Electro-reductive Fragmentation of Oxidized Lignin Models.

Lignin provides a potential sustainable source for production of electron-rich aromatic compounds. Recently, electrochemical lignin degradation via an oxidation/reduction sequence under mild conditions has garnered much attention within the lignin community, as electrochemistry simplifies redox reactions and offers an electron source/sink for synthesis without using stoichiometric oxidants or reductants. This paper describes a fundamental approach for the electrochemical fragmentation of the primary connection in native lignin, ß-O-4. Potential-controlled electrolysis enables selective reduction and provides fragmentation products and/or coupling products in isolated yields of 59-92%.

Organocatalytic Approach to Photochemical Lignin Fragmentation.

Herein, an organocatalytic method for photochemical C-O bond cleavage of lignin systems is reported. The use of photochemistry enabled fragmentation of the ß-O-4 linkage, the primary linkage in lignin, provides the fragmentation products in good to high yields. The approach was merged with reported oxidation conditions in a one-pot, two-step platform without any intermediary purification, suggesting its high fidelity. The future utility of the organocatalytic method was illustrated by applying the visible light-mediated protocol to continuous flow processing.

Redox Catalysis Facilitates Lignin Depolymerization.

Lignin is a recalcitrant and underexploited natural feedstock for aromatic commodity chemicals, and its degradation generally requires the use of high temperatures and harsh reaction conditions. Herein we present an ambient temperature one-pot process for the controlled oxidation and depolymerization of this potent resource. Harnessing the potential of electrocatalytic oxidation in conjugation with our photocatalytic cleavage methodology, we have developed an operationally simple procedure for selective fragmentation of ß-O-4 bonds with excellent mass recovery, which provides a unique opportunity to expand the existing lignin usage from energy source to commodity chemicals and synthetic building block source.

Transition-metal catalyzed valorization of lignin: the key to a sustainable carbon-neutral future.

The development of a sustainable, carbon-neutral biorefinery has emerged as a prominent scientific and engineering goal of the 21st century. As petroleum has become less accessible, biomass-based carbon sources have been investigated for utility in fuel production and commodity chemical manufacturing. One underutilized biomaterial is lignin; however, its highly crosslinked and randomly polymerized composition have rendered this biopolymer recalcitrant to existing chemical processing. More recently, insight into lignin's molecular structure has reinvigorated chemists to develop catalytic methods for lignin depolymerization. This review examines the development of transition-metal catalyzed reactions and the insights shared between the homogeneous and heterogeneous catalytic systems towards the ultimate goal of valorizing lignin to produce value-added products.