Aerobic Oxidation of 2-Phenoxyethanol Lignin Model Compounds Using Vanadium and Copper Catalysts

Abstract

Lignin is the most abundant renewable aromatic-containing macromolecule in Nature. Intensive research efforts are underway to obtain additional value from lignin beyond current low-value heating. Aerobic oxidation has emerged as one promising alternative for the selective depolymerization of lignin, and a variety of models for the most abundant β-O-4 linkage have been employed. In this work, aerobic oxidation of the simple β-O-4 lignin models 2phenoxyethanol (2) and 1-phenyl-2-phenoxyethanol (3) were investigated using the oxovanadium complex (HQ) 2 V V (O)(O i Pr) (HQ = 8-oxyquinolinate) and CuCl/TEMPO/2,6-lutidine as catalysts in several different solvents at 100 °C (TEMPO = 2,2,6,6tetramethylpiperidine-1-oxyl). Using the vanadium catalyst, reactions proceed more readily in pyridine (vs dimethyl sulfoxide) presumably via an initial base-assisted alcohol dehydrogenation followed by oxidative C−C and C−O bond cleavage to afford phenol, formic acid and CO 2 . In contrast, the copper-catalyzed reactions suffer from extensive formylation of the substrate and radical coupling to give TEMPO-functionalized products. These results suggest that use of more complex β-O-4 lignin models is required for accurate comparison of selective oxidation catalysts.

Oxidative C−C Cleavage of Ketols over Vanadium–Carbon Catalysts

“…As a result of the non‐green nature of such oxidants, catalytic systems in which O 2 is used as an oxidant have been sought. Typical catalysts for oxidative C−C cleavage with O 2 are based on Ru, Mn, Cu, and V . Ru and Mn catalysts can convert aromatic vicinal diol to aldehydes in good yields and they are less effective for aliphatic substrates, especially under neutral or acidic conditions.…”

“…[1,4,5] As ar esult of the non-greenn ature of such oxidants, catalytic systemsi nw hich O 2 is used as an oxidant have been sought. Typical catalysts foro xidative CÀCc leavage with O 2 are based on Ru, [6][7][8][9][10] Mn, [11] Cu, [12][13][14] and V. [10,13,[15][16][17][18][19][20][21][22] Ru and Mn catalysts can convert aromatic vicinald iol to aldehydes in good yields and they are less effective for aliphatic substrates, especially under neutralo ra cidic conditions. Substrates for Cu-catalyzed CÀCo xidation are also aromatic compounds, typicallyt hose with benzoyl groups.H owever,h omogeneous vanadium catalysts are active in the oxidative CÀCc leavage of aliphatic ketols such as 2-hydroxycyclohexanone [10,15,16] and sugars [17][18][19][20][21][22] to adipic acid and formic acid, respectively (Scheme 1).…”

Selective Cα Alcohol Oxidation of Lignin Substrates Featuring a β‐O‐4 Linkage by a Dinuclear Oxovanadium Catalyst via Two‐Electron Redox Processes

“…We have summarized the catalytic selectivity and conversion between complex 2 and some representative mononuclear vanadium catalysts in Table . [7b], [7d], [8b], In particular, we focus on the alcohol oxidation of non‐phenolic lignin model substrates with a β‐O‐4 linkage mode and primary C γ alcohol. Since the alcohol oxidations of this research work is using air as oxidant, we compared the results of benzyl alcohol oxidation of lignin model substrates in literature that use air as oxidant.…”

Review on Catalytic Cleavage of C–C Inter-unit Linkages in Lignin Model Compounds: Towards Lignin Depolymerisation

“…9) resulted in C-C cleavage under basic condition. However, under their reaction conditions, CuOTf + TEMPO and TEMPO did not result in C-C cleavage [68,69]. Another interesting method for the cleavage of C α -C β linkage was reported by Patil et al [70,71].…”