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-ZnI 2 combo-catalyzed mild Cu 1 -Cu III switching activation of sp 2 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 Cu I -Cu III combo-catalysis for a rapid three-component cyclization is expected to proceed through the formation of a flexible propargyl amine intermediate, which provides a Cu I -procatalyst for rapid sp 2 C-H activation with cyclization involving transient Cu III species. The in situ generation of transient Cu III species was confirmed through online ultraviolet-visible spectroscopy (UV-vis), electrospray ionization mass spectrometry (ESI-MS), and X-ray photoelectron spectroscopy (XPS) analyses.