Oxovanadium-Catalyzed Epoxidation of Methyl Oleate: Ligand Effects

The development of catalytic reactions based on earth-abundant first-row transition metals that use chemicals from renewable feedstocks aligns with current principles of sustainable chemistry. Here, we employ oxovanadium-(IV) salen-type complexes ([VO-(Ln)], n = 1-5: 1-5) as catalysts for the selective epoxidation of biodiesel-derived methyl oleate, where Ln are tetradentate salen-type ligands with different diamine linkers, namely ethylenediamine (1), 1,3-diaminopropane (2 and 5), diaminomaleonitrile (3), and 1,2-diaminocyclohexane (4). The 1,3-diaminopropane system was examined with both unsubstituted (p-H) (2) and substituted (p-OMe) (5) salicylaldehyde-aromatic rings. DFT analysis revealed the influence of the ligand on the electronics of the VO moiety, with [VO-(L3)] exhibiting the most electron-deficient vanadium center. Notably, this complex also proved to be the most efficient epoxidation catalyst under optimized conditions (1 mol % catalyst, 3.5 equiv oxidant-TBHP, no added solvent). UV-Vis spectroscopy monitoring of the reaction between the complexes 1-5 with excess oxidant (pseudo-first order conditions) highlighted pronounced ligand-dependent differences in reactivity. Linear kinetics were observed only for [VO-(L2)] and [VO-(L5)], both containing a 1,3-diaminopropane bridge. In contrast, compounds with saturated two-carbon bridges [VO-(L1)] and [VO-(L4)] reacted slowly with the oxidant, displaying an induction period. Finally, [VO-(L3)] does not react with the oxidant under the same conditions, suggesting an alternative epoxidation mechanism via a V-(IV) center in the initial stage of catalysis. These results demonstrate that vanadium salen-type complexes, although structurally similar, enable epoxidation through either the commonly proposed V-(V)-peroxide pathway or a V-(IV) Lewis-acidic center, depending on the nature of the moiety bridging the two imine nitrogens.