Cu(II)-Cu(I) synergistic cooperation to lead the alkyne C-H activation

Article abstract of DOI:10.1021/ja5097489

An efficient alkyne C-H activation and homocoupling procedure has been studied which indicates that a Cu(II)/Cu(I) synergistic cooperation might be involved. In situ Raman spectroscopy was employed to study kinetic behavior, drawing the conclusion that Cu(I) rather than Cu(II) participates in the rate-determining step. IR, EPR, and X-ray absorption spectroscopy evidence were provided for structural information, indicating that Cu(I) has a stronger interaction with alkyne than Cu(II) in the C-H activation step. Kinetics study showed Cu(II) plays a role as oxidant in C-C bond construction step, which was a fast step in the reaction. X-band EPR spectroscopy showed that the coordination environment of CuCl₂(TMEDA) was affected by Cu(I). A putative mechanism with Cu(I)-Cu(II) synergistic cooperation procedure is proposed for the reaction.

Full text of DOI:10.1021/ja5097489

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Communication
Cu(II)/Cu(I)-Synergistic Cooperation to Lead the Alkyne C-H Activation
Ruopeng Bai, Guanghui Zhang, Hong Yi, Zhiliang Huang, Xiaotian Qi, Chao Liu,
Jeffrey T Miller, Arthur Jeremy Kropf, Emilio E Bunel, Yu Lan, and Aiwen Lei
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja5097489 • Publication Date (Web): 10 Nov 2014
Downloaded from http://pubs.acs.org on November 12, 2014
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Cu(II)/Cu(I)-Synergistic Cooperation to Lead the Alkyne C-H Activa-
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Ruopeng Bai,† Guanghui Zhang,†,‡ Hong Yi,† Zhiliang Huang,†,‡ Xiaotian Qi,§ Chao Liu,† Jeffrey
T. Miller,‡ A. Jeremy Kropf,‡ Emilio E. Bunel,‡ Yu Lan,*,§ and Aiwen Lei*,†,‡
†College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, P.R. China
‡Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 S. Cass Ave., Argonne, Illinois
60439, United States
§School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400030, P.R. China
Supporting Information Placeholder
different from other isolated intermediates, catalytic behav-
ABSTRACT: An efficient alkyne C-H activation and homo-
ior of Cu-acetylide is not that active. Herein, we communi-
cate an efficient autocatalytic alkyne C-H activation proce-
dure via Cu(II)-Cu(I) synergistic cooperation.
coupling procedure has been studied which indicates that a
Cu(II)/Cu(I) synergistic cooperation might be involved. In
situ Raman spectroscopy was employed to study kinetic be-
havior, drawing the conclusion that Cu(I) rather than Cu(II)
participates in the rate determining step. IR, EPR and X-ray
absorption spectroscopy evidence were provided for struc-
tural information, indicating that Cu(I) has a stronger inter-
action with alkyne than Cu(II) in the C-H activation step.
Kinetics study showed Cu(II) plays a role as oxidant in C-C
bond construction step, which was a fast step in the reaction.
X-band EPR spectroscopy showed that the coordination en-
vironment of CuCl2(TMDEA) was affected by Cu(I). A puta-
tive mechanism with Cu(I)-Cu(II) synergistic cooperation
procedure was proposed for the reaction.
Our initial effort focused on the kinetic behaviors of the
homocoupling of terminal alkynes with Cu(II). Raman spec-
troscopy is a powerful method to monitor Raman-active C-C
triple bonds. Herein, in situ FT-Raman spectroscopy was
employed instead to detect the changes of alkyne substances
(Figure 1). CuCl2(TMEDA) (TMEDA = tetramethylethylenedi-
amine) 2 could be reduced by phenylacetylene 1 promptly to
o
form biphenylacetylene 3 at 37 C (Figure 1a). In Figure 1b, a
short induction period, less than 150s, was observed, together
with an auto-acceleration curve. Similar kinetic behavior was
observed when lowering the reaction to 0 oC, however, with a
longer induction period and longer reaction time. Upon fur-
ther cooling to -20 oC, the reaction was totally restrained
unless rising the temperature.
Copper salts are well accepted as remarkable, important
and promising catalysts,1 which are widely used to synthesize
a variety of compounds, such as natural products, medicine
molecules, agrochemicals, and organic functional materials.2
Among these, coupling reactions involving terminal alkynes
is a significant branch, and have a history of over one century,
and several named reactions were developed to build Csp-C
single bonds, such as Glaser-Hay reaction3 and Sonogashira
reaction.4 These processes have received extensive attention
and been widely applied as simple and convenient methods
to synthesize internal alkynes and diynes.5 There have also
been a number of studies that have investigated the reaction
mechanism.6
As the kinetic behavior was so close to the autocatalytic
reaction module, a similar autocatalytic procedure involving
Cu(I) was proposed. To verify our hypothesis, a large concen-
tration of Cu(I) was added to in the reaction at -20oC. The
induction period disappeared and the reaction finished in
about 10 min, and the reaction rate was evidently improved
with 0.5 mmol [CuCl(TMEDA)]2 (Figure 1c). Moreover, when
decreasing the amount of [CuCl(TMEDA)]2 to 0.1 mmol, the
reaction was moderated and the induction period appeared
again. These results indicated that Cu(I) was involved in the
reaction, same as orthodox autocatalytic procedures.
To clear the roles Cu(I) and Cu(II) play in the reaction, we
tried to investigate the interaction between Cu species and
different substances. As Cu(II) is widely used to catalyze
terminal alkynes coupling reactions,6e, 7a, 7b to elucidate the
effect of Cu(II) specie in the reaction, detailed kinetic exper-
iments of CuCl2(TMEDA) and phenylacetylene were carried
out for the observed initial rate constant of the reaction in
the presence of same initial amount of [CuCl(TMEDA)]2. As
shown in Figure 2a, variation of the initial concentration of
CuCl2(TMEDA) displayed little influence upon the initial rate
However, the mechanistic research of these reactions still
remains a challenge and attracted numerous attention. Usu-
ally, in most Cu-catalyzed coupling reactions with alkynes, it
is generally proposed that the C-H activation of alkyne, to
form Cu(I)-acetylide, is a smooth and facile step. It is gener-
ally accepted that Cu(I) acetylides are key intermediates for
numerous Cu-catalyzed synthetic reaction,7 however, some-
times without convincing evidences. As is well known, syn-
thetic Cu-acetylide are polymers,8 and are not kinetically
active as catalytic intermediates in most reactions. As a result,
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of the reaction, presenting a zero-order dependent on
considered as direct evidence that Csp-Csp bond construc-
1
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CuCl2(TMEDA). This indicated that it is more likely Cu(II)
didn’t participate in the rate determining step (RDS). Mean-
while, as shown in Figure 2b, a first-order kinetic plot was
obtained on the initial concentration of phenylacetylene,
suggesting that phenylacetylene was involved in the rate
determining step.
tion was a fast, non-rate-determining step in the reaction.
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C
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Figure 2. (a) Kinetic profiles with different concentration of
Time / s
CuCl2(TMEDA) at -20 oC. (b) Kinetic profiles and fitting curve of
o
initial rate with different concentration of phenylacetylene at -20 C,
and the Kobv = 4.485×10-4 s-1. All of the reactions were carried out
with 1 (2.0 mmol or as shown), [CuCl(TMEDA)]2 (0.25 mmol),
iPr2NH (2.0 mmol), CuCl2(TMEDA) (2.0 mmol or as shown) in 10 mL
DMF. See Supporting Information for details.
0.05
0.04
0.03
0.02
0.01
0.00
without 4
with 0.01 mmol 4
with 0.05 mmol 4
According to the kinetic behavior above, we put our efforts
into discerning the catalytic intermediates in the catalytic
process engaging various analytic methods. When phenyla-
cetylene was introduced into the DMF solution of
[CuCl(TMEDA)]2, a weak Infrared (IR) shift of the peak of
[CuCl(TMEDA)]2 was detected from 796 cm-1 to 800 cm-1.
Similar effect was not observed by substitution
CuCl2(TMEDA) for Cu(I). This phenomenon could be re-
curred by electron paramagnetic resonance (EPR): the addi-
tion of phenylacetylene didn’t cause the peak shift of
CuCl2(TMEDA). This phenomena can be explained by densi-
ty functional theory (DFT) calculations: the coordination
energy of phenylacetylene-Cu(II)-TMEDA adduct (+5.9
kcal/mol) is much higher than phenylacetylene-Cu(I)-
TMEDA adduct (+1.2 kcal/mol),9 ,which means that the co-
ordination between phenylacetylene and Cu(II)-TMEDA
adduct is more difficult than Cu(I)-TMEDA adduct.
0
100
200
300
400
500
600
Time / s
Figure 1. (a) Raman spectra change in the reduction process at 37 oC.
(b) Kinetic profiles of the reduction process at 37 oC (black) and -20
oC (red). (c) Kinetic profiles with 0 mmol (red), 0.05 mmol (green),
0.5 mmol (blue) 4 at -20 oC. All of the reactions were carried out with
1 (2.0 mmol), 2 (1.0 mmol), iPr2NH (2.0 mmol), in 10 mL DMF. See
Supporting Information for details.
Moreover, for the purpose of investigating the reaction
rate of C-C bond formation, stoichiometric reaction between
CuCl2(TMEDA) and lithium phenylacetylide was carried out.
On addition of CuCl2(TMEDA) solution into lithium phe-
nylacetylide solution at -70 oC, the increase of diphenylacety-
lene was detected immediately, and the reaction was com-
plete within 2 minutes monitored by in situ IR spectroscopy.
Compared with the kinetic behavior above, this could be
In addition, we came to the same conclusion when investi-
gating the interaction between Cu catalyst and phenylacety-
lene via X-ray absorption spectroscopy (XAS). As shown in
Figure 3, in the R-space EXAFS spectra, a shorter average
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bond distance was observed once upon the addition of phe-
adding stoichiometric [CuCl(TMEDA)]2 into CuCl2(TMDEA)
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nylacetylene, which suggested the coordination of phenyla-
cetylene to Cu(I). Moreover, the shape of the XANES spec-
trum also changed, suggesting that phenylacetylene coordi-
nated to Cu(I) center. On the contrary, when phenylacety-
lene was added into the CuCl2/TMEDA solution, no obvious
change was found in neither the XANES nor the EXAFS spec-
tra. It can be concluded that Cu(I) has stronger interaction
with phenylacetylene than Cu(II).
DMF solution. That indicates that the coordination envi-
ronment of CuCl2(TMDEA) was affected by added stoichio-
metric Cu(I). These results showed that the coordination
environment of CuCl2(TMDEA) could be affected by Cu(I),
directing to a possibility of Cu(I)-Cu(II) intermediate struc-
ture.
Based on the investigations and clues above, putative
mechanism was proposed (Scheme 1). First, Cu(I) coordinat-
ed with terminal alkyne reactants, to generate the coordina-
tion adduct intermediate I. By this step, inactive C-H bond
could be activated for C-H activation. Subsequently, in the
presence of base, I was deprotonated, and this step was con-
sidered as the rate determining step according to previous
kinetic studies. Meanwhile, C-Cu(II) bond was constructed,
to generate intermediate II. Next, III was generated from II
by a dimerization step. Finally, III participated in the elec-
tron transfer and C-C bond formation step, to generate the
final coupling product 3 and the reduced Cu(I) species.
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0.8
(a)
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R / Å
1.5
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Scheme 1. Key steps of the putative mechanism.
R / Å
In conclusion, we have discovered an efficient terminal al-
kyne C-H activation and homocoupling procedure via Cu(I)-
Cu(II) synergistic cooperation. By studying the kinetic behav-
ior with in situ Raman spectroscopy, we came to the conclu-
sion that Cu(I) participated in the rate determining step and
accelerate the reduction procedure efficiently; and Cu(II)
took part in the C-C bond formation step. IR, EPR, X-ray
absorption spectroscopy experiments and DFT calculation
were proceeded for observing the coordination between dif-
ferent Cu species and alkyne, giving a direct evidence that
Cu(I) has a stronger coordination effect with terminal al-
kynes than Cu(II) to promote the C-H activation step. By
kinetic studies, we made it clear that Cu(II) played a role as
oxidant in C-C bond construction step, which was proved as
a fast step in the reaction. Moreover, the difference of X-
band EPR spectroscopy showed that the coordination envi-
ronment of CuCl2(TMDEA) was affected by Cu(I), drawing a
possibility of Cu(I)-Cu(II) intermediate structure, which
could be an efficient way to shorten the distance between
Cu(II) oxidant and alkyne. All these clues described the cata-
lytic circle: Cu(I) is more possible than Cu(II) to play a role as
catalyst: coordinating with alkynes to promote C-H activa-
tion procedure, followed by a fast electron transfer step from
Cu(II), to construct the homocoupling product. The overall
reaction was proceeded with the assist of Cu(I)-Cu(II) syner-
Figure 3. (a) k2-weighted magnitude of the fourier transform (FT) of
[CuCl(TMEDA)]2 without (red) and with (black) phenylacetylene. (b)
k2-weighted magnitude of the fourier transform (FT) of
CuCl2(TMEDA) without (red) and with (black) phenylacetylene.
CuCl2(TMEDA) + 1/2[CuCl(TMEDA)]2
CuCl2(TMEDA)
2600 2700 2800 2900 3000 3100 3200 3300
B / G
Figure 4. X-band EPR spectra of CuCl2(TMEDA)-[CuCl(TMEDA)]2
DMF solution (black, 0.01M CuCl2(TMEDA) and 0.005M
[CuCl(TMEDA)]2), and CuCl2(TMEDA) DMF solution (red, 0.01M).
Cu(I) could not only coordinate with the terminal alkynes,
but affect the coordination environment of Cu(II). As shown
in Figure 4, slight EPR spectra differences were observed by
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ASSOCIATED CONTENT
Supporting Information
The experimental procedure; detailed IR, EPR and
XANES/EXAFS results; detailed information of DFT calcula-
tion. This material is available free of charge via the Internet
at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
lanyu@cqu.edu.cn
aiwenlei@whu.edu.cn
(9) For detailed information of DFT calculation, please refer Supporting
Information.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was supported by the “973” Program from the
MOST of China (2012CB725302), the National Natural Sci-
ence Foundation of China (21390400, 21025206, 21272180
21302148 and 21372266), the Research Fund for the Doctoral
Program of Higher Education of China (20120141130002) and
the Program for Changjiang Scholars and Innovative Re-
search Team in University (IRT1030).The Program of Intro-
ducing Talents of Discipline to Universities of China (111 Pro-
gram) is also appreciated. Use of the Advanced Photon
Source was supported by the U. S. Department of Energy,
Office of Science, Office of Basic Energy Sciences, under Con-
tract No. DE-AC02-06CH11357. MRCAT operations are sup-
ported by the Department of Energy and the MRCAT mem-
ber institutions. Partial funding for J.T.M. was provided by
the Chemical Sciences, Geosciences and Bioscience Division,
U.S. Department of Energy, under contract no. DE-AC0-
06CH11357. This work was also funded by the Chemical Sci-
ences and Engineering Division at Argonne National Labora-
tory.
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