Rearrangements of (chloromethyl)organylsilanes induced by Lewis acids or by Lewis bases: Regioselective methylenation of allylic systems

Abstract

AbstractIn order to study the scope and mechanisms of the migration of organyl groups from silicon to its adjacent methylene carbon, a series of derivatives of the type RSiMe2CH2Cl were synthesized, where R is vinyl, benzyl, methallyl, phenylethynyl, aryloxy, and N‐methylanilino. Attempts were then undertaken to induce such Si → C shifts by either the Lewis acid, methylaluminum dichloride, or the Lewis base, potassium fluoride. Under Lewis acidic conditions, only the benzyl and methylallyl groups could be induced to migrate; clearly, in the case of benzyl (and probably in the case of methallyl), migration was accompanied by allylic rearrangement to produce the o‐tolyl group. Under the agency of KF, all the groups investigated underwent rearrangement to produce FSiMe2CH2R. By a crossover experiment, it was shown that, when R = aryloxy, the rearrangement occurs intermolecularly. Experimental evidence indicates that all KF‐induced migrations are probably intermolecular and all MeAlCl2‐induced migrations considered here are intramolecular. The advantages for organosilicon synthesis of utilizing so‐called “relay substitution reactions” with the ClSiR2CH2Cl system are discussed: equation image With unsymmetrical allylic systems, regioselective methylenation now becomes possible, simply by inducing the Si → C rearrangement by MeAlCl2 or by KF.