Hydrogenolysis of β-O-4 lignin model dimers by a ruthenium-xantphos catalyst

Abstract

Hydrogenolysis reactions of so-called lignin model dimers using a Ru-xantphos catalyst are presented (xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene). For example, of some nine models studied, the alcohol, 2-(2-methoxyphenoxy)-1-phenylethanol (1), with 5 mol% Ru(H)(2)(CO)(PPh(3))(xantphos) (18) in toluene-d(8) at 135 °C for 20 h under N(2), gives in ~95% yield the C-O cleavage hydrogenolysis products, acetophenone (14) and guaiacol (17), and a small amount (<5%) of the ketone, 2-(2-methoxyphenoxy)-1-phenylethanone (4), as observed by (1)H NMR spectroscopy. The in situ Ru(H)(2)(CO)(PPh(3))(3)/xantphos system gives similar findings, confirming a recent report (J. M. Nichols et al., J. Am. Chem. Soc., 2010, 132, 12554). The active catalyst is formulated 'for convenience' as 'Ru(CO)(xantphos)'. The hydrogenolysis mechanism proceeds by initial dehydrogenation to give the ketone 4, which then undergoes hydrogenolysis of the C-O bond to give 14 and 17. Hydrogenolysis of 4 to 14 and 17 also occurs using the Ru catalyst under 1 atm H(2); in contrast, use of 3-hydroxy-2-(2-methoxyphenoxy)-1-phenyl-1-propanone (7), for example, where the CH(2) of 4 has been changed to CHCH(2)OH, gives a low yield (≤15%) of hydrogenolysis products. Similarly, the diol substrate, 2-(2-methoxyphenoxy)-1-phenyl-1,3-propanediol (9), gives low yields of hydrogenolysis products. These low yields are due to formation of the catalytically inactive complexes Ru(CO)(xantphos)[C(O)C(OC(6)H(4)OMe)=C(Ph)O] (20) and/or Ru(CO)(xantphos)[C(O)CH=C(Ph)O] (21), where the organic fragments result from dehydrogenation of CH(2)OH moieties in 7 and 9. Trace amounts of Ru(CO)(xantphos)(OC(6)H(4)O), a catecholate complex, are isolated from the reaction of 18 with 1. Improved syntheses of 18 and lignin models are also presented.

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

“…However, attempts to cleave models more similar to the actual lignin structural motifs afforded cleavage products only in low yield. [356] Instead, ad ouble dehydrogenated substrate was found to have chelated to the ruthenium metal centre, inhibiting further catalytic activity.A na cetylated keto-bether model on the other hand did undergo cleavage,yielding both the acetophenone and propiophenone,b ut also al arge amount of condensation products.P reliminary small-scale experiments on an acetylated Kraft lignin did seem to suggest cleavage of the lignin, but the products could not be unambiguously identified. [357] Ruthenium-triphos complexes have also proved competent catalysts for the cleavage of 2-aryloxy-1-arylethanols.…”

Lignin depolymerization (LDP) in alcohol over nickel-based catalysts via a fragmentation–hydrogenolysis process

“…30 Zhang et al have shown that woody lignin is catalytically hydrogenated to guaiacol and syringols at 235 C under 6 MPa H 2 , and has obtained 46% (based on lignin) phenolic compounds. 31 Other catalysts, such as CuCr oxide, 32 Co-Mo-S/Al 2 O 3 , 33 activated carbon-, alumina-or silica-supported Ru [34][35][36][37] or Pt, 38,39 have also been reported in hydrogenation of lignin or model compounds to monomeric phenols. Nickel-based catalysts have shown excellent chemoselectivity for aromatic products or high activity for C-O bond cleavage, as reported by Hartwig 40 and Rinaldi.…”

Degradation of β-O-4 model lignin species by vanadium Schiff-base catalysts: Influence of catalyst structure and reaction conditions on activity and selectivity

“…A wide range of metal complexes have been shown to effectively cleave ␤-O-4 model lignin compounds [12][13][14][15][16]. For the production of monomeric phenols, vanadium(V) complexes with O/N donor ligands are amongst the most promising homogeneous catalysts, Fig.…”