Regioselective Hydrosilylations of Propiolate Esters with Tris(trimethylsilyl)silane
journal contributionposted on 21.01.2005, 00:00 by Yang Liu, Shoko Yamazaki, Shinichi Yamabe
Lewis acid and substituent dependency on the regioselectivity of hydrosilylation of propiolate esters 1a−c with tris(trimethylsilyl)silane (2a) was found. The reaction of methyl and ethyl propiolate esters and 2a without Lewis acid and in the presence of EtAlCl2 and Et2AlCl gave β-silicon-substituted Z-alkenes 3 selectively. On the other hand, reaction in the presence of AlCl3 in dichloromethane gave α-silicon-substituted alkenes 4. In the case of trifluoroethyl propiolate ester 1c, reaction with aluminum chloride-based Lewis acids gave α-silicon-substituted alkenes 4 exclusively. Two competitive mechanisms, free-radical and ionic, are proposed as the source of the complementary regioselectivity displayed in these reactions. A transition state of the radical-forming step was obtained computationally. The reaction of various reactive acetylene substrates and 2a without Lewis acid and without solvent at room temperature gave β-silicon-substituted Z-alkenes 3 selectively.