Oxidation of olefins with H₂O₂ catalysed by salts of group III metals (Ga, In, Sc, Y and La): epoxidation versus hydroperoxidation

The catalytic activity of aqua complexes of the group III metals [M(H2O)n]3+ (M = Ga, In, Sc, n = 6; M = Y, n = 8; M = La, n = 9) towards the oxidation of olefins with H2O2 was investigated in detail by theoretical (DFT) methods. It was predicted and then confirmed in a preliminary experiment that these complexes formed from simple soluble salts in aqueous medium are able to efficiently catalyse the olefin oxidation. The reaction occurs via two competitive reaction channels which are realized concurrently, i.e. (i) hydroperoxidation of the allylic C atom(s) via a radical Fenton-like mechanism involving HO˙ radicals and leading to alkyl hydroperoxides ROOH and (ii) epoxidation of the CC bond through a one-step mechanism involving oxygen transfer from the hydroperoxo ligand in an active catalytic form [M(H2O)n−k(OOH)]2+ (M = Ga, In, Y, La, k = 2; M = Sc, k = 1) to the olefin molecule and leading to epoxides and/or trans-diols. Other concerted and stepwise mechanisms of the epoxidation were also considered but found less favourable.