The oxidation of pyridines catalyzed by surfactant-encapsulated polyoxometalate [(C18H37)2(CH3) 2N]8[HBW11O39] with the temperature-responsive property of solubility

Article abstract of DOI:10.1039/c3nj00628j

Temperature-responsive characterization of solubility based on a surfactant-encapsulated polyoxometalate ([(C₁₈H₃₇) ₂(CH₃)₂N]₈[HBW₁₁O ₃₉]) in tert-butyl alcohol was described and used in catalytic oxidation of pyridines. The catalyst could be recovered and reused several times by controlling the temperature.

Full text of DOI:10.1039/c3nj00628j

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LETTER
The oxidation of pyridines catalyzed by
surfactant-encapsulated polyoxometalate
[(C18H37)2(CH3)2N]8[HBW11O39] with the
temperature-responsive property of solubility†
Cite this: NewJ.Chem., 2013,
37, 2614
Received (in Porto Alegre, Brazil)
12th June 2013,
Accepted 4th July 2013
Wei Zhao,*a Chunxia Yang,a Yong Ding*b and Baochun Mab
DOI: 10.1039/c3nj00628j
www.rsc.org/njc
Temperature-responsive characterization of solubility based on great contribution in the field of SEPs. In the multitudinous
a surfactant-encapsulated polyoxometalate ([(C18H37)2(CH3)2N]8- applications of polyoxometalates, catalysis is the most important
[HBW11O39]) in tert-butyl alcohol was described and used in and the main one, especially the catalytic oxidation reaction.
catalytic oxidation of pyridines. The catalyst could be recovered Although a lot of studies about SEPs have been reported, the
and reused several times by controlling the temperature.
catalytic applications of SEPs for oxidation reaction are less.4,6b,h
Herein, we reported for the first time a temperature-responsive
Polyoxometalates (POMs), as a class of functional inorganic characterization of a surfactant-encapsulated polyoxometalate in
transition-metal oxide clusters with abundant compositions, tert-butyl alcohol. Firstly, the polyoxometalate anion [HBW11O39]8À
have been employed widely in catalysis, medicine, imaging, was encapsulated by the common surfactant of dimethyldioctadecyl-
sensing, and multifunctional materials.1 To study the unique ammonium (DODA) (see Fig. 1). The procedure of formation
properties of POMs, much effort has been made to incorporate was similar to the reported methods.6,7 Organic surfactants and
them into organic matrices through chemical and physical inorganic cores are combined together through electrostatic
approaches, containing covalent2 and non-covalent electrostatic interaction ((DODA)8[HBW11O39], denoted as SEP-BW11). The
interactions.3 Because of the simple and rapid synthesis principle surfactant-encapsulated polyoxometalate of SEP-BW11 has a
in the latter field, the methods of electrostatic interaction have temperature-responsive characterization in tert-butyl alcohol.
been used widely to improve the surface properties of the POM At room temperature, SEP-BW11 was insoluble in the solvent,
clusters. POMs can be conveniently modified by exchanging but dissolved in tert-butyl alcohol with the elevation of reaction
their counterions with cationic surfactants, generating surfactant- temperature. After cooling, the solution gradually became
encapsulated polyoxometalate (SEP) complexes. The improvement emulsive. When the emulsion was heated, a clear solution was
of the surface properties of the inorganic clusters makes the obtained. The solution and emulsion were transformed to each
complexes applicable for hybrid self-assembling and grafting other by heating or cooling (see Fig. 2). This special temperature-
into organic and inorganic matrices. Various morphologies of responsive property of SEP-BW11 in tert-butyl alcohol could be
surfactant-encapsulated POM nanoclusters, such as, fullerene-
like spheres, disks, cones, tubes, flowers, and wires, were
obtained and studied.4 Many researchers including Kurth’s,5
Wu’s6 and Wang’s7 research groups, among others, have made
a College of Chemistry and Pharmaceutical Engineering, Nanyang Normal
University, Nanyang 473061, China. E-mail: zhaowei1231@163.com;
Fax: +86-377-63525055; Tel: +86-377-63525056
b Key Laboratory of Nonferrous Metals Chemistry and Resources Utilization of
Gansu Province, State Key Laboratory of Applied Organic Chemistry and College
of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000,
China. E-mail: dingyong1@lzu.edu.cn; Fax: +86-931-8912582;
Tel: +86-931-8619200
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
Fig. 1 Schematic drawing of the chemical structure of SEC-BW11
.
c3nj00628j
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SEP-BW11 in tert-butyl alcohol was low (entries 10 and 11).
If this behavior is used in catalytic oxidation systems using
polyoxometalates with hydrogen peroxide, an advanced system
containing high catalytic activity in homogeneous medium and
reusable catalyst in heterogeneous medium will be developed.
The oxidation of pyridines was used to model reactions to
check the applications of temperature-responsive systems
based on SEP-BW11. Pyridine N-oxides are important and
versatile synthetic intermediates, which are used widely in
fields of chemistry and biology.8 Pyridine N-oxides were usually
prepared by oxidation of pyridines. Several oxidation methods
have been developed in the past.9 Catalytic oxidation using a
homogeneous catalyst with hydrogen peroxide of pyridines to
N-oxides has received much attention in recent years.10
To overcome the problems of catalyst separating from the
homogeneous solution, some heterogeneous catalysts were
demonstrated for the oxidation of pyridines.11 Among these
catalytic systems, polyoxometalates showed good activity.12
Generally, homogeneous approaches provide a high catalytic
efficiency but suffer from difficult catalyst recovery and separation.
In contrast, the catalyst can be recovered in heterogeneous systems
easily, but the catalytic activities are usually not satisfactory for the
multiphase and the lower mass transfer. So, if the system is
homogeneous during reaction and then becomes heterogeneous
after reaction, the catalytic efficiency will be improved by the
combination of two processes.
The oxidations of pyridines catalyzed by SEP-BW11 in tert-
butyl alcohol were carried out. As shown in Table 2, the yields
were high to excellent for some representative substrates for a
short reaction time such as pyridine, 2-picoline, quinoline,
isoquinoline and 4-methyl-quinoline. For pyridine and 2-picoline,
high yields of 96% and 92% were obtained after 3 h (entries 1
and 2). However, di-substituted pyridine derivatives, such as
2,3-lutidine and 2,6-lutidine, were more difficult to be oxidized
over this catalyst (entries 3 and 4), 59 and 51% yields were got
after 6 h. This may be attributed to the solubility behavior of
the catalyst during reaction. Because of the coexistence of
the organic substrates and the aqueous solution of hydrogen
peroxide, the catalyst SEP-BW11 cannot dissolve totally in all
systems for different substrates. For 2,3-lutidine and 2,6-lutidine,
less part of the catalyst precipitated from the system during
reaction, which lead to lower yields of N-oxides. Even though
the catalyst is not completely soluble in some systems because of
the complicacy during reactions, the catalyst was completely
dispersed very well during reaction upon heating. Interestingly,
for quinoline and its derivatives (entries 6–9), high to excellent
yields of corresponding N-oxides were obtained for a short
reaction time (5 h). For these substrates, the catalyst SEP-BW11
is soluble in the solvent during reaction upon heating. Good
temperature-responsive properties were found in oxidation of
these substrates over heating based on catalyst SEP-BW11.
For all oxidation reactions using different substrates, the
catalyst SEP-BW11 could precipitate from the reaction medium
after reaction upon cooling (see Fig. 3). After the reaction
finished with the temperature dropping, the system gradually
changed turbid, and the catalyst precipitated itself from the
Fig. 2 Photos of the temperature-responsive transition of SEP-BW11 between
solution and emulsion.
used in catalytic reaction, which is different from the traditional
catalytic methods.
Firstly, the solubility behaviors of SEP-BW11 were evaluated
in various solvents during heating and cooling, which are
usually used in catalytic oxidation over polyoxometalates.
The data summarized in Table 1 reveal that the complex of
SEP-BW11 shows different solubility behaviors in different
solvents. For dichloromethane, chloroform and toluene,
SEP-BW11 can dissolve in the solvent at high temperature upon
heating (entries 1–3). After cooling to room temperature,
SEP-BW11 was still soluble in the above solvents. However, for
ethyl acetate, acetone, acetonitrile and water, the surfactant-
encapsulated polyoxometalate of SEP-BW11 cannot dissolve in
these solvents even after heating (entries 4–7). Of course, it
is insoluble after cooling in those solvents. Especially, the
solubility behaviors of SEP-BW11 in hexane and tert-butyl
alcohol are different when they were heated or cooled. For
hexane, SEP-BW11 dissolved in the solvent during heating, and
a gel was observed after cooling to a lower temperature (4 1C)
gradually (entry 8). The supramolecular gels of surfactant-
encapsulated polyoxometalates have been studied by Wu’s
group in detail.6g What we focused on is the behaviors of
SEP-BW11 in tert-butyl alcohol (entry 9). SEP-BW11 is insoluble
in tert-butyl alcohol at room temperature, but dissolves gradually
in it upon heating. The sample of SEP-BW11 started to dissolve in
tert-butyl alcohol at 45–50 1C, and the solid was dissolved
completely at 55 1C. After cooling to room temperature, an
emulsion was obtained based on SEP-BW11 in tert-butyl alcohol.
The same results were obtained when the concentration of
Table 1 Solubility behaviors of SEP-BW11 in various solvents during heating and
coolinga
Entry
Solvent
Heating
Cooling
1
2
3
Dichloromethane
Chloroform
Toluene
Soluble
Soluble
Soluble
Soluble
Soluble
Soluble
4
5
6
Ethyl acetate
Acetone
Acetonitrile
Water
Heterogeneous
Heterogeneous
Heterogeneous
Heterogeneous
Soluble
Soluble
Soluble
Heterogeneous
Heterogeneous
Heterogeneous
Heterogeneous
Gel
Emulsion
Emulsion
Emulsion
7
8b
9
Hexane
tert-Butyl alcohol
tert-Butyl alcohol
tert-Butyl alcohol
10c
11d
Soluble
a
b
c
50 mg of SEP-BW11 + 3 ml of solvent. Cooled to 4 1C. 25 mg of
d
SEP-BW11 + 3 ml of solvent. 25 mg of SEP-BW11 + 6 ml of solvent.
c
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a
Table 2 Oxidation of pyridines with hydrogen peroxide catalyzed by SEP-BW11
Entry
Substrate
Product
Time (h)
Yieldb (%)
1
3
96
92
59
51
33
2
3
4
5
3
6
6
6
Fig. 3 Representation of the catalytic system based on temperature-responsive
surfactant-encapsulated polyoxometalate.
Table 3 Recycling of the catalytic system for the oxidation of quinolinea
Yield of the
recycled catalyst (%)
Cycle
Yieldb (%)
1
2
3
97
96
96
92
90
90
a
Reaction conditions: 4 mmol quinoline, 20 mmol (30% aq.) H2O2,
b
28 mmol catalyst, 20 ml of tert-butyl alcohol, 65 1C, 5 h. Isolated yields;
assignments of products were analyzed by H NMR and 13C NMR.
1
6
7
5
5
97
92
The catalyst with the counter cation tetrabutylammonium
was also synthesized and used in acetonitrile for quinoline
oxidation as a homogeneous catalyst. A 96% yield of quinoline-
N-oxide was achieved in the above system. However, the catalyst
was soluble in the solvent after reaction upon cooling (Table 2,
entry 10). The solid of K8[BW11O39H]Á13H2O was used as a
heterogeneous catalyst in tert-butyl alcohol for the oxidation of
quinoline, and a 29% yield of N-oxide was obtained (Table 2,
entry 11). The results indicate that the catalytic system
described in this paper is advanced.
8
5
5
94
82
9
In summary, a peculiar temperature-responsive surfactant-
encapsulated polyoxometalate (SEP-BW11) catalyst was developed
using tert-butyl alcohol as a solvent and used for the catalytic
oxidation of pyridines with hydrogen peroxide. This green,
effective and reusable catalytic system possesses the advantages
of both homogeneous and heterogeneous catalysts. The surfactant-
encapsulated polyoxometalate catalyst described in this paper
provides a useful solution to the difficult problem of the separation
of homogeneous catalysts in the future.
10c
5
5
96
29
11d
a
Reaction conditions: 1 mmol substrate, 5 mmol (30% aq.) H2O2,
7 mmol catalyst, 5 ml of tert-butyl alcohol, 65 1C. b Isolated yields; assign-
ments of products were analyzed by 1H NMR and 13C NMR. 1 mmol
c
substrate, 5 mmol (30% aq.) H2O2, 7 mmol [(n-C4H9)4N]8[HBW11O39], 5 ml of
acetonitrile, 65 1C. d 1 mmol substrate, 5 mmol (30% aq.) H2O2, 7 mmol
K8[BW11O39H]Á13H2O, 5 ml of tert-butyl alcohol, 65 1C.
Experimental section
system. The solid catalyst could be separated from the solution Synthesis
of
surfactant-encapsulated
polyoxometalate:
by centrifugation and used directly for the next cycle. Table 3 K8[BW11O39H]Á13H2O was synthesized by the procedure
lists the catalytic activity in oxidation of quinoline based on reported in ref. 13. Surfactant-encapsulated polyoxometalate
SEP-BW11 for different cycles. As shown in the table, the (SEP-BW11) was synthesized according to the previously
catalytic activities of the recovered catalysts are almost the reported method.6,7 K8[BW11O39H]Á13H2O was dissolved in
same as that of the fresh one. A 96% yield of quinoline-N-oxide water, and then the chloroform solution of DODAÁBr (dimethyl-
was achieved after three cycles, indicating the system is an dioctadecylammonium bromide) was added slowly under stirring.
active and reusable catalytic one for pyridine oxidation based The initial molar ratio of DODAÁBr to K8[BW11O39H]Á13H2O was
on surfactant-encapsulated polyoxometalate.
controlled at 7.2 : 1, after 10 h of stirring, polyoxometalate was
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D.-L. Long and L. Cronin, Angew. Chem., Int. Ed., 2007,
46, 3900.
transferred into chloroform spontaneously, and the electro-
static complex with a defined ratio of components was formed.
The organic phase was then separated and dried with anhydrous
sodium sulfate, giving the solid [(C18H37)2(CH3)2N]8[HBW11O39]
(denoted as SEP-BW11) by evaporating the solution to dryness.
The product was placed in vacuum for the measurements. Calcd
for [(C18H37)2(CH3)2N]8[HBW11O39]: C, 51.67; H, 9.14; N, 1.59%.
Found: C, 51.38; H, 9.14; N, 1.56%.
3 (a) A. Haimov, H. Cohen and R. Neumann, J. Am. Chem. Soc.,
2004, 126, 11762; (b) W. F. Bu, L. X. Wu, X. L. Hou, H. L. Fan,
C. W. Hu and X. Zhang, J. Colloid Interface Sci., 2002,
251, 120; (c) D. Volkmer, A. Du Chesne, D. G. Kurth,
H. Schnablegger, P. Lehmann, M. J. Koop and A. Mu¨ller,
J. Am. Chem. Soc., 2000, 122, 1995.
4 A. Nisar and X. Wang, Dalton Trans., 2012, 41, 9832.
5 (a) D. Volkmer, A. Du Chesne, D. G. Kurth, H. Schnablegger,
P. Lehmann, M. J. Koop and A. Mu¨ller, J. Am. Chem. Soc.,
2000, 122, 1995; (b) D. G. Kurth, P. Lehmann, D. Volkmer,
A. Mu¨ller and D. Schwahn, J. Chem. Soc., Dalton Trans.,
2000, 3989; (c) D. G. Kurth, P. Lehmann, D. Volkmer,
H. Colfen, M. J. Koop, A. Mu¨ller and A. Du Chesne,
Chem.–Eur. J., 2000, 6, 385.
6 (a) H. L. Li, H. Sun, W. Qi, M. Xu and L. X. Wu, Angew.
Chem., Int. Ed., 2007, 46, 1300; (b) Y. Yang, Y. Z. Wang,
H. L. Li, W. Li and L. X. Wu, Chem.–Eur. J., 2010, 16, 8062;
(c) X. K. Lin, Y. L. Wang and L. X. Wu, Langmuir, 2009,
25, 6081; (d) W. F. Bu, H. L. Li, H. Sun, S. Y. Yin and
L. X. Wu, J. Am. Chem. Soc., 2005, 127, 8016; (e) H. Sun,
W. Li, L. Wollenberg, B. Li, L. X. Wu, F. Y. Li and L. Xu,
J. Phys. Chem. B, 2009, 113, 14674; ( f ) H. Sun, W. Li and
L. X. Wu, Langmuir, 2009, 25, 10466; (g) Y. Wang, W. Li and
L. X. Wu, Langmuir, 2009, 25, 13194; (h) Y. Z. Wang, H. L. Li,
W. Qi, Y. Yang, Y. Yan, B. Li and L. X. Wu, J. Mater. Chem.,
2012, 22, 9181.
Catalytic reaction: catalyst (7 mmol), tert-butyl alcohol (5 ml),
substrate (1 mmol), and H2O2 (5 mmol, 30% aq.) were charged
in the reaction flask. The reaction was carried out at 65 1C for
3–6 h. The reaction was detected by TLC. After reaction finished
with temperature dropping, the catalyst precipitated itself from
the system. The catalyst was separated by centrifugation and
washed with Et2O, which was then used for the next cycle. The
organic products were separated from the aqueous phase
by extraction. The combined organic layers were dried over
anhydrous Na2SO4, and the pure products were obtained by
evaporation or column chromatography. The products were
1
analyzed by H NMR and 13C NMR.
Acknowledgements
This work was supported by the Natural Science Foundation of
Nanyang Normal University (No. ZX2012012) and the National
Natural Science Foundation of China (Grant No. 20803032 and
21173105).
7 (a) A. Nisar, Y. Lu and X. Wang, Chem. Mater., 2010,
22, 3511; (b) A. Nisar, J. Zhuang and X. Wang, Adv. Mater.,
2011, 23, 1130; (c) A. Nisar, Y. Lu, J. Zhuang and X. Wang,
Angew. Chem., Int. Ed., 2011, 50, 3187.
Notes and references
1 (a) I. V. Kozhevnikov, Chem. Rev., 1998, 98, 171; (b) J. T.
Rhule, C. L. Hill, D. A. Judd and R. F. Schinazi, Chem. Rev.,
1998, 98, 327; (c) M. Sadakane and E. Steckhan, Chem. Rev.,
8 (a) A. Albini, Synthesis, 1993, 263; (b) A. V. Malkov, M. Bell,
ˇ
F. Castelluzzo and P. Kocovsk´y, Org. Lett., 2005, 7, 3219;
´
(c) L.-C. Campeau, S. Rousseaux and K. Fagnou, J. Am. Chem.
Soc., 2005, 127, 18020; (d) R. Sarma, A. Karmakar and
J. B. Baruah, Inorg. Chim. Acta, 2008, 361, 2081;
(e) F. F. Bamoharram, M. M. Heravi, M. Roshani and
N. Tavakoli, J. Mol. Catal. A: Chem., 2006, 252, 219;
( f ) J. Balzarini, M. Stevens, E. De Clercq, D. Schols and
C. Pannecouque, J. Antimicrob. Chemother., 2005, 55, 135.
9 (a) J. G. Robhe and E. J. Behrman, J. Chem. Res., Synop.,
1998, 98, 219; (d) E. Coronado, C. Gimenez-Saiz and
´
´
C. J. Gomez-Garcıa, Coord. Chem. Rev., 2005, 249, 1776;
(e) C. Ritchie, C. Streb, J. Thiel, S. G. Mitchell, H. N.
Miras, D.-L. Long, T. Boyd, R. D. Peacock, T. McGlone and
L. Cronin, Angew. Chem., Int. Ed., 2008, 47, 6881;
( f ) C. Fleming, D.-L. Long, N. McMillan, J. Johnston,
¨
N. Bovet, V. Dhanak, N. Gadegaard, P. Kogerler, L. Cronin
and M. Kadodwala, Nat. Nanotechnol., 2008, 3, 289;
(g) D.-L. Long, R. Tsunashima and L. Cronin, Angew. Chem.,
Int. Ed., 2010, 49, 1736; (h) J. Song, Z. Luo, D. K. Britt,
H. Furukawa, O. M. Yaghi, K. I. Hardcastle and C. L. Hill,
J. Am. Chem. Soc., 2011, 133, 16839; (i) F. Y. Song, Y. Ding,
B. C. Ma, C. M. Wang, Q. Wang, X. Q. Du, S. Fu and J. Song,
Energy Environ. Sci., 2013, 6, 1170.
´
1993, 412; (b) M. Ferrer, F. Sanchez-Baeza and A. Messgure,
Tetrahedron, 1997, 53, 15877; (c) P. Brougham, M. S. Cooper,
D. A. Cummerson, H. Heaney and N. Thomson, Synthesis,
1987, 1015; (d) H. Ritter and H. H. Licht, J. Heterocycl.
Chem., 1995, 32, 585; (e) R. A. Hollins, L. H. Merwin,
R. A. Nissan and W. S. Wilson, J. Heterocycl. Chem., 1996,
33, 895.
2 (a) P. Yin, P. Wu, Z. Xiao, D. Li, E. Bitterlich, J. Zhang,
P. Cheng, D. V. Vezenov, T. Liu and Y. Wei, Angew. Chem., 10 (a) V. Boekelheide and W. J. Linn, J. Am. Chem. Soc., 1954,
´
Int. Ed., 2011, 50, 2521; (b) H. Liu, C. Qin, Y.-G. Wei, L. Xu,
G.-G. Gao, F.-Y. Li and X.-S. Qu, Inorg. Chem., 2008, 47, 4166;
(c) J. Hao, L. Ruhlmann, Y. Zhu, Q. Li and Y. Wei, Inorg.
Chem., 2007, 46, 4960; (d) Z. Peng, Angew. Chem., Int. Ed.,
2004, 43, 930; (e) H. Kwen, J. V. G. Young and E. A. Maatta,
Angew. Chem., Int. Ed., 1999, 38, 1145; ( f ) Y.-F. Song,
76, 1286; (b) C. Coperet, H. Adolfsson, T. V. Khuong,
A. K. Yudin and K. B. Sharpless, J. Org. Chem., 1998,
63, 1740; (c) D. Rong, V. A. Phillips, R. S. Rubio, M. Castro
and R. T. Wheelhouse, Tetrahedron Lett., 2008, 49, 6933;
(d) X. Z. Zhu, K. D. Kreutter, H. P. Hu, M. R. Player and
M. D. Gaul, Tetrahedron Lett., 2008, 49, 832.
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013
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View Article Online
NJC
Letter
11 (a) K. Neimann and R. Neumann, Chem. Commun., 2001, 12 (a) D. Sloboda-Rozner, P. Witte, P. L. Alsters and R. Neumann,
487; (b) D. J. Robinson, P. McMorn, D. Bethell, P. C.
Bulman-Page, C. Slyd, F. King, F. E. Hancock and
G. J. Hutchings, Catal. Lett., 2001, 72, 233; (c) L. Rout and
T. Punniyamurthy, Adv. Synth. Catal., 2005, 347, 1958.
Adv. Synth. Catal., 2004, 346, 339; (b) Y. Ding and W. Zhao, J. Mol.
Catal. A: Chem., 2011, 337, 45; (c) Y. Ding, W. Zhao, W. F. Song,
Z. X. Zhang and B. C. Ma, Green Chem., 2011, 13, 1486.
´ ´
´
13 A. Teze, M. Michelon and G. Herve, Inorg. Chem., 1997, 36, 505.
c
2618 New J. Chem., 2013, 37, 2614--2618
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