Profoundly new approaches are required to upgrade lignin, which contains up to 40 % of the energy of lignocellulose, into useful fuels and chemicals. The activation and selective cleavage of aromatic ether bonds are important for lignin depolymerization and selective deoxygenation of the resulting fragments. Herein, we report a uniquely selective vector of heterogeneous palladium catalysis with up to 80 % solvolysis at complete conversion of aryl ethers (Ar-OR) in water or alcohols (R’-OH) under H<sub>2</sub> at relatively mild conditions. Isotopic labeling shows the oxygen from ether is eliminated with the R group, which indicates the aromatic C-O bond is cleaved. And kinetic analysis illustrates the primary products from solvolysis are cyclyhexenyl-O-R’ and R-OH. By running cyclohex-1-enyl phenyl ether as the potential intermediate, we propose a hitherto unconsidered mechanism involving partial hydrogenation of the aryl ring and addition of R’-OH to form ketals which rapidly eliminate R-OH (Scheme 1).<br/>We tested different aryl ethers on Pd/C at the same reaction conditions in water and methanol. Reductive solvolysis was found as one of the major reaction, other products are only hydrogenated without C-O cleavage. Ethanol, 1-propanol and 2-propanol were also tested as different solvents for diphenyl ether. The selectivities are determined by the competition between solvolysis and hydrogenation of the intermediate enol ether as we proposed in Scheme 1. Thus, we suggest that the addition of R’-OH to the double bond in cyclohexenyl phenyl ether becomes slower than hydrogenation when the size of R’ group increases (from H to isopropyl), because of the steric hindrance induced in the transition state and ketal product. These observations indicate the reductive solvolysis on Pd is generally available to aryl ethers, although alcohols that have bulky alkyl groups are expected to lead to low yields.

Palladium-catalyzed reductive solvolysis of aryl ethers

256th National Meeting (2018)

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