Theoretical studies on the stabilities of 1,2- and 1,3-isomers of cyclic H₄A₂X₂ (A=C, Si; X=O,S,CH₂): the anomeric effect at silicon

Abstract

1,2- and 1,3-isomers of dioxetanes, dithietanes, and their silicon analogues (1, 2) have been studied by Molecular Orbital methods (using both HF/3-21G and MNDO). The 1,3-isomers (2) are found to be more stable than their 1,2-counterparts (1) except for dithietane where the energy difference is negligible. The relative energies of the 1,2- and 1,3-isomers are also estimated using bond energy differences (eqn. 1) at the 3-21G level on models. These are corrected for the anomeric effect (eqn. 2) present in the 1,3-isomer and compared to the 3-21G estimate of the relative energies of 1 and 2. The discrepancy between the two estimates is equated to the strain energy difference between the two four-membered rings. This is found to be maximum for dioxetane and disilacyclobutane. The anomeric effect at silicon (10.0 kcal mol^-1 at the 3-21G level for oxygen substitution) is smaller than that at carbon (17.4 kcal mol^-1 at the same level) for the same substituent.