Thermal Aromatizations of 2-Vinylmethylenecyclopropane and 3-Vinylcyclobutene

A comprehensive theoretical investigation of thermal rearrangements of 2-vinylmethylenecyclopropane and 3-vinylcyclobutene is carried out employing density functional theory and high level ab initio methods, such as the complete active space self-consistent field, multi-reference second-order Moller-Plesset perturbation theory, and coupled-cluster singles and doubles with perturbative triples. In all computations, Pople's polarized triple-zeta split valence basis set, 6-311G(d,p), is utilized. The potential energy surface for the relevant system is explored to provide theoretical insights for the thermal aromatizations of 2-vinylmethylenecyclopropane and 3-vinylcyclobutene. 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 at high temperatures thermal aromatizations of 2-vinylmethylenecyclopropane (at 700 degrees C and higher) and 3-vinylcyclobutene (at 500 degrees C and higher) are feasible under appropriate experimental conditions. However, at low temperatures (at 500 degrees C and lower), 2-vinylmethylenecyclopropane yields 3-methylenecyclopentene as a unique product, kinetically, and the formation of benzene is not favorable. Similarly, at 300 degrees C and lower temperatures, 3-vinylcyclobutene can only yield trans-1,3,5-hexatriene (major) and cis-1,3,5-hexatriene (minor). At 300 < T < 500 degrees C, 3-vinylcyclobutene almost completely yields 1,3-cyclohexadiene. Hence, our computations provide a useful insight for the synthesis of substituted aromatic compounds. Further, calculated energy values (reaction energies and activation parameters) are in satisfactory agreement with the available experimental results.