Mechanism of the alkoxycarbonylation of alkynes in the presence of the Pd(OAc)2/PPh2Py/CH3SO3H catalytic system

The mechanism of the carbonylation of alkynes promoted by the Pd(OAc)2/2-pyridyldiphenylphosphine/methanesulfonic acid catalytic system has been studied. The carbonylation of 2-butyne in the presence of methanol affords stereospecifically the methyl ester of (E)-2-methyl-2-butenoic acid, indicating that the addition of H and COOCH3 moieties proceeds with cis stereochemistry. Experiments carried out using 1-alkynes and CH3OD reveal that the catalyst also promotes the exchange of the terminal hydrogen of the alkyne with the deuterium of the alcohol. H-1 NMR experiments show that upon addition of phenylacetylene to a CD2Cl2 solution containing the catalyst and CH3OH a palladium complex having a 2-styryl group bound to the metal center, Pd-C(C6H5)=CH2, is formed. This species can be invoked as an intermediate to account for both the H/D exchange and the carbonylation reaction. Carbonylation of 2-butyne in the presence of a 1/1 mixture of CH3OH and CH3OD indicates that a fairly large isotope effect (k(H)/k(D)=6.4) is operative. All these results suggest that the carbonylation of alkynes proceeds via the protonation of a Pd(O)-alkyne species to give a Pd-vinyl complex, followed by CO insertion and alcoholysis.