Reductive Chlorination and Bromination of Ketones via Trityl Hydrazones.

A method is presented for the direct transformation of a ketone to the corresponding reduced alkyl chloride or bromide. The process involves the reaction of a ketone trityl hydrazone with tBuOCl to give a diazene which readily collapses to the α-chlorocarbinyl radical, reduction of which by a hydrogen atom source gives the alkyl chloride product. The use of N-bromosuccinimide provides the corresponding alkyl bromide. This unique transformation provides a reductive halogenation that complements Barton's redox-neutral vinyl halide synthesis.