CuBr–ZnI(2) Combo-Catalysis for Mild Cu(I)–Cu(III) Switching and sp(2) C–H Activated Rapid Cyclization to Quinolines and Their Sugar-Based Chiral Analogues: A UV–Vis and XPS Study

[Image: see text] 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.