Transition‐Metal‐Free Synthesis of Functionalized Quinolines by Direct Conversion of β‐O‐4 Model Compounds

Abstract

Direct production of heterocyclic aromatic compounds from lignin β‐O‐4 models remains a huge challenge due to the incompatible catalysis for aryl ether bonds cleavage and heterocyclic ring formation. Herein, the first example of quinoline synthesis from β‐O‐4 model compounds by a one‐pot cascade reaction is reported in yields up to 89 %. The reaction pathway involves selective cleavage of C−O bonds, dehydrogenation, aldol condensation, C−N bond formation along with heterocyclic aromatic ring construction. The control experiments suggest that both imine and chalcone were identified as the key intermediates, and the rate determining step as well as the preferred pathway were experimentally clarified and supported by density functional theory (DFT) calculations. Based on this protocol, the conversion of β‐O‐4 polymer delivered 56 wt % yield of quinoline derivative in three steps. This transformation provides a potential petroleum‐independent choice for heterocyclic aromatic chemicals.

Cation Bridge Mediating Homo- and Cross-Coupling in Copper-Catalyzed Reductive Coupling of Benzaldehyde and Benzophenone

“…This change suggests that the strong binding of the sodium cation to the two O atoms of the carbonyl group may be a concern. This indicates that the sodium cation bridge present in the solution plays an important role in mediating stereoselectivity in the C–C coupling step. − We also explored the C–C coupling mechanism with IMesCuO t Bu and observed it to be less favorable than that without IMesCuO t Bu (Figure ). TS9_10a comprises a moderate barrier (5.9 kcal/mol), which is higher than TS9_10 by 4.5 kcal/mol, indicating that IMesCuO t Bu should dissociate in this step.…”

Hf4+-exchanged montmorillonite-boosted Pd-catalyzed reductive aminolysis of aryl ethers to efficiently synthesize cyclohexylamines

“…, phenols and guaiacols) are often employed as alternatives to synthesize amines, by which innovative strategies can be developed for converting lignin into useful amines. 19–33 As reported, lignin-derived phenols could be transformed into cyclohexylamines via reductive amination in the catalytic system of Pd/C-sodium formate. 19–22 Besides, aryl amines could be synthesized from the direct coupling of lignin-derived phenols with amines over Pd/C–sodium formate–trifluoroacetic acid.…”

Advances in Catalytic Depolymerization of Lignin

“…The reaction pathway involves selective cleavage of CÀ O bonds, dehydrogenation, aldol condensation, and CÀ N bond formation along with heterocyclic aromatic ring construction. [78] The synthesis of pyrimidines from lignin β-O-4 model compounds, mediated by NaOH through a one-pot multi-component cascade reaction is also reported. Mechanism study suggests that the transformation starts by NaOH-induced deprotonation of C α -H bond in β-O-4 model compounds, and involves highly coupled sequential cleavage of CÀ O bonds, alcohol dehydrogenation, aldol condensation, and dehydrogenative aromatization.…”

“…It was found that quinoline was synthesized from β‐O‐4 model compounds via a one‐pot cascade reaction in high yields of 89 %, imine and chalcone were identified as the key intermediates. The reaction pathway involves selective cleavage of C−O bonds, dehydrogenation, aldol condensation, and C−N bond formation along with heterocyclic aromatic ring construction [78] . The synthesis of pyrimidines from lignin β‐O‐4 model compounds, mediated by NaOH through a one‐pot multi‐component cascade reaction is also reported.…”