Diels−Alder Reaction of an Ene−Phospha−Yne

Abstract

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 (ΔGrxn = −2.70 kcal mol-1), reaction 2 is endothermic (ΔGrxn = 9.38 kcal mol-1, though ΔHrxn = −3.23 kcal mol-1). The activation barrier for reaction 2 is, however, 4 kcal mol-1 lower than for reaction 1 (ΔG⧧ = 35.14 vs 39.22 kcal mol-1, respectively). The ring strain energies for 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.