SUBSTITUENT EFFECTS ON TORQUOSELECTIVITY - COMPUTATIONAL INVESTIGATIONS OF THE THERMAL RING OPENINGS OF CYCLOBUTENES FUSED TO SUBSTITUTED CYCLOHEXENES

Ab inito (RHF/3-21G to MP2/6-31G*) and force field (MacroModel/MM2*, MM3*) calculations were carried out to investigate the torquoselectivity of ring openings of cyclobutenes with substituents which sterically influence the selectivity (Piers ct al. J. Org. Chem. 1989, 54, 2267; Tetrahedron Lett. 1993, 34, 1875). Transition state modeling gave a good correlation with experimental selectivity, while relative product stabilities did not correlate with selectivities. The difference between ab initio and force field relative activation energies of monosubstituted cyclobutene systems gave the electronic preference for inward or outward substituent rotation. By adding this electronic preference to the relative activation energies of highly substituted systems calculated with MM3*, a reasonable approximation for the torquoselectivity of these systems was made.