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. 1H 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 (kH/kD = 6.4) is operative. All these results suggest that the carbonylation of alkynes proceeds via the protonation of a Pd(0)-alkyne species to give a Pd-vinyl complex, followed by CO insertion and alcoholysis.