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

摘要

An unprecedented CuBr–ZnI2 combo-catalyzed mild Cu¹–Cu^III 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 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 sp² 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.