A competition between carbo and hetero Diels-Alder reactions of E-2-phenyl-1-cyano-1-nitroethene to cyclopentadiene in the light of B3LYP/6-31G(d) computational study

The competitive reaction paths (Scheme 1) 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.