Torsional Control of Epoxidation Stereoselectivity in 1,2-Dihydronaphthalenes. Transition State Modeling with Semiempirical Quantum Mechanics

Recently, highly diastereoselective epoxidations of chiral 1,2-dihydronaphthalenes were reported. Crystallographic characterization of the major product indicated that the selectivity cannot be attributed to long-range steric effects. We examined this system through ab initio and semiempirical quantum mechanical modeling. Corroborating experimental data, our investigations explain the apparent irrelevance of direct interactions with substituents and address the insensitivity of the stereoselectivity to oxidant structure. Furthermore, extension of this model to other related epoxidation reactions indicates that pi-facial discrimination may be interpreted as a result of a preference for a staggered asynchronous transition state, obtainable only through axial attack.