Sequential Catalytic Modification of the Lignin α‑Ethoxylated β‑O‑4 Motif To Facilitate C–O Bond Cleavage by Ruthenium-Xantphos Catalyzed Hydrogen Transfer

• Published in: ACS sustainable chemistry & engineering Vol. 7; no. 14; pp. 12105 - 12116
• Main Authors: Zhang, Zhenlei, Lahive, Ciaran W, Zijlstra, Douwe S, Wang, Zhiwen, Deuss, Peter. J
• Format: Journal Article
• Language: English
• Published: American Chemical Society 15-07-2019
• Subjects: Depolymerization; Lignin; Hydrogen borrowing; Decarbonylation; Ethanosolv; Aerobic oxidation
• ISSN: 2168-0485; 2168-0485
• DOI: 10.1021/acssuschemeng.9b01193

Lignin is an abundant natural biopolymer that has the potential to act as a renewable feedstock for valuable aromatic compounds via selective catalytic depolymerization. In recent years, elegant, mild, catalytic hydrogen neutral C–O bond cleavage methodologies have been developed on model compounds yielding acetophenone derivatives. However, none of these have been reported to be effective once applied to lignin. One of the reasons for this is the highly functionalized nature of the native lignin β-O-4 motif, which is often not taken into account in the β-O-4 model compounds used for methodology development. In this work, we demonstrate the development of a stepwise modification protocol on lignin β-O-4 model compounds to overall yield a partially defunctionalized β-O-4 motif. This was achieved by making use of an α-ethoxylated β-O-4 motif that is readily available from ethanosolv extraction of lignocellulosic biomass. This specific motif allowed us to apply selective copper catalyzed aerobic oxidation and subsequent rhodium catalyzed decarbonylation of the primary hydroxyl group in the γ position. The obtained partially defunctionalized β-O-4 lignin motif allowed effective homogeneous ruthenium catalyzed hydrogen neutral C–O bond cleavage (>99% of 3,4-dimethoxyacetophenone and >99% of guaiacol). The stepwise modification strategy was extended to walnut ethanosolv lignin, demonstrating that the specific structural motifs are accessible from such a readily available lignin. Overall, this work illustrates that the structure of lignin can be strategically modified to allow access to otherwise inaccessible specific aromatic compounds via selective depolymerization methodologies.