Reductive Cleavage of C-O Bond in Model Compounds of Lignin

Uranyl-Photocatalyzed Hydrolysis of Diaryl Ethers at Ambient Environment for the Directional Degradation of 4-O-5 Lignin

Uranyl-photocatalyzed hydrolysis of diaryl ethers has been established to achieve two types of phenols at room temperature under normal pressure. The single electron transfer process was disclosed by a radical quenching experiment and Stern-Volmer analysis between diphenyl ether and uranyl cation catalyst, followed by oxygen atom transfer process between radical cation of diphenyl ether and uranyl peroxide species. The ¹⁸O-labeling experiment precisely demonstrates that the oxygen source is water. Further application in template substrates of 4-O-5 linkages from lignin and 30-fold efficiency of flow operation display the potential application for phenol recovery via an ecofriendly and low-energy consumption protocol.

Visible-light photoredox-catalyzed C-O bond cleavage of diaryl ethers by acridinium photocatalysts at room temperature

Cleavage of C-O bonds in lignin can afford the renewable aryl sources for fine chemicals. However, the high bond energies of these C-O bonds, especially the 4-O-5-type diaryl ether C-O bonds (~314 kJ/mol) make the cleavage very challenging. Here, we report visible-light photoredox-catalyzed C-O bond cleavage of diaryl ethers by an acidolysis with an aryl carboxylic acid and a following one-pot hydrolysis. Two molecules of phenols are obtained from one molecule of diaryl ether at room temperature. The aryl carboxylic acid used for the acidolysis can be recovered. The key to success of the acidolysis is merging visible-light photoredox catalysis using an acridinium photocatalyst and Lewis acid catalysis using Cu(TMHD)₂. Preliminary mechanistic studies indicate that the catalytic cycle occurs via a rare selective electrophilic attack of the generated aryl carboxylic radical on the electron-rich aryl ring of the diphenyl ether. This transformation is applied to a gram-scale reaction and the model of 4-O-5 lignin linkages.

Amine-Mediated Bond Cleavage in Oxidized Lignin Models

Introducing amines/ammonia into lignin cracking will allow novel bond cleavage pathways. Herein, a method of amines/ammonia-mediated bond cleavage in oxidized lignin β-O-4 models was studied using a copper catalyst at room temperature, demonstrating the effect of the amine source on the selectivity of products. For primary and secondary aliphatic amines, lignin ketone models underwent oxidative C_α -C_β bond cleavage and C_α -N bond formation to generate aromatic amides. For ammonia, the competition between oxygen and ammonia determined the selectivity between C_α -N and C_β -N bond formation, generating amides and α-keto amides, respectively. For tertiary amines, the lignin models underwent oxidative C_α -C_β bond cleavage to benzoic acids. Control experiments indicated that amines act as nucleophiles attacking at the C_α or C_β position of the oxidized β-O-4 linkage to be cleaved. This study represents a novel example that the breakage of oxidized lignin model can be regulated by amines with a copper catalyst.

Downstream processing of lignin derived feedstock into end products

This review provides critical analysis on various downstream processes to convert lignin derived feedstock into fuels, chemicals and materials.