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