Metal-free electrochemical Reformatsky reaction in water: further evidence for a radical mechanism

The first report of an electrochemical Reformatsky reaction in aqueous medium and in the absence of metal is presented. Ethyl bromoisobutyrate (1) and benzaldehyde (2) were electrolyzed in a divided cell at a potential of −1.2 V versus Ag/AgCl, corresponding to the reduction wave of 1. Several types of carbon cathodes and a platinum anode were used in a 0.1 M KBr solution in either water+methanol (5:1) or water. The Reformatsky adduct ethyl 2,2-dimethyl-3-hydroxy-3-phenyl propionate (5) was obtained with yields up to 46% (relative to consumed 2) together with varying amounts of ethyl isobutyrate (3) and diethyl 2,2,3,3-tetramethylsuccinate (4). The probable mechanism starts with a radical species generated from 1 which adds to benzaldehyde giving a radical intermediate, which is further reduced to the final product 5. A radical chain through bromine abstraction from 1 is also possible. No products resulting from reduction of benzaldehyde were observed. On zinc electrodes, in an undivided cell, an organometallic reaction producing the same coupling product 5 becomes more important. The results represent a strong additional support for a radical mechanism postulated in previous work for the Zn-promoted chemical Reformatsky reaction in water.