Palladium(II)-catalyzed catalytic aminocarbonylation and alkoxycarbonylation of terminal alkynes: Regioselectivity controlled by the nucleophiles

The aminocarbonylation and alkoxycarbonylation reactions of terminal alkynes took place smoothly and efficiently using a catalyst system Pd(OAc) ₂-dppb-p-TsOH-CH₃CN-CO under relatively mild experimental conditions. The catalytic system was tested and optimized using two different nucleophiles: alcohols and amines. Phenylacetylene (1a) was considered as an alkyne along with diisobutylamine (2b₁) andmethanol (2c₁) as nucleophiles. The results showed significant differences in the conversion of 1a and in the selectivity towards the gem or trans unsaturated esters or amides with these nucleophiles. The effects of the type of palladium catalysts, the type of ligands, the amount of dppb and the solvents were carefully studied. With diisobutylamine (2b₁), excellent regioselectivity towards the 2-acrylamides (gem isomer, 3ab₁) was almost always observed, while trans-α,β-unsaturated esters 4ac₁ was the predominant product with methanol (2c₁) as a nucleophile. This remarkable sensitivity in the selectivity of the reaction indicates two different possible mechanistic pathways for these carbonylation reactions.