Competition between the one-step and two-step, zwitterionic mechanisms in the [2+3] cycloaddition of gem-dinitroethene with (Z)-C,N-diphenylnitrone: a DFT computational study

DFT calculations have proven clearly the polar nature of gem-dinitroethene cycloaddition to (Z)-C,N-diphenylnitrone. The formation of the azolidine ring on both analysed [2+3] cycloaddition pathways occurs in the gas phase according to the one-step mechanism. When toluene is added as the reaction medium, the kinetic pathway preference does not change. However, the mechanism for the formation of the 2,3-diphenyl-4,4-dinitroisoxazolidine heterocyclic ring does change. In the first reaction stage, a zwitterionic intermediate forms, which is only later cyclized to the [2+3] cycloadduct.