Ab initio studies on the mechanism of cycloaddition reactions of isocyanic acid and methylenimine

Abstract

The cycloaddition reactions of isocyanic acid and methylenimine leading to four- or six-membered ring products were studied theoretically. If the reaction is equimolar, a two-step mechanism via a cis intermediate is confirmed. The second step is a rate-controlling one with an energy barrier of 28.3 kcal mol −1. In addition, the cis intermediate (INT) can further react with another methylenimine or isocyanate molecule to form a six-membered ring adduct. For the system INT + H 2 C NH, the energy barrier is only 2.76 kcal mol −1, while for the system INT + HN C O, it is 5.02 kcal mol −1. This is consistent with the experimental fact that when one of the reactants is in excess, a six-membered ring adduct is formed exclusively.