ABINITIO STUDY OF THE CONROTATORY RING-OPENING OF PHOSPHACYCLOBUTENES AND AZACYCLOBUTENES .1. MONOPHOSPHACYCLOBUTENES AND MONOAZACYCLOBUTENES

The allowed conrotatory ring-opening reactions of 1,2-dihydrophosphete (1), 2,3-dihydrophosphete (2), 1,2-dihydroazete (3), and 3,4-dihydroazete (4) were examined at the HF/6-31G* and MP2/6-31G* levels. Reasonable activation barriers were obtained only with the inclusion of electron correlation; however, geometry optimization at MP2 did not significantly change the geometry from those obtained at the HF level. The opening of the dihydrophosphetes is thermoneutral or slightly endothermic, while the opening of the dihydroazetes is exothermic. The calculated activation barriers for the opening of 2 and 4 are 40.76 and 37.08 kcal mol-1, respectively. The opening of 1 and 3 can occur via two diastereomeric pathways. Inward rotation of the heteroatom lone pair is favored for both systems; the lower barrier is 24.59 kcal mol-1 for 1 and 29.76 kcal mol-1 for 2. The differences in these reactions are compared and explained in terms of ring strain and orbital interactions.