AB-INITIO STUDY OF THE DIELS-ALDER REACTION BETWEEN PHOSPHOLES AND ETHYNE

The reaction of cyclopentadiene with ethyne (reaction 1) and 1H-, 2H-, and 3H-phosphole with ethyne (reactions 2-4) were examined at the MP4SDQ/6-31G*//MP2/6-31G* level. The quality of this computational level was evaluated by comparison of the geometries and energetics of reaction 1 with experimental results. The activation energies of reactions 1-4 are 27.94, 30.62, 17.93, and 28.14 kcal mol(-1). These reactions all proceed through a common mechanism-the classic concerted [4 + 2] cycloaddition-as determined by analysis of the geometry, activation energy, and electron distribution. These calculations are then used to evaluate the mechanistic proposal of Mathey concerning the reactions of substituted 1H-phosphole with alkynes.