Photoinduced Copper-Catalyzed Alkylation of Amines: A Mechanistic Study of the Cross-Coupling of Carbazole with Alkyl Bromides
We have recently reported that a variety of couplings of nitrogen, sulfur, oxygen, and carbon nucleophiles with organic halides can be achieved under mild conditions (−40 to 30 °C) through the use of light and a copper catalyst. Insight into the various mechanisms by which these reactions proceed may enhance our understanding of chemical reactivity and facilitate the development of new methods. In this report, we apply an array of tools (EPR, NMR, transient absorption, and UV-vis spectroscopy;
... SI-MS; X-ray crystallography; DFT calculations; reactivity, stereochemical, and product studies) to investigate the photoinduced, copper-catalyzed coupling of carbazole with alkyl bromides. Our observations are consistent with pathways wherein both an excited state of the copper(I) carbazolide complex ([Cu I (carb) 2 ] − ), and an excited state of the nucleophile (Li(carb)), can serve as photoreductants of the alkyl bromide. The catalytically dominant pathway proceeds from the excited state of Li(carb), generating a carbazyl radical and an alkyl radical. The cross-coupling of these radicals is catalyzed by copper via an out-of-cage mechanism in which [Cu I (carb) 2 ] − and [Cu II (carb) 3 ] − (carb = carbazolide), both of which have been identified under coupling conditions, are key intermediates, and [Cu II (carb) 3 ] − serves as the persistent radical that is responsible for predominant cross-coupling. This study underscores the versatility of copper(II) complexes in engaging with radical intermediates that are generated by disparate pathways, en route to targeted bond constructions. The UV-vis spectrum of [Cu I (carb) 2 ]Li in CH 3 CN exhibits an absorption at 365 nm with ε = 4300 M −1 cm −1 , which overlaps with the highest-energy emission band of the mediumpressure Hg lamp used for the catalytic reactions. This absorption profile has enabled us to Ahn et al.