Carbocations in the β-Lactam and β-Thiolactam Series

Abstract

Solvolyses of mesylate and trifluoroacetate derivatives of 3-aryl-3-hydroxy-substituted β-thiolactams and β-lactams proceeds to give substitution products in a process which exhibits a large solvent effect as well as large substituent effects. A common ion rate suppression as well as largely racemic products from solvolyses of an optically active substrate all point to the involvement of β-lactam derived carbocationic intermediates. Unlike acyclic analogs, these cations are captured by solvent to give simple substitution products with no competing proton loss. Computational studies suggest that proton loss from these β-lactam derived cationic intermediates is an unfavorable process due to antiaromatic character in the potential elimination product. Computational studies also suggest that CS and CO conjugative stabilization of these cations is minimal or non-existent. Substituent effect studies show that the major mode by which these cations derive stabilization is by aryl group charge delocalization. Azide ion in DMSO or DMF reacts with N-methyl-3-mesyloxy-3-phenylazetidine-2-thione via a bimolecular substitution mechanism to give the corresponding 3-azido substitution product, which can be converted to N-methyl-3-amino-3-phenylazetidine-2-thione.