Cu(II) salts accelerate azide-alkyne cycloaddition reactions in alcoholic solvents without reductants such as sodium ascorbate. Spectroscopic observations suggest that Cu(II) undergoes reduction to catalytic Cu(I) species via either alcohol oxidation or alkyne homocoupling, or both, during an induction period. The reactions involving 2-picolylazide are likely facilitated by its chelation to Cu(II). The highly exothermic reaction between 2-picolylazide and propargyl alcohol completes within 1-2 min in the presence of as low as 1 mol % Cu(OAc)(2).

We described in a previous communication (ref. 13) a variant of the popular Cu I -catalyzed azidealkyne cycloaddition (AAC) process where 5 mol% Cu(OAc) 2 in the absence of any added reducing agent is sufficient to enable the reaction. 2-Picolylazide (1) and 2-azidomethylquinoline (2) were found to be by far the most reactive carbon azide substrates that convert to 1,2,3-triazoles in as short as a few minutes under the discovered conditions. We hypothesized that the abilities of 1 and 2 to chelate Cu II contribute significantly to the observed high reaction rates. The current work examines the effect of auxiliary ligands near the azido group other than pyridyl for Cu II on the efficiency of the Cu(OAc) 2 -accelerated AAC reaction. The carbon azides capable of binding to the catalytic copper center at the alkylated azido nitrogen in a chelatable fashion were indeed shown to be superior substrates under the reported conditions. The chelation between carbon azide 11 and Cu II was demonstrated in an X-ray single crystal structure. In a limited set of examples, the ligand tris (benzyltriazolylmethyl)amine (TBTA), developed by Fokin et al. for assisting the original Cu Icatalyzed AAC reactions (ref. 8), also dramatically enhances the Cu(OAc) 2 -accelerated AAC reactions involving non-chelating azides. This observation leads to the hypothesis of an additional effect of chelating azides on the efficiencies of Cu(OAc) 2 -accelerated AAC reactions, which is to facilitate the rapid reduction of Cu II to highly catalytic Cu I species. Mechanistic studies on the AAC reactions with particular emphasis on the role of carbon azide/copper interactions will be conducted based on the observations reported in this work. Finally, the immediate utility of the product 1,2,3-triazole molecules derived from chelating azides as multidentate metal coordination ligands is demonstrated. The resulting triazolyl-containing ligands are expected to bind with transition metal ions via the N(2) nitrogen of the 1,2,3-triazolyl group to form non-planar coordination rings. The Cu II complexes of bidentate T1 and tetradenetate T6, and the Zn II complex of T6 were characterized by X-ray crystallography. The structure of [Cu(T1) 2 (H 2 O) 2 ](ClO 4 ) 2 reveals the interesting synergistic effect of hydrogen bonding, π-π stacking interactions, and metal coordination in forming a one-dimensional supramolecular construct in the solid state. The tetradentate coordination mode of T6 may be incorporated into designs of new molecule sensors and organometallic catalysts.

1,2,3-Triazol-4-yl (triazolyl)-containing tetradentate ligand 1 undergoes fluorescence enhancement upon binding to zinc ion (Zn(2+)) in both organic (acetonitrile) and aqueous solutions. A 1:1 complex of 1 with a trigonal bipyramidal Zn(2+) was characterized by X-ray crystallography. The cyclic voltammogram (CV) of 1 suggests that an intramolecular photoinduced electron transfer (PET) process is thermodynamically feasible which would quench the fluorescence of the 2-anthryltriazolyl fluorophore. On the basis of the X-ray and CV data, it was initially postulated that the 1:1 binding between Zn(2+) and ligand 1 shuts down the PET quenching pathway of the free ligand, which leads to the fluorescence enhancement of 1. However, the nuance of the interaction between 1 and Zn(2+) was revealed by isothermal titration calorimetry (ITC) and (1)H NMR titration experiments. A two-step binding process was observed which proceeds through an intermediate species of 2:1 (ligand/Zn(2+)) stoichiometry. Upon close examination of the fluorescence spectra of 1 during the Zn(2+) titration experiment, the fluorescence profile is in fact consistent with a two-step binding process in which a moderate fluorescence enhancement was observed during the early stage of the titration, followed by a bathochromic shift in conjunction with a more pronounced enhancement as Zn(2+) concentration increases. The studies on compounds 2-5 support the amended hypothesis that upon increasing Zn(2+) concentration, compound 1 first undergoes fluorescence enhancement because of the formation of a 2:1 (ligand to Zn(2+)) complex which slows down the PET quenching process. As Zn(2+) concentration increases, the 2:1 complex is converted into a 1:1 complex which facilitates an intramolecular exciplex formation between the excited 2-anthryltriazolyl fluorophore and the Zn(2+)-bound pyridyl moiety. Finally, the potential of compound 1 as an intracellular fluorescent indicator for Zn(2+) was evaluated. HeLa cells loaded with compound 1 grown in Zn(2+)-rich media show stronger fluorescence than those grown under Zn(2+)-deprived conditions, confirming the promise that the triazolyl-containing polyaza fluoroionophores can be developed into intracellular fluorescent indicators targeting biological Zn(2+).

Polytriazole ligands such as the widely used tris[(1‐benzyl‐1 H‐1,2,3‐triazol‐4‐yl)methyl]amine (TBTA), are shown to assist copper(II) acetate‐mediated azide–alkyne cycloaddition (AAC) reactions that involve nonchelating azides. Tris(2‐{4‐[(dimethylamino)methyl]‐1 H‐1,2,3‐traizol‐1‐yl}ethyl)amine (DTEA) outperforms TBTA in a number of reactions. The satisfactory solubility of DTEA in a wide range of polar and nonpolar solvents, including water and toluene, renders it advantageous under copper(II) acetate‐mediated conditions. The copper(II) acetate‐mediated formation of the three triazolyl groups in a tris(triazolyl)‐based ligand occurs sequentially with an inhibitory effect in the last step. The kinetic investigations of the ligand‐assisted reactions reveal an interesting mechanistic dependence on the relative affinity of azide and alkyne to copper (II). In addition to expanding the scope of the copper(II) acetate‐mediated AAC reactions to include nonchelating azides, this work offers evidence for the mechanistic synergy between the title reaction and the alkyne oxidative homocoupling reaction. The elucidation of the structural details of the polytriazole‐ligand‐bound reactive species in copper(I/II)‐mediated AAC reactions, however, awaits further characterization of the metal coordination chemistry of polytriazole ligands.

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