Olefin epoxidation with transition metal η²-Peroxo complexes: The control of reactivity

The activation energies for olefin epoxidation with Mimoun-type η²-peroxo complexes have been calculated using density functional methods. Six degrees of freedom of the complex [MOL(O₂)(OER₃)] and the olefin CH₂CHR′ have been systematically modified. The calculations were based on the assumptions that the reaction follows a concerted oxygen-transfer mechanism suggested by Sharpless and that a peroxo oxygen atom trans to the phosphane oxide ligand is transferred. This was recently proved for the epoxidation of ethylene with the parent complex [MoO(O₂)₂{OP(CH₃)₃}]. It has been found that the diperoxotungsten complexes (M = W; L = O₂) are more reactive than the diperoxomolybdenum complexes (M = Mo; L = O₂). The activation barriers for the monoperoxomolybdenum complexes (M = Mo; L = O) are significantly higher than the barriers for the corresponding diperoxo complexes (M = Mo; L = O₂), whereas equal activation energies have been predicted for both tungsten compounds (M = W; L = O₂ and O). The influence of the pnicogen oxide OER₃ on epoxidation activity is comparably small, while electron-releasing substituents R′ at the C=C bond reduce the activation barrier. The transition states for the epoxidation of alkenes with conjugated double bonds show a large extent of asymmetry, with the C-O bond at the terminal carbon atom being formed first. Additional ligands L′ coordinating to the metal center inhibit oxygen transfer. The results are in agreement with an electrophilic attack of the oxidant on the C=C double bond.