A Qualitative Examination of the Effects of Silicon Substituents on the Efficiency of Cross-Coupling Reactions

A qualitative study of the effects of various substituents on the silicon atom in cross-coupling reactions of alkenylsilanes has been carried out. In intermolecular competition experiments, the influence of carbon-based groups (methyl, ethyl, isopropyl, <i>tert</i>-butyl, phenyl, and 3,3,3-trifluoropropyl) and alkoxy groups (monoethoxydimethyl-, diethoxymethyl-, and triethoxy) on the silicon have been evaluated under activation by two different methods, fluoride (TBAF) and silanolate (TMSOK). The influence of the substituents was highly dependent on the method of activation. In the presence of TBAF, there was only a modest steric effect (except for <i>tert-</i>butyl substituents), and the efficiency decreased slightly with increasing numbers of alkoxy groups. In the presence of TMSOK, a significant steric effect was noted, but the number of alkoxy groups had almost no influence. These trends were interpreted in terms of the divergent mechanisms for the cross-coupling process.