Ab initio studies on the mechanism of cycloaddition reactions of isocyanates and ethylene

Abstract

The cylcoaddition reactions of ethylene and isocyanates (HNCO, FNCO or CH 3NCO) leading to 2-azetidiones were studied theoretically. For the HNCO + H 2C CH 2 system, the stationary points are located by RHF/3-21G, RHF/6-31G, RHF/6-31G ∗, MP2/3-21G and MP2/6-31G respectively. However, for the FNCO + H 2C  CH 2 or CH 3NCO + H 2C  CH 2 systems, HF/6-31G and HF/6-31G ∗ have been used. All reactions studied are believed to be concerted. Four orbitals are involved in these reactions, which are of the “2 × [1 + 1]” type. The order of reactivity of the different isocyanates is FN C O > HN C O or CH 3NCO, which is explained in terms of frontier orbital interactions.