Computational study of the "stable" bis(amino)silylene reaction with halomethanes. A radical or concerted mechanism?

The reaction of bis(amino)silylene with XCH3, X = Cl, Br, I, has been studied computationally using DFT with flexible basis sets. A radical process where a halogen atom is abstracted from halomethane is predicted to be much more favorable than oxidative addition of the halomethane to the divalent silicon center. A chain mechanism is proposed that consists of a chain-initiation step (halogen abstraction) followed by competing chain-propagation steps. In one branch, the methyl-substituted bis(amino)silylene abstracts a halogen from XCH3 to form an observed product (the 1:1 adduct), releasing a methyl radical. In the other branch, the methyl-substituted bis(amino)silylene is intercepted by another bis(amino)silylene, which, in turn, can abstract a halogen from XCH3 to form the other observed product (the 2:1 adduct) and release a methyl radical. In the series, XCH3, X = Cl, Br, and I, we predict an increase of the 1:1 adduct-producing pathway over the 2:1 adduct-producing pathway, which is consistent with experimental observations. The reactivity of bis(amino)silylene indicates a greater similarity to disilene rather than to previously suggested phosphines.