Isolation of Functionalized Phenolic Monomers through Selective Oxidation and CO Bond Cleavage of the β‐O‐4 Linkages in Lignin

Abstract

Functionalized phenolic monomers have been generated and isolated from an organosolv lignin through a two-step depolymerization process. Chemoselective catalytic oxidation of β-O-4 linkages promoted by the DDQ/tBuONO/O2 system was achieved in model compounds, including polymeric models and in real lignin. The oxidized β-O-4 linkages were then cleaved on reaction with zinc. Compared to many existing methods, this protocol, which can be achieved in one pot, is highly selective, giving rise to a simple mixture of products that can be readily purified to give pure compounds. The functionality present in these products makes them potentially valuable building blocks.

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

“…[325] Hence,t he maximum yield of am onomer can be estimated by Equation (1), as reported in Ref. [326]:…”

“…[325] Fort he processing of Birch lignin, an alternative solvent to that used in the selective cleavage of model compounds was required, an important practical issue that is not to be underestimated. Amixture of 2-methoxyethanol/1,2-dimethoxyethane was found not only capable of dissolving the lignin feedstock, but also allowed for the catalytic DDQ reactions demonstrated on the model compounds.2DHSQC NMR revealed acomplete disappearance of the a-C-H cross-peak of the b-O-4 linkages,w ith ac oncomitant appearance of as ignal associated with the oxidised linkage.Asubsequent stoichiometric reduction using metallic zinc performed in the same pot yielded am onomeric syringyl-derived compound in 5wt% isolated yield;i tw as also shown that the highly-functionalised 3-hydroxy-1-phenylpropan-1-one product provides many opportunities for further conversion into value-added fine chemicals (Scheme 11 c).…”

“…Amixture of 2-methoxyethanol/1,2-dimethoxyethane was found not only capable of dissolving the lignin feedstock, but also allowed for the catalytic DDQ reactions demonstrated on the model compounds.2DHSQC NMR revealed acomplete disappearance of the a-C-H cross-peak of the b-O-4 linkages,w ith ac oncomitant appearance of as ignal associated with the oxidised linkage.Asubsequent stoichiometric reduction using metallic zinc performed in the same pot yielded am onomeric syringyl-derived compound in 5wt% isolated yield;i tw as also shown that the highly-functionalised 3-hydroxy-1-phenylpropan-1-one product provides many opportunities for further conversion into value-added fine chemicals (Scheme 11 c). [325] Thep roduction of this highlyfunctionalised derivative,a nd therefore,t he possibility of generating high-value fine chemicals from this phenone product, may eventually justify the use of more expensive reagents and protocols,a fter which the remaining,l owerquality and lower-value condensed lignin could be further converted by harsher catalytic methods,i .e., by ac ascading approach.…”

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

“…Compared to hardwood and softwood lignins, grass lignins contain more H units [7,17,20]. For any given lignin sample, the H:G:S ratio can be calculated using wet chemical methods such as acidolysis, nitrobenzene oxidation (NBO), permanganate oxidation, cupric oxide and thioacidolysis [21][22][23] or spectroscopic techniques such as FT-IR [24] or 2D-NMR [25,26]. For example, a method for the quantitative analyses of lignin samples was reported by Kline et al using their normalized Fourier Transform Infra-Red (FTIR) spectra [24].…”

“…S/G molar ratios can also be semi-quantitatively estimated by Heteronuclear Single Quantum Coherence (HSQC) 2D-NMR spectroscopy [25,26].…”