The reaction of hydrazones with chlorine and bromine

Abstract

The reactions of six hydrazones with bromine in the presence of pyridine, and of two hydrazones with chlorine under a variety of conditions, were investigated. Geminal dihalides, haloalkenes, rearranged products, azines, and carbonyl compounds account for up to 85% of the products of the reaction with bromine, but the reactions with chlorine give rise to more complex mixtures. The mechanisms of these reactions are discussed and compared with the analogous reactions with iodine. The differences in the distribution of products for different halogens and bases can be rationalized in terms of competing a-eliminations and nucleophilic substitutions of common intermediate X-halogeno compounds. However, the reaction with chlorine is not entirely straightforward. Further evidence is presented for the intermediacy of halocarbonium ions in the reactions of hydrazones with halogens in the presence of bases.