Reaction Pathway Discrimination in Alkene Oxidation Reactions by Designed Ti-Siloxy-Polyoxometalates

Titanium complexes of silanol functionalized polyoxometalates (THA)(3)[SbW9O33(RSiO)(3)Ti((OPr)-Pr-i)] (Ti-SiloxPOMs) catalyze alkene oxidation with tert-butyl hydrogen peroxide (TBHP). However catalytic activity and product distribution in the oxidation of allylic alcohols are shown to depend on the steric surrounding generated by the SiloxPOM (R=Bu-t, Pr-i, Pr-n). Combined experimental and computational studies clarify how steric repulsions between the oxidant (Bu-t group) and the surrounding SiloxPOM govern the reaction pathways leading either to oxidation of the alcohol function (R=Bu-t) or to alkene epoxidation (R=Pr-n). Moreover, another consequence of this steric repulsive interactions is that outer-sphere mechanisms become competitive with the inner-sphere ones (coordination of allylic alcohol), whether for the oxidative dehydrogenation reaction or for the epoxidation reaction. In the case of unfunctionalized olefins (linear and cyclic), we show that reducing the bulkiness surrounding the active site leads to higher conversion to epoxide, emphasizing that these Ti-SiloxPOMs may behave as structural and functional models for metal single-site in Ti-Silicates.