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Regioselective Hydrosilylations of Propiolate Esters with Tris(trimethylsilyl)silane

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journal contribution
posted 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 1ac 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.