Clean and selective oxidation of alcohols with n-Bu4NHSO 5 catalyzed by ionic liquid immobilized TEMPO in ionic liquid [bmim][PF6]

Article abstract of DOI:10.1016/j.catcom.2010.05.002

An efficient oxidation of alcohols with tetra-n-butylammonium peroxymonosulfate catalyzed by ionic liquid immobilized TEMPO in ionic liquid [bmim][PF₆] was reported. TEMPO-IL serves as a homogeneous catalyst to enhance the reaction remarkably.

Full text of DOI:10.1016/j.catcom.2010.05.002

Catalysis Communications 11 (2010) 1017–1020
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Clean and selective oxidation of alcohols with n-Bu4NHSO5 catalyzed by ionic liquid
immobilized TEMPO in ionic liquid [bmim][PF6]
⁎
Chenjie Zhu, Lei Ji, Yunyang Wei
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 April 2010
Received in revised form 29 April 2010
Accepted 5 May 2010
Available online 13 May 2010
An efficient oxidation of alcohols with tetra-n-butylammonium peroxymonosulfate catalyzed by ionic liquid
immobilized TEMPO in ionic liquid [bmim][PF6] was reported. TEMPO-IL serves as a homogeneous catalyst to
enhance the reaction remarkably. The oxidation proceeded to afford aldehydes or ketones in excellent yields
and high selectivity without remarkable over-oxidation to carboxylic acids. It is noteworthy to mention that
the catalyst and solvent could easily be recycled and reused without loss of activity. A possible mechanism
for the oxidation is proposed.
Keywords:
Alcohol
© 2010 Elsevier B.V. All rights reserved.
TEMPO
Ionic liquid
Oxidation
1. Introduction
an effective reagent for the oxidation of alcohols in CH2Cl2 or H2O [17].
Although oxidation of secondary alcohols led to ketones in good
The selective oxidation of alcohols to the corresponding aldehydes
or ketones is a fundamental transformation both in laboratory
synthesis and industrial production [1–3]. Numerous oxidizing
reagents (for example, CrO3, KMnO4, MnO2, etc.) in stoichiometric
amounts have been traditionally employed to accomplish this
transformation with considerable drawbacks such as the use of
expensive reagents, volatile organic solvents, and discharge of
environmentally pernicious wastes [4]. Transition metal-catalyzed
oxidations of alcohols using aqueous H2O2 or gaseous O2 as oxidant
have also been developed [5–8]. From economic and environmental
perspectives, the development of new catalytic oxidation system with
cheap and green oxidant is particularly attractive.
yields, oxidation of primary alcohols was proceeded with low
conversion and selectivity.
On the other hand, it is well known that nitroxyl radicals, such as
2,2,6,6-tetramethyl-1-piperidinyloxyl free radical (TEMPO) promote
oxidation of various alcohols to the corresponding carbonyl com-
pounds effectively under mild reaction conditions [18–20]. However,
separation of TEMPO from the products and recycling could be
problematic. Numerous immobilized TEMPO variants on both organic
and inorganic supports have been synthesized [21–24]. Nevertheless,
TEMPO immobilized on inorganic supports in some cases shows lower
catalytic activity, and prolonged reaction times are needed for high
conversion [25].
Oxone (2KHSO5·KHSO4·K2SO4) has drawn considerable attention
as a mild and efficient reagent for various organic transformations in
recent years [9–15]. However, oxone is practically insoluble in all
solvents except water. The need for aqueous or pH-controlled reac-
tions is perhaps the most significant drawback in the use of oxone for
applications in synthetic chemistry [16]. To overcome the need for
aqueous conditions, several salts of oxone have been reported, such as
tetra-n-butylammonium peroxymonosulfate (n-Bu4NHSO5, TBA-OX),
tetra-n-pentylammonium peroxymonosulfate, tetra-n-hexylammo-
nium peroxymonosulfate and benzyltriphenylphosphonium perox-
ymonosulfate. Recently, TBA-OX was demonstrated by Lei et al. to be
Room temperature ionic liquids have received recognition as novel
and promising solvents for synthesis chemistry. They have also been
referred to as “designer solvents” as their chemical and physical
properties could be adjusted by careful choice of the cation or anion.
Recent advances in ionic liquids provided efficient routes for the
preparation of task-specific ionic liquids (TSILs). Muzart has reviewed
the development of ionic liquids as solvents for catalyzed oxidations
of organic compounds, and some methods about using ionic liquid
immobilized TEMPO were summarized [26].
In continuation of our interest in exploring systems on the
oxidation of organic compounds [27], especially alcohols [28–32],
here, we would like to report a facile procedure for the oxidation of
alcohols to the corresponding aldehydes or ketones with TBA-OX
catalyzed by TEMPO or TEMPO-IL (ionic liquid immobilized TEMPO)
in 1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6]
(Scheme 1). The catalyst TEMPO-IL, ionic liquid [bmim][PF6] and the
⁎
Corresponding author. Fax: +86 25 84315514.
E-mail address: ywei@mail.njust.edu.cn (Y. Wei).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.05.002

1018
C. Zhu et al. / Catalysis Communications 11 (2010) 1017–1020
Scheme 1. Oxidation of alcohols catalyzed by TBA-OX/TEMPO-IL in [bmim]PF6.
reduction product n-Bu4NHSO4 from the oxidant TBA-OX are easily
recovered and reused in the procedure.
Fig. 1. Photographic plates. (1) [bmim][PF6]: 3 mL; (2) 0.25 g TBA-OX in 3 mL [bmim]
[PF6] at RT; (3) 0.25 g TBA-OX in 3 mL [bmim][PF6] at 60 °C; (4) Photo 3 was washed
with 2 mL H2O after being cooled to RT, the upper layer was H2O; (5) After completion,
the aqueous phase was lyophilized to obtain the n-Bu4NHSO4.
2. Experimental
All chemicals (AR grade) were obtained from commercial resources
and used without further purification. TBA-OX was prepared from
commercially available oxone and n-Bu4NHSO4 by a procedure
described in the literature [16]. TEMPO-IL was synthesized according
to literature [23,24]. Gas chromatography (GC) analysis was performed
on an Agilent GC-6820 chromatograph equipped with a 30 m×
0.32 mm×0.5 μm HP-Innowax capillary column and a flame ionization
detector. GC–MS spectra were recorded on Thermo Trace DSQ GC–MS
spectrometer using TRB-5MS (30 m×0.25 mm×0.25 µm) column.
Products were all known compounds and were identified by comparing
their physical and spectra data with those reported in the literature.
(Table 1, entries 1–2), neutral ionic liquids (Table 1, entries 3–4) and
basic ionic liquids (Table 1, entries 5–7) were tested to dissolve TBA-
OX, the results are shown in Table 1. Interestingly, TBA-OX is nearly
insoluble in [bmim][PF6] at low temperature, but miscible at high
temperature (see Fig. 1). Furthermore, [bmim][PF6] is immiscible with
water. This could perhaps be beneficial since it can provide an easy
route to remove the oxidant and its by-product (n-Bu4NHSO4) after
completion by simple washing of the reaction mixture with water.
3.2. Effects of the catalysts on the reaction rate
2.1. Typical experimental procedure for oxidation of alcohols
The initial experiments were carried out using 4-nitrobenzyl
alcohol as the model substrate. When 4-nitrobenzyl alcohol was
oxidized with TBA-OX/TEMPO in [bmim][PF6] at 60 °C for 1 h, the
formation of 4-nitrobenzaldehyde as the exclusive product with 94%
yield was established. As a control experiment, the same reaction was
carried out in the absence of TEMPO and no oxidation product was
observed even after stirring for 8 h. The use of TEMPO-IL instead of
TEMPO in the oxidation led to similar results, without apparent loss of
catalytic activity (see Fig. 2).
TBA-OX (0.95 g, 1.00 mmol) was dissolved in [bmim][PF6] (4 mL)
at 60 °C for several minutes, alcohol (0.5 mmol) and TEMPO-IL
(0.05 g, 0.1 mmol) were added with stirring. The mixture was stirred
at 60 °C for a given time (see Table 2). After completion, the mixture
was extracted with ether (3×10 mL) and analyzed by GC–MS. The
organic layer was dried and concentrated to provide the
corresponding product in an almost pure state. If necessary, the
crude product was purified by column chromatography on silica gel
(petroleum ether/ethyl acetate=10/1) to afford the analytically pure
aldehydes or ketones.
3.3. Oxidation of alcohols catalyzed by TEMPO or TEMPO-IL
In order to evaluate the versatility of this novel catalytic system,
we applied the procedure to the oxidation of a wide range of alcohols.
As shown in Table 2, most alcohols underwent oxidation to afford the
3. Results and discussion
3.1. The solubility properties of TBA-OX
The solubility properties of TBA-OX in some traditional solvents
were first investigated. It is completely soluble in water, MeOH,
acetone, CH2Cl2, CHCl3, MeCN, MeNO2, DMF, and DMSO. Other
solvents such as EtOAc, benzene, CCl4, hexane, THF, toluene and
ether are not able to dissolve TBA-OX. To the best of our knowledge,
there is no report on the solubility properties of TBA-OX in ionic
liquids. So, a variety of ionic liquids, including acidic ionic liquids
Table 1
Solubility of TBA-OX in ionic liquids.a,b
Entry
Ionic liquid
T/°C
25
40
60
80
1
2
3
4
5
6
7
[Hmim][HSO4]
[Hmim][CF3COO]
[bmim][PF6]
s
s
i
i
i
s
s
i
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
[bmim][BF4]
[bmim][OH]
[H3N+–CH2–CH2–OH][CH3CH(OH)COO−
]
s
s
[H3N+–CH2–CH2–OH][CH3COO−
]
Fig. 2. Comparison with the catalytic activity of TEMPO and TEMPO-IL. Reaction
conditions: 4-nitrobenzyl alcohol (0.07 g, 0.5 mmol), TBA-OX (0.95 g, 1 mmol), TEMPO
or TEMPO-IL (0.1 mmol), and [bmim][PF6] (4 mL) at 60 °C.
a
s: soluble; i: insoluble.
0.25 g TBA-OX in 3 mL ionic liquid.
b

C. Zhu et al. / Catalysis Communications 11 (2010) 1017–1020
1019
Table 2
TBA-OX/TEMPO (TEMPO-IL) catalyzed oxidation of alcohols.
Entry Alcohols
Products
Catalytic Time Yieldb
systema (h)
(%)
1
A
B
0.5
0.5
92
90
2
A
B
1
1
95
94
3
4
5
A
B
1
1
96
94
A
B
1
1
97
98
A
B
1.5
1.5
90
85
6
A
B
1
1
92
90
Fig. 3. Recycling of the catalytic system for the oxidation of 4-nitrobenzyl alcohol.
7
8
A
B
1
1
96
95
phase transfer catalyst and can be reused in the preparation of TBA-
OX. The ionic liquid layer was distilled under vacuum to remove
residual ether and water. Fresh substrates were then recharged to the
residual liquid layer and the mixture was heated to react once again.
The procedure was repeated 5 times in the oxidation of 4-nitrobenzyl
alcohol. It was found that if TEMPO-IL was used as a catalyst, there is
no apparent loss of catalytic activity in the oxidation (see Fig. 3), and
only 2.3% loss of weight was observed after five times recycling.
However, when the reaction catalyzed by TEMPO, yields drastically
decreased from the first cycle of 95% to the second cycle of 63% and the
third cycle of 27%. These results demonstrated that TEMPO is easily
removed from the ionic liquid layer by ether extraction during the
work up process and TEMPO-IL is not. Therefore, from the point of
view on the greener chemical processes, the use of the catalytic
system of TBA-OX/TEMPO-IL/[bmim][PF6] do not lead to three major
sources of waste: organic solvents, catalysts and any hazardous by-
products.
A
B
3
4
98
94
9
A
B
3
4
85
80
10c
11c
A
B
5
7
78
73
A
B
6
8
48
40
12c
13c
CH3(CH2)6CH2OH
CH3(CH2)6CHO
A
B
A
B
6
8
6
8
72
68
78
71
a
Catalytic system A: TBA-OX/TEMPO, B: TBA-OX/TEMPO-IL.
Yields of isolated products unless otherwise noted.
Yields were determined by GC.
b
c
3.5. Mechanistic aspects
corresponding aldehydes or ketones in excellent yields. Benzylic
alcohols underwent smooth oxidation (Table 2, entries 1–6). Allylic
alcohols such as cinnamyl alcohol (Table 2, entry 7) were also
oxidized efficiently to the corresponding aldehyde without any
observable reaction at the double bond functionality. Electron-rich
and electron-deficient benzylic alcohols did not show obvious
differences. Aliphatic and heterocyclic alcohols were less reactive
than benzylic and allylic alcohols toward this system. Aliphatic
alcohols were oxidized to the corresponding aldehydes or ketones
in reasonable yields in prolonged reaction times (Table 2, entries 9, 12,
and 13). For the oxidation of furan-2-yl methanol, a heterocyclic
alcohol, the yield obtained was quite low even in prolonged reaction
time (Table 2, entry 11). Use of TEMPO-IL instead of TEMPO as catalyst
in this reaction was advantageous because of its similar reactivity to
TEMPO but with the additional advantages of simple workup
procedure and easy recovery.
For the present TBA-OX/TEMPO catalytic oxidation system, both
primary and secondary alcohols give carbonyl compounds. TEMPO
must play a key role for this selectivity. TEMPO would be the active
oxidant in this reaction. The role of TBA-OX is to regenerate TEMPO or
TEMPO-IL. A plausible mechanism of this reaction is depicted in
Scheme 2.
4. Conclusion
In summary, an efficient, facile and rapid oxidation of alcohols
with TBA-OX/TEMPO-IL system in the ionic liquid [bmim][PF6] has
been developed, which allowed the oxidation of alcohols to aldehydes
or ketones in excellent yields and high selectivity without remarkable
3.4. Recycling of the catalysts and ionic liquid
Extraction of the product with ether led to the separation of the
catalyst and ionic liquid from the product. The ionic liquid layer was
then washed with water to remove n-Bu4NHSO4 (the reduction
product of TBA-OX). The aqueous phase was removed by decantation
and lyophilized to obtain n-Bu4NHSO4 (see Fig. 1). n-Bu4NHSO4 is a
Scheme 2. A plausible reaction pathway.

1020
C. Zhu et al. / Catalysis Communications 11 (2010) 1017–1020
[8] J. Piera, J.-E. Bäckvall, Angew. Chem. Int. Ed. 47 (2008) 3506.
[9] S.E. Denmark, D.C. Forbes, D.S. Hays, J.S. DePue, R.G. Wilde, J. Org. Chem. 60 (1995)
1391.
over-oxidation to carboxylic acids. It is noteworthy to mention that
the catalyst and solvent could easily be recycled and reused without
loss of activity and the oxidant's by-product (n-Bu4NHSO4) could also
be recycled. The scope and synthetic and industrial applications of the
oxidation procedure are currently under investigation.
[10] K.S. Webb, S.J. Ruszkay, Tetrahedron 54 (1998) 401.
[11] M. Curini, F. Epifano, M.C. Marcotullio, O. Rosati, Synlett (1999) 777.
[12] B.R. Travis, B. Borhan, Tetrahedron Lett. 42 (2001) 7741.
[13] B.R. Travis, R.S. Narayan, B. Borhan, J. Am. Chem. Soc. 124 (2002) 3824.
[14] B.R. Travis, M. Sivakumar, G.O. Hollist, B. Borhan, Org. Lett. 5 (2003) 1031.
[15] M. Uyanik, M. Akakura, K. Ishihara, J. Am. Chem. Soc. 131 (2009) 251.
[16] B.R. Travis, B.P. Ciaramitaro, B. Borhan, Eur. J. Org. Chem. (2002) 3429.
[17] Z. Lei, Y. Yang, X. Bai, Adv. Synth. Catal. 348 (2006) 877.
[18] R.A. Sheldon, I.W.C.E. Arends, G.-J. Ten Brink, A. Dijksman, Acc. Chem. Res. 35
(2002) 774.
Acknowledgement
We are grateful to Nanjing University of Science and Technology
for financial support.
[19] R.A. Sheldon, I.W.C.E. Arends, Adv. Synth. Catal. 346 (2004) 1051.
[20] R.A. Sheldon, I.W.C.E. Arends, J. Mol. Catal. A Chem. 251 (2006) 200.
[21] T. Fey, H. Fischer, S. Bachmann, K. Albert, C. Bolm, J. Org. Chem. 66 (2001) 8154.
[22] G. Pozzi, M. Cavazzini, S. Quici, M. Benaglia, G. Dell Anna, Org. Lett. 6 (2004) 441.
[23] X.E. Wu, L. Ma, M.X. Ding, L.X. Gao, Synlett (2005) 607.
[24] W.X. Qian, E.L. Jin, W.L. Bo, Y.M. Zhang, Tetrahedron 62 (2006) 556.
[25] A. Dijksman, I. Arends, R.A. Sheldon, Synlett (2001) 102.
[26] J. Muzart, Adv. Synth. Catal. 348 (2006) 275.
References
[1] J.E. Bäckwall, Modern Oxidation Methods, Wiley-VCH, New York, 2004.
[2] G. Tojo, M. Fernandez, Oxidation of Alcohols to Aldehydes and Ketones, Springer,
Berlin, 2006.
[3] G. Tojo, M. Fernandez, Oxidation of Primary Alcohols to Carboxylic Acids, Springer,
Berlin, 2006.
[4] M. Hudlický, Oxidations in Organic Chemistry, American Chemical Society
Science, Washington, DC, 1990.
[5] I.E. Marko, P.R. Giles, M. Tsukazaki, S.M. Brown, C.J. Urch, Science 274 (1996) 2044.
[6] G.-J. ten Brink, I.W.C.E. Arends, R.A. Sheldon, Science 287 (2000) 1636.
[7] D.I. Enache, J.K. Edwards, P. Landoin, B. Solsona-Espriu, A.F. Carley, A.A. Herzing,
M. Watanabe, C.J. Kiely, D.W. Knight, G.J. Hutchings, Science 311 (2006) 362.
[27] Y. Wei, M. Cai, C. Lu, Catal. Lett. 90 (2003) 81.
[28] L. Lin, J. Liuyan, W. Yunyang, Catal. Commun. 9 (2008) 1379.
[29] L. Lin, M. Juanuan, J. Liuyan, W. Yunyang, J. Mol. Catal. A Chem. 291 (2008) 1.
[30] L. Liu, L.-Y. Ji, Y.-Y. Wei, Monatsh. Chem. 139 (2008) 901.
[31] C. Zhu, L. Ji, Q. Zhang, Y. Wei, Can. J. Chem. 88 (2010) 1.
[32] C. Zhu, L. Ji, Y. Wei, Monatsh. Chem. 141 (2010) 327.