FACILE SYNTHESIS OF ALPHA,BETA-UNSATURATED NIOBIUM ACYLS VIA GAMMA-HYDRIDE ABSTRACTION

The niobocene ketene complexes [C5H4(SiMe3)]2Nb(Cl)(eta-2-C,O-O=C=C(R)CH2R') (1) undergo a facile reaction with Ph3C+BF4- to give the cationic alpha,beta-unsaturated metal acyls of general formula [[C5H4(SiMe3)]2Nb(Cl)(eta-2-C(O)CR=CHR')][BF4]. The reaction is stereospecific for the production of E-alkenes, but these undergo slow isomerization to a mixture of E and Z isomers. If both ketene substituents contain accessible allylic hydrogens, the reaction shows little regioselectivity. Preliminary mechanistic studies are consistent with a direct hydride abstraction rather than an electron-transfer/H atom transfer sequence.