Reactivity of (E)-2-aryl-1-cyano-1-nitroethenes in carbo and hetero Diels-Alder reactions with cyclopentadiene: A DFT study

The competitive reaction paths for carbo- (CDA) and hetero (HDA) Diels-Alder-type cycloadditions of cyclopentadiene to (E)-2-aryl-1-cyano-1-nitroethenes with different electrophilicity were examined on the basis of reactivity indexes and PES calculations. The calculations indicate that the reaction studied shows polar character, which deepens with increase of the nitroalkene electrophilicity. The reaction course is controlled by nucleophilic attack of cyclopentadiene on the nitroalkene with concomitant ring closure. PES analysis has confirmed that the transition state on kinetically most favored reaction path exhibit highly asymmetric zwitterionic character. However, all attempts to find a zwitterionic intermediate failed, even in the case of the most electrophilic (E)-2-(p-nitrophenyl)-1-cyano-1-nitroethene.