Kinetic evidence for the occurrence of a stepwise mechanism in the aminolysis of maleimide. General acid and base catalysis by secondary amines

Abstract

The reaction rates for nucleophilic cleavage of maleimide have been studied for buffered solutions of eight secondary amines. The reactions of secondary amines with ionized and non-ionized malemide are subject to general acid and general base catalysis, respectively. Non-liner plots of the apparent second-order rate constants at a given pH against total buffer concentrations reveal that the rate-determining step chages as the buffer concentration is increased; this provides evidence for the existence of an intermediate on the reaction path. The rate constants for specific acid-catalysed cleavage of an oxy monoanionic tetrahedral addition intermediate (T–) display a Brønsted plot of slope (βnuc) 0.37. The rate constants for general base-catalysed cleavage of non-ionized maleimide (SH) reveal a Brønsted plot of slope (βnuc1) 0.79. The significantly high value of βnuc1 is attributed to a normal stepwise mechanism (i.e. diffusion-contraolled trapping mechanism) in which a zwitterionic tetrahedral addition intermediate (T±) is converted into T– by general base catalysis. The rate constants for general acid-catalysed