Mechanistic Insights into Visible-Light-Induced ATRA Reactions Powered by the Symbiotic Relationship between Cu(II)/Cu(I) Phenanthroline Complexes

As photoredox catalysis continues to yield promising chemical transformations, there is an increased need to understand how specific photocatalysts function to improve reaction efficiencies while expanding their scope. Copper-phenanthroline-based photocatalysts such as CuII(dap)Cl2 (dap = 2,9-di(p-anisyl)-1,10-phenanthroline) and [CuI(dap)2]Cl were both found to be equally capable of olefin activation through electrophilic atom transfer radical addition (ATRA) reactions. Although these molecular catalysts have proven successful, many intermediates suggested in the proposed catalytic cycles have never been detected. One undetermined aspect in this chemistry is related to how one equivalent of CuII(dap)Cl2 generates half of an equivalent of [CuI(dap)2]+ during the photocatalytic sequence. To this end, we initially used more synthetically accessible model systems, namely, [CuI(dpp)2]Cl and CuII(dpp)Cl2 (dpp = 2,9-diphenyl-1,10-phenanthroline), to glean detailed mechanistic insights into this unusual symbiotic relationship. We directly detected several intermediates involved in the ATRA photocatalytic cycle using these model chromophores in conjunction with electronic spectroscopy, infrared matrix-assisted laser desorption electrospray ionization (IR-MALDESI) mass spectrometry, electronic structure calculations, EPR spin-trap experiments, and 1H NMR spectroscopy. We found that the unique ligand lability and coordinating properties of acetonitrile enable both the in situ oxidation of [CuI(dpp)2]+ by tosyl chloride into CuII(dpp)Cl2 and the visible-light-induced homolysis of the CuII–Cl bond, which initiates the conversion to the CuI species [CuI(dpp)2][CuICl2]. The combined findings from the present study of the catalytic cycle demonstrate that the symbiotic relationship between CuII(dpp)Cl2 and [CuI(dpp)2]+, as well as between CuII(dap)Cl2 and [CuI(dap)2]+, is the critical factor enabling the ATRA photoreaction by departing from either photocatalyst.