Advancements and Perspectives toward Lignin Valorization via O‐Demethylation

Abstract

Lignin represents the largest aromatic carbon resource in plants, holding significant promise as a renewable feedstock for bioaromatics and other cyclic hydrocarbons in the context of the circular bioeconomy. However, the methoxy groups of aryl methyl ethers, abundantly found in technical lignins and lignin‐derived chemicals, limit their pertinent chemical reactivity and broader applicability. Unlocking the phenolic hydroxyl functionality through O‐demethylation (ODM) has emerged as a valuable approach to mitigate this need and enables further applications. In this review, we provide a comprehensive summary of the progress in the valorization of technical lignin and lignin‐derived chemicals via ODM, both catalytic and non‐catalytic reactions. Furthermore, a detailed analysis of the properties and potential applications of the O‐demethylated products is presented, accompanied by a systematic overview of available ODM reactions. This review primarily focuses on enhancing the phenolic hydroxyl content in lignin‐derived species through ODM, showcasing its potential in the catalytic funneling of lignin and value‐added applications. A comprehensive synopsis and future outlook are included in the concluding section of this review.

Hydrosulfonylation of Unactivated Alkenes Involving Sulfonyl Radical Generation via Photocatalytic Activation of Symmetrical Disulfones by an Energy Transfer Mimicry

“…Interestingly, biorenewable O-methyleugenol (2r), eugenol (2s), and 4-allylsyringol (2t) featuring oxidation-sensitive electron-rich arene moieties also delivered the desired product in 70−88% yield. 17 Particularly, both 2s and 2t show that phenolic moieties are tolerated, which is remarkable considering these are widely known as radical stabilizers. 18 Also 5−7-membered cyclic internal alkenes (2u− w) were hydrosulfonylated successfully in high yields (84− 95%), irrespective of their ring size.…”

“…The corresponding products ( 3d – q ) were obtained in good to excellent yields. Interestingly, biorenewable O -methyleugenol ( 2r ), eugenol ( 2s ), and 4-allylsyringol ( 2t ) featuring oxidation-sensitive electron-rich arene moieties also delivered the desired product in 70–88% yield . Particularly, both 2s and 2t show that phenolic moieties are tolerated, which is remarkable considering these are widely known as radical stabilizers .…”

Selective C(aryl)–O bond cleavage in biorenewable phenolics

“…19,20 Interestingly, these guaiacols and syringols can be further transformed into catechol and pyrogallol via O - and C -dealkylation using catalytic HCl or an acidic zeolite in water at high temperature. 16–18 Alternatively, catechol is obtainable from glucose by fermentation yielding protocatechuic acid and subsequent decarboxylation or from C-lignin. 21,27,35,36 Catechol is a major commodity chemical (40 kilotons year −1 ) petrochemically mainly produced through hydroxylation of phenol, 37,38 synthesized from benzene by the cumene Hock process (Scheme 2A).…”