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

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.