} } < < Ab Initio Investigation of the Diels ] Alder Reaction between 2H-Phosphole and Phosphaethene: A Model for Phosphole Dimerization ULRIKE SALZNER, STEVEN M. BACHRACH,* and DEBBIE C. MULHEARN Department of Chemistry, Northern Illinois University, DeKalb, Illinois 60115 Received 16 January 1996; accepted 24 April 1996 ABSTRACT All four possible Diels ] Alder reactions between 2 H-phosphole and phosphaethene were examined at various theoretical levels, including HF, Ž. MP4SDQ, CCSD T , and CASSCF. MP2r6-31G* 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 Ž . 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 MP4SDQr6- 31G*rrHFr6-31G* these reactions are exothermic by 34 ] 38 kcal mol y1 and have very low activation energies, 5 ] 7 kcal mol y1 . The P }PrC}C regioisomer products are lower in energy than the C}P 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 C}P endo product. Larger basis sets, electron correlation, and solvent favor the transition state leading to the experimentally observed P }PrC}P endo isomer. The dimerization of phosphole is, therefore, kinetically controlled. Based on geometric and electron density analysis, the reactions are concerted and synchronous. Q 1997 by John Wiley & Sons, Inc. Introduction n 1982, Mathey et al. 1 reported the attempted I synthesis of P-unsubstituted 1 H-phospholes by * Author to whom all correspondence should be addressed. E-mail: smb@smb.chem.niu.edu protonation of 2,5-diphenyl- and 3,4-dimethyl- phospholyl anions. In contrast to the expected 1 H- phospholes, they isolated the dimer I as a colorless solid. The composition of I was determined by elementary analysis, mass, and NMR spec- troscopy. The presence of a P }P bond was con- firmed by 31 P NMR spectroscopy. Crystal struc- ture analysis 2 revealed that I has the endo struc- ( ) Journal of Computational Chemistry, Vol. 18, No. 2, 198 ]210 1997 Q 1997 by John Wiley & Sons CCC 0192-8651 / 97 / 020198-13