Effect of methyl and vinyl substitution on the geometries, relative stabilities and Diels-Alder reactivities of phospholes: a DFT study

Abstract

Equilibrium geometries and the relative stabilities of 1H-, 2H- and 3H-phospholes and their methyl- and vinyl-substituted analogs were computed at the B3LYP/6–31G* level. The transition states and products were located for the Diels–Alder reactions of all the dienes with ethylene. The effect of substitution on the geometries, relative stabilities and Diels–Alder reactivities of 1H-, 2H- and 3H-phospholes were analyzed. The relative stabilities predict that the substitution on phospholes prefers phosphorus over carbon. Computations show that the activation energies are not altered much by substitution of either methyl or vinyl groups on phospholes, but most of the substituted phospholes require slightly higher activation energies than the corresponding parent phospholes. 2H-phospholes require lower activation energies than 1H- and 3H-phospholes. The reactivity ordering based on FMO energy gaps and the activation energies are similar and follows the sequence 2H- > 3H- > 1H-phospholes. The quantaum of charge-transfer values from diene to dienophile at the transition states and distortion energies were calculated.