The Diels-Alder reaction of 1,3,5-triphosphabenzene with phosphaacetylene

The Diels-Alder reaction between 1,3,5-triphosphabenzene and phosphaacetylene to yield tetraphosphabarrelene was examined at MP4SDQ/6-31G*//M2/6-31G*. The results were compared with those for the carbon analogue: benzene+acetylene to give barrelene. The phosphorus reaction is more exothermic than the carbon one, as is expected, as benzene possesses more resonance energy than triphosphabenzene. The activation barrier for the phosphorus reaction is 17.5 kcal mol(-1), significantly smaller than the barrier in the carbon case (44.6 kcal mol(-1)). This difference is consistent with our previous studies, which suggest a 10 kcal mol(-1) decrease in activation energy for each C=P involved in the Diels-Alder reaction. Topological electron density analysis suggests that both reactions are synchronous and concerted.