Experimental investigation on the mechanism of chelation-assisted, copper(II) acetate-accelerated azide-alkyne cycloaddition

Abstract

A mechanistic model is formulated to account for the high reactivity of chelating azides (organic azides capable of chelation-assisted metal coordination at the alkylated azido nitrogen position) and copper(II) acetate (Cu(OAc)(2)) in copper(II)-mediated azide-alkyne cycloaddition (AAC) reactions. Fluorescence and (1)H NMR assays are developed for monitoring the reaction progress in two different solvents, methanol and acetonitrile. Solvent kinetic isotopic effect and premixing experiments give credence to the proposed different induction reactions for converting copper(II) to catalytic copper(I) species in methanol (methanol oxidation) and acetonitrile (alkyne oxidative homocoupling), respectively. The kinetic orders of individual components in a chelation-assisted, copper(II)-accelerated AAC reaction are determined in both methanol and acetonitrile. Key conclusions resulting from the kinetic studies include (1) the interaction between copper ion (either in +1 or +2 oxidation state) and a chelating azide occurs in a fast, pre-equilibrium step prior to the formation of the in-cycle copper(I)-acetylide, (2) alkyne deprotonation is involved in several kinetically significant steps, and (3) consistent with prior experimental and computational results by other groups, two copper centers are involved in the catalysis. The X-ray crystal structures of chelating azides with Cu(OAc)(2) suggest a mechanistic synergy between alkyne oxidative homocoupling and copper(II)-accelerated AAC reactions, in which both a bimetallic catalytic pathway and a base are involved. The different roles of the two copper centers (a Lewis acid to enhance the electrophilicity of the azido group and a two-electron reducing agent in oxidative metallacycle formation, respectively) in the proposed catalytic cycle suggest that a mixed valency (+2 and +1) dinuclear copper species be a highly efficient catalyst. This proposition is supported by the higher activity of the partially reduced Cu(OAc)(2) in mediating a 2-picolylazide-involved AAC reaction than the fully reduced Cu(OAc)(2). Finally, the discontinuous kinetic behavior that has been observed by us and others in copper(I/II)-mediated AAC reactions is explained by the likely catalyst disintegration during the course of a relatively slow reaction. Complementing the prior mechanistic conclusions drawn by other investigators, which primarily focus on the copper(I)/alkyne interactions, we emphasize the kinetic significance of copper(I/II)/azide interaction. This work not only provides a mechanism accounting for the fast Cu(OAc)(2)-mediated AAC reactions involving chelating azides, which has apparent practical implications, but suggests the significance of mixed-valency dinuclear copper species in catalytic reactions where two copper centers carry different functions.

Figure 1

A mechanistic proposal by Fokin et al.^, L: a ligand or a counter ion associated with copper(I/II). i.p.: induction period; r.d.s.: rate-determining step. The numbering of azide and acetylide are marked blue. The brackets around “CuL_n” indicate that bi- or polynuclear copper(I) species may be involved.

Figure 2

(A) Fluorescence spectral changes (λ_ex 320 nm) during the fluorogenic CuAAC reaction between alkyne 7 and azide 1. Reaction conditions: 7 (10 µM), 1 (5 mM), Cu(OAc)₂·H₂O (10 µM) in CH₃OH. (B) ¹H NMR spectral evolution during the CuAAC reaction between 7 and 1. Conditions: 7 (10 mM), 1 (10 mM), Cu(OAc)₂·H₂O (1 mM) in CD₃CN.

Figure 3

(A) Growth of fluorescence intensity at 400 nm of the mixture of 1 (1 mM), 7 (10 µM), and Cu(OAc)₂ (10 µM) in CH₃OH over time with (red) or without (blue) pre-mixing azide 1 and Cu(OAc)₂ for 2 h. (B) Product (8) generation over time monitored by ¹H NMR with (red) or without (blue) pre-mixing azide 1 and Cu(OAc)₂. Conditions: [1] = 10 mM, [7] = 10 mM, [Cu(OAc)₂] = 1 mM in CD₃CN.

Figure 4

(A) Growth of fluorescence intensity at 400 nm of the mixture of 1 (10 mM), 7 (10 µM), and Cu(OAc)₂·H₂O (10 µM) in CH₃OH (cornflower squares), CH₃OD (garnet diamonds), and CD₃OD (lime triangles), respectively.

Figure 5

(A) Absorption spectra of alkyne 7 (cornflower), triazole 8 (garnet), and diyne 9 (lime) in CH₃CN at 2 µM each. (B) The growth of absorption at 374 nm in CH₃CN of the mixture of 7 (10 mM) and Cu(OAc)₂·H₂O (1 mM) in the presence (garnet) and absence (cornflower) of pyridine, and of the mixture of 7 (10 mM) and CuSO₄·5H₂O (1 mM) in the presence of pyridine (lime), respectively. The absorbance values at time = 0 are zeroed.

Figure 6

(A) The dependence of fluorescence time course (λ_ex 320 nm, λ_em 400 nm) on [7]. Conditions: [1] = 5 mM, [Cu(OAc)₂·H₂O] = 10 µM, and [7] = 1–10 µM in CH₃OH at 25 °C. (B) Plot of lnV_int vs. ln[7]. The slope yields the kinetic order of 7. V_int: initial observed rate = d[8]/dt (M·s⁻¹). The spectra taken after 48 h, assuming full conversion, are included in Figure S6.

Figure 7

(A) The dependence of fluorescence time course (λ_ex 320 nm, λ_em 400 nm) on [1]. Conditions: [7] = 10 µM, [Cu(OAc)₂·H₂O] = 10 µM, and [1] = 0.5–3.5 mM in CH₃OH. (B) Plot of lnV_int vs. ln[1]. The slope yields the kinetic order of 2-picolylazide 1. V_int: initial observed rate = d[8]/dt (M·s⁻¹).

Figure 8

(A) The dependence of fluorescence time course (λ_ex 320 nm, λ_em 400 nm) on [Cu(OAc)₂·H₂O]. Conditions: [7] = 50 µM, [Cu(OAc)₂·H₂O] = 0.8–2.5 µM, and [1] = 5 mM in CH₃OH. (B) Plot of lnV_int vs. ln[Cu(OAc)₂·H₂O]. The slope yields the kinetic order of Cu(OAc)₂·H₂O. V_int: initial observed rate = d[8]/dt (M·s⁻¹).

Figure 9

(A) The dependence of reaction time course on [7]. Conditions: [1] = 10 mM, [Cu(OAc)₂·H₂O] = 1 mM, and [7] = 3–10 mM in CD₃CN at 25 °C. Full conversions of each reaction would afford 8 at 3, 4, 6, 8, and 10 mM, respectively. (B) Plot of lnV_int vs. ln[7]. The slope yields the kinetic order of 7. V_int: initial observed rate = d[8]/dt (M·s⁻¹).

Figure 10

(A) The dependence of reaction time course on [1]. Conditions: [1] = 4–15 mM, [Cu(OAc)₂·H₂O] = 1 mM, and [7] = 10 mM in CD₃CN at 25 °C. Full conversions of each reaction would afford 8 at 4, 6, 8, 10, and 10 mM, respectively. (B) Plot of lnV_int vs. ln[1]. The slope yields the kinetic order of 1. V_int: initial observed rate = d[8]/dt (M·s⁻¹).

Figure 11

(A) The dependence of reaction time course on [Cu(OAc)₂·H₂O]. Conditions: [1] = 25 mM, [Cu(OAc)₂·H₂O] = 0.05–0.12 mM, and [7] = 25 mM in CD₃CN at 25 °C. Full conversions of each reaction would afford 8 at 25 mM. (B) Plot of ln V_int vs. ln[Cu(OAc)₂·H₂O]. The slope yields the kinetic order of Cu(OAc)₂·H₂O. V_int: initial observed rate = d[8]/dt (M·s⁻¹).

Figure 12

The time courses of triazole product formation determined by integrating ¹H NMR signals. Conditions: [1] = 20 mM, [alkyne] = 20 mM, and [Cu(OAc)₂·H₂O] = 1 mM in CD₃CN at 40 °C. (A) phenylacetylene (cornflower diamonds); phenylacetylene-d (garnet squares). (B) 1-ethynyl-4-nitrobenzene (cornflower diamonds); 1-ethynyl-4-fluorobenzene (garnet squares); phenylacetylene (lime triangles); 1-ethynyl-4-methoxybenzene (purple crosses); 1-ethynyl-4-dimethylaminobenzene (turquoise crosses).

Figure 13

Postulated catalytic cycles accounting for the results of the kinetic studies on the CuAAC reactions in Scheme 1 in CD₃CN (A) and CH₃OH (B). L: copper-binding ligand or the counter ion. py: 2-pyridyl. Blue, orange, and purple represents the +2, +1, and +3 oxidation states of copper, respectively. The steps of copper(I) triazolide protonation are in green.

Figure 14

Chelation between copper(II) and azides 1,^, 5, and 6 found in the solid state structures. L represents a counter ion (Cl⁻, NO₃⁻, SO₄²⁻, or BF₄⁻), a second ligand, or a water molecule.

Figure 15

ORTEP views (50% probability ellipsoids) of the asymmetric units of (A) [Cu₂(1)₂(OAc)₄]. Selected distances (Å): Cu1-N1 2.236, Cu1-Cu1ⁱ 2.644, Cu1-O1 1.975, Cu1-O2 1.977, Cu1-O3 1.960, Cu1-O4 1.977. (B) [Cu2(6)₂(OAc)₄]. Selected distances (Å): Cu1-N1 2.239, Cu1-Cu2 2.641, Cu1-O1 1.979, Cu1-O2 1.962, Cu1-O3 1.973, Cu1-O4 1.954. (C) [Cu₂(3)₆]²⁻. Selected distances (Å): Cu1-N4 2.219, Cu1-Cu1ⁱ 2.587, Cu1-O1 1.957, Cu1-O2 1.965, Cu1-O3 1.967, Cu1-O4 1.958.

Figure 16

Models of dinuclear pathways for azide/alkyne ligation. L: a bridging ligand or counter ion (e.g. acetate, acetylide, or iodide) that is not participating in the redox reaction, i.e., no electron exchange between L and copper. The formal charges on individual atoms are noted. All bonds surrounding L are considered to be coordinative rather than covalent for the ease in formal charge calculations. Orange: copper(I); purple: copper(III). The carbon and nitrogen atoms that engage in bonding are numbered 2 and 3, respectively.

Figure 17

Postulated Glaser/Eglinton coupling sequence enabled by Cu(OAc)₂/pyridine complex. A delocalized negative charge is implied in the drawing of the acetate.

Figure 18

Mechanistic model of the 2-picolylazide (1) involved, Cu(OAc)₂-accelerated CuAAC reaction. Structure I could be generated via the OHC sequence depicted in Figure 17 in an induction period. Orange: +1 oxidation state; purple: +3 oxidation state. Formal charges on individual atoms are noted. Cu_A – oxidation state is undefined. See text.

Figure 19

Known structural modes for carboxylate-bridged dinuclear copper complexes. Left: copper(II) acetate, middle: mixed-valency copper(II)/copper(I) acetate; right: copper(I) acetate.

Figure 20

The dependence of reaction time course on sodium ascorbate (NaAsc). Conditions: [1] = 10 mM, [7] = 10 mM, [Cu(OAc)₂·H₂O] = 1 mM, [NaAsc] = 0 (lime triangles); [NaAsc] = 4 mM (garnet squares); [NaAsc] = 0.25 mM (cornflower diamonds).

Figure 21

A tetranuclear copper(II) cluster structure [Cu₄(OAc)₄(OCH₃)₄] (50% probability ellipsoids) isolated after mixing Cu(OAc)₂·H₂O and azide 1 in CH₃OH. Selected distances (Å): Cu1-O1 1.973, Cu1-O3 1.938, Cu1-O5 1.935, Cu1-O5ⁱ 1.924, Cu1-Cu2 2.958, Cu1-Cu1ⁱ 2.987, Cu1-O2 (opposite to Cu2 on the axial position; not shown) 2.586.

Figure 22

(A) A copper(I)₁₄ cluster structure isolated via mixing Cu(OAc)₂·H₂O with 3,3-dimethylbutyne in CH₃OH. (B) The co-crystallized 2,2,7,7-tetramethylocta-3,5-diyne.

Scheme 1

(a) 5 mol% Cu(OAc)₂·H₂O, CH₃OH or CD₃CN. The fluorescence quantum yields (ϕ) were measured in CH₃OH.

Scheme 2

Conditions: (a) 5 mol% Cu(OAc)₂·H₂O, CD₃CN.

Scheme 3

Conditions: (a) 5 mol% Cu(OAc)₂·H₂O, CD₃CN. R = -NO₂, -F, -H, -OCH₃, and –N(CH₃)₂.