Kinetics and mechanism of tertiary amine-catalysed aqueous cleavage of maleimide

Abstract

The reaction rates of the aqueous cleavage of maleimide in the presence of buffer solutions of trimethylamine, 1,4-diazabicyclo[2.2.2]octane, triethylamine, and triethanolamine have been studied at 30 °C. Both the ionized (S) and non-ionized (SH) forms of maleimide are reactive towards the free base component of buffer solutions of trimethylamine and 1,4-diazabicyclo[2.2.2]octane. These amines have exhibited general acid-catalysed and uncatalysed nucleophilic attack at carbonyl carbon of both SH and S^–. Triethylamine has been found to be essentially unreactive towards SH and S^– while triethanolamine has revealed significant nucleophilic reactivity towards SH and S^–. The observed nucleophilic reactivity of triethanolamine and absence of such reactivity in the case of triethylamine have led to the proposal that the nucleophilic site in triethanolamine is the internally hydrogen-bonded hydroxy group rather than the free amine group. The absence of intermolecular general acid catalysis in the reactions of SH and S^– with triethanolamine is attributed to both the steric effect as well as probable intramolecular general acid catalysis. A pre-association stepwise mechanism is proposed for the reactions of tertiary amines with S^–while a stepwise mechanism has been favoured for the reactions of amines with SH.