Reactions between Tantalum- or Niobium-Alkyne Complexes and Carbonyl Compounds

A variety of tantalum-alkyne complexes are generated in situ by treatment of alkynes with low-valent tantalum derived from TaCl₅ and zinc. These complexes add regioselectively to carbonyl compounds in a one-to-one fashion to yield (E)-allylic alcohols stereoselectively. Iodinolysis of the oxatantalacyclopentene, which is postulated as an intermediate of the reaction, gives a (Z)-3-iodo-2-propen-1-ol derivative. In contrast to tantalum-alkyne complexes, niobium-alkyne complexes, prepared with low-valent niobium derived from NbCl₅ and zinc, add in situ to aldehydes in a one-to-two fashion to give 1,3-diene derivatives. The dienes are produced through (i) addition of the alkyne complexes with 2 equiv of aldehydes at cis vicinal positions of the alkenes and (ii) deoxygenative elimination of 2,7-dioxanioba-4-cycloheptene complexes.