Thermal denitrogenation of 7-isopropylidene-2,3-diaza-norbornene: formation of substituted 3-methylene-(1,4)-pentadienes

A theoretical study of the thermal denitrogenation of 7-isopropylidene-2,3-diaza-norbornene is carried out by employing density functional theory and high level ab initio methods, such as the complete active space self-consistent field, multi-reference second-order Møller–Plesset perturbation theory, and coupled-cluster singles and doubles with perturbative triples. In all computations Pople's polarized triple-ζ split valence basis set, 6-311G(d,p), is utilized. The potential energy surface for the relevant system is explored to provide a theoretical account for the formation of trienes (major products). For this purpose, a mechanism has been proposed and theoretically studied. The rate constant for each isomerization reaction is computed using the transition state theory. The simultaneous first-order ordinary-differential equations are solved numerically for the considered system to obtain time-dependent concentrations, hence the product distributions at a given temperature. Our results demonstrate that E- and Z-2-methyl-3-ethylidene-1,4-pentadienes are formed via a reaction path originating from 3-isopropylidene-1,4-pentadiene. The result of kinetic analysis agrees well with the experimental observations of Berson et al. (M. R. Mazur, S. E. Potter, A. R. Pinhas and J. A. Berson, J. Am. Chem. Soc., 1982, 104, 6823–6824). Further, the computed percentage of trienes is in general agreement with experiment. Therefore, we conclude that our proposed mechanism satisfactorily explains the experimental observations.