Copper Catalysts for Selective CC Bond Cleavage of β‐O‐4 Lignin Model Compounds

Abstract

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

Paving the Way for Lignin Valorisation: Recent Advances in Bioengineering, Biorefining and Catalysis

“…[344] Acoppercatalysed oxidative process on the same model compound, by contrast, led primarily to an a-ketone. [345] These examples again outline the need of acareful choice of model compound and catalytic parameters,a st he modification of as ingle substituent or metal complex can exert aprofound influence on the predominant depolymerisation pathway(s).…”

Towards lignin valorisation: comparing homogeneous catalysts for the aerobic oxidation and depolymerisation of organosolv lignin

“…The CuOTf/TEMPO catalyst 4, consisted of 10 wt% CuOTf, 10 wt% TEMPO and 100 wt% 2,6lutidine in dimethylformamide (DMF), the mixture was then heated at 100 C for 18 h. 26 For catalysts 3, 5 and 6, we used the reported conditions (solvents, temperature and catalyst loading) in order to avoid variability in the catalyst performance, although O 2 pressure in our experiment was lower than that reported for the Co catalyst. Aer 18 h, 400 mL aliquots were removed and analyzed by GPC, q-HSQC and 31 P NMR spectroscopy (see ESI † for details).…”

“…In previous studies we [22][23][24] and Hanson and co-workers 39 showed that oxovanadium(V) complexes, 1 and 2 ( Fig. 27 In contrast, the Cu catalyst system 4 leads to direct cleavage of the C a -C b bond to give primarily the aldehyde product, presumably through a single-electron transfer pathway 26,32,33 reminiscent of peroxidase enzymes. Subsequent cleavage of the C a -C b bond in the aryl ketone then gives aldehyde and acid products.…”

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

“…Sedai and Baker reported an effective combined catalytic system containing CuCl and TEMPO for the oxidation of 1,2-diphenyl-2-methoxyethanol (model 8, W = OCH 3 , Fig. 8), having β-1 linkage, using O 2 as the oxidant [64]. After 48 h reaction at 100 °C, they achieved more than 80% of β-1 cleavage.…”