Non-food based biomass (lignocellulose) is an attractive renewable carbon feedstock for the production of chemicals or fuels. Lignin, a key constituent (15À30%) of lignocellulose, is an irregular polymer composed of methoxy-substituted phenyl and phenolic subunits. 1,2 Because of its structural complexity and inherent resistance to chemical reactivity, lignin has been considered a major obstacle in the production of biofuels from lignocellulose.Recently, increasing attention has been focused on the development of methods to convert lignin into more valuable and useful products. 3,4 Reductive approaches using hydrogen or formic acid have been developed to transform lignin into mixtures of monomeric phenols and alkanes suitable for use as fuel additives. 5À9 Catalytic oxidation of lignin using environmentally friendly oxidants (O 2 , H 2 O 2 ) has also been explored as a means to break lignin down into monomeric products. 10À15 Crestini, Chen, Dolphin, and others have investigated the oxidation of lignin and lignin model compounds using H 2 O 2 and transition metal catalysts, including Mn and Fe complexes of porphyrin and TACN ligands (TACN = triazacyclononane). 16À21 While the reduction of lignin is well suited for the production of fuels, aerobic oxidation is advantageous in that it requires no added reagents and could preserve a high degree of the functionality present in the original lignin polymer.Oxidation is also the primary pathway by which lignin is broken down in nature. Both the enzymes lignin peroxidase and manganese-dependent peroxidase are thought to mediate the oxidative disassembly of lignin by wood-rotting fungi. 22À24 To establish the mechanisms of these enzymes, detailed studies have been carried out of the oxidation of arylglycerol β-aryl ether compounds, 25À28 which are models for the lignin β-O-4 linkage, a predominant structural feature representing approximately 50% of the linkages occurring in the natural polymer. 29 For example, oxidation of lignin model compound 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-1,3-diol (1) (Scheme 1) by lignin peroxidase affords products resulting from cleavage of the

The reactivity of homogeneous oxovanadium and copper catalysts toward aerobic oxidation of phenolic and nonphenolic β-1 lignin model compounds has been investigated. Aerobic oxidation of diastereomeric, nonphenolic β-1 lignin models (1T, 1E) using the sixcoordinate vanadium complex (HQ) 2 V V (O)(O i Pr) (HQ = 8-oxyquinolinate) as a precatalyst in pyridine afforded ketone (3) and dehydrated ketone (4) derived from oxidation of the secondary alcohol. In contrast, using CuOTf/2,6-lutidine/TEMPO (OTf = trifluoromethanesulfonate, TEMPO = 2,2,6,6-tetramethyl-piperidin-1-yl-oxyl) in toluene for the same reaction afforded 3,5dimethoxybenzaldehyde ( 5) and 4-methoxybenzaldehyde (6) as major products resulting from C α −C β bond cleavage. Reactions of the corresponding phenolic lignin model compounds (2T, 2E) with 10 mol % CuOTf/2,6lutidine/TEMPO gave ketone (9) as the major product, whereas 10 mol % (HQ) 2 V V (O)(O i Pr) or a stoichiometric amount of CuOTf/2,6-lutidine/TEMPO yielded 2,6-dimethoxybenzoquinone (10) as the major product, arising from cleavage of the C aryl − C α bond. Different selectivity was observed in the oxidation of 2T, 2E using the five-coordinate complex (dipic)V V (O)(O i Pr) (dipic = dipicolinate), with α,β-unsaturated aldehyde ( 14) as the major product (formed from oxidation of the primary alcohol and dehydration). The key differences in chemoselectivity between the vanadium and copper catalysts in the oxidations of these phenolic and nonphenolic β-1 lignin models are discussed.

The reactivity of homogeneous copper catalysts towards the selective CC bond cleavage of both phenolic and non‐phenolic arylglycerol β‐aryl ether lignin model compounds has been explored. Several copper precursors, nitrogen ligands, and solvents were evaluated in order to optimize the catalyst system. Using the optimized catalyst system, copper(I) trifluoromethanesulfonate [Cu(OTf)]/L/TEMPO (L=2,6‐lutidine, TEMPO=2,2,6,6‐tetramethyl‐piperidin‐1‐yl‐oxyl), aerobic oxidation of the non‐phenolic β‐O‐4 lignin model compound proceeded with good selectivity for CαCβ bond cleavage, affording 3,5‐dimethoxybenzaldehyde as the major product. Aerobic oxidation of the corresponding phenolic β‐O‐4 lignin model proceeded with different selectivity, affording 2,6‐dimethoxybenzoquinone and α,β‐unsaturated aldehyde products resulting from cleavage of the CαCaryl bond. At low catalyst concentrations, however, a change in selectivity was observed as oxidation of the benzylic secondary alcohol predominated with both substrates.magnified image

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.

No abstract