Ni- and Ni/Pd-Catalyzed Reductive Coupling of Lignin-Derived Aromatics to Access Biobased Plasticizers

Lignin-derived aromatic chemicals offer a compelling alternative to petrochemical feedstocks, and new applications are the focus of extensive interest. 4-Hydroxybenzoic acid (H), vanillic acid (G), and syringic acid (S) are readily obtained via oxidative depolymerization of hardwood lignin substrates. Here, we explore the use of these compounds to access biaryl dicarboxylate esters that represent biobased, less toxic alternatives to phthalate plasticizers. Chemical and electrochemical methods are developed for catalytic reductive coupling of sulfonate derivatives of H, G, and S to access all possible homo- and cross-coupling products. A conventional NiCl₂/bipyridine catalyst is able to access the H-H and G-G products, but new catalysts are identified to afford the more challenging coupling products, including a NiCl₂/bisphosphine catalyst for S-S and a NiCl₂/phenanthroline/PdCl₂/phosphine cocatalyst system for H-G, H-S, and G-S. High-throughput experimentation methods with a chemical reductant (Zn powder) are shown to provide an efficient screening platform for identification of new catalysts, while electrochemical methods can access improved yields and/or facilitate implementation on larger scale. Plasticizer tests are performed with poly(vinyl chloride), using esters of the 4,4'-biaryl dicarboxylate products. The H-G and G-G derivatives, in particular, exhibit performance advantages relative to an established petroleum-based phthalate ester plasticizer.