AM1 and PM3 transition structures for the epoxidation of alkenes and allene by methylated dioxiranes

AM1 and PM3 calculations reveal that epoxidations of ethylene (ET) by dimethyldioxirane (DMD) and methyl(trifluoromethyl)dioxirane (MTMD), and 1,2-dichloroethylene (DCE) and allene by DMD pass through a spiro transition structure (TS) and in the reactions, peroxide bond sigma* level takes part at a very early stage. MTMD is found to be the most reactive dioxirane as the CF3 group in it stabilizes this sigma* level. Differential preference of cis- to trans-DCE in the reaction is explained as due to steric repulsion by the chlorine atom on the approaching dioxirane during the reaction and the calculated results are thus more favorable for Z-epoxide formation over E-epoxide. Compared to ethylene, allene is found to be less reactive in the epoxidation by DMD. Computed AM1 and PM3 barriers agree reasonably with the available experimental results. Deformation energy analysis provide new insight into the factors involving in the activation process. The bond order analysis clearly reveals the extent of bond make-break at TS, and through that, explains the reaction barrier. (C) 1997 Elsevier Science B.V.