Origins of the Loss of Concertedness in Pericyclic Reactions:  Theoretical Prediction and Direct Observation of Stepwise Mechanisms in [3 + 2] Thermal Cycloadditions

Several [3 + 2] thermal cycloadditions between azomethine ylides and nitroalkenes have been studied both theoretically and experimentally. When the N-metalated 1,3-dipoles are used, the reaction is stepwise. The corresponding zwitterionic intermediates have been located computationally and observed by NMR monitoring. In the case of N-unsubstituted azomethine ylides, the reaction can be concerted or stepwise, depending upon the ability of the substituents to stabilize zwitterionic intermediates. A general model is proposed to explain the observed phenomena. This simple model can be extended to other thermal cycloadditions to predict the stepwise or concerted nature of their mechanisms without computing complete reaction coordinates.