Photocatalytic Cleavage of Aryl Ether in Modified Lignin to Non-phenolic Aromatics

Abstract

Depolymerization of lignin meets the difficulty in cleaving the robust aryl ether bond. Herein, through installing an internal nucleophile in the β-O-4′ linkage, the selective cleavage of aryl ether was realized by the intramolecular substitution on aryl rings affording non-phenolic arylamine products. In particular, nitrogen-modified lignin models and lignin samples were employed to generate the iminyl radical under photocatalytic reduction, which acted as the internal nucleophile inducing aryl migration from O to the N atom. The following hydrolysis released primary arylamines and α-hydroxy ketones. Mechanism studies including electron spin resonance (ESR), fluorescence quenching experiments, and density functional theory (DFT) calculations proved the aryl migration pathway. This method enables access to non-phenolic arylamine products from lignin conversion.

Photocatalytic Conversion of Lignin into Chemicals and Fuels

“…For example, Li and co‐workers used nitrogen‐modified lignin models and lignin samples to produce nonphenolic arylamine products by selective cleavage of the C 4 −O bond in β‐O‐4 linkages under photocatalytic reduction. The introduction of an internal nucleophile by nitrogen modification induced aryl groups to migrate from O to N atoms . As shown in Scheme , the reaction started when the excited catalyst (phenothiazine, PTH) reduced the N−O bond, and then the generated imino radicals attacked the aryl C−O bond, where the ortho substituent promoted spirocyclization.…”

Selective valorization of lignin to phenol by direct transformation of C sp2 –C sp3 and C–O bonds

“…These results highlighted the deconstructive strategy for the C sp2 –C sp3 bonds in the H-, G (guaiacyl)–, and S (syringyl)–derived building blocks (fig. S7) ( 33 , 34 ), establishing lignin as a renewable source for phenol. Notably, the scission of C sp2 –C sp3 bonds in G and S units is more difficult due to the steric and electronic effects, which restrained the respective formation of 2-methoxyphenol and 2,6-dimethoxyphenol and afforded the high selectivity of phenol ( 13 ).…”

Amine‐Mediated Bond Cleavage in Oxidized Lignin Models

“…[7] Pre-functionaliza-tion to construct acyl oxime in lignin could generate non-phenolic aryl amines. [8] Despite these successes, the variability of products is limited because of the adopted nitrogen source. The variety of amines is much wider than other N-containing reagents,a nd transformation of lignin by using amines will afford access to diverse N-containing products.…”

“…Direct reaction between sodium azide and lignin benzyl alcohol structure provided primary aryl amines . Pre‐functionalization to construct acyl oxime in lignin could generate non‐phenolic aryl amines . Despite these successes, the variability of products is limited because of the adopted nitrogen source.…”

“…(3)].T hen, substrate 7 was selected as as ingle-hydrogen-abstracted substrate, and it smoothly delivered the amide with high selectivity [Scheme 3, Eq. (4)].M eanwhile, methyl benzoylformate(8) could not be converted into the amide [Scheme3,E q. (5)].…”