C-N Bond Formation at Discrete Cu^III-Aryl Complexes

Copper(III) aryl species are widely proposed as intermediates in Cu-catalyzed C-C and C-heteroatom bond formation reactions. Despite their wide utility, mechanistic aspects of C-heteroatom formation at Cu-III centers as well as factors that lead to byproducts, e.g., Ar-H, Ar-Ar, remain elusive due to the rarity of discrete Cu-III-Ar complexes. Herein, we report the synthesis and reactivity of a series of Cu-II and Cu-III aryl complexes that closely mimic the intermediates in Cu-catalyzed C-N coupling reactions. Copper(II) aryl complexes [TBA][LCuII-Ar (R)] were synthesized via the treatment of Cu-II with a range of aryl donors, such as ZnAr2 (R), TMS-Ar (R), and Ar (R)-Bpin. Oxidation of [TBA][LCuII-Ar (R)] produces formal copper(III) aryl complexes LCuIII-Ar (R). Treatment of copper(III) aryl complexes with neutral nitrogen nucleophiles produces the C-N coupling product in up to 64% yield, along with commonly observed byproducts, such as Ar-H and Ar-Ar. Hammett analysis of the C-N bond formation performed with various N-nucleophiles shows a rho value of -1.66, consistent with the electrophilic character of LCuIII-Ar (R). We propose mechanisms for common side reactions in Cu-catalyzed coupling reactions that lead to the formation of Ar-Ar and Ar-H.