An efficient and recyclable heterogeneous catalytic system for the synthesis of 1,2,4-triazoles using air as the oxidant

Article abstract of DOI:10.1039/c3ra47029f

Copper-zinc supported on Al₂O₃-TiO₂ was found as a simple and efficient heterogeneous catalyst for the oxidative synthesis of 1,2,4-triazole derivatives using air as the green oxidant under ligand-, base- and additive-free conditions. The heterogeneous reactions carried out smoothly with a large range of substrates, including NO₂-, vinyl-, pyrimidine- and imidazole-contained starting materials, and provided corresponding triazoles in moderate to excellent yields with low catalyst loading (1.6 mol%). Furthermore, the catalyst can be simply recycled many times without significant loss in catalytic activity. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3ra47029f

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An efficient and recyclable heterogeneous catalytic
system for the synthesis of 1,2,4-triazoles using air
as the oxidant†
Cite this: RSC Adv., 2014, 4, 8612
*
Xu Meng, Chaoying Yu and Peiqing Zhao
Copper–zinc supported on Al2O3–TiO2 was found as a simple and efficient heterogeneous catalyst for the
oxidative synthesis of 1,2,4-triazole derivatives using air as the green oxidant under ligand-, base- and
additive-free conditions. The heterogeneous reactions carried out smoothly with a large range of
substrates, including NO2-, vinyl-, pyrimidine- and imidazole-contained starting materials, and provided
corresponding triazoles in moderate to excellent yields with low catalyst loading (1.6 mol%).
Furthermore, the catalyst can be simply recycled many times without significant loss in catalytic activity.
Received 26th November 2013
Accepted 17th January 2014
DOI: 10.1039/c3ra47029f
www.rsc.org/advances
Introduction
Although signicant advances have been brought in homoge-
neous catalysis in decades, this knowledge has not largely been
applied in an industrial environment and industrial processes
still employ heterogeneous catalysts.1 Nevertheless, heteroge-
neous catalysis is widely employed in industrial processes
because it has economic advantages and combats practical
issues, such as catalyst handing, recyclability and the separa-
tion of catalyst from the products.2–6 More importantly,
heterogeneous catalysis meets the requirements of green
sustainable chemistry due to the use of unrecoverable metal-
and ligand-free reaction conditions.2–5 Therefore, the develop-
ment of economic and environmentally friendly heterogeneous
catalytic systems is highly desirable.
The 1,2,4-triazole derivatives are valuable structural motifs
present in a variety of functionalized molecules which have
applied into organocatalysis and material science.6 Moreover,
they are a signicant class of heterocycles with broad utilities in
the pharmaceutical industry because of their biological activi-
ties.7 Commonly, the 1,2,4-triazole derivatives are prepared via
multistep intramolecular condensations of N-acylamidorazones
that are obtained from carboxylic acid derivatives and hydra-
zines.8 With the development of transition-metal-catalysis, the
synthesis of 1,2,4-triazoles turns to be more efficient, simple,
atom-economic and environmentally friendly. The rst transi-
tion-metal-catalytic synthesis of 1,2,4-triazoles using readily
available starting materials was discovered by Nagasawa's
group, although the homogeneous catalytic system suffers the
Scheme 1 Heterogeneous Cu–Zn/Al–Ti-catalyzed cyclization.
use of additional ligand, base, additive and alternative solvents
as the change of substrates.9 Most recently, Fu's group devel-
oped a Cu-catalyzed sequential method of synthesis of 1,2,4-
triazole from amidines.10 In the course of investigating novel
activity of heterogeneous catalysts in our research group,4a,11 we
envisioned a heterogeneous catalytic system for single-step
synthesis of 1,2,4-triazoles from readily available starting
materials using a simple solid-supported catalyst, which can be
a complement to the previous methods.
Here we describe the heterogeneous oxidative catalytic
synthesis of 1,2,4-triazole derivatives via addition-oxidative
cyclizations from 2-aminopyridines and nitriles using solid-
supported CuOx–ZnO/Al2O3–TiO212 (Cu–Zn/Al–Ti) as the catalyst
(Scheme 1). The catalytic system is ligand-, base-, additive-free,
and can tolerate a large range of functional groups as well as
successfully employs air as the oxidant. More importantly, Cu–
Zn/Al–Ti is likely to be heterogeneous in nature and can recycle
many times without losing of activity.
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of
Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R China. E-mail:
zhaopq@licp.cas.cn; Fax: +96 931 8277008; Tel: +86 931 4968688
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ra47029f
Results and discussion
Our initial addition-oxidative cyclizations of 2-aminopyridine
and benzonitrile were carried out by using 1,2-dichlorobenzene
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(DCB) as the solvent, air as the oxidant at 140 ꢀC for 20 h to
optimize the catalytic system (Table 1). Firstly, it was found
using mono-metal solid-supported catalyst, such as Cu/Al–Ti,
Zn/Al–Ti, Ru/Al–Ti and Pt/Al–Ti, did not give any desired
product (Table 1, entries 1–4). Pleasingly, the reaction provided
desired product of 52% yield, when Cu–Zn/Al–Ti (0.4 mol%, Cu
¼ 3.25 wt%, Zn ¼ 3.40 wt%) was used as the heterogeneous
catalyst (Table 1, entry 5). Obviously, the cyclization has a
relatively high reaction energy barrier since the reaction failed if
the reaction temperature was decreased (Table 1, entry 5). To
better the yield of reaction, we tried to use other green oxidants
although the optimization conrmed air was the best and most
economic green oxidant (Table 1, entries 5–7). Subsequently,
the blank experiment was performed by only using support
Al2O3–TiO2 as the catalyst, which gave us a result that Al2O3–
TiO2 did not efficiently catalyze the reaction at all (Table 1, entry
8). Furthermore, the other bimetallic solid-supported catalysts
did not make the reaction perform smoothly (Table 1, entries 9
and 10). In an attempt to increase the yield of reaction, we
proposed a variety of solvents to enhance reactivity of reaction
Table 2 The examination of other supports for the heterogeneous
systema
Entry
Catalyst (1.6 mol%)
Isolated yield (%)
1
Cu–Zn/Al–Ti
Cu–Zn/CeO2
Cu–Zn/ATP
Cu–Zn/MWCNT
Cu–Zn/nano-r-Al2O3
Cu–Zn/nano-TiO2
Cu–Zn/nano-ZrO2
83
51
15
75
21
25
12
2
3b
4c
5
6
7
a
Reaction Conditions: benzonitrile (0.6 mmol), 2-aminopyridine (0.72
b
mmol), catalyst (24 mg, 1.6 mol%), 140 ꢀC, 20 h, air. ATP ¼
c
Attapulgite. MWCNT ¼ Multi-wall carbon nanotube.
system, including polar, nonpolar, protic solvents and H2O, and 1.6 mol%, the yield of reaction arrived at 83% (Table 1, entries
the results showed that polar organic solvent DCB was the best 21 and 22). However, keeping increasing the catalyst loading
choice (Table 1, entries 11–20). Finally, the examination of the did not better the reaction anymore.
catalyst loading bettered the yield of the reaction signicantly.
On the other hand, the supports we investigated could affect
Consequently, when the loading of catalyst enhanced to reactivity of the catalyst obviously (Table 2). In Table 2, we
loaded Cu–Zn on various solid supports (CeO, ATP, charcoal,
nano-r-Al2O3, nano-TiO2 and nano-ZrO2) and performed the
cyclization under the established reaction conditions. As a
consequence, all heterogeneous catalysts could catalyze the
reaction to some extent and Cu–Zn/Al–Ti was proved to be the
most efficient heterogeneous catalyst among them (Table 2).
With the optimized conditions established we explored the
scope of the reaction (Table 3). Electron-poor and electron-
neutral benzonitriles could react with 2-aminopyridine
successfully and gave excellent yields, while electron-rich ben-
zonitriles offered relatively low yields of 1,2,4-trizaoles (Table 3,
b–n). Generally, the steric hindrance of the substrates could low
the yields of the reactions and the reaction did not even work
when 3-methyl-2-aminopyridine was used as the substrate
(Table 3, d, e and n). Unfortunately, the reaction failed to give
the desired product when 4-(bromomethyl)benzonitrile was
used as the substrate. Notably, imidazole-subtituted benzoni-
trile also could react with 2-aminopryridine very well and gave
high yield of 1,2,4,-triazole which is important structure in
medicine intermediate (Table 3, o).13 Delightfully, the reactions
performed very smoothly when the heterocyclic nitriles were
employed as substrates and offered good yields of products,
such as substituted cyanopyridines, cyanofuran and cyano-
thiophene (Table 3, p–t), although sterically hindered cyano-
pyridine failed the reaction. Interestingly, the heterogeneous
system not only beard aryl nitriles, but also tolerated vinyl
nitrile and provided single E-isomer product in excellent yield of
85% (Table 3, w). Impressively, the heterogeneous catalytic
system also could tolerate cyanopyrimidine as the substrate and
provided very good yield of 1,2,4-triazole (Table 3, y), which offer
a simple way to synthesize pyrimidine-substituted triazole.
However, benzonitriles substituted by certain sensitive
Table 1 Optimization of the reaction conditionsa
Entry Catalyst (0.5 mol%) Oxidant
Solvent
Isolated yield (%)
1
2
3
4
Cu/Al–Ti
Zn/Al–Ti
Ru/Al–Ti
Pt/Al–Ti
Cu–Zn/Al–Ti
Cu–Zn/Al–Ti
Cu–Zn/Al–Ti
Al–Ti
Cu–Fe/Al–Ti
Cu–Ni/Al–Ti
Cu–Zn/Al–Ti
Cu–Zn/Al–Ti
Cu–Zn/Al–Ti
Cu–Zn/Al–Ti
Cu–Zn/Al–Ti
Cu–Zn/Al–Ti
Cu–Zn/Al–Ti
Cu–Zn/Al–Ti
Cu–Zn/Al–Ti
Cu–Zn/Al–Ti
Cu–Zn/Al–Ti
Cu–Zn/Al–Ti
Air
Air
Air
Air
Air
DCB
DCB
DCB
DCB
DCB
Trace
0
0
0
5
52 (0b)
6
O2 (1 atm) DCB
57
<5
Trace
0
0
8
0
10
7c
8
H2O2
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
DCB
DCB
DCB
DCB
9
10
11
12
13
14
15
16
17
18
19
20
21
22
DMSO
CH3NO2
AcOH
o-Xylene
Dioxane
DMF
Pyridine
EtOAc
EtOH
H2O
0
0
Trace
0
0
0
10
DCB
DCB
75d
83e (80 f )
a
Reaction Conditions: benzonitrile (0.6 mmol), 2-aminopyridine (0.72
b
mmol), catalyst (6 mg, 0.4 mol%), DCB (0.6 mL), 140 ꢀC, 20 h, air. At
c
d
80 ꢀC. Using 0.06 mL H2O2 (30%) as oxidant.
Increasing
e
the amount of catalyst to 15 mg, 1 mol%. Increasing the amount of
catalyst to 24 mg, 1.6 mol%. Using 60 mg, 4 mol% catalyst.
f
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Table 3 The scope of the cyclization between 2-aminopyridine and Table 4 The scope of the cyclization between amidines and nitrilesa
nitrilesa
a
Reaction Conditions: benzonitrile (0.6 mmol), 2-aminopyridine (0.72
mmol), Cu–Zn/Al–Ti (24 mg, 1.6 mol%), DCB (0.6 mL), 140 ꢀC, 20 h,
air, isolated yields.
electron-rich ones did and steric hindrance of substrates led to
low yields of the products. In terms of amidines, not only aryl
amidine but also alkyl amidine could be employed to synthesize
1H-1,2,4-triazoles under the heterogeneous catalytic system
successfully. Importantly, nitro-substituted benzonitrile, the
sensitive substrate for the previous synthetic methods,9 can be
employed into this catalytic system (Table 4, d).
To conrm this procedure catalyzed by Cu–Zn/Al–Ti is
unambiguously heterogeneous in nature, an additional experi-
ment was carried out (Scheme 2). Aer ltering a totally con-
verted reaction mixture (4-Cl-benzonitrile as a substrate) to
remove the catalyst, 1.0 equiv. of a different aryl nitrile was
added as an additional substrate to the ltrate, then the ltrate
a
was treated with the remaining amount of 2-aminopyridine
(>1.2 equiv.). Therefore, only trace amount of another triazole
3o was discovered, while 78% yield of 3o would be obtained, if
the fresh Cu–Zn/Al–Ti was added to the ltrate.
Reaction Conditions: benzonitrile (0.6 mmol), 2-aminopyridine (0.72
mmol), Cu–Zn/Al–Ti (24 mg, 1.6 mol%), DCB (0.6 mL), 140 ꢀC, 20 h,
air, isolated yields.
functional groups, shcu as –NH2, –OH and –COOH, could not be
applied into this reaction.
Next, 2-aminopyridines with substituted groups were exam-
ined under our heterogeneous catalytic system. In general, 2-
aminopyridines with electron-donating and electron-with-
drawing groups could react with benzonitrile successfully and
gave 1,2,4-triazoles in good yields (Table 3, aa–ad). Especially,
nitro-substituted 2-aminopyridine could be tolerated in this
reaction, which can provide an efficient method to synthesis
nitro-substituted 1,2,4-triazoles (3ad) as a complement to
previous methods.
To our delight, application of this heterogeneous catalytic
system can be employed to synthesize 1H-1,2,4-triazoles without
changing the standard reaction conditions (Table 4). Speci-
cally, electron-poor benzotriles brought better yields than Scheme 2 The test of heterogeneous system.
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Table 5 Recycling of the Cu–Zn/Al–Ti catalysta
Cycle
1
2
3
4
5
Isolated yield of 3b (%)
92
91
91
88
85
a
Reaction conditions: 2-aminopyridine (0.72 mmol), 4-Cl-benzonitrile
(0.6 mmol), Cu–Zn/Al–Ti (24 mg, 1.6 mol%), DCB (0.6 mL), 140 ꢀC,
air, 20 h.
Scheme 4 The proposed mechanism of the reaction.
For the practical applications of such heterogeneous system,
the lifetime of the catalyst and its level of reusability are key
factors. To testify this issue, we performed a set of experiments
of 2-aminopyridine and 4-chlorobenzonitrile using the Cu–Zn/
Al–Ti catalyst under the standard reaction conditions (Table 5).
Aer the completion of the rst reaction, the reaction mixture
was ltered and the catalyst was washed by EtOAc and water,
then it was dried at 100 ꢀC for 2 h for being subjected to the next
run of the same reaction process. Aer ve cycles, the recovered
catalyst remained highly catalytic reactivity and the desired
product was obtained above 85% yield (Table 5, entry 5). Thus,
the Cu–Zn/Al–Ti catalyst could be used at least 5 times without
any signicant change in its activity.
nitrile promoted by copper happens rstly by forming coordi-
nated intermediate A probably (Scheme 4). Next, intermediate A
might provide cyclic intermediate B aer amino attacks the
carbon of cyano and proton-transfer process. Then, the intra-
molecular oxidative cyclization takes place induced by copper,
which gave the desired 1,2,4-triazole and reduced copper
species. Finally, the reduced copper species was oxidized by
oxygen of air to give active copper species for nishing the
catalytic cycle of the reaction.
When the mechanism of this reaction was investigated, we
proposed the amidine 6 formed from 2-aminopyridine and
nitrile, would be the intermediate.9,14,15 We used amidine 6 as
the starting materials to perform the cyclization under the
heterogeneous catalytic system (Scheme 3). The results showed
the cyclization could happen smoothly and gave higher yield of
88% (Scheme 3A), while the desired product could also be
obtained by using Cu/Al–Ti catalyst in moderate yield (Scheme
3B). Moreover, the cyclization achieved efficiently and provided
higher yields of products by using other substituted benzoni-
triles under Cu–Zn/Al–Ti-catalyzed conditions (Scheme 3C and
D). Therefore, it is hypothesized that the zinc probably assists
the initial amidine formation step.9 Additional, due to the high
energy barrier of the oxidative cyclization, zinc probably plays a
role of electron-transfer mediate for decreasing the energy
barrier between the reduced transition-metal and oxidant,
thereby increasing the efficiency of the reaction and making air
as the efficient oxidant.16
Conclusions
In summary, we have described an efficient and simple
heterogeneously catalyzed addition-oxidative cyclization
between 2-aminopyridines or amidines and nitriles via C–N/N–
N bond-formation using air as the green oxidant. The hetero-
geneous catalytic system is ligand-, base- and additive-free,
which tolerate a large range of substrates, including heterocy-
cles, –NO2, vinyl, pyrimidine and imidazole groups. The cata-
lytic system also can be applied into the synthesis of 1,2,4-
triazoles directly from N-(pyridin-2-yl)benzamidines. Further-
more, the Cu–Zn/Al–Ti catalyst can recycle many times without
losing activity, which provides the potential for practical
applications.
Experimental
General
According to our observed results and a number of related
literatures involving oxidative C–N/N–N formation,9,17 it is
plausible that the nucleophilic attack of 2-aminopyridine on the
All reagents were purchased from commercial suppliers and
used without further purication. Metal salts and catalyst
supports were commercially available and were used directly.
All experiments were carried out under air. Flash chromatog-
raphy was carried out with Merck silica gel 60 (230–400 mesh).
Analytical TLC was performed with Merck silica gel 60 F254
plates, and the products were visualized by UV detection. 1H
NMR and 13C NMR (300 or 400 and 75 or 100 MHz respectively)
spectra were recorded in CDCl3. Chemical shis (d) are reported
in ppm using TMS as internal standard, and spin–spin coupling
constants (J) are given in Hz.
Scheme 3 The direct cyclization of amidine 6.
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(b) B. K. Trivedi and R. F. bruns, J. Med. Chem., 1988, 31,
1011; (c) K. Makino, G. Sakata and K. Morimoto,
Heterocycles, 1985, 23, 2025.
7 (a) D. R. Armour, D. A. Baxter, J. S. Bryans, K. N. Dack,
P. S. Johnson, R. A. Lewthwaite, J. Newman, D. J. Rawson
and T. Ryckmans, WO 2004037809, 2004, Chem. Abstr.,
2004, 140, 370921; (b) J.-C. Pascal and D. Carniato,
General procedure for the synthesis of CuOx–ZnO/Al2O3–TiO2
Support Al2O3–TiO2 powder (8 g) was added to a 250 mL round-
bottom ask. A solution of Cu(NO3)2–3H2O (1.205 g) and
Zn(NO3)2–6H2O (1.462 g) in deionized water (100 mL) was
added to Al2O3–TiO2 powder, and additional deionized water
(50 mL) was added to wash down the sides of the ask. Then the
ask was submerged into an ultrasound bath for 3 h at room
temperature. Aer that, the water was distilled under reduced
pressure on a rotary evaporator at 90 ꢀC for more than 2 h.
Subsequently, the white powder was dried into an oven under
the condition of increasing the temperature from 25 ꢀC to
110 ꢀC within 1 h and keeping the temperature for another 5 h.
Finally, the dried white powder was calcined under the condi-
tion of increasing the temperature from 25 ꢀC to 350 ꢀC within
1 h and keeping the temperature for 2 more hours to get the
catalyst Cu (3.25 wt%)–Zn(3.40 wt%)/Al–Ti. For the catalyst
characterization, see ESI.†
EP
1099695,
2001,
Chem.
Abstr.,
2001,
134,
353312.
8 (a) J. Balsells, L. DiMichele, J. Liu, M. Kubryk, K. Hansen and
J. D. Armstrong, Org. Lett., 2005, 7, 1039; (b) M. J. Stocks,
D. R. Cheshire and R. Reynold, Org. Lett., 2004, 6, 2969; (c)
S. D. Larsen and B. A. DiPaolo, Org. Lett., 2001, 3, 3341.
9 S. Ueda and H. Nagasawa, J. Am. Chem. Soc., 2009, 131,
15080.
10 H. Xu, Y. Jiang and H. Fu, Synlett, 2013, 24, 125.
11 (a) J. Liu, C. Yu, P. Zhao and G. Chen, Appl. Surf. Sci., 2012,
258, 9096; (b) C. Yu, P. Zhao, G. Chen and H. Bin, Appl.
Surf. Sci., 2011, 257, 7727.
General procedure for Cu–Zn/Al–Ti-catalyzed cyclization
12 For the selected papers on the properties of Al2O3–TiO2 and
relative solid-supported catalyst see: (a) Y. Saih, M. Nagata,
T. Funamoto, Y. Masuyama and K. Segawa, Appl. Catal., A,
2005, 295, 11; (b) K. Segawa, M. Katsuta and F. Kameda,
Catal. Today, 1996, 29, 215; (c) C. Pophal, F. Kameda,
K. Hoshino, S. Yoshinaka and K. Segawa, Catal. Today,
1997, 39, 21; (d) S. Yoshinaka and K. Segawa, Catal.
Today, 1998, 45, 293; (e) K. Segawa, K. Takahashi and
S. Satoh, Catal. Today, 2000, 63, 123; (f) K. Takahashi,
Y. Saih and K. Segawa, Stud. Surf. Sci. Catal., 2002, 145,
311; (g) Y. Saih and K. Segawa, Catal. Today, 2003, 186,
61; (h) Y. Saih and K. Segawa, Catal. Surv. Asia, 2003, 7,
235; (i) A. Duan, R. Li, G. Jiang, J. Gao, Z. Zhao, G. Wan,
D. Zhang, W. Huang and K. H. Chung, Catal. Today, 2009,
140, 187.
Cu–Zn/Al–Ti (24 mg, 1.6 mol%), 2-aminopyridine (0.72 mmol),
benzonitrile (0.6 mmol) and DCB (0.6 mL) were added to a ask
with a bar. The ask was stirred at 140 C for indicated time
ꢀ
under air. Aer cooling to room temperature, the mixture was
diluted with ethyl acetate and ltered. The ltrate was removed
under reduced pressure to get the crude product, which was
further puried by silica gel chromatography (petroleum/ethyl
acetate ¼ 3/2 as eluent) to yield corresponding product. The
identity and purity of the products was conrmed by 1H and 13
C
NMR spectroscopic analysis.
Notes and references
1 (a) H. U. Blaser and E. Schmidt, Asymmetric Catalysis on
Industrial Scale, Wiley-VCH, Weinheim, 2004; (b) 13 (a) P. N. Craig, in ComprehensiVe Medicinal Chemistry, ed. C.
R. A. Sheldon and H. van Bekkum, Fine Chemicals through
Heterogeneous Catalysis, Wiley-VCH, Weinheim, 2001.
2 A. Thayer, Chem. Eng. News, 2005, 83, 55.
3 Review on heterogeneous palladium-catalyzed couplings:
L. Yin and J. Liebscher, Chem. Rev., 2007, 107, 133.
4 For the selected papers on the application of solid-
J. Drayton, Pergamon Press, New York, 1991, vol. 8; (b)
P. Cozzi, G. Carganico, D. Fusar, M. Grossoni,
M. Menichincheri, V. Pinciroli, R. Tonani, F. Vaghi and
P. Salvati, J. Med. Chem., 1993, 36, 2964.
14 G. Rousselet, P. Capdevielle and M. Maumy, Tetrahedron
Lett., 1993, 34, 6395.
supported heterogeneous catalysts see: (a) X. Meng, 15 (a) J. Wang, F. Xu, T. Cai and Q. Shen, Org. Lett., 2008, 10,
X. Xu, T. Gao and B. Chen, Eur. J. Org. Chem., 2010,
5409; (b) R. B. N. Baig and R. S. Varma, Green Chem.,
445; (b) F. Xu, J. Sun and Q. Shen, Tetrahedron Lett., 2002,
43, 1867.
2013, 15, 1838; (c) D. D. Tang, K. D. Collins and 16 For the selected papers on the applications of electron-
F. Glorius, J. Am. Chem. Soc., 2013, 135, 7450; (d) C. Liu,
X. Rao, Y. Zhang, X. Li, J. Qiu and Z. Jin, Eur. J. Org.
Chem., 2013, 4345.
transfer mediates in oxidative reactions see: (a) J. Piera
¨
and J.-E. Backvall, Angew. Chem., Int. Ed., 2008, 47, 3506;
¨
(b) B. P. Babu, X. Meng and J.-E. Backvall, Chem. – Eur.
5 For the selected reviews on the application of nanocatalysts,
see: (a) E. Gross, J. H.-C. Liu, F. D. Toste and G. A. Somorjai,
Nat. Chem., 2012, 4, 947; (b) V. Polshettiwar, R. Luque,
A. Fihri, H. Zhu, M. Bouhrara and J.-M. Basset, Chem. Rev.,
J., 2013, 19, 4140; (c) X. Meng, C. Li, B. Han, T. Wang
and B. Chen, Tetrahedron, 2010, 66, 3029; (d) T. Oishi,
K. Yamaguchi and N. Mizuno, ACS Catal., 2011, 1,
1351.
2011, 111, 3036; (c) K. V. S. Ranganath and F. Glorius, 17 (a) O. Prakash, H. K. Gujral, N. Rani and S. P. Singh, Synth.
¨
Catal. Sci. Technol., 2011, 1, 13; (d) A. Schatz, O. Reiser and
Commun., 2000, 30, 417; (b) G. Jones and D. R. Sliskovic, J.
Chem. Soc., Perkin Trans. 1, 1982, 4, 967; (c) S. Mineo,
S. Kawamura and K. Nakagawa, Synth. Commun., 1976,
6, 69.
W. J. Stark, Chem. – Eur. J., 2010, 16, 8950.
6 (a) R. Sarges, H. R. Howard, R. G. Browne, L. A. Lebel,
P. A. Seymour and B. K. Koe, J. Med. Chem., 1990, 33, 2240;
8616 | RSC Adv., 2014, 4, 8612–8616
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