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.

Advances in Catalytic Depolymerization of Lignin

“…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.…”

“…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.…”

Catalytic conversion of chitin-based biomass to nitrogen-containing chemicals

“…Furthermore, the coupling of ammonia or organic amines as the external nitrogen source with lignocellulose or lignocellulose-derived biomass feedstocks to produce high-value nitrogen-containing chemicals is an emerging hot field. 20 , 21 , 22 , 23 Typical reactions include the amination of platform chemicals derived from lignocellulosic biomass into corresponding amines. 24 , 25 For example, pyrrolidinone can be synthesized in one pot through the reductive amination of cellulose-derived platform chemical LA using hydrogen gas.…”