Crystallographic, NMR and ab initio calculation studies of tautomerism among substituted thiazol-2-yl-hydrazines

The structures of (+/-)-dihydrothiazolyl hydrazones 3a, b have been fully characterized by combining crystallography, NMR measurements in solution, and computational methods. The thiazolidine structure of the heterocycle is confirmed both by crystallography and in solution. The stability of the endocyclic N3 tautomeric form is confirmed by energy calculation at the MP2/6-31+G(d) level. The anti-E conformation observed in the crystal structures is retained in solution, in agreement with stability prediction.