A highly recyclable magnetic core-shell nanoparticle-supported TEMPO catalyst for efficient metal- and halogen-free aerobic oxidation of alcohols in water

Article abstract of DOI:10.1002/chem.201100047

The selective oxidation of primary and secondary alcohols to the corresponding carbonyl compounds is one of the most challenging reactions in organic chemistry. Many oxidizing reagents such as stoichiometric CrVI salts, DMSOcoupled reagents, and hypervalent iodine have been traditionally used to accomplish this transformation. However, these reagents show poor atom efficiency and are often toxic; their widespread use thereby causes significant environmental concerns that render them impractical. As a consequence, due to ever-increasing environmental standards and economic pressures, there is substantial interest towards the use of heterogeneous and recyclable catalysts to achieve the efficient oxidation of alcohols with molecular oxygen or air as the oxidant. Whereas methodologies for the improvement of catalytic activities and selectivities have been developed considerably, they may possibly leave toxic traces of heavy metals in the products. Moreover, many of the metal-based catalyst systems are often deactivated because of the occupation of coordination sites with the by-produced water and so they generally require the use of organic solvents. Therefore, it seems that there is still a great interest in developing efficient and non-metallic catalysts for the aerobic oxidation of alcohols from the viewpoint of socalled green and sustainable chemistry. Of particular interest in this area is the application of the stable nitroxyl radical 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in combination with various types of non-metallic, stoichiometric co-oxidants as an alternative to metal-based oxidants.

Full text of DOI:10.1002/chem.201100047

DOI: 10.1002/chem.201100047
A Highly Recyclable Magnetic Core-Shell Nanoparticle-Supported TEMPO
catalyst for Efficient Metal- and Halogen-Free Aerobic Oxidation of
Alcohols in Water**
Babak Karimi* and Elham Farhangi[a]
The selective oxidation of primary and secondary alcohols
to the corresponding carbonyl compounds is one of the
most challenging reactions in organic chemistry.[1] Many oxi-
dizing reagents such as stoichiometric CrVI salts,[2] DMSO-
coupled reagents,[3] and hypervalent iodine[4] have been tra-
ditionally used to accomplish this transformation. However,
these reagents show poor atom efficiency and are often
toxic; their widespread use thereby causes significant envi-
ronmental concerns that render them impractical. As a con-
sequence, due to ever-increasing environmental standards
and economic pressures, there is substantial interest towards
the use of heterogeneous and recyclable catalysts to achieve
the efficient oxidation of alcohols with molecular oxygen or
air as the oxidant.[5] Whereas methodologies for the im-
provement of catalytic activities and selectivities have been
developed considerably, they may possibly leave toxic traces
of heavy metals in the products. Moreover, many of the
metal-based catalyst systems are often deactivated because
of the occupation of coordination sites with the by-produced
water and so they generally require the use of organic sol-
vents. Therefore, it seems that there is still a great interest
in developing efficient and non-metallic catalysts for the
aerobic oxidation of alcohols from the viewpoint of so-
called green and sustainable chemistry. Of particular interest
in this area is the application of the stable nitroxyl radical
2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in combina-
tion with various types of non-metallic, stoichiometric co-ox-
idants as an alternative to metal-based oxidants.[6]
methods are inefficient for the oxidation of aliphatic alco-
hols and/or require high reaction temperatures, which are
not suitable for thermally unstable substrates.[7] Meanwhile,
the use of TEMPO in the absence of transition metals has
received increasing attention as a possible alternative to the
widely used traditional TEMPO/Metal/O2 in the aerobic ox-
idation of alcohols.[10] Although, these findings presented an
interesting breakthrough in the field of metal-free aerobic
oxidation of alcohols, the methods still require homogene-
ous TEMPO, which is quite expensive, and therefore the ef-
fective recovery of TEMPO is highly desirable for many
practical applications. In addition, these methods require
acidic additives such as HBr, HCl, Br2, and chlorinated sol-
vents in most cases.[9b,10a–d,11] To address the recovery issues,
TEMPO was chemically anchored onto various supports
such as silica,[12] organic polymers,[13] mesoporous silica,[14]
functionalized ionic liquids,[15] and perfluoroalkyl systems
with multiple triazole moieties.[16] Although these methods
have partly addressed the recycling problem of the homoge-
neous TEMPO, only a few of them have shown acceptable
activities in the aerobic oxidation of alcohols under transi-
tion-metal-free conditions.[14,15] Nevertheless, these methods
mainly employ acidic conditions in which oxidation of acid
sensitive substrates such as allylic alcohols can be problem-
atic. Furthermore, although these protocols represent con-
siderable advances, they still need halogen containing co-re-
agents,[14,15] and more importantly, the supported TEMPO
catalyst is difficult to separate from the reaction solution by
classical methods such as extraction, filtration, or centrifuga-
tion.[17]
Accordingly, the use of TEMPO-based catalyst systems in
combination with transition metals such as Ru,[7] Cu,[8] and
MnII–CoII,[9] as co-catalysts has been well documented for
the oxidation of alcohols using molecular oxygen as terminal
oxidant. However, these methods still require transition-
metal co-catalysts, which are intrinsically difficult to sepa-
rate from the product and recycle. Moreover, many of these
A possible strategy to circumvent these problems is to
change the traditional supported matrix to materials that
have magnetic properties, thus allowing easy separation of
the catalysts by simply applying an external magnetic
field.[17,18] Besides the easy separation, an interesting feature
of magnetically functionalized catalysts is that they show
identical and sometimes even higher activity than their cor-
responding homogeneous analogues.[19] Quite recently, this
approach has been applied for anchoring TEMPO on gra-
[a] Prof. Dr. B. Karimi, E. Farhangi
Department of Chemistry, Institute for Advanced
Studies in Basic Sciences (IASBS)
Zanjan 45137-6731 (Iran)
phene-coated nanobeads with
a magnetic cobalt core
through a “click-chemistry” approach.[20] This catalytic mate-
rial was then successfully applied to the TEMPO-mediated
oxidation of alcohols under the standard Anelliꢀs oxidation[6]
and magnetically separated and recycled. However, to the
best of our knowledge there has been no report on the use
Fax : (+98)241-415-3225
E-mail: karimi@iasbs.ac.ir
[**] TEMPO=2,2,6,6-tetramethylpiperidine-1-oxyl.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100047.
6056
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Chem. Eur. J. 2011, 17, 6056 – 6060

COMMUNICATION
of supported aminoxyl radicals on magnetic nanoparticles as
recyclable catalysts in the transition-metal-free and halogen-
free aerobic oxidation of alcohols.
Herein, we present for the first time the aerobic oxidation
of a wide range of alcohols by using a novel, magnetically
separable TEMPO catalyst 1, which is anchored onto silica-
coated Fe3O4 nanoparticles, in the absence of any transition-
metal and halide co-catalyst (Scheme 1).[21]
(AMNP).[21] This analysis, in combination with microanalysis
data, indicated the successful anchoring of the TEMPO
groups on the surface of magnetic nanobeads. Unfortunate-
ly, due to the magnetic properties of MNST it is actually im-
possible to further characterize this material by using solid-
state NMR spectroscopy.
The catalytic activity of MNST was first tested in the aero-
bic oxidation of benzyl alcohol. In this regard, we were very
interested to combine the advantages of using molecular
oxygen as a terminal oxidant under non-halogenated and
aqueous conditions, with the possibility to magnetically re-
cover and recycle the N-oxyl radical catalyst. To determine
the optimal experimental conditions, the impact of various
nitrite sources such as tert-butylnitrite (TBN) and NaNO2
were examined; it was found that TBN was better suited for
this purpose, with water as reaction solvent (Table 1, en-
tries 1 and 2). Notably, decreasing the amount of water to
less than 3 mL resulted in significantly higher yields; the op-
timum result was obtained by using 0.3 mL of H2O (Table 1,
entry 3). Although the oxidation of benzyl alcohol in either
CH3COOH (0.3 mL) or H2O (0.3 mL) led to complete sub-
Scheme 1. General procedure for the aerobic oxidation of alcohols using
catalyst 1.
To prepare the catalyst, we
have chosen a known silica-
Table 1. Optimization of the conditions for the oxidation of benzyl alcohol (1 mmol) with 1 (MNST).
coated nanomagnet, which can
be easily prepared from cheap
starting materials according to
the reported procedure with
slight modifications.[21,22] The
resulting silica-coated Fe3O4
nanoparticles were then al-
lowed to react under vigorous
stirring with an appropriate
concentration of (3-aminopro-
Entry
1
AcOH
[mL]
H2O
[mL]
TBN
[mol %]
NaNO2
[mol %]
Conditions
t
T
Yield
[%][a]
A
R
G
[h]
[oC]
1
2
3
4
5
6
0.2
0.2
0.2
0.2
0.14
0.2
–
–
–
0.3
–
3
3
0.3
–
0.3
0.3
–
4
4
4
4
4
10
–
–
–
–
O2
O2
O2
O2
O2
O2
8
7
4
2
6
4
50
50
50
50
50
RT
10
70
100
100
57
–
–
50
[a] GC yields were obtained using internal standard method.
pyl)triethoxysilane
to
give
amino-functionalized silica-coated nanomagnets (Fe3O4–
SiO2PrNH2). To this end, the obtained aminopropyl-coated
nanomagnets were subjected to reductive amination with 1-
hydroxy-4-oxo-2,2,6,6,-tetramethylpiperidine in the presence
of NaBH3CN to prepare the corresponding magnetic nano-
particle-supported TEMPO (MNST) (Scheme 2).[21] The
loading of the TEMPO function in MNST can be calculated
from quantitative elemental analysis, which confirmed a
loading of approximately 0.3 mmol gÀ1. The IR spectrum of
MNST shows peaks that are characteristic of a functional-
ized TEMPO group, which clearly differs from that of the
unfunctionalized silica-coated nanomagnets (SMNP) and
strate conversion and afforded the corresponding aldehyde
in almost quantitative yield, the acidity of acetic acid and
the environmentally friendliness of water resulted in the
latter being preferable for subsequent studies (Table 1, en-
tries 3 and 4). It was also noted that the decrease of either
the MNST loading or reaction temperature resulted in sig-
nificantly lower yields (Table 1, entries 5 and 6). With the
optimal reaction conditions for transition-metal-free and
halogen-free catalysis to hand, we then proceeded to investi-
gate the scope of this new aerobic oxidation protocol for
other alcohols.
As shown in Table 2, various types of primary benzylic al-
cohols, including those with both electron-withdrawing and
electron-donating groups, were
aminopropyl-functionalized
magnetic
nanoparticles
selectively converted to the cor-
responding benzaldehydes in
excellent yields under the opti-
mal
reaction
conditions
(Table 2, entries 1–13). Similar-
ly, MNST was also a very active
catalyst for the aerobic oxida-
tion of substituted secondary
benzylic alcohols under such
Scheme 2. Synthesis of magnetic nanoparticle-supported TEMPO 1; a) tetraethyl orthosilicate, EtOH, aqueous
NH3 (10%), 408C, 12 h; b) (3-aminopropyl)triethoxysilane, toluene, 1058C, 20 h; c) 4-oxo-TEMPO, MeOH,
NaBH3CN, 48 h.
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www.chemeurj.org
6057

B. Karimi and E. Farhangi
Table 2. Aerobic oxidation of various alcohols using 1 (MNST).
carbonyl compounds in good to excellent yields with a high
level of selectivity under the same reaction conditions
(Table 2, entries 21–25). Remarkably, this method is highly
selective for the oxidation of primary alcohols to the corre-
sponding aldehydes, without any detectable over-oxidation
to their carboxylic acids, even after prolonged reaction
times (Table 2, entries 1–13, 21 and 22).
Entry
R1
R2
t
Yield
[h]
[%][a,b]
1
Ph
H
4
100
100
100
100
100
100
100
100[c]
92
99
99
100
95
100
89
96
80
77
100
86
100
100
97
100
91
90
100
61
2
3
4
5
6
7
8
9
4-ClC6H4
3-ClC6H4
2,4-Cl2C6H3
2,6-Cl2C6H3
2-MeC6H4
4-MeOC6H4
4-MeOC6H4
4-NO2C6H4
2-NO2C6H4
1-naphthyl
4-MeSC6H4
3-pyridyl
C6H5
C6H5
cyclohexyl
C6H5
C6H5CO
C6H4CH2
4-ClC6H4CO
H
H
H
H
H
H
H
H
H
H
H
H
Me
Et
C6H5
C6H5
C6H5
C6H5
4-ClC6H4
H
H
Me
Me
7
10
16
29
3
2.5
2.5
22
30
15
15
35
9
14
24
24
28
24
25
29
12
24
25
24
12
13
48
30
In general, heteroatom-functionalized alcohols are regard-
ed as highly challenging substrates for aerobic oxidation in
most transition-metal catalyst systems because the strong co-
ordination to metal centers deactivates the catalyst. Howev-
er, we found that our protocol is also highly effective in the
oxidation of 4-(methylthio)benzyl alcohol and 3-pyridineme-
thanol to afford the expected aldehydes, which further con-
firmed the superior capability of MNST in oxidizing similar
substrates (Table 2, entries 12 and 13). Furthermore, allylic,
propargylic, and cyclopropylcarbinols, which are highly sen-
sitive to acidic conditions, could be effectively oxidized
using the present method to give moderate to excellent
yields of the corresponding carbonyl products; a feature not
observed with most of the previous halogen-based TEMPO-
catalyzed protocols (Table 2, entries 26–30).[10a–c,13] Encour-
aged by these results, we then managed to perform the aero-
bic oxidation of a number of highly challenging, sterically
hindered alcohols in the presence of MNST. To our delight,
sterically demanding substrates were also successfully oxi-
dized to their corresponding carbonyl compounds, although
in these circumstances somewhat longer reaction times were
required (Table 2, entries 5, 16 and 17, and 31–33). Impor-
tantly, the same transformations are very difficult to achieve
using previously developed heterogeneous TEMPO/O2 sys-
tems,[23] and to the best of our knowledge, there are only
two reports on the application of supported TEMPO in the
aerobic oxidation of highly hindered secondary aliphatic al-
cohols.[14,24] For practical purposes, a long catalyst lifetime
and the ability to easily recycle the catalyst are highly desir-
able. To investigate this issue, we turned our attention to the
reusability of MNST and found that this catalyst demon-
strated remarkably excellent recyclability; after the first use
of MNST in the aerobic oxidation of benzyl alcohol to give
benzaldehyde in quantitative yield, the catalyst was magneti-
cally separated, washed thoroughly with EtOH and H2O,
and the recycled catalyst exhibited consistent efficiency in
20 subsequent reaction runs (Figure S1, see the Supporting
Information).[21]
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
C6H
CH3A
C6H5A
CH3A(CH2)4
5A
(CH2)6
(CH2)2
E
R
cyclohexanol
cyclohexene-3-ol
G
H
Me
À ꢀ
C CH
C
58
30
36
94
28
36
64
83
31
28
82
32
33
36
48
90
94
[a] GC yields were obtained using internal standard method. [b] Condi-
tions: Method A: primary benzylic alcohols (1 mmol), tBuONO
(4 mol%), and catalyst 1 (0.2 mol%) in H2O (0.3 mL) at 508C; Method
B: aliphatic and secondary benzylic alcohols (1 mmol), tBuONO
(5 mol%), and catalyst 1 (0.3 mol%) in H2O (0.3 mL) at 508C; Method
C: allylic and hindered alcohols (1 mmol), tBuONO (5 mol%), and cata-
lyst 1 (0.35 mol%) in H2O (0.3 mL) at 508C. [c] Reactions were run on a
5 mmol scale.
TEM images of both the fresh and reused catalyst
(Figure 1) also indicated that no detectable changes of the
catalyst occurred during the reaction and the recycling
stages. Moreover, one of the attractive features of this novel
catalyst system is the rapid (within 5 seconds) and efficient
separation of the catalyst by using an appropriate external
magnet, which minimizes the loss of catalyst during separa-
tion (Figure 2).
In conclusion, we have developed a highly efficient cata-
lyst system for the water-mediated aerobic oxidation of a
wide range of alcohols by using magnetically core-shell
nanoparticle-supported TEMPO (MNST) as the catalyst
mild reaction conditions to give the corresponding ketones
in good to excellent yields (Table 2, entries 14–20). These re-
sults show that electronic effects do not seem to have a sig-
nificant effect on the final product yields for electron-rich
and electron-deficient benzylic substrates. Notably, this
method was equally applicable to the oxidation of both pri-
mary and secondary aliphatic alcohols, giving the respective
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Chem. Eur. J. 2011, 17, 6056 – 6060

Highly Recyclable Magnetic Core-Shell Nanoparticle-Supported TEMPO
COMMUNICATION
graphic purification. The separated catalyst was successively washed with
EtOH and H2O, and then used directly for subsequent reaction runs
without further purification.
Acknowledgements
We gratefully acknowledge financial support from the Iranian National
Science Foundation (INSF) and the IASBS Research Council. We also
acknowledge Dr. Saeed Emadi from IASBS for reading the manuscript
and for his friendship and help.
Keywords: aerobic oxidation
·
alcohols
·
magnetic
properties · nanoparticles · supported catalysts
Figure 1. TEM images of MNST before (left; scale bar=100 nm) and
after (right; scale bar=100 nm) recycling experiments.
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Figure 2. Easy separation of magnetic nanoparticle-supported TEMPO
catalyst 1 (MNST) within 5 s.
under transition-metal- and halogen-free reaction condi-
tions. In particular, the present catalytic system shows excel-
lent activity in the aerial oxidation of acid-sensitive and
highly challenging sterically hindered alcohols, a feature not
generally observed in most of the previous halogen-based
TEMPO-catalyzed protocols. The system also couples the
advantages of heterogeneous (easy separation, and excellent
reusability of up to 20 runs) and homogeneous TEMPO-
based systems (high activity and reproducibility), which
make it as a promising material for practical and large-scale
applications. Studies aimed at extending the scope of MNST
to other useful transformations are currently underway in
our laboratories.
Experimental Section
General procedure:
A mixture of alcohol (5 mmol), tBuONO (4–
5 mol%), and catalyst 1 (0.2–0.35 mol%) in H2O (1.5 mL) was prepared
in a two-necked flask at 508C. The flask was then filled with pure oxygen
(balloon filled). The resulting mixture was stirred at 508C under an
oxygen atmosphere. After completion of the reaction, Et2O (3ꢂ15 mL)
was added to the reaction flask and the catalyst was separated by an ex-
ternal magnet. The resulting solution was then dried with sodium sulfate
and the excess solvent was removed under reduced pressure to give the
corresponding carbonyl compounds. In most cases, the purity of the prod-
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Received: January 5, 2011
Published online: April 19, 2011
6060
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Chem. Eur. J. 2011, 17, 6056 – 6060