CuBr–ZnI2 Combo-Catalysis for Mild CuI–CuIII Switching and sp2 C–H Activated Rapid Cyclization to Quinolines and Their Sugar-Based Chiral Analogues: A UV–Vis and XPS Study

An unprecedented CuBr–ZnI2 combo-catalyzed mild Cu1–CuIII switching activation of sp2 C–H of highly electron-rich arenes is reported. Anilines, aldehydes, and terminal alkynes were rapidly coupled together at ambient temperature to construct a ubiquitous quinoline framework through cyclization of the CC bond. This smart solvent-free strategy was exploited for the direct synthesis of valuable 4-substituted, 2,4-disubstituted, and thermally labile sugar-based chiral quinolines in good yields. In contrast to the frequently used imine–alkyne cyclization reaction, this uncommonly mild CuI–CuIII combo-catalysis for a rapid three-component cyclization is expected to proceed through the formation of a flexible propargyl amine intermediate, which provides a CuI-procatalyst for rapid sp2 C–H activation with cyclization involving transient CuIII species. The in situ generation of transient CuIII species was confirmed through online ultraviolet–visible spectroscopy (UV–vis), electrospray ionization mass spectrometry (ESI-MS), and X-ray photoelectron spectroscopy (XPS) analyses.

本研究报道了一种前所未有的溴化铜-碘化锌（CuBr–ZnI2）协同催化体系，可在温和条件下通过Cu(I)-Cu(III)价态切换活化富电子芳烃的sp2型碳氢键（sp2 C–H）。苯胺类、醛类与末端炔烃可在室温下快速发生三组分偶联，通过碳碳三键环化构建广泛存在的喹啉骨架。该智能无溶剂策略可直接合成具有应用价值的4-取代、2,4-二取代以及热不稳定的含糖基手性喹啉类化合物，且收率良好。与当前常用的亚胺-炔烃环化反应不同，该格外温和的Cu(I)-Cu(III)协同催化三组分快速环化过程，可通过生成柔性炔丙基胺中间体完成：该中间体可作为Cu(I)预催化剂，实现快速sp2 C–H活化，并伴随涉及瞬态Cu(III)物种的环化步骤。研究通过在线紫外-可见分光光度法（ultraviolet–visible spectroscopy, UV–vis）、电喷雾电离质谱（electrospray ionization mass spectrometry, ESI-MS）以及X射线光电子能谱（X-ray photoelectron spectroscopy, XPS）分析，证实了瞬态Cu(III)物种的原位生成过程。