Deep eutectic solvent supported TEMPO for oxidation of alcohols

Article abstract of DOI:10.1039/c4ra05598e

A novel deep eutectic solvent supported TEMPO (DES-TEMPO) composed of N,N-dimethyl-(4-(2,2,6,6-tetramethyl-1-oxyl-4-piperidoxyl)butyl)dodecyl ammonium salt ([Quaternium-TEMPO]⁺Br^-) and urea was prepared. An efficient catalytic system for the oxidation of alcohols with molecular oxygen as terminal oxidant has been developed from DES-TEMPO and Fe(NO₃)₃·9H₂O. The DES-TEMPO/Fe(NO₃)₃system showed good performances on the selective oxidation of various alcohols to the corresponding aldehydes and ketones under mild and solvent-free conditions. The DES could be recovered easily and recycled up to five times in the oxidation of benzyl alcohol without significant loss of catalytic activity. This journal is

Full text of DOI:10.1039/c4ra05598e

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Deep eutectic solvent supported TEMPO for
oxidation of alcohols†
Cite this: RSC Adv., 2014, 4, 40161
Yuecheng Zhang, Fenglian Lu, Xiaohui Cao and Jiquan Zhao*
¨
A novel deep eutectic solvent supported TEMPO (DES–TEMPO) composed of N,N-dimethyl-(4-(2,2,6,6-
+
ꢀ
tetramethyl-1-oxyl-4-piperidoxyl)butyl)dodecyl ammonium salt ([Quaternium-TEMPO] Br ) and urea
was prepared. An efficient catalytic system for the oxidation of alcohols with molecular oxygen as
Received 11th June 2014
Accepted 18th August 2014
3 3 2 3 3
terminal oxidant has been developed from DES–TEMPO and Fe(NO ) $9H O. The DES–TEMPO/Fe(NO )
system showed good performances on the selective oxidation of various alcohols to the corresponding
aldehydes and ketones under mild and solvent-free conditions. The DES could be recovered easily and
recycled up to five times in the oxidation of benzyl alcohol without significant loss of catalytic activity.
DOI: 10.1039/c4ra05598e
www.rsc.org/advances
be regarded as a good alternative. Therefore, TEMPO has been
1
. Introduction
21–28
immobilized onto various inorganic
and organic supports or
29–32
The oxidation of alcohols to the corresponding aldehydes and polymers,
affording diversied catalysts so as to realize the
ketones is one of the most widely used reactions in organic separation of TEMPO from the reaction mixture. However, the
chemistry, and the carbonyl compounds are used in production immobilization of TEMPO on solid supports generally results in
of bulk and ne chemicals such as perfumes, pharmaceuticals decrease of activity or extending reaction time in some cases.
1–3
and organic intermediates. Traditional methods to carry out For resolving this problem TEMPO has been also supported on
3
3–35
this reaction involve the use of stoichiometric inorganic several ionic liquids.
Due to their good mobility the ionic
1,4
oxidants, such as chromium(VI) oxide, MnO or hypervalent liquid-supported TEMPO showed performances similar to those
iodine compounds, which would generate large amount of by- of non-supported counterpart in terms of activity and
products and cause pollution to the environment. From both selectivity.
2
5
–9
economic and environmental viewpoints, the development of
effective catalytic oxidation processes with molecular oxygen as liquids have been receiving great attention. DESs, which were
Deep eutective solvents (DESs) as a novel class of ionic
36
37
the terminal oxidant is most attractive, because molecular rst reported by Abbott and co-workers, are generally obtained
oxygen is inexpensive and water is produced as the only by- by the interfusion of quaternary ammonium salts and hydrogen
product in principle. Many catalytic systems have been devel- bond donors (e.g. amides, amines, alcohols, and carboxylic
oped and showed high efficiency in the aerobic oxidation of acids). When the components are mixed the melting point is
alcohols to the corresponding aldehydes and ketones. Among drastically decreased. The melting point depression, relative to
all the systems the ones based on stable nitroxyl radical 2,2,6,6- the melting points of the individual components in the mixture,
tetramethylpiperidine-1-oxyl (TEMPO) in combination with results from the charge delocalization of hydrogen bonding
10–19
metals as co-catalysts are the most reported.
Of particular between the halide anion and the hydrogen donor moiety. DESs
interest are the catalytic systems involving iron as co-catalyst have their unique properties except those similar to ionic
because iron is very cheap and readily available. Several TEMPO liquids, which make them excellent choices for the preparation
15–20
38–44
based catalytic systems involving iron as co-catalyst
have of materials in different elds.
Their ionic nature and rela-
been found being effective in the aerobic oxidation of alcohols. tively high polarity show high solubility for many ionic species,
Although these systems are successful for efficient aerobic such as metal salts. They also have some momentous advan-
oxidation of alcohols, the separation of TEMPO from reaction tages over other types of ionic liquids, especially their simple
products remains a problem especially on a large scale and easy preparation as pure phases from cheap and easily
production. Immobilization of TEMPO on some supports can available components and their relative chemical stability
36,37
towards atmospheric moisture and temperature.
Therefore,
we attempt to immobilize TEMPO on a DES to obtain a novel
TEMPO-based catalyst with advantages of easy separation and
School of Chemical Engineering and Technology, Hebei University of Technology,
Tianjin 300130, People's Republic of China. E-mail: zhaojq@hebut.edu.cn;
yczhang@hebut.edu.cn; Fax: +86 22 60202926; Tel: +86 22 60202926; +86 good mobility for the oxidation of alcohols. Encouragingly, a
2
†
1
260204279
low catalytic amount of the DES supported TEMPO in conju-
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4ra05598e
gation with Fe(NO
3
)
3
has shown excellent catalytic properties in
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the aerobic oxidation of various alcohols under mild and Table 1 Melting points and viscosities of DESs with different molar
+
ꢀ
ratios of [Quaternium-TEMPO] Br to urea
solvent-free conditions. Herein, we report the preparation and
catalytic performances of the DES supported TEMPO on the
oxidation of alcohols with molecular oxygen as a terminal
oxidant.
+
ꢀa
ꢁ
Entry
Urea: [Quaternium-TEMPO] Br
T
f
( C)
Viscosities (cP)
ꢁ
1
2
3
4
5
6
1 : 0.5
1 : 1
80
70
221(80 C)
ꢁ
329(70 C)
ꢁ
635(60 C)
2. Results and discussion
ꢁ
1 : 2
1 : 3
60
60
355(70 C)
ꢁ
262(80 C)
2.1 Synthesis of N,N-dimethyl-(4-(2,2,6,6-tetramethyl-1-oxyl-
ꢁ
660(60 C)
4
-piperidoxyl)butyl)dodecyl ammonium salt ([Quaternium-
+
ꢀ
a
TEMPO] Br )
Molar ratio.
With the aim of preparation of DES supported TEMPO, a
precursor quaternary ammonium with a TEMPO moiety, which
is N,N-dimethyl-(4-(2,2,6,6-tetramethyl-1-oxyl-4-piperidoxyl)butyl)
ꢁ
to urea was 2 : 1, a DES with a melting point of 60 C was
received and the melting point maintained constant with
+
ꢀ
dodecyl ammonium bromide ([Quaternium-TEMPO] Br ) was
synthesized in two steps as shown in Scheme 1. First, 4-(2,2,6,6-
tetramethyl-1-oxyl-4-piperidoxyl)butyl bromide was synthesized
from 4-OH-TEMPO and 1,4-dibromobutane followed the stan-
+
ꢀ
further increase of [Quaternium-TEMPO] Br . It is conceivable
that each amino group in the urea structure is combined with a
Quaternium-TEMPO] Br through a hydrogen bond in this
case (Scheme 2). Here, we can say that the TEMPO was sup-
ported on DES. This DES supported TEMPO ([Quaternium-
+
ꢀ
[
45
dard procedure in literature. Then quaternization of N,N-
dimethyldodecylamine with 4-(2,2,6,6-tetramethyl-1-oxyl-4-
+
ꢀ
piperidoxyl) butyl bromide gave [Quaternium-TEMPO] Br in
+
ꢀ
TEMPO] Br –urea ¼ 2 : 1), which is denoted as DES–TEMPO, is
+
ꢀ
7
6% yield. The structure of [Quaternium-TEMPO] Br was
miscible with various alcohols and capable of dissolving
1
13
characterized by H NMR, C NMR and HR-MS (ESI). Due to the
interference of the free radical present in the structure of
Quaternium-TEMPO] Br , the H NMR and C NMR were
obtained from the sample of [Quaternium-TEMPO] Br
reduced by phenylhydrazine.
3 3
Fe(NO ) .
The viscosities of the DES–TEMPOs have also been listed in
Table 1. It can be found that the viscosities of the DES–TEMPOs
+
ꢀ
1
13
[
+
ꢀ
+
ꢀ
increased with the molar ratio of [Quaternium-TEMPO] Br to
urea. In addition, the viscosities of the DES–TEMPOs decreased
with the increase of temperature.
2
.2 Preparation of DES supported TEMPO based on
+
ꢀ
[Quaternium-TEMPO] Br and urea
+
ꢀ
2.3 Catalytic activity
With the [Quaternium-TEMPO] Br in hand, a series of DESs
with a TEMPO moiety (DES supported TEMPO) were prepared The catalytic performance of the DES–TEMPO in combination
+
ꢀ
3 3 2
by mixing [Quaternium-TEMPO] Br with urea in different with Fe(NO ) $9H O as co-catalyst was evaluated for the aerobic
36
molar ratios. Here, urea acts as a hydrogen bond donor. In the oxidation of benzyl alcohol as a model substrate. The reaction
preparation process the two solids were mixed together rstly. was carried out under atmospheric O pressure and solvent-free
2
ꢁ
ꢁ
ꢁ
The mixture was heated slowly and maintained at 60 C to 80 C conditions at 60 C—the melting point of DES–TEMPO. The
for a period of time resulting in the formation of eutectic benzyl alcohol (10 mmol) was completely converted to benzal-
solvent with 100% atom economy. From Table 1 it can be seen dehyde with a selectivity of higher than 99% in 1.5 h in the
that the melting points of the resulted DESs with a TEMPO presence of DES–TEMPO (1.25 mol%) and Fe(NO
3 3 2
) $9H O
moiety (DES–TEMPO) are dependent on the molar ratio of (3 mol%) (Table 2, entry 4). This corresponds to a TOF of
+
ꢀ
ꢀ1
[
Quaternium-TEMPO] Br to urea. If the molar ratio of [Qua- 26.4 h which is an excellent result for an aerobic oxidation of
+
ꢀ
ternium-TEMPO] Br to urea was lower than 2 : 1, the melting alcohols involving a TEMPO catalyst. Decreasing the DES–
points of DESs were always higher than 60 C. With the increase TEMPO loading from 1.25 mol% to 1 mol% led to slow reaction,
of [Quaternium-TEMPO] Br , the melting points of the DESs the reaction time required to nish the reaction increased from
decreased. When the molar ratio of [Quaternium-TEMPO] Br
ꢁ
+
ꢀ
+
ꢀ
1.5 h to 2.5 h (Table 2, entries 4 and 6). It is worth noting that
+
ꢀ
Scheme 1 General synthetic route for [Quaternium-TEMPO] Br .
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Scheme 2 The preparation and structure of DES-supported TEMPO.
a
3 3
Table 2 Effect of each component in the DES–TEMPO/Fe(NO ) catalyst on the aerobic oxidation of benzyl alcohol
b
b
c
d
ꢀ1
Entry
DES–TEMPO (mol%)
Fe(NO
3
)
3
(%)
Time (h)
Conv. (%)
Select. (%)
TON
TOF (h
)
1
2
3
4
5
6
0
3
0
3
3
2
3
3
1
1
1
1.5
4.5
2.5
1.5
8.7
0.4
93
>99
>99
>99
86
>99
>99
>99
>99
>99
>99
>99
0
0.2
0
0.2
1.25
1.25
1.25
1.25
1.00
2.50
37.2
39.6
39.6
49.5
34.4
37.2
26.4
8.8
19.8
23.0
e
7
a
ꢁ
b
Reaction conditions: benzyl alcohol 10 mmol, reaction temperature 60 C, atmospheric oxygen pressure. Conversions and selectivity were
c
d
e
determined by GC (area normalization method). TON ¼ moles of product/2(moles of DES–TEMPO). TOF ¼ TON/reaction time. TEMPO was
used instead of DES–TEMPO.
x
the use of TEMPO instead of DES–TEMPO resulted in a decrease that iron counterion is critical to the reaction, and NO is
of the conversion of benzyl alcohol under the same reaction essential in the reaction. It should be mentioned that excess
conditions (Table 2, entry 7), conrming the favorable role of water retarded the reaction (Table 3, entry 4).
the DES moiety to the oxidation reaction. Either only use of
DES–TEMPO without Fe(NO $9H as co-catalyst or system, the aerobic oxidation of various alcohols including
Fe(NO $9H O in the absence of DES–TEMPO afforded very secondary aliphatic alcohols, benzylic alcohols, heterocyclic
poor results, which indicated that both DES–TEMPO and alcohols and also primary alcohols to the corresponding
Fe(NO were essential in the aerobic oxidation of benzyl carbonyl compounds was explored, and the results are shown in
In order to evaluate the versatility of this novel catalytic
3
)
3
2
O
a
3
)
3
2
3 3
)
alcohol (Table 2, entries 1 and 2). It can be concluded that 1.25 Table 4. It can be seen from Table 4 that various types of primary
mol% of DES–TEMPO in combination with 3% Fe(NO ) $9H O benzylic alcohols, including those bearing both electron-with-
3
3
2
as catalytic system was very effective in the oxidation of benzyl drawing and electron-donating groups, were selectively con-
alcohol to benzaldehyde with oxygen as an oxidant. verted to their corresponding aromatic aldehydes in excellent
The reaction was also performed under different tempera- yields under the optimal reaction conditions. However, the time
ture. Generally, the reaction rate increased with the increase of required to nish the reaction was different (Table 4, entries 1,
reaction temperature. Detailed results can be found in ESI.†
2, 5, 6, 8), which is different from aerobic oxidation of alcohols
46
Several metal nitrates and ferric chloride were also evaluated catalyzed by a metal-free catalyst system. These results
as co-catalyst in the oxidation of benzyl alcohol. As shown in demonstrated that electronic effects seem to have some effects
Table 3, only Fe(NO ) $9H O and Cr(NO ) $9H O worked well on the reaction of substrate with a substituent para to the
3
3
2
3 3
2
and had similar efficiency (Table 3, entries 3 and7). The reaction -hydroxymethyl. As observed in the case of non-supported 4-OH-
15
catalyzed by co-catalysts such as Cu(NO ) $3H O and TEMPO as a catalyst, electron-donating groups were in favor of
3
2
2
Co(NO
3 2 2
) $6H O produced unfavorable results (Table 3, entries 5 the reaction, whereas electron-withdrawing groups were unfa-
3
+
3+
and 6). The common feature of Fe and Cr is that both of vorable to the reaction. In addition, the position of the
2
+
2+
them have higher oxidation potentials than Cu and Co . substituent on the benzene ring has certain effects on the
FeCl $6H O itself as co-catalyst did not work too (Table 3, entry reactivity of benzylic alcohols, too. The substrate with an
). However, the reaction proceeded in moderate efficiency o-substituent showed poor reactivity compared to that with an
when NaNO was added (Table 3, entry 2). The results indicate m- or p-substituent due to the steric hindrance (Table 4, entries
3
2
1
2
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a
Table 3 DES–TEMPO catalyzed aerobic oxidation of benzyl alcohol with different metal salts as co-catalysts
b
b
c
d
ꢀ1
Entry
Metal salts
Time (h)
Conv. (%)
Select. (%)
TON
TOF (h
)
1
2
3
FeCl
FeCl
Fe(NO
Fe(NO
Cu(NO
Co(NO
Cr(NO
3
$6H
$6H
2
O
1.5
2
3.1
>99
>99
>99
>99
>99
>99
>99
1.2
31.6
39.6
2.1
9.6
1.2
0.8
15.8
26.4
1.4
0.5
0.8
3
2
O/NaNO
2
79.2
>99
5.3
24.0
3.1
3
)
)
3
$9H
$9H
2
O
O
1.5
1.5
17.5
1.5
1.5
e
4
3
3
2
5
6
7
3
)
2
$3H
$6H
$9H
2
O
3
)
2
2
O
3
)
3
2
O
>99
39.6
26.4
a
b
Reaction conditions: benzyl alcohol 10 mmol, 1.25% DES–TEMPO, 3% salt(s), atmospheric oxygen pressure. Conversions and selectivity were
c
d
e
determined by GC (area normalization method). TON ¼ moles of product/2(moles of DES–TEMPO). TOF ¼ TON/reaction time. 2 ml H
2
O
was added.
2–4, 8–9). Overall, the reaction rate was determined by the
To demonstrate practicality of this methodology, a multi-
combination of the electronic and steric effects. This catalytic gram scale reaction was carried out with benzyl alcohol as
system also showed good tolerance for the heterocyclic substrate. Same results were received when the loading amount
substrates, and good to excellent yields of the corresponding of benzyl alcohol was increased to 40 mmol (Table 4, entry 1).
aldehydes were received by increasing the loadings of DES–
TEMPO and Fe(NO ) or (and) extending reaction time (Table 4,
entries 12 and 13). 2-Thioylmethyl alcohol, which is usually
3
3
2.4 Recycling of DES–TEMPO
Finally, the recyclability of DES–TEMPO was investigated using
benzyl alcohol as a model substrate. Aer each catalytic run the
product benzaldehyde was distilled off in vacuum and the
residue was used directly in subsequent runs. It is worth
mentioning that fresh Fe(NO3)3 was required to add in each
successive run. The results listed in Table 5 show that no
decreases in the rate and selectivity were observed in the rst 5
runs. However, the activity of the catalytic system gradually
decreased with further recycling and the conversion of benzyl
alcohol to benzaldehyde was only 27% in 1.5 h under the
described conditions in the eighth run. The decrease of the
activity of the catalytic system may be due to the accumulation of
the solid substance derived from Fe(NO ) with recycling times.
regarded as a difficult substrate in the aerobic oxidation cata-
47
lyzed by transition metals, was smoothly converted into
-thenaldehyde in high yield (Table 4, entry12); 2-pyridyl
2
methanol was also converted to 2-pyridinecarboxaldehyde in a
conversation of 81% and a selectivity of 98% when both the
amounts of DES–TEMPO and Fe(NO ) were increased to 8
3 3
mol% (Table 4, entry13). Cinnamic alcohol, which is a repre-
sentative of allylic alcohols, was totally converted to the corre-
sponding aldehyde in 4 h when the loadings of DES–TEMPO
and Fe(NO
entry11). The increase of the loadings of DES–TEMPO and
Fe(NO indicated that the oxidation of allylic alcohols was
more difficult than that of benzylic alcohols. In the oxidation of
-butanol longer reaction time was required to nish the reac-
3 3
) were 1.5 mol% and 5 mol%, respectively (Table 4,
3 3
)
3
3
1
2.5 Mechanistic aspect
tion, and small amounts of over oxidation products butyric acid
and butyl butyrate were detected by GC (Table 4, entry14). It is
very important to note that this system is also effective in the
aerobic oxidations of both secondary benzylic and aliphatic
alcohols which are poor substrates in most of the transition-
The detailed mechanism for the aerobic oxidation of alcohols
catalyzed by this catalytic system is not clear. However, it has
ꢀ
been found from Table 3 that NO
Therefore, it is likely that NO
3
is essential in the reaction.
ꢀ
x
derived from NO
3
is playing a
10–12,48,49
metal catalyst systems,
giving the respective ketones in
key role in this system and this has been suggested in the other
1
5,51
good to excellent yield with slight modications (Table 4,
entries 15–21). Various types of secondary benzylic alcohols,
including those bearing both electron-withdrawing and elec-
tron-donating groups, were selectively converted to the corre-
sponding ketones in excellent yields under relative higher
Fe/TEMPO systems by others.
It has also been suggested that
2
+
3+ 51
NO can assist in the re-oxidation of Fe to Fe . From Table 3
2
3
+
3+
it has also been found that only the metal ions (Fe and Cr )
with higher oxidation potentials can promote the aerobic
oxidation of benzyl alcohol. Furthermore, it has been conrmed
that Fe can oxidize 4-OH-TEMPO to oxammonium cation (4-
OH-TEMPO ), a highly efficient oxidizing ingredient for alco-
hols, while itself was reduced to Fe . Based on the above facts,
a possible overall mechanism of this catalytic oxidation can be
described as a cascade of redox reactions involving three cycles
in Scheme 3. Initially, Fe oxidizes [Quaternium-TEMPO] Br
to oxammonium cation [Quaternium-TEMPO ] Br . Then
3
+
loadings of DES–TEMPO (2.5 mol%) and Fe(NO
3 3
) (5 mol%)
+
(Table 4, entries 15–18). Secondary aliphatic alcohols were also
2+
15
oxidized to the corresponding ketones in good to excellent
yields (Table 4, entries19 and 20). Even cyclohexanol, which is
regarded as a difficult substrate for TEMPO-mediated oxida-
50
3+
+
ꢀ
tion, was converted to cyclohexanone in a good yield of 86%
when the loadings of DES–TEMPO and Fe(NO ) were increased
+
+
ꢀ
3
3
+
+
ꢀ
to 4 mol% and 8 mol%, respectively (Table 4, entry19). These
results demonstrate that the DES–TEMPO based catalyst system
has broad applicability in the aerobic oxidations of alcohols.
[
Quaternium-TEMPO ] Br serves as an oxidant to transform
alcohol to the corresponding carbonyl compound, while itself is
+
ꢀ
ꢀ
3
reduced to [Quaternium-TEMPO] Br . The role of the NO
is to
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a
Table 4 Aerobic of alcohols to carbonyl compounds catalyzed by TEMPO-supported DES in combination of Fe(NO
3
)
3 2
$9H O
b
b
c
d
ꢀ1
Entry
Substrate
Catalyst
t (h)
Conv. (%)
Select. (%)
TON
TOF (h
)
e
1
1.25% DES–TEMPO/3%Fe(NO
3
)
)
3
3
1.5
>99(93)
>99
>99
>99
39.6
26.4
52.8
2
3
1.25% DES–TEMPO/3%Fe(NO
3
0.75
39.6
39.6
1.25% DES–TEMPO/3%Fe(NO
3
3
)
)
3
3
2.5
1.5
>99
>99
>99
15.8
26.4
4
5
1.25% DES–TEMPO/3%Fe(NO
>99(91)
39.6
1.25% DES–TEMPO/3%Fe(NO
1.25% DES–TEMPO/3%Fe(NO
1.25% DES–TEMPO/3%Fe(NO
1.25% DES–TEMPO/3%Fe(NO
1.25% DES–TEMPO/3%Fe(NO
3
)
)
)
)
)
3
3
1.5
2.5
2
>99
>99
>99
>99
>99
>99
39.6
39.6
39.6
39.6
39.6
26.4
15.8
19.8
19.8
9.9
f
6
3
>99(91)
>99(83)
>99(92)
>99(87)
f
7
3
3
3
3
3
3
8
9
2
4
1
0
1.25% DES–TEMPO/3%Fe(NO
3
)
3
1.5
>99
>99
39.6
26.4
1
1
1
2
1.5% DES–TEMPO/5%Fe(NO
3
)
3
4
>99(92)
>99(94)
>99
>98
33.0
39.6
8.3
1.25% DES–TEMPO/3%Fe(NO
3
)
)
3
2.5
9
15.8
1
1
3
4
4% DES–TEMPO/8%Fe(NO
3
)
3
81
97
99
94
10.1
38.8
1.1
3.5
g
1.25% DES–TEMPO/3%Fe(NO
3
3
11
1
5
6
2.5% DES–TEMPO/5%Fe(NO
3
3
)
3
1.5
0.5
>99(91)
>99
>99
>99
19.8
19.8
13.2
39.6
1
2.5% DES–TEMPO/5%Fe(NO
)
3
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Table 4 (Contd.)
b
b
c
d
ꢀ1
Entry
Substrate
Catalyst
t (h)
Conv. (%)
Select. (%)
TON
TOF (h
)
1
7
8
2.5% DES–TEMPO/5%Fe(NO
3
)
)
3
3
2.5
>99
>99
>99
19.8
7.9
1
2.5% DES–TEMPO/5%Fe(NO
3
1
>99(83)
19.8
19.8
1
2
9
0
4% DES–TEMPO/8%Fe(NO
3
)
3
1
2
86
>99
>99
10.8
19.4
10.8
9.7
2.5% DES–TEMPO/5%Fe(NO
3
)
3
97(89)
2
1
4% DES–TEMPO/8%Fe(NO
3
)
3
9
80
>99
10.0
1.1
a
ꢁ
b
Reaction conditions: alcohol 10 mmol, reaction temperature 60 C, atmospheric oxygen pressure. Conversions and selectivity were determined
1
c
by GC (area normalization method); data in parentheses were isolated yields; all products were determined by H NMR. TON ¼ moles of product/
d
e
f
2
7
(moles of DES–TEMPO). TOF ¼ TON/reaction time. The loading amount of benzyl alcohol was 40 mmol. The reaction temperature was kept on
ꢁ ꢁ
g
0 C until the substrates was melted, then resumed to 60 C. Acid (4.6%) and esters (2.4%) were detected.
molar ratio of 2 : 1 to give a DES supported TEMPO (DES–
TEMPO) with a melting point of 60 C. Catalytic amount of DES–
TEMPO in combination with Fe(NO ) as co-catalyst showed
3 3
Table 5 Recycling of DES–TEMPO in the aerobic oxidation of benzyl
alcohol
ꢁ
a
b
b
c
d
ꢀ1
Run
Conversion (%)
Selectivity (%)
TON
TOF (h
)
excellent performances on the oxidation of various alcohols to
their corresponding carbonyl compounds with molecular
oxygen as oxidant under solvent-free conditions. The DES–
TEMPO performed better than TEMPO under the same reaction
conditions. The DES–TEMPO could be easily recovered from the
reaction mixture in the oxidation of benzyl alcohol, and reused
up to ve times with no signicant loss of catalytic activity. The
novel catalyst system not only offers the advantages of simpli-
1
2
3
4
5
6
7
8
>99
>99
>99
>99
>99
85
>99
>99
>99
>99
>99
>99
>99
>99
39.6
39.6
39.6
39.6
39.6
34.0
26.4
10.8
26.4
26.4
26.4
26.4
26.4
22.7
17.6
7.2
66
27

ed workup procedure and easy recycling generally associated
a
ꢁ
Reaction conditions: alcohol 10 mmol, reaction temperature 60 C, with heterogenized TEMPO with inorganic and organic poly-
b
atmospheric oxygen pressure.
Conversions c and selectivity
mers, but also has high activity and selectivity due to its solu-
bility in the substrate. This process thus represents a greener
pathway for aerobic oxidation of alcohols into carbonyl
compounds, which manifested in the aspects of recyclability of
determined by GC (area normalization method). TON ¼ moles of
d
product/2(moles of DES–TEMPO). TOF ¼ TON/reaction time.
52
2
be a source of NO , which is responsible for the oxidation of
the catalyst, use of non-toxic and cheap Fe(NO
and absence of solvent in reaction.
3 3
) as co-catalyst,
2
+
3+
3ꢀ
ꢀ
ꢀ
Fe to Fe and NO is reduced to NO
2
. Finally, NO
3
can be
ꢀ
easily regenerated by the oxidation of NO
oxygen.
2
with molecular
52
4
. Experimental section
3. Conclusion
4.1 General remarks
A quaternary ammonium graed with a TEMPO moiety was 4-OH-TEMPO was purchased from Energy Chemical. 1,4-
synthesized. The quaternary ammonium mixed with urea in a Dibromobutane was obtained from Tianjin Keruisi Fine
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Scheme 3 Proposed mechanism for the catalytic aerobic oxidation of alcohols.
Chemical Co., LTD. Alcohols were obtained from Alfa Aesar Me Si) d: 1.14 (6H, s, piperidine-Me), 1.20 (6H, s, piperidine-
4
China (Tianjin) Co., Ltd. All the chemicals were used as Me), 1.42 (2H, t, J ¼ 11.6 Hz, piperidine-CHH), 1.66–1.70 (2H, m,
received. Solvents were puried by standard methods and dried O–CH –CH –CH –CH –Br), 1.87–1.94 (4H, m, O–CH –CH –
2
2
2
2
2
2
if necessary.
CH
2
2 2
–CH –Br, piperidine-CHH), 3.42–3.47 (4H, m, OCH ,
2
), 3.50–3.57 (1H, m, piperidine-CH).
1
13
H MNR and C MNR spectra were recorded with TMS as BrCH
internal standard on a Bruker AC-P 400 spectrometer. ESI mass
spectra were recorded on a LCQ Advanced high-resolution
4.3 Synthesis of N,N-dimethyl-(4-(2,2,6,6-tetramethyl-1-
spectrometer. The viscosity was measured on a NDJ-79 rotary
viscometer, Shanghai Changji Geological Instrument Co., LTD.
Oxidation reaction samples were analyzed on a Shandong
Lunan Ruihong Gas Chromatograph (SP-6800A) equipped with
a FID detector and a SE 30 column (30 m ꢂ 0.5 mm). The
conditions used in gas chromatography were temperature of the
detector 280 C, column temperature 130–220 C (varying with
alcohols), pressure of the carrier gas 0.05–0.07 MPa (varying
with alcohols), split ratio of injection 10 : 1.
opiperidoxylxyl-4-)butyl)dodecyl ammonium salt
+
ꢀ
([Quaternium-TEMPO] Br )
Under stirring, N,N-dimethyldodecylamine (2.15 g, 7 mmol) was
slowly added to a solution of 4-(2,2,6,6-tetramethyl-1-oxyl-4-
piperidoxyl)butyl bromide (1.86 g, 8.4 mmol) in 20 ml of tetra-
hydrofuran. The mixture was then heated to reux and stirred
for 6 h. Aer cooling to room temperature, tetrahydrofuran was
removed by reduced pressure. Then water (20 ml) was added to
dissolve the quaternary ammonium salt and ethyl acetate (20
ml) was added to dissolve the un-reacted raw materials. The
aqueous layer was extracted with ethyl acetate several times.
Finally water was distilled by reduced pressure to give a red
ꢁ
ꢁ
4.2 Synthesis of 4-(2,2,6,6-tetramethyl-1-oxyl-4-piperidoxyl)
butyl bromide
To a solution of 4-hydroxy-TEMPO (2.58 g, 0.015 mol) in anhy- viscous liquid (2.66 g, 76.6% yield). The sample of the product
1
13
drous acetone (20 ml) was added NaH (0.6 g, 0.015 mol) and the was reduced by phenylhydrazine to record the H NMR and
C
1
resulting slurry was stirred at room temperature for 10 min. 1,4- NMR. H NMR (400 MHz; CDCl ; Me Si) d: 0.88 (3H, t, J ¼ 6.8
3
4
Dibromobutane (4.86 g, 0.0225 mol) was then added and the Hz, Me), 1.26 (12H, s, piperidine-Me), 1.28–1.48 (18H, m, N–
mixture was stirred at room temperature for 3 h. Aer reaction CH
2
–CH
2
–(CH
2
)
9
–CH
–CH –N, N–CH
–CH –CH –CH –N), 3.35 (6H,s, NMe), 3.43–3.47
–CH –C 18–CH
), 3.55 (2H, t, J ¼ 6 Hz, O–CH
–N), 3.63–3.68 (2H, m, OCH ), 3.71–3.76 (1H, m,
) d: 14.11 (Me), 19.89
tion was separated by ash chromatography (10 : 1 (v/v), (O–CH –CH –CH –CH –N), 20.69 (N–CH –CH –C H –CH –
3
), 1.65–1.88 (8H, m, piperidine-CH
2
, O–
acetone was removed under reduced pressure. Water (30 ml) CH
2
–CH
2
–CH
2
2
2
–CH –C 18–CH ), 2.00–2.13
2
9
H
3
was then added to dissolve the solid residue. The aqueous layer (2H, m, O–CH
was extracted with methylene chloride (15 ml ꢂ 3). The (2H, m, N–CH
combined organic extracts were dried over Na SO and CH –CH –CH
concentrated under vacuum. The resulting concentrated solu- piperidine-CH). C NMR (100 MHz, CDCl
2
2
2
2
2
2
9
H
3
2
–
2
4
2
2
2
2
1
3
3
2
2
2
2
2
2
8
16
2
petroleum–ethyl acetate) to obtain red viscous liquid (1.15 g, CH ), 22.64 (N–CH –CH –C H –CH ), 22.66 (N–CH –CH –
3
2
2
9
18
3
2
2
2
5% yield). The sample of the product was reduced by phenyl- CH
2
–C
8
H
16–CH
3
), 26.22 (O–CH
2
2 2 2
–CH –CH –CH –N), 26.54
1
1
hydrazine to record the H NMR. H NMR (400 MHz; CDCl
3
;
(piperidine-Me), 29.22, 29.28, 29.41, 29.46, 29.56, 31.85, 31.99
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(
N–CH
2
–CH
2
–CH
2
–(CH
2
)
7
–CH
2
–CH
3
), 44.60 (piperidine-CH
2
), 11 P. Gamez, I. W. C. E. Arends, R. A. Sheldon and J. Reedijk,
5
1.26 (NMe), 59.29 (piperidine-C), 63.50 (N–CH –CH –C H –
Adv. Synth. Catal., 2004, 346, 805–811.
2
2
9
18
CH ), 63.84 (O–CH –CH –CH –CH –N), 66.80 (piperidine-CH), 12 P. J. Figiel, M. Leskel ¨a and T. Repo, Adv. Synth. Catal., 2007,
3
2
2
2
2
7
0.71 (OCH ). HR-MS (ESI): m/z 440.4336. C H N O calcu-
349, 1173–1179.
2
27 56 2 2
lated m/z: 440.4342.
13 M. N. Kopylovich, K. T. Mahmudov, M. Haukka, P. J. Figiel,
A. Mizar1, J. A. L. da Silva and A. J. L. Pombeiro, Eur. J. Inorg.
Chem., 2011, 2011, 4175–4181.
4.4 Preparation of DES–TEMPO
1
4 Y. Jing, J. Jiang, B. Yan, S. Lu, J. Jiao, H. Xue, G. Yang and
G. Zheng, Adv. Synth. Catal., 2011, 353, 1146–1152.
Into a round-bottom ask equipped with a magnetic stirrer was
added N,N-dimethyl-(4-(2,2,6, 6-tetramethyl-1-oxyl-4-piper-
idoxyl)butyl)dodecyl ammonium salt (0.4 mmol, 0.2082 g) and
1
1
5 X. Wang and X. Liang, Chin. J. Catal., 2008, 29, 935–939.
6 N. Wang, R. Liu, J. Chen and X. Liang, Chem. Commun., 2005,
ꢁ
urea (0.2 mmol, 0.015 g). The mixture was heated at 60 C for a
5322–5324.
period of time until a clear liquid was appeared, which resulted
in the formation of DES–TEMPO with 100% yield. The obtained
DES–TEMPO was used without any further purication.
1
1
7 J. Liu and S. Ma, Tetrahedron, 2013, 69, 10161–10167.
8 W. Yin, C. Chu, Q. Lu, J. Tao, X. Liang and R. Liu, Adv. Synth.
Catal., 2010, 352, 113–118.
1
9 R. A. Sheldon, I. W. C. E. Arends, G.-J. T. Brink and
A. Dijksman, Acc. Chem. Res., 2002, 35, 774–781.
4.5 Representative procedure for the oxidation of alcohols
In a typical process, into a 5 ml two-necked, round-bottom ask 20 J. Liu and S. M. Ma, Org. Lett., 2013, 15, 5150–5153.
equipped with a magnetic stirrer and an oxygen balloon was 21 A. Heeres, H. A. van Doren, K. F. Gotlieb and I. P. Bleeker,
added DES–TEMPO (0.25 mmol, 0.1375 g). Then, benzyl alcohol
Carbohydr. Res., 1997, 299, 221–227.
O (0.3 mmol, 0.1231 g) was added 22 C. Bolm and T. Fey, Chem. Commun., 1999, 1795–1796.
successively at 60 C under stirring. The oxygen from the 23 B. Karimi and E. Farhangi, Chem.–Eur. J., 2011, 17, 6056–
(
10 mmol) and Fe(NO
3
)
3
$9H
2
ꢁ
balloon was introduced and controlled through a triple valve.
The reaction was monitored by GC equipped with a suitable 24 B. Karimi and E. Badreh, Org. Biomol. Chem., 2011, 9, 4194–
6060.
column. The sample was diluted with acetonitrile before
4198.
injection.
25 T. Fey, H. Fischer, S. Bachmann, K. Albert and C. Bolm,
J. Org. Chem., 2001, 66, 8154–8159.
2
6 L. Wang, J. Li, X. Zhao, Y. Lv, H. Zhang and S. Gao,
Tetrahedron, 2013, 69, 6041–6045.
Acknowledgements
The authors are grateful for nancial supports from the 27 B. Karimi and E. Farhangi, Adv. Synth. Catal., 2013, 355, 508–
National Natural Science Foundation of China (no. 21276061)
516.
and Natural Science Foundation of Hebei Province, China (no. 28 B. Karimi, A. Biglari, J. H. Clark and V. Budarin, Angew.
B2013202158).
Chem., Int. Ed., 2007, 46, 7210–7213.
29 Z. Zheng, J. Wang, M. Zhang, L. Xu and J. Ji, ChemCatChem,
2
013, 5, 307–312.
References
3
0 K. Saito, K. Hirose, T. Okayasu, H. Nishide and
1
2
3
M. Hudlicky, Oxidations in Organic Synthesis, Washington
DC, 1990, pp. 114–149.
R. C. Larock, Comprehensive Organic Transformations: a guide
M. T. W. Hearn, RSC Adv., 2013, 3, 9752–9756.
31 A. Dijksman, I. W. C. E. Arends and R. A. Sheldon, Chem.
Commun., 2000, 271.
to functional group preparations, Wiley-VCH, New York, 1999. 32 C. Tanyeli and A. G u¨ m u¨ s, Tetrahedron Lett., 2003, 44, 1639–
G. Tojo and M. Fernandez, Oxidations of Alcohols to Aldehydes 1642.
and Ketones: a guide to current common practice, Springer 33 C. Zhu, L. Ji and Y. Wei, Catal. Commun., 2010, 11, 1017–
Science+Business Media, Inc., Boston, MA, 2006.
1020.
R. A. Sheldon and J. K. Kochi, Metal Catalyzed Oxidation of 34 W. Qian, E. Jin, W. Bao and Y. Zhang, Tetrahedron, 2006, 62,
Organic Compounds, Academic Press, New York, 1984.
556–562.
J. March, Advanced Organic Chemistry: Reactions, Mechanisms, 35 L. Liu, L. Ji and Y. Wei, Monatsh. Chem., 2008, 139, 901–903.
4
5
6
7
8
9
and Structure, Wiley, New York, 4th edn, 1992.
D. B. Dess and J. C. Martin, J. Org. Chem., 1983, 48, 4155–
36 Q. Zhang, K. D. O. Vigier, S. Royer and F. J ´e r ˆo me, Chem. Soc.
Rev., 2012, 41, 7108–7146.
37 A. P. Abbott, D. Boothby, G. Capper, D. L. Davies and
R. K. Rasheed, J. Am. Chem. Soc., 2004, 126, 9142–9147.
38 S. Ramesh, R. Shanti and E. Morris, J. Mol. Liq., 2012, 166,
40–43.
39 H. Wang, Y. Jing, X. Wang, Y. Yao and Y. Jia, J. Mol. Liq.,
2012, 170, 20–24.
40 A. P. Abbott, G. Capper, D. L. Davies, R. K. Rasheed and
V. Tambyrajah, Chem. Commun., 2003, 70–71.
4
156.
D. B. Dess and J. C. Martin, J. Am. Chem. Soc., 1991, 113,
277–7287.
7
R. Alvarez, B. Iglesias, S. L ´o pez and A. R. de Lera, Tetrahedron
Lett., 1998, 39, 5659–5662.
I. Paterson, O. Delgado, G. J. Florence, I. Lyothier, J. P. Scott
and N. Sereinig, Org. Lett., 2003, 5, 35–38.
10 P. Gamez, I. W. C. E. Arends, J. Reedijk and R. A. Sheldon,
Chem. Commun., 2003, 2414–2415.
40168 | RSC Adv., 2014, 4, 40161–40169
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
4
4
4
4
4
4
1 A. P. Abbott, J. C. Barron, K. S. Ryder and D. Wilson, Chem.– 47 T. Hara, J. Sawada, Y. Nakamura, N. Ichikuni and
Eur. J., 2007, 13, 6495–6501. S. Shimazu, Catal. Sci. Technol., 2011, 1, 1376–1382.
2 Z. Chen, B. Zhou, H. Cai, W. Zhu and X. Zou, Green Chem., 48 Z. Lu, T. Ladrak, O. Roubeau, J. van der Toorn, S. J. Teat,
2
009, 11, 275–278.
3 Z. Chen, W. Zhu, Z. Zheng and X. Zou, J. Fluorine Chem.,
010, 131, 340–344.
C. Massera, P. Gamez and J. Reedijk, Dalton Trans., 2009,
3559–3570.
49 O. Das and T. K. Paine, Dalton Trans., 2012, 41, 11476–11481.
2
4 P. M. Pawar, K. J. Jarag and G. S. Shankarling, Green Chem., 50 B. Karimi and E. Badreh, Org. Biomol. Chem., 2011, 9, 4194–
011, 13, 2130–2134. 4198.
5 W. Zhang, L. Qiu, X. Chen and F. Yan, Electrochim. Acta, 51 S. Ma, J. Liu, S. Li, B. Chen, J. Cheng, J. Kuang, Y. Liu,
2
2
014, 117, 48–54.
6 X. Wang, R. Liu, Y. Jin and X. Liang, Chem.–Eur. J., 2008, 14,
679–2685.
B. Wan, Y. Wang, J. Ye, Q. Yu, W. Yuan and S. Yu, Adv.
Synth. Catal., 2011, 353, 1005–1017.
52 L. I. Rossi and S. E. Mart ´ı n, Appl. Catal., A, 2003, 250, 271–
2
278.
This journal is © The Royal Society of Chemistry 2014
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