Pentacoordinate silicon intermediates in relay substitution reactions of organosilanes: successive nucleophilic attack at silicon and its adjacent carbon

Chloromethylsilyl chlorides of the type, ClCH 2 R 2 SiCl, have been shown to undergo kinetically controlled nucleophilic attack at silicon, with either phenols or amines (PhEH, with E  O or NMe), to yield products of the type, PhESiR 2 CH 2 Cl. Subsequent treatment of the latter products with KF in acetonitrile causes the nucleophile to be shifted from silicon to carbon with the formation of FSiR 2 CH 2 EPh. Alternatively, treatment of ClCH 2 R 2 SiCl simultaneously with PhEH and KF yields the same thermodynamically controlled product directly, FSiR 2 CH 2 EPh. This possibility of directing nucleophilic attack toward silicon or its adjacent carbon has great importance in the synthesis of substituted organosilanes. The mechanism of these so-called relay nucleophilic substitutions is proposed as involving pentacoordinate silicon intermediates. This concept of initial nucleophilic attack at silicon and the subsequent relaying of the nucleophile from the pentacoordinate silicon to the adjacent carbon is also shown to be applicable to the unusual reactions of α,β-epoxyalkylsilanes.