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336 J. Phys. Chem. A, Vol. 114, No. 32, 2010
Sun and Wu
basis of the catalytic cycle proposed by Sharpless have been
made, wherein it is noted that the consumption of Cu(I)-
coordinated alkyne 2 was earlier than that of free alkyne 1,
contrary to the reacting direction. That is because the coordina-
tion of Cu(I) and alkyne to afford Cu(I) acetylide 3 proceeded
very fast and 2Dcos cannot discern this sequence. The sequence
Acknowledgment. We gratefully acknowledge the financial
support National Science Foundation of China (NSFC) (20934002,
20774022), the National Basic Research Program of China (Nos.
2005CB623800, 2009CB930000).
Supporting Information Available: Determination of rate
orders, operation details of sequence order determination from
2Dcos results, spectral comparison of azido vibrations. This
material is available free of charge via the Internet at http://
pubs.acs.org.
+
-
between the two azido resonance structures of -NdN dN 5
+
-
and NdN dN 4 is also very meaningful in that it was the
+
-
resonance structure -NdN dN 5 that participates in the
complexation between azide and alkyne. It is presumed that
the coordination of Cu(I) acetylide to 5 may lead to a relatively
lower activation energy in the following rate-determining step
from the perspective of charge distribution. Along with the
formation of 1,2,3-triazole 8, phenyl and CdO were affected
successively, with CdO being the slowest response. According
to the catalytic cycle proposed by Sharpless, the azide-alkyne
References and Notes
(
1) (a) Moses, J. E.; Moorhouse, A. D. Chem. Soc. ReV. 2007, 36, 1249.
(
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(
2) (a) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B.
1
:1 complex 6 would first produce a preproduct or triazolyl-
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copper derivative 7 followed by its proteolysis to afford 1,2,3-
triazole 8. The slowest response of CdO in triazole indicates
that this proteolysis process was also very fast and 2Dcos cannot
discern it. Thus, the rate-determining step can be easily
confirmed only to the transition of azide-alkyne 1:1 complex
(
3) Himo, F.; Lovell, T.; Hilgraf, R.; Rostovtsev, V.; Noodleman, L.;
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(
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6
to the preproduct 7. This is the first experimental proof for
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(
5) Rodionov, V. O.; Fokin, V. V.; Finn, M. G. Angew. Chem., Int.
2
, the structure of azide-alkyne 1:1 complex 6 has also been
6a
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(
4
4
. Conclusion
A designed ligand-accelerated CuAAC reaction was moni-
(
(
tored for the first time by real time infrared analysis technique
based on ATR-FTIR principles. Three reacting groups directly
participating in the reaction (azido, alkynyl, and 1,2,3-triazole)
are all IR-responsive and can be traced primely. The absorbance-
time curves with different molar ratios of reactants as well as
PCA calculations on the fingerprint region showed that the
consumption of alkyne and azide took place successively
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(
(
4
(
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1
:1 complex of two reactants or the intermediate would be
formed in the reaction process, in good conformity with current
catalytic mechanism that Sharpless proposed. 2Dcos discerned
the sequence of all the reacting species and confirmed the rate-
determining step of CuAAC reaction to be the transition of the
azide-alkyne 1:1 complex to the preproduct 1,2,3-triazole. A
modified catalytic cycle has also been plotted to get a better
understanding of the CuAAC reaction.
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(
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1
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