Polyoxometalate Photocatalysis

Thomas BrewerMacMillan Group Meeting

December 13, 2017

What are polyoxometalates (POMs)?

≥ 3 transition metal oxyanions linked by shared oxygen atoms closed, 3-dimensional frameworks

typically group 6 (Mo, W) and group 5 (V, Nb, Ta) type I & III POMs shown to be reversibly reduced to “heteropoly blues”

Type Imonooxo terminal groups

Ex: [M10O28]6- (M = Nb, Ta, V)

Type IIcis-dioxo terminal groups

Ex: [W7O24]6-

Type IIImonooxo and cis-dioxo terminal groups

Ex: [H2W12O42]10-

Pope, M. T. Inorg. Chem., 1972, 11, 1973structures from http://verahill.blogspot.com/search/label/molecules

Outline

Polyoxometalate Fundamentals

POM

1S0h!

1(O→M LMCT)

1S1ISC

3(O→M LMCT)

3T1

C–H Oxygenation/Fluorination Reactions

H X

C–C Bond-Forming Reactions

H R

POM synthesis

can include heteroatoms “X” to form hetero-POMs frequently involve equilibria between several POMs

controlled by pH, counterion identity, temperature, cosolvent, M/X ratio

Berzelius, J. J. Pogg. Ann. 1826, 6, 380Weinstock, I. A. Chem. Rev. 1998, 98, 113

Hill, C. L.; McCartha-Prosser, C. M. Catal. Met. Complexes

[H+][MO4]n- [XOpHh]q- [XxMmOr]t-H2O

via olation:HO O

MOO

HO OM

OO

O OM

OOM

O

OO+ H2O

[H+][MO4]n- [MmOr]t-H2O

isopolyoxometalatesonly TM and oxide ions

Ex: [W10O32]4-

heteropolyoxometalatescontain ≧1 ‘heteroatom’

Ex: [PMo12O40]3-

(fun fact: discovered in 1826!)

Type I POMs have rich redox chemistry

(n-1)d

ns

np

!(z)

"(xy)

"(z)

2a1* + e*

a1*

b1* (x2-y2)

e* (xz, yz)

b2 (xy)

e

3a1 + b1 + e

MOL5 MO diagram (C4v, no xy !-bonding)

M

L

L LO

LL

idealized local symmetry

TM atoms in highest (d0) oxidation state ⇒ POMs act as oxidants Type I POMs have formally nonbonding LUMO (b2)

In type II POMs, former dxy orbital is !*

Polyoxometalate Molecular Science; Almenar-Borras, J. J., Coronado, E., Muller, A., Pope, M., Eds.; Springer: Dordrecht, 2003

POM photoredox reactivity

Potentials are very approximatec

V, Mo POMs strong oxidants, but difficult to re-oxidize to d0 V, Mo POMs can be sufficiently oxidizing in ground state to react with oxidizable intermediates

POMs typically show long (100s of nm), tailing absorbance

apotential vs. NHE | birreversible reduction | cvarious reference electrodes and experimental conditions; literature not in agreementDe Waele, V., Poizat, O., Fagnoni, M., Bagno, A., Ravelli, D. ACS Catal., 2016, 6, 7174

Hou, D., Hagen, K. S., Hill, C. L., J. Am. Chem. Soc., 1992, 114, 5864Polyoxometalate Molecular Science; Almenar-Borras, J. J., Coronado, E., Muller, A., Pope, M., Eds.; Springer: Dordrecht, 2003

POM

HOMO

LUMO

1S0h!

1(O→M LMCT)

1S1ISC

3(O→M LMCT)

3T1D–H

3(POM H∙ ∙ ∙D•)–

3(RP)2(POM H) + 2(D•)–

2D

0,0 transition

[W10O32]4- [PW12O40]3- [PMo12O40]3- [V10O28]6- [V13O34]3-

450 nm 415 nm 415 nm 490 nm 560 nm

ground state reductiona -0.97 V -0.37 V +0.14 V -0.24 Vb +0.63 V

excited state reductiona 2.44 V 2.63 V 2.67 V 2.01 V 2.84 V

"max 324 nm 220 nm263 nm 240 nm 202 nm

A closer look at decatungstate photochemistry

apotential vs. SCEDe Waele, V., Poizat, O., Fagnoni, M., Bagno, A., Ravelli, D. ACS Catal. 2016, 6, 7174

Yamase, T., Takabayashi, N., Kaji, M. J. Chem. Soc. Dalton Trans. 1984, 793

[W10O32]4-

[W10O32]4-*

1S0

3T1

[W10O32]4-*1S1

[HW10O32]4-

[W10O32]4-*1S1

hot

h!

τ < 1 ps

τ ~ 10 ps

τ ~ 50 ns

η ~ 0.5

η ~ 0.5

ISC

D–H3T1 HOMO-1 (A1g)

oxidation

disproportionation

[H2W10O32]4-k = 6 x 105 M-1 s-1

E½ = -1.4 Va

E½ = -0.97 Va

after excitation, fast (τ ~ 10 ps) ISC yields the reactive 3T1 state 3T1 HOMO-1 has highest (accessible) density on apical oxo ligands Singly-reduced [HW10O32]4- can disproportionate to [W10O32]4- and [H2W10O32]4-

Decatungstate redox potentials vary widely

apotential vs. Ag/AgCl | protonation-dependent CV potentials vs. Ag/AgNO3Renneke, R. F., Kadkhodayan, M., Pasquali, M., Hill, C. L. J. Am. Chem. Soc. 1991, 113, 8357

Yamase, T., Takabayashi, N., Kaji, M. J. Chem. Soc. Dalton Trans. 1984, 0, 793

MeCN : H2O

10 : 0

9 : 1

8 : 2

7 : 3

E½(1)a E½(2)a

-0.94

-0.72

-0.62

-0.59

-1.37

-0.96

-0.81

-0.74

strongly dependent on protonation state strongly dependent on solvent

[W10O32]4- e[W10O32]5- [W10O32]6-

e

E½(1) E½(2)

POM photoredox catalysis

Fox, M. A., Cardona, R., Gaillard, E. J. Am. Chem. Soc. 1987, 109, 6347Papaconstantinou, E. Chem. Soc. Rev. 1989, 18, 1

Polyoxometalate Molecular Science; Almenar-Borras, J. J., Coronado, E., Muller, A., Pope, M., Eds.; Springer: Dordrecht, 2003

POM

h!

POM Sub

POM* Sub

Sub

Subox

POMred

O2, H2O, oxidants

O2-, H2, etc.

generalized POMphotoredox cycle

"K[Sub]R =

1 + K[Sub]

initia

l rat

e

[Sub]

K

" ~ 1012 M-1s-1 � preassociation shows similar dependence on [POM]

Kinetics of POM photoredox

Common POM photoredox catalysts

[XM12O40]n-

X = P, Si, Fe, H2, etc.M = W, Mo

[W10O32]4-

Selected substrate classes

alcohols

H

alkanes

alkenes

Cl

alkyl-Cl

OHO

ethers

CN

nitriles

amines

Ar R

benzylic

NR2

Site selectivity in decatungstate-mediated C–H abstraction

aMichael addition | boxygenation | decatungstate chemdraw provided by Ian PerryRavelli, D., Fagnoni, M., Fukuyama, T., Nishikawa, T., Ryu, I. ACS Catal. 2017, just accepted

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

O

Hh!

H O WO

O O

O

"+ "-

‡

Polar effects

MeMe

MeH

H

19% / 81%a

NH

H

14% / 86%a

HO Me

MeH

100%a

O H

100%a

O

Me

MeMe

HH

29% / 71%b

H

100%a

O

O H

15% / 85%a

O

H

NC CN

n.r.a

N

7% / 93%a

H

O H

NC HH

9% / 91%a

Me

H

Site selectivity in decatungstate-mediated C–H abstraction

aMichael addition | decatungstate chemdraw provided by Ian PerryRavelli, D., Fagnoni, M., Fukuyama, T., Nishikawa, T., Ryu, I. ACS Catal. 2017, just accepted

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

O

Hh!

H O WO

O O

O

"+ "-

‡

Steric effects

t-Bu

H

H

H

Me

MeH

30% / 70%a

48% / 52%a

O

H

100%a

O

MeMe

Me

n.r.a

MeMe

Me

H

H

[W10O32]4-* O

MeMe

H

100%a

OMe

Me

[W10O32]4-*

weak C–H bonds

H2N H

O

100%a

O

H

H

20% / 80%a

H

100%a

Outline

Polyoxometalate Fundamentals

POM

1S0h!

1(O→M LMCT)

1S1ISC

3(O→M LMCT)

3T1

C–H Oxygenation/Fluorination Reactions

H X

C–C Bond-Forming Reactions

H R

Early work in decatungstate-catalyzed C–H oxygenation

Chambers, R. C., Hill, C. L. Inorg. Chem. 1989, 28, 2509

(Bu4N)4W10O32 (0.4 mol%)

h!, MeCN, O2

[W12O32]4-*

HAT

Me

Me H

Me

Me OOH

Me

Me

Me

Me OO

Me Me

Me Me

InitiationMe

Me HMe

PropogationO O

Me

MeMe

Me

MeHMe

Me

Me OOHMe

Me

MeMe+

55%, sole product

TerminationMe

MeMe

MeMe

HMe

MeHMe

Me

Me+

Me

MeMe

Me

MeOO

Met-BuOOt-Bu

[HW10O32]4- turned over by O2 Alternative to organic radical intitiators

Further studies on decatungstate-catalyzed C–H oxygenation

Tzirakis, M. D., Lykakis, I. N., Orfanopoulos, M. Chem. Soc. Rev., 2009, 38, 2609

H(Bu4N)4W10O32

h!, O2OH + O

Me Me

OH

Me Me

O

OH

OH O

RR

OH

R

O

Me OH OMe Me

OHOH

O

2° alcohol ketone

1° benzylic alcohol benzoic acid

3° alcohol3° C–H

2° benzylic C–H 2° alcohol aryl ketone

2° C–H 2° alcohol ketone

2° allylic C–H allylic alcohol enone

Oxygenative transformations

Oxyfunctionalization of remote C–H bonds of aliphatic amines

Schultz, D. M., Davies, I. W., et al. Angew. Chem. Int. Ed. 2017, 56, 15274

[W10O32]4-

h!N

RHN

H+

most hydridic C–H

RN

RH[W10O32]4-

h!NR

Protonation inverts polarity of α-amino C–H bonds ⇒ distal C–H abstraction possible

HN

Na4W10O32 (1 mol%)H2SO4 (1.5 equiv.), H2O2 (2.5 equiv.)

MeCN/H2O, h!

N

O

HHHSO4 derivitization N

O

R HN

RN

or

Uses desirable unprotected aliphatic amines keto-amine substitution pattern common motif in biologically-relevant molecules amenable to flow (0.7 M, 5 g scale, 5.0 bar O2 as oxidant)

α-amino C–H polarity inversion

Distal C–H oxygenation

Oxyfunctionalization of remote C–H bonds of aliphatic amines

aAY = assay yield before derivitization; IY = isolated yield after derivitizationSchultz, D. M., Davies, I. W., et al. Angew. Chem. Int. Ed. 2017, 56, 15274

Substrate scopea

NBoc

O

64% AY43% IY

N

O

Boc

68% AY (2:1 γ:β)43% IY (γ)

NBoc

O

58% AY43% IY

HN CO2H

NOBn

53% AY43% IY

HN

O

29% AY

CO2Me

BocHN CO2Me

Me

O

48% AY26% IY

Me NHBoc

O

60% AY50% IY

MeNH2

NPhHN

57% AY47% IY

MeNH2

O

36% AY (2:1 δ:γ)

NEt

N

46% AY33% IY

BnO

NHCBz

O

51% AY41% IY

HN

Me OH

64% AY

TBADT-catalyzed fluorination of unactivated C–H bonds

Halperin, S. D., Fan, H., Chang, S., Martin, R. E., Britton, R. Angew. Chem. Int. Ed. 2014, 53, 4690

(TBA)4W12O32 (2 mol%), NaHCO3 (10 mol%)

h!, MeCN

synergistic H atom transfer/F atom transfer fluorinates C–H bonds potentially prone to oxidative metabolism

[W10O32]4-

[W10O32]4-* [HW10O32]4-

R’R

H

R’R

h!

PhO2SN

SO2Ph

FR

H

R’

1.5 equiv.

R R’

F

PhO2SN

SO2Ph

F

PhO2SN

SO2Ph

R’R

F

PhO2SNH

SO2Ph

Proposed mechanism

58% (4.1:1 β:α)

OMe

Me Me

Me

O

F

H

O

F

O

OMe

Me Me

Me

O

Me

TBADT-catalyzed fluorination of unactivated C–H bonds

Halperin, S. D., Fan, H., Chang, S., Martin, R. E., Britton, R. Angew. Chem. Int. Ed. 2014, 53, 4690

(TBA)4W12O32 (2 mol%), NaHCO3 (10 mol%)

h!, MeCNPhO2S

NSO2Ph

FR

H

R’

1.5 equiv.

R R’

F

C–H substrate scopeMeMe

Me O

MeMe

Me O

F

Me

MeF O

17% (2:1 β:α)Me

OAc

MeMe

OAc

MeMe

MeF Me

OAc

MeMe

F

14%

13% 15% (2.4:1 β:α)

OEt

O

MeF

Me OEt

O

3Me OEt

O

3

F

33% 17%

OMe

OMe

MeOMe

OMe

MeF

40%

79%

TBADT-catalyzed fluorination of leucine on large scale

Halperin, S. D., Kwon, D., Holmes, M., Regalado, E. L., Campeau, L.-C., DiRocco, D. A., Britton, R. Org. Lett. 2015, 17, 5200

Na4W12O32 (1 mol%)

h!, MeCN:H2O (9:1), 2 h residence time

PhO2SN

SO2Ph

F

1.5 equiv.

NH3HSO4

OMe

OMe

Me

H

NH3HSO4

OMe

OMe

Me

F

90% isolated yield44.7 g

large scale for the synthesis of odanacatib (development now discontinued) previously processes required 2-6 steps (F introduced by alkene or epoxide hydrofluorination)

MeO2S

NH

CF3 HN CN

O

Me

MeF

odanacatibpotent and selective inhibitor of cathpesin K

Outline

Polyoxometalate Fundamentals

POM

1S0h!

1(O→M LMCT)

1S1ISC

3(O→M LMCT)

3T1

C–H Oxygenation/Fluorination Reactions

H X

C–C Bond-Forming Reactions

H R

Early work in TBADT-catalyzed C–C bond-forming reactions

Jaynes, B. S., Hill, C. L. J. Am. Chem. Soc., 1993, 115, 12212Jaynes, B. S., Hill, C. L. J. Am. Chem. Soc., 1995, 117, 4704

Zheng, Z., Hill, C. L. Chem. Commun., 1998, 2467Hill, C. L. Synlett, 1995, 1995, 127

H

Et

H

O

low yield & selectivity competitive product decomposition limited scope

3 – 25%

18 – 26%

2 – 8%

[W10O32]4-

h!

H

H H

H

HH

C O

NC OMe

O

CNCO2Me

O

3 – 11% (exclusively b)4 – 6% (>13:1 a:b)

22 °C or 90 °C

a b

TBADT-catalyzed acetylation

Prosser-McCartha, C. M., Hill, C. L. J. Am. Chem. Soc. 1990, 112, 3671

Na4W12O32

h!, MeCN

Me

O

48% yield, 77% selectivity

[W12O32]4-*

H

[HW12O32]4- or [H2W12O32]4-

HAT

SET

MeCN,then hydrolysis

rare example of a carbanion intermediate in POM photoredox product deactivated toward further functionalization

Mechanistic evidence

Na4W12O32

h!, 5% D2O

H D

0

50

100

183 184 185 186 187 188

abun

danc

e /

%

m/zbefore reaction partially reacted

TBADT-catalyzed alkylation of electrophilic alkenes

Dondi, D., Fagnoni, M., Molinari, A., Maldotti, A., Albini, A. Chem. Eur. J. 2004, 10, 142Dondi, D., Fagnoni, M., Albini, A. Chem. Eur. J. 2006, 12, 4153

R

R’

HEWG+

(Bu4N)4(W10O32) (2 mol%) R

R’

EWG

Me Me

OHH O H O

O

HH

1 equiv.5 equiv.h!, MeCN

72%CN

CN

Me CN

Me50%

78%

75%

76%

50%

63%

65%

Reaction scope

nucleophilic alkyl radicals trapped efficiently by electrophilic alkenes complete conversion " ranges from 0.06–0.58 depending on alkyl radical nucleophilicity

TBADT-catalyzed alkylation of electrophilic alkenes

Dondi, D., Fagnoni, M., Molinari, A., Maldotti, A., Albini, A. Chem. Eur. J. 2004, 10, 142Dondi, D., Fagnoni, M., Albini, A. Chem. Eur. J. 2006, 12, 4153

R

R’

HEWG+

(Bu4N)4(W10O32) (2 mol%) R

R’

EWG

1 equiv.5 equiv.h!, MeCN

Proposed mechanism

[W10O32]4-

[W10O32]4-*

[HW10O32]4-

R’R

H

R’Rh!

EWG

R

R’

EWG R

R’

EWG

R

R’

EWG

R

R’

EWG H+

electrophilic radical turns over [HW10O32]4- by SET

Further studies on TBADT-catalyzed alkylation of electrophilic alkenes

Ravelli, D., Protti, S., Fagnoni, M.. Acc. Chem. Res. 2016, 49, 2232Ravelli, D., Fagnoni, M., Fukuyama, T., Nishikawa, T., Ryu, I. ACS Catal. 2017, just accepted

Scope has been extended to many classes of C–H nucelophile

Site selectivity is governed by a combination of steric and polar effects

CO2MeCO2Me

NN

CO2iPr

iPrO2C

SO2Ph

CO2Et

CN

CO2Et

Me

O

O

Michael acceptorsC–H nucleophiles

Me

MeH

CN

3° C–H

H

OR

aldehyde C–H

H

benzylic C–H

NMe

O

H H

α-amino C–H

O

Me

MeMe

H

bridged bicyclic C–H

N

H

β-pyridyl C–H

O H

β-keto C–H

SiHPh

Me Me

silane Si–H

H2N

O

H

formamide C–H

TBADT-catalyzed cross-dehydrogenative coupling

Quattrini, M. C., Fujii, S., Yamada, K., Fukuyama, T., Ravelli, D., Fagnoni, M., Ryu, I. Chem. Commun. 2017, 53, 2335

(TBA)4W12O32 (4 mol%), K2S2O8 (2 equiv.)

h!, MeCN/DCM 5:1 (0.1 M), 40 °C

Minisci-type reaction

requires 20 equiv. C–H nucleophile

15 examples

40-81% yield

H

N Me

N Me20 equiv.

N Me

[W10O32]4-

[W10O32]4-* [HW10O32]4-

[S2O8]2-

[SO4]2- + [SO4]•- +

H

H

N MeH

N Me+

N MeH

N MeH

h!

[SO4]•-

-H+

TBADT-catalyzed cross-dehydrogenative coupling

Quattrini, M. C., Fujii, S., Yamada, K., Fukuyama, T., Ravelli, D., Fagnoni, M., Ryu, I. Chem. Commun. 2017, 53, 2335

(TBA)4W12O32 (4 mol%), K2S2O8 (2 equiv.)

h!, MeCN/DCM 5:1 (0.1 M), 40 °C

H

N Me

N Me20 equiv.

C–H nucleophile scope

74% 77% 66%

53%

O

O

O

73%

NH

MeO

47%

NMe

MeO

73%

MeO

67%

69%

O

N

Me

N

81% 40%

N

N

43% (14% bis-Cy)

N

N

60% (24% bis-Cy)

NN

28% (20% bis-Cy)

N

S

60%

Heteroarene scope

A final comment on visible light-excitable POMs

Visible light photocatalysis with polyoxovanadates

a80 °CForster, J., Rösner, B., Khusniyarov, M. M., Streb, C. Chem. Commun. 2011, 47, 3114

in situ thermal conversion of {V4} to {V5} allows visible light photoredox catalysis turnover by O2 is quite slow (weeks), but turnover by H2O2 is fast (seconds)

[V4O12]4- [V5O14]3- [V10O26]4-

ΔTa

MeOH H2CO + 2 H+

h!

1/2 O2 + 2 H+H2O

UV/vis spectra of [V4] ⇌ [V5]

Visible light photoredox catalysis with heteropolyoxovanadates

Seliverstov, A., Streb, C. Chem. Eur. J. 2014, 20, 9733

explored for oxidation of pollutants

[(Ce(dmso)3)2V12O33Cl]2-

( 1 )[(Ce(dmso)4)2V11O30Cl]

( 2 )[(Ce(nmp)4)2V12O32Cl]

( 3 )

Cerium vanadate oxide clusters

UV/vis spectra

NH

HN

O

Oindigo (model pollutant)

! ~ 2% for photooxidation of indigo at "ex = 450 nm

POM–photosensitizer co-catalyzed water oxidation

Yin, Q., Hill, C. L., et al. Science 2010, 328, 342Huang, Z., Hill, C. L., Lian, T., et al. J. Am. Chem. Soc. 2011, 133, 2068

[Co4(H2O)2(PW9O34)2]10-

key: Co, O, PO4, WO6Co-POM

4 [Ru(bpy)3]2+

2 S2O82-

h! >420 nm

4 [Ru(bpy)3]3+

2 H2OO2 + 4 H+

Co-POM

Water oxidation catalyst

POM catalysis can be coupled to external photoredox processes [Ru(bpy)3]2+ acts as electron shuttle between Co-POM and stoichiometric oxidant