Effects of silyl substituents on the palladium-catalyzed asymmetric synthesis of axially chiral (allenylmethyl)silanes and their S _E2′ chirality transfer reactions

摘要

A series of axially chiral 4-substituted-1-silyl-2,3-butadienes [(allenylmethyl)silanes] were synthesized from 3-bromo-5-silylpenta-1,3-dienes by a Pd-catalyzed asymmetric reaction with a soft nucleophile. The optically active (allenylmethyl)silanes react with an acetal in the presence of TiCl _4 to give the enantiomerically enriched 1,3-diene derivatives through an S _E2′ pathway. Effects of the silyl groups on the enantioselectivity of the asymmetric allene synthesis and the subsequent S _E2′ chirality transfer reaction were studied. It was found that as the steric bulk of the silyl groups in the 3-bromo-5-silylpenta-1,3-dienes was increased from -SiMe _3 to -SiiPr _3, the enantioselectivity of the two enantioselective processes also improved.