Reinvestigation of tandem dimerization and 1,3-alkyl shift in 3-benzylbenzothiazolylidene carbene: Experimental and theoretical results

Abstract

The reaction of 3-benzylbenzothiazolium bromide with triethylamine generates initially two products as revealed by the TLC. However, on purification, only one compound is obtained, which is the same compound as reported by Baldwin and co-worker. A theoretical investigation at the B3LYP/6–31+G(d) level could identify two possible routes involving successive dimerization and 1,3-benzyl shift of the N-heterocyclic carbene, namely 3-benzylbenzothiazolylidene leading to two isomeric products. The second step is found to be the rate-determining step. The activation free energy barriers of both steps for the isomer actually obtained are comparatively lower than for the other product. Furthermore, the two isomers are found to be atropisomers separated by a small activation free energy barrier which is insufficient to keep the identities of the two isomers intact, with the result that the less stable atropisomer changes into the more stable one. Thus it has been possible to rationalize the experimental results by the theoretical investigation.