Biorefining of lignocellulosic biomass via a lignin first approach delivers a range of products with high oxygen content. Besides pulp, a lignin oil rich in guaiacols and syringols is obtained bearing multiple C(aryl)–OH and C(aryl)–OMe groups, typically named phenolics. Similarly, technical lignin can be used but is generally more difficult to process providing lower yields of monomers. Removal of the hydroxy and methoxy groups in these oxygenated arenes is challenging due to the inherently strong C–O bonds, in addition to the steric and electronic deactivation by adjacent –OH or –OMe groups. Moreover, chemoselective removal of a specific group in the presence of other similar functionalities is non-trivial. Other side-reactions such as ring saturation and transalkylation further complicate the desired reduction process. In this overview, three different selective reduction reactions are considered. Complete hydrodeoxygenation removes both hydroxy and methoxy groups resulting in benzene and alkylated derivatives (BTX type products) which is often complicated by overreduction of the arene ring. Hydrodemethoxylation selectively removes methoxy groups in the presence of hydroxy groups leading to phenol products, while hydrodehydroxylation only removes hydroxy groups without cleavage of methoxy groups giving anisole products. Instead of defunctionalization via reduction transformation of C(aryl)–OH, albeit via an initial derivatization into C(aryl)–OX, into other functionalities is possible and also discussed. In addition to methods applying guaiacols and syringols present in lignin oil as model substrates, special attention is given to methods using mixtures of these compounds obtained from wood/technical lignin. Finally, other important aspects of C–O bond activation with respect to green chemistry are discussed.

中文翻译：

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生物可再生酚醛树脂中选择性 C(芳基)–O 键断裂

通过木质素优先方法对木质纤维素生物质进行生物精炼，可生产一系列高氧含量的产品。除纸浆外，还获得富含愈创木酚和丁香酚的木质素油，其带有多个 C(芳基)-OH 和 C(芳基)-OMe 基团，通常称为酚类。类似地，可以使用工业木质素，但通常更难以加工，单体产率较低。由于固有的强 C-O 键，以及相邻的 -OH 或 -OMe 基团的空间和电子失活，去除这些含氧芳烃中的羟基和甲氧基具有挑战性。此外，在存在其他类似功能的情况下化学选择性去除特定基团并非易事。其他副反应如环饱和和烷基转移使所需的还原过程进一步复杂化。在本概述中，考虑了三种不同的选择性还原反应。完全加氢脱氧去除羟基和甲氧基，产生苯和烷基化衍生物（BTX型产品），这通常因芳烃环的过度还原而变得复杂。加氢脱甲氧基化在羟基存在下选择性地除去甲氧基，产生苯酚产物，而加氢脱羟基仅除去羟基，而不裂解甲氧基，产生苯甲醚产物。尽管通过初始衍生化为 C(芳基)-OX，但不是通过 C(芳基)-OH 的还原转化来去官能化，而是可能并讨论了其他官能团。除了应用木质素油中存在的愈创木酚和丁香酚作为模型底物的方法外，还特别关注使用从木材/工业木质素获得的这些化合物的混合物的方法。最后，讨论了与绿色化学有关的 C-O 键活化的其他重要方面。