Homolytic Reduction of Various Haloalkanes

The Hydrosilane Reduction II

Abstract

Reduction of a variety of haloalkanes with organosilicon hydrides was carried out under homolytic conditions. While the fluorides and iodides employed were found to be quite inert, the bromides and chlorides could be readily reduced. The mode of the reduction is consistent with a radical chain mechanism previously proposed. The apparent reactivities (Br>Cl>>F, I) can be reasonably understood by applying the Hirschfelder rules to the halogen atom abstraction step in the presence of the organosilyl radicals. The vicinal dibromide underwent radical debromination rather than bromine-hydrogen exchange, presumably due to the reversible addition of bromine atoms to olefins. Simple alkyl bromides, however, are less reactive than carbon tetrachloride. This seems to imply that the polar factor is also important in determining the relative rates of the halogen atom abstraction.