Electrooxidative N-halogenation of 2-azetidinone derivatives

Electrolysis of 2-azetidinone 3 in AcOH/CH₃CN containing NaBr (2-10 equiv.) was carried out in an undivided cell fitted with two platinum electrodes to afford the corresponding N-bromo-2-azetidinones 2a, while a similar electrolysis in a divided cell afforded no appreciable amount of 2 but a small amount of 4-acetoxy-2-azetidinone 1 together with a complex mixture. Ring-expansion of 2-azetidinone 3 leading to 4 took place exclusively by electrolysis in methanol containing AcONa. On the other hand, N-iodination proceeded efficiently only in a divided cell in the presence of 2.5 equiv. of NaI. Reaction of N-bromo-2-azetidinone 2 with various nucleophiles, e.g., acetate, thiolate, and alkoxides, afforded no substitution products, resulting in the reductive debromination leading to 3, whereas with diphenyl disulfide, N-phenylsulfenyl-2-azetidinone 5 was mainly obtained.