Activity of peroxo and hydroperoxo complexes of Ti-IV in olefin epoxidation: A density functional model study of energetics and mechanism

Epoxidation of olefins by Ti-IV peroxo and hydroperoxo (alkylperoxo) comple xes was investigated using a hybrid DFT method (B3LYP). Reaction energies a nd activation barriers for direct oxygen transfer to ethylene as a model ol efin were computed for various model complexes to compare the epoxidation a ctivity of Ti(eta(2)-O-2) and TiOOR (R = H, CH3) moieties. The activity of complexes with a Ti(O-2) peroxo group is shown to be essentially quenched w hen the coordination sphere of the complex is saturated by strongly basic ( sigma-donor) ligands. In contrast, the activity of a TiOOH functional group depends only weakly on the saturation of the coordination sphere of the Ti center. Substitution of methyl for hydrogen in a TiOOH group is found to s lightly increase the activation barrier of epoxidation. The computational r esults give preference to reaction paths that involve TiOOR species. The fa ctors governing the activity of Ti(O-2) and TiOOR groups, in particular the effects of donor Ligands, are discussed on the basis of a molecular orbita l analysis.