Mechanistic study of ester olefinations using dimethyltitanocene

Olefination of esters using dimethyltitanocene is proposed to proceed via the titanium carbene, Cp(2)Ti=CH2, on the basis of the following evidence: (1) No H/D exchange or C-13 scrambling occurs in the reactions of ethyl acetate or dodecyl acetate using either labeled ester or labeled dimethyltitanocene, (2) kinetics are zero order in ester and first order in Cp(2)TiMe(2), (3) the reactions of ethyl acetate and dodecyl acetate with Cp(2)Ti(CD3)(2) produce substantial kinetic isotope effects of 9-10, (4) esters of differing electronic and steric environments have similar reactivity, and (5) there is regiochemical preference for the least stable exo-methylene product. These findings are inconsistent with the methyl addition mechanism proposed in the literature.