Reaction of diazomethane with selenoesters : Preparation of α-(alkyl- or arylseleno)methyl ketones and methyl ketones

Abstract

The reaction of diazomethane with a series of selenoesters 1 in the presence of CuI, CuSePh or Cu powder produced α-(alkyl- or arylseleno)methyl ketones 2 in yields of 41–65%. Methyl ketones 3 and bis(arylseleno)methanes 9 or 14 were formed as by-products. The direct conversion of selenoesters to methyl ketones was accomplished in high yield by the usual reaction with diazomethane, followed by workup with HBr solution. The simultaneous copper-catalyzed reactions of selenoesters 1c and 1i with diazomethane resulted in crossover, with the formation of all four possible α-seleno ketones 2b, 2c, 2h and 2i. A non-concerted mechanism involving attack by the diazo compound upon the acyl carbon atom of an activated selenoester with the formation of a tetrahedral intermediate 11 has been suggested. The reaction of the selenothiocarbamate 4 with diazomethane resulted in 1,3-dipolar cycloaddition to afford 5 instead of insertion into the acyl-selenium bond.