Chemistry of First-Row Transition Metal Photocatalystschemlabs.princeton.edu/macmillan/wp-content/... · photochemistry with copper ... Properties of Organometallic Photocatalysts

Chemistry of First-Row Transition Metal Photocatalysts

David Kornfilt

MacMillan Group Meeting10/17/2018

Introduction

� Triplet Sensitizationwith chromium

� Ligand-directed photochemistry with copper

� Direct photo-HAT with iron

Cr Fe Cu

Why should we care about first-row transition metal photocatalysts?

Properties of Organometallic Photocatalysts

Wenger, O. Chem. Eur. J. 2018, 24, 2039

� Long lived phosphorescent T1 state

� Tunable oxidation and reduction potential

� Singlet ground state

� Highly optimized for SET

� Weak fluorescence, usually TADF

� Ligand dependent absorption spectra

� Singlet or higher spin ground states

� Can do SET and other chemistry

NIr

N

NCr N

NMe

N

NMe

N

N

NMe

N

NMe

N

2nd and 3rd row TM Photocatalysts First row transition metal photocatalysts

Introduction




Cr Fe Cu


Chromium-Based Photocatalysts

Heinze, K., Angew. Chem. Int. Ed., 2015, 54, 11572

Cr N

NMe

N

NMe

N

N

NMe

N

NMe

N

4A2 ground statet2g

3

(t2g)2(eg)1 4T2

ISC 2T22T12E435 nm 500 nm

775 nm

�Quartet ground state (3 unpaired electrons)

�High photon absorption, Φ = 12.1% (Cr[bpy]3+, Φ = 0.09%)

�Long-lived lifetimes of 899, 898, 1164 μs (Cr[bpy]3+, 63 μs)

Oxygen Sensitization with Organic Dyes

3O2

light

photosensitizer

1O2

O

O

O

ClCl

Cl

Cl I

OHII

I

HO

Rose bengal

Spin statistics suggest that TTA produces an overall singlet state only 11% of the time (exclusions apply)

� Singlet Oxygen Production

Photobleaching of photosensitizer can be problematic

3D 3A3D 1A

1D 1A

pentet

triplet

singlet

*

*

*

Heinze, K., Angew. Chem. Int. Ed., 2015, 54, 11572

Oxygen Sensitization with Chromium

Quartet ground state carries away spin, so 4 out of 6 microstates lead to singlets

6-fold increase in collision efficiency

2D 3A

2D 1A

quartet

doublet

*

*

Heinze, K., Angew. Chem. Int. Ed. 2015, 54, 11572

Cr N

NMe

N

NMe

N

N

NMe

N

NMe

N

ISC 2T22T12E

775 nm

Excited state doublet

Amine α-cyanation with Cr

Opatz, T. ChemPhotoChem, 2017, 1, 344

Cr N

NMe

N

NMe

N

N

NMe

N

NMe

NR N

R

R O2, TMSCN 462 nm lightMeCN, 6 h

[Cr(ddpd)2](BF4)3 (1 mol %)R N

R

R

5 examples

CN

N

CN

Me N

CN

Me

Me

47 % yield99% yield

N Pr

Pr

Me

CN

99% yield

NNC

O

O

OH

Ph

63% yield

Radical Cation [4+2] with Chromium Photocatalysts

Ferreira, E. M. Angew. Chem. Int. Ed. 2015, 54, 6506

Me

MeO

Me Cr(Ph2phen)3(BF4)3 (1 mol %)

300-420 nm lightCH3NO2, air,

Ar

Me Me

N N

Ph Ph

Ph2Phen

Ar

EtO2C

Me

Reversed regioselectivity with respect to Diels Alder!

Φ = 0.21in degassed solvent

MeOTs

PMP

Me MeOMe

Me Me

OTBS

Me

PMPC6H13

Me

PMP

Me

Me

Me

PMP

Me

C6H13

89% yield15:1 endo/exo

86% yield19:1 d.r.

74% yield6:1 endo:exo

96% yield 76% yield 78% yield23W CFL, 50% yield

Radical Cation [4+2] with Chromium Photocatalysts

Shores, M. P. J. Am. Chem. Soc. 2016, 138, 5451

CO2Et

MeO

Me Cr(Ph2phen)3(BF4)3

CH3NO2, air, 23W CFL Ar

EtO2C Me

N NAr

EtO2C Me

Ar

HO2C Me

Ar

OHC Me

Ar

EtO2C Me

CH2OAc

75% yield19:1 r.r.

78% yield 6:1 r.r.

Ph Ph

Ph2Phen57% yield 17:1 r.r.

53% yield11:1 r.r.

Ar

EtO2C

Me

Reversed regioselectivity with respect to Diels Alder!

Ar

Ph(O)COAc

Ar

Ph(O)CO

MeMe

61% yield 7:1 r.r.

72% yield 7:1 r.r.

Φ = 0.21in degassed solvent

Oxygen Sensitization for [4+2] Cycloadditions

Shores, M. P. J. Am. Chem. Soc. 2016,138, 5451

LnCr3+*

LnCr3+

1O2

3O2 O2-

LnCr3+ LnCr3+*

LnCr2+

MeAr

MeAr

SET

Me

Me

Ar

Me

Me

Ar

Me

dienophile

diene[4+2] adduct

ET SET

� Double photon mechanism

Energy transfer and electron transfer from same photocatalyst

Formation of Radical Cation Intermediate

CO2Et

MeO


2.00 eV 1.98 eV

CO2Et

MeO1.33 eV vs. SCE


Ph(O)C

Ar Me

CO2Et

MeO

Me hv Me

Ar

Ph(O)C

Ph(O)C C(O)Ph

PMP PMP

1.40 eV

1.66 eV

CO2Et

MeO

Me

Possible Alternative Oxidations

Cr Me

Ar

Ph(O)C

Cr

hv

530 nm LED


Convergent Reactivity

PMP

Ph(O)C

Me23W CFL, airCH3NO2, 10 h

67% yieldMe

Me

MePh(O)C

PMP

MePh(O)C

PMP Me

not detected

Ph(O)C C(O)Ph

PMP PMP 23W CFL, airCH3NO2, 6 h PMP

Ph

O

16% yield

MePh(O)C

PMP

Me20% yield

� Cyclobutane A is competent

� Cyclobutane B is competent

Cr Cr

Cr Cr

A

B


All roads lead to Rome!

Introduction




Cr Fe Cu


Ligand Substitution on Organometallic Photocatalysts

Wenger, O. Chem. Eur. J. 2018, 24, 2039

� Ir,Ru tris-complexes are usually inert to ligand exchange

� Ligand exchange is usually through two-point binding

� LMCT, MLCT bands interrupted by substitution

� Cu substitution is facile

� Ligand-Metal complex can be directly excited

NIr

N

N

NN

NN

RuN

N

N

N

N

N

N

N

Ir(ppy)3 Ru(bpz)3

NN Cu

LiCuCz2

N

NCu

PMP

PMP

Cl

Cu(dap)Cl

Ligand Directed Cu Photocatalysts

Han, S. B. Org. Lett. 2014, 16, 1310Reiser, O. Angew. Chem. Int. Ed. 2015, 127, 6999

F3CS

Cl

OO

F3CS

ONa

O

Langlois Reagentsource of CF3 radical

ATRA reagentfor CF3 and Cl transfer

RCF3SO2Cl

RCF3

Cl

Me

Cu(dap)2Cl

CF3SO2ClBlue LEDs Me

SO2ClCF3

N NPMP PMP

Ligation to photoactive metal center can directly affect radical stability

Ru

� Trifluoromethylchlorination

� Trifluoromethylsulfonylation

dap ligand

Trifluoromethylchlorosulfonylation with Copper

Me

L2CuCl (1 mol %)

K2HPO4, CF3SO2Cl530nm LED, MeCN, 24 h Me

SO2ClCF3

N NPMP PMP

SO2ClCF3

CF3

SO2ClMeO

SO2ClCF35

CF3

SO2Cl

67 % yield7:3 rr ratio

76% yield

71% yield

56% yield

No Lewis Basic donors

OCF3

Cl

57% yield

Cl

Cl

N

O

O

Cl CF3

53% yield

Lewis basic donors

Reiser, O. Angew. Chem. Int. Ed. 2015, 127, 6999

Ligand-Directed Photocatalysis with Copper

Reiser, O. J. Org. Chem. 2016, 81, 7139

HOOMe

SO

F3C

OO

OMe

SO

F3C

OO

OOH

NHtBu

HOS

F3C

OOO

��Sultone formation is rare

� SO2Cl moiety tolerates alcohols

SO

F3C

OO Me

MeS

O

F3C

OO S

O

F3C

OO S

O

F3C

OO

73% yield 74% yield

H

H

90% yield1.1:1 d.r. 67% yield

*

Cu

530 nm LED

Cu

530 nm LEDβ-blocker derivative

Ligand Excitation of Copper Complexes

CuCl

Non-absorptive

NLi

NN Cu

Absorbs at short and long UVCompetent intermediate for cross-coupling

I CuI (10 mol %), CzLi

CFL or Hg lamp, 10 h

NHAr� Light necessary for reaction

��Amine deprotonation requires base

��Yield increased with Hg lamp

� Radical pathway65 % yield

Formation of UV-active Cu Complexes

Fu, G. Science, 2012, 338, 647

Photoinduced Ullmann Couplings

N-Arylation of Heteroarenes

MeCN, rt, 10h

40% yield

N

Fu, G. Science, 2012, 338, 647

NPh

CuPPh3

PPh3

PhBr, 13W CFL

254 nm lightMeCN, rt, 10h

56% yield

10% CuI, LiOtBuNH

N

Br

� Catalytic copper can be used

Photoinduced Ullmann Couplings

Fu, G. J. Am. Chem. Soc. 2014, 136, 2162Fu, G. Science, 2012, 338, 647

254 nm lightMeCN, rt, 10h

56% yield

10% CuI, LiOtBuNH

N

Br

� Catalytic Ullmann couplings

OH SH

NH2

O

� Nucleophiles

Br Cl

I

� Electrophile

BrC N

Copper Arylation with UV Light

Fu, G. J. Am. Chem. Soc. 2013, 135, 13107

NN Cu

Cu NR2R2N*

Cl

CNCu NR2R2N

CN

Cu NR2X

Cz

CN

CzHLiOtBu

C-N productLiCl

SET

Aryl halide

LiCl

CopperPhotocatalytic

Cycle

Cu

UV light mediated C-N coupling

254-365 nmlight

Mechanistic Investigation of CuCz System

Fu, G. J. Am. Chem. Soc. 2017 139 12716

� Copper carbazolide system selected for further study

Me

Br

LiCz (1.5 equiv)Cz2CuLi (5 mol %)

100W Hg LampMeCN, 8 h, 0 oC

Me

Cz

64% yield

Me

Br 100W Hg LampMeCN, 8 h, 0 oC

Me

Cz

96% yield

Cz2CuILi

catalytic

stoichiometric

� 5% product obtained in absence of copper

� Yield increases over time

What is the source of this background reactivity ?

Cz2CuLi is quenched by alkyl bromide

� kq = 4.8 x 106 M-1 s-1

� t1/2, Cu = 910 ns


Fu, G. J. Am. Chem. Soc. 2017 139 12716

LiCz also quenches alkyl bromide!

� kq = 4.9 x 108 M-1 s-1

� t1/2, CzLi = 31 ns

What is the source of this background reactivity ?


Fu, G. J. Am. Chem. Soc. 2017 139 12716

Cz2CuLi is quenched by alkyl bromide

� kq = 4.8 x 106 M-1 s-1

� t1/2, Cu = 910 ns

� Alternative catalytic cycle

N

LiCuICz2 LiCuIICz3

Cz

LiCuIICz3 functions as a persistent radical

Br

LiCz*

SET

LiCz


Fu, G. J. Am. Chem. Soc. 2017 139 12716

In photocatalysis with 1st row transition metals, multiple pathways are possible!

� Off-cycle reactivity observed as concentration of key LiCuCz3 builds up

Formation and change in absorption due to LiCuCz3 Ratio of Product/dimer increases over time


Fu, G. J. Am. Chem. Soc. 2017 139 12716

Traditional SET with Copper Catalysts

Photoactive Ligands

� Formation of photoactive copper catalysts

CuX

Nu

Cu

Photoactive Cu Complex

� Substrate generality

� Wavelength modulation

� Catalyst Stability

OHOH

O

Me Me

PPh2PPh2

PPh NH

tBu

tBu

P(iPr)2

P(iPr)2

Traditional SET with Copper Catalysts

OtBu

O

NH2

1 mol % L110 mol % CuBrLiOtBu (3 equiv)

Blue LED, DME, rtOtBu

O

NHCy

Br

81% yield

NH

tBu

tBu

P(iPr)2

P(iPr)2

N

OCl

Et Ph

carbazoleligand (1.2 mol %)

CuCl (1 mol%)LiOtBu, blue LEDtoluene, -40 oC

N

ON

Et Ph

90% yield94% ee

PPh

NH2

CuI (5 mol%)BINOL (10 mol %)

BTPP, blue LEDMeCN/DMF, -10 oC, 24 h

I HN

Cy

92% yield

OHOH

Fu, G. J. Am. Chem. Soc. 2017, 139, 17707

Fu, G. Science 2016, 351, 681

Fu, G. J. Am. Chem. Soc. 2017, 139, 18101

Introduction




Cr Fe Cu


Direct HAT with Photocatalysts

Wenger, O. S. Chem. Eur. J. 2018, 24 2039

MO

Photo HAT

MO

* H

MOH

+

alkyl radical

Non-Photo HAT

PivO

Fe(PDP) (5 mol %)

AcOH, H2O2, MeCN, 0.5 h, rt PivO

OHMeMe White, 2007

51% yield

Mn(TMP)Cl (1 mol%)

NaOCl, TBAClDCM, 12 h, rt

Cl51 yield4:1 r.r.

Groves, 2015

Direct HAT with Photocatalysts

� ~390 nm excitation

� Powerful oxygen centered abstractor

� Selective C-H abstraction

� Electronic properties not tunable

� ~420 nm Soret band

� Powerful oxygen centered abstractor

� Selective C-H abstraction

� Electronic properties highly tunable

� Short-lived photoexcited state!

WO

O

OW

W

OO

WO

O

O

OW

O

O

O

O

O

W

W

OO

WO

O

O

OW

O

O

O

WO

O

O

O

O

O

O

O

4–

O

Fagnoni, M. Acc. Chem. Res. 2016, 49, 2232Emily Scott Lab Webpage, umich.edu

Iron Photosensitizers for HAT

NN N

NFeIII

O

N

N NN

FeIII

C6F5

C6F5

C6F5 O C6F5

C6F5

C6F5

Pacman-Complex

Pacman (0.0007 mol%)Me

1 M in pyridine

Me

425 nm lightO2 (1 atm), 18 h

76 TONΦp =0.00151

425 nm light

NN N

NFeIV

O

N

N NN

FeII

C6F5

C6F5

C6F5O

C6F5

C6F5

C6F5

Active oxidation complex

reclamp

� Sole observed product

� Competitive reclamping

� KH/KD = 1.55

Light-driven HAT from Toluene

Nocera, D. G. J. Am. Chem. Soc. 2006, 128, 6546


Me

287 TONΦp =0.0152

235 TONΦp = ND

160 TONΦp =0.00276

143 TONΦp =0.00199

H H H

HAT from weak C-H bonds

NN N

NFeIII

O

N

N NN

FeIII

C6F5

C6F5

C6F5 O C6F5

C6F5

C6F5

Pacman-Complex

425 nm light

NN N

NFeIV

O

N

N NN

FeII

C6F5

C6F5

C6F5O

C6F5

C6F5

C6F5

Active oxidation complex

reclamp



Fe FeO

Fe FeO

Me

Fe FeOH

OH

Fe Fe

O2

� Radical rebound

� No backgroud reaction

� Reclamp t1/2 = 2.3 ns

� KH/KD > 1 suggests PCET

Substrate

Product

Photocatalytic Fe H Atom Abstraction

Fe

Iron Photocatalytic

Cycle


Introduction




Cr Fe Cu


First row transition metal catalysts enable new photochemistry!

Introduction




Cr Fe Cu


First row transition metal catalysts enable new photochemistry!

? ?

? ??Questions?

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Chemistry of First-Row Transition Metal Photocatalystschemlabs.princeton.edu/macmillan/wp-content/... · photochemistry with copper ... Properties of Organometallic Photocatalysts