Interaction of Organic Radicals with Transition...

Chris Prier

MacMillan Group Meeting

March 23, 2011

Interaction of Organic Radicals with Transition Metals

IrICOPPh3Cl

Ph3P R•IrII

COPPh3Cl

IrIIICOPPh3Cl

Ph3PRRX

oxidation of alkyl radicals

CH3CH2•H2C=CH2

CH3CH2X

radical mechanism ofoxidative addition !-carbonyl radicals

metal-radical interactionsin asymmetric catalysis coenzyme vitamin B12

L PdII IBr

Ar1•

L PdIII IBr

Ar1 Ph

PhCoII

CH3CH2•H2C=CH2

CH3CH2X

L PdII IBr

Ar1•

L PdIII IBr

Ar1 Ph

PhCoII

R• MnXm

addition to the metalR Mn+1Xm

ligand transferR X Mn-1Xm-1

electron transferR± Mn±1Xm

! What are the mechanisms by which an alkyl radical can interact with a transition metal?

Kochi, J. K. Science. 1967, 27, 415-424.

Transition Metals and Organic Radicals

! Different CuII sources show divergent reactivity with respect to ethyl radical

CH3CH2•CuCl2CuSO4

CH3CH2• H2C=CH2 CH3CH2Cl

! Oxidation by CuSO4 is an electron transfer processes (outer-sphere)

Kochi, J. K. Science. 1967, 27, 415-424.

Oxidation of Alkyl Radicals by CuII

CH3CH2•+CuII CH3CH2+CuI +

Cu(OAc)2exclusive product

Cu(OAc)2

t-BuO Me

80% 20%

Me Cu(OAc)2 Me

55% 23% 22%thermodynamic distribution at 30 ºC: 2% 75% 20%

! Stabilities of the oxidation products do not control the selectivity of oxidative elimination

H2C=CH2 + H+

! Radicals which would give more stabilized carbocations give more substitution product

! Oxidation by CuCl2 is a ligand transfer processes (inner-sphere)

Kochi, J. K. Science. 1967, 27, 415-424.

Oxidation of Alkyl Radicals by CuII

CH3CH2•+CuII CH3CH2Cl

! Product ratios match those obtained with atom transfer reagents

CuICl Cl Cl

CuCl2Cl

Cu(OAc)2

! Oxidation of neopentyl radical gives no rearranged products with CuCl2

exclusive productcarbocation rearrangement products

CuCl2:t-BuOCl:

70%70%

30%30%

! Analogy with Taube inorganic ligand transfer process

[CrII(H2O)5]2+

[CoIII(NH3)5Cl]2+[(H2O)5CrII–Cl–CoIII(NH3)5]4+

[CrIII(H2O)5Cl]2+

[CoII(NH3)5]2+

CuIICl ClH3CH2C

! Metal-alkyl bonds are characteristically weak, correlate with degree of steric crowding

Metal-Alkyl Bond Strengths

bond dissociation energy (kcal/mol)2

25201822241917

3741412121

SALOPH = N,N'-disalicylidene-o-phenylenediamine, (DH)2 = dimethylglyoxime

(py)(SALOPH)Co–CH2CH2CH3(py)(SALOPH)Co–CH(CH3)2(py)(SALOPH)Co–CH2C(CH3)3(py)(SALOPH)Co–CH2C6H5(PMe2Ph)(DH)2Co–CH(CH3)C6H5(PEtPh2)(DH)2Co–CH(CH3)C6H5(PPh3)(DH)2Co–CH(CH3)C6H5

(CO)5Mn–CH3(CO)5Mn–CF3(CO)5Mn–C6H5(CO)5Mn–CH2C6H5(CO)5Mn–COC6H5

(CO)5Re–CH3

Halpern, J. Inorg. Chim. Acta. 1985, 100, 41-48.Brown, D. L. S.; Connor, J. A.; Skinner, H. A. J. Organomet. Chem. 1974, 81, 403-409.

Connor, J. A. et al. Organometallics. 1982, 1, 1166-1174.

CH3CH2•H2C=CH2

CH3CH2X

L PdII IBr

Ar1•

L PdIII IBr

Ar1 Ph

PhCoII

CH3CH2•H2C=CH2

CH3CH2X

L PdII IBr

Ar1•

L PdIII IBr

Ar1 Ph

PhCoII

! Concerted pathway: cis insertion via a three-center, two-electron bond

Hegedus, L. S. Transition Metals in the Synthesis of Complex Molecules. 1999, 2nd ed.

Two-Electron Mechanisms of Oxidative Addition

! SN2-type substitution: highly nucleophilic metal complexes attack primary or secondary halides

L2PdBr

L2PdIIBr

IrICOPPh3Cl

Ph3PCH3Br

IrCOPPh3Cl

Ph3PCH3

Br– IrIII COPPh3Cl

Ph3PCH3

! Radical pathway (two inner sphere one-electron processes)

Tsou, T. T.; Kochi, J. K. J. Am. Chem. Soc. 1979, 101, 6319-6332.

One-Electron Mechanisms of Oxidative Addition

IrI COPPh3Cl

Ph3PIn•

IrII COPPh3Cl

Ph3PIn

IrIIICOPPh3Cl

Ph3PInRX

Mn+1 Mn+1X–

X– Mn+2

! Electron-transfer mechanism (outer sphere one-electron process, then inner sphere one-electron process)

IrI COPPh3Cl

Ph3P R•IrII CO

PPh3ClPh3P

IrIIICOPPh3Cl

Ph3PRRX

initiation:

propagation:

Labinger, J. A.; Osborn, J. A.; Coville, N. J. Inorg. Chem. 1980, 19, 3236-3243.

! Concerted pathway: requires retention of configuration

Stereochemical Consequence of Oxidative Addition Pathways

! SN2-type substitution: requires inversion of configuration

! Radical pathways: likely to proceed with loss of stereochemistry

Mn BrR1

Mn+2R1

Br– Mn+2R1

R2R3Br

R3 Mn+2

Mn R1R2

R3Mn+1

R3Mn+2

! Oxidative addition of alkyl halides to an IrI complex proceeds with loss of stereochemistry

Stereochemical Consequence of Oxidative Addition Pathways

IrICOPMe3Cl

Me3PIrI CO

PMe3ClMe3P

1:1 mixture

Labinger, J. A.; Osborn, J. A. Inorg. Chem. 1980, 19, 3230-3236.

IrI COPMe3Cl

! Cross-coupling of endo- and exo-2-norbornane leads to the same exo product

González-Bobes, F.; Fu, G. C. J. Am. Chem. Soc. 2006, 5360-5361.

6% NiI26% trans-2-aminocyclohexanol

91%NaHMDS (2 equiv.)isopropanol, 60 ºC

! Cis- and trans-1,2-dichloroethylene give the same isomeric mixture of oxidative addition product

Fitton, P.; McKeon, J. E. Chem. Commun. 1968, 4-6.

! Complete retention of configuration is observed in the oxidative addition to Pd(PPh3)4

Evidence for Radical Chain Process

IrI COPMe3Cl

IrIIICOPMe3Cl

40:60 cis:trans

IrIIICOPMe3Cl

Pd(PPh3)4

PdIIPPh3Cl

! Radical initiators promote the oxidative addition of alkyl halides to IrI complexes

! Radical inhibitors depress the oxidative addition of alkyl halides to IrI complexes

IrI COPMe3Cl

Me3PPh

IrIIICOPMe3Cl

Phinitiator

noneAIBN

benzoyl peroxide

conversion (%)

after 100 min5

after 46 h5

36100100

65 ºC

IrI COPR3Cl

IrIIICOPR3Cl

Ph inhibitor555

noneduroquinone

galvinoxyl

conversion (%)after 5 min

421419

FCO2Et EtO2C

! Rate of reactivity of alkyl halides is consistent with radical process, inconsistent with SN2

! Trapping of radical intermediates with acrylonitrile

IrI COPMe3Cl

Me3PIrIIICO

PMe3ClMe3P

RX relative reactivity2

1.0 (defined)5.46.8

n-BuBrs-BuBrt-BuBr

IrICOPMe3Cl

Me3PCN

IrIIICOPMe3Cl

Me3Pi-Pr

10 equiv.

IrICOPMe3Cl

Me3PCN

IrIIICOPMe3Cl

Me3PMe

not observed

! Alkyl iodides bearing tethered alkenes undergo radical cyclization concomitant with oxidative addition

Phapale, V. B.; Buñuel, E.; García-Iglesias, M.; Cárdenas, D. J. Angew. Chem. Int. Ed. 2007, 46, 8790-8795.

Radical Cyclizations in Oxidative Addition

BrZn O

[Ni(py)4Cl2] (10%)(S)-(s-Bu)-Pybox (10%)

THF, 23 ºC

O O O O

NiIII I

NiIO O

NiI*L I

! Isopropyl iodide accelerates the Kumada cross-coupling of aryl Grignard reagents

Manolikakes, G.; Knochel, P. Angew. Chem. Int. Ed. 2009, 48, 205-209.

Alkyl Iodide Accelerated Kumada Couplings

Br OMe

rate enhancementdue to presence

of i-PrI

conversion after 15 min

L Pd0 L PdI I

Ar1 Br

L PdII IBr

Ar1•

L PdIII IBr

Ar1L PdIII X

Ar2MgX

Ar1 Ar2 L PdI X

! Rate acceleration arises from initiation of radical pathway by isopropyl iodide

PEPPSI (2 mol%)

THF, 0 ºC

Grignard prepared from PhCl and Mg/LiClGrignard prepared from PhI and i-PrMgCl•LiCl

! Aryl halides bearing an alkene tether undergo radical cyclization

Manolikakes, G.; Knochel, P. Angew. Chem. Int. Ed. 2009, 48, 205-209.

Radical Cyclizations in the Accelerated Kumada Coupling

OMeOMe

additive

nonei-PrI (1.1 equiv.)

80%34%

60 min5 min

PEPPSI (2 mol%)

THF, 25 ºC

! No cyclized products are observed when the alkene tether is on the Grignard reagent

PEPPSI (2 mol%)

THF, 25 ºC, 5 minI

i-PrMgCl•LiCl

THF, -20 ºC, 1h

CH3CH2•H2C=CH2

CH3CH2X

L PdII IBr

Ar1•

L PdIII IBr

Ar1 Ph

PhCoII

CH3CH2•H2C=CH2

CH3CH2X

L PdII IBr

Ar1•

L PdIII IBr

Ar1 Ph

PhCoII

! Synthesis of !-lactones by reaction of Mn(OAc)3 with olefins

Oxidative Reactions with Manganese(III) Acetate

inner-sphereoxidation

MnIII OMnII

MnIII O

Mn(OAc)3•2H2O (2 equiv.) O

PhHOAc, KOAc135 ºC, 1 h

proposed intermediate:

Heiba, E. I.; Dessau, R. M.; Koehl, W.J. J. Am. Chem. Soc. 1968, 90, 5905-5906.Heiba, E. I.; Dessau, R. M.; Rodewald, P. G. J. Am. Chem. Soc. 1974, 96, 7977-7981.

MnIII OMnIII

MnIII O

Me MnIII OMnII

MnIII O

! Key mechanistic question: Is the species that adds to the olefin a free or metal-complexed radical?

styrene

outer-sphereoxidation

styrene

! No polymerization of styrene observed

Fristad, W. E.; Peterson, J. R.; Ernst, A. B.; Urbi, G. B. Tetrahedron. 1986, 42, 3429-3442.

Evidence Against Intermediacy of Discrete Radicals

no polymerization observed

OEtClH2Cno rxn OEt

C6H13C6H13

(Ot-Bu)2

155 ºC

Mn(OAc)3•2H2O

HOAc, Ac2Oreflux

Mn(OAc)3•2H2O

HOAc, Ac2Oreflux

Bush, J. B., Jr.; Finkbeiner, H. J. J. Am. Chem. Soc. 1968, 90, 5903-5905.Allen, J. C.; Cadogan, J. I. G.; Hey, D. H. J. Chem. Soc. 1965, 1918-1932.

! Acetate esters add to olefins upon initiation with (Ot-Bu)2, but not Mn(OAc)3•2H2O

! No dimerization of acetic acid radicals observed

H3C OH

O Mn(OAc)3•2H2O

Ac2O, refluxno alkene

succinic acid not observed

Bush, J. B., Jr.; Finkbeiner, H. J. J. Am. Chem. Soc. 1968, 90, 5903-5905.

! H/D exchange experiments show no rate dependence on the metal

Evidence for Metal-Complexed Radical

OD DH2C

! !-lactone formation exceeds total solution H/D exchange

! Results are only consistent with rate-determining enolization of complexed acetate

rateA = rateB

KOAc, reflux

A: 0.25 M Mn(OAc)3•2H2OB: no metal

HOAc-d2

! Rate-determining enolization followed by single-electron oxidation generates metal-bound radical

Consensus Mechanism of Manganese(III) Acetate Oxidation

inner-sphereoxidation

MnIII OMnII

MnIII O

OCH2MnIII O

MnIIIO

MnIII O

MnIII OMnII

MnIII O

styrene

enolizationMnIII O

MnIII O

oxidative lactonization

MnIII OMnII

! Mn(OAc)3 initiates radical cyclizations of !-keto esters and 1,3-diketones

Kates, S. A.; Dombroski, M. A.; Snider, B. B. J. Org. Chem. 1990, 55, 2427-2436.

Oxidative Cyclization of !-Dicarbonyls

Mn(OAc)3•2H2O (2 equiv.)Cu(OAc)2•H2O (1 equiv.)

HOAc, 50 ºC, 1 h

Cu(OAc)2

! Cu(OAc)2 is used to oxidize the alkyl radical to the alkene

O!-hydride

elimination O

Cu(OAc)

71% (mixture of tautomers)

Mn(OAc)3•2H2O (2 equiv.)Cu(OAc)2•H2O (1 equiv.)

HOAc, 25 ºCO

OCO2Me

(OAc)2CuMe

OCO2Me !-hydride

elimination

! Mn(OAc)3 can initiate radical cascade cyclizations

Zoretic, P. A.; Shen, Z.; Wang, M.; Riberio, A. A. Tetrahedron Lett. 1995, 36, 2925-2928.

Oxidative Radical Cascade Cyclizations

(14% without Cu(OAc)2)

MnIIMn(OAc)3•2H2O (2 equiv.)Cu(OAc)2•H2O (1 equiv.)

HOAc, r.t., 26 h

OCO2Et

Me MeMn(OAc)3•2H2O (2 equiv.)Cu(OAc)2•H2O (1 equiv.)

HOAc, r.t., 24 hCO2EtH

Dombroski, M. A.; Kates, S. A.; Snider, B. B. J. Am. Chem. Soc. 1990, 112, 2759-2767.

! Baran's Cu(II)-mediated indole-carbonyl coupling

Baran, P. S.; Richter, J. M. J. Am. Chem. Soc. 2004, 126, 7450-7451.Baran, P. S.; Richter, J. M.; Lin, D. W. Angew. Chem. Int. Ed. 2005, 44, 609-612.

Richter, J. M.; Whitefield, B. W.; Maimone, T. J.; Lin, D. W.; Castroviejo, M. P.; Baran, P. S. J. Am. Chem. Soc. 2007, 129, 12857-12869.

Indole Coupling via !-Carbonyl Radicals

2 equiv.

54%, 5:1 d.r.

53%, single diastereomer(5 equiv. indole: 77%)

THF-78 ºC to r.t.

LHMDS (3.3 equiv.)Cu(II)2-ethylhexanoate (1.5 equiv.)

! Oxidation of enolate to !-ketoradical enables coupling with copper-coordinated indole

Proposed Mechanism of Indole-Carbonyl Coupling

THF-78 ºC to r.t.

LHMDS (3.3 equiv.)Cu(II)2-ethylhexanoate (1.5 equiv.)

! Free N-H is required for reactivity under copper conditions

Evidence for Electron Transfer via Copper Enolate

! Reaction proceeds with a known outer-sphere oxidant

no reaction

discrete !-carbonyl radical

implicates inner-sphere oxidation via indole-bound copper enolate:

LHMDSCu(II)2-ethylhexanoate

-78 ºC to r.t.

LHMDS, NEt3

11%, 4.5:1 d.r.(after benzoylation)

NOCuII

! Enables the synthesis of 1,4-dicarbonyl compounds via !-carbonyl radicals

Oxidative Enolate Coupling

! Reaction does not proceed via formation of the !-bromoester

t-BuOOt-Bu

Rathke, M. W.; Lindert, A. J. J. Am. Chem. Soc. 1971, 93, 4605-4606.

t-BuOOt-Bu

! Proposed to proceed via single-electron oxidation of enolate to !-carbonyl radical

lithium amide baseCuBr2 (2 equiv.)

Me t-BuO

! Heterocoupling can be achieved in the coupling of imides or amides with ketones or esters

Oxidative Enolate Coupling

Fe(III): 57%, 1.8:1 d.r.Cu(II): 55%, 1:1.6 d.r.

Fe(acac)3 (2.0 equiv.) orCu(2-ethylhexanoate)2 (2.0 equiv.)

LDA (2.1 equiv.)

THF, -78 ºC to r.t.

Fe(III): 60%, 2.6:1 d.r.

1 equiv. 1 equiv.

Fe(III): 73%, 2:1 d.r.

CO2t-Bu

Phi-Pr

Cu(II): 62%, 1.2:1 d.r.(1.75 equiv. ester)

Baran, P. S.; DeMartino, M. P. Angew. Chem. Int. Ed. 2006, 45, 7083-7086.DeMartino, M. P.; Chen, K.; Baran, P. S. J. Am. Chem. Soc. 2008, 130, 11546-11560.

! Formation of ketone-iron enolate followed by single-electron oxidation furnishes !-carbonyl radical

Proposed Mechanism of Oxidative Enolate Coupling

LDAFe(III)

FeIII FeII

Baran, P. S.; DeMartino, M. P. Angew. Chem. Int. Ed. 2006, 45, 7083-7086.DeMartino, M. P.; Chen, K.; Baran, P. S. J. Am. Chem. Soc. 2008, 130, 11546-11560.

! Fe(III) and Cu(II) show divergent reactivity in cyclopropane radical clock studies

DeMartino, M. P.; Chen, K.; Baran, P. S. J. Am. Chem. Soc. 2008, 130, 11546-11560.

Mechanism of Oxidative Enolate Coupling

Bn PhPh

OFe(III) or Cu(II)

LDA (2.1 equiv.)

Fe(III): traceCu(II): 20% (mixture

of ring-opened products)

Fe(III) or Cu(II)LDA (2.1 equiv.)

Fe(III): traceCu(II): 14%

MeMe Fe(III) or Cu(II)LDA (2.1 equiv.)

Fe(III): 27%Cu(II): 28%

! Fe(III) and Cu(II) promote cyclization with tethered olefins

Mechanism of Oxidative Enolate Coupling

! Extent to which metal enolate behaves like an !-carbonyl radical depends on the identity of the metal

Mn-1•R

CH3CH2•H2C=CH2

CH3CH2X

L PdII IBr

Ar1•

L PdIII IBr

Ar1 Ph

PhCoII

CH3CH2•H2C=CH2

CH3CH2X

L PdII IBr

Ar1•

L PdIII IBr

Ar1 Ph

PhCoII

! Racemic starting material is converted to single enantiomer product via radical intermediate

Asymmetric Negishi Coupling of !-Bromoamides

n-Hex ZnBr

90%, 95% eeracemic !-bromoamide

achiral radical intermediate

Fischer, C. F.; Fu, G. C. J. Am. Chem. Soc. 2005, 127, 4594-4595.

Ph NiII

Ph NiIII

10% NiCl2•glyme13% (R)-(i-Pr)-Pybox

DMI/THF (7:1)0 ºC, 12 h

Ph NiIII

Xn-Hex

n-Hex ZnBr

1.3 equiv.

NiIII Br

! Wide range of asymmetric cross couplings of racemic secondary alkyl halides has been developed

Asymmetric Cross Coupling of Alkyl Halides

!-bromoketones

J. Am. Chem. Soc. 2005, 127, 10482-10483.

n-HexMe

89%, 96% ee81%, 92%

Angew. Chem. Int. Ed. 2009, 48, 154-156.J. Am. Chem. Soc. 2010, 132, 1264-1266.

Phn-Bu

84%, 94% ee

homobenzylic halides

J. Am. Chem. Soc. 2008, 130, 6694-6695.

secondary allylic chlorides

95%, 87% ee

J. Am. Chem. Soc. 2008, 130, 2756-2757.

acylated halohydrins

80%, 94% ee

J. Am. Chem. Soc. 2010, 132, 11908-11909.

!-bromoesters

80%, 99% ee

J. Am. Chem. Soc. 2008, 130, 3302-3303.

secondary benzylic halides

! A copper/chiral diamine catalyst controls the dimerization of 2-naphthol derivatives

Enantioselective Oxidative Biaryl Coupling

N HCuIOMeO

CO2MeN

10 mol%

CuI (10 mol%), O2MeCN, 48 h, 40 ºC

enolization and catalyst release

85%, 93% ee

CuIIHO I

Li, X.; Yang, J.; Kozlowski, M. C. Org. Lett. 2001, 3, 1137-1140.Hewgley, J. B.; Stahl, S. S.; Kozlowski, M. C. J. Am. Chem. Soc. 2008, 130, 12232-12233.

! Iron(salan)-catalyzed process enables oxidative biaryl heterocoupling

Egami, H.; Matsumoto, K.; Oguma, T.; Kunisu, T.; Katsuki, T. J. Am. Chem. Soc. 2010, 132, 13633-13635.

Enantioselective Oxidative Biaryl Coupling

(salan)FeIII

FeIII(salan)OH

O(salan)FeIII

O2•–

O(salan)FeIV

O(salan)FeIII

X = H, Y = Br: 52%, 89% eeX = Ph, Y = CO2Me: 70%, 90%eeX = Ph, Y = Ac: 65%, 88%ee

•–

CH3CH2•H2C=CH2

CH3CH2X

L PdII IBr

Ar1•

L PdIII IBr

Ar1 Ph

PhCoII

CH3CH2•H2C=CH2

CH3CH2X

L PdII IBr

Ar1•

L PdIII IBr

Ar1 Ph

PhCoII

! Homolysis of cobalt-carbon bond generates carbon-centered radical capable of performing catalysis

Buckel, W.; Golding, B. T. Annu. Rev. Microbiol. 2006, 60, 27-49.

Coenzyme Vitamin B12 is a Source of Radicals

BDE = 30 kcal/mol

5'-deoxyadenosyl radical

"latent radical reservoir"

! Glutamate mutase converts (S)-glutamate to (2S,3S)-3-methylaspartate

Buckel, W.; Golding, B. T. Annu. Rev. Microbiol. 2006, 60, 27-49.

Catalysis by Coenzyme Vitamin B12

CO2--O2C

AdoCO2

glycyl radical

(S)-glutamate(2S,3S)-3-methylaspartate

! Methylmalonyl CoA mutase converts L-methylmalonyl CoA to succinyl CoA

Banerjee, R.; Ragsdale, S. W. Annu. Rev. Biochem. 2003, 72, 209-247.

succinyl CoAL-methylmalonyl CoA

! D-ornithine aminomutase converts D-ornithine to (2R,4S)-2,4-diaminopentanoic acid

2,4-diaminopentanoic acidD-ornithine

R = pyridoxal phosphate

Banerjee, R.; Ragsdale, S. W. Annu. Rev. Biochem. 2003, 72, 209-247.Chen, H.-P.; Wu, S.-H.; Lin, Y.-L.; Chen, C.-M. J. Biol. Chem. 2001, 276, 44744-44750.

CH3CH2•H2C=CH2

CH3CH2X

L PdII IBr

Ar1•

L PdIII IBr

Ar1 Ph

PhCoII

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