Iodine oxidation of arylacetones in alkaline methanol

Abstract

The reactions of deoxybenzoin (1) and phenylacetones (2) with iodine have been studied in methanol containing sodium methoxide. The reaction of (1) gives 1,2-dibenzoyl-1,2-diphenylethane (4)(an oxidative coupling product) and benzoin (3)(an epoxidation product) in the ratio 0.2–1.6. The ratio is proportional to the initial concentration of (1), suggesting the intermediacy of α-benzoyl-α-iodotoluene (12). Diphenylacetone afforded only an intramolecular oxidative coupling product, methyl 2,3-diphenylpropionate. The reaction of PhCHMeCOCH₃ yields PhCHMeCO₂Me (7)(a halogenoform reaction product). PhCHMeCH₂CO₂Me (5)(a Favorskii reaction product), and a small amount of PhCMeCHCO₂Me (6). The ratio of products (7) and (5) is proportional to the concentration of the remaining I₂ in the alkaline methanol, which implies that the iodination of ketones involves an attack by molecular iodine on the enolate ion. The effect of the ring substituents in PhCHMeCOCH₃ on the (7) : (5) ratio (ρ 0.5–0.40; r 0.992) along with the observed position of iodination imply a transition state containing bridged H⁺ and I₂ for iodination of the enolate ion of arylacetones.