Barriers to rotation in acylguanidines and acylguanidinium ions

Abstract

Using both ¹H and ¹³C n.m.r. spectroscopy, the free energies of activation (ΔG_c^‡) values for barriers to rotation in 2-dimethylamino-2-imidazolin-4-one (1a), an acylguanidine, and its hydrochloride salt (1b), an acylguanidinium ion, have been determined to be 15.6 ± 0.1 and 17.6 ± 0.2 kcal mol^–1, respectively. In both cases, the rotational barrier about the exocyclic carbon–nitrogen bond was involved. Resonance arguments are used to rationalize these experimental results, and ab initio theoretical calculations are presented that successfully reproduce the relative order of experimentally determined barriers to rotation in guanidinium ions, acylguanidines, and acylguanidinium ions.