PALLADIUM-CATALYZED HYDROCARBOXYLATION OF ALKYNES WITH FORMIC-ACID

Alkynes are hydrocarboxylated with formic acid in the presence of catalytic amounts of Pd-(OAc)2 and suitable phosphine ligands (120 psi of CO gas pressure, 100-110-degrees-C) to produce the corresponding unsaturated carboxylic acids in 60-90 % yields. The products of terminal alkyne reactions are CR(COOH)=CH2, 1, and (E)-CHR=CH(COOH), 2. The regioselectivity is approximately 90:10 in favor of 1 when R is a phenyl or a straight chain alkyl group; 2 is the favored product when R is t-Bu and the exclusive one when R is SiMe3. The products of internal alkyne reactions are (E)-CR(COOH)=CHR', 3, and (E)-CHR=CR'(COOH), 4; the regioselectivity in this case is not as high as for terminal alkynes. Oxalic acid can be used instead of formic acid in both of these reactions. The most suitable phosphine ligands for alkyne hydrocarboxylation in the present system are PPh3 and dppb (1,4-bis(diphenylphosphino)butane). In some cases, using a mixture of these two ligands remarkably improves the reaction yields; the implications of such ligand synergism are discussed. On the basis of deuterium labeling studies and other experimental results, a reaction mechanism has been proposed which involves the addition of the O-H bond of formic acid to the palladium center, forming a cationic hydrido(alkyne)palladium intermediate. This intermediate is believed to undergo a sequence of reactions, including hydride and CO insertions, to furnish the organic product(s) and regenerate the active catalyst. The postulated insertion steps have been discussed in light of the present results and literature precedents.