Ab initio investigation of the diels‐alder reaction between 2H‐phosphole and phosphaethene: A model for phosphole dimerization

All four possible Diels-Alder reactions between 2H-phosphole and phosphaethene were examined at various theoretical levels, including HF, MP4SDQ, CCSD(T), and CASSCF. MP2/6-31G* geometry optimizations could not be employed since the potential energy surface is qualitatively incorrect at this level of theory, due to the inherent underestimation of the activation energies (ameliorated at higher-order MP or coupled-cluster levels). Solvent effects were examined employing the Onsager, polarized continuum, and isodensity and surface polarized continuum models. At MP4SDQ/6-31G*//HF/6-31G* these reactions are exothermic by 34–38 kcal mol−1 and have very low activation energies, 5–7 kcal mol−1. The PP/CC regioisomer products are lower in energy than the CP isomers and, within each pair, the exo isomer is lower in energy. At low computational levels the smallest activation energy is for the reaction leading to the CP endo product. Larger basis sets, electron correlation, and solvent favor the transition state leading to the experimentally observed PP/CP endo isomer. The dimerization of phosphole is, therefore, kinetically controlled. Based on geometric and electron density analysis, the reactions are concerted and synchronous. © 1997 by John Wiley & Sons, Inc.