The mechanism of C--X (X=F, Cl, Br, and I) bond activation in CX4 by a stabilized dialkylsilylene.

The potential-energy surfaces for the abstraction and insertion reactions of dialkylsilylene with carbon tetrahalides (CX4) have been characterized in detail using density functional theory (B3LYP), including zero-point corrections. Four CX4 species, CF4, CCl(4), CBr4, and CI(4), were chosen as model reactants. The theoretical investigations described herein suggest that of the three possible reaction paths, the one-halogen-atom abstraction (X abstraction), the one-CX3-group abstraction (CX3 abstraction), and the insertion reaction, the X-abstraction reaction is the most favorable, with a very low activation energy. However, the insertion reaction can lead to the thermodynamically stable products. Moreover, for a given stable dialkylsilylene, the chemical reactivity has been found to increase in the order CF4<<CCl(4)<CBr4<CI(4), that is, the heavier the halogen atom (X), the more facile is its reaction with a stable dialkylsilylene. In particular, halogen abstraction is always predicted to be much more favorable than abstraction of a CX3 group from both energetic and kinetic viewpoints. In brief, electronic as well as steric factors play a crucial role in determining the chemical reactivity of the haloalkane species, kinetically as well as thermodynamically. Our conclusions based on the results of our theoretical investigations are in accordance with available experimental observations. Furthermore, a configuration-mixing model based on the work of Pross and Shaik has been used to rationalize the computational results. The results obtained allow a number of predictions to be made.