Reassessment of the level of theory required for the epoxidation of ethylene with dioxiranes

High level ab initio and CASSCF calculations on the epoxidation of ethylene with dioxirane (DO) and dimethydioxirane (DMDO) have been carried out to distinguish between a symmetrical versus an unsymmetrical spiro orientation of the dioxirane in the transition structure for oxygen atom transfer. The optimized C-1 DO/ ethylene unsymmetrical spiro CASSCF(12,12)/6-31G(d) transition structure is a first-order saddle point that is 5.5 kcal/mol lower in energy than the corresponding constrained C-S symmetrical approach (a second-order saddle point) after correction for dynamic correlation [CASSCF(MP2)]. However, a single-point energy correction at the BD(T)/6-311 +G(d,p) level on these CASSCF geometries suggests that the C-1 TS is only 0.6 kcal/mol lower in energy than an symmetrical spiro TS. Both the BD(T) correction to the CAS(8,8)/6-31+G-(d,p) and RSPT2 energy correction on CAS(8,8)/6-31G(d) DO/ethylene structures slightly favors the C-S structures. BD(T)/6-311+G(d,p) single point calculations on the C-1 and C-S transition structures optimized at the CCSD(T)/6-31G(d) level slightly favor the unsymmetrical TS. These combined data suggest that the potential energy surface for the approach of dioxirane to the C=C of ethylene is very soft with the C-1 transition structure being slightly favored. For DMDO epoxidation, we conclude that the approach of DMDO to a symmetrically substituted alkene should result in an essentially symmetrical transition structure.