An efficient photocatalyst for the azide-Alkyne click reaction based on direct photolysis of a copper(II)/carboxylate complex

Article abstract of DOI:10.1246/cl.140240

Photochemical processes provide a temporal and spatial control of the reaction, which offers an efficient way for chemical synthesis and affords extensive applications in materials science. In this study, a new photocatalyst system based on a Cu(II)/carbo

Full text of DOI:10.1246/cl.140240

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Received: March 18, 2014 | Accepted: April 8, 2014 | Web Released: April 15, 2014
CL-140240
An Efficient Photocatalyst for the Azide-Alkyne Click Reaction Based
on Direct Photolysis of a Copper(II)/Carboxylate Complex
Xin Guan, Jing Zhang, and Yuechuan Wang*
College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials and Engineering,
Sichuan University, Chengdu 610065, P. R. China
(E-mail: ycwang@scu.edu.cn)
Photochemical processes provide a temporal and spatial
control of the reaction, which offers an efficient way for
chemical synthesis and affords extensive applications in
materials science. In this study, a new photocatalyst system
based on a Cu(II)/carboxylate complex was developed for
photoinduced CuAAC reaction. The results show that the
conversion was over 90% after irradiation for 30 min.
Click chemistry is defined as a class of chemical reactions
that are modular, wide in scope, give very high yields, generate
little or no by-product, and are stereospecific.1 The most typical
case of a click chemistry reaction is the copper(I)-catalyzed
azide-alkyne cycloaddition (CuAAC) reaction, which was
discovered by the groups of Sharpless2 and Meldal3 in 2001.
The unique properties of the CuAAC reaction, such as high
yields and modular synthesis make it a powerful tool for
polymer chemistry,4 drug synthesis,5 and surface modification.6
Photochemical processes provide a temporal and spatial
control of the reaction, which can offer an efficient way for
chemical synthesis and afford extensive applications in materials
science.7 Recently, Bowman8 and Yagci9 have developed a
photoinduced CuAAC reaction using a Cu(II)/N,N,N¤,N¤¤,N¤¤-
pentamethyldiethylenetriamine (PMDETA) complex and a pho-
toinitiator as catalyst. During irradiation, the photoinitiator
generates radicals that can reduce the Cu(II) to Cu(I) complex
that subsequently catalyzes the CuAAC reaction. The compre-
hensive temporal and spatial control and high-efficiency of the
photoinduced CuAAC reaction have been proved by standard
photolithographic techniques.8a
It should be noted that the reaction can also be catalyzed by
a direct photoelectron transfer from metal ligand PMDETA to
Cu(II), reducing Cu(II) to Cu(I).10 Since the direct photolysis of
the Cu(II) complex makes possible a simple catalyst system and
a rapid reducing of Cu(II), to accelerate the CuAAC reaction,
a new photocatalyst system is called for. In this study, we
developed a highly efficient photocatalyst for the CuAAC
reaction based on the Cu(II)/carboxylate complex.
In this study, benzyl azide and 1-hexyne (1:1.1 molar ratio)
were chosen as simple model click components. After adding
the preprepared solution of CuCl2¢2H2O/sodium benzoate
(10 mol % of benzyl azide) in DMSO-d6, the reaction mixture
was irradiated by a high-pressure mercury UV lamp (8.05
mW cm¹2) for 30 min. The 1H NMR spectra of the reaction
mixture before and after the irradiation are shown in Figure 1.
The formation of triazole is confirmed by the disappearance of
the -CH2- group signal in benzyl azide (4.44 ppm) and the
appearance of the -CH2- group signal in benzyl triazole
(5.53 ppm). After 30 min irradiation, the conversions of the
reaction in DMSO and ACN (Figure S1),11 are 97% and 93%,
Figure 1. 1H NMR spectra of benzyl azide and 1-hexyne in DMSO-d6
(a) before and (b) after irradiation for 30 min using 10 mol % CuCl2/
C6H5COONa as catalyst, and (c) the corresponding isolated product, in
which the © denotes residual CH2Cl2.
respectively. Besides, the 1H NMR spectrum of the isolated
product is shown in Figure 1c, and the purification process is
provided in the Supporting Information.
The photoinduced CuAAC reaction of benzyl azide and 1-
hexyne was investigated by real-time FT-IR to reveal the kinetic
¹1
process. The absorbance of the azide group at 2096 cm was
monitored during UV irradiation at room temperature. The
conversion curves of the photoinduced CuAAC reaction with
various catalyst systems are shown in Figure 2a. As can be seen
from the figure, the reaction only occurs under UV irradiation
at room temperature. Different from the Cu(II)/acetate catalyst
system reported by Zhu’s group,12 the Cu(II)/benzoate does not
undergo the Glaser reaction; therefore, it is stable without UV
irradiation. Although the reaction can take place using CuCl2 in
DMSO without any additional ligand, the reaction rate is much
slower than the same reaction catalyzed by CuCl2/C6H5COONa.
The conversion reaches 90% in 5 min with CuCl2/C6H5COONa
as catalyst, whereas the reaction with CuCl2/PMDETA as
catalyst, which was commonly used in ATRP and recently
employed in photoinduced CuAAC reaction,10 reaches the same
percentage after irradiation for 40 min.
The efficiency of the photocatalyst is revealed by the effect
of the concentration of CuCl2/C6H5COONa on the reaction, as
shown in Figure 2b. When the content of CuCl2/C6H5COONa
ranges from 1 to 10 mol % of the benzyl azide, the conversion
of the reaction reaches 90% after 5 min irradiation. Even if
the content of CuCl2/C6H5COONa reduced to 0.1 mol %, the
conversion can still reach 90% in 10 min. Based on these results,
Chem. Lett. 2014, 43, 1073–1074 | doi:10.1246/cl.140240
© 2014 The Chemical Society of Japan | 1073

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¹1
Figure 3. UV-vis spectra of 0.002 mol L
CuCl2/C6H5COONa in
DMSO (solid line) before and (dash line) after UV irradiation for
30 min. Insert: zoomed spectrum between 625 to 800 nm.
Scheme 1. Photoinduced CuAAC reaction via direct photoelectron-
transfer route.
reducing chlorocopper(II) complexes ([CuIIClx]2¹x) to Cu(I)
complexes and chloro radicals14 (Figure S3).
In summary, we reported a simple and efficient photo-
catalyst for the photoinduced CuAAC reaction based on the
Cu(II)/carboxylate complex. The results show that the CuAAC
reaction containing CuCl2 can take place without any additional
ligand. However, the CuCl2/C6H5COONa catalyst accelerates
the reaction over ten times, and the conversion exceeds 90%
in 30 min. Based on these results, we believe that the direct
photoelectron transfer from the metal ligand to Cu(II) can
accelerate the catalytic process; thus accelerating the CuAAC
reaction. Further studies on Cu(II) complex catalysts for the
photoinduced CuAAC reaction are in progress.
Figure 2. The conversion curves of photoinduced CuAAC reaction of
benzyl azide and 1-hexyne (a) with various catalyst systems, and (b) with
different concentration of CuCl2/C6H5COONa.
we can safely draw the conclusion that CuCl2/C6H5COONa is
an efficient photocatalyst for the azide-alkyne cycloaddition.
To confirm the generation of Cu(I) via photoelectron transfer
from the ligand to the Cu(II) center, we recorded the UV-vis
absorbance of the CuCl2/C6H5COONa solution before and
after irradiation using a UV-vis spectrophotometer (Figure 3).
The strong absorbance and bright green color of the solution
indicates the existence of the Cu(II)/carboxylate complex, which
has also been reported by other groups.13 After irradiation, the
UV-vis spectrum shows a decrease in the absorbance of the
Cu(II) complex in the near-infrared region and near UV region,
indicating the generation of the Cu(I) complex. The light yellow
color of the solution after irradiation also suggests the generation
of the Cu(I) complex. After it was left open to air overnight,
the solution turned to green, implying the formation of Cu(II)
by oxidation of Cu(I). Photograph of the color changes of the
photoinduced redox reaction of the Cu(II)/Cu(I) species during
the entire process can be seen in the Supporting Information
(Figure S2). Based on these results, we proposed a mechanism
for the photoinduced CuAAC reaction via direct photolysis of
the Cu(II) complex, as shown in Scheme 1. UV irradiation
drives the electron transfer from the ligand to the Cu(II) directly,
resulting in the formation of Cu(I) that subsequently catalyzes
the CuAAC reaction. It can be seen from Figure 2a that CuCl2
alone can catalyze the CuAAC reaction in DMSO without
sodium benzoate, which can also be explained by the mecha-
nism. Specifically, irradiation of the system results in photo-
References and Notes
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1074 | Chem. Lett. 2014, 43, 1073–1074 | doi:10.1246/cl.140240
© 2014 The Chemical Society of Japan