Elimination–addition mechanisms of acyl-group transfer: the neutral and alkaline decomposition of 1-(N-methylcarbamoyl)imidazoles

Abstract

Aqueous hydrolysis of 1-(N-methylcarbamoyl)imidazoles exhibits a pH-dependence consistent with E1cB pathways through neutral and cationic forms of the substrate. The derivatives of the most basic imidazoles have the greatest overall hydrolytic reactivity. There is little steric requirement for decomposition via either mechanism despite the relatively high crowding in the most hindered species studied; the slight enhancement of the hydrolysis rate for 1-(N-methylcarbamoyl)-2,4,5-trimethylimidazole is judged to be due to a gain in rotational entropy during departure of the leaving imidazolyl group. The ionisation of imidazoles and of their conjugate acids have nearly identical selectivities to substituent change.