Weak Bonds Joint Effects Catalyze the Cleavage of Strong C-C Bond of Lignin-Inspired Compounds and Lignin in Air by Ionic Liquids

Experimental and theoretical investigation of synergistic effects in a binary ionic liquid system for the selective production of benzaldehyde from lignin model compound

Lignin is an abundant and cost-effective aromatic polymer, which makes the production of high-value aromatic aldehydes from it highly significant. However, a challenge is that these aldehydes can easily over-oxidation into aromatic acids during aerobic oxidation. Therefore, preventing over-oxidation is crucial. In this study, a binary ionic liquid system ([Emim][Ac] and [Bmim][FeCl4]) was employed for synergistic catalysis to mitigate this issue. Notably, density functional theory analyses have revealed that [Ac]- and [FeCl4]- can engage in hydrogen bonding and van der Waals interactions with the β-O-4 linkages in lignin. These interactions can effectively reduce the bond energy of the Cβ-H bonds, facilitating its homolysis to generate free radicals. Furthermore, under mild conditions, these free radicals can readily break CC bonds, producing aromatic aldehydes while minimizing over-oxidation. In this system, the lignin model compound 2-Phenoxy-1-phenylethanone (PP) was cleaved to produce benzaldehyde (46.4 %) and phenol (73.4 %), enzymatic lignin was degraded to yield aromatic products like vanillic aldehyde, all without over-oxidation. To meet the growing demand for biomass upgrading, this work introduces a green and controlled conversion technology capable of cleaving inert CC bonds under mild conditions, establishing a foundation for future advancements in renewable energy production.

Photoredox-catalyzed trifluoromethylation of 2H-indazoles using TT-CF3+OTf- in ionic liquids

A protocol for metal and oxidant free photoredox catalyzed trifluoromethylation of 2H-indazoles was developed by using Eosin Y as the photocatalyst and recoverable ionic liquids as the solvents. A series of trifluoromethylated products were obtained in moderate to good yields in this protocol under mild conditions. The reaction proceeded via a free-radical mechanism with a broad substrate range, excellent regioselectivity, and good functional group tolerance. Furthermore, the utility of this protocol was demonstrated by the synthesis of a highly selective ligand for estrogen receptor beta (ERβ) and the drug granisetron. The protocol provides a mild and environmentally friendly solution for trifluoromethylation reaction.

Enhanced oxidative depolymerization of lignin in cooperative imidazolium-based ionic liquid binary mixtures

The aerobic oxidation of lignin model 2-phenoxyacetophenone (2-PAP) in cooperative ionic liquid mixtures (CoILs) with 1-ethyl-3-methylimidazolium acetate ([C₂C₁im]OAc) and 1-benzyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([B_ZC₁im]NTf₂) was investigated. Complete degradation of 2-PAP was achieved with [C₂C₁im]OAc/[B_ZC₁im]NTf₂ molar ratio (R_IL) of 1/1 and 1/2 at 100 °C for 2 h. The conversion and product yields from CoILs were higher than those in pure ILs, indicating the cooperative effects of [C₂C₁im]OAc/[B_ZC₁im]NTf₂ on cleaving aryl-ether bonds. [C₂C₁im]OAc promoted the catalytic cleavage of aryl-ether bonds and solvation, and [B_ZC₁im]NTf₂ induced the formation of alkyl radicals and enhanced the product selectivity. Accordingly, the highest conversion of alkali lignin (79.8%) was obtained with R_IL of 5/1 at 100 °C for 2 h, and phenol monomers (306 mg/g) were selectively produced. The CoILs exhibited good catalytic capacities for oxidative depolymerization of lignin, which strongly depends on the changes in intermolecular interactions and structural organization with varying R_IL.