TEMPO and carboxylic acid functionalized imidazolium salts/ sodium nitrite: An efficient, reusable, transition metal-free catalytic system for aerobic oxidation of alcohols

Article abstract of DOI:10.1002/adsc.200900285

An effective catalytic system comprising a 2,2,6,6-tetramethylpiperidine-1- oxyl (TEMPO) functionalized imidazolium salt ([Imim-TEMPO]⁺X ^-), a carboxylic acid substituted imidazolium salt ([ImimCOOH] ⁺X^-), and sodium nitrite (NaNO₂) was developed for the aerobic oxidation of aliphatic, allylic, heterocyclic and benzylic alcohols to the respective carbonyl compounds with excellent selectivity up to >99%, even at ambient conditions. Notably, the catalyst system could preferentially oxidize a primary alcohol to the aldehyde rather than a secondary alcohol to the ketone. Moreover, the reaction rate is greatly enhanced when a proper amount of water is present. And a high turnover number (TON 5000) is achieved in the present transition metal-free aerobic catalytic system. Additionally, the functionalized imidazolium salts are successfully reused at least four times. This process thus represents a greener pathway for the aerobic oxidation of alcohols into carbonyl compounds by using the present task-specific ionic liquids in place of the toxic and volatile additive, such as hydrogen bromide, bromine, or hydrogen chloride (HBr, Br₂ or HCl), which is commonly required for the transition metal-free aerobic oxidation of alcohols.

Full text of DOI:10.1002/adsc.200900285

FULL PAPERS
DOI: 10.1002/adsc.200900285
TEMPO and Carboxylic Acid Functionalized Imidazolium Salts/
Sodium Nitrite: An Efficient, Reusable, Transition Metal-Free
Catalytic System for Aerobic Oxidation of Alcohols
a
a,
a
a
Cheng-Xia Miao, Liang-Nian He, * Jin-Quan Wang, and Jing-Lun Wang
a
State Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, Peopleꢀs Republic
of China
Fax : (+86)-22-2350-4216; phone: (+86)-22-2350-4216; e-mail: heln@nankai.edu.cn
Received: April 22, 2009; Revised: June 29, 2009; Published online: August 13, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900285.
Abstract: An effective catalytic system comprising a catalytic system. Additionally, the functionalized imi-
2
,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) func- dazolium salts are successfully reused at least four
+
À
tionalized imidazolium salt ([Imim-TEMPO] X ), a times. This process thus represents a greener path-
carboxylic acid substituted imidazolium salt ([Imim- way for the aerobic oxidation of alcohols into car-
COOH] X ), and sodium nitrite (NaNO ) was de- bonyl compounds by using the present task-specific
+
À
2
veloped for the aerobic oxidation of aliphatic, allylic, ionic liquids in place of the toxic and volatile addi-
heterocyclic and benzylic alcohols to the respective tive, such as hydrogen bromide, bromine, or hydro-
carbonyl compounds with excellent selectivity up to gen chloride (HBr, Br or HCl), which is commonly
2
>
99%, even at ambient conditions. Notably, the cat- required for the transition metal-free aerobic oxida-
alyst system could preferentially oxidize a primary tion of alcohols.
alcohol to the aldehyde rather than a secondary alco-
hol to the ketone. Moreover, the reaction rate is
greatly enhanced when a proper amount of water is Keywords: alcohol oxidation; imidazolium salts; mo-
present. And a high turnover number (TON 5000) is lecular oxygen; 2,2,6,6-tetramethylpiperidine-1-oxyl
achieved in the present transition metal-free aerobic (TEMPO); water
Introduction
Transition metals like copper, ruthenium, iron in
conjugation with TEMPO have been reported for the
[4]
The selective oxidation of primary and secondary al- oxidation of alcohols under the aerobic conditions.
cohols into the corresponding aldehydes or ketones is However, deactivation of the transition metal cata-
undoubtedly one of the most important and challeng- lysts and product contamination with heavy metals
[
1]
[5]
ing transformations in organic chemistry. And one hinder the development of these systems. Therefore,
of particular interest in this field is the use of the developing a transition metal-free system for aerobic
stable nitroxyl radical 2,2,6,6-tetramethylpiperidine-1- oxidations would be promising from the viewpoint of
oxyl (TEMPO) in combination with terminal oxi- green chemistry and sustainable development. In this
[2]
dants. However, the many systems that exist could context, Liang and Hu et al. made a great break-
suffer from the large amounts of waste produced be- through in this field. At first, they developed three-
cause stoichiometric amounts of terminal oxidants component catalyst systems, including TEMPO/Br /
2
[
6]
such as bleach, sodium chlorite, m-chloroperoxyben- NaNO2,
TEMPO/1,3-dibromo-5,5-dimethylhydan-
zoic acid (m-CPBA), hypervalent iodine(III) com- toin/NaNO2, TEMPO/HCl/NaNO , TEMPO/HBr/
pounds has been used. From economic and environ- tert-butyl nitrite (TBN), which would be considered
mental perspectives, using molecular oxygen or hy- to go through a sequential tricycle with a two-electron
drogen peroxide as a terminal oxidant has received transfer mechanism. Then, a TEMPO/TBN double-
great attention nowadays since only water as a sole component and sequential bicycle catalyst system was
[
7] [8]
AHCTUNGTRENNUNG
2
[
3]
[9]
by-product is formed.
successfully exploited for alcohol oxidations, and even
has been applicable to substrates containing acid-sen-
Adv. Synth. Catal. 2009, 351, 2209 – 2216
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2209

Cheng-Xia Miao et al.
FULL PAPERS
sitive functional groups without the need of the toxic >99%) for the oxidation of aliphatic, allylic, hetero-
[
10]
halogen source and acid. The TEMPO used could cyclic and benzylic alcohols to the respective carbonyl
not be recovered, however. On the other hand, compounds, even at ambient conditions.
Karimi et al. developed a heterogeneous catalyst
system, consisting of the SBA-15-supported TEMPO/
Bu NBr/NaNO under acidic conditions, allowing the Results and Discussion
4
2
[
11]
supported TEMPO to be recoverable. In addition,
the acidic conditions were essential to the reaction The exploratory experiments were started by screen-
when NaNO was used as the NO source. And certain ing the designed catalyst system Cat-1a/Cat-2a/
2
toxic and high volatile substances, such as HBr, Br2, NaNO . And benzyl alcohol was chosen as a model
2
HCl, CH COOH, are commonly needed for this tran- substrate. The results are summarized in Table 1. It is
3
sition metal-free aerobic oxidation according to the found that Cat-1a, Cat-2a, NaNO and oxygen were
2
literature. Consequently, it is still desirable to develop essential for the aerobic oxidation of benzyl alcohol
a cleaner, highly efficient and recyclable homogene- (Table 1, entries 1–4). However, the yield of benzyl al-
ous catalyst system without the need of a transition dehyde was just 21% in the absence of water
metal for this process.
(entry 5). Interestingly, a 53% yield was obtained as a
Ionic liquids as an environmentally friendly solvent result of adding 0.05 mL water (entry 6), presumably
represent interesting properties such as high thermal due to an improvement of the solubility of the cata-
stability, negligible vapor pressure, high loading ca- lyst system and an acceleration of the ionization of
pacity and easy recyclability. Various chemical reac- the Cat-2 in water.
tions have been well performed in ionic liquids. More-
In this respect, control experiments were carried
over, the task-specific ionic liquid could be designed out in aprotic solvents like DMSO (dimethyl sulfox-
for specific purposes on the basis of the reaction ide) and DMF (N,N-dimethylformamide). DMSO
[12]
mechanism.
However, only a few reports about which is capable of forming a homogeneous system
TEMPO functionalized imidazolium salts for alcohol enhanced the reaction (entry 9), whereas DMF could
[
13]
oxidations are available in the literature.
Mean- not dissolve the catalyst and demonstrated a poor per-
while, acidic ionic liquids also have attracted much at- formance under identical conditions (entry 10). Con-
[14]
tention.
sequently, the solubility of the catalyst would likely
A three-component catalyst system, consisting of a be more important for this reaction in comparison
TEMPO functionalized imidazolium salt ([Imim- with the ionizing ability of the Cat-2a. Notably, the
+
À
TEMPO] X , Cat-1a or Cat-1b), a carboxylic acid oxidation could also be successfully carried out under
substituted imidazolium-based ionic liquid ([Imim- mild conditions, even at room temperature and an air
+
À
COOH] X , Cat-2a or Cat-2b) to provide the acidic atmosphere (entries 7 and 8). Moreover, the effect of
conditions, and NaNO as the NO resource to activate the Cat-1a loading was evaluated (entries 11–16). The
2
oxygen, was designed for the aerobic oxidation of al- reaction was completed within 30 min in the presence
cohols in water (Scheme 1). To our delight, the cata- of 5% catalyst Cat-1a (entry 11). The reaction was op-
+
À
+
À
lyst system ([Imim-TEMPO] X /[Imim-COOH] X / erative in the range of the catalyst loading from 1 to
NaNO ) was effective and highly selective (almost 0.01% (entries 12 to 15). Specifically, a high turnover
2
number (TON 5000) can be also attained (entry 16).
The initial results showed that water played an ac-
celerative effect for the aerobic oxidation. According-
ly, the effect of the quantity of water was examined.
The yield and selectivity were found to be at the
same level when the amount of water was altered
from 0.05 mL to 0.2 mL (Table 2, entries 3–5). How-
ever, a more or a lesser amount of water can lead to a
drop of the yield (entries 1, 2, 6 and 7). This is reason-
able that the catalysts could not be completely dis-
solved if the added water is not enough, and thus re-
tarded the ionization of Cat-2a, and therefore resulted
in a decrease in the yield. On the other hand, an ex-
cessive amount of water could form an obvious bipha-
sic system, that is catalysts/H O with the substrate,
2
leading to the decrease in yield.
To examine the utility and generality of the catalyst
Scheme 1. Aerobic oxidation of alcohol catalyzed by [Imim- system for the alcohol oxidation, we applied the new
+
À
+
À
TEMPO] X /[Imim-COOH] X /NaNO .
catalyst system to a variety of alcohols, and the results
2
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Adv. Synth. Catal. 2009, 351, 2209 – 2216

TEMPO and Carboxylic Acid Functionalized Imidazolium Salts/Sodium Nitrite
FULL PAPERS
+
À
+
À
[a]
Table 1. Aerobic oxidation of alcohol catalyzed by [Imim-TEMPO] X /[Imim-COOH] X /NaNO .
2
Entry PO2
Amount of Cat-1a
[mol%]
Amount of Cat-2a
[mol%]
Solvent T
[K]
t
Conv.
Yield
[%]
TON
[
b]
[b]
[MPa]
[h] [%]
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
1
1
–
1
1
0.1
–
1
1
1
1
1
1
1
1
1
1
–
5
5
5
5
5
5
5
5
5
5
1
10
10
–
10
10
10
10
10
10
10
10
10
10
10
10
10
–
–
–
–
–
333
333
333
333
333
333
303
303
0.25 –
0.25 –
0.25 –
0.25 -
0.25 21
0.25 53
–
–
–
–
–
–
–
–
4
10
5
3
14
8
20
40
168
660
2200
5000
9.6
9.8
[
c]
21
53
25
17
71
39
>99
40
84
66
22
5
H O
H O
H O
DMSO 333
DMF
H O
H O
H O
H O
H O
H O
H O
2
6
8
25
17
2
[
d]
2
0.25 71
0.25 39
0.5 100
0.5 40
0
1
2
3
4
5
6
7
8
333
333
333
333
333
333
333
333
333
2
2
0.5
0.1
0.01
0.001
5
2
3
88
66
2
[
[
[
[
[
e]
f]
2
24 23
36
2
g]
h]
i]
5
2
10
0.25 48
0.25 49
48
49
2
5 (Cat-1b)
5 (Cat-2b)
H O
2
[
a]
Unless otherwise noted, all the experiments were carried out with benzyl alcohol (0.2 mL, 1.93 mmol), NaNO (6.7 mg,
2
5
mol%).
[
[
[
[
[
[
[
[
b]
Determined by GC using biphenyl as an internal standard.
Without NaNO2.
1
Benzyl alcohol (1 mL, 9.65 mmol), NaNO (33.5 mg, 5 mol%).
Benzyl alcohol (4 mL, 48.25 mmol), NaNO (167.5 mg, 5 mol%).
Benzyl alcohol (5 mL, 48.25 mmol), NaNO (167.5 mg, 5 mol%).
NaCl (11.2 mg, 10 mol%) was added.
Cat-1b (36.9 mg, 5 mol%)/Cat-2b (44.0 mg, 10 mol%) was used instead of Cat-1a/Cat-2a.
c]
d]
e]
f]
atm of air.
2
2
g]
h]
i]
2
[
a]
Table 2. Effect of the amount of water.
tivity of secondary benzylic alcohols, such as 1-phe-
nylethanol, benzhydrol, was affected by the a sub-
stituent group due to steric hindrance. And both sub-
strates could be smoothly oxidized to the correspond-
ing acetophenone and benzophenone, respectively,
after prolonging the reaction time compared with the
primary benzylic alcohols (entries 11 and 12). Besides,
the catalytic system was applicable to the oxidation of
other activated primary alcohols such as allylic,
[
b]
[b]
Entry
VH2O [mL]
Conv. [%]
Yield [%]
1
2
3
4
5
6
7
1
0.5
0.2
0.1
0.05
0.025
0.01
16
29
47
51
53
16
15
16
29
47
51
53
16
15
hetero ACHUTNRGNENUGc yclic alcohols (entries 13 and 14). To our de-
[
a]
Reaction conditions: benzyl alcohol (0.2 mL, 1.93 mmol), light, aliphatic alcohols could be converted to the
Cat-1a (31.9 mg, 5 mol%), Cat-2a (34.0 mg, 10 mol%), target products with high selectivity. Unfortunately,
NaNO2 (6.7 mg, 5 mol%), PO2 =1 MPa, T=333 K, t= the oxidations of 1-heptanol and 2-octanol were
1
5 min.
unsatisfac ACHNTUGRNENUGt ory even after elongating the reaction time
[
b]
Determined by GC using biphenyl as an internal stan-
dard.
because of the further oxidation of the corresponding
aldehydes into acids and/or ortho esters (entries 16
[
15]
and 23).
Moreover, in the cases of aliphatic alco-
are summarized in Table 3. Obviously, the activities of hols, the reaction rate also relied on the steric hin-
primary benzylic alcohols were best, and the activities drance as well as the chain length (entries 15–24). The
of those primary benzylic alcohols were also not sig- longer the chain was, poorer was the activity (en-
nificantly affected by the electronic properties and tries 15–20). In order to examine the chemoselectivity
steric hindrance of the substituents on the benzene of the catalytic system, a 1:1 molar mixture of primary
ring (Table 3, entries 1–7). Even substrates with an o- and secondary alcohols was used in the present cata-
or p-nitro group showed good activity at an elevated lytic system. Interestingly, only primary alcohols, such
temperature (entry 9 vs. 8, 10). In addition, the reac- as benzyl alcohol and 1-heptanol were converted into
Adv. Synth. Catal. 2009, 351, 2209 – 2216
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asc.wiley-vch.de
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Cheng-Xia Miao et al.
Time Conv. Yield
FULL PAPERS
[
a]
Table 3. Catalytic aerobic oxidation for various alcohols.
Entry Substrate
Product
T
[b]
[b]
[K]
[%]
[%]
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
Benzyl alcohol
Benzyl aldehyde
333 30 min 100
373 10 min 100
>99
>99
96
53
85
78
92
23
94
72
96
66
93
4-Methoxybenzyl alcohol
4-Methoxybenzaldehyde
333 1 h
97
373 20 min 54
333 30 min 87
333 30 min 80
333 20 min 93
2-Methoxybenzyl alcohol
3-Methoxybenzyl alcohol
4-Methylbenzyl alcohol
4-Nitro benzyl alcohol
2-Methoxybenzaldehyde
3-Methoxy benzaldehyde
4-Methylbenzaldehyde
4-Nitrobenzaldehyde
353 1 h
26
373 20 min 94
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
2-Nitrobenzyl alcohol
1-Phenylethanol
Benzhydrol
2-Furfurylmethanol
Cinnamyl alcohol
1-Heptanol
2-Nitrobenzaldehyde
Acetophenone
Benzophenone
Furfuryl aldehyde
Cinnamaldehyde
1-Heptaldehyde
353 1 h
333 3 h
353 12 h
333 1 h
333 6 h
333 6 h
333 12 h
333 6 h
333 12 h
333 6 h
333 12 h
333 6 h
333 6 h
333 12 h
333 6 h
72
100
68
100
24
51
94
36
79
6
45
59
29
81
26
[
c]
22
[
[
c]
c]
45
76
[d]
Dodecanol
Dodecanal
35
72
6
1-Hexadecanol
1-Hexadecanal
41
56
23
[
[
[
[
[
[
c]
c]
c]
c]
c]
c]
2-Phenyl ethanol
2-Octanol
Phenylacetaldehyde
2-Octanone
[e]
66
Cyclohexanol
Cyclohexanone
26
4-Methoxybenzyl alcohol+1-Phenylethanol 4-Methoxybenzyl aldehyde+Acetophenone 333 30 min 62+3 62+2
1-Heptanol+Cyclohexanol 1-Heptaldehyde+Cyclohexanone 333 5 h 35+0 34+0
[
a]
Reaction conditions: alcohol (1.93 mmol), Cat-1a (31.9 mg, 5 mol%), Cat-2a (34.0 mg, 10 mol%), NaNO2 (6.7 mg,
5 mol%), VH2O =0.05 mL; PO2 =1 MPa, T=333 K.
b]
[
[
[
[
Isolated yield.
c]
d]
e]
Determined by GC.
1
5
2% acid.
% isooctyl acid and 5% isooctyl isooctanoate were observed.
[a]
the corresponding aldehydes, and the secondary alco- Table 4. Recycling of Cat-1a and Cat-2a.
hols were almost completely recovered (entries 25
and 26).
Next, we examined the recyclability of this catalyst
system (Table 4). In each cycle, the catalysts were dis-
solved in water, and the product could be extracted
by diethyl ether or supercritical carbon dioxide. After
being recovered, the catalyst, the fresh substrate, i.e.,
[
b]
[b]
Entry
Conv. [%]
Yield [%]
1
52
2
53
49
48
43
52
1
52
49
47
43
[
a]
2
3
4
5
6
[
[
[
d]
e]
f]
2-nitrobenzyl alcohol was used for the second run
[a]
(
Table 4, entry 2), and the substrate was almost com-
The procedure for recycling the catalyst system is given
in the Experimental Section. Reaction conditions: 2-ni-
trobenzyl alcohol (1.93 mmol), Cat-1a (31.9 mg,
mol%), Cat-2a (34.0 mg, 10 mol%), NaNO2 (6.7 mg,
5 mol%), VH2O =0.5 mL; PO2 =1 MPa, T=333 K.
Isolated yield.
pletely recovered. This is probably because NaNO2
would decompose into NO and NO under the acidic
2
5
conditions, and be lost at the stage of product separa-
[6,7,8]
tion.
In this regard, NaNO was replenished for
2
[
[
[
b]
c]
d]
the subsequent run. The results showed that the yield
and selectivity of the products just had a slight de-
crease after the fourth run (entries 4–6). As a result,
the functionalized imidazolium salts are successfully
reused at least four times.
The second run after entry 1.
The second run after entry 3 and NaNO2 (6.7 mg,
5
mol%) was added.
[e]
The third run after entry 3 and NaNO (6.7 mg, 5 mol%)
2
was added.
[
6–11]
[f]
Based on previous studies
and the above-men-
The fourth run after entry 3 and NaNO2 (6.7 mg,
5 mol%) was added.
tioned results, a tentative mechanism of this aerobic
2212
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Adv. Synth. Catal. 2009, 351, 2209 – 2216

TEMPO and Carboxylic Acid Functionalized Imidazolium Salts/Sodium Nitrite
FULL PAPERS
Scheme 2. The proposed mechanism.
oxidation is proposed as shown in Scheme 2. The de- Experimental Section
signed transition metal-free aerobic system was com-
pleted by a sequential bicycle (cycle I) or tricycle General Information
(
cycle II) involving a two-electron transfer step.
NMR spectra were recorded on a Bruker 300 or 400 spec-
Indeed, the control experiments showed that the chlo-
ride anion gave a comparable activity with BF4
(
1
13
trometer in CDCl or D O. H and C NMR chemical shifts
3
2
À
1
13
(d) are given in ppm relative to TMS. H and C NMR posi-
tive chemical shifts (d) in ppm are downfield from tetrame-
Table 1, entry 18 vs. 6), whereas increasing the con-
centration of chloride anion had no effect on the reac- thylsilane (CDCl : d =77.0 ppm; residual CHCl in CDCl :
3
C
3
3
tion (Table 1, entry 17 vs. 6). In other words, cycle I
d =7.26 ppm). ESI-MS were recorded on a Thermo Finni-
H
was necessary. On the other hand, as shown in gan LCQ Advantage spectrometer in ESI mode with a
À
spray voltage of 4.8 kV. GC-MS were measured on a Finni-
gan HP G1800 A. GC analyses were performed on a Shi-
madzu GC-2014 equipped with a capillary column (RTX-5,
Scheme 2, NO2 could release NO and NO under
2
acidic conditions, and the oxidation of NO into NO2
could proceed easily with molecular oxygen. In this
context, the experimental results for reusing the cata-
3
0 mꢂ0.25 mm) using a flame ionization detector. Column
chromatography was performed by using silica gel 200–300
mesh with ethyl acetate/petroleum as eluent. And melting
points were measured on an X4 apparatus and uncorrected.
lytic system showed the fresh NaNO must necessarily
2
be added for the subsequent run as discussed above
(
Table 4, entries 4–6), suggesting the existence of the
essential process (I and II ), so that the NO pro-
1
2
2
Chemical Reagents
duced could oxidize the chorine anion to Cl (II ) or
2
2
Alcohols were purchased from Alfa Aesar Company. 1-
Methylimidazole, ethyl chloroacetate, chloroacetic acid and
hydrochloric acid (37%) were purchased from Tianjin
Guangfu Fine Chemical Research Institute. 4-Hydroxy-
TEMPOH to oxoammonium cation (I ).
2
Conclusions
2
,2,6,6-tetramethyl-piperidine-1-oxyl from Liansheng Chem-
ical Company of Jiangsu province was used without further
A three-component catalyst system, consisting of
purification. NaBF was purchased from Hengye Zhongyuan
4
TEMPO functionalized imidazolium salt, carboxylic Chemical Research Institute. The other organic compounds
acid functionalized imidazolium salt and NaNO , was from the sixth reagent company of Tianjin were used with-
2
developed for the aerobic oxidation of alcohols with out further purification except for the solvents, which were
distilled by the known procedure prior to use.
avoidance of using a toxic and high volatile substance,
such as HBr, Br , or HCl. The present homogeneous
2
catalyst system was recyclable, highly efficient and se- Preparation and Characterization of [Imim-
+
À[13,16]
lective for a wide set of aliphatic, allylic, heterocyclic TEMPO] X
and benzylic alcohols, even at ambient conditions.
This process thus represents a greener pathway for
aerobic oxidation of alcohols into carbonyl com-
Procedure for preparation of Cat-1a: To a stirred solution of
4
-hydroxy-2,2,6,6-tetramethylpiperdine-1-oxyl
(1,
4.3 g,
2
5 mmol) and chloroactic acid (2.0 g, 25 mmol) in CH Cl2
2
pounds by using the present task-specific ionic liquids (80 mL) at 08C under argon, DCC (5.15 g, 25 mmol) and
without the need of a transition metal.
DMAP (0.75 g, 6.25 mmol) were dropwise added. And the
Adv. Synth. Catal. 2009, 351, 2209 – 2216
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2213

Cheng-Xia Miao et al.
FULL PAPERS
mixture was stirred for 12 h at room temperature. After re-
action, the precipitate was filtered, and the filtrate was
The synthetic procedure of Cat-2b is similar to the proce-
dure for Cat-1b.
1
3
washed with 1M HCl (25 mL), saturated NaHCO (50 mL)
3: H NMR (300 MHz, D O): d=1.22 (t, JH,H =7.2 Hz,
3
2
and brine (50 mL). The organic phase was dried over
3H), 3.88 (s, 3H), 4.23 (q, 2H), 5.10 (s, 2H), 7.45 (s, 2H),
1
3
MgSO , and evaporated under reduced pressure. Further
8.76 (s, 1H); C NMR (75 MHz, D O): d=13.3, 36.1, 50.0,
4
2
purification was through a short flash chromatography (the
eluent: EtOAc-petroleum ether 1:10) providing 2,2,6,6-tetra-
methyl-1-oxyl-piperidin-4-yl 2-chloroacetate (2) as a red
powder. Then 1-methylimidazole (0.46 g, 5.6 mmol) was
added to a solution of 2 (1.00 g, 4 mmol) in MeCN (30 mL),
and the resulting solution was stirred for 48 h at 808C. After
concentration to about half the original volume, diethyl
ether was added to get a precipitate. Then the solid was fil-
tered and washed with acetone, diethyl ether, respectively to
give Cat-1a as a light red powder; yield: 76%.
63.7, 123.6, 137.5, 168.3.
Cat-2a: mp 191–1938C (Ref.
[16]
1
:
2048C); H NMR
(300 MHz, D O): d=3.82 (s, 3H), 5.03 (s, 2H), 7.40 (d,
2
3
13
JH,H =2.7 Hz), 8.70 (s, 1H); C NMR (75 MHz, D O): d=
2
36.09, 50.0, 123.6, 137.4, 169.9; ESI-MS (4.8 kV): m/z (%)=
+
141 (100) [MÀCl] .
1
Cat-2b: H NMR (300 MHz, D O): d=3.85 (s, 3H), 5.01
2
1
3
(s, 2H), 7.40 (s, 2H), 8.69 (s, 1H); C NMR (75 MHz,
D O): d=36.0, 50.2, 123.5, 137.4, 170.3.
2
Procedure for preparation of Cat-1b: A round-bottomed
flask was charged with 0.4132 g (1.25 mmol) of Cat-1a,
Representative Procedure for the Aerobic Oxidation
of Alcohols
0
.1373 g (1.25 mmol%) of NaBF in 1 mL of distilled water.
4
The reaction mixture was stirred at room temperature for
h affording a heterogeneous mixture. The water was re-
Safety Warning: Experiments using oxygen are potentially
hazardous and must only be carried out by using the appro-
priate equipment and under rigorous safety precautions.
A mixture of substrate (1.93 mmol), Cat-1a (31.9 mg, 5
mol%), Cat-2a (34.0 mg, 10 mol%), NaNO2 (6.7 mg, 5
mol%) and 0.05 mL water was placed in a 25-mL autoclave
4
moved under reduced pressure and then 1 mL of dichloro-
methane was added. The organic phase was dried over
MgSO4 and evaporated under reduced pressure to afford
Cat-1b as a red powder. One drop of neat phenylhydrazine
was added into the samples containing the nitroxyl radical
prior to NMR analysis in order to reduce in situ the para-
magnetic center to the corresponding hydroxylamine spe-
cies.
equipped with an inner glass tube. 1 MPa O was introduced
2
into the autoclave and the system was heated to the reaction
temperature. The mixture was stirred continuously for the
designated reaction time. After cooling, products were then
extracted by diethyl ether, and analyzed by gas chromatog-
raphy (Shimadzu-2014) equipped with a capillary column
1
-Hydroxy-2, 2, 6, 6-tetramethylpiperidin-4-yl 2-chloroace-
1
tate: H NMR (400 MHz, CDCl ): d=1.18 (s, 12H), 1.58 (t,
JH,H =12 Hz, 2H), 1.93 (d, J =5.4 Hz, 2H), 3.98 (s, 2H),
.08–5.14 (m, 1H); C { H} NMR (100.6 MHz, CDCl ): d=
3
3
3
H,H
(
RTX-5 30 mꢂ0.25 mm) using a flame ionization detector.
1
3
1
5
3
The structure and the purity of the products were further
identified using NMR (Bruker-300 MHz, 400 MHz), GC-MS
2
0.4, 31.7, 40.9, 43.5, 58.9, 69.0, 166.7.
1
Cat-1a-TEMPOH: H NMR (400 MHz, CDCl ): d=1.13
3
(
HP G1800 A), HPLC-MS (LCQ Advantage) and GC,
3
3
(
1
5
s, 12H), 1.37 (t, J =9 Hz, 1H), 1.59 (t, J =9 Hz, 2H),
H,H H,H
3
HPLC by comparing retention times and fragmentation pat-
terns with those of authentic samples.
.96 (t, J =9 Hz, 2H), 3.98 (s, 3H), 5.05 (s, 1H), 5.11–
H,H
13 1
.21 (m, 2H), 7.40 (s, 2H); C { H} NMR (100.6 MHz,
CDCl ): d=20.8, 29.8, 36.0, 42.2, 50.0,60.2, 70.0, 123.5, 123.6,
3
1
28.4, 167.4.
Procedure for Reusing the Task-Specific Ionic
[13b]
Cat-1a: mp 213–2168C (Ref. : mp 225–2278C); ESI-MS Liquids
+
(
4.8 kV): m/z (%)=295 (100) [MÀCl] .
1
After the addition of diethyl ether (10 mLꢂ3) to the result-
ing mixture upon completion of the reaction, the aqueous
layer containing the catalyst was separated by simple de-
cantation of the ether phase containing the oxidized prod-
ucts. Consequently, the aqueous phase containing the cata-
lyst was bubbled with argon. We conducted further oxida-
tions by the addition of successive portions of 2-nitrobenzyl
Cat-1b-TEMPOH: H NMR (400 MHz, D O): d=1.10 (s,
2H), 1.57 (t, JH,H =12 Hz, 2H), 1.94 (t, JH,H =11.2 Hz,
H), 3.76 (s, 3H), 4.97 (s, 2H), 5.11 (m, 1H), 7.32(s, 2H);
C {H} NMR (100.6 MHz, D O): d=20.7, 30.0, 35.8, 42.1,
2
3
3
1
2
AHCTUNGTRENNUNG
13
2
4
9.8, 60.1, 69.8, 123.3, 123.5, 128.5, 167.3.
alcohol and NaNO to the water phase followed by stirring
2
under identical reaction conditions.
Preparation and Characterization of [Imim-
COOH] X
1
+
À[14]
Benzaldehyde: Light yellow oil liquid, H NMR (CDCl
,
3
3
3
4
00 MHz): d=7.50 (t, JH,H =7.7 Hz, 2H), 7.60 (t, JH,H =
3
Under an inert atmosphere of argon, a mixture of 1-methyl-
imidazole (0.020 mol) and ClCH COOCH (0.020 mol) was
7.4 Hz, 1H), 7.84 (d, JH,H =7.9 Hz, 2H), 9.98 (s, 1H);
1
3
1
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
C { H} NMR (CDCl , 100.6 MHz): d=128.9, 129.6, 134.3,
2
3
3
stirred at room temperature for 6 h. The resultant mixture
turned to a solid during the reaction. The solid was washed
with diethyl ether (3ꢂ30 mL) and dried under vacuum for
136.2, 192.3.
4-Methoxybenzaldehyde: Colorless liquid,
(300 MHz, CDCl ): d=3.87
1
H NMR
3
A
H
U
G
R
N
U
G
3
H,H
3
24 h to give 1-ethoxycarbonylmethyl-3-methylimidazolium
2H), 7.82 (d, JH,H =8.78 Hz, 5H), 9.87 (s, 1H, CHO);
1
3
1
chloride (3). In a typical procedure, a mixture of 3 (0.020
mol) and 37% HCl aqueous solution (0.022 mol) was re-
fluxed for 4 h. The solvent was removed and the residue was
washed with acetone and diethyl ether to give the product
as a white powder Cat-2a.
C { H} NMR (75 MHz, CDCl ): d=55.5, 114.3, 130.0,
3
131.9, 164.6, 190.7.
2-Methoxybenzaldehyde: Light yellow solid, mp 36–388C,
1
H NMR (300 MHz, CDCl ): d=3.84 (s, 3H), 6.94 (m, 2H),
3
3
3
7.48 (t, J =8.4 Hz, 1H), 7.75 (d, J =3.9 Hz, 1H), 10.4
H,H
H,H
2214
asc.wiley-vch.de
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2009, 351, 2209 – 2216

TEMPO and Carboxylic Acid Functionalized Imidazolium Salts/Sodium Nitrite
FULL PAPERS
1
3
1
(
1
s, 1H); C { H} NMR (75 MHz, CDCl ): d=55.4, 111.5, References
3
20.4, 124.6, 128.2, 135.8, 161.6, 189.5.
-Methoxybenzaldehyde: Light yellow oil liquid, H NMR
300 MHz, CDCl ): d=3.83(s, 3H), 7.14 (m, 1H), 7.36 (d,
=1.05 Hz, 1H), 7.42 (m, 2H), 9.94 (s, 1H);
C { H} NMR (75 MHz, CDCl ): d=55.3, 112.0, 121.3,
23.4, 129.9, 137.7, 160.1, 192.0.
1
3
[1] M. Hudlicky, Oxidations in Organic Chemistry, Ameri-
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ACHTUNGTRENNUNG
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H,H
13
1
3
1
1
4
-Methylbenzaldehyde:
300 MHz, CDCl ): d=2.44 (s, 3H), 7.33 (d, J =3.9 Hz,
Colorless
liquid,
H NMR
3
(
3
H,H
3
2
H),1 7.78 (d, JH,H =4.05 Hz, 2H), 9.97 (s, 1H);
13
C { H} NMR (75 MHz, CDCl ): d=21.9, 129.7, 129.9,
3
1
34.2, 145.6, 192.0.
-Nitrobenzaldehyde: Light yellow acicular crystals, mp
05–1068C, H NMR (CDCl , 300 MHz): d=8.08 (d, J
.35 Hz, 2H), 8.39 (d, J =4.35 Hz, 2H), 10.16 (s, 1H);
C { H} NMR (CDCl , 75 MHz) : d=124.2, 130.4, 140.0,
51.1, 190.2.
4
1
3
1
=
H,H
3
3
4
H,H
13
1
3
1
AHCTUNGTRENNUNG
2
-Nitrobenzaldehyde: Light yellow solid, mp 45–468C,
5
1
H NMR (CDCl , 300 MHz): d=7.78 (m, 2H), 7.96 (d,
3
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1
3
J
=2.7 Hz, 1H), 8.13 (d, J =3 Hz, 1H), 10.43 (s, 1H);
H,H
H,H
3
C {1H} NMR (CDCl , 75 MHz): d=124.5, 129.6, 131.4,
34.1, 188.1.
Acetophenone: Light yellow liquid, H NMR (400 MHz,
CDCl ): d=2.60 (s, 3H), 7.46 (t, J =7.7 Hz, 2H), 7.56 (t,
3
1
2
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H,H
3
1
3
J
=7.5 Hz, 1H), 7.96 (d,
JH,H =7.8 Hz, 2H);
H,H
3
1
C { H} NMR (CDCl , 100.6 MHz): d=26.5, 128.2, 128.5,
33.0, 137.1, 198.1.
Benzophenone: White crystals, mp 47–488C, H NMR
300 MHz, CDCl ): d=7.48 (t, J =7.9 Hz, 4H), 7.58 (t,
3
1
[
4] a) M. F. Semmelhack, C. R. Schmid, D. A. CortCs, C. S.
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1
3
(
3
H,H
3
3
J
=7.9 Hz,
2H),
7.81
(t,
JH,H =5.7 Hz,
4H);
H,H
8
2
25; c) A. Gassama, N. Hoffmann, Adv. Synth. Catal.
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13
1
C { H} NMR (75 MHz, CDCl ): d=128.3, 130.1, 132.4,
37.6, 196.7.
Cinnamaldehyde:
3
1
J. Mol. Catal. A: Chem. 2008, 291, 1–4; e) A. Dijks-
man, A. Marino-Gonzaꢀlez, A. M. i Payeras, I. W. C. E.
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1
Light
yellow
liquid,
H NMR
(
300 MHz, CDCl ): d=6.69–6.77 (m, 1H), 7.43–7.47 (m,
3
3
4
H), 7.52–7.59 (m, 2H), 9.71 (d, JH,H =3.9 Hz, 1H);
6
826–6833; f) A. Dijksman, I. W. C. E. Arends, R. A.
13
1
C { H} NMR (75 MHz, CDCl ): d=128.5, 128.6, 129.1,
31.3, 134.0, 152.8, 193.8.
Dodecanal: White solid, mp 43–448C,
3
Sheldon, Chem. Commun. 1999, 1591–1592; g) T. Mat-
sumoto, M. Ueno, J. Kobayashi, H. Miyamura, Y. Mori,
S. Kobayashi; Adv. Synth. Catal. 2007, 349, 531–534;
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1
1
H NMR
3
(
(
9
2
300 MHz, CDCl ): d=0.86 (s, J =5.1 Hz, 3H), 1.25–1.28
3 H,H
3
m, 16H), 1.61 (t, J =5.4 Hz, 2H), 2.38–2.42 (m, 2H),
H,H
13
1
.75 (s, 1H); C { H} NMR (75 MHz, CDCl ): d=14.1, 22.1,
2.6, 29.1, 29.2, 29.3, 29.4, 29.6, 31.9, 43.9, 202.9.
3
[
[
[
[
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1
1-Hexadecanal: White solid, 33–358C,
H NMR
3
(
(
9
300 MHz, CDCl ): d=0.87 (s, J =6.9 Hz, 3H), 1.19–1.28
m, 24H), 1.62 (t, J =7.2 Hz, 2H), 2.39–2.44 (m, 2H),
.76 (t, J =2.1 Hz, 1H); C { H} NMR (75 MHz, CDCl ):
d=14.1, 22.1, 22.7, 29.2, 29.3, 29.4, 29.6, 29.7, 31.9, 43.9,
02.9.
3
H,H
3
H,H
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Adv. Synth. Catal. 2009, 351, 2209 – 2216