Highly efficient and recyclable catalyst for the direct chlorination, bromination and iodination of terminal alkynes

Article abstract of DOI:10.1016/j.jcat.2017.07.019

Direct halogenation, including chlorination, bromination and iodination of terminal alkynes, are of great importance in organic synthesis. Here an efficient and recyclable nano-Ag/g-C₃N₄ catalyst system was developed and proved to be remarkably active with 39 examples varied from chlorination, bromination to iodination, of which 14 runs have yielded more than 95% of the product. Recycling of the catalyst was also achieved without obvious activity loss after several runs: 99% yield was observed even after 5 runs in the bromination of phenylacetylene. The catalysts system is of low cost and easy to be prepared, making this procedure versatile, convenient and economic.

Full text of DOI:10.1016/j.jcat.2017.07.019

Journal of Catalysis 353 (2017) 199–204
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Highly efficient and recyclable catalyst for the direct chlorination,
bromination and iodination of terminal alkynes
⇑
Wei Shi, Zhipeng Guan, Peng Cai, Hao Chen
Department of Chemistry, College of Science, Huazhong Agricultural University Wuhan, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Direct halogenation, including chlorination, bromination and iodination of terminal alkynes, are of great
importance in organic synthesis. Here an efficient and recyclable nano-Ag/g-C3N4 catalyst system was
developed and proved to be remarkably active with 39 examples varied from chlorination, bromination
to iodination, of which 14 runs have yielded more than 95% of the product. Recycling of the catalyst was
also achieved without obvious activity loss after several runs: 99% yield was observed even after 5 runs in
the bromination of phenylacetylene. The catalysts system is of low cost and easy to be prepared, making
this procedure versatile, convenient and economic.
Received 25 April 2017
Revised 7 July 2017
Accepted 20 July 2017
Keywords:
Ag/C3N4
Ó 2017 Elsevier Inc. All rights reserved.
1-Haloalkyne
Terminal alkyne
Heterogeneous catalysis
Halogenation
Alkyne
1. Introduction
the PTC. Organic base, such as DBU (1,8-Diazabicyclo[5.4.0]undec-
7-ene) was also reported as the catalyst for the preparation of
1-haloalkynes are a series of important and powerful building
blocks in organic chemistry. They are not only the precursors of a
variety of advanced structures such as conjugated diynes, enynes,
substituted alkenes, heterocycles and functional polymers, but also
conceived as a dual functionalized molecules due to their unique
structures involving both controllable electrophilic and nucle-
ophilic properties [1–4], as shown in Fig. 1.
Considering about the importance of haloalkynes, different
methods of the preparation of them are developed during decades
[1]. Among these protocols, the most commonly and widely used
methods are the direct halogenations of CAH bonds in terminal
alkynes [5,6]. However, satisfactory results can be observed only
for bromination and iodination in a long period [7–9]. The chlori-
nation of terminal alkynes under mild conditions, however,
remains a challenge. Typical procedures often require the involve-
ment of highly unstable alkyl lithium reagents under À78 °C
[10,11]. Other improvements of chlorination of terminal alkynes
including using hypochlorites [12] or PTC (phase transfer catalyst)
[13–15]. For example, Szafert et al. have reported a Silver nitrate
catalyzed chlorination of terminal alkynes, using TBAF (tetrabuty-
lammonium fluoride) as the PTC [16]. It is noteworthy that PTC in
this process was crucial, and no reaction occurred in the absence of
chloroalkynes using NCP (N-chlorophthalimide) as the chlorination
reagent [9], although only limited examples were achieved.
Recently our group have developed a practical method for the chlo-
rination of terminal alkynes using Ag2CO3 as the catalyst and
K2CO3 as the base [17]. Although the recycled silver salts have lost
the catalytical activity, good to high isolated yields were achieved
for a variety of terminal alkynes, encouraging us to further explore
the halogenation of terminal alkynes. Despite of all the methods
mentioned above, there is still a lack of a general and convenient
method suitable for the bromination, iodination as well as chlori-
nation, as Fig. 2 showed. Here we would like to report our latest
discoveries on the halogenation (including chlorination, bromina-
tion and iodination) of terminal alkynes using a universal catalyst.
2. Results and discussions
2.1. Bromination of alkynes
During our exploration, the recycling of catalyst stands as the
first priority to fulfill the purpose of sustainable chemistry. The
immobilization of metal catalyst is one of the choices. Since
g-C3N4 (graphitic carbon nitride) has shown a series of advantages
such as low cost, easy to be prepared and functionalized, insoluble
in most solvents and potential photocatalytic properties, and rep-
resents a series of heterogeneous catalyst in many fields [18–22],
⇑
Corresponding author.
E-mail address: hchenhao@mail.hzau.edu.cn (H. Chen).
http://dx.doi.org/10.1016/j.jcat.2017.07.019
0021-9517/Ó 2017 Elsevier Inc. All rights reserved.

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W. Shi et al. / Journal of Catalysis 353 (2017) 199–204
Fig. 3. TEM image of Ag@C3N4-10.8 wt%.
Fig. 1. Important organic structures from haloalkynes.
Fig. 2. Direct halogenation of terminal alkynes.
we have also applied it as the co-catalyst in our study. Ag was cho-
sen as the metal catalyst due to the high activities in alkyne syn-
thesis [6,17,23], and we have thus prepared the Ag/g-C3N4 to
initialize our research. The g-C3N4 catalyst can be easily prepared
by one-step polymerization of urea at 550 °C for 2 h in a muffle fur-
nace with a heating rate of 5 °C/min. Ag (10.8 wt%) was introduced
through a chemical deposition method, using NaBH4 as the reduc-
tant, and the residue was dried at 60 °C for 12 h, resulting a light
yellow powder as the final form. TEM image (Fig. 3) shows that
the Ag particles are immobilized at the surface of C3N4 as small
nanoparticles with diameter of about 10–40 nm.
To test the catalytic activity of the prepared catalyst, we initiate
the investigation with the bromination of terminal alkynes with
NBS (N-bromosuccinimide), which yields 1-bromoalkynes, a kind
of important and widely used precursors in organic synthesis
[24–28]. Phenylacetylene (1a) was used as the substrate to opti-
mize the reaction conditions, and the results are shown in
Scheme 1. The catalyst has shown excellent activities even under
room temperature, with 4 mol% of the catalyst loading (catalyst
loading is calculated according to Ag) to generate 99% of the pro-
duct 2a in less than 3 h.
Scheme 1. Bromination of phenylacetylene under different conditions.
withdrawing or electron-donating ones, do not have obvious influ-
ences on the yields. Heteroaryl substituted alkynes, such as 2m and
2n, also gave excellent isolated yields. Terminal diynes, which are
less stable than ordinary terminal alkynes, also gave excellent
yields (2o and 2p).
2.2. Iodination of alkynes
We have then further applied the catalyst in the iodination
experiments, and found that the catalyst performed excellently
again. Using NIS (N-iodosuccinimide) as the iodination reagent,
high yields of 1-iodoalkynes were observed in all runs, ranging
from 88% to 99% with 12 different examples, and details are listed
in Scheme 3. Aryl substituents on the alkynes with either electron
withdrawing or donating groups all gave satisfactory yields (3b–
3e). The positions of the substitution group on the aryl ring of
Different substrates were then used under the optimized condi-
tions, and we are pleased to observe that all the substrates we
tested have achieved excellent results, and the yields are listed in
Scheme 2. The substituents on the aryl moieties, either electron-

W. Shi et al. / Journal of Catalysis 353 (2017) 199–204
201
Scheme 2. Bromination of different substrates.
the substrate have no obvious effect to this reaction (3f–3h). Het-
eroaryl groups were tolerated in this reaction too (3i and 3j).
With these results in hand, we have applied the optimized con-
ditions to the chlorination of a variety of terminal alkynes, and sat-
isfying yields were observed again, as shown in Scheme 5. Both
EWG and EDG substituted aryl group are tolerable in these trans-
formations, and the heterocyclic substituted acetylenes have also
performed well (entries 7, 8), indicating the universality of the cat-
alyst. Alkynes with formyl group could also be good substrates
(entry 9).
2.3. Chlorination of alkynes
Compared with bromination and iodination, the chlorination of
terminal alkynes is of more challenging, and has attracted much
more attentions in recent years [9,29,30]. We have recently
reported an efficient method of the direct chlorination of terminal
alkynes using Ag2CO3 as the catalyst and NCS (N-
chlorosuccinimide) as the chlorine source [17]. Although high
yields and broad substrate scope were achieved, considerable loss
in catalytic activity was observed for the recycled catalyst. Here we
have also explored the activity of this new catalyst for the chlori-
nation of terminal alkynes, and results are listed below.
Considering that the reactivity of NCS is relatively lower com-
pared with NIS or NBS, we have set the reaction temperature at
50 °C comparing to the previous room temperature. However, only
moderate yield was observed (entry 5, 67%). Further optimizations
were then conducted, and the results were shown in Scheme 4. It is
found that MeCN performs the best among various solvents, and
base is crucial to this process, which K2CO3 was proved to be a suit-
able one. Best result was obtained when 2.0 eq of NCS was
employed under 50 °C, with almost quantitative transformation
of terminal alkynes (entry 16, 99%). Lower temperature led to the
decline of the reaction rate (entries 16–19).
2.4. Recycling and discussion of the catalyst
The recycling tests of the catalyst activity for the bromination of
terminal alkynes have also been explored. At first, we have simply
isolated the catalyst from the reaction system by centrifugation
and applied it in the next bromination process, but unfortunately
no reactivity was observed at all. Following XRD analysis of the cat-
alyst showed that after the first bromination, the valence of Ag
have changed from 0 to 1, indicating that the immobilized Ag (0)
have been transformed into AgBr during the bromination, as
shown in Fig. 4. Similar results were also observed in the process
of chlorination and iodination, showing a valence change in the
reaction. Thus we suppose that the reduction of the recycled Ag
(I) may reactivate the catalyst, and NaBH4 aqueous solution was
used in this process as the reductant.
After these reduction steps, the recycled catalysts did have
shown excellent activities in the halogenation process. Fig. 5 listed

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W. Shi et al. / Journal of Catalysis 353 (2017) 199–204
Scheme 3. Iodination of terminal alkynes.
Scheme 4. Optimization of chlorination.

W. Shi et al. / Journal of Catalysis 353 (2017) 199–204
203
99%
99%
97%
97%
95%
Run ꢀmes
Fig. 5. Recycle test for the catalyst in the bromination of phenylacetylene.
lized AgBr, and the reduction of AgBr to nano silver particles would
reactive the catalyst.
The leaching of the Ag was also discussed. After the bromina-
tion of phenylacetylene using our catalyst, the mixture was cen-
trifuged, and hydrochloric acid (0.1 mol/L) was added dropwise
to the supernatant. However, the supernatant remained clear.
ICP-MS analysis showed that the Ag lost was 0.0128% after one cat-
alytic circle. There are also reported examples that confirmed the
absence of metal leaching including Ag deposit on g-C3N4
[31,32], indicating the excellent stability and recyclability of the
catalyst.
3. Conclusion
Scheme 5. Chlorination of terminal alkynes.
In summary, we have developed a general, versatile and highly
efficient Ag/C3N4 catalyst for the chlorination, bromination and
iodination of the terminal alkynes. The yields of the halogenated
products are above 90% in most cases, proved by nearly 40 exam-
ples, including the relatively challenging chlorination process.
Moreover, the catalyst could be reused after reduction for more
than 5 times without any obvious loss in activity.
AgBr
a
b
g-C3N4
Ag
Acknowledgments
Ag
g-C3N4
This work was supported by the National Natural Science
Foundation of China (NSFC) (51572101, 21202050) and the Natural
Science Foundation of Hubei Province of China (2014CFB930).
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2017.07.019.
20
40
60
80
2θ (Dgree)
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