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-Bu-t(2))(2)NC5H3) and the bis(phenolate)amine ligand H(2)BPA (H(2)BPA = N,N-bis(2-hydroxy-4,5dimethylbenzyl)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)V-V(O)((OPr)-Pr-i) (4) with pinacol afforded the monomeric vanadium(IV) product (BPP)V-IV (3)((HOPr)-Pr-i) (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)V-V(O)(2)]HNEt3. Complex (BPA)V-V(O)((OPr)-Pr-i) (5) reacted with pinacol at room temperature, to afford acetone and the vanadium(IV) dimer [(BPA)V-IV(O)((HOPr)-Pr-i)](2). Complexes 4 and 5 were evaluated as catalysts for the aerobic oxidation of 4-methoxybenzyl alcohol and arylglycerol beta-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.