Oxovanadium-Catalyzed Epoxidation of Methyl Oleate: Ligand Effects

High Resolution Image Download MS PowerPoint Slide 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.