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Abstract: 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. [Copyright &y& Elsevier]

Abstract: The competitive reaction paths () for the carbo and hetero Diels–Alder reaction of E-2-phenyl-1-cyano-1-nitroethene (1) to cyclopentadiene (2) were examined using the B3LYP exchange–correlation functional and the 6-31G(d) basis set. The calculated activation enthalpies indicate that preference of the paths increases in the order: A = C > B > D > F > E. In the gas phase, all reactions occur via pre-reaction complexes, which on the paths A, B, C and E resemble orientation complexes. On path A the initially formed 2-phenyl-3-cyano-4-aza-5-oxy-bicyclo-[3,4,0]-nona-3,7-diene N-oxide (5) is converted to endo-nitronorbornene 3 as a result of a [3.3]-sigmatropic shift. On path B, which yields exo-nitronorbornene 4, and pathways C–F, which yield 2-phenyl-3-cyano-4-aza-5-oxy-bicyclo-[3,4,0]-nonadienes N-oxides 6–8, the reaction proceeds according to a concerted mechanism. When a solvent is introduced into the reaction environment, activation barriers are slightly reduced and the degree of formation of new σ-bonds in transition structures is lowered. The solvent effect however is not sufficient to induce a change of the reaction mechanism or the reaction path preference. The global electrophilicity and electron chemical potential of the reagents 1 and 2 harmonise with the data of the B3LYP/6-31G(d) simulations. [Copyright &y& Elsevier]