Model theoretical study of 2 + 2 cycloadditions of dialkoxyethynes with heterocumulenes

Abstract

The 2 + 2 cycloaddition reactions of dihydroxyethyne (4), as a model for a dialkoxyalkyne, with hydroxyketene (5), isocyanic acid (6), formyl isocyanate (7), and isothiocyanic acid (8), as models for heterocumulenes, have been studied by the MNDO method. All the reactions are predicted to take place stepwise through very short-lived dipolar intermediates. Since MNDO tends to describe reactions as non-concerted due to overestimation of interatomic repulsions at distances around the van der Waals separation, these reactions can be properly classified as a borderline situation between a highly non-synchronous concerted mechanism and a stepwise one.