Abstract
A series of well‐defined copper(I) complexes bearing ring‐expanded N‐heterocyclic carbene (NHC) ligands has been applied to the azide–alkyne cycloaddition reaction. The obtained results notably showed that the six‐membered NHC ligands outperform well‐established five‐membered ones. [CuI(Mes‐6)] displayed a remarkable catalytic activity while respecting the strict criteria for click reactions.
Highlights
The development of copper(I) catalysts for the regioselective cycloaddition of azides and alkynes is one of the latest success stories of organometallic catalysis, and it exemplifies the concept of click chemistry.[1]
The novel copper catalysts were prepared by the addition of the appropriate CuX salt to the N-heterocyclic carbene (NHC) ligand formed in situ by prior deprotonation of the required NHC·HX in THF solution (Scheme 1)
In this particular case, the halogen, and the speciation in solution needs to be taken into account. It appears that the specific behaviour of [CuBr(Mes-6)] in solution is linked to its diminished catalytic activity and [CuX(RENHC)] are privileged complexes for this cycloaddition reaction when compared to [Cu(RENHC)2]X complexes
Summary
The development of copper(I) catalysts for the regioselective cycloaddition of azides and alkynes is one of the latest success stories of organometallic catalysis, and it exemplifies the concept of click chemistry.[1]. Considering the relatively low configurational stability of these ligands, the actual active species and the rate-determining step in this reaction might differ depending on the reaction conditions and the employed copper source/ligand combination.[9] it is not overly surprising that strongly coordinating ligands, and N-heterocyclic carbenes (NHCs) in particular,[10] have played a major role in the development of highly performing catalytic systems, and in important mechanistic studies for this transformation.[11]. RENHCs are known to improve the catalytic efficiency in important transformations such as cross-coupling,[19] or allylic boronation[20] reactions, as well as allowing the study of elusive species (that is, three-coordinated nickel(I) complexes,[21] or copper(I)–hydrides[22]). We report the remarkable catalytic activity of several [CuX(RENHC)] complexes in the formation of triazoles under click-suitable conditions
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