Mechanism of the ligand-free CuI-catalyzed azide-alkyne cycloaddition reaction

Article abstract of DOI:10.1002/anie.200461496

(Chemical Equation Presented) A powerful engine: The Cu-catalyzed azide-alkyne cycloaddition depends on rapid formation of Cu^I- acetylide complexes from terminal alkynes 2 and their ability to activate organic azides 1. A kinetics study uncover

Full text of DOI:10.1002/anie.200461496

Angewandte
Chemie
Communications
Two copper centers acting in concert are required for azide–alkyne
cycloaddition at high rates. When applied to solid-phase oligopeptides
bearing azide and alkyne units at opposite ends of each chain, dimeric
cyclic structures of unprecedented ring size are produced selectively.
For more information, see the Communications by M. G. Finn and co-
workers on the following pages.
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DOI: 10.1002/anie.200461496
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Table 1: Observed rate orders in the variable component derived from
kinetics measurements.[a]
Azide–Alkyne Cycloadditions
Mechanism of the Ligand-Free CuI-Catalyzed
Azide–Alkyne Cycloaddition Reaction**
Valentin O. Rodionov, Valery V. Fokin,* and
M. G. Finn*
Entry Component[b] [PhC CH] [BnN3] [Cu]
Rate order
ꢂ
[mm]
[mm]
[mm]
[c]
[c]
ꢂ
1
2
3
4
5
6
7
8
9
PhC CH
0.4–1.0
0.04 10
1.3ꢀ0.2
0
0
The 1,3-dipolar cycloaddition of azides and alkynes[1] is an
effective way to make connections between structures that
bear a wide variety of functional groups.[2] The process is
strongly favored in thermodynamic terms, yet both azide and
alkyne units are highly selective in their reactivity; they are
inert to most chemical functionalities and are stable in wide
ranges of solvents, temperatures, and pH values. The discov-
ery of copper(i) catalysis of this process[3] has opened a myriad
of applications in bioconjugation,[4] organic synthesis,[5] mate-
rials and surface science,[6] and combinatorial chemistry.[7]
Herein we report the results of the first experimental
investigation of the mechanism of the fundamental reaction
catalyzed by copper(i) species generated in situ from cop-
per(ii) and ascorbate.[8] This “ligand-free” process, which lacks
the heterocyclic chelates that have been found to accelerate
the reaction,[9] has been used in many situations and
represents a good starting target for mechanistic study.
Pseudo-first-order (initial rate) kinetics experiments were
performed on the reaction of benzyl azide (1) and phenyl-
acetylene (2). Aliquots taken at intervals from the reactions
by an automated liquid handler under inert atmosphere were
quenched with H2O2. Quantitative LC–MS analysis of the
resulting samples (Agilent Zorbax C18 column, MS detection
in single-ion mode) allowed the collection of reliable kinetic
data for reactions with concentrations of organic starting
materials ranging from 0.4 to 100 mm. Examples of the data
are shown in Figure 1, and the results are summarized in
Table 1.[10]
ꢂ
PhC CH
1–2.5
10–25
0.04
1
1
1
100
1
5
0.1
ꢂ
PhC CH
[c]
BnN3
0.4–1 10
1–25 10
1.0ꢀ0.2
BnN3
BnN3
0
[c]
1–25
1
0.1
1–10
ꢁ0.25ꢀ0.1
0.1ꢀ0.1
0.6ꢀ0.2
2.0ꢀ0.1
Cu
Cu
10
0.4
10
0.4
0.5–2
0.04–0.16
Cu[c]
[a] Reactions were performed at 20ꢀ28C in DMSO/H2O (4:1), [Na
ascorbate]=20 or 40 mm. Addition of organic reagents did not change
the pH value of the solution. [b] Reaction component with varied initial
concentration. [c] Performed with the addition of [D7]3 as internal
standard.
presence or absence of [D7]3 made no difference in the
observed rate law, except in the cases of catalytic copper. In
the absence of supplemental triazole, the reaction rate
increased more slowly than expected with increasing [Cu]
(Figure 1c), suggesting that less-reactive species—presuma-
bly higher aggregates—accumulate at higher metal concen-
trations. The addition of 30 mm [D7]3 prevents this phenom-
enon, which permits the observation of a clean second-order
dependence on [Cu] (Figure 1b). Benzyl azide was found to
be slightly inhibitory when present in large excess over copper
(Figure 1d–f and Table 1, entry 6), which reveals the presence
of an unproductive Cu–azide association.
The kinetics results are consistent with a stepwise
mechanism that involves a dynamically exchanging family
of Cu–acetylide complexes.[11] We propose the broad outline
shown in Scheme 1. In the presence of excess copper, no
catalyst turnover is required and all of the organic compo-
nents are engaged with the metal, making the reaction zero
order in CuI. Under these conditions, the reaction was found
to be first order in azide, and between first and second order
in alkyne. We have not yet determined if two pathways are
operative (requiring one and two alkynes, respectively) or if
the preferred catalytic mechanism involves two alkynes but is
inhibited by excess alkyne, giving rise to the intermediate
observed rate order. (While s-acetylide–Cu complexes are
shown in Scheme 1 and are proposed to be the active form,[12]
p complexes of alkynes may also play a role. Commercially
available copper acetylides have so far proven to be
ineffective in this reaction.)
Under conditions of excess CuI (Table 1, entries 1, 4, 7),
the experimental rate law was:
rate = k[alkyne]1.3ꢀ0.2[azide]1ꢀ0.2[Cu]0.
With catalytic CuI under saturating conditions (Table 1,
entries 2, 3, 5, 9; rate independent of [alkyne]), the reaction
was second order in metal. A small amount of deuterated
triazole product [D7]3, prepared from perdeuterobenzyl
azide, was added as an internal standard in some cases. The
[*] V. O. Rodionov, Prof. V. V. Fokin, Prof. M. G. Finn
Department of Chemistry
The Scripps Research Institute
10550 N. Torrey Pines Rd., La Jolla, CA 92037 (USA)
Fax: (+1)858-784-8850
It is envisioned that azide is activated by the appropriate
Cu–acetylide; compound 4 represents one of many possible
structures consistent with density functional theory calcula-
E-mail: fokin@scripps.edu
tions performed earlier.[12] Formation of the resulting Cu C
ꢁ
mgfinn@scripps.edu
bond-containing (triazole) species is then followed by pro-
[**] We thank the Skaggs Institute for Chemical Biology for support of
this work.
teolysis of the Cu C bond to regenerate the catalyst.[13] When
ꢁ
the amount of azide and alkyne was increased to stoichio-
metric levels with respect to metal (Table 1, entry 5), the rate
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Figure 1. Examples of initial-rate kinetics measurements: a) data and b) rate-order plots showing second-order dependence on [Cu] under catalytic
conditions; c) analogous to b), except that the two sets of reactions shown were performed without the deuterated product as internal standard;
d) data and e) rate-order plots showing the inhibitory effect of large amounts of azide under catalytic conditions; f) data showing zero-order
dependence on azide under noncatalytic conditions. AP/AS =peak area ratio of product/standard; AP =peak area of product determined by single-
ion mode LC–MS.
Scheme 1. Outline of species involved in the copper catalytic cycle, and the known trinuclear Cu–acetylide complex 5.
order in azide was zero, consistent with an increase in the rate
of capture by azide and a concomitant change in the rate-
determining step to the formation of activated acetylide.
Under catalytic conditions at saturating levels (rates
independent of [azide] and [alkyne]), the reaction was
second order in copper,[13] consistent with parallel observa-
tions for a highly active Cu–phenanthroline catalyst.[9b] There
are several possible explanations; perhaps the simplest is the
activation of azide and alkyne by different metal centers as
suggested in 4. However, multinuclear Cu–acetylide species
are common. One example, the known[14] phenylacetylide
respect to total alkyne. p-Tolylacetylene was also incorpo-
rated into its respective 1,4-disubstituted triazole, but cata-
lytic activity (> 100% yield with respect to Cu) was not
observed. The reactivities of other model Cu–acetylide
complexes are currently being explored to test various
mechanistic hypotheses. The details of the interaction of
organic azide with monomeric Cu–acetylide have been
explored by computational methods.[12]
Further insight was gained by examination of the reac-
tivities of diazides and dialkynes (Scheme 2). Compound 6
provided ditriazole 8 as the major product, even when
10 equivalents of diazide over alkyne were used, whereas
the analogous dialkyne 9 gave statistically expected quantities
of 10 and 11. Of the six additional diazides shown in
1
+
ꢂ
cation [Cu3(m3-h -C CPh)2(m-dppm)3] (5, prepared as the
triflate salt, Scheme 1), was found to react with excess benzyl
azide at room temperature to give clean 3 in 50% yield with
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Scheme 2. Comparison of diazide and dialkyne reactivity.
Scheme 2, only the 1,2 compound 12 and the 1,3 compound 13
(but not 14) exhibited the same strong preference as 6 for the
formation of ditriazole products.[15] Therefore, the first
triazole formed, and not a pendant hydroxy group, is the
critical accelerating element. Conformational constraints
present in 6 and 13, but not 14, are required to hold the
triazole and the second azide in close proximity. The rates of
reactions between 1 and 2 were not affected by the addition of
8. Furthermore, cycloaddition reactions of 6 did not exhibit
any features of autocatalysis. Thus, Cu·8 complexes do not
catalyze the process. When the accelerating ligand 18[4a] was
added, the overall rate of disappearance of 6 was increased
(reactions were complete within 20 min), but the ratio of 8 to
7 remained unchanged.
an intermediate in the conversion of 6 into 8. Rather, we
suggest that the Cu–triazole organometallic precursor to 7
may be the active structure.
The conversion of 7 into 8 was slowed neither by toluene,
nor when benzyl azide was used in the presence of a large
excess of alkyne (a better ligand for CuI than the azide). These
observations, along with the inhibitory behavior of benzyl
azide under catalytic conditions in metal (Table 1, entry 6),
suggest that benzyl azide binds reversibly to CuI centers
through interactions with both the aromatic ring[16] and azide
group.[12] This model is supported by a recent report of an
inorganic Ru–N3 moiety that participates in a cycloaddition
reaction with alkynes.[17]
A proposed series of steps consistent with the data
obtained thus far is shown in Scheme 3. Diazide 6 is converted
Reactions of 9 and 1 were carefully monitored. Product 10
initially built up, and then diminished to give 11 in a manner
expected for sequential reactions of approximately the same
rate. In contrast, the reaction of 6 with 2 was found to
generate a small amount (< 3 mol%) of 7, which persisted
throughout the reaction. Compound 7 was prepared inde-
pendently, and the rate of its disappearance in reaction with
phenylacetylene was found to be significantly faster than that
of 6 (complete reaction in 30 min for 7, versus 30–40%
completion in the same time for 6). However, the rate
difference was not large enough to account for the predom-
inant formation of ditriazole 8 from diazide 6, especially in
the presence of a large excess of 6 relative to alkyne.
Moreover, reactions of 2 with a mixture of 1 and 7 were
found to proceed with consumption of the two azides at an
approximately equal rate, and much more slowly than
reactions of 2 and 7 in the absence of 1. In other words, the
reaction of azide 7 was inhibited by benzyl azide, but no
increase in the amount of 7 was detected when benzyl azide
was added to reactions of 6. Therefore, free 7 is unlikely to be
ꢁ
into the Cu C bond-containing triazole intermediate 19, from
ꢁ
which free 7 can be obtained by proteolysis of the Cu C
bond.[13] However, 19, a CuI complex, can also bind alkyne[18]
to give a system of the form 20 or Cu–acetylide[18a,19] to give
21, with subsequent rapid intramolecular capture of the
remaining azide to afford 8 without the intermediacy of free 7,
when conformational factors permit. The fact that the triazole
unit of 7 can direct Cu–acetylide to the pendant azide
(structure 22) also serves to accelerate the reactions of 7
relative to 6. The observed inhibition of the 7!8 conversion
by added benzyl azide is proposed to result from competitive
binding of active Cu–acetylide in a chelate such as structure
23. Finally, the necessary intermediate 24 from dialkyne 9
cannot easily recruit a reaction partner, as Cu–azide coordi-
nation alone is too weak. The binding of CuI by the triazole
moiety of 10 offers little advantage, as the Cu–alkyne
interaction is rapid even without pendant-group assistance.
In summary, a mechanistic outline that requires the
ordered interaction of two copper centers, with one or two
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Scheme 3. Proposed steps in the transformations of 6 and 9 into mono- and ditriazoles.
alkyne/acetylide units and one azide, has been uncovered by
kinetics measurements. For certain 1,2- and 1,3-diazides, the
formation of the second triazole ring was found to be much
faster than the first. An unusual inhibitory effect of benzyl
azide was also observed, which suggests the presence of two
pathways to ditriazole, one via free monotriazole and the
other via a copper(i) organometallic intermediate. Much, of
course, remains to be explained. For example, in addition to
employing only one of its two acetylide fragments in cyclo-
addition, complex 5 was found to mediate the exclusive
transformation of 6 into 7, in remarkable contrast to all other
systems examined so far. The most important unresolved
issue is the precise nature of the putative binuclear copper
system responsible for efficient catalysis—a question we are
addressing with copper-chelating ligands. Preliminary meas-
urements with ligand-accelerated catalysts are currently
underway and show kinetic parameters similar to those
described above. The requirement for two copper centers and
the advantages of intramolecular positioning of azide groups
are highlighted in the selective synthesis of cyclic peptide
dimers, described in the accompanying paper.[20] These
principles also provide guidance for ongoing efforts in our
laboratories to develop new ligands and self-accelerating
(autocatalytic) systems.
[1] R. Huisgen in 1,3-Dipolar Cycloaddition Chemistry, Vol. 1 (Ed.:
A. Padwa), Wiley, New York, 1984, pp. 1 – 176.
[2] a) W. G. Lewis, L. G. Green, F. Grynszpan, Z. Radic, P. R.
Carlier, P. Taylor, M. G. Finn, K. B. Sharpless, Angew. Chem.
2002, 114, 1095 – 1099; Angew. Chem. Int. Ed. 2002, 41, 1053 –
1057; b) H. C. Kolb, K. B. Sharpless, Drug Discovery Today 2003,
8, 1128 – 1137.
[3] a) C. W. Tornøe, C. Christensen, M. Meldal, J. Org. Chem. 2002,
67, 3057 – 3062; b) V. V. Rostovtsev, L. G. Green, V. V. Fokin,
K. B. Sharpless, Angew. Chem. 2002, 114, 2708 – 2711; Angew.
Chem. Int. Ed. 2002, 41, 2596 – 2599.
[4] a) Q. Wang, T. R. Chan, R. Hilgraf, V. V. Fokin, K. B. Sharpless,
M. G. Finn, J. Am. Chem. Soc. 2003, 125, 3192 – 3193; b) A. J.
Link, D. A. Tirrell, J. Am. Chem. Soc. 2003, 125, 11164 – 11165;
c) A. E. Speers, B. F. Cravatt, Chem. Biol. 2004, 11, 535 – 546;
d) A. E. Speers, G. C. Adam, B. F. Cravatt, J. Am. Chem. Soc.
2003, 125, 4686 – 4687; e) L. V. Lee, M. L. Mitchell, S.-J. Huang,
V. V. Fokin, K. B. Sharpless, C.-H. Wong, J. Am. Chem. Soc.
2003, 125, 9588 – 9589; f) A. Deiters, T. A. Cropp, M. Mukherji,
J. W. Chin, J. C. Anderson, P. G. Schultz, J. Am. Chem. Soc. 2003,
125, 11782 – 11783; g) R. Breinbauer, M. Kꢀhn, ChemBioChem
2003, 4, 1147 – 1149.
[5] a) F. Fazio, M. C. Bryan, O. Blixt, J. C. Paulson, C.-H. Wong, J.
Am. Chem. Soc. 2002, 124, 14397 – 14402; b) K. D. Bodine, D. Y.
Gin, M. S. Gin, J. Am. Chem. Soc. 2004, 126, 1638 – 1639; c) T. S.
Seo, Z. Li, H. Ruparel, J. Ju, J. Org. Chem. 2003, 68, 609 – 612;
d) Z. Zhou, C. J. Fahrni, J. Am. Chem. Soc. 2004, 126, 8862 –
8863; e) T. Jin, S. Kamijo, Y. Yamamoto, Eur. J. Org. Chem. 2004,
3789 – 3791.
Received: July 31, 2004
Revised: October 18, 2004
Published online: February 3, 2005
[6] a) P. Wu, A. K. Feldman, A. K. Nugent, C. J. Hawker, A. Scheel,
B. Voit, J. Pyun, J. M. J. Frꢁchet, K. B. Sharpless, V. V. Fokin,
Angew. Chem. 2004, 116, 4018 – 4022; Angew. Chem. Int. Ed.
2004, 43, 3928 – 3932; b) D. D. Dꢂaz, S. Punna, P. Holzer, A. K.
McPherson, K. B. Sharpless, V. V. Fokin, M. G. Finn, J. Polym.
Sci. Part A 2004, 42, 4392 – 4403; c) J. P. Collman, N. K. Devaraj,
Keywords: alkynes · azides · click chemistry · copper ·
.
cycloaddition
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C. E. D. Chidsey, Langmuir 2004, 20, 1051 – 1053; d) T. Lum-
merstorfer, H. Hoffmann, J. Phys. Chem. B 2004, 108, 39663 –
33966; e) J. K. Lee, Y. S. Chi, I. S. Choi, Langmuir 2004, 20,
3844 – 3847.
[7] S. Lꢀber, P. Rodriguez-Loaiza, P. Gmeiner, Org. Lett. 2003, 5,
1753 – 1755.
[8] a) C. Creutz, Inorg. Chem. 1981, 20, 4449 – 4452; b) M. B. Davies,
Polyhedron 1992, 11, 285 – 321.
[9] a) T. R. Chan, R. Hilgraf, K. B. Sharpless, V. V. Fokin, Org. Lett.
2004, 6, 2853 – 2855; b) W. G. Lewis, F. G. Magallon, V. V. Fokin,
M. G. Finn, J. Am. Chem. Soc. 2004, 126, 9152 – 9153.
[10] For details, see Supporting Information.
[11] B. M. Mykhalichko, O. N. Temkin, M. G. Mysꢃkiv, Russ. Chem.
Rev. 2001, 69, 957 – 984.
[12] F. Himo, T. Lovell, R. Hilgraf, V. V. Rostovtsev, V. V. Fokin, L.
Noodleman, K. B. Sharpless, J. Am. Chem. Soc. 2005, 127, 210 –
216.
ꢂ
[13] The use of PhC CD in protic solvent gave no deuterium
incorporation in the triazole product, whereas reactions in D2O
or MeOD provided high levels of deuteration at the C5 triazole
position.
[14] J. Diez, M. P. Gamasa, J. Gimeno, E. Lastra, A. Aguirre, S.
Garcia-Granda, Organometallics 1993, 12, 2213 – 2220.
[15] Geminal diazides also show this behavior. This chemistry will be
described separately in a report on asymmetric induction with
chiral ligands.
[16] a) F.-B. Xu, Q.-S. Li, L.-Z. Wu, X.-B. Leng, Z.-C. Li, X.-S. Zeng,
Y. L. Chow, Z.-Z. Zhang, Organometallics 2003, 22, 633 – 640;
b) M. Mascal, J.-L. Kerdelhuꢁ, A. J. Blake, P. A. Cooke, Angew.
Chem. 1999, 111, 2094 – 2096; Angew. Chem. Int. Ed. 1999, 38,
1968 – 1971; c) S. M. Hubig, S. V. Lindeman, J. K. Kochi, Coord.
Chem. Rev. 2000, 200, 831 – 873; d) T. Osako, Y. Tachi, M. Taki,
S. Fukuzumi, S. Itoh, Inorg. Chem. 2001, 40, 6604 – 6609; e) H.
Schmidbaur, W. Bublak, B. Huber, G. Reber, G. Mueller, Angew.
Chem. 1986, 98, 1108 – 1109; Angew. Chem. Int. Ed. Engl. 1986,
25, 1089 – 1090; f) R. W. Turner, E. L. Amma, J. Am. Chem. Soc.
1966, 88, 1877 – 1882.
[17] C.-W. Chang, G.-H. Lee, Organometallics 2003, 22, 3107 – 3116.
[18] a) F. Olbrich, U. Behrens, E. Weiss, J. Organomet. Chem. 1994,
472, 365 – 370; b) W. Frosch, S. Back, G. Rheinwald, K. Kꢀhler,
H. Pritzkow, H. Lang, Organometallics 2000, 19, 4016 – 4024;
c) K. Kꢀhler, H. Pritzkow, H. Lang, J. Organomet. Chem. 1998,
553, 31 – 38.
[19] V. W.-W. Yam, S. W.-K. Choi, C.-L. Chan, K.-K. Cheung, Chem.
Commun. 1996, 2067 – 2068.
[20] See the following Communication in this issue: S. Punna, J.
Kuzelka, Q. Wang, M. G. Finn, Angew. Chem. 2005, 117, 2255;
Angew. Chem. Int. Ed. 2005, 44, 2215 (DOI: 10.1002/
anie.200461656).
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