Electrophilic additions to styrylsilanes: the effect of changing the ligands on silicon

Styrylsilanes readily undergo addition of carbon electrophiles and protons. The products of the reaction depend upon the non-participating substituents on silicon. Thus, while (E)-β-(trimethylsilyl)styrene 4 readily reacts with electrophiles, the reaction products did not contain silicon or new CC bonds; even in the presence of aryl-substituted carbon electrophiles, the favored reaction was protiodesilylation. In contrast, (E)-β-(trichlorosilyl)styrene 2 did not participate in the reaction with carbon electrophiles or reasonably strong protic acids. However, with triflic acid, 2 cleanly and diastereoselectively dimerized producing after methylation 25, as shown by an X-ray crystal structure analysis. The simple change of a methyl for a chloro group 3 under the same conditions produced a different diastereomer 19 along with a trimer 21. The reasons for the changes in reaction mechanism are discussed. © 1995.