ABINITIO STUDY OF THE CONROTATORY RING-OPENING OF PHOSPHACYCLOBUTENES AND AZACYCLOBUTENES .2. DIPHOSPHACYCLOBUTENES AND DIAZACYCLOBUTENES

The thermally allowed conrotatory ring opening of 1,2-dihydro-1,3-diphosphate (1), trans- and cis-1,2-dihydro-1,2-diphosphete (2 and 3), 1,2-dihydro-1,3-diazete (4), and trans- and cis-1,2-dihydro-1,2-diazete (5 and 6) is examined using ab initio calculations. The rings, transition structures, and products were fully optimized at HF/6-31G* with single-point energy calculations performed at MP2. The opening of the dihydrodiphosphetes is endothermic while the opening of the dihydrodiazetes is exothermic. The calculated activation barrier for the opening of 3 and 6 is 19.78 and 24.58 kcal mol-1, respectively. The ring opening of 1, 2, 4, and 5 can occur via two diastereomeric pathways. Inward rotation of the heteroatom lone pair is favored for all four compounds. The lower barriers are: 28.78 kcal mol-1 for 1, 18.00 kcal mol-1 for 2, 27.82 kcal mol-1 for 4, and 22.95 kcal mol-1 for 5. The structural and energetic differences and trends among these compounds are interpreted in terms of ring strain and orbital interactions.