An electron localization function analysis of the molecular mechanism and the C-O bond formation in the [3+2] cycloaddition reaction involving zwitterionic type between a nitrone and an electron deficient ethyne

The mechanistic nature of a [3 + 2] cycloaddition reaction involving zwitterionic species has been investigated, and the changes of electron density related to the O-C and C-C bond formation along the intrinsic reaction coordinate have been characterized. This polar [3 + 2] cycloaddition reaction, which takes place through a non-concerted two-stage one-step mechanism, proceeds with a moderate Gibbs free activation energy of 21 kcal mol(-1). The reaction begins by the creation of a pseudoradical centre at the central carbon, first on the dimethyl acetylenedicarboxylate, and second on the nitrone framework. This immediately favours the formation of the first O-C single bond by donation of some electron density of the oxygen atom lone pairs, which represents the most attractive centre in this cycloaddition reaction.