Structural and Electronic Control of the Bidentate 1‐(2‐pyridyl)benzotriazole Ligand in Copper Chemistry with Application to Catalysis in the A³ Coupling Reaction

Abstract

The hybrid bidentate 1‐(2‐pyridyl)benzotriazole (pyb) ligand was introduced into 3d transition metal catalysis. Specifically, [CuII(OTf)2(pyb)2]⋅2 CH3CN (1) enables the synthesis of a wide range of propargylamines by the A3 coupling reaction at room temperature in the absence of additives. Experimental and high‐level theoretical calculations suggest that the bridging N atom of the ligand imposes exclusive trans coordination at Cu and allows ligand rotation, while the N atom of the pyridine group modulates charge distribution and flux, and thus orchestrates structural and electronic precatalyst control permitting alkyne binding with simultaneous activation of the C−H bond via a transient CuI species.

Discrete Benzotriazole‐Copper(II) Complexes in Chelated and Non‐Chelated Coordination Modes: Structural Analysis and Catalytic Application in Click and A3 Coupling Reactions

“…To the best of our knowledge, these are the first examples of benzotriazolebased discrete copper catalysts in click as well as A 3 coupling reactions. [5] Scheme 2. Catalytic application of 1 and 2 in azide-alkyne click reaction.…”

“…The generation of active Cu(I) species from the starting Cu(II) complexes via single electron transfer (SET) pathway was hypothesized in line with the proposal recently provided by Vargas and Kostakis with similar Cu(II) complexes. [5] The active Cu(I) intermediates might involve monodentate-coordinated BTZ-2P and BTZ-2Q via the benzotriazole nitrogen, thus making the two catalytic systems very similar in terms of activity. A similar structural change during the reduction process might plausibly be the main reason behind the similar catalytic activity observed for both 1 and 2-catalyzed A 3 coupling as well (Figure S47, Supporting Information).…”

“…In this study, we utilized 1heteroaryl-substituted benzotriazole ligands, BTZ-2P and BTZ-2Q, to synthesize two new Cu(II)-complexes -1 with chelated BTZ-2P and 2 with non-chelated BTZ-2Q coordination modes and investigated their catalytic applications in these reactions (Figure 1c). [5] Both the air-stable complexes were found to be effective in the click as well as A 3 -coupling reactions. These results are in support of the fact that benzotriazole-based ligands are efficient to stabilize the low oxidation state Cu(I), which is the key intermediate for these reactions.…”

“…[1] The N-donor ligands possessing modular electronic and steric properties are particularly suitable in stabilizing such variable oxidation states of Cu, thus justifying their immense significance in efficient operation of the relevant catalytic cycles. Among several N-donor heterocyclic ligands, benzotriazole-based motifs, [2,3,4,5] owing to some excellent attributes such as inexpensive, simple, stable, and ready accessibility, have been well-explored as suitable auxiliary ligands in various Cu-catalyzed organic syntheses. In addition to exhibiting very good σ-donor property, these nitrogen-enriched benzotriazole ligands also act as excellent π-acceptors, and thus they are able to stabilize low-valent Cu(I) intermediates which are key to several catalytic reactions.…”

“…In fact, recently Vargas and Kostakis established the unique properties of this class of ligands as efficient charge sinks due to the availability of extended π-conjugation as well as uncoordinated N atoms within the ligand backbone, which facilitates stabilization of low oxidation states of Cu, such as Cu(I), during reductive catalysis. [5] Notably, a balanced σ-donating and π-accepting capability is expected to be the key in such reductive catalysis. For example, Cu(I)-MIC (MIC = mesoionic carbene) complexes were found to be better catalysts than Cu(I)-NHC (NHC = Nheterocyclic carbene) complexes in copper-catalyzed azidealkyne cycloaddition (CuAAC) reactions, probably due to stronger σ-donating and π-accepting efficiency of the former ligands than the later ones.…”