C–N Bond Formation at Discrete Cu<sup>III</sup>–Aryl Complexes
Maxwell S. Reese, Mitchell G. Bonanno, Jamey K. Bower, Curtis E. Moore, Shiyu Zhang
Abstract
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][ L Cu II –Ar R ] were synthesized via the treatment of Cu II with a range of aryl donors, such as ZnAr 2 R, TMS–Ar R, and Ar R –Bpin. Oxidation of [TBA][ L Cu II –Ar R ] produces formal copper(III) aryl complexes L Cu III –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 ρ value of −1.66, consistent with the electrophilic character of L Cu III –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.