Hydrodefluorination and Other Hydrodehalogenation of Aliphatic Carbon-Halogen Bonds Using Silylium Catalysis

Trialkylsilylium cation equivalents partnered with halogenated carborane anions (such as Et3Si[HCB11H5Cl6]) function as efficient and long-lived catalysts for hydrodehalogenation of C-F, C-Cl, and C-Br bonds with trialkylsilanes as stoichiometric reagents. Only C(sp(3))-halogen bonds undergo this reaction. The range of C-F bond-containing substrates that participate in this reaction is quite broad and includes simple alkyl fluorides, benzotrifluorides, and compounds with perfluoroalkyl groups attached to an aliphatic chain. However, CF4 has proven immune to this reaction. Hydrodechlorination was carried out with a series of alkyl chlorides and benzotrichlorides, and hydrodebromination was studied only with primary alkyl bromide substrates. Competitive experiments established a pronounced kinetic preference of the catalytic system for activation of a carbon halogen bond of a lighter halide in primary alkyl halides. On the contrary, hydrodechlorination of C6F5CCl3 proceeded much faster than hydrodefluorination of C6F5CF3 in one-pot experiments. A solid-state structure of Et3Si[HCB11H5Cl6] was determined by X-ray diffraction methods.