Reactions from a-halocarbonyl compounds

The carbon-halogen bonds next to a carbonyl group undergo easy reduction in the presence of a metal, which is significantly improved by sonication. 

From a,a -dibromoketones, oxyallyl cations are the intermediates leading to a-acyloxy ketones (Eq. 73), along with the dehalogenated ketone 40 319320 Q O 

Formally the process is comparable to an electrochemical reduction on mercury drops (p. 290), but lower yields (30-75%) are obtained sonochemically than electro-chemically ( 90%). With unsymmetrical dibromoketones, mixtures of two isomeric a-acetoxy ketones are obtained, but the selectivity is enhanced by using bulkier acids, e.g., trimethyl- or triethyl acetic acid. Oxyallyl cations from a,a -dibromoketones add to olefins or dienes in a [3 + 2] or [3 + 4] mode, the Noyori or Hoffmann-Noyori reaction. 21,322 

The generation of the oxyallyl cation is generally accomplished from polybromo ketones and the toxic and sensitive iron carbonyls. Hoffmann et al introduced a simple and safer sonochemical preparation in the presence of the zinc-copper couple. The reactions are rapid in dioxane and the cycloadducts are obtained in 

The Reformatsky reaction is one of the first organometallic reactions studied under ultrasound irradiation,323 because of the known difficulties in the initiation step. Han and Boudjouk observed rates and yield (98% in 5 min of sonication in a bath) much higher in some typical reactions (Fig. 63) than commonly reached conventionally.324 

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