Regioselectivity in the Reaction of Tantalum-Unsymmetrical Acetylene Complexes with Carbonyl Compounds. Stereoselective Preparation of 1-Alkenyl Sulfides, α,β-Unsaturated Esters, and Amides

Tantalum-alkyne complexes, derived by treatment of alkynes with low-valent tantalum (TaCl₅ and zinc), react in situ with carbonyl compounds to give (E)-allylic alcohols stereoselectively. When unsymmetrical acetylenes are employed in the reaction, two regioisomeric allylic alcohols are produced. The regioselectivity of the reaction depends on the steric and electronic effects of the substituents on the acetylenes. For example, treatment of tantalum-alkyne complexes derived from methyl alkynyl sulfides with carbonyl compounds yields (E)-3-hydroxy-1-propenyl methyl sulfides in a regioselective manner. Tantalum-alkyne complexes derived from acetylenic esters react with carbonyl compounds regioselectively at the α-position of the esters to give Z-isomers of trisubstituted. α,β-unsaturated esters. In contrast, tantalum-alkyne complexes derived from acetylenic amides react with carbonyl compounds predominantly at the β-position of the amides. The regioselectivity of the reaction between acetylenic amides and aldehydes, however, cannot be explained solely in terms of the steric and electronic effects of the substituents. Strong coordination of the amide group to the tantalum center could also be responsible for the observed selectivity, which is opposite to that observed with tantalum-acetylenic ester complexes.