Effect of the substituents at the silicon of (omega-chloroalkyl)silanes on the alkylation to benzene

Abstract

(omega-Chloroalkyl)silanes [Cl3-mMemSi(CH2)(n)-Cl: m = 0-3, n = 1-3] underwent Friedel-Crafts alkylation with benzene in the presence of aluminum chloride to give alkylated products. Such alkylation reactions took place at temperatures ranging from room temperature (m=0-1, n =2, 3; m = 3, n= 1) to 80 (m = 1, 2; n := 1) and 200 degrees C (m =0; n = 1), depending on the substituent(s) of the silicon and the alkylene-chain s:pacer between the silicon and C-CI bond of (omega-chloroalkyl)silanes. In the alkylation to benzene, the reactivities of (omega-chloroalkyl)silanes increase as the number (m) of methyl-group(s) at the silicon and the alkylene length between the silicon and C-CI bond increases. While decomposition of alkylation products was observed at two more methyl groups substituted at silicon in the cases of (chloromethyl)silanes such as (chloromethyl)dimethylchlorosilane and (chloromethyl)trimethylsilane. The reaction with (chloromethyl)trimethylsilane occurred at room temperature to give trimethylchlorosilane, toluene and xylene via a decomposition reaction of the products. No (trimethylsilylmethyI)benzene was formed. In the alkylation to benzene, the reactivity of (omega-chloroalkyl)silanes decreases in the following order: m = 3 > 2 > 1 > 0; n=3>2 much greater than 1. The results are consistent with the stability of the carbocation generated by the complexation of (omega-chloroalkyl)silanes with aluminum chloride. (C) 2000 Elsevier Science S.A. All rights reserved.