R.P.B. and G.A.B. thank GSK and the Engineering and Physical Sciences Research Council (EPSRC) for an industrial CASE studentship (EP/P51066X/1). G.A.B., F.P., and A.J.B.W. thank the Leverhulme Trust (RP-2020-380). A.T.S. and G.A.B. thank the Biotechnology and Biological Research Council (BBSRC) for its support (BB/V017586/1; BB/T000627/1). A.J.B.W. and M.J.A. thank the EPSRC for its support (EP/R025754/1). A.J.B.W. thanks the Leverhulme Trust for a Research Fellowship (RF-2022-014).The Cu-catalyzed azide–alkyne cycloaddition (CuAAC) reaction is used as a ligation tool throughout chemical and biological sciences. Despite the pervasiveness of CuAAC, there is a need to develop more efficient methods to form 1,4-triazole ligated products with low loadings of Cu. In this paper, we disclose a mechanistic model for the ynamine-azide (3 + 2) cycloadditions catalyzed by copper(II) acetate. Using multinuclear nuclear magnetic resonance spectroscopy, electron paramagnetic resonance spectroscopy, and high-performance liquid chromatography analyses, a dual catalytic cycle is identified. First, the formation of a diyne species via Glaser–Hay coupling of a terminal ynamine forms a Cu(I) species competent to catalyze an ynamine-azide (3 + 2) cycloaddition. Second, the benzimidazole unit of the ynamine structure has multiple roles: assisting C–H activation, Cu coordination, and the formation of a postreaction resting state Cu complex after completion of the (3 + 2) cycloaddition. Finally, reactivation of the Cu resting state complex is shown by the addition of isotopically labeled ynamine and azide substrates to form a labeled 1,4-triazole product. This work provides a mechanistic basis for the use of mixed valency binuclear catalytic Cu species in conjunction with Cu-coordinating alkynes to afford superior reactivity in CuAAC reactions. Additionally, these data show how the CuAAC reaction kinetics can be modulated by changes to the alkyne substrate, which then has a predictable effect on the reaction mechanism.Peer reviewe
