SBA-15-functionalized TEMPO confined ionic liquid: An efficient catalyst system for transition-metal-free aerobic oxidation of alcohols with improved selectivity

Article abstract of DOI:10.1039/c1ob05178d

A novel SBA-15-functionalized TEMPO confined ionic liquid [BMIm]Br was found to be a highly efficient and recyclable catalyst system for the transition-metal-free aerobic oxidation of a wide range of structurally diverse alcohols. Thanks to the strong physical confinement of the ionic liquid inside the mesochannels of SBA-15-supported TEMPO, the resulting solid catalyst showed improved selectivity in the aerobic oxidation of allylic alcohols. The catalyst can be recovered and re-used for at least 11 reaction runs without significant loss of either activity or confined IL.

Full text of DOI:10.1039/c1ob05178d

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PAPER
SBA-15-functionalized TEMPO confined ionic liquid: an efficient catalyst
system for transition-metal-free aerobic oxidation of alcohols with improved
selectivity†
Babak Karimi* and Ebrahim Badreh
Received 31st January 2011, Accepted 16th March 2011
DOI: 10.1039/c1ob05178d
A novel SBA-15-functionalized TEMPO confined ionic liquid [BMIm]Br was found to be a highly
efficient and recyclable catalyst system for the transition-metal-free aerobic oxidation of a wide range of
structurally diverse alcohols. Thanks to the strong physical confinement of the ionic liquid inside the
mesochannels of SBA-15-supported TEMPO, the resulting solid catalyst showed improved selectivity
in the aerobic oxidation of allylic alcohols. The catalyst can be recovered and re-used for at least 11
reaction runs without significant loss of either activity or confined IL.
6
,7
Introduction
using molecular oxygen as terminal oxidant. Along these lines,
the first breakthrough metal-free aerobic oxidation of alcohols
The selective oxidation of alcohols into carbonyl compounds
is one of the most important reactions in both the industrial
synthesis of fine chemicals and laboratory research. This trans-
formation has traditionally been performed with stoichiomet-
ric amounts of high-valence transition-metal salts or organic-
based oxidants that generate relatively large amounts of metal-
containing hazardous wastes and the reactions are usually run in
using a TEMPO/NaNO
2
/Br
2
catalyst system was uncovered by
8
Liang and Hu and co-workers. However, the use of chlorinated
solvents and homogeneous, non-recyclable TEMPO make this
route less attractive for practical purposes. To address some of
these limitations, the same research group also developed various
1
NaNO
2
- and t-butylnitrite (TBN)-based systems in combination
9
with TEMPO to catalyze the aerobic oxidation of alcohols.
Meanwhile, He and co-workers have disclosed a metal-free
catalytic system comprising TEMPO-functionalized imidazolium
2
chlorinated solvents. However, the recent stringent environmental
and legislative concerns demand greener methods with a high
degree of chemoselectivity that minimize or eliminate the use
of toxic and/or corrosive reagents and stop the formation of
undesired by-products. As a consequence, the oxidation of
alcohols with molecular oxygen in the presence of metal-catalysts
has attracted a great deal of attention in recent years. Although
interesting developments in improving catalytic activities and
selectivities are evident, these methods are still using transition
metal catalysts, which may possibly leave toxic traces of heavy
metals in the products. Therefore, in order to avoid the use of
transition metals, the development of new efficient and non-
metallic catalysts for the aerobic oxidation of alcohols appears
veryappealing inviewofso-called green and sustainablechemistry.
Of particular interest in this field are catalytic systems involving
the stable nitroxyl radical 2,2,6,6,-tetramethylpiperidine-1-oxyl
salt/NaNO
2
/CO
2
/H O for oxidation of alcohols in the presence
2
10
of molecular oxygen. While this protocol demonstrates excellent
performance in the oxidation of primary benzylic alcohols, the
same transformations with either primary aliphatic or secondary
alcohols gave moderate to low yields of the corresponding
carbonyl compounds. On the other hand, quite recently, Studer
and Wertz have described an innovative metal-free catalyst system
consisting of homogeneous TEMPO and aqueous NH OH to
accomplish the aerobic oxidation of a relatively wide range of
alcohols, with good to excellent results. However, despite these
interesting achievements, this method requires a relatively high
concentration of non-recyclable and expensive TEMPO (up to
3
4
,5
2
11
4
mol%) and oxygen pressure up to 3 bar.
To address the recyclability of TEMPO while emphasizing the
importance of metal-free aerobic oxidation of alcohols, we recently
reported a durable and recyclable catalyst system comprising
(
TEMPO) in combination with a non-metallic co-catalyst, which
have been increasingly explored for the oxidation of alcohols
SBA-15 supported TEMPO/NaNO
2
/n-Bu NBr for the aerobic
4
12
oxidation of alcohols in acetic acid. However, despite the high
catalytic performance and recyclability of this system, our studies
showed that the method is not suitable for acid-sensitive substrates
and, more importantly, the oxidation of allylic alcohols to a,b-
unsaturated carbonyl compounds could not be achieved using
this protocol (vide supra).
Department of Chemistry, Institute for Advanced Studies in Basic Sci-
ences (IASBS), Gava Zang, Zanjan, 45137-6731, Iran. E-mail: karimi@
iasbs.ac.ir; Fax: +98-241-4214949; Tel: +98-241-4153225
†
Electronic supplementary information (ESI) available: details of exper-
imental procedure, N adsorption–desorption isotherm for 1, 2, and the
recovered 2, BJH diagrams, and TEM image of the recovered 2. See DOI:
0.1039/c1ob05178d
2
1
4
194 | Org. Biomol. Chem., 2011, 9, 4194–4198
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Herein we wish to report that filling mesochannels of SBA-
5-functionalized TEMPO with 1-methyl-3-butylimidazolium
BMImBr) ionic liquid results in an efficient and recyclable
On the other hand, the N
well as the BET surface area, was considerably different from
2
adsorption isotherm of 2, as
1
(
2
-1
1. The decrease in the BET surface area (29 m g ) and
3
-1
catalyst system for the aerobic oxidation of various types of
mesopore volumes (0.1 cm g ) and the loss of the capillary
condensation/evaporation N isotherm clearly demonstrate that
13
alcohols with improved selectivity under metal-free conditions.
2
In particular, we found that the catalyst shows excellent selectivity
and performance in the oxidation of allylic alcohols
the loading of ionic liquid into the mesopores of 1 has successfully
been achieved (Fig. 2).
Results and Discussion
The SBA-15 supported TEMPO 1 was initially prepared according
12a
to our previously reported protocol. The obtained TEMPO-
functionalized SBA-15 was then mixed with an acetone solution
containing appropriate amounts of BMImBr and the mixture was
14,15
stirred for 3 h at room temperature.
To this end, the solvent was
evaporated under reduced pressure to give a very fine bright yellow
and free-flowing solid denoted as IL@SBA-15-TEMPO (Scheme
1, 2).
2
Fig. 2 The N adsorption–desorption isotherm for SBA-15-function-
alized TEMPO confined ionic liquid 2.
The catalytic performance and selectivity of 2 was evaluated in
the aerobic oxidation of benzyl alcohol as a model substrate, in
the presence of molecular oxygen. The reactions were carried out
Scheme 1 A schematic pathway for the preparation of SBA-15-functiona-
lized TEMPO confined ionic liquid 2.
in a batch reactor under an O
2
◦
atmosphere at normal pressure, in
the temperature range 30–80 C. In this regard, the oxidation of
benzyl alcohol took place smoothly in the presence of 1 mol% of
The nitrogen adsorption–desorption experiments of 1 showed
◦
2, 8 mol% of NaNO
2
and AcOH (0.3 mL) at 80 C to give, after
2
the BET (Brunauer–Emmett–Teller) surface area of 480 m
3
h, the corresponding benzaldehyde in only 60% yield (Table 1,
-
1
3
-1
g , the primary mesopore volume of 0.91 cm g , and BJH
entry 1).
Interestingly, the use of TBN (8 mol%) instead of NaNO
14
(
Barrett–Joyner–Halenda) average pore diameter of 6.18 nm.
2
, gave
The materials typically showed a type IV N
with a sharp hysteresis loop at relative pressure range P/P
2
sorption isotherm
~ 0.6–
.8, which is characteristic of ordered mesoporous solids with a
narrow pore size distribution (Fig. 1).
quantitative yields of benzaldehyde (Table 1, entry 2). It should
be also underlined that no significant oxidation took place in the
absence of either TBN or 2 under the same reaction conditions
0
0
Table 1 Optimization of aerobic oxidation of benzyl alcohol catalyzed
by 2
◦
a
Run T/ C 2(mol %) NO source (mol %) Time (h) Yield (%)
1
2
3
4
5
6
7
8
9
1
1
80
80
80
80
80
60
50
40
30
40
40
1
1
1
—
1
1
1
1
1
NaNO
TBN (8)
—
2
(8)
3.0
2.0
3.0
3.0
2.5
2.8
2.8
3.3
5.0
5.0
3.0
60
>99
<1
TBN (8)
TBN (4)
TBN (4)
TBN (4)
TBN (4)
TBN (4)
TBN (4)
TBN (4)
8
>99
>99
>99
>99
59
0
1
0.5
1
30
<5
b
Fig. 1 The N
lized TEMPO 1.
2
adsorption–desorption isotherm for SBA-15-functiona-
a
b
GC yields using internal standard method. The catalyst 1 was used.
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(
Table 1, entries 3, 4). The impact of the reaction temperature and
2 showed excellent activity for the oxidation of substituted
secondary benzylic alcohols using molecular oxygen to furnish
the corresponding ketones in good to excellent yields (Table 2,
entries 14–16). These results clearly demonstrate that electronic
effects do not seem to have a significant effect on the reaction time
and final product yields for electron-rich and electron-deficient
benzylic substrates.
TBN loading were also surveyed, and we found that by decreasing
the reaction temperature to 40 C, the oxidation reaction still went
◦
to completion, with a slight increase in reaction times (Table 1,
entries 5–8). It is also worth mentioning that decreasing the TBN
loading from 8 mol% to 4 mol% did not significantly affect the total
efficacy of the process (Table 1, entry 5). In a separate experiment,
the use of 1 without the loaded ionic liquid as a catalyst was shown
to afford much worse results, further confirming the beneficial role
of the ionic liquid in obtaining acceptable conversions (Table 1,
entry 11).
Having the optimized reaction conditions, we then investigated
the generality of our catalyst system with respect to the alcohol
structure (Table 2).
Along these lines, we found that various types of primary ben-
zylic alcohols, including those bearing both electron-withdrawing
and electron-donating groups, were selectively converted to the
corresponding aromatic aldehydes in excellent yields under the
optimal reaction conditions (Table 2, entries 1–13). Similarly,
In general, the aerobic oxidation of alcohols that contain
heteroatoms are considered highly challenging in most of the
transition-metal catalyst systems, because their strong coordina-
tion to metal centers deactivates the catalyst. However, we found
that our catalyst system is also highly effective for the oxidation
of 4-(methylthio)benzyl alcohol, and 3-pyridinemethanol and
affords the expected aldehydes in excellent yields and selectivities,
further confirming the superior capability of 2 in oxidizing similar
substrates (Table 2, entries 11, 12). Furthermore, allylic alcohols
that are highly sensitive to acidic conditions could be effectively
oxidized using the current method and gave excellent yields of the
corresponding a,b-unsaturated carbonyl products, a feature not
observed with most of the previous halogen-based TEMPO cat-
9a–c,12a
alyzed protocols (Table 2, entries 17–19).
It is very important
Table 2 Metal-free aerobic oxidation of alcohols catalyzed by SBA-15-
functionalized TEMPO confined ionic liquid (2)
a
to note that the same transformation using our previously reported
SBA-15–TEMPO catalyst system 1 led to worse results for the
aerobic oxidation of cinnamyl alcohol as a model substrate under
the same reaction conditions (Table 2, entries 20, 21). These results
clearly confirm that the confinement of ionic liquid molecules
inside the mesochannels of SBA-15-functionalized TEMPO have
a high impact in achieving improved selectivity in the oxidation
1
2
b
Run
R
R
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
C
6
H
5
H
H
H
H
H
H
H
H
H
H
H
H
H
Me
Et
3.5
3
>99
96
>99
95
98
99
96
>99
94
>99
>99
93
16
of allylic substrates using the present protocol. Notably, this
method was equally applicable to the oxidation of both primary
and secondary aliphatic alcohols with slight modifications, giving
the respective carbonyl compounds in moderate to excellent yield
(Table 2, entries 22–26). Even cyclohexanol, which is considered
a difficult substrate for TEMPO-mediated oxidation could be
smoothly converted to cyclohexanone in a moderate yield of
4-Me-C
6
H
4
4-MeO-C
3-NO
4-NO
3-Cl-C
3-Br-C
4-Br- C
2,4-Cl-C
2-Naphthyl
4-MeS-C H
4
3-Pyridyl
2-Furyl
C
C
C
6
H
H
H
4
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
6
6
3
6
6
2
-C
-C
6
4
2
6
4
6
H
H
4
6
4
6
H
4
6
H
3
0
1
2
3
4
5
6
7
8
9
6
7% under the described modified protocol (Table 2, entry 26).
6
Remarkably, for all of the substrates studied, the method was also
found to be highly selective for the oxidation of primary alcohols to
the corresponding aldehyde without any detectable over oxidation
to their carboxylic acids, even after prolonged reaction times.
The recyclability of 2 was also examined by isolating it from
the reaction mixture of aerobic oxidation of benzyl alcohol
through the successive microfiltration, washing with n-heptane
98
92
91
6
6
6
H
H
H
5
5
5
C
6
H
5
5
9
89
99
99
>99
c
Ph-CH CH
Ph-CH CH
H
15
17
17
c
C
6
H
c
17
and drying. The recycled catalyst was then successfully used in
1 successive reaction runs without any detectable loss of activity
Table 1S, ESI†).
Notably, N adsorption–desorption analysis of the recovered
1
(
d
2
2
2
2
2
2
2
0
1
2
3
4
5
6
Ph-CH CH-
Ph-CH CH-
H
H
H
H
H
15
15
3
3
3
5
15
—
e
2
18
94
95
87
>99
67
f
Ph-CH
2
CH
2
catalyst 2 demonstrated that the leaching of ionic liquid [BMIm]Br
was negligible in the recovered catalyst (Fig. 3). Moreover,
elemental microanalysis data of the recovered catalyst clearly
shows the loss of IL from recovered 2 under our described
recovery procedure was less than 8% of the initial content in fresh
IL@SBA-15-TEMPO, thus confirming the results obtained from
f
CH
CH
CH
3
3
3
(CH
(CH
(CH
2
2
2
)
)
)
6
3
5
f
f
Me
f
a
b
◦
c
N
2
-sorption analysis.
Furthermore, no appreciable catalytic activity was observed
Conditions: 2 (1 mol%), TBN (4 mol%) at 40 C unless otherwise stated.
GC yields using internal standard method. Conditions: 2 (3 mol%),
◦
d
TBN (4 mol%) at 60 C. Conditions: 1 (3 mol%), NaNO
2
(10 mol%), n-
from the residue of the n-heptane extraction of 2, even after
prolonged reaction times. Moreover, TEM images of the reused
catalyst also indicate that the 2D-hexagonal (honeycomb) nanos-
tructures of 2 largely survived and no detectable changes in
◦
Bu
4
NBr (8 mol%) at 60 C; mixture of unidentified products were formed.
e
◦
f
Conditions: 1 (3 mol%), TBN (4 mol%) at 60 C. Conditions: 2 (2 mol%),
◦
TBN (15 mol%), NaBr (15 mol%), at 80 C.
4
196 | Org. Biomol. Chem., 2011, 9, 4194–4198
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moieties in achieving such selectivity. This study is currently
ongoing in our laboratories.
Conclusions
In conclusion, we have demonstrated that the confinement of
the ionic liquid [BMIm]Br inside the mesopores of SBA-15-
functionalized TEMPO results in a highly recyclable catalyst
system 2 for the selective and transition-metal-free aerobic oxi-
dation of alcohols at normal oxygen pressures. In particular, this
heterogeneous catalyst system remarkably showed much higher
selectivities and efficiencies in the oxidation of allylic alcohols,
in comparison with SBA-15–TEMPO not charged with IL. The
catalyst can be recovered and reused for at least 11 reaction cycles
without any significant loss of activity or selectivity. Notably,
N adsorption–desorption analysis of the recovered catalyst 2
2
Fig.
3
The
N
2
adsorption–desorption isotherm of the recovered
demonstrated that the leaching of ionic liquid [BMIm]Br was
negligible in the recovered catalyst. TEM images of the reused
catalyst also demonstrated that in the nanostructures of 2, no
detectable changes in the catalyst nano-architecture occurred
during the catalysis and recycling stages. Further studies to
explore this strategy for other catalyst systems are ongoing in
our laboratories.
SBA-15-functionalized TEMPO confined ionic liquid 2 from the aerobic
oxidation of benzyl alcohol.
Acknowledgements
The authors acknowledge the IASBS Research Council and Iran
National Science Foundation (INSF) for support of this work.
Notes and references
1
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4
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J.-E. B a¨ ckvall, Modern Oxidation Methods, Wiley-VCH, Weinheim,
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Fig. 4 A TEM image of recovered SBA-15-functionalized TEMPO
confined ionic liquid 2 from the aerobic oxidation of benzyl alcohol.
2
006, 311, 362; (e) A. Abad, P. Concepcion, A. Corma and H. Garcia,
the catalyst occurred during the catalysis and recycling stages
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(
Fig. 4)
(
4
(
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The exact role of the IL is not precisely clear to us at this
stage. One explanation for the improved selectivity of 2 in
comparison with the un-modified SBA-15–TEMPO in oxidizing
allylic alcohols might be due to the highly polar environment
provided by the IL inside the mesoporous channels, which likely
speeds up the oxidation of TEMPO-OH by using in situ generated
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2
N. Theyssen, A. Brinkmann, K. V. Klementiev, W. Gr
u¨ nertr, M. B u¨ hl,
This may in turn prevent (or slow down) the addition of bromine
to double bonds in allylic systems. However, further in-depth
investigation would be necessary to clarify the actual synergistic
effect between the coated ionic liquid and immobilized TEMPO
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14 See Electronic Supplementary Information† for details.
15 The optimum amounts of ionic liquid needed to fill mesopores of
SBA-15–TEMPO catalyst 1 was obtained based on the performance of
IL@SBA-15-TEMPO 2 containing different amounts of [BMIm]Br in
a typical aerobic oxidation of benzyl alcohol.
2
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6
(
2
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7
For a recent review on aerobic oxidations in nitroxide-based oxidation
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4
7, 3506. For a review on aerobic oxidation of alcohols using TEMPO
in the presence of metal co-catalysts see: R. A. Sheldon and I. W. C.
E. Arends, J. Mol. Catal. A: Chem., 2006, 251, 200.
8
9
R. Liu, X. Liang, C. Dong and X. Hu, J. Am. Chem. Soc., 2004, 126,
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112.
(a) R. Liu, C. Dong, X. Liang and X. Hu, J. Org. Chem., 2005, 70, 729;
b) X. Wang, R. Liu, Y. Yin and X. Liang, Chem.–Eur. J., 2008, 14,
679; (c) Y. Xie, W. M. Mo, D. Xu, Z. L. Shen, N. Sun, B. Hu and X.
(
2
16 It has been shown that the coating of ionic liquid onto the silica
supported sulfonic acid led to a catalyst system with improved
selectivity in acid-catalyzed reactions: Y. Gu, A. Karam, F. J e´ r oˆ me
and J. Barrault, Org. Lett., 2007, 9, 3145.
17 In order to minimise the leaching of loaded ionioc liquid during the
recycling process it is crucial to use a non-polar solvent like n-heptane
in washing stages.
Hu, J. Org. Chem., 2007, 72, 4188; (d) X. He, Z. Shen, W. Mo, N. Sun,
B. Hu and X. Hu, Adv. Synth. Catal., 2009, 351, 89.
1
0 (a) C.-X. Miao, L.-N. He, J. L. Wang and F. Wu, J. Org. Chem., 2010,
5, 257. For the use of recyclable acidic imidazolium salts in TEMPO-
7
catalyzed aerobic oxidation of alcohols see: (b) C.-X. Miao, L.-N. He,
J.-Q Wang and J.-L. Wang, Adv. Synth. Catal., 2009, 351, 2209.
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198 | Org. Biomol. Chem., 2011, 9, 4194–4198
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