Reformatsky reaction in water: Evidence for a radical chain process

The Reformatsky reaction of 2-bromo esters and carbonyl compounds in the presence of zinc can be carried out in concentrated aqueous salt solutions without any cosolvent. The reaction of bromoacetates is greatly enhanced by catalytic amounts of benzoyl peroxide or peracids and gives satisfactory yields with aromatic aldehydes. Preparative yields comparable to those of the traditional procedure are obtained with ethyl 2-bromoisobutyrate. This ester needs no catalyst and reacts even with saturated aldehydes and aromatic ketones, although in law yields. A radical chain mechanism, initiated by electron abstraction from the organometallic Reformatsky reagent, is proposed. Two nonchain pathways, involving radicals directly produced od the metal surface, may compete, especially in the case of secondary and tertiary halides.