Ab initio and density functional theory (DFT) study on [1,5] sigmatropic rearrangements in pyrroles, phospholes, and siloles and their Diels−Alder reactivities

Abstract

CCSD(T) and B3LYP calculations were done on 1H-, 2H-, and 3H-pyrroles, phospholes, and siloles to account for the relative stabilities, activation energies for sigmatropic hydrogen shifts, and Diels−Alder reactivities with two model dienophiles. The computed barrier heights for sigmatropic rearrangements in phospholes account for the observed reversible equilibrium at higher temperatures. Both CCSD(T) and B3LYP calculations support the experimental observation that 1H- and 2H-phospholes lead to the Diels−Alder adduct of 2H-phosphole and validate Mathey's proposal that 1H-phosphole (1P) converts to 2H-phosphole (2P) prior to the participation in the Diels−Alder reactions. In contrast to what is observed in phospholes, the sigmatropic shifts in pyrroles and siloles require higher activation energies compared to the Diels−Alder reactions, which indicate no sigmatropic shifts prior to cycloadditions. Distortion energies of dienophiles explain the small discrepancy between the activation energies and exothermicities in the reactions involving ethylene and acetylene. The frontier molecular orbital (FMO) analysis and the extent of charge transfer values from diene to dienophile are also used to assess the Diels−Alder reactivities of all of the dienes considered in this study.