Stereoselective Synthesis of Trisubstituted α,β-Unsaturated Esters and Amides via Reactions of Tantalum-Alkyne Complexes Derived from Acetylenic Esters and Amides with Carbonyl Compounds

Treatment of acetylenic esters with low-valent tantalum (TaCl₅ and Zn) in DME and benzene produces tantalum-alkyne complexes (not isolated), which react with carbonyl compounds regioselectively at the α-position of the esters to give Z isomers of trisubstituted α,β-unsaturated esters in a stereoselective manner. In contrast, tantalum-alkyne complexes derived from acetylenic amides react with carbonyl compounds at the β-position of the amides predominantly.