Reaction of tantalum-alkyne complexes with isocyanates or acyl cyanides. Stereoselective functionalization of carbon-carbon triple bonds

Treatment of alkynes with low-valent tantalum derived from TiCl5 and zinc produces tantalum-alkyne complexes (not isolated), which react in situ with phenyl isocyanate (or butyl isocyanate) to give (E)-α,β-unsaturated amides stereoselectively. The tantalum–alkyne complexes also react with acyl cyanides in the presence of BF3·OEt2 to give α-cyanohydrins. In both reactions, filtration of the reaction mixture containing the tantalum–alkyne complexes before addition of isocyanates (or acyl cyanides) is indispensable to obtain good yields.