Diels-Alder reaction of an ene-phospha-yne

The Diels-Alder reactions of ethylene with but-3-en-1-yne (reaction 1) or 1-phosphabut-3-en-1-yne (Reaction 2) were examined at the MP4SDTQ/6-311G**/MP2/6-311G** level. While reaction 1 is exothermic (Delta G(rxn) = -2.70 kcal mol(-1)), reaction 2 is endothermic (Delta G(rxn) = 9.38 kcal mol(-1), though Delta H-rxn = -3.23 kcal mol(-1)). The activation barrier for reaction 2 is, however, 4 kcal mol(-1) lower than for reaction 1 (Delta G(double dagger) = 35.14 vs 39.22 kcal mol(-1), respectively). The ring strain energies far the two cyclic cumulene products are 29.56 kcal mol(-1) for 1,2-cyclohexadiene and 23.24 kcal mol(-1) for 1-phospha-1,2-cyclohexadiene.