Olefin epoxidation with transition metal eta(2)-peroxo complexes: The control of reactivity

The activation energies for olefin epoxidation with Mimoun-type eta (2)-per oxo complexes have been calculated using density functional methods. Six de grees of freedom of the complex [MOL(O-2)(OER3)] and the olefin CH2CHR ' ha ve been systematically modified. The calculations were based on the assumpt ions that the reaction follows a concerted oxygen-transfer mechanism sugges ted by Sharpless and that a peroxo oxygen atom irans to the phosphane oxide ligand is transferred. This was recently proved for the epoxidation of eth ylene with the parent complex [MoO(O-2)(2){OP(CH3)(3)}]. It has been found that the diperoxotungsten complexes (M = W; L = O-2) are more reactive than the diperoxomolybdenum complexes (M = Mo; L = O-2). The activation barrier s for the monoperoxomolybdenum complexes (M = Mo; L = O) are significantly higher than the barriers for the corresponding diperoxo complexes (M = Mo; L = O-2), whereas equal activation energies have been predicted for both tu ngsten compounds (M = W; L = O-2 and O). The influence of the pnicogen oxid e OER3 on epoxidation activity is comparably small, while electron-releasin g substituents R ' at the C=C bond reduce the activation barrier. The trans ition states for the epoxidation of alkenes with conjugated double bonds sh ow a large extent of asymmetry, with the C-O bond at the terminal carbon at om being formed first. Additional ligands L ' coordinating to the metal cen ter inhibit oxygen transfer. The results are in agreement with an electroph ilic attack of the oxidant on the C=C double bond.