CARBOCATIONS IN THE BETA-LACTAM AND BETA-THIOLACTAM SERIES

Solvolyses of mesylate and trifluoroacetate derivatives of 3-aryl-3-hy droxy-substituted beta-thiolactams and beta-lactams proceeds to give s ubstitution products in a process which exhibits a large solvent effec t as well as large substituent effects. A common ion rate suppression as well as largely racemic products from solvolyses of an optically ac tive substrate all point to the involvement of beta-lactam derived car bocationic intermediates. Unlike acyclic analogs, these cations are ca ptured by solvent to give simple substitution products with no competi ng proton loss. Computational studies suggest that proton loss from th ese beta-lactam derived cationic intermediates is an unfavorable proce ss due to antiaromatic character in the potential elimination product. Computational studies also suggest that C=S and C=O conjugative stabi lization of these cations is minimal or non-existent. Substituent effe ct studies show that the major mode by which these cations derive stab ilization 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 s ubstitution product, which can be converted to N-methyl-3-amino-3-phen ylazetidine-2-thione.