Concerted Albeit Not Pericyclic Cycloadditions: Understanding the Mechanism of the (4+3) Cycloaddition between Nitrones and 1,2-Diaza-1,3-dienes

The mechanism of (4+3) cycloaddition reactions of nitrones with 1,2-diaza-1,3-dienes has been studied by using density functional theory (DFT) methods. The cycloaddition reaction takes place through an asynchronous concerted transition state that reflects a two-stage process in which the formation of the first bond occurs close to the transition state, while the second bond forms well after the transition state. The alternative stepwise mechanism is higher in energy than the concerted process. In this mechanism, the nitrone oxygen acts as a nucleophile by attacking the more electrophilic terminal carbon of the 1,2-diaza-1,3-diene to form an intermediate. The second step of the reaction is the rate-limiting one, which is higher in energy than that observed for the concerted process. Topological analysis of the gradient field of electron localization function (ELF) provides a complete characterization of the electron density changes during the course of the reaction.