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 e vidence: (1) No H/D exchange or C-13 scrambling occurs in the reaction s of ethyl acetate or dodecyl acetate using either labeled ester or la beled dimethyltitanocene, (2) kinetics are zero order in ester and fir st order in Cp(2)TiMe(2), (3) the reactions of ethyl acetate and dodec yl acetate with Cp(2)Ti(CD3)(2) produce substantial kinetic isotope ef fects of 9-10, (4) esters of differing electronic and steric environme nts have similar reactivity, and (5) there is regiochemical preference for the least stable exo-methylene product. These findings are incons istent with the methyl addition mechanism proposed in the literature.