CuI-Zn(OAc)2 catalyzed C(sp2)-H activation for the synthesis of pyridocoumarins through an uncommon CuI-CuIII switching mechanism: A fast, solvent-free, combo-catalytic, ball milling approach

CuI-Zn(OAc)(2) catalyzed, a fast, solvent-free synthetic protocol has been developed for the oxidative C-C and C-N coupling via C(sp(2))-H activation. In this work, an aldehyde, terminal alkyne and 3-aminocoumarin were coupled together to form pyridocoumarin framework through a greener ball milling process under very mild condition. In contrast to the frequently used imine-alkyne cyclization reactions, this uncommon mild Cu-I-Cu-III switching combo-catalysis is expected to proceed through the formation of a flexible propargylic amine intermediate, which leads to a rapid C(sp(2))-H activation for cyclization involving transient Cu-III species. The in-situ formation of transient Cum species was confirmed through ultraviolet-visible spectroscopy (UV-Vis), electrospray ionization mass spectrometry (ESI-MS), and X-ray photoelectron spectroscopy (XPS) analyses of the reaction mixture. (C) 2017 Elsevier Ltd. All rights reserved.