OTBDPS, R2 = H95c : R1 = H, R2 = (CH2)2OBn95d : R1 = Ph, R2 = H

TBDPS = ter t-butyldiphenylsilyl

Scheme 57. Borylstannation of 1,3-enyles followed by boryl alcoholysis [159].

R H E EPh

Ph+

1-2 mol% [Pd(PPh3)4]

benzene, 80 οC R

E E

H

Ph Ph

96

R H E EPh

Ph+

2 mol% [Pd(PPh3)4]or [Cl2Pd(PPh3)]

CO atmosphere, benzene, 80 οC H

EO

EPh

Ph

R97

96a : R = (CH2)2CH(CH3)2, E = S/Se96b : R = C(CH3)2OH, E = S/Se96c : R = CH(CH3)OH, E = S96d : R = CH2OH, E = S96e : R = (CH2)2OH, E = S/Se

96f : R = SiMe3, E = S/Se96g : R = CH2NMe2, E = S/Se96h : R = Ph, E = S96i : R = CH2Br, E = Se

97a : R = nHex, E = S97b : R = (CH2)2CH(CH3)2, E = Se

97c : R = Ph, E = S/Se97d : R = (CH2)3OH, E = Se'

Scheme 58. Dichalcogen and carbonylative dichalcogen additions to alkynes [166].

M.B. Ansell et al. / Coordination Chemistry Reviews 336 (2017) 54–77 71

the corresponding Z-1-silyl-2-stannyl-1-alkenes (82) (Scheme 49).In all cases the silyl moiety added regioselectively at the terminalcarbon [149].

Ito extended the use of [Pd(PPh3)4] as a catalyst in the reactionof the disilanylstannane 83 with alkynes affording the (b-disilanylalkenyl)stannanes 84. The reaction proceeded with theZ-addition of the SiASn bond to the CAC triple bond. 84, in the pres-ence of phenylacetylene and further quantities of [Pd(PPh3)4], thenunderwent regioselective cyclization to form the silastannacylo-hexadiene 85 as a single isomer (Scheme 50) [150]. Ito lateraccomplished the silylstannation of 1-alkoxyalkynes employingthe pre-catalytic combination of [Pd(OAc)2]/tert-octylisocyanide.The reactions proceeded at room temperature and yielded thesyn-addition products 86, with the silyl group regioselectively intro-duced at the carbon atom bearing the alkoxy moiety (Scheme 51).

[Pd(PPh3)4] was inactive in these transformations at both roomand elevated temperatures [151]. The resulting alkene adductswere then exposed to a range of reactions including Stille cross-couplings, iodination at the CASn bond and the formation ofacylsilanes.

Singer reported the silylstannation of terminal alkynes withBu3SnSiMe2Ph using catalytic quantities of [Pd(PPh3)4] immo-bilised in the ionic liquid 1-nbutyl-3-methylimidazolium hexafluo-rophosphate ([bmim][PF6]). High stereo- and regioselectivitieswere observed and simple ether extraction resulted in isolationof the Z-1-silyl-2-stannyl-1-alkenes 87 (Scheme 52) [152]. Thepalladium(0)-ionic liquid combination was recyclable with no lossof activity even after 10 cycles [153].

The bismetallative cyclization of 1,3-enynes, using Bu3SnSiMe3,is dependent on the choice of ligand and palladium source. The useof [Pd(PPh3)4] results in ‘normal’ silylstannation of the alkyneaffording 88. However, upon removing the phosphine and using[Pd2(dba)3.CHCl3] or [Pd(OH)2/C], cyclized compounds 89 can beisolated as the major products of the reaction (Scheme 53). The bis-metallative cyclization can also be observed on employing nucle-ophilic N-heterocyclic carbenes with bulky alkyl N-substituents[154].

It is possible to tune the regioselectivity of alkyne silylstanna-tion by changing the ligand of a palladium(II) catalyst. Treatmentof fluorine containing internal alkynes with Bu3SnSiMe3 in thepresence of 2.5 mol% [Cl2Pd(PPh3)2] yields the silylstannylatedadducts 90. However, by switching the palladium catalyst to

[Cl2Pd(tBuNC)2], the opposite regioselectivities 91 are observed(Scheme 54) [155].

2.6.2. CopperThe only example of a copper-catalyzed alkyne silylstannation

was carried out by Yoshida and co-workers. The authors detaileda three-component coupling reaction employing terminal alkynes,a silylborane (PhMe2SiBpin) and a tin alkoxide (nBu3SnOtBu) in thepresence of a Cu(I) catalyst ([CuCl-PtBu3]). The observed regioselec-tivities were inverse to those of conventional silylstannation underpalladium catalyzed conditions, with the stannyl moiety predomi-nantly adding to the terminal carbon as shown in 92. A range ofalkyl branched and unbranched alkynes bearing cyano, bromo,hydroxyl or amino functionalities were accessible (Scheme 55).The authors proposed a similar mechanism to the distannation ofalkynes mediated by a copper(I) catalyst. A silylcopper species,CuSiMe2Ph, is initially formed via a sigma-bond metathesisbetween a copper alkoxide and a silylborane. An alkyne wouldthen insert into the CuASi bond to give a b-silylalkenylcopperintermediate, which is subsequently trapped by a tin alkoxide tofurnish the silylstannation alkene adduct and regenerated the cop-per alkoxide [156].

2.7. Tin–Boron (SnAB)

2.7.1. PalladiumThe borylstannation of alkynes results in the formation of alke-

nes with a new CASn and CAB bond. The first palladium-catalyzedexample was shown by Tanaka and co-workers in 1996. The boryl-stannane 1,3-dimethyl-2-(trimethylstannyl)-2-bora-1,3-diazacyclopentane (Me3SnB[NMe{CH2CH2}NMe]) was added to alkynesusing catalytic quantities of [Pd(PPh3)4] (Scheme 56). The reagentswere added together in benzene at 0 �C and then warmed to roomtemperature. Terminal alkynes yielded syn-1-boryl-2-stannyl-1-alkenes 93 as the sole product. Internal alkynes were also accessi-ble, although a higher temperature (80 �C) was necessary [157].Weber later extended this protocol to the more sterically hinderedborylstannane, 1,3-di-tert-butyl-2-(trimethylstannyl)-2-bora-1,3-diazacyclopentane (Me3SnB[NtBu{CH2CH2}NtBu]) [158].

RajanBabu developed Tanaka’s methodology and extended it tothe borylstannation of 1,3-enynes. This protocol resulted in the iso-lation of the syn-1-boryl-2-stannyl-1-alkenes (94) in chemo-,regio- and stereoselective fashions with no complications arisingdue to the adjacent alkene. However, the boryl group in 94 washydrolytically unstable. In situ treatment with pinacol andp-toluenesulfonic acid (PTSA) yielded the hydrolytically stable 95(Scheme 57) [159].

R H + S SSi(iPr)3

Si(iPr)3

5 mol% [Pd(PPh3)4]

benzene, reflux R

(iPr)3SiS SSi(iPr)3)

H

TBAF, 4eq. MeI

THF, 0 οC R

MeS SMe

H

98

99

98a/99a : R = nOctyl98b/99b : R = CH2Ph98c/99c : R = cyclohexyl98d/99d : R = cyclohex-1-enyl98e/99e : R = tBu

98f /99f : R = C(CH2)2OH98g/99g : R = (CH2)10OH98h/99h : R = (CH2)3Cl'

98i /99i : R = Ph98j /99j : R = (CH2)2CO2Ph

Scheme 59. Synthesis of Z-1,2-bis(methylthio)alkenes via silyl-protected dichalcogens [167].

R

(iPr)3SiS SSi(iPr)3

H98b/e/g

SS

S R

R

5 mol% [Zn(OTf)2]1.0 M HCl

DCM, r.t.100

100a : R = CH2Ph100b : R = tBu100c : R = (CH2)10OH

Scheme 60. Reactivity of 98 with acid/Lewis acid combination forming 2,5,7-trithiabicyclo[2.2.1]heptane adducts [169].

OH

R3HO

R2R1 + E E

Ph

Ph

20 mol% [FeCl3.H2O]

DCE, r.t. O

PhE EPh

R1R2

101101a : R1 = R2 = R3 = H, R4 = Ph, E = Se/S101b : R1 = R2 = R3 = H, R4 = 4-MeC6H5, E = Se/S101c : R1 = R2 = R3 = H, R4 = 4-ClC6H5, E = Se/S101d : R1 = R2 = R3 = H, R4 = 2-naphthyl, E = Se101e : R1 = R2 = Me, R3 = Ph, R4 = H, E = Se101f : R1 = R2 = R3 = Me, R4 = Ph, E = Se101g : R1 = R2 = R3 = R4 = Me, E = Se

R3R4

OH

PhHORSe SeR+

20 mol% [FeCl3.6H2O]

DCE, r.t.O Ph

SeRSeR

102102a : R = Ph102b : R = 4-MeC6H5'

102c : R = 4-FC6H5102d : R = nBu

OH

PhTsHNRSe SeR+

20 mol% [FeCl3.6H2O]

DCE, r.t. N

RSe SeR

Ph

Ts103103a : R = Ph

103b : R = 4-MeC6H5'103c : R = 4-FC6H5103d : R = nBu

R1 R2 BuSe SBu+2 eq. FeCl3

DCM, 40 οC SeR1 R2

BuSe SeBu

104104a : R1 = R2 = Ph104b : R1 = R2 = 4-MeC6H5104c : R1 = R2 = 4-MeOC6H5'

104d : R1 = R2 = 4-ClC6H5104e : R1 = R2 = 4-FC6H5104f : R1 = R2 = 2-MeC6H5104g : R1 = R2 = 1-naphthyl

104h : R1 = R2 = 2-naphthyl104i : R1 = R2 = nBu104j : R1 = nBu, R2 = Ph104k : R1 = nBu, R2 = 4-MeC6H5104l : R1 = nBu, R2 = 4-ClC6H5104m : R1 = Ph, R2 = 4-MeC6H5104n : R1 = Ph, R2 = 2-MeC6H5

Scheme 61. Synthesis of bis(organochalcogen)-dihydrofurans, dihydro-2H-pyrans,dihydro 1H-pyrroles and selenophanes [176,177].

R H E EAr

Ar+

5 mol% CuI1 eq. Zn

glycerol, 110 οC

E

R E

HAr

Ar105

105a : R = Ph, Ar = Ph, E = S/Se105b : R = Ph, Ar = 2-MeC6H5, E = Se105c : R = Ph, Ar = 2,4,6-Me3C6H2, E = Se105d : R = 4-MeC6H5, Ar = Ph, E = Se105e : R = 4-ClC6H5, Ar = Ph, E = Se

Scheme 62. Copper catalyzed E-dithiolation and diselenation of terminal alkynes[178].

72 M.B. Ansell et al. / Coordination Chemistry Reviews 336 (2017) 54–77

2.7.2. CopperYoshida detailed the copper(II) catalyzed borylstannation of

alkynes. A three-component coupling reaction employed analkyne, a diboron (B2pin2) and a tin alkoxide (nBuSnOMe) withthe aid of a copper(II)acetate/tricyclohexylphosphine combination.A range of internal and terminal alkynes were accessible. Internal

alkynes with one aryl and one alkyl substituent resulted in perfectregioselectivities with the boryl moiety geminal to the alkyl group.In the case of terminal alkynes, the boryl group added to the termi-nal carbon [160]. All reactions proceeded at room temperaturewith catalyst loadings as low as 1 mol%. The authors proposed thatthese reactions proceeded through a similar mechanism to that ofthe silylstannation [156].

2.8. Sulfur–Sulfur and Selenium–Selenium (SAS and SeASe)

2.8.1. PalladiumOrganochalcogens are known to exhibit a range of pharmaco-

logical activity profiles including as potential anticancer [161],anti-inflammatory [162], and antibacterial agents [163]. The intro-duction of chalcogens into alkynes to form 1,2-bis(chalcogen)alkenes is challenging. The formation of 1,2-bis(chalcogen)alkenes often requires the use of heavy metals, high temperaturesand results in a mixture of stereoisomers. Such methods includethe reaction vinyldichlorides with thiolate anions [164] and radicalreactivity between chalcogen species and alkynes [165]. Thetransition-metal catalyzed addition of dichalcogens to alkynes isa possible alternative to synthesizing 1,2-bis(chalcogen)alkenesin a stereoselective and atom-economical manner.

Sonoda reported the first palladium mediated addition of diaryldisulphides and diselenides to terminal alkynes to yield the corre-sponding Z-1,2-bis(arylthio) and Z-1,2-bis(arylseleno)-1-alkenes(96), respectively (Scheme 58). This protocol tolerated functional-ities such as hydroxyl, trimethylsilyl and amino groups. The inclu-sion of a carbon monoxide (CO) atmosphere in these reactions leadinstead to the isolation of the carbonylative addition adducts Z-1,3-bis(arylchalcogen)-2-alken-1-ones (97) (Scheme 58). A stepwiseattempt at carbonylation of 96 with CO to yield 97 resulted inthe isolation of only 96, suggesting that CO insertion was part ofdichalcogen additions in the first instance [166].

Gareau and co-workers established a procedure that effectivelyintroduced ‘dialkyl’-disulphides in the dithiolation of alkynes.Upon protecting the disulphide with a bulky silyl group, the dithi-olation of terminal alkynes with bis(triisopropylsilyl)disulphideresulted in the isolation of the corresponding Z-1,2-bis(thio)alkenes (98). Subsequent treatment with tetra-nbutylammoniumfluoride (TBAF) in the presence of excess methyl iodide (MeI)

SRe SS

S

-

R R +1/8 S8

acetonitrile, r.t.

SReS S

S S

R

RR

R

-

106106a : R = Ph106b : R = Me

106c : R = CO2Me

Scheme 63. Rhenium mediated stoichiometric dithiolation of internal alkynes[180].

R H BuS SBu+

3 mol% [RhH(PPh3)4]12 mol% P(p-MeOC6H4)3

3 mol% CF3SO3H

acetone, reflux

BuS

R H

SBu

107107a : R = nHex107b : R = cyclohexyl107c : R = (CH2)2OH107d : R = (CH2)2OSiMe2tBu

107e : R = (CH2)3CN107f : R = CH2Ph107g : R = Ph107h : R = SiMe3'

Scheme 64. Rhodium(I) catalyzed dithiolation of terminal alkynes [181].

R H

PhS SPh

+

+

PhSe SePh

5 mol% [Rh(PPh3)4]10 mol% dppf

acetone, reflux

PhS

R H

SePh

108

108a : R = nHex108b : R = (CH2)2Ph108c : R = tBu

108d : R = (CH2)2OH108e : R = (CH2)2OAc108f : R = (CH2)2OSiMe2tBu

Scheme 65. Cross-over addition of diphenyldisulphide/diselenide to terminalalkynes [182].

S

R

Si(iPr)3

2-25 mol% AuCl

toluene, 45 οC SR

Si(iPr)3

S

R

Si(iPr)3

ClAu

S+R

Si(iPr)3

AuCl-cyclizat ion

π-coordination silyldemetalat ion

109 110

110a : R = nPr110b : R = cyclohexyl110c : R = 4-(iPr)2NC6H4110d : R = 2,4-(MeO)2C6H3

110e : R = 4-MeOC6H4110f : R = 4-MeC6H4110g : R = Ph'

110h : R = 4-CF3C6H4

Scheme 66. Gold(I) catalyzed cyclization of (o-alkynylphenylthio)silanes [184].

Ar2GeXGeAr2R R +

10 mol% [Pd(PPh3)4]benzene, reflux

GeAr2Ar2Ge

R R111

111a : R = MeO2C111b : R = H

Ar = 2,6-Et2C6H3

Ar2GeAr2Ge

S

H

H

112

10 mol% [Pd(PPh3)4]benzene, reflux

Scheme 67. Linker-atom dependent addition strained digermane to alkynes [185].

M.B. Ansell et al. / Coordination Chemistry Reviews 336 (2017) 54–77 73

de-protected/alkylated the sulfur atoms affording 99 (Scheme 59)[167].

Additionally, Gareau investigated the reactivity of 98 towardsother electrophiles including halides, epoxides and acyl chlorides[168]. Furthermore, treatment of 98 with HCl in the presence ofa Lewis acid ([Zn{OTf}2]) yielded the bicyclic adducts 2,5,7-trithiabicyclo[2.2.1]heptane (100, Scheme 60) [169].

Beletskaya and co-workers reported an alternative methodol-ogy in the addition of SAS and SeASe bonds to alkynes. Diphenyldisulphide (Ph2S2) and diphenyl diselenide (Ph2Se2) were reactedwith a variety of terminal alkynes in the presence of catalyticquantities of [Cl2Pd(PPh3)2], PhEH (E = S or Se) and triethylamine(NEt3) to yield the Z-1,2-bis(arylthio) and Z-1,2-bis(arylseleno)-1-alkenes. Both PhEH and NEt3 were essential for the success of thereaction. The yields increased on the addition of excess PPh3, whichcontradicts the general trend observed for other EAE additions toalkynes within the literature [170]. It was later shown that excessPPh3 prevented the rapid polymerization to [Pd(EAr)2]n and there-fore the inhibition of the palladium catalyst [171].

Beletskaya also reported the dithiolation of terminal alkynesutilizing a Pd(0) catalyst supported by a triphenylphosphine resinunder conventional [172], and microwave heating conditions[173]. Simple filtration resulted in the isolation of the Z-1,2-bis(thio)-1-alkenes. This approach was not applicable to diaryl dise-lenides. Other palladium(0) supported mediators for the dithiola-tion of terminal alkynes include the MCM-41-supportedbidentate phosphine Pd(0) catalyst reported by Cai [174].

2.8.2. IronZeni described the addition of diorganyl diselenides and

disulphides to terminal alkynes in the presence of an iron(III)

chloride (FeCl3) catalyst. The best results were observed using dia-ryl diselenides bearing neutral electron-donating and withdrawinggroups. The electronic nature of the terminal alkyne substituent didnot have an effect on the rate or yield of the reaction [175]. Iron cat-alyzed addition to 1,4-butyn-diols, pentyne-1,5-diol and 4-aminobutynol afforded 3,4-bis(organochalcogen)-2,5-dihydrofurans(101), 4,5-bis(organochalcogen)-3,6-dihydro-2H-pyrans (102) and2,5-dihydro 1H-pyrrole derivatives (103), respectively, under mildaerobic conditions (Scheme 61) [176]. 1,3-Diynes in the presenceof dibutyl diselenide or dimethyldisulfide and stoichiometric quan-tities of FeCl3 yielded symmetrical and unsymmetrical 3,4-bis(butylselanyl)chalcogenophanes (104). In the synthesis of the 104, thecyclization was stereoselective providing exclusively the desiredE-selenoenynes as intermediates. The selenophanes then formedvia an intramolecular 5-endo-dig cyclization [177].

2.8.3. CopperThe addition of the catalytic mixture of CuI, zinc dust and

glycerol resulted in the stereoselective addition of diaryldichlcogenides to form a variety of E-1,2-bis-chalcogen alkenes(105) (Scheme 62). Zinc and glycerol were essential to the reaction;Zn reduced Cu(I) to Cu(0) while glycerol acted as a solvent, but alsoas a possible reducing agent for the reduction of Zn(II) to Zn(0)[178].

2.8.4. NickelThe only examples of nickel-catalyzed addition of diaryldisul-

phides to alkynes were developed by Beletskaya and co-workers.

74 M.B. Ansell et al. / Coordination Chemistry Reviews 336 (2017) 54–77

The use of 3 mol% [Ni(acac)2] and 30 mol% PMePh2 at 100 �C undersolvent-free conditions resulted in the stereoselective dithiolationof both internal and terminal alkynes to form Z-dithiolated alkeneproducts. The reaction temperature was important: too low meantincomplete reactivity and too high led to a mixture of stereoiso-mers [179].

2.8.5. RheniumThe stoichiometric reaction between the tetrathiometallate

anion [ReS4]� and diphenylacetylene, 2-butyne and bis(trimethylsilyl)acetylene in the presence of elemental sulfuryielded the dithiolation adducts 106 (Scheme 63) [180].

Ph H ClR2Ge GeR2Cl+5 mol% [Pd(PPh3)4]

toluene, 120 οC Ph

ClR2Ge GeR2Cl

H

2.8.6. RhodiumYamaguchi and co-workers showed that it was possible to

dithiolate terminal alkynes using the dialkyl disulphide,Bu2S2, employing catalytic quantities of a Rh-phosphine complex,tris(p-methoxyphenyl)phosphine and trifluoromethane sulfonicacid affording the corresponding Z-bis(alkylthio)alkenes (107)(Scheme 64). A range of functionality at the terminal alkynesubstituent was accessible including hydroxyl, tert-butyldimethylsiloxy and nitrile. However, internal alkynes werenot accessible with this protocol [181].

Yamaguchi and co-workers extended their studies to the addi-tion of disulphides and diselenides to alkynes in cross-over exper-iments. A 1:1 mixture of diaryl disulphides and diaryl diselenideswere reacted with terminal alkynes using the same Rh complexand 1,10-bis(diphenylphosphino)ferrocene (dppf). This resulted inthe formation of Z-1-arylseleno-2-(arylthio)-1-alkenes (108) asthe major product (Scheme 65). The amounts of minor by-products Z-2-arylseleno-1-(arylthio)-1-alkene, Z-1,2-bis(arylthio)alkene and Z-1,2-bis(arylseleno)alkene were insubstantial.However, the minor product ratio became significant uponremoval of trifluorosulfonic acid or when increasing the steric hin-drance surrounding the alkynes [182].

116

Na

toluene, 120 οC Ph H

R2Ge GeR2

117Ph

ClR2Ge GeR2Cl

H116

Ph H

Et2Ge GeEt2

117b

5 mol% [Pd(PPh3)4]

benzene, r.t.GeEt2Et2Ge GeEt2Et2Ge

Ph

PhH

H

Ph H

Ph H

+

118 119

116a/117a : R = Me116b/117b : R = Et116c/117c : R = iPr

Scheme 69. Digermylation, reductive cyclization employing dichlorodigermanesand terminal alkynes [188].

2.9. Sulfur–Silicon (SASi)

2.9.1. GoldNakamura and co-workers developed the AuCl-catalyzed

cyclization of (o-alkynylphenylthio)silanes (109) to form the corre-sponding 3-silylbenzo[b]thiophenes (110). The reaction was pro-posed to proceed initially by coordination of the gold species tothe alkynyl moiety. The sulfur atom then acts as an intramolecularnucleophile, attacking the electron deficient alkyne which resultsin a silylsulfonium intermediate. Subsequently, [1,3]-migration ofthe silyl group and elimination of AuCl yielded 110 (Scheme 66).The yield was highly dependent on the natural of the alkyne sub-stituents with electron rich aromatic rings producing higher yieldsthan electron poor or bulky groups (which inhibited the reaction)[183].

R H

(iPr)2Ge(iPr)2Ge Ge(iPr)2

Ge(iPr)2

+

5 mol% [Pd(dba)2]20 mol% ETBP

benzene, 120 οC(iPr)2Ge(iPr)2Ge

1

113a/114a/115a : R = nBu113b/114b/115b : R = 4-MeC6H4

Scheme 68. Addition of a tetragerm

2.10. Sulfur–Boron (SAB)

2.10.1. PalladiumSuzuki and Miyuara employed 9-(alkylthio)-9-borabicyclo[3.3.

1]nonane in the palladium(0) catalyzed thioboration of terminalalkynes to produce 9-[Z-2-(alkylthio)-1-alkenyl]-9-borabicyclo[3.3.1]nonane derivatives. These reactions were highly regio- andstereoselective with the boryl group adding to the terminal carbonin all cases. The reactions were sufficiently mild that a variety offunctionalities were tolerated [184].

2.11. Germanium–Germanium (GeAGe)

2.11.1. PalladiumIn contrast to SiASi and SnASn bonds, the insertion of alkynes

into GeAGe bonds has been investigated to a much lesser extent.The resulting compounds are expected to have a reactivity profilesomewhere in-between their SiASi and SnASn analogues. Themajority of alkyne digermylations in the literature are palladium-catalyzed. The first example was reported by Ando and co-workers. In their work, a strained cyclic digermirane was reactedwith acetylene and dimethyl acetylenedicarboxylate in the pres-ence of 10 mol% [Pd(PPh3)4] resulting in the formation of the diger-macyclopentene 111. When X was a sulfur atom, it was possible toselectively cleave the GeAS bond to afford 112 (Scheme 67) [185].

Mochida and co-workers reacted 1,1,2,2,3,3,4,4-octaisopropyltetragermetane ({iPr2Ge}4) with various terminal alkynes in the pres-ence of palladium complexes to synthesise 1,2,3,4-tetrahydro-1,2,3,4-tetragermins (113), D4-1,2,3-trigemolene (114) and 1H-germoles (2,4-, 3,4- and 2,3-disubstituted) (115). The yields of114 and 115 increased with time, a fact attributed to the thermoyl-sis of 113 in the presence of excess alkyne. The formation of 114from 113 suggested extrusion of diisopropylgermylene (iPrGe:),

Ge(iPr)2

(iPr)2Ge

R

H

R

H(iPr)2Ge

(iPr)2GeGe(iPr)2

+ + (iPr)2Ge

R

R

13 114 115

etane to internal alkynes [186].

M.B. Ansell et al. / Coordination Chemistry Reviews 336 (2017) 54–77 75

which was readily trapped by two equivalents of alkyne to give115 [186] (Scheme 68).

In 1991, Tanaka used linear non-strained digermanes in thedigermylation of alkynes. 1,2-Dichloro-1,1,2,2-tetramethyldigermane was reacted with phenylacetylene in the presence of a palla-dium(0) catalyst to form 116 (Scheme 69). The extension of theprotocol to hexamethyldigermane resulted in very low conversions[187]. Following studies accomplished the conversion of 116 to

R1 R2 Me3Ge GeMe3+5 mol% [Pt(acac)2]

toluene, 120 οC R1

Me3Ge GeMe3

R2120

120a : R1 = Ph, R2 = H120b : R1 = nBu, R2 = H

120c : R1 = SiMe3, R2 = H120d : R1 = CO2Me', R2 = H

Scheme 70. Platinum catalyzed digermylation of alkynes with hexamethyldiger-mane [189].

R1 CO2R2

+

Bu3Sn GeMe3

4 mol% [Pd(PPh3)4]

neat, 85 οC R1

Me3Ge CO2R2

SnBu3121

121a : R1 = Me, R2 = Et121b : R1 = iPr, R2 = Me121c : R1 = (CH2)3OTHP, R2 = Me

121d : R1 = (CH2)5Cl, R2 = Me121e : R1 = (CH2)4SiMe2tBu

Me

Me3Ge CO2Et

SnBu3121a

1) nBuLi, THF, −98 οC

2) R-X, THF, −98 οC Me

Me3Ge CO2Et

R122

122a : R = Me122b : R = CH2CH=CH2

122c : R = CH2CH=C(Me)2122d : R = (CH2)3Cl

Scheme 71. E-Germylstannation of a,b-acetyleneic esters [190].

R H + Bu3Sn GeEt3

5 mol% [Pd(dba)2]10 mol% P(OCH2)3CEt

THF, 80 οC R

Bu3Sn GEt3

H123

123a : R = Ph123b : R = 4-ClC6H4123c : R = 3-ClC6H4123d : R = 4-FC6H4

123e : R = 3-F3CC6H4123f : R = 4-NCC6H4123g : R = 4-O2NC6H4123h : R = 3,4-(MeO)2C6H3

Scheme 72. Z-Germylstannation of aryl terminal alkynes [191].

nBu H

+

PhMe2Ge BO

O

5 mol% [Ni(acac)2]10 mol% DIBALH

2 mol% [Pd(acac)2]8 mol% tOcNC

2 mol% [Pt(PPh3)2(C2H4)]

nBu H

+

PhMe2Ge BO

O

nBu H

+

PhMe2Ge BO

O

Scheme 73. Catalyst dependent ger

1,2-digermacyclobut-2-enes (117) by reductive cyclization in thepresence of sodium metal. The treatment of 117 with alkynes inthe presence of palladium catalysts resulted in the digermylationand the formation of the corresponding 1,4-digermacyclohex-2,5-dienes 118 and 119 (Scheme 69) [188].

2.11.2. PlatinumThe digermylation of terminal with hexamethyldigermane has

only been accessible employing a platinum catalyst at 120 �C,affording the corresponding Z-1,2-bis(trimethylgermyl)ethenes(120) (Scheme 70). Lowering the temperature resulted in deterio-ration of the yields [189]. Internal alkynes were unreactive.

2.12. Germanium–Tin (GeASn)

2.12.1. PalladiumPiers and co-workers reported the germylstannation

of a,b-acetyleneic esters with Bu3SnGeMe3 to affordE-2-(tri-nbutylstannyl)-3-(trimethylgermyl)alk-2-enoates (121) asthe major product. The reactivity of the resulting germyl and stan-nyl groups were separately assessed. 121 was treated with nBuLiand an alkyl halide to form 122 via the transmetallation of Bu3Sn.The germyl moiety was also transformed into a CAI bond uponaddition of iodine (Scheme 71) [190].

Nakano synthesized Z-1-aryl-2-germyl-1-stannylethenes (123)by adding tributyl(triethylgermyl)stannane to aryl terminal alky-nes in the presence of catalytic amounts of [Pd(dba)2] and 4-ethyl-1-phospha-2,6,7-trioxabicyco[2.2.2]octane (Scheme 72). Thisprotocol was extended to ethynylthiophene and 2-methyl-3-butyn-2-ol. The regioselective addition of the germyl moiety atthe terminal carbon was favored in all cases [191].

2.13. Germanium–Boron (GeAB)

2.13.1. Nickel, palladium and platinumIn their investigation into the silaborative dimerization of alky-

nes catalyzed by nickel complexes, Ito and co-workers developedthe analogous germylborane reaction. The product/s obtained inthe germylboration of 1-hexyne were highly dependent on themetal catalyst used. In the presence of [Ni(acac)2]/DIBALH, thegermylborated dimerized product 124 was obtained. By alteringthe catalyst to [Pd(OAc)2]/isocyanide, a 1:1 mixture of 124 andthe germylboration adduct 125 was isolated, whereas catalytic-quantities of Pt(PPh3)2(C2H4) resulted in exclusively 125(Scheme 73) [131].

nBu

nBuBpin

H

GeMe2Ph

H

nBu

nBuBpin

H

GeMe2Ph

H

nBu

PhMe2Ge Bpin

H+

nBu

PhMe2Ge Bpin

H

124

124 125

125

mylboration of 1-hexyne [131].

76 M.B. Ansell et al. / Coordination Chemistry Reviews 336 (2017) 54–77

3. Conclusions

This review presents the state-of-the-art in homo- and hetero-geneous transition metal catalyzed hetero element–element0

additions to alkynes. These reactions yield highly functionalizedmulti-substituted alkenes with high regio- and stereoselectivities.The early literature was limited to reactive or unstable EAE0 bonds,and reactions were primarily mediated by platinum grouptransition metal complexes with phosphine or isocyanide ligands.Developments over the past decade made it possible to employother transition metals such as coinage metal complexes, and uti-lize commercially available EAE0 bonds that are air and moisturestable. Furthermore, the application of NHCs as ligands hasvastly improved conditions in an unprecedented manner.However, EAE0 bonds are still restricted in their application andthe state-of-the-art catalysts remain flawed. Continuing effortsare likely to focus on earth abundant transition metal complexesand new EAE0 bonds that improve current regio- andstereoselectivities.

Acknowledgment

M.B.A. is funded as an EPSRC Standard Research Student (DTG)under Grant No: EP/L505109/1.

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