Aerobic Oxidation Reactions Catalyzed by Vanadium Complexes of Bis(Phenolate) Ligands

Vanadium­(V) complexes of the tridentate bis­(phenolate)­pyridine ligand H2BPP (H2BPP = 2,6-(HOC6H2-2,4-tBu2)2NC5H3) and the bis­(phenolate)­amine ligand H2BPA (H2BPA = N,N-bis­(2-hydroxy-4,5-dimethylbenzyl)­propylamine) have been synthesized and characterized. The ability of the complexes to mediate the oxidative C–C bond cleavage of pinacol was tested. Reaction of the complex (BPP)­VV(O)­(OiPr) (4) with pinacol afforded the monomeric vanadium­(IV) product (BPP)­VIV(O)­(HOiPr) (6) and acetone. Vanadium­(IV) complex 6 was oxidized rapidly by air at room temperature in the presence of NEt3, yielding the vanadium­(V) cis-dioxo complex [(BPP)­VV(O)2]­HNEt3. Complex (BPA)­VV(O)­(OiPr) (5) reacted with pinacol at room temperature, to afford acetone and the vanadium­(IV) dimer [(BPA)­VIV(O)­(HOiPr)]2. Complexes 4 and 5 were evaluated as catalysts for the aerobic oxidation of 4-methoxybenzyl alcohol and arylglycerol β-aryl ether lignin model compounds. Although both 4 and 5 catalyzed the aerobic oxidation of 4-methoxybenzyl alcohol, complex 4 was found to be a more active and robust catalyst for oxidation of the lignin model compounds. The catalytic activities and selectivities of the bis­(phenolate) complexes are compared to previously reported catalysts.