Reactions of αα′-dibromocycloalkanones with bases

Abstract

Reactions of αα′-dibromocycloalkanones [(1) and (2)] with various bases in several solvents have been studied. Treatment of 2,5-dibromocyclopentanone (1) with bases such as sodium hydrogen carbonate, sodium acetate, sodium phenoxide, and secondary amines afforded the corresponding 2-substituted cyclopent-2-enones [(3), (4), (5), and (8)]. Similar treatment of 2,6-dibromocyclohexanone (2) with weak nucleophilic bases gave 2-substituted cyclohex-2-enones (9) and (10), but reactions with secondary amines possessing both strong basicity and high nucleophilicity gave 2-aminocyclohex-2-enones (13a–c) and cyclopent-1-enecarboxamides (14a–c). The ratio of (13) to (14) is strikingly affected by the reaction solvent: (13) or (14) was obtained selectively when hexamethylphosphoramide or chloroform was used, respectively. Allied reactions are described and mechanisms are suggested.