mechanism of the oxidative carbonylation of terminal alkynes

8/13/2019 Mechanism of the Oxidative Carbonylation of Terminal Alkynes

1/17

ISSN 0023-1584, Kinetics and Catalysis, 2007, Vol. 48, No. 2, pp. 228244. MAIK Nauka/ Interperiodica (Russia), 2007.Original Russian Text V.R. Khabibulin, A.V. Kulik, I.V. Oshanina, L.G. Bruk, O.N. Temkin, V.M. Nosova, Yu.A. Ustynyuk, V.K. Belskii, A.I. Stash, K.A. Lysenko, M.Yu. Antipin,2007, published in Kinetika i Kataliz, 2007, Vol. 48, No. 2, pp. 244260.

228

The oxidative carbonylations of terminal alkynes atthe

bond (I), which yield esters of arylpropiolicand alkylpropiolic acids, are of considerable theoreticaland applied interest, since these esters have great syn-thetic potential [1].

. (I)

Various oxidizers (Ox) can be used in this reaction.In the particular case of Ox = quinone, the oxidizerreduction product is Red + 2H

+

= hydroquinone.

Comparatively selective catalytic systems for reac-tion (I) were reported for the first time by Tsuji et al.[2]. The most effective system is PdCl

2

CuCl

2

NaOAc

, on which the following reactions proceed with70% alkyne selectivity:

(II)

It was found earlier [3] that this catalytic system ismultifunctional and reaction (II) is autocatalytic. Cu(I)chloride, which results from CuCl

2

reduction, is thecomponent that catalyzes the palladium alkynyl com-plex formation step in mechanism (III):

(III)

Mechanism (III) was confirmed by the synthesis andreactivity studies of Cu(I), Ag(I), and Hg(II) alkynylcomplexes in reaction (II), by kinetic studies [3, 4], andby determination of the kinetic isotope effect [5]. Fur-thermore, these studies made it possible to carry out anew catalytic reaction using oxygen as the oxidizer andPd(II), Cu(I), and Cu(II) as catalysts:

(IV)

This reaction, which does not yield any acid and,therefore, does not need a base for acid neutralization,can provide a basis for small-scale production of estersof alkynylcarboxylic acids (productivity, ~40

g l

1

h

1

;

RCCH + CO + MeOH + Ox

RCCC(O)OMe + Red + 2H+

RCCH CO MeOH 2NaOAc 2CuCl2+ + + +

RCCC(O)OMe 2NaCl 2CuCl 2AcOH.+ + +

PdCl2

RCCH + CuCl RCCCu RCCPdCl

RCCC(O)PdCl HPdCl PdCl2+ 2CuCl + HCl

RCCC(O)OMe

HCl

PdCl2

CO

CuCl

MeOH 2CuCl2

RCCH + CO + MeOH + 1/2O2

RCCCOOMe + H2O.PdCl2, CuCl, CuCl2

Mechanism of the Oxidative Carbonylation of Terminal Alkynesat the

CH Bond in Solutions of Palladium Complexes

V. R. Khabibulin

a

, A. V. Kulik

a

, I. V. Oshanina

a

, L. G. Bruk

a

, O. N. Temkin

a

, V. M. Nosova

b

,Yu. A. Ustynyuk

b

, V. K. Belskii

c

, A. I. Stash

c

, K. A. Lysenko

d

, and M. Yu. Antipin

d

a

Lomonosov State Academy of Fine Chemical Technology, Moscow, 117571 Russia

e-mail: [email protected]

b

Moscow State University, Moscow, 119899 Russia

c

Karpov Research Institute of Physical Chemistry, Moscow, 103064 Russia

d

Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Moscow, 117813 Russia

Received January 11, 2006

Abstract

The formation mechanism of the active catalyst in the oxidative carbonylation of terminal alkynesat the

CH

bond has been investigated for the catalytic system Pd(OAc)

2

PPh

3

p

-benzoquinone (Q)MeOH.It has been demonstrated by NMR spectroscopy, X-ray crystallography, and kinetic measurements that the cat-alytically active palladium is in the oxidation state 0 and is bound into complexes stabilized byp

-benzoquinone

(

PdL

2

Q

, where L = PPh

3

). A mechanism is suggested for the catalytic process, which includes the formation ofthe complex PdL

2

Q

, the oxidative addition of the alkyne to this complex at the

CH

bond, the insertion of COinto the PdC bond, and steps in which hydride hydrogen is intramolecularly transferred to thep

-quinone.

DOI: 10.1134/S0023158407020073


2/17

KINETICS AND CATALYSIS

Vol. 48

No. 2

2007

MECHANISM OF THE OXIDATIVE CARBONYLATION OF TERMINAL ALKYNES 229

desired-product selectivity, 7585%, depending on thealkyne) [7].

For reaction (I), Tsuji et al. [2] suggested the funda-mentally different catalytic system Pd(OAc)

2

p

-benzo-quinone (Q), which seems simpler at first glance. Thesubject of this publication is the mechanism of reaction

(I) involving this system as the catalyst and phenylacet-ylene as the substrate.

PRELIMINARY EXPERIMENT: CHOOSING THECOMPOSITION OF THE CATALYTIC SYSTEM

In our preliminary experiments, we studied how theoxidative carbonylation of phenylacetylene (PA) into

methyl ester of phenylpropiolic acid (MEPA) dependson the natures of the ligand, of the solvent, and of the

p

-quinone.

A p

-quinone must be present in the Pd(OAc)

2

MeOH system for reaction (I) to occur. Other oxidizers,for example, Cu(II) and Fe(III) compounds, do notensure the synthesis of MEPA. Besides p

-benzo-quinone, 2,3,5,6-tetrachloro-

p

-benzoquinore (chlo-ranil) was tested in the synthesis. The rate and MEPAselectivity of the process were found to depend stronglyon the quinone (Table 1). Therefore, the quinone playsa significant role in the process.

Phosphine admixtures, which serve as efficientligands in the Pd(OAc)

2

MeOH system, cause differenteffects depending on their nature (Table 2). The greatestspeedup of the reaction is observed with PPh

3

, and thiseffect is stronger for the chloranyl-containing system. Itis essential that PPh

3

does not decrease the selectivityof the reaction. Likewise, P(cyclo-C

6

H

11

)

3

added to the

p

-benzoquinone-containing system speeds up the reac-tion significantly without causing a decrease in theselectivity. For the chloranil-containing system,P(cyclo-C

6

H

11

)

3

affords a greater increase in the reac-tion rate but diminishes the selectivity of the reaction.P(

n

-C

3

H

7

)

3

exerts the opposite effect. Diphosphines

Table 1. Solvent effects on MEPA synthesis in thePd(OAc)

2

PPh

3

p

-quinoneMeOHsolvent systems

p

-Quinone SolventInitial MEPA

formation rate,mol l

1

h

1

PA-to-MEPAconversion

selectivity, %

p

-Benzo-quinone

CH

3

CN 0.03 51

CH

3

COCH

3

0.05 56

CHCl

3

0.05 47

1,4-dioxane 0.05 82

THF 0.032 74

C

6

H

6

0.042 65

CH

3

OH 0.065 51

Chloranil CH

3

CN 0.054 59

CH

3

COCH

3

0.36 80

CHCl

3

0.40 81

1,4-dioxane 0.40 85

THF 0.40 85

CH

3

OH 0.25 72

Note: T= 31C, PCO = 1 atm, [Pd] = 0.006 mol/l, [L] = 0.010 mol/l,[p-quinone] = 0.1 mol/l, and [PA]0 = 0.1 mol/l.

Table 2. Ligand effects on MEPA synthesis in the Pd(OAc)2p-quinoneMeOH systems

Quinone Ligand Initial MEPA formation rate,mol l1h1PA-to-MEPA

conversion selectivity, %

p-Benzoquinone PPh3 0.065 51

P(n-C3H7)3 0.01 44

P(cyclo-C6H11)3 0.05 69

PPh2(2-Py) 0* 25

Ph2PCH2CH2PPh2 0 0

Ph2PCH2CH2CH2PPh2 0 14

o-phenanthroline 0 0

Chloranil PPh3 0.25 72P(n-C3H7)3 0 82

P(cyclo-C6H11)3 0.14 54

PPh2(2-Py) 0 40

Ph2PCH2CH2CH2PPh2 0 0

o-phenanthroline 0 0

Note: T= 31C, PCO= 1 atm, [Pd] = 0.006 mol/l, [L] = 0.010 mol/l, [p-quinone] = 0.1 mol/l, and [PA]0= 0.1 mol/l.* The processes with a zero initial rate have an induction period.


3/17

230

KINETICS AND CATALYSIS Vol. 48 No. 2 2007

KHABIBULIN et al.

and nitrogen-containing ligands do not produce a favor-able effect: with these ligands, PA is not converted intoMEPA. Further studies were carried out with triphe-nylphosphine (hereafter, L = PPh3).

The effect of the nature of the solvent on the processwas again studied for the p-benzoquinone- and chlo-ranil-containing systems (Table 1). In all runs involving

a solvent, PA is completely converted in 12 h. Noteagain that the same solvent causes different effects insystems with differentp-quinones. For the system con-tainingp-benzoquinone, the initial reaction rate is lowerin a mixed solvent (MeOH : solvent = 1 : 1 vol/vol) thanin pure methanol. For the chloranil-containing system,the initial MEPA formation rate is higher in a mixedsolvent than in methanol. Most solvents (exceptCH3CN and CHCl3in the p-benzoquinone-containingsystem) enhance the selectivity of the process. Thegreatest increase in selectivity is achieved by adding1,4-dioxane or tetrahydrofuran (THF). The strongdependence of reaction (I) on the kind ofp-quinone andthe fact that a solvent added to methanol produces dif-ferent effects in systems containing different p-quino-nes suggest that the catalytically active complexes inthese similar systems contain a p-quinone or that thisp-quinone is involved in reaction steps preceding therate-limiting step.

HYPOTHETIC MECHANISMSOF MEPA SYNTHESIS

IN THE Pd(OAc)2PPh3p-QUINONEMeOHSYSTEMS

When constructing hypotheses, we took into consid-eration the sets of mechanisms obtained by using a for-malized procedure involving the ChemNet program [5,8]. All hypothetic mechanisms can be divided into thefollowing three groups.

(1) Mechanisms involving a key alkoxycarbonyl(methoxycarbonyl) intermediate:

(V)

(2) Mechanisms involving the formation of a keyPd(II) alkynyl complex at the stage of the electrophilicsubstitution of palladium(II) for hydrogen in thealkyne:

(VI)

PdX2+ MeOH + CO XPdC(O)OMe

XPdC(R)=CHC(O)OMe RCCC(O)OMe.

HX

HPdX

RCCH

PdX2+ RCCH XPdCCR

XPdC(O)CCR RCCC(O)OMe.CO MeOHHPdX

HX

(3) Mechanisms involving the formation of a keyPd(II) ethynyl hydrido complex at the stage of the CHoxidative addition of the alkyne to Pd(0):

(VII)

A first-group MEPA formation mechanism was sug-gested by Heck [9] in 1972. No second-group mecha-nisms have been reported. Palladium(II) forms alkynylcomplexes unreadily (much less readily than Cu(I) orAg(I); see above). However, some Pd(II) alkynyl com-plexes do exist, so there is no reason to rule out the for-mation of such a complex by the electrophilic substitu-tion of palladium for a hydrogen atom in the alkyne.There is only very little information concerning thepossibility of the CH oxidative addition of 1-alkynesto Pd(0) [10]. For Ni(0) and Rh(0), the possibility ofthis step is beyond any doubt [11].

DISCRIMINATION OF HYPOTHETICALMECHANISMS

The above hypotheses can be verified and discrimi-nated by experiments aimed at gaining the followinginformation:

(1) the state of the catalyst during the oxidative car-bonylation of alkynes (the oxidation state of palladiumand the composition and structure of palladium com-plexes);

(2) the kinetic isotope effect (KIE) arising from thereplacement of 3with 3ODor D3OD(if afirst-group mechanism takes place, KIE will be farabove unity);

(3) kinetics observed while varying the concentra-tions of the reactants and of the components of the cat-alytic system.

State of the Catalyst

The IR spectrum of the reaction solution in themixed solvent MeOHCHCl3changes mainly owing toMEPA bands (CO group, 1717 cm1; CC bond,2200 cm1) appearing during the process.

By contrast, the changes in the 1H and 31P NMRspectra are very sophisticated. For example, some 31PNMR spectra show ten to twenty time-variable phos-

phorus resonances. Table 3 lists the multicomponentsystems studied by NMR and their designations.In order to identify the intermediates, we investi-

gated less complicated, model systems, including sys-tems into which reactants were introduced one afteranother. NMR spectra were recorded during the reac-tion. Table 4 lists chemical shifts () and spinspin cou-pling constants (J) for the components of the catalyticsystems and for the complexes obtained by independentsynthesis.

Pd(0) + RCCH RCCPdH

HPdC(O)CCR RCCC(O)OMe.CO MeOHHPdH


4/17



The actual component ratios in the reactions studiedby NMR were derived from total integrated intensitiesin the following three 1H NMR spectral regions:

(1) 12.3 ppm, where the signals from the OAcgroups of various palladium complexes and the signalfrom the OAcanion are observed;

(2) 77.9 ppm, the aromatic range accommodatingthe signals from the phenyl groups of triphenylphos-phine, both free and coordinated to palladium in vari-ous complexes, and from the phenyl groups of the otherPPh3-containing compounds; integration in this rangeallows the total amount of triphenylphosphine involvedin the reaction to be determined;

(3) 5.27.1 ppm, the range accommodating the sig-nals from p-benzoquinone, hydroquinone, and all of

their derivatives, including the products resulting fromtheir interaction with PPh3and palladium acetate.

Palladium acetate (I) is known to be readily reduc-ible with triphenylphosphine (II) [12]; carbon monox-ide [1316]; and, probably, methanol, all of which arepresent in the catalytic system. Indeed, we observedtriphenylphosphine oxide (III) signals in nearly allreaction systems, although the amount of IIIdid notexceed 4% of the amount of triphenylphosphineinvolved in the reaction (see below). The 31P chemicalshift for O=PPh3depends strongly on the solvent, theconcentration of the compound, and temperature. Thetrue position of the triphenylphosphine oxide signalwas determined against 13C satellites (1JPC = 102104 Hz) in the 31P NMR spectrum and from character-istic chemical shifts and constants in the 13C NMRspectrum.

Table 3. Multicomponent systems studied by 1H, 13C, and 31P NMR spectroscopy

Designation System

Three-component1.1 [PPh3QCD3OD], 25C1.2 [PPh3QCDCl3], 25C

1.3 [Pd(OAc)2QCD3OD], 23C

1.4 [Pd(OAc)2PPh3CD3OD], 25C1.5 [Pd(OAc)22PPh3CDCl3], 25C1.6 [Pd(PPh3)410QCDCl3], 25C

Four-component2.1 [Pd(OAc)2PPh3QCH3OD], 25C

2.2 [Pd(OAc)2PPh3QCD3OD], 27C

2.3 [Pd(PPh3)4QCOCD3OD], 30CFive-component

3.1 [Pd(OAc)2PPh3QCOCH3OD], 30C3.2 [Pd(OAc)2PPh3QCOCD3OD], 30C

Six-component

4.1 [Pd(OAc)2PPh3QCOCH3OHCDCl3], 25C4.2 [Pd(PPh3)4QCOPhCCHCH3OHCDCl3], 25C

Seven-component5.1 [Pd(OAc)2PPh3QCOPhCCHCH3OHCDCl3], 25C

Systems into which the reactants were introduced in sequence6.1 [Pd(OAc)2QCD3OD] + [PPh3], 23C

6.2 [Pd(OAc)2PPh3CD3OD] + [Q], 23C

6.3 [Pd(OAc)2PPh3QCD3OD] + [CO], 24C6.4 [Pd(OAc)2PPh3CD3OD] + [Q] + [CO], 26C6.5 [Pd(OAc)2PPh3CD3OD] + [QCOPhCCH], 27C6.6 [Pd(OAc)2CDCl3] + [PPh3] + [Q], 25C

6.7 [Pd(OAc)23PPh3CDCl3] + [CH3OH] + [Q] + [CO], 25C6.8 [PPh3QCDCl3] + [CH3OH] + [CO] + [PhCCH], 25C

Note: The components that were mixed or added together are bracketed.Q =p-benzoquinone.


5/17

232


KHABIBULIN et al.

Table4.

1H,1

3C,and31PNMRdata:chemicalshifts()andspin

spincouplingconstants(J)(CDCl3,25C)

Compound

Fragment

Group

1H

13C

31Pa

,ppm(1JPC,Hz,derived

from12Csatellites

1Hatom

,ppm,intensity,

(splitting,Hz)

13Catom

,ppm(JPCormultiplet

splitting,bHz)

I

Pd(OAc)2

OAc

2.026;1.981c;

1.987c

II

Ph

ortho

7.317.38m,2H

ortho

133.59d(19.3)

6.36

PPh3

meta,para

7.227.31m,3H

meta

128.44d(6.7)

para

128.70brds

ipso

136.58d(9.4)

III

Ph

ortho

7.587.68m,2H

ortho

131.92d(10.6)

30.86(104);33.66d

O=PPh3

meta

7.427.49m,2H

meta

128.48d(12.3)

para

7.517.58m,1H

para

132.04d(2.5)

ipso

131.74d(104.4)

VI

ortho

7.657.73m,12H

ortho

134.44vt(6.66)

14.92

Pd(PPh3)2(OAc)2

O

C(O)CH3

meta,para

7.337.47m,18H

meta

128.13vt(5.05)

(yellow)

para

130.37brds

ipso

128.83vt(23.34)

CH3

0.806vt,6H,(0.51)

21.73vt(1.2)

C=O

175.77s

VIII

CH=

6.80s

136.48

p-Benzoquinone

C=O

187.52

IX

CH=

6.643s,4H;6.706e

115.74;116.16e

Hydroquinone

COH

9.00brs,2H

149.67

Xf

[Pd(PPh3)2Q]2H2Q

MeOH(red)

Ph

ortho

7.130m,12H

ortho

133.52vt(6.83)

31.17;31.30g;31.24h

meta

7.193m,12H

meta

128.03vt(4.94)

33.10d

para

7.305m,6H

para

129.69brds

ipso

132.15vdd(17.2;18.4)

CH=

5.352vti,4H,(1.98)

104.79t(3.2)

C=O

184.69t(2.35)

XIf

Ph

ortho

7.135m,12H

ortho

133.36vt(6.47)

16.95;17.99g;18.29d

[Pd(PPh3)Q]2

1.5MeOH

(black)

meta

7.243m,12H

meta

128.61vt(5.05)

para

7.318m,6H

para

130.47brds

ipso

131.99vdd(17.2;18.5)

CH=

4.02m,4H

78.04t(3.69)

CH=

4.34m,4H

81.94t(1.35)

C=O

185.61t(3.67)

C=O

193.94t(1.12)


6/17



Table4.(Contd.)

Compound

Fragment

Group

1H

13C

31Pa

,ppm(1JPC,Hz,derived

from12Csatellites

1Hatom

,ppm,intensity,

(splitting,Hz)

13Catom

,ppm(JPCormultiplet

splittingb,Hz)

XII

Ph

ortho

7.637.72m,2H

ortho

134.86vt(6.28)

23.88

Pd(PPh3)2Cl2

(yellow)

meta,para

7.347.48m,3H

meta

128.03vt(5.3)

para

130.55brds

ipso

128.97vt(24.8)

XVI

ortho

7.627.71m,24H

ortho

134.30vt(6.58)

18.34

[Pd(COOCH3)

(OAc)(PPh3)2]2

H2QCH3OH

Ph

meta,para

7.327.44m,36H

meta

128.15vt(5.24)

para

130.35brds

ipso

129.76vt(23.04)

OC(O)CH3

CH3

0.894s,6H

22.87vt(1.1)

C=O

177.17s

C(O)OCH3

OCH3

2.434s,6H

51.81s

C=O

181.12vt(2.3)

pa

ra-(OH)2Q

CH=

6.624s,4H

115.73

COH

9.146brds,2H

149.71

XVII

CCH

Ph

ortho

7.477.53m,2H

ortho

131.96

PhCCH

meta,para

7.307.38m,3H

meta

128.25

para

128.79

ipso

121.59

PA

CH

3.119

77.26

C

83.44

XIX

Ph

7.47.63m,

MEPAg,j

OMe

3.803

Note:ssinglet,ddoublet,v

ddvirtualdoubletofdoublets,ttriplet,vtvirtualtriplet,brdsbroadenedsinglet,brsbroadsinglet,andmmultiplet.

aMeasuredrelativetothephosphorussignalfromouter85%H3PO

4inH2O.

bVirtualsplittingsareno

tspinspincouplingconstants,buttheyindicatethepresenceofABXspinsystems.

cSystem6.1,CD5OD,1.981(26C),1.987(23C).

dCD3OD,27C.

eCDCl3,24C.

f Thechemicalshiftsfor

protonsarederivedfrom1H{31P}NMRspectra.

gAcetone-D6,25C.

hCDCl3,30C.

i SpinsystemA2

XX'.

j MEPA+ethylbenzoate

(1:1)mixture.

A2'


7/17

234


KHABIBULIN et al.

Two phosphorus signals not assignable to palladiumcomplexes (21.7 and 16.9 ppm) appeared in the NMRspectra of all systems containing triphenylphosphine,excess p-benzoquinone, and methanol (Table 3, sys-tems 2.12.3, 3.1, 3.2, 6.16.5). As was demonstratedin earlier works [1719], the interaction between triph-enylphosphine and p-benzoquinone is a 1,4-additionreaction yielding (2,5-dihydroxyphenyl)phosphoniumbetaine (IV); however, no NMR data for IV werereported in those works. We found that the main reac-tion product in the simplest system 1.1 (PPh3QCD3OD, +25C) is indeed compound IV, which isreadily identifiable by 1H NMR. The spectrum of IV

exhibits signals from three nonequivalent protons at6.13 ppm (4JHH= 3.18 Hz, 5JHH = 0.38 Hz, 3JH =15.02 Hz), 6.60 ppm (3JHH= 9.04 Hz, 5JHH= 0.38 Hz,4JH= 7.23 Hz), and 7.01 ppm (3JHH= 9.04 Hz, 4JHH=3.18 Hz, 5JH= 0.71 Hz). In the 31P NMR spectrum ofthe reaction mixture, compound IVgives rise to a reso-nance at 21.7 ppm. In the reaction mixture containingexcess benzoquinone, IV is converted into anothercompound (V), which manifests itself as a 31P reso-nance at 16.9 ppm. The structure of V is being deter-mined now.

The nature of the complexes resulting from theinteraction between palladium diacetate and triphe-

nylphosphine was found to depend strongly on the sol-vent and on the PPh3: Pd ratio.Amatore et al. [12, 20] studied the interaction

between Pd(OAc)2and a tenfold excess of PPh3in THFand dimethylformamide as solvents. It was demon-strated that the complex Pd0(PPh3)(OAc) initiallyresults from the reaction between Pd(OAc)2and 2 molof PPh3in these solvents and that this complex reactswith excess triphenylphosphine to yield the complexPd0(PPh3)3(OAc)

(which has a pyramidal structure). Itwas noted that, immediately after mixing Pd(OAc)2with10 mol PPh3 in THF, signals from the complexPdII(PPh3)2(O)2 (VI) (14.48 ppm) and triphenylphos-phine oxide are also observed in the 31P NMR spectrum.

The interaction between Pd(OAc)2 and PPh3 inCDCl3at 25C (Table 3, system 1.5: 2 mol PPh3; sys-tem 6.7: 3 mol PPh3) yields compound VI(7080%,

singlet at 14.92 ppm in the 31P NMR spectrum).1A sig-

1 Hereafter, when considering a compound present in the reactionmixture, we present the percentage of its phosphorus nuclei in thetotal number of phosphorus nuclei. From these data, one canreadily derive the molar concentration of this compound in thereaction mixture.

nal from triphenylphosphine oxide (12%, 30.98 ppm)and several weak signals from unidentified compounds(~3%) are present in the spectrum along with the sig-nals due to VI. Unreacted triphenylphosphine (1626%, 6.37 ppm) was also detected in system 6.7.Complex VIwas synthesized using a standard proce-dure [21] and was characterized by 1H, 13C, and 31PNMR, so its identification based on NMR data for cat-

alytic systems in CDCl3was not a particular problem.In the 1H NMR spectrum of VI, the methyl signal of theacetoxy group (0.806 ppm) is split into a triplet becauseof the spinspin coupling of the protons with the phos-phorus nuclei of the two phosphine ligands (5JHP =0.51 Hz).

In CD3OD(system 1.4, PPh3: Pd ~ 3.5), the domi-nant complex at 2327C is VII, which likely formsfrom complex VI. The 31P NMR spectrum of this sys-tem shows two broad signals (halfwidth, ~10 Hz; totalamount of phosphorus nuclei, 90%), one at 30.81 ppm(triplet, 1P) and the other at 29.32 ppm (doublet, 2P),both arising from the spin system A2(JPP= 18.1 Hz),

as well as signals from free triphenylphosphine (4.2%,broad singlet at 4.47 ppm) and triphenylphosphineoxide (3.8%, 33.66 ppm). The 1H NMR spectrumexhibits broad and smooth signals from phenyl protons(7.087.55 ppm) and two signals from acetate groups(a broad one at 1.87 ppm and the other at 0.29 ppm),and the ratio of these signals is 45 : 3 : 3. These datasuggest that the molecule of complex VII containsthree phosphine ligands and two acetoxy groups, one ofwhich is coordinated to the palladium atom.

An AX2spin system in the 31P NMR spectrum wasalso observed for the planar cationic complex[(PPh3)3PdH]

+and for the complex resulting from the

interaction between palladium diacetate and triphe-nylphosphine in a highly polar medium (80%CF3COOHin water) [22]. It was assumed that the latteris also a cationic complex ([(PPh3)3PdII()]+ or[(PPh3)3Pd

II(S)]2+, where X = Oac or F3and S isprobably an 2molecule) [22]:

A planar Pd(II) complex with three triphenylphos-phine ligands and one acetoxy group in the coordina-

tion sphere of palladium can also form in system 1.4.The second OAc group is likely an anion (Fig. 1).

The signals of complex VIIare observed in the 1Hand 31P NMR spectra of systems 6.2, 6.4, and 6.5immediately after mixing Pd(OAc)2with PPh3(L : Pd =58). For all four systems in methanol-D4, complexVIIis the dominant palladium-containing product beforequinone and CO are introduced. Besides the signals ofVII, triphenylphosphine oxide, and unreacted triphe-nylphosphine, two unidentified signals at 16.3 and

Pd(OAc)2+ PPh3 [(PPh3)3PdIIX]+

or [(PPh3)3PdII(S)]2+.

80%, CF3COOH H2O

PPh3PdPh3P

PPh3

OAc

+

OAc

Fig. 1.Presumable structure of the complex forming in sys-tem 1.4.


8/17



17.9 ppm (total intensity, 3%) are observed in the 31PNMR spectrum. The signal at 16.3 ppm is likely due tocomplex VI.

In all the [Pd(OAc)2+ PPh3+ methanol-D4] systemsexamined, including the systems containing benzo-quinone and CO (1.4, 6.2, 6.4, and 6.5), the amount oftriphenylphosphine oxide does not exceed 4% of thetotal concentration of phosphorus-containing com-pounds (according to 31P NMR data); therefore, triphe-nylphosphine is not the main Pd(II) reducer. In sys-tem 1.5, the main product of the interaction betweenpalladium diacetate and triphenylphosphine in CDCl3is complex VI. Therefore, palladium is not reduced toany significant extent because there is no nucleophile(OAc, MeOH, OMe) necessary for PR3 oxidationwith palladium.

p-Benzoquinone (VIII) does not form any complexwith palladium diacetate in methanol (NMR data forsystem 1.3, IR spectroscopic data), chloroform (IRspectroscopic data), or acetone (IR spectroscopic data).As far as we know, the literature contains no informa-tion concerning complexation between Pd(II) andp-benzoquinone. Only one complex between Pd(I) andp-benzoquinone has been reported [23], and its struc-ture has not been determined.

Complexes between Pd(0) and p-benzoquinonehave long been known, but hypotheses as to the struc-ture of Pd(0)PPh3 compounds have been based only

on indirect data [24, 25]. There is no information con-cerning compounds between palladium and chloranil.For identification of palladium complexes withp-quinones and triphenylphosphine in catalytic solu-tions, it was necessary to obtain, by independent syn-thesis, complexes that could form in the [Pd(OAc)2PPh3QMeOH] and [Pd(PPh3)4QMeOH] systems.Earlier, we synthesized several palladium complexeswith p-benzoquinone, triphenylphosphine, and p-hyd-roquinone (IX) and studied their structures by X-raycrystallography [26]. In the study reported here, wecharacterized these complexes in detail by NMR spec-troscopy.

The red complex [Pd(PPh3)2Q]2H2Q MeOH (X),where 2Q is hydroquinone (Table 4, Fig. 2), wasobtained by reacting Pd(PPh3)4with hydroquinone in amethanol solution in air at room temperature. Thep-benzoquinone that appears in this compound resultsfrom the oxidation of p-hydroquinone with atmo-spheric oxygen. According to X-ray diffraction data,the structure of Xis composed of two Pd(PPh3)2Qmol-

O2q

C4qC5q

C6q

C1qC2q

C3q

Pd2P2

P1

O1q H1h O1h

O1mH1m

C1m

C1hC2h

C3hPd1

Fig. 2.Molecular structure of the complex [Pd(PPh3)2Q]2H2Q MeOH (X).

Table 5. NMR parameters for the ABX spin systems of the phenyl carbon atoms in complex X

Spin system ABXSpinspin coupling constant, Hz

X A B13C nucleus , ppm 31P(13C), ppb 31P(12C), ppb 2JAB JAX JBX

ipso 133.02 0 14 2 30.45 33.95, 1J 0.55, 3J132.15* 35.1* 34.7* 0.65*

ortho 133.73 0 0 30.45 13.5, 2J 0.5, 4Jmeta 128.13 0 0 30.45 9.7, 3J 0.01, 5J

Note: CDCl3 , 27C, Bruker AM-360, 90.568 MHz.* CDCl3, 25C, Bruker DPX-300, 75.468 MHz.


9/17

236


KHABIBULIN et al.

ecules linked by a p-hydroquinone bridge throughhydrogen bonding (Fig. 2). Pure complex Xdissolvedin CDCl3at a low temperature in an argon atmospheregives rise to a 31P resonance at 31.17 ppm (92.6%). The1H NMR signal ofX occurs at 7.147.32 ppm. The sig-

nal from the protons of coordinated p-benzoquinone(A2 part of the A2 XX'spin system (X, X' = 31P))is a triplet occurring at 5.35 ppm with 1.98-Hz splitting.

A2' A2'

The signal intensity ratio Ph : Q = 30H : 4H is consis-tent with the structure of complex X. Furthermore, the1H NMR shows ap-hydroquinone signal at 6.58 ppm.

For X and other palladium complexes containingtwo triphenylphosphine ligands, the 13C NMR spectralregion pertaining to the phenyl carbon nuclei of theseligands deserves special attention (Fig. 3).

The 13C NMR spectrum displays signals from allfour nonequivalent types of carbon atoms, which arethe X-parts of ABX spectra (A, B = 31P). These spectraarise from isotopomers each containing a single 13Cisotope (this is possible because the natural abundanceof this isotope is as low as 1.1%). As a consequence, thephosphorus atoms are nonequivalent. The 13C/12C iso-tope effect on 31P estimated by analysis of the AB partof the spin system in the phosphorus spectrum is 14 2ppb (Table 5). The profile of the ipso-13C multiplet isthe most informative. An analysis of this multipletallowed us to determine JAXand JX. As was expected,

these parameters are very different. Therefore, the pro-files of the multiplets from the phenyl carbon nuclei intriphenylphosphine complexes of palladium and othertransition metals provide a reliable analytical means fordetermining the number of phosphine ligands coordi-nated to the metal atom.

The dissolution of complex X in acetone at roomtemperature yields black crystals of the compound[Pd(PPh3)Q]2 1.5MeOH (XI), in which the Pd(PPh3)frag-ments are linked by twop-benzoquinone bridges (Fig. 4).

10000 9980 9960 9940 H, Hz

133.5 133.0 132.5 132.0 , ppm134.0 131.5 131.0 130.5 130.0 129.5 129.0 128.5 128.0

*

**

10 Hz

meta-

para-

ipso-

orto-

Fig. 3.13C NMR spectrum of complex X in the phenyl range. The signals of triphenylphosphine oxide are starred. The arrows pointto the 9th and the 14th lines in the X-part of the ABX spin system of the ipso carbon atom.

C5

C6

C4

O1

C2

C3

P1

Pd2 Pd1

C1

O2P2

O3C7

C12

C11

O4

C10C9

C8

Fig. 4.Molecular structure of the complex [Pd(PPh3)Q]21.5MeOH (XI).


10/17


11/17

238


KHABIBULIN et al.

quinone involved in steps (2) and (3) plays a significantrole: it favors the binding of part of the PPh3 intobetaines IVand Vin systems 6.1, 6.2, and 6.4and theformation of the dimeric complex; as a ligand, it stabi-lizes Pd(0). In systems 6.1and 6.2, 3151% of the PPh3reacts with quinone to yield betaines. Complex VIIforms via reaction (4) in methanol, a polar solvent, inthe presence of excess phosphine. It also turns intodimeric complexes upon addition of Q (scheme, reac-

tion (5)).The formation of palladium methoxide complexesfrom the cationic dimer is beyond question, since meth-anol is oxidized to formaldehyde and then formic acideven by palladium chloride via the formation of palla-dium methoxide [27].

Contrary to the reports on Pd(II) reduction withPPh3 in THF and DMF [12, 20], PPh3 in methanolapparently does not play any significant role in Pd(II)reduction at 25C.

As CO is passed through the reactor (systems 6.3and 6.4), the signals from complexes Xand XIdisap-pear. A signal at 19.3 ppm appears in the 31P NMRspectrum. The carbon signals corresponding to this sig-nal (X-multiplets of the orthoand metacarbon atoms ofABX spin systems; see above) indicate that the newcomplex (XV) contains a Pd(PPh3)2 fragment and itscomposition can be, e.g., ()2Pd(PPh3)2or (Ph3)PdPd(PPh3). This complex does not contain quinone as aligand. A Pd(0) benzoquinone complex containing nophosphine ligands presumably forms along with com-plex XV, as is indicated by a 1H singlet at 5.34 ppmfrom coordinated p-benzoquinone. It is likely that the

phosphine-free complex results from the replacementof the triphenylphosphine ligands by carbon monoxidein X. The amount of complex XV in systems 3.2, 6.3,and 6.4 is 14 to 60%. Complex XV is stable in solution.After systems 6.3 and 6.4 were stored in the cold for ashort time, their 31P NMR spectra indicated the sameamounts of the complex.

The concentration of hydrogen ions (acid) in thesystems examined is very low. The only process that

can cause the acid concentration in the reaction sys-tem to exceed the acid concentration in the initialmethanolic solution is palladium reduction involvingPPh3and methanol, CO and H2O, or CO and metha-nol, for example,

(VIII)

The mechanism of reaction (VIII) includes the for-mation of an intermediate methoxycarbonyl complex,whose decomposition yields dimethyl carbonate andPd(0) [15, 16, 28]. The methoxycarbonyl complex canalso be an intermediate in the process examined (see the

first-group hypotheses). The dimeric methoxycarbonylcomplex [Pd(COOMe)(OAc)(PPh3)2]2C6H4(OH)2(VI) was isolated from the Pd(OAc)2PPh3QMeOHsystem treated with CO (Fig. 6; see Experimen-tal).

The compound XVI was characterized by X-raycrystallography and 1H, 13C, and 31P NMR spectros-copy. The 1H NMR spectrum of XVI(Table 4) showssignals from p-hydroquinone (6.624 ppm, 4H, CH=;9.146 ppm, 2H, OH). As deduced from the integrated

Pd(OAc)2+ CO + 2MeOH

Pd(0) + O=C(OMe)2+ 2AcOH.

P4

O9

C85 O10Pd2

O3

C81

O4

C82

H4O

H823

O6

C76

C77C78 C73

C75

C74

O2

O5

H5OC80

C79

O1

Pd1

P2

C83

O8

O7

C84AP1

Fig. 6.Molecular structure of the complex [Pd(COOCH3)(OAc)(PPh3)2]2H2Q H3OH (XVI). The disordered methanol moleculeis not shown.


12/17



line intensities in the proton spectrum, the complexcontains two acetoxy groups (0.894 ppm, 6H) and fourtriphenylphosphine ligands (7.627.71 ppm, 24H,orthoprotons; 7.327.44 ppm, 36H, metaand ppro-tons) per p-hydroquinone molecule. The 13C NMRspectrum of XVIexhibits signals from all these func-tional groups, and their intensities are consistent withthe proton spectrum.

According to X-ray structure determination data(Fig. 6; see Experimental), complex XVIis a dimer inwhich two Pd(COOMe)(OAc)(PPh3)2fragments arep-hydroquinone-bridged through hydrogen bondsbetween the hydroxyl hydrogen atoms of p-hydro-quinone and the oxygen atoms of the acetate groups.

Thus, CO brings about the formation of complexXVI and its decomposition in the presence of ap-quinoneanother route for the formation of Pd(0)complexes:

Likely Mechanisms of Reaction (I) (Ox = O)

The concentration of hydrogen ions in the systemand its effect on the parameters of the process are themajor discriminating factors governing the possibilityof Pd(0) oxidation by ap-quinone (the oxidation poten-tial ofp-quinones decreases greatly as pH is raised to56) and the realizability of first- and second-grouphypotheses. pH measurements during the processesdemonstrated that, at the early stages of the reaction,

the solution acidity does not rise and pH remains invari-able (~6). Investigation of the effects of acetic acid andsodium acetate admixtures on the pH value of the reac-tion solution and on the process parameters demon-strated that the system as a whole has the properties ofa buffer and all its components can react with aceticacid (Table 6). The pH of the system remains almostinvariable as 1 to 4 mol of CH3COOH per mole of pal-ladium is added. The process parameters also vary onlyslightly (Table 6). Only addition of 2 mol of the strongacid CF3COOH per mole of palladium reduces pH to0.4 and completely terminates the carbonylation reac-tion. These data are consistent with the third-groupmechanisms, since the acetic acid resulting from palla-

dium(II) acetate reduction is neutralized through reac-tions with the components of the catalytic system.

Likewise, adding up to 10 mol of a base(CH3COONa) per mole of palladium causes only slightchanges in pH and in the initial rate and selectivity ofthe process (Table 6). As pH is changed from 6 to 7.7,the process parameters do not change significantly.These results are most consistent with the third-group

Pd(II) (PPh3)nPdQ

XVI (PPh3)2Pd(CO)2

(presumably)

MeOH

PPh3, Q

PPh3, CO

MeOHCO

MeOH, Q

QPd(CO)m

XV

Table 6. Effects of acid and base admixtures on the param-eters of reaction (I) with the catalytic system Pd(OAc)2PPh3QMeOH

n* pHInitial MEPA

formation rate,mol l1h1

PA-to-MEPAconversion

selectivity, %

Reference system

6.0 0.07 51Admixture of CH3COOH

1 6.0 0.12 472 5.9 0.10 453 6.0 0.08 46

Admixture of CF3COOH2 0.4 0.00 0

Admixture of CH3COONa1 7.0 0.07 552 7.3 0.12 534 7.5 0.13 61

6 7.7 0.10 5910 7.7 0.09 62

* Number of moles of an acid or a base per mole of Pd(II) at[PPh3] : [Pd]= 2.

Table 7. Effect of the nature of the initial palladium compound on the parameters of reaction (I)

Catalyst precursor Quasi-steady-statereaction rate, mol l1h1 Reaction time, minPA-to-MEPA conversion

selectivity, %

Pd(PPh3)2Cl2 0.000 120 TracesPd(OAc)2+ 2PPh3 0.043 200 58Pd(PPh3)4 0.070 127 54[Pd(PPh3)2Q]2H2Q MeOH 0.080 104 63[Pd(PPh3)Q]21.5MeOH + 2PPh3 0.042 184 68Pd(dba)2+ 2PPh3 0.031 219 72

Note: T= 31C, PCO= 1 ata, [Pd] = 0.006 mol/l, [PA]0= 0.1 mol/l, [Q] : [Pd] = 17.dba = dibenzylideneacetone.


13/17

240


KHABIBULIN et al.

mechanisms, since the first- and second-group mecha-nisms imply that the reaction rate depends significantlyon the proton concentration (they involve reversibleand, probably, quasi-equilibrium steps in which H+ isreleased). The termination of the process by CF3COOHin a third-group mechanism can be caused by the

decomposition of the palladium alkynyl complexresulting from the oxidative addition of the alkyne toPd(0) at the CH bond.

It is possible that the weak dependence of the pro-cess parameters on the acidity of the reaction solutionis evidence that thep-quinone protonation step does notplay any significant role in the oxidation of the reduced

form of the catalyst. Therefore, thep-quinone does notoxidize Pd(0) in the process examined. The presence ofcomplex X(48%) in the Pd(PPh3)410QCDCl3solu-tion (system 1.6) is further evidence that excess p-quinone does not cause Pd(0) oxidation, apparentlybecause of the nearly complete absence of hydrogenions.

Comparing the catalytic activities of the initial com-

pounds (precursors) containing Pd(II) or Pd(0) suggeststhat the compounds tested show similar catalytic prop-erties in reaction (I) (Table 7). The most plausibleexplanation for this fact is that all precursors butPd(PPh3)2Cl2turn into the same or similar compounds.These compounds are most likely Pd(0) p-quinonecomplexes containing a PdLnQfragment. Since reduc-ing agents (CO, MeOH, PPh3) are present in the systemand thep-quinones have a low oxidation potential, thereduction of Pd(II) to Pd(0) is much more probable thanthe reverse process.

The above results suggest that the third-group mech-anisms are the most likely. We have not detected any

hydrido complexes of palladium in the catalytic solu-tion. It is possible that, after phenylacetylene (XVII) iscoordinated to palladium, its proton is rapidly trans-ferred intramolecularly to the quinone coordinated tothe same metal atom, involving or not involving anintermediate hydride, to yield the alkynylp-hydro-quinolate palladium(II) derivative Pd(OC6H4OH)(CCPh).

The complex (PPh3)2ClPdC6Cl2O2(OMe) (XVIII),which was isolated from the Pd(OAc)2PPh3MeOHCOchloranil system, provides further evidence thatPd(0) is present in the reaction system examined. Themost likely route for the formation of XVIIIis the oxi-dative addition of chloranil to the Pd(0) complex at the

CCl bond followed by the substitution of a methoxygroup for a chlorine atom in the chloranil molecule.The composition and structure of the(PPh3)2ClPdC6Cl2O2(OMe)complex were determinedby X-ray crystallography (Fig. 7; see Experimental).

As was noted above, the first-group hypotheses canbe checked by KIE measurements. The kinetic experi-ments carried out for the Pd(OAc)2PPh3chloranilCH3OH (CH3OD)chloroform (95%) system demon-strated that KIEH/D is close to unity (Table 8). Thisresult rules out the first-group hypotheses.

The totality of our data allow us to exclude fromconsideration the hypotheses involving a Pd(II) com-

pound as an active species, the steps involving protons,and Pd(0) reoxidation withp-benzoquinone (because ofthe insufficient proton concentration) and to suggest areaction mechanism consistent with the third-grouphypotheses.

The following two schemes of the catalytic cyclecan be suggested:

(1) Hydridoalkynyl mechanism (Fig. 8, outer cycle).PA adds oxidatively to the Pd(0)PdL2Q intermedi-

ate to yield an organopalladium hydride for which two

Cl6

C20A O3

C13

C200

O11C7B

C10A

O2C1B

C14A

Cl5

C18

P1

C10C6

Pd1

Cl2

C4B

C12P2

C5

Fig. 7. Molecular structure of the complex(PPh3)2ClPdC6Cl2O2(OMe) (XVIII).The solvate moleculeof 2,5-dichloro-3,6-dimethoxy-1,4-benzoquinone is notshown.

Table 8. MEPA formation rates in the system Pd(OAc)2chloranilPPh3CH3OH/CH3ODchloroform (95%)

Alcohol Reaction rate,mol l1h1 AverageStandarddeviation

CH3OH 0.25 0.22 0.020.210.210.20

CH3OD 0.17 0.19 0.010.20

0.190.20


14/17



further conversion pathways are possible, namely, theoxidation of the hydrido ligand by thep-quinone, yield-ing palladium hydroquinolate (Fig. 8, mechanism 2),and the insertion of carbon monoxide into the palla-diumalkynyl -bond. The alcoholysis of the PdC(O)bond in the propioloyl derivative of palladium results inthe decomposition of the organopalladium intermediateinto molecules of the product (MEPA, XIX) and thedihydridop-quinone complex (Q)LPdH2. The hydridoligands are oxidized by the coordinatedp-quinone mol-ecule, and the phosphine ligands andp-quinone presentin the system ensure the regeneration of the L2PdQcomplex.

(2) Hydroquinolatealkynyl mechanism (Fig. 8,inner cycle).

Mechanism 2 does not imply the formation of ahydrido complex from PA. In this mechanism, the pro-ton of the coordinated alkyne is intramolecularly trans-ferred to the coordinatedp-quinone molecule, resultingin Pd(0) oxidation and the formation of a hydroquino-late derivative. This is followed by CO insertion intothe Pdalkynyl -bond and, next, the alcoholysis of thePdC(O) bond yielding a molecule of the product(MEPA) and the hydroquinolate complex L2(HQ)PdH.The complex L2(HQ)PdH is reduced to L2Pd0via the

reductive elimination ofp-hydroquinone, and the lattercomplex reacts withp-quinone to turn into L2PdQ.Both mechanisms are in agreement with our data,

and discrimination between them will be the subject offorthcoming studies.

Our data concerning the oxidative carbonylation ofPA suggest that the reaction system possesses the prop-erties of a buffer at pH 67.7. The findings thatKIEH/D 1is close to unity and that the initial systembased on Pd(OAc)2and the complexes PdLn, X, and XI

show similar catalytic activities are evidence thatLnPdQ or (CO)mPdQpalladium(0) complexes partici-pate in reaction (I) as catalysts. The role of p-quinonein this system is not only to oxidize the hydrido com-plexes of palladium. It is likely that p-quinone isinvolved in the reaction steps yielding the active Pd(0)complex and, as a ligand, in the alkyne activation step,followed by the intramolecular transfer of hydridehydrogen or a proton from the alkyne to the oxygenatoms of coordinated Q.

EXPERIMENTALThe PA and MEPA concentrations in catalytic solu-

tions were determined using an LKhM-80 gas chro-matograph (thermal-conductivity detector; columnpacked with Porapak P, l= 1 m, d= 3 mm; carrier gas,helium; column, injector, and detector temperatures,230, 250, and 240, respectively). Isobutyl benzoateor ethyl benzoate was used as the standard. The gas incontact with the solution was analyzed on an LKhM-8MD gas chromatograph (thermal-conductivity detec-tor; column with activated carbon AR-3, l= 3 m, d=3 mm; carrier gas, argon; column and detector temper-ature, 160; injector temperature, 200).

Infrared spectra were recorded on a Specord M82spectrophotometer.

The 1H, 1H{31P}, 13C{1H}, and 31P{1H} NMR spec-tra of compounds IXIX in CDCl3 (25C) wereobtained on a Bruker DPX 300 spectrometer operatingat 300.13, 75.468, and 121.495 MHz, respectively.Chemical shifts for phosphorus nuclei were measuredrelative to the signal from external 85% 34in 2.Reactions were carried out in an Ar or N2atmosphere.

Pd(II)

Q L, COQH2 PhCCH

L2PdQQ, L

Q

PhCCH

L2Pd(H)(QH) L2Pd(HQ)CCPh

H2Pd(Q)L H Pd(Q)(CCPh)

PhCCCOOMe CO,MeOH

L

PhCCCOOMeH Pd(Q)(COCCPh) CO

MeOH

1

QH2

2

L

L

L

L

Fig. 8.MEPA synthesis mechanism including the oxidative addition of PA to Pd(0) at the bond. Q =p-quinone and L = PPh3.


15/17

242


KHABIBULIN et al.

Kinetic experimentswere carried out in a closed,thermostated glass reactor fitted with a precessingmechanical stirrer. The weighed components of the cat-alytic system were charged into the reactor, and metha-nol and, if necessary, another solvent were added. Thereactor was thermostated, carbon monoxide was passedthrough the reaction system, and phenylacetylene wasinjected using a microsyringe. During the reaction, wemeasured the volume of the gas consumed and sampledthe gas and the solution for GC analysis.

Synthesis of complex XVI.A mixture of Pd(OAc)2(0.15 mmol) and PPh3(0.31 mmol) in methanol (4 ml)was stirred for 5 min. This resulted in a light green pre-cipitate. p-Benzoquinone (0.573 mmol) was added tothe precipitate, and CO was passed through the mixturefor 1015 min while stirring. This yielded a light yel-low solution. The solution was filtered, and hexane(0.5 ml) was added. The resulting solution was cooled

to +5. Yellow crystals formed in 48 h.Synthesis of complex XVIII. A mixture of

Pd(OAc)2 (0.114 mmol) and PPh3 (0.23 mmol) inmethanol (4 ml) was stirred for 5 min. This resulted ina light green precipitate. CO was passed through thesystem for ~1015 min while stirring. This yielded ahomogeneous light yellow solution. Chloranil(0.55 mmol) was added to the solution. The resultingsolution was stirred for 1520 min and was then cooledto +5. Yellow crystals resulted in 2 months.

X-ray structure determination of complexes XVIand XVIII. The X-ray diffraction experiment for[Pd(COOCH3)(OAc)(PPh3)2]2H2Q H3OH (XVI)wascarried out on a CAD-4 automated diffractometer(-filter, Mo K, = 0.71073 , T = 293(2) K,/2scan mode). The crystal disintegrated slowly dur-ing the experiment, and this caused a 36% change in thereference reflection intensities.

The intensity losses caused by crystal breakdownwere compensated for by recovering integrated intensi-ties from reflection profiles [29]. Crystal data and thestructure determination parameters for complex XVIare presented in Table 9.

The structure was solved by direct methods usingthe SHELXS97 program [30] and was refined using theSHELXL program [31]. The positions and thermalparameters of the non-hydrogen atoms were refinedisotropically and then anisotropically using full-matrixleast squares. The hydrogen atoms were placed in theirgeometrically calculated positions and were included inthe refinement in the riding model.

In the dimeric noncentrosymmetric complex, twometal complex fragments of compositionPd(C(O)OCH3)(OAc)(PPh3)2 are linked by a hydro-quinone bridge (Fig. 6).p-Hydroquinone is hydrogen-bonded with the carbonyl oxygen atoms of the acetategroups coordinated to the metal atom. The distancesbetween oxygen atoms involved in hydrogen bondingare similar (O5O2 2.679(4) , O4O6 2.632(4) ).At the same time, HOH angles differ significantly(O4H40O6 157.15(1), O2H50O5 129.95(2)).Note that one proton ofp-hydroquinone (H40) is nearlycompletely shared between an oxygen atom ofp-hyd-roquinone (H40O4 1.260(1) ) and an oxygen atom of

the acetate group (H40O6, 1.425(2) ). This is not thecase for the other hydrogen bond (O5H50 0.674(1) ,O2H50 2.195(2) ). However, the sharing of the pro-ton exerts only a slight effect on the CO bond lengthinp-hydroquinone (in the case of proton sharing, C76O6 1.375(1) ; in the case of the normal hydrogenbond, C73O5 1.366(1) ) and a stronger effect on theC=O bond of the acetate ligand. The length of this bondis 1.275(2) in the ligand that shares the proton and1.245(2) in the ligand that does not share the proton.

In the aromatic ring of the bridgingp-hydroquinonemolecule, all CC bond lengths are between 1.389 and1.391 , within the range typical of aromatic rings. Themetal atom is in a square-planar coordination environ-ment, which is typical of Pd(II) complexes. The phos-phine ligands are trans, and the PdP bond lengths liein the range 2.327(4)2.236(4) . The aceto group iscoordinated to Pd through one oxygen atom, and theO3Pd2 bond length is 2.085(8) . The methoxycarbo-nyl group is transto the aceto group, and the length ofthe Pd2C85 -bond is 1.966(2) . The angles betweenthe PdC bond and the carbonyl and ester oxygenatoms are O9C85Pd2 125.2(1)and O10C85Pd2113.4(1), respectively. In one of the two metal com-

Table 9. Crystal data and X-ray structure determination pa-rameters for complex XVI

Formula C87H80O11P4Pd2Molecular weight 1638.19

System Triclinic

Space group P-1

a, 10.594(2)

b, 14.119(3)

c, 26.705(5)

, deg 85.57(3)

, deg 89.40(3)

, deg 76.82(3)

V, 3 3877.5(13)

Z 2

dcalc, g/cm3 1.403

, cm1 0.607

Total number of reflections

measured (Rint)

14428 (Rint= 0.0208)

Number of reflections withI > 2(I) 5922

Number of refined parameters 775

R1 (I > 2(I)) 0.0677

wR2 (for all reflections) 0.1831


16/17



plex fragments, the coordinated methoxycarbonylgroup is partially disordered. A heavily disorderedmethanol molecule is also present in the crystal struc-ture.

Comparing this structure with the structure of trans-acetatomethoxycarbonyl(bistriphenylphosphine)palla-dium [32], which has no p-hydroquinone bridge, sug-gests that the most important structural features of themetal complex moiety, the mutual arrangement and thecoordination modes of the ligands, the geometry of theenvironment of the metal atom, and the bond lengthsand bond angles are invariable and are independent ofthe synthetic route, the crystallization procedure, andthe presence of a supramolecular environment.

The most significant structural distinction betweenthe metal complex moieties is that the C=O bondlengths in their acetate ligands differ by 0.044 . Thegreatest bond angle difference is 2.7, which isobserved for the angle between the PdC bond and theCOC bond in the methoxycarbonyl ligand.

X-ray structure determination for the complex(PPh3)2ClPdC6Cl2O2(OMe) (XVIII) was carried out

according to a standard procedure [33] on a BrukerAXS SMART 100 diffractometer fitted with a CCDdetector (Mo; graphite monochromator; 110 K; -scan mode; 2max= 60; 0.3 increments; frame mea-surement time, 15 s). The structure was solved by directmethods using the SHELXS97 program [30] and wasrefined by anisotropic full-matrix least squares usingthe SHELXL97 program [31] (the H atoms werelocated geometrically and were fixed in their positionswith UH= 0.08 2). Crystal data and structure determi-nation parameters for complex XVIIIare presented inTable 10.

According to X-ray diffraction data, XVIII(Tables 10,

11) is a tetracoordinated palladium(II) complex. Themetal atom is in a square-planar coordination environ-ment, which is typical of Pd(II) complexes. The coordi-nation sphere of the metal atom consists of two transtriphenylphosphine ligands, a chlorine atom, and2,5-dichloro-3-methoxy-1,4-benzoquinonyl in thetransposition relative to the chlorine atom. The averageplane of the 2,5-dichloro-3-methoxy-1,4-benzo-quinonyl ring and the plane passing through the phos-phorus and chlorine atoms bonded to palladium makean angle close to 90. The methyl fragment of the meth-oxy group of 2,5-dichloro-3-methoxy-1,4-benzo-quinonyl is partially disordered. The CC bonds incoordinated 2,5-dichloro-3-methoxy-1,4-benzo-

quinonyl have lengths usual for the p-hydroquinoneseries. The double bonds are C20AC7B 1.317(1) and C14AC13 1.323(1) . The lengths of the ordinarybonds are between C13C10A 1.521(1) and C20C10 1.490(1) . The length of the Pd1C13 -bond is1.996(8) .

ACKNOWLEDGMENTS

This work was supported by the Russian Foundationfor Basic Research, grant nos. 01-03-32883, 04-03-33014, 05-03-08134, and 05-03-33151.

REFERENCES

1. Yashina, O.G. and Vereshchagin, L.I., Usp. Khim., 1978,vol. 47, p. 557.

2. Tsuji, J., Takahashi, M., and Takahashi, T., TetrahedronLett., 1980, vol. 21, no. 9, p. 849.

3. Zung, T.T., Bruk, L.G., and Temkin, O.N., MendeleevCommun., 1994, no. 2, p. 2.

4. Zung, T.T., Bruk, L.G., and Temkin, O.N., Izv. Akad.Nauk, Ser. Khim., 1993, no. 42, p. 1730.

Table 10. Crystal data and X-ray structure determinationparameters for complex XVIII

Formula C47H33Cl4O5P2Pd

Molecular weight 987.87

System Monoclinic

Space group P2(1)/c

a, 12.382(2)b, 10.745(19)

c, 32.206(6)

, deg 90.00

, deg 92.770(5)

, deg 90.00 (3)

V, 3 4279.6(13)

Z 4

dcalc, g/cm3 1.533

, cm1 0.805

Total number of reflections

measured (Rint)

34176 (Rint= 0.0948)

Number of reflections withI > 2(I) 3905

Number of refined parameters 531

R1 (I > 2(I)) 0.0645

wR2 (for all reflections) 0.1484

Table 11. Selected bond lengths (d) and bond angles () forXVIII

Bond d, Angle , deg

Pd1P2 2.307(2) C7BO11C200 120.4(110)

Pd1P1 2.314(2) P2Pd1P1 175.55(7)Pd1Cl2 2.361(2) P2Pd1Cl2 88.54(7)

P2C12 1.809(7) C13Pd1Cl2 177.8(2)P2C5 1.814(7) P2Pd1Cl2 88.54(7)P2C4B 1.822(7)


17/17

244


KHABIBULIN et al.

5. Bruk, L.G., Gorodsky, S.N., Valdes-Perez, R.E., andTemkin, O.N.,J. Mol. Catal. A: Chem., 1998, no. 130,p. 29.

6. Bruk, L.G. and Temkin, O.N.,Inorg. Chim. Acta, 1998,vol. 280, p. 202.

7. RF Patent 2051896.8. Shalgunov, S.I., Zeigarnik, A.V., Bruk, L.G., and

Temkin, O.N.,Izv. Akad. Nauk, Ser. Khim., 1999, no. 10,

p. 1891.9. Heck, R.F., J. Am. Chem. Soc., 1972, vol. 94, no. 8,p. 2712.

10. Chukhadzhyan, G.A., Evoyan, Z.K., and Melkonyan, L.N.,Zh. Obshch. Khim., 1975, vol. 45, p. 1114.

11. Nesmeyanov, A.N. and Kocheshkov, K.A.,Metody ele-mentoorganicheskoi khimii(Methods of OrganoelementChemistry), Moscow: Nauka, 1978.

12. Amatore, C., Carre, E., Jutand, A., and MBarki, M.A.,Organometallics, 1995, vol. 14, no. 4, p. 1818.

13. Moiseev, I.I., Stromnova, T.A., Vargaftik, M.N.,Mazo, G.Ja., Kuzmina, L.G., and Struchkov, Y.T.,J. Chem. Soc., Chem. Commun., 1978, p. 27.

14. Chernysheva, T.V., Stromnova, T.V., Vargaftik, M.N, andMoiseev, I.I.,Izv. Akad. Nauk, Ser. Khim., 1996, no. 10,p. 2456.

15. Rivetti, F. and Romano, U.,J. Organomet. Chem., 1978,vol. 154, p. 323.

16. Rivetti, F. and Romano, U.,J. Organomet. Chem., 1979,vol. 174, p. 221.

17. Arshad, M., Beg, A., and Siddiqui, M.S., Tetrahedron,1966, vol. 22, no. 7, p. 2203.

18. Ramirez, F. and Dershowitz, S., J. Am. Chem. Soc.,1956, vol. 78, p. 5614.

19. Kutyrev, A.A. and Moskva, V.V., Usp. Khim., 1987,vol. 56, no. 11.

20. Amatore, C. and Jutand, A., Acc. Chem. Res., 2000,

vol. 33, p. 314.

21. Dzhemilev, U.M., Popodko, N.R., and Kozlova, E.V.,Metallokompleksnyi kataliz v organicheskom sinteze:alitsiklicheskie soedineniya (Metal Complex Catalysisin Organic Synthesis: Alicyclic Compounds), Moscow:Khimiya, 1999.

22. Zudin, V.N., Chinakov, V.D., Nekipelov, V.M., Rogov, V.A.,Likholobov, V.A., and Yermakov, Yu.I.,J. Mol. Catal. A:Chem., 1989, no. 52, p. 27.

23. Shitova, N.B., Kuznetsova, L.N., Yurchenko, E.N.,Ovsyannikova, I.A., and Matveev, K.I.,Izv. Akad. NaukSSSR, Ser. Khim., 1973.

24. Takahashi, S. and Hagihara, N.,J. Chem. Soc. Jpn., PureChem. Sect., 1967, no. 88, p. 1306.

25. Milani, B., Anzilutti, A., Vicentini, L., Sessanta o Santi, A.,Zangrando, E., Geremia, S., and Mestroni, G., Organo-metallics, 1997, no. 16, p. 5064.

26. Kulik, A.V., Bruk, L.G., Temkin, O.N., Khabibulin, V.R.,Belsky, V.K., and Zavodnik, V.E.,Mendeleev Commun.,2002, p. 47.

27. Moiseev, I.I., -Kompleksy v zhidkofaznom okislenii ole-finov(-Complexes in Liquid-Phase Alkene Oxidation),Moscow: Nauka, 1970.

28. Zhir-Lebed, L.N., Kuzmina, L.G., Struchkov, Yu.T.,Temkin, O.N., and Golodov, V.A., Koord. Khim., 1978,vol. 4, p. 1046.

29. Streltsov, V.A. and Zavodnik, V.E., Kristallografiya,1989, vol. 34, no. 6, p. 1369.

30. Sheldrick, G.M., SHELXS97: Program for the Solutionof Crystal Structures, Gttingen: Univ. of Gttingen,1997.

31. Sheldrick, G.M.,SHELXL97: Program for Refinementof Crystal Structures, Gttingen: Univ. of Gttingen,1997.

32. Del Piero, G. and Cesari, M.,Acta Crystallogr., Sect. B,1979, no. 35, p. 2411.

33. SMART (Control) and SAINT (Integration) Software,

Version 5.0, Madison, WI: Bruker AXS, 1997.

mechanism of the oxidative carbonylation of terminal alkynes

Documents