The nucleophilic and reductive properties of thiolates make them ideal candidates as redox mediators via the thiol/disulfide couple. In natural lignin depolymerization, reduction of keto aryl ether bonds occurs enzymatically via an SN2 mechanism with the thiol redox mediator glutathione. Mimicking this chemistry in a simple, mild, protein-free process, we surveyed several small organic thiols for their ability to cleave different aryl ether phenolic dimers. High yields were obtained using thiols such as β-mercaptoethanol and dithiothreitol under conditions that favor nucleophilic attack. We assessed the effects of aromatic functional groups and various ether derivatives. Direct observations of the expected intermediate, the effects of aryl ring substituents, and the activation barriers from quantum chemical simulations all support for the SN2 mechanism. Under similar conditions, aryl ether phenolic polymers (i.e., lignin) were cleaved with a polymer average molecular weight reduction of up to 68%. Initial experiments have also demonstrated that the redox mediator can be recycled via electrochemical reduction, highlighting the potential for an electrocatalytic process for the green and mild reduction of keto aryl ether bonds.

Survey and mechanism of the reductive cleavage of lignin-relevant aryl ethers by small molecule thiols

2019 Fall National Meeting

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