Ab Initio Studies of the Contrasting Butadiene Cheletropic and Diels−Alder Cycloaddition Reactivities Observed for “Carbenic” Phosphorus (Phosphenium) and Arsenic (Arsenium) Cations

Abstract

Quantum-chemical studies (UMP2/6-311G*//UHF/6-311G*) on a series of phosphenium and arsenium derivatives confirm singlet ground states and reveal an unexpected model for the bonding in their lowest triplet state. Models of the novel solid-state dimer structures that are experimentally observed for some arsolanium cations reveal large positive dimerization energies (P, 3, 277 kJ/mol; As, 7, 207 kJ/mol), implying that the observed dimers are imposed by crystal-packing phenomena. The surprising contrast observed in the butadiene cycloaddition reactivities for phosphenium and arsenium cations are understood in terms of the calculated absolute energies of the observed structural arrangements for the cycloadducts in each case; the cheletropic adduct 4 is 52 kJ/mol more stable than the Diels−Alder adduct for the phospholanium cation, while in the case of the arsolanium cation the Diels−Alder adduct 9 is favored by 59 kJ/mol.