Selective Hydrogenation of Quinizarins to Leuco-quinizarins and Their Direct Derivatization Using Flow-Batch-Separator Unified Reactors under Continuous-Flow Conditions

Anthraquinone derivatives are classes of compounds employed in the production of valuable materials. Leuco-quinizarin, the 2-electron-reduced form of quinizarin (1,4-dihydroxy-anthraquinone), is a highly active and useful reactant for the synthesis of variety of anthraquinone derivatives. However, conventional methods of leuco-quinizarin synthesis require stoichiometric amounts of inorganic reductants and acids or bases. We developed Pt/DMPSi-Al₂O₃ and Pt-Ni/DMPSi-Al₂O₃ as highly active and selective heterogeneous catalysts for the hydrogenation of quinizarins to leuco-quinizarins under neutral and continuous-flow conditions. Remarkably, bimetallic effects of Ni and Pt nanoparticle systems were highlighted in the selective hydrogenation of the substituted quinizarin. In addition, the continuous-flow synthesis of leuco-quinizarin and its derivatization reaction were integrated to be a fully continuous process using flow-batch-separator unified reactors.