Monohydride catalysts - Harvard Universitysites.fas.harvard.edu/~chem153/lectures/week7.pdf ·...

M.C. White, Chem 153 Hydroformylation -204- Week of October 29, 2002

Monohydride catalysts

H2Rh(CO)(PPh3)n

Cl

H

H

δ−

δ+

HCl

σ-bond metathesis

external amine base may drive the rxn forward by forming insoluble amine hydrochloride salts

Ph3P

Rh(I)

Ph3P PPh3

Cl

CO

Addition of CO, a strong π-acid, enhances the electrophilic character of the Rh promotingheterolytic σ-bond metathesis with H2.

Rh(I)PPh3

PPh3

Cl

CO

Ph3P Rh(I)PPh3

PPh3

H

CO

Ph3P

commercially availablehydroformylation pre-catalyst

COPPh3

Rh(I)PPh3

PPh3

H

CO

OC

PPh3

OC

Rh(I)H PPh3

CO

active monohydride catalyst

A dihydride catalyst may result inreductive elimination of theintermediate metal alkyl with a second hydride (favored over CO insertion).This would result in undesiredhydrogenation product.

Hydroformylation proceeds with RhCl(PPh3)3, however there is an induction period. It was shown that in thepresence of a base the induction period is removed. What is the role of the base? Indicate the active catalyst for this process and specify how it is formed under the reaction conditions. (Question 3, Q&A pg. 200).

Ph3P

Rh(I)

Ph3P PPh3

Cl

C3H7 C3H7

O

Hcat.

H2:CO (1:1, 100 atm), 70oC

16 h, >99% conversion

C3H7

O

H+

linear: branched (2.7:1)


Isomerization/hydroformylationA rhodium hydroformylation catalyst was reported that hydroformylates internal alkenes to produce linear aldehydes preferentially. Provide a detailed mechanism for this transformation. Rationalize the effect of the bulky phosphineligand in the observed regioselectivity. (note: Question 5; Q&A pg. 202)

O

HC5H11 C5H11

Rh(I)CO

COO

O

R

R 0.1 mol%

Ligand, 1 mol%

H2/CO (20 atm), 80oC, toluene

18h n

+ mixture of branched aldehydes

iso

O

t-But-Bu

O

P

O

P

1

Ligand

PPh3

1

n:iso

0.9

9.2

% 1-nonanal

46%

90%

H2

Rh(I)H CO

COR

O

OH

R

sigma-bond metathesis?

1O

t-But-Bu

R

RPR

R

P

Rh(I)

H CO

note: formationof the dihydridewould result incompetitive hydrogenation

R

RPR

R

P

Rh(I)

HR'

R

RPR

R

P

Rh(I)

R'

H

R

RPR

R

P

Rh(I)R'

H

R

RPR

R

P

Rh(I)

OC

R

R

RPR

R

P

Rh(I)

OC

O

R

R

CO

H2

n-aldehyde

bulky ligand may promote hydrometallation to form less sterically hindered primary Rh alkyl.

Proposed mechanism

CO

CO

CO

CO

via Rh(III) dihydride

van Leeuwen ACIEE 1999 (38) 336.


One-Pot Hydroaminomethylation

Rh(I) (BF4-)

R RN

P

P

F3C CF3CF3

CF3

CF3

CF3F3C CF3

+

+ Ligand

0.1 mol%0.4 mol%

H2:CO (50 atm:10 atm), piperdine (1 eq), tol/THF,

120oC, 24h R= Me, 97% yieldn:iso (90:10)

Subtle ligand effects observed.Comparable ligand with p-CF3substituted aryls gave only 11% yield of the amine products. The majorside product was the enamine.

Non-cyclic 1o and 2o amines also resulted in high yields of linearhydroaminomethylated products.

Beller Science 2002 (297) 1676.

R

R

RPR

R

P

Rh(I)

H CO

R

R CHO

R

CHO

R

CHO

R

NR2

RNR2

R

NR2

R

NR2

RNR2

R

RPR

R

P

Rh(I)

H CO

H2

H2

H2

R

NR2

possible monohydride catalyst

olefin isomerization

H2/CO

H2/CO

hydroformylation

hydroformylation

HNR2

HNR2

HNR2

non-catalyzed

non-catalyzed

non-catalyzed

Most hydroformylation catalysts showlow isomerization activity in the presenceof strong σ-donor ligands such as amines. This constitutes the first report of olefinisomerization/ hydroformylation in thepresence of amines.

possible monohydride catalyst

linear amines are valuable chemical feedstocks.

Thermodynamic equilibrium mixturecontains less than <5% of terminal olefin,indicating that hydroformylation of theterminal olefin must occur at significantly higher rates than the process with internalolefins and with high n-selectivity


Hydroformylation of alkynesAcetylenes have lower reactivities than alkenes towards hydroformylation. This is attributed largely to formation of stable metal-acetylene complexes that resist hydroformylation under typical conditions.

(OC)3Co Co(CO)3

C

C

O

O

(OC)4Co Co(CO)4

R'R

+2 CO

(OC)3Co Co(CO)3

R R'

Takahashi Bull. Chem. Soc. Jpn. 1988 (61) 4353.

Under forcing conditions of high CO pressure and high temperatures, terminal acetylenes are converted to fully saturated aldehydes.

CH3H3C

Rh(I)PPh3

PPh3

H

CO

Ph3PO

O

PPh2

PPh2

H

H

+

(0.01 mol%) (0.05 mol%)

H2/CO (1:1, 100 atm), 80oC, 24 hrs, 20% conversion CHO32%

CHO

(S) +

68%

CHO

H2/CO H2hydroformylation of cis-2-butene gave analdehyde with the same configuration. Theisolation of (E)-2-methylbutenal is taken tobe the strongest evidence that the fullysaturated aldehyde arrises via hydrogenation of an initially formed unsaturated aldehydeintermediate.

internal alkynes

terminal alkynes

HC6H13

Rh(I)PPh3

PPh3

H

CO

Ph3PO

O

PPh2

PPh2

H

H

+

(0.01 mol%) (0.05 mol%)

H2/CO (1:1, 100 atm), 80oC, 24 hrs, 78% conversion C6H13

CHO

(S)

27%C6H13 73%

CHO+

no 1-octenal or 2-methyloctenal was observed, suggesting that the fully saturated aldehydes arrise via hydroformylation of highlyreactive alkene intermediates. However, when 1-octene washydroformylated under these conditions, (R)-2-methyloctanal wasformed as the major enantiomer.Salomon TL 1974 (49) 4285.


Hydroformylation of internal alkynesC3H7C3H7

C3H7 C3H7

CHO

C3H7 C3H7

CHO

C3H7C3H7C3H7 C3H7

CHO

C3H7 C3H7

CHO

PdCl2(PCy3)2 (2mol%), NEt3 (60 mol%)

H2/CO (1:1, 35 atm), benzene 150oC, 1h

20% conv. 1, 16 % 2, <1 %

PdCl2(PCy3)2- Co2(CO)8 (2mol%)

NEt3 (60 mol%), H2/CO (1:1, 35 atm)

benzene 150oC, 1h100% conv. 1, 95 % 2, 2 %

note: after 6 hrs, reaction went to 84% conversion generating 83% 1 and <1 % of 2.

Co metal is thought to promote the CO insertion into the Pd-Cbond of the vinylpalladiumintermediate to form anacylpalladium species.

+

+

A possible mechanism:

ClPd(II)

(Cy)3P Cl

P(Cy)3 ClPd(II)

(Cy)3P H

P(Cy)3

H2 HCl

heterolytic cleavage

ClPd(II)

(Cy)3P HR

R

P(Cy)3

ClPd(II)

(Cy)3P

R

R

(OC)4CoPd(II)

(Cy)3P

(OC)4CoPd(II)

(Cy)3P

R

R

O

R

R

(OC)4CoPd(II)

(Cy)3PH

(OC)3CoPd(II)

(Cy)3PR

R

C

O

(OC)4CoPd(II)

(Cy)3P HR

R

migratoryinsertion

[Co(CO)4]-

H2

P(Cy)3

P(Cy)3

PCy3

CO

σ-bondmetathesis

Because the productselectivity obtained for themonometallic Pd catalyst andthat of the heterobimetalliccatalytic system are similar,the chemistry is thought toproceed on the Pd center.The Co2(CO)8 accelerates the rate of the reaction. It's rolemay be to deliver CO to thePd center, therebyaccelerating CO insertion.

Hidai J. Chem. Soc. DaltonTrans. 1995 3489. Reaction ofPdPh(PMe3)3(OTf) with[Co(CO)4]- results in facile COinsertion into the P-aryl bond togive (PMe3)2(PhCO)PdCo(CO)4.

Hidai JACS 1997 (119) 6448. For a Rh system w/bulky ligand see: Buchwald ACIEE 1995 (34) 1760.

M.C. White, Chem 153 Silylformylation -209- Week of October 29, 2002

Silylformylation of alkynes

Me H

Me H

SiMe2PhOHC

CsFEtOH

Me H

HOHC

C3H7 Me

C3H7 Me

SiMe2PhOHC

CsFEtOH

C3H7 Me

HOHC

Rh4(CO)12 (1 mol%)

Me2PhSiH (1 eq)

NEt3 (1 eq), CO (29 atm)

2h, 100oC

terminal sp C is selectively silylated

99% yieldZ:E (80:20)

Z-isomer is the kinetic product. E-isomer arises from isomerization under thecarbonylation conditions.

terminal alkynes:

internal alkynes:

Formal product ofhydroformylation ofterminal alkynes withopposite regioselectivity.

Rh4(CO)12 (1 mol%)

Me2PhSiH (1 eq)

NEt3 (1 eq), CO (29 atm)

2h, 100oC85% yield

Z:E (70:30)sp C bearing thebulkier substituent isselectively formylated

tri-substituted olefin

Other substituents tested: Ph, CO2R.

Matsuda JACS 1989 (111) 2332.

Matsuda OM 1997 (16) 4327.

Rh4(CO)12metal CO clusters are known todecompose to lower nuclearity metal CO complexes under CO pressureR3SiH

(CO)4Rh(I) SiR3

(CO)3Rh(I ) SiR3

R'

(CO)4Rh(I)

R'

R3Si

(CO)3Rh(I)

O

R'

R3Si

(CO)3(SiR3)Rh(III)

O

R'

R3Si

H

R' H

SiR3OHC

CO

COR3SiH

oxidativeaddition

migratory insertion

migratoryinsertion

When R3Si-D was used,alkenal deuterated at theformyl carbon was produced >98%. When deuteratedalkyne was used, alkenaldeuterated at the vinylcarbon was selectivelyisolated (>94%).


Silylformylation of epoxides

Murai JOC 1993 (58) 4187.

OCRh

OC Cl

ClRh

CO

COCl[Rh] SiR3

H

HSiR3exact structurenot known

O

O SiR3

+

[RhI II]

OSiR3

RhIII

OSiR3

H

H

O

RhIII HCl

Cl

Cl

CO

[RhI]Cl

HSiR3

OSiMe2Ph

CHO

nucleophilic epoxide ring

opening

stereospecificmigratory insertion

Proposed mechanism:

O

n= 0,1

OCRh

OC Cl

ClRh

CO

CO1 mol%

HSiMe2Ph (1.2 eq), CO (50 atm)

N

N 40 mol% n= 0,1

OSiMe2Ph

O

H

Stereospecific epoxide ring opening: no cis product observed.

n = 0, 72% 1, 82%

O

OCRh

OC Cl

ClRh

CO

CO1 mol%

HSiMe2Ph (1.2 eq), CO (50 atm)

N

N 40 mol%

CHO

OSiMe2Ph+

OSiMe2Ph

CHO

60% yieldlinear: branched (77:23)

note: 1-methylpyrazole is essential toring-opening silylformylation. w/out it cyclopentanol silyl ether is the mainproduct observed. Although NEt3 waseffective at promoting the rxn withcyclohexene, 1-methylpyrazole wasuniquely effective at promotingring-opening silylformylation over awide range of substrates (other aminestried: pyridine, pyrrole, DBU failed).Itis suggested that 1-methylpyrazolepromotes CO incorporation, howeverno discussion of it's mechanism ofaction is presented (perhaps it's actingas a ligand to the Rh).

Co2(CO)8 was alsotried; however yieldswere significantlylower (51% based onsilane, 17% based onepoxide which wasused in 3-fold excess).

Muria Synlett 1996 414.


“Silylformylation” of alkenes

Observed product: silyl enol ether of formylated alkene

C4H9

RhCl(PPh3)3 (1.3 mol%)

HSiEt2Me (1 eq)

Et3N (3 mol%), CO (x atm),

benzene, 140oC, 20h

C4H9

OSiEt2Me

88%

linear:Z: 56%; E: 23%

+

C4H9

MeEt2SiO

branched:Z: 18%; E : 13%

linear alkenes

cyclic alkenes

Similar product ditributions were obtained with CO2(CO)8, Ru3(CO)12; however theoverall yields were lower (57% and 40%respectively).

Murai ACIEE 1977 (16) 881.

Co2(CO)8 (0.7 mol%)

HSiEt2Me (1 eq)

CO (X atm)

benzene, 140oC, 20h

OSiEt2Me

89%

OSiEt2Me

product appears if rxncarried out at highertemperatures or with a high concentration ofcatalyst

Murai ACIEE 1977 (16) 174.

Desired silylformylation product:

Rcatalyst*

CO, R3Si-H R

O

H

SiR3

R

O

H

OH* *Tamao

oxidation

acetate aldol equivalentpolyacetate polyol

M.C.White Chem 153 Silylformylation -212- Week of October 29, 2002

MechanismOSiEt2Me

DCo2(CO)8 (0.7 mol%)

DSiEt2Me (1 eq)

CO (X atm)

benzene, 140oC, 20h

91% incorporation of the deuterium in the vinylicposition

Proposed mechanism:

(OC)3Co Co(CO)3

C

C

O

O

(OC)4Co Co(CO)4

D SiR3

(OC)4Co Co(CO)4

σ-bond metathesis

D-SiEt2Me DCo(CO)4+ R3SiCo(CO)4

catalyst activation:

1st catalytic cycle:

DCo(CO)4D-SiEt2Me HCo(CO)4

OSiEt2Me

D

D

+ CO (X atm) +

catalytic cycle:

HCo(I )(CO)4H

Co(CO)4

O

Co(CO)3

O

Co(III)SiR3(CO)3

D

O

D

R3Si Co(CO)4Co(CO)4

SiR3

O

D

OSiR3

Co(CO)4

D

H

OSiEt2Me

D

+

DSiR3

Muria ACIEE 1979 (18) 837.


Intramolecular alkene silylformylation

Alkyl

OSi

Ph Ph

H

Alkyl

O SiPh2 O

H

ORh(I)O CO

CO

alkyl = Me allyl i-Pr -CH2CH2OTBS

alkyl = Me, 67%, 4.5:1 (cis: trans) allyl, 64%, 4:1 i-Pr, 79%, 6:1 -CH2CH2OTBS, 60%, 4:1

1 mol%

1000 psi CO,

benzene, 60oC

i-Pr

OSi

i-Pr i-Pr

H ORh(I)

O CO

CO1 mol%

i-Pr CH3

O SiPh2

1000 psi CO,

benzene, 60oC

Silylformylation reactivity depends on the natureof the silicon substituents. If isopropyl is replacedfor Ph in this system, hydrosilylation results.Interestingly, when phenyl homoallylic alcoholsare used as substrates, phenyl substituted silaneslead to hydrosilylation whereas diisopropyl silanes result in silylformylated products.

R groups on Si in conjunction w/R' maydictate which olefin binding mode ispreferable. Binding perpendicular toRh-Si bond results in poor orbitalalignment with Rh-Si but good orbitalalignment w/Rh-H. This may lead topreferential migratory insertion of H that results in hydrosilylated product.

Leighton JACS 1997 (119) 12416.

Proposed mechanism:

R'

OSi

R R

H

ORh(I)O CO

CO

ORh(Iii)

O

H

CO

R2Si

OR'

ORh(Iii)

O

H

CO

CO

O

R2Si

R'

ORh(Iii)

OH

CO

OR2Si R'

O

R'

O SiR2 O

H

silane migratory insertion

oxidative addition

CO migratoryinsertion

reductive elimination

ORh(Iii)

O

H

CO

R2Si

OR'

or

i-Pr CH3

O SiPh2


Intramolecular silylformylation-allylsilylation

i-Pr

OSi

HO

Rh(I)O CO

CO1 mol%

1000 psi CO,

benzene, 60oC

i-Pr

O Si O

H i-Pr

O Si O

H Hi-Pr

OH OHOHH2O2,NaHCO3MeOH

alkenes

59% overall yield77:23 (syn,syn triol: rest)

Leighton JACS 2000 (122) 8587.

i-Pr

OSi

HO

Rh(I)O CO

CO1 mol%

1000 psi CO,

benzene, 60oC

i-Pr

O Si O

H i-Pr

O Si O

i-Pr

OAc OAc

1. TBAF2. Ac2O

alkynes

70% overall yield23:1 (1,5-anti: 1,5-syn)

Leighton ACIEE 2001 (40) 2915.

Leighton JACS 2001 (123) 341.Schreiber JACS 1993 (115) 3360.

OTIPSOSi

HO

Rh(I)O CO

CO

H2O2, NaHCO3MeOH

OTIPSOHOHOHOPMB O O O O O O O O OH1 mol%

1000 psi CO, benzene, 60oC1.

2.55% overall yield

Formal synthesis of Mycoticin A

Schreiber intermediate to Mycoticin A

M.C. White, Chem 153 Hydrocarbonylation -215- Week of October 29, 2002

Hydroesterification

CO (300 atm), MeOH

100oCMe Me

EtPh3P

Pd(II)Cl PPh3

Cl catalytic

Me Me

CO2MeH

Et

only source of hydrogen is protic solvent

94:6 dr ( hydroesterification is predominantly cis)

High pressure conditions:

Consiglio Gazz. Chim. Ital. 1975 (105) 1133.

Ph3P

Pd(II)Cl PPh3

Cl

Ph3P

Pd(II)Cl PPh3

O

Cl

Ph3P

Pd(II)Cl PPh3

O

OMe

HCl

Pd(II)Cl PPh3

O

OMeR'

Pd(II)Ph3P Cl

R'

CO2Me

PPh3

MeOH

PPh3

HCl

RCO2Me

Proposed carboxylate insertion mechanism:

Stille Comprehensive Organic Synthesis 1993; Volume 4; Chapter 4.5

Insertion of the esteroccurs at the leaststerically hinderedolefinic carbon

M.C. White, Chem 153 Hydrocarbonylation -216- Week of October 29, 2002

CO (150-400 atm), H2NR'

150-200oC

catalytic

only source of hydrogen is H2NR'

Natta JACS. 1952 (74) 4496.

High pressure and high temperature conditions:

Co2(CO)8

O

NHR'+

O NHR'

MeOH can also be used under these conditions to obtain the methyl ester

Hydroamidation

The clear limitation of these systemsis the extreme temperatures andpressures that are necessary toeffect the hydrocarbonylative process

Proposed hydride mechanism:

Stille Comprehensive Organic Synthesis 1993; Volume 4; Chapter 4.5

Independently prepared Co-H has beenshown to effect both hydroamidation and hydroesterification processes.

Hemiacetal intermedaite is speculative and basedon the reaction of ananalogous manganesecomplex with alcohols.

(OC)3Co Co(CO)3

C

C

O

O

(OC)4Co Co(CO)4

H2NR'

?

(OC)4Co H

(OC)3Co

RH

R

CO

(OC)3CoR

(OC)4CoR

CO

(OC)4Co R

O

H2RN

(OC)4Co R

O NHRH

R

O

NHR'

M.C. White, Chem 153 Carbonylation -217- Week of October 29, 2002

The palladium catalyzed carboxylation of a variety of aryl iodides can be performed under mild conditions of temperature (rt) and CO pressure (1 atm). Propose a catalytic cycle for this transformation.

I

X

Pd(OAc)2 1 mol%K2CO3 (4 eq), H2O/DMF, rtCO (1 atm)

CO2H

X

X = NO2, 90% Cl, 92% CN, 98% NH2, 68% OH, 92%Pd(Cl)2(PPh3)2 used

Beletskaya JOMC 1988 (358) 563.

Palladium catalyzed carbonylation of Ar-X

PdII

X

I

PdII

IO

X

-OH

PdII

I

X

-O

O

H

PdII

H I

Pd0Ln

L = DMF

Ln

Ln

Ln

Ln

A similar system has been developed that uses aryl triflates:

O

TfO

Pd(OAc)2 3 mol%dppf (6 mol%), NEt3 (2 eq)MeOH,DMF,CO (1 atm)

60oC, 2h

O

MeO2C81%

Treatment with piperdine under the same conditions yielded theamide in 61% yield.

Ortar TL 1986 (27) 3931

Proposed mechanism:

M.C. White, Chem 153 Carbonylation -218- Week of October 29, 2002

Carbomethoxylation of terminal epoxides

(OC)4Co Co(CO)4

?

(OC)4Co H

MeOH

R

O

R

O+

H

Co(CO4)

R

OH

Co(CO4)

R

OH

Co(CO4)

O

MeOH

R

OH

Co(CO4)

O OH

Me

R

OH O

OMe

Proposed mechanism:

Using the same catalyst, the addition of3-hydroxypyridine (based on a patent report)results in significantly improved yields and appearsto allow the system to be run under milderconditions of temperature (55-65oC) and COpressure (41 atm). The role of this co-catalyst is not addressed; however, we've seen this amine"additive"effect before in the ring-openingsilylformylation of epoxides.

3-hydroxypyridine

CO (41 atm), MeOH

55-65oC

5 mol%Co2(CO)8

R

O

R

OH O

OMe

R = CH3, 92% yield C6H13, 96% CH2Cl, 96% CH2OBn, 86%

>99% ee >99% ee

N

OH

10 mol%

Reaction was highly regioselective andstereoretentive. Enantiomerically pure terminalepoxides result in enantiomerically pureβ-hydroxy carbonyls (valuable synthetic building blocks).

CO (238 atm), MeOH

190oC

catalyticCo2(CO)8O

OH O

OMe

β-hydroxybutyrate22% yield

+ < 4% methoxypropanol

The original report:

Eisenmann JACS 1961 (26) 2102.

Jacobsen JOC 1999 (64) 2164.

M.C.White/M.S. Taylor Chem 153 Carbonylation -219- Week of October 29, 2002

Pd catalyzed hydroxycyclization/carbonylation/lactonization

C6H13

OH OH O

OO

H

H

H

C6H13CO

NaOAc / AcOH

PdCl2, CuCl 2

Kitching JOC. 2001 (66) 7487

C6H13

OH OHPdCl2 OH

OHH

H

C6H13

O

OO

H

H

H

C6H13

PdCl2

O

OH

H

H

H

C6H13

Pd

CO O

OH

H

H

H

C6H13

O

PdCl

CO

Wacker-typereaction

carbonylation

NaOAc O

OH

H

H

H

C6H13

PdCl

Cl

+ NaCl, AcOH

O

O

H

H

H

C6H13

O

Pd ClH

+

+ PdHCl

After reductive elimination to Pd0, reoxidation with CuCl2regenerates PdIICl2

Proposed mechanism:

M.C. White, Chem 153 Decarbonylation -220- Week of October 29, 2002

H

O

MePd-C

MeMe

Me

An early report using palladium metal on carbon (heterogeneous catalysis) to effect decarbonylation of aldehydes yielding mixtures of alkene and alkane products..."Recently the usefulness of palladium in organic syntheses has attracted much attention" (1968).

6

190 oC, 2 h

6 6+

57 % 27 %

+ H2 + CO

CH3R

Pd

H

H CO

R

PdLn CO

CO

R

Pd

H Ln

H CO

H2

Pd

H

Ln CO

R

O

HR

Pd

H

O

R

L n

oxidativeaddition

deinsertionreductive

elimination

Pd(0)metallic Pd

β-hydrideelimination

reductiveelimination

Proposed mechanism

The oxidative addition of aldehydesto metallic palladium to form the acyl complex was not a known reaction.This mechanism was proposed byTsuji and coworkers based onestablished analogous reactions withacyl halides. Part of the evidencepresented for this oxidative additionwas that metallic palladium partiallydissolves in acyl halides.

This system has clear limitations including high reaction temperatures, theformation of product mixtures, and heterogeneity which makes it difficultto tune the catalyst's reactivity. Thus a more useful, homogeneouscatalytic system was sought. Based on this postulated mechanism fordecarbonylation with palladium, it was reasoned that a complex that couldeffect this process efficiently should have the following properties: (1) low valent, coordinately unsaturated metal (e.g. 4-coordinate d8 complex) tomake possible the oxidative addition of aldehydes or acid halides and (2)the complex should be able to coordinate carbon monoxide. Wilkinson'scatalyst satisfies both of these requirements.

CO is thought to readily dissociate at

200 oC, thus rendering this process

catalytic in metallic palladium at these

temperatures.

Tsuji TL 1965 4565, JACS 1968 94-98.

Palladium-mediated decarbonylation of aldehydes


RhCl PPh3

Ph3P PPh3

O

H

CH2Cl2

RhCl PPh3

Ph3P CO PPh3+ + +

roomtemp

+ H2

78 % Yieldratio of hexane:hexene 6:1

+

dark red solutionin CH2Cl2

yellowcolor change signifies

reaction completion

This reaction is very senstitve to sterics. Whereas primary aldehydes can be decarbonylated at room temperature, secondary aldehydes require high temperatures (>100 oC).

Wilkinson's catalyst

RCH3

RhClPh3P

PPh3

CO

X

RhPh3PPh3P

Cl

RhHPh3P

CO

Cl

PPh3

R

RhCl PPh3

Ph3P PPh3

RhPh3PPh3P

H

Cl O

R

R

O

Hoxidative addition

deinsertionreductiveelimination

CO

Proposed mechanism:

The carbon monoxide ligand does not dissociate atmild temperatures rendering this processnon-catalytic. Attempts to effect CO dissociation viathermal and photochemical approaches met with little success.

Tsuji JACS 1968 99-107.

Decarbonylation of primary aldehydes mediated by Wilkinson's catalyst


Catalytic decarbonylation of primary aldehydes at room temperature

RhCl PPh3

Ph3P CO R N3

O

RhCl PPh3

Ph3P N

CO

N N

O

R

RhCl PPh3

Ph3P N2

R NCO

ON2

RhC

NCl

PPh3

PPh3

O

R

O

O'Connor and coworkers have found a way to close this catalytic cycle (25 years after Tsuji's initial report)...it was known in the literature that chlorocarbonylbis-

(triphenylphosphine)rhodium reacted with butanoyl azide at -70 oC to give the corresponding dinitrogen complex which decomposes in chloroform solution at room

temperature.

+

Ukhin Izv. Akad. Nauk SSSR, Ser. Khim. 1967 957.Collman JACS 1968 90 5430.

O

PN3PhO

PhO

DPPA

RCH3

RhCl

Ph3PPPh3

CO

O

PNCO

PhOPhO

RhH

Ph3P

CO

Cl

PPh3

R

RhPh3P

Ph3PCl

RhCl PPh3

Ph3P PPh3

R

O

H

RhPh3P

Ph3P

H

Cl O

R

oxidativeaddition

deinsertionreductiveelimination

O'Connor and coworkers found that in contrast to other azideswhich were decompsed under the reaction conditions, the relativelystable azide diphenylphosphoryl azide (DPPA) was effective inremoving the CO ligand from chlorocarbonylbis(triphenyl-phosphine)rhodium and was thus able to render thedecarbonylation reaction catalytic with Wilkinson's catalyst.DPPA is a readily available, non-explosive, high-boiling azide used as a reagent in peptide synthesis.

Limitations of this catalytic system include the requirement for theuse of a syringe pump to add the DPPA in addition to the generation of stoichiometric isocyanate byproduct

O'Connor JOC 1992 5075-5077.


Pd-catalyzed decarbonylative olefination of aryl esters

O

ONO2

N

RR

HO

NO2

CO

PdCl2, LiCl

NMP, 160 oC

16 h

++ +

R = p-C(O)CH 3 85 % yieldR = p-Cl 74 % yieldR = m-CH3 52 % yield

Gooβen and coworkers recently reported a method that makes it possible to use aryl esters as substrates in a Heck coupling. As shown above, the reaction is more efficient for substrates with electron withdrawing groups than those with electron donating groups. Propose a detailed catalytic cycle to account for this overall transformation. (Hint: the Pd catalyst formed in this process is capable of inserting into the single carbon-oxygen bond of an ester.

Gooββββen ACIEE 2002 (41) 1237.

PdLL

PdRO

L

PdRO

LH

R'

PdRO

LH

L

ROH

R'

R'

PdRO

L

O

O

R

PdLOR

O

CO

PdRO

L CO

R'

oxidativeaddition

deinsertion

migratoryinsertion


reductiveelimination

R

H

H

Ph

M.C. White, Chem 153 Hydroacylation -224- Week of October 29, 2002

C4H9

O

H

RhCl PPh3

Ph3P PPh3

+

R

O

H+ R'

catalystR

O

R'

Direct route to di-substituted ketones from unactivated precursors

Observed intermolecularly: decarbonylation

saturatedsolution

CHCl3Rh

Cl PPh3

Ph3P CO+ C5H12

Miller JACS 1976 (98) 1281.

stiochiometric

O

HRh

Cl PPh3

Ph3P PPh3

CHCl3

10 mol%

O

14%

When the solution was saturated w/ethylene, the yield increased to 78%. Thought to occupy free coordinationsite on Rh complex necessary fordecarbonylation.

Hydroacylation

RhCl PPh3

Ph3P PPh3

RhCl PPh3

Ph3P

RhPh3P

Ph3P

H

Cl

RhPh3P

Ph3P

H

Cl

O

O

RhPh3P

Ph3P

Cl O

RhPh3P

Ph3PCl

O

O

H

Proposed mechanism:

An improved process was later developed byLarock which utilizes a Rh(I) catalystwith electron rich trarylphosphines.Larock JACS 1980 (102) 190.


Chelate-assisted intermolecular hydroacylation

R

O

H+

N NH2

N N

2-amino-3-picoline RH

THF, reflux3Å mol. sieves

Rh(I)

Cl PPh3

Ph3P PPh3

THF, 55oC Rh(III)

H

C

N N

RCl

Ph3P

PPh3

R = Ph, >90% p-OMePh, >90% p-ClPh, >90%

isolated and characterized by NMR and IR.

N N

PhH

C6H13

Rh(I)

Cl PPh3

Ph3P PPh3

5 mol%

160oC, THF (sealed)

1.

2. H+ workup

Ph

O

C6H13

10%

+

First report Suggs JACS 1979 (101) 489.

Ph

O

H

C4H9+

Rh(I)

Cl PPh3

Ph3P PPh3

2 mol%

160oC, toluene (sealed)

N NH2 7 mol%

Ph

O

C6H13

72%

One-Pot: Jun JOC 1997 (62) 1200. What's the difference?

R

O

H+ R' R

O

R'R

N

H

masked carbonyl can no longer decarbonylate

A Lewis basic group on thesubstrate binds to the metal and directs it towardsinsertion into the imine C-H .

LB

H2OM


Chelate-assisted intermolecular hydroacylation

Ph

O

H

C4H9+

Rh(I)

Cl PPh3

Ph3P PPh3

2 mol%

160oC, toluene (sealed)

N NH2 7 mol%

Ph

O

C6H13

72%

Proposed mechanism:

One-Pot: Jun JOC 1997 (62) 1200. What's the difference?

N NH2

Ph

O

H

N N

Ph

Rh(I)

Cl PPh3

Ph3P PPh3

H

Rh(III)

H

N N

PhCl

Ph3P

PPh3

Rh(III)

H

N N

PhCl

Ph3P

R

Rh(III)

N N

PhCl

Ph3P

R

N N

PhR

oxidativeaddition

H2O

H2O

Ph

O

C6H13

+


Hydroacylation of 1-alkynes with aldehydes: chelation assistance

R H

OMe

H

NH2N

Rh(PPh)3Cl (5 mol%)

OH

O

R

O

Me+

40 mol%

20 mol%

Toluene, 80 oC, 12 h

R = Ph 92 % yieldR = p-CF3C6H4 95 % yieldR = p-MeOC6H4 76 % yield

best substrates are aromatic aldehydes (non-aromatic aldehydes givemixtures of branched and unbranchedproducts.

R H

O

N N

RH

H2O

Rh(I) PPh3Cl

PPh3

Ph3P

N N

RMe

R

O

Me

N NH2

H2O

Rh(III)Cl

PPh3

Ph3PNN

RH

Rh(III)Cl

Ph3PNN

R

Me

Rh(III)Cl

Ph3PNN

RH

Me

Me

H

migratory insertion to give moresterically hindered metal alkenyl. It is unclear why this regioselectivity isobserved.

Jun ACIEE 2002 (41) 2146

M.C. White, Chem 153 Hydroacylation-228- Week of October 29, 2002

Synthesis of cyclopentenones: Rh-catalyzed intramolecular trans hydroacylation of an alkyne

R

Me O

H

O

Me

R

R = n-C6 H13 75% yieldR = Ph 88% yield

[Rh(dppe)]2(BF4)2

acetone, CH3CN

rt (R = n-C6 H13)

100 oC (R = Ph)

LnRh(I) H

O

Rh(III)

O

HRh(III)

H

O

O

H

oxidativeaddition

trans hydrometalation?


it is unclear how the observed trans hydrometalation is achieved

Ln

Proposed mechanism The following studies support this mechanistic proposal

n-C6 H13

Me O

D

O

Me

n-C6 H13

D

[Rh(dppe)]2(BF4)2

acetone, CH3CN100 oC

n-C6 H13

O

H

n-C6 H13

Me O

D

Me

O

n-C6 H13

H [Rh(dppe)]2(BF4)2

acetone, CH3CN100 oC

O

Me

n-C6 H13

D

1:1

Me

none of the crossover products were observed, supporting the hypothesis that this transfer process proceeds intramolecularly

aldehde proton (deuterium) istransfered cleanly to the βposition of the cyclopentenone

Fu JACS 2001 46 11492-11493..

M.C. White, Chem 153 Hydrovinylation -229- Week of October 29, 2002

Hydrovinylation of vinyl arenes

X X1 atm

+[Ni(CH3CN)6](BF4-)2, 0.25 mol%

PPh3 (1 mol%), AlEt2Cl (1.25 mol%)CH2Cl2, rt

+ linear product isomerized product

branched productX = H, 87% 3-Me, 82% 4-OMe, >5% 4-Cl, NR

Monteiro TL 1996 (37) 1157.

NiBr

NiBr

X X

0.35 mol%PPh3/AgOTf (0.35 mol%)

CH2Cl2, -56oC1 atm

+

branched productX = H, >95% 3-Me, >95% 4-OMe, >95% 4-Cl, 81% 4-Br, >95%

RajanBabu JACS 1998 (120) 459.

elimination of a Lewisacid in this system isthought to increase thereaction scope to include substrates with Lewisbasic fuctionality.

Hydride insertion gives thebranched product which canbe electronically stabilized asan η-3 intermediate. Note that this is a common trend inhydrometallations of arylolefins.

Bulky phosphine ligand may promote binding of ethylenevs. binding of another arylolefin which would result inoligomerization product


Ni

Br

Ni

Br

pre-catalyst

PPh3Ni(II)

Br

PPh3

AgOTfNi(II)

OTf

PPh3

Ni(II)

PPh3

+(OTf-)

metathesis

insertion

NiPPh3

OTf

H

Ni(II)

OTf

PPh3

H

nickel hydridecatalytically active

species

Ni(II)

OTf

PPh3H

Ni(II)

OTf

PPh3

Ni(II)

OTf

PPh3

Ni(II)

OTf

PPh3

Ni(II)

OTf

PPh3

H


Hemilabile bidentate chelation/counterion effects

NiBr

Ni

Br0.35 mol%

L/AgX (0.35 mol%)

CH2Cl2, -56oC1 atm

+MeO MeO

P

A systematic study in the hydrovinylation of styrene:

Counterion Yield

OTfClO4SbF6BARF

94%95%<2%<2%

ligand w/outhemilabile substituent

P

CH2OCH2Ph Counterion Yield

OTfClO4SbF6BARF

<4%<2%94%97%

ligand w/ hemilabilesubstituent


With ligands devoid of hemilabile bidentatechelation, weakly coordinatng counterions must beused to prevent catalyst deactivation. Counterionssuch as OTf or ClO4 are small enough to transientlyoccupy a site of coordinative unsaturation on the Nicomplex. Because of their highly diffuse electrondensity, OTf and ClO4 weakly coordinate to the Nicomplex and can be readily displaced by ethyene.Alternatively, bulky counterions such as BARF orSbF6 cannot enter the coordination sphere of the Nicomplex and lead to rapid complex decomposition.The use of a hemilabile bidentate ligand inconjunction with coordinating counterion generates acatalytically inactive Ni complex. This may be due todiminished binding of the ethylene to the Ni complex.

O

PPh2

Ni

+

(BARF-)

97% isolated yield80% ee

Ph

PPh2 BARF = BAr4Ar = 3,5-(CF3)-Ph

Ni

+

(BARF-)

12% conversion0% ee

Hemilabile ether substituent on themonophosphine ligand stabilizescoordinatively unsaturated intermediates in the absence of coordinatingcounterions or solvents and readilydissociates to to bind ethylene andinitiate the formation of the active Ni-Hcatalyst. In the absence of hemilabileligation by the ligand, NMR studiesindicate that the highly dissociatedBARF complex readily decomposes.

hydrovinylation of 2-methoxy-6-vinylnaphthalene


Asymmetric hydrovinylation

NiBr

NiBr

0.35 mol%L/AgX (0.35 mol%)CH2Cl2, low temp

1 atm+

X X

Monodentate phosphporus ligands (bidentate phosphines lead to catalytically inactive species)

O

PPh2

Ph

P

CH2OCH2Ph

P

N

Ph

2

OO

O

OBnNH

Ac

OAr2P

1 2 3 4 (monophosphite)

O

O

P N

5

X = i-Bu, 40% ee X = H, 50% ee X = H, 86% ee X = H, 81% ee Br, 89% ee i-Bu, 74% ee

X = H, 95% ee Br, 92% ee i-Bu, 68% ee

(monophosphoamidite)

Asymmetric synthesis of (R)-ibuprofen

Br

NiBr

NiBr

0.35 mol%4/NaBARF (0.35 mol%)

CH2Cl2, low temp1 atm

+

Br

98% yield>99% selectivity for iso89% ee

(dppp)NiCl21.6 mol%i-BuMgCl

i-Bu i-Bu

O

O3, MeOHMe2S

KMnO4

acetone95% 96% 66%

i-Bu

O

HO

(R)-ibuprofen89% ee

i-Bu

? Takaya's asymmetric hydroformylation?(see hydroformylation -194-)

High levels of asymmetric induction inhydrovinylation with the current catalytic systemsappears to be limited to electron-neutral or deficient styrenes. 4-isobutylstyrene undergoeshydrovinylation with 4 in high yield (99%) butmodest ee (74%).

RajanBabu JACS 1999 (121) 9899. Ligand 1.RajanBabu JACS 2002 (124) 734. Ligand 4.Leitner JACS 2002 (124) 736. Ligand 5.

M.C. White, Chem 153 Hydrocyanation -232- Week of October 29, 2002

Hydrocyanation

OO

O

MeO MeO

CN Very narrow substrate scope reportedcomprised primarily of isomericvinylnaphthalene substrates. Yieldsrarely reported and typical runs were runwith substiochiometric amounts of HCN. Zerovalent nickel phosphite catalyst areknown to undergo irreversible oxidationwith HCN to Ni(CN)2. Since catalystdeactivation is second order in HCN,slow addition of HCN is required foroptimal yields.

OPh

O

PAr2

OAr2P

Ph

Proposed catalytic cycle:

Ni0

OO

O OPh

O

PAr2

OAr2P

Ph

COD

PNi0

P*

PNiIIP H

CN

*

PNi0

P*

COD

HCNCOD

PNiIIP*

H

CN Ar

Ar

PNiIIP*

CN

CN

CN

HCN

Ar

HCN

ligated Ni0 complex

observed by NMR



Nugent JOC 1985 (50) 5370. Ni0[(P(O-o-tolyl)3]4.

HCN

H2, L

NiII(CN)2

Ar = 3,5(CF3)2-Phnote when Ar = 3,5-(CH3)2-Ph, 16% ee

Industrial hydrocyanation to form adiponitrile:

Ni[P(O-o-tolyl)3]3 cat.

HCN, LA (e.g.AlCl3, ZnCl2) NiLCN LA

Spectroscopic studies indicate that the LA coordinates to the Ni cyanide intermedaites via the CN nitrogen . Coordination is thought to enhance both the rate and selectivity (n:iso) of reductive elimination. Selectivity is enhanced by destabilizing therelatively bulky branched alkyl intermediates.

5 mol%L (6.5 mol%)

benzene, low temp

+ Ni0(COD)2HCN (2.5 eq)

>99% regioselectivity85 % yield

85% eeL =

aimed towards synthesis of Naproxin and it's derivatives

η3-organo nickel intermedaite identified by NMR

NCNi(0)

NC

isomerizationhydrocyanation

hydrocyanationNC

CNadiponitrile: key

intermediate in Nylon-6,6Houpis Tetrahedron 2000 (56) 817.

Tolman OM 1984 (3) 33.

M.C. White,Q. Chen Chem 153 Q&A -233- Week of October 29, 2002

Hydrogenation: dihydride mechanism

CoH P(OMe)3

P(OMe)3

H

P(OMe)3

< 1 atm H2, 25 °C, rt.

cat.

Mild hydrogenation of benzene can be achieved with the cobalt complex below. Also, the rate of cyclohexadiene reduction is similar to the rate of benzene hydrogenation while cyclohexene is reduced at three to four timesslower than benzene. Propose 2 mechanisms for the arene hydrogenation.

Similar rates for benzene and cyclohexadiene suggests an η4-benzene complex is involved.

Muetterties JACS 1975, 97, 237.

CoIIIH P(OMe)3

P(OMe)3

H

P(OMe)3

HH

CoIII(MeO)3P HCo(MeO)3P Co(MeO)3P

HH

Co(MeO)3P

HCo(MeO)3PCo(MeO)3PCo(MeO)3P

H

H

Co(MeO)3P

P(OMe)3

H Co(MeO)3P

P(OMe)3

P(OMe)3

CoH P(OMe)3

P(OMe)3

H

P(OMe)3

H2

P(OMe)3

P(OMe)3

-C6H12

H2

H2

-2 P(OMe)3

η4-benzene, 18 e-

Dihydride mechanism:

H

H

H

H

H

H

M.C. White/Q. Chen Chem 153 Q&A -234- Week of October 29, 2002

CoH P(OMe)3

P(OMe)3

H

P(OMe)3

< 1 atm H2, 25 °C, rt.

cat.

Hydrogenation: monohydride mechanism

CoH P(OMe)3

P(OMe)3

H

P(OMe)3

CoP(OMe)3

P(OMe)3

H

P(OMe)3

P(OMe)3CoH

P(OMe)3

P(OMe)3Co

P(OMe)3

P(OMe)3Co

P(OMe)3

H HP(OMe)3CoH

P(OMe)3

P(OMe)3Co

P(OMe)3

H H

CoP(OMe)3

P(OMe)3

H

P(OMe)3

P(OMe)3

-C6H12

H2

Monohydride mechanism:

16e-

-2P(OMe)3

η4-benzene, 18 e-

repeat

The following experimental evidence supports the dihydride mechanism: HCo(P(OMe)3)3 has been independently synthesized and is inactive for the hydrogenation of arenes. Additionally, propene evolution is not observed in presence of arene, and the original Co allyl complex can be recovered in almost quantitative yield after the hydrogenation.

Muetterties JACS 1975, 97, 237.

M.C. White/M.S. TaylorChem 153 Q&A -235- Week of October 29, 2002

CO2H

NHAc

H2 (1 atm), 25oC, MeOH

CO2Et

NHAc

N-acetyl-(R)-phenylalanine>95% ee

(R)

Rh(I)P

P

(ClO4-)

+

H3C

H3C

(S,S-CHIRAPHOS)

Rh(I)P

P O

NH

(ClO4-)

+

H3C

H3C

MeO2C

Ph

olefin bound to Rh via its

si-face

stereospecific H migratory insertion /stereospecific H/C reductive elimination

both to the olefin face bound to Rh

minor diastereomer formednone detected by NMR (must be less than 5% present in solution)

Rh(I)P

PO

HN

(ClO4-)

+

CH3

CH3

CO2Me

Ph

olefin boundto Rh via its

re-face

major diastereomer formedin solution (identified by NMR

and x-ray crystallography)

It was concluded from kinetic measurements thatthe minor diastereomer was 580 fold more reactivetowards H2 oxidative addition (recall the RDS at rt). This factor offsets its lower concentration insolution and results in a 60:1 product ratio in favorof the R enantiomer.

Halpern's seminal mechanistic studies on the Rh+ - DIPAMP system for asymmetric dehydroamino acid hydrogenation permitted improvement of the reaction conditions to

increase enantiomeric excess. Provide a rationalization for the following observations, in light of Halpern's results.i) Decreasing the H2 pressure of the reaction increases the enantiomeric excess.

ANSWER: Oxidative addition of hydrogen to the DIPAMPRh-enamide complex is the rate-determining and ee determining step in this catalytic cycle. At low H2 pressures,the rate of interconversion between the major and minor diatstereomeric enamides is faster than the reaction of either complex with H2 (recall that the minor diastereomericenamide complex leads to the formation of the major product). As the hydrogen pressure is increased, the rate of oxidative addition of H2 becomes competitive with themajor/minor interconversion. Because it is present in excess amount, the reaction with the major diastereomer (leading to the minor enantiomer) becomes more favorable.Stated differently, at high H2 pressures the minor Rh-enamide diastereomer (which leads to the major product enantiomer, and exists in very low concentration) exhibitssaturation kinetics while the major Rh-enamide diastereomer (which leads to the undesired product enantiomer) is consumed at a rate which increases linearly with H2 pressure.

ii) Increasing the reaction temperature increases the enantiomeric excess.

The enthalpies of activation for oxidative additionH2 to the Rh-enamide complexes is lower than theenthalpic barrier for diastereomer interconversion(requires enamide dissociation). Thus as thetemperature of the reaction is increased, theequilibration between the major and minordiastereomeric enamide complexes is promotedmore than the oxidative addition of H2. Thus, it ispredicted that increasing the temperature will offset the effect of increasing the H2 pressure and lead toan enhancement in enantioselectivity.

Halpern JACS 1987, 109, 1746.Blackmond JACS 1996, 118, 1348.

M.C. White/Q. Chen Chem 153 Q&A -236- Week of October 29, 2002

Si(OiPr)3

OSi

OiPrOiPr

2%

Rh

Si(OiPr)3(PrOi)3Si

Rh(I)

Cp*

Si(OiPr)2

O

Me Me

Rh(III)

Cp*

Si(OiPr)2H

O

Me

Si O

Rh(III)

Me

OiPr

OiPr

Cp*

Si O

Rh(III)

OiPr

OiPr

Cp* H

Me

O

Me

SiOiPr

OiPr

Me liganddissociation

migratoryinsertion



oxidative addition(C-H activation)

Si(OiPr)3

Rh

Si(OiPr)3(PrOi)3Si

Vinyl silane isomerization via C-H activationThe rhodium complex below catalyzes the isomerization of vinyl silanes to silyl enol ethers. Propose a mechanism for the transformation.

Brookhart JACS 1999 (121) 4385.

M.W. Kanan/M.C. White, Chem 153 Q&A -237- Week of October 29, 2002

Question 1

Pt

N

N Me

Me

Me

Me Me

iPr

iPr

Pt

N

N D

Me

Me iPr-d7

iPr-d7

iPr-d7CD3

DD

1

Provide a mechanism for the formation of 2-d27. Note that very little CH3D is formed under the reaction conditions.

iPr

iPr

C6D6 150°C

10 min.

2-d27

+ C2H6, CH4

M.W. Kanan/M.C. White, Chem 153 Q&A -238- Week of October 29, 2002

O

TESO

OH

ROTf

O

TESO

OH

R

OTf

Pd(PPh3)4800 psi CO

i-Pr2NEt, PhCN65 to 110°C

or

O

O

O

R

TESO

Provide a mechanism for this one-pot transformation. Explain why both the E- and Z-tetrasubstituted enol triflates react to form the desired product in comperable yields.

Question 2

Q. Chen/M.C. White, Chem 153 Q&A -239- Week of October 29, 2002

R

O

HN

O

N

H

CH3

H

H3C

NN

O

O

CH3

H3C

R+ PdBr2(PPh3)2 (0.25 mol %)

30% LiBr, 1% H2SO4, CO

Substituted hydrantions can be prepared by the ureidocarbonylation of an aldehyde in the presence of LiBr and H2SO4. Provide a mechansim for this transformation.

Question 3

Q. Chen/M.C. White, Chem 153 Q&A -240- Week of October 29, 2002

Question 4

Hn-Hex n-HexPPh2

O

Hn-HexPPh2

O

n-Hex

Hydrophosphinylation of internal and terminal alkynes with Ph2P(O)H can be catalyzed by a variety of palladium sources. Pd(PPh3)4 is selective for the anti-Markovnikovproduct. Propose a mechanism for the hydrophosphinylation of 1-octyne below.

The regioselectivity of the hydrophosphinylation can be completely reversed by usingPdMe2(dmpe) (dmpe = dimethylphosphinoethane) and the phosphinic acid, Ph2P(O)OH.Propose a mechanism for the formation of the Markovnikov product under these conditions.

+ Ph2P(O)H5% Pd(PPh3)4

+ Ph2P(O)H5% PdMe2(dmpe)

5% Ph2P(O)OH

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