Tacticity

m

r

Dyad Tacticity

isotactic (meso, m)

syndiotactic (racemic, r)

• Isotactic and syndiotactic polymers are crystalline, atactic is amorphous.

• NMR spectroscopy is a powerful tool for studying polymer stereochemistry.

• Ziegler/Natta catalysts allow control of tacticity.

•Tacticity of polymer is determined by % m or r dyads. (Perfectly isotactic polypropylene has 100% m dyads)

isotactic

syndiotactic

atactic

m m

m r

r r

Triad Tacticity

isotactic(mm)

atactic (mr)

syndiotactic (rr)

Site vs. chain-end control

Site control mechanism: the catalytic center determines the stereospecificity of the polymer.

Chain-end control mechanism: the polymer chain determines the stereospecificity of the final polymer.

Ti

Cl

ClCl

Cl

CH3

Ti

Ti

Ti

P

re face

si face

12

MP*

+

MP*

*

MP*

*

Chain-end control

TiP*

*

VP**

1

2

V

P*

* VP

Ti

P**1

2Ti

P

Chain-end control through 2,1-insertions leading to syndiotactic polymer

Chain-end control through 1,2-insertions leading to isotactic polymer

trans

trans

trans

syntranstrans

trans

V = VCl4 / AlClEt2 or V(acac)3 / AlClEt2

Ti = Cp2TiPh2 / MAO at -45 oC; atactic at 25 oC

13C NMR spectroscopy

Atactic: 9 signals (10 expected, 2 coincidental) based on pentads

# # # # ###o o

#

m m m m m m m m mr

o

mmmm

mmmrmmrm

Chain-end control

# # # # ##o

#

m m m m m r m m mr

o

mmmm

mmmr mmrr

Site control

o

mrrm

Late transition metal catalysts

N N

M

Pol

+

ArAr N

N NM Ar

Cl Cl

MAO

EthenePropene

Polymers

M = Ni, Pd

N N

M

S Pol

+

ArAr

M = Ni, Pd

COOMe

COOMe

MeOOC

Brookhart M = Fe, Co

Ethylene polymerization catalysts

Ethylene-acrylate copolymerization catalysts

M=C complexes

Textbook H: Chapter 13.1 – 13.2

Textbook A: Chapter 9

2005 Nobel prize in chemistry

Yves ChauvinInstitut Français du Pétrole Rueil-Malmaison, France

Robert GrubbsCalifornia Institute of Technology

(Caltech) Pasadena, CA, USA

Richard SchrockMassachusetts Institute

of Technology (MIT) Cambridge, MA, USA

http://nobelprize.org/chemistry/laureates/2005/index.html

"for the development of the metathesis method in organic synthesis"

Outline Carbene complexes

Bonding Structural and spectroscopic features Synthesis Reactivity

Olefin metathesis Mechanism Reaction overview

ROMP RCM

Grubbs catalysts Stereochemistry Catalyst decomposition

Alkyne metathesis Reactions related to olefin metathesis

M=C complexes: comparison

Property Fischer-type

(carbene)

Schrock-type

(alkylidene)

Nature of carbene C

Typical R groups

Typical metal

Typical ligands also attached to the metal

Electrophilic

donor (e.g. OR)

Mo(0), Fe(O), Cr(0)

Good acceptor (e.g. CO)

Nucleophilic

Alkyl, H

Ti(IV), Ta(V), W(VI)

Good or donor (Cl, Cp, Alkyl)

C

R

R

M C

R

R

M

Fischer carbene Schrock carbene

(OC)5WOR

R

W(0), 18e

Cp2(Me)TaH

H

Ta(V), 18e

Fischer carbenes: synthesis Nucleophilic attack at a carbonyl ligand: most common method

Activation of a neutral acyl complex

From activated olefins

(OC)5Cr

OLi

Ph

Cr(CO)6 + PhLi (OC)5Cr

OMe

Ph

Me3OBF4

Re

OCPh3P R

O

FSO3CH3Re

OCPh3P R

OCH3

FSO3-

NR

RN

RN

NR

+ RuCl2(PPh3)2 Cl2Ru

RN

NR 4

TaCl5 + 3Li

tBu

Ta

Cl

Cl

tButBu

tBu

Zn

tBu

2Ta

tButBu

tButBu

tBuHHH

H

Ta

tButButBu

tBu

-abstraction: induced by steric bulk

Rearrangement of coordinated ligands

From activated olefins

Schrock carbenes: synthesis

LnM R + H+ LnM C

R

Hvinylidene

Cp2Ti(PMe3)2 + Cp2(PMe3)Ti

-unsaturated carbene

M=C complexes: comparison

Property Fischer-type

(carbene)

Schrock-type

(alkylidene)

Nature of carbene C

Typical R groups

Typical metal

Typical ligands also attached to the metal

Electrophilic

donor (e.g. OR)

Mo(0), Fe(O), Cr(0)

Good acceptor (e.g. CO)

Nucleophilic

Alkyl, H

Ti(IV), Ta(V), W(VI)

Good or donor (Cl, Cp, Alkyl)

C

R

R

M C

R

R

M

Fischer carbene Schrock carbene

(OC)5WOR

R

W(0), 18e

Cp2(Me)TaH

H

Ta(V), 18e

Carbene complexes: Fischer type

x

z

y *

*

dz2 ( )

dyz ( )dxz ( )

dxy

COH

H

COH

H

COH

H

(OC)5Mo=CH(OH) CH(OH)Mo(CO)5

Alkylidene complexes: Schrock type

x

z

y *

*

dz2 ( )

dyz ( ) CH

H

CH

H

Cp(Cl)2Nb=CH2 CH2NbCpCl2

Structural and spectroscopic features

1H NMR: 5 – 15 ppm13C NMR: 220 – 260 ppmJCH typical: 150 – 160 HzJCH agostic: 90 – 120 Hz

Schrock typeFischer type

MCC angle: 160 – 170°

MC distance: longer than M=C, but shorter than M-CCX distance: shorter than C-X

(OC)5Cr

OEt

NMe2

2.13 Å 1.35 Å

1.33 Å

LnM

OR

R'

LnM

OR

R'

C

R'

OR

M

LnM

R

H

LnM

R

H

Fischer carbenes: reactivity Heteroatom substitution: related to the aminolysis of esters to give amides

Cyclopropanation

The Dötz reaction

(OC)5Cr

OMe

Ph

:NH2R (OC)5Cr

OMe

PhNHR

H - MeOH(OC)5Cr

NHR

Ph

(OC)5Cr

OMe

Ph

or h(OC)4Cr

OMe

Ph

X

Y(OC)4Cr

OMePh

XY MeO

Ph

X

Y- "Cr(CO)4"

(OC)5Cr

OMe R1

R2

+50 oC

- CO

OH

R1

R2

OMe Cr(CO)3

+

OH

R1

R2

OMe(OC)3Cr

Mechanism of the Dötz reaction

(OC)5Cr

OMe R

R

+ (OC)4Cr

OMe

R

R

(OC)4Cr

OMe

R

R

CO insertion

(OC)4Cr

OMe

RR

O

Cr(CO)4

OMe

RR

C

OO

R

R

OMe Cr(CO)4

H

- COproducts

enolize

- CO

+ CO

Schrock carbenes: reactions with electrophiles

(tBuH2C)3Ta CH

tBu+ O

R

R'

Wittig-like reactivity:

[(tBuCH2)3TaO]n +R

R'

H

tBu

Reactions with Bronsted acids:

Mo C CH

R

CpMe3P

Br PMe3

HBF4Mo C HC

H

R

CpMe3P

Br PMe3

+

BF4-

WMe3P

ClPMe3

CR

CO

PMe3

HCl

- PMe3

WMe3P

ClPMe3

CHR

CO

Cl

Other reactions

(R3P)2Cl3Ta CH

tBu+ O W(OtBu)4

Alkylidene transf er:

TaCl(OtBu)4 + WCl

ClO

CHtBu

PR3

PR3

C-C bond f ormation:

Cp2Ta

CH2

+ Me3P CH2

Me

Ta + PMe3

Me

IrR3PBr

PR3

CH2

CH3

PR3

+

Br- IrR3PBr

PR3

CH2CH3

Br

PR3

Cp2

Olefin metathesis

Reference: Chem. & Eng. News 2002, Dec 23, 34-38http://nobelprize.org/nobel_prizes/chemistry/laureates/2005/animation.html

Mechanism:

Such cycloaddition reactions between two alkenes to give cyclobutanes are symmetry forbidden and occur only photochemically. However, the presence of d-orbitals on the metal alkylidene fragment breaks this symmetry and the reaction is quite facile.

Normally, the products are statistical, unless the reaction can be driven in some way or the two alkenes have different reactivities.

R1

R1

R1

R1

R2

R2

R2

R2

+catalyst R2

R2

R1

R1

2

LnMR

R'R'

LnMR

R'R'

MLnR

R'R'

R1

R1

+catalyst R1

R1

R1

R1

2 C2H4

Timeline

Titanium

Tungsten

Molybdenum

Ruthenium

AcidsAlcohols, water

AldehydesKetones

Esters, amidesOlef ins

AcidsAlcohols, water

AldehydesKetonesOlef ins

Esters, amides

AcidsAlcohols, water

AldehydesOlef insKetones

Esters, amides

Olef insAcids

Alcohols, waterAldehydesKetones

Esters, amides

1950

1960

1970

1980

1990

2000

discovery of olef in metathesis

RuCl3 performs ROMP

Chauvin proposes metal alkylidene

-based mechanismevidence for Chauvin's

mechanism found

single-componentcatalysts developed

synthesis of Mo-alkylidene

synthesis of Ru-alkylidene(Cy3P)2Cl2Ru=CHPhmechanism

of (Cy3P)2Cl2Ru=CHPh investigated mono(NHC) Ru

catalysts developed