A mechanistic study of reactions of stable disilenes with haloalkanes

Mechanisms of the reactions of three tetrakis(trialkylsilyl)disilenes and a tetraaryldisilene with various haloalkanes such as carbon tetrachloride, c hloroform, dichloromethane, which gave the corresponding 1-alkyl-2-chlorodi silanes and/or 1,2-dichlorodisilanes, were investigated in detail. As evide nced by an ESR observation of an intermediate radical, these reactions were quite unusual, farming neutral radical pairs from two closed shell molecul es at the first step; no similar reactions have been observed between alken es and haloalkanes. Low oxidation potentials of these disilenes, large nega tive activation entropies, and solvent effects for the rates are in good ac cord with the direct halogen abstraction of disilenes from haloalkanes inst ead of single-electron transfer at the rate-determining first step. The str ucture-reactivity relationship of the reactions and the Hammond postulate s uggest that the transition state structures for the first step are similar to those for the halogen abstraction by silyl radicals, but more product-li ke.