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.

中文翻译：

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硝酮和缺电子乙炔间两性离子型[3+2]环加成反应中分子机制和C-O键形成的电子局域化函数分析

研究了涉及两性离子物种的 [3+2] 环加成反应的机械性质，并表征了与 O-C 和 C-C 键形成相关的电子密度沿本征反应坐标的变化。这种极性 [3+2] 环加成反应通过非协调的两阶段一步机制发生，以 21 kcal mol-1 的中等吉布斯自由活化能进行。反应开始于在中心碳上产生假自由基中心，首先在乙炔二甲酸二甲酯上，其次在硝酮骨架上。这立即有利于通过氧原子孤对电子密度的捐赠形成第一个 O-C 单键，这代表了该环加成反应中最具吸引力的中心。