The replacement of petrochemicals with lignin is challenging due, in part, to the expensive inputs and caustic depolymerization techniques typically required. Nature, however, provides an example of lignin depolymerization under relatively mild conditions. In the β-aryl ether cleavage pathway of wood degrading bacteria, the cleavage of β-O-4 bonds occurs in a three-step process: 1) oxidation of the α-carbon hydroxyl group, 2) glutathione nucleophilic attack on the β-carbon, displacing the phenoxide, and 3) reduction of the above-formed glutathione ether’s S-C bond with a second glutathione, releasing the second lignin fragment and the glutathione disulfide. This work focuses on replicating the nucleophilic and reductive cleavage of the β-O-4 bond using only small organic thiols, without the aid of proteins or metals. Oxidized lignin dimer models treated with β-mercaptoethanol and related thiols undergo cleavage with yields ranging from 30% to 100% depending on the specific reaction conditions. The sensitivity of this cleavage to lignin-relevant functional groups was probed to determine the scope of the reaction. As in the enzymatic pathway, oxidation of the α-carbon hydroxyl group is required for reductive cleavage by the thiols. Primary and secondary beta aryl ether bonds were cleaved while tertiary sites were unreactive as expected for a nucleophilic substitution reaction. Lastly, the reaction tolerates a variety of functional groups on both the phenolic and keto aromatic rings (<i>e.g.</i>, authentic S-S and G-G dimers). Having mechanistically analyzed the reactions in well-defined model dimers, we applied this process to real lignin, achieving approximately 68% molecular weight reduction. This work exemplifies a reductive biomimetic approach to lignin depolymerization by mimicking the nucleophilic thiol-mediated ether cleavage found in the enzymatic β-aryl ether cleavage pathway.

Biomimetic cleavage of β–O–4 bonds using small organic thiols

2019 Fall National Meeting

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