Palladium‐Catalyzed Reductive Insertion of Alcohols into Aryl Ether Bonds

Lignin Compounds to Monoaromatics: Selective Cleavage of C-O Bonds over a Brominated Ruthenium Catalyst

The cleavage of C-O linkages in aryl ethers in biomass-derived lignin compounds without hydrogenation of the aromatic rings is a major challenge for the production of sustainable mono-aromatics. Conventional strategies over the heterogeneous metal catalysts require the addition of homogeneous base additives causing environmental problems. Herein, we propose a heterogeneous Ru/C catalyst modified by Br atoms for the selective direct cleavage of C-O bonds in diphenyl ether without hydrogenation of aromatic rings reaching the yield of benzene and phenol as high as 90.3 % and increased selectivity to mono-aromatics (97.3 vs. 46.2 % for initial Ru) during depolymerization of lignin. Characterization of the catalyst indicates selective poisoning by Br of terrace sites over Ru nanoparticles, which are active in the hydrogenation of aromatic rings, while the defect sites on the edges and corners remain available and provide higher intrinsic activity in the C-O bond cleavage.

Mechanistic Study of Diaryl Ether Bond Cleavage during Palladium-Catalyzed Lignin Hydrogenolysis

Hydrogenolysis has emerged as one of the most effective means of converting polymeric lignin into monoaromatic fragments of value. Reported yields may be higher than for other methods and can exceed the theoretical yields estimated from measures of the content of lignin's most readily cleaved alkyl-aryl ether bonds in β-ether units. The high yields suggest that other units in lignin are being cleaved. Diaryl ether units are important units in lignin, and their cleavage has been examined previously using simple model compounds, such as diphenyl ether. Herein, the hydrogenolysis of model compounds that closely resemble the native lignin 4-O-5 diaryl ether units was analyzed. The results provided unexpected insights into the reactivity and partial cleavage of these compounds. The models and lignin polymer produced not only monomers, but also unusual 1,3,5-meta-substituted aromatics that appear to be diagnostic for the presence and the cleavage of the 4-O-5 diaryl ether unit in lignin.

The Critical Role of Reductive Steps in the Nickel-Catalyzed Hydrogenolysis and Hydrolysis of Aryl Ether C-O Bonds

The hydrogenolysis of the aromatic C-O bond in aryl ethers catalyzed by Ni was studied in decalin and water. Observations of a significant kinetic isotope effect (kH /kD =5.7) for the reactions of diphenyl ether under H2 and D2 atmosphere and a positive dependence of the rate on H2 chemical potential in decalin indicate that addition of H to the aromatic ring is involved in the rate-limiting step. All kinetic evidence points to the fact that H addition occurs concerted with C-O bond scission. DFT calculations also suggest a route consistent with these observations involving hydrogen atom addition to the ipso position of the phenyl ring concerted with C-O scission. Hydrogenolysis initiated by H addition in water is more selective (ca. 75 %) than reactions in decalin (ca. 30 %).