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

Abstract

Vanadium(V) complexes of the tridentate bis(phenolate)pyridine ligand H(2)BPP (H(2)BPP = 2,6-(HOC(6)H(2)-2,4-(t)Bu(2))(2)NC(5)H(3)) and the bis(phenolate)amine ligand H(2)BPA (H(2)BPA = 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)V(V)(O)(O(i)Pr) (4) with pinacol afforded the monomeric vanadium(IV) product (BPP)V(IV)(O)(HO(i)Pr) (6) and acetone. Vanadium(IV) complex 6 was oxidized rapidly by air at room temperature in the presence of NEt(3), yielding the vanadium(V) cis-dioxo complex [(BPP)V(V)(O)(2)]HNEt(3). Complex (BPA)V(V)(O)(O(i)Pr) (5) reacted with pinacol at room temperature, to afford acetone and the vanadium(IV) dimer [(BPA)V(IV)(O)(HO(i)Pr)](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.

Hydrogenation and cleavage of the C-O bonds in the lignin model compound phenethyl phenyl ether over a nickel-based catalyst

“…For example, Toste's group [1] developed a series of vanadium-oxo complexes and applied them to the non-oxidative cleavage of the C-O bonds of dimeric lignin model compounds to give aryl enones with 95% conversion. Hansen's group [2][3][4] also studied a vanadium complex catalyzed lignin model oxidation reaction, and reported that the C-O and C-C bonds could be selectively cleaved by changing the ligand involved in the catalysis [5]. The use of hydrogenation as another potential method for the conversion of lignin has also been the subject of considerable attention.…”

Vanadium catalyzed aerobic carbon–carbon cleavage

“…Studies on the stoichiometric oxidation of pinacol have been carried out also with complexes V2 and V3 [67]. Reaction of V2 with pinacol (5 equiv.)…”

“…In this case, the complete transformation of V3 into light purple reduced V16 was observed (Scheme 14), even if the reaction required longer E. Amadio et al / Coordination Chemistry Reviews xxx (2015) xxx-xxx Scheme 10. Vanadium complexes V1-11 used in aerobic oxidative carbon-carbon cleavage of diols [66,67,[86][87][88][89].…”

“…Very interesting and promising results have recently been reported by both the groups of Hanson and co-workers [67,[86][87][88][89][90].…”

Reductive Cleavage of C—O Bond in Model Compounds of Lignin

“…[3][4][5] The 4-O-5 linkage (e.g., diaryl and aryl alkyl ethers) is a representative C-O linkage in lignin, and its catalytic cleavage is considered as an efficient way to depolymerize lignin for production of aromatics. [3,4,[6][7][8][9][10][11] Various metal catalysts such as Ni, [12][13][14][15] Ru, [16,17] Cu, [18][19][20][21] Pd/Zn, [22] V [23][24][25][26] and Fe, [27] have been reported for catalysing the cleavage of aryl C-O in lignin model compounds including diaryl ethers and aryl alkyl ethers via oxidation or hydrogenlysis. However, harsh reaction conditions including high temperature and complicated reaction systems are generally required, and the target product yields are relatively low.…”