Novel [4-hydroxy-TEMPO + NaCl]/SiOas a reusable catalyst for aerobic oxidation of alcohols to carbonyls

Article abstract of DOI:10.1055/s-0031-1290980

A convenient method for the preparation of [4-hydroxy-TEMPO + NaCl]/SiOwas developed. And [4-hydroxy-TEMPO + NaCl]/SiOwith Fe(NOH was used for an aerobic oxidation of alcohols to carbonyls under mild reaction conditions. Alcohols were converted to the corresponding carbonyls in good to excellent yields. [4-Hydroxy-TEMPO + NaCl]/SiOcan be easily separated from the reaction mixture by filtration and reused at least six times. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0031-1290980

LETTER
▌1397
letter
Novel [4-Hydroxy-TEMPO + NaCl]/SiO as a Reusable Catalyst for Aerobic
2
Oxidation of Alcohols to Carbonyls
a
A Reusable Catalyst for Aerobic Oxidation of Alcohols to Carbonyls
a,b
a
a
Department of Materials and Applied Chemistry, College of Science and Technology, Nihon University, Kanda Surugadai, Chiyoda-ku,
Tokyo 101-8308, Japan
Fax +81(3)32590818; E-mail: aoyama.tadashi@nihon-u.ac.jp
b
The Center for Creative Materials Research, Research Institute of Science and Technology, College of Science and Technology, Nihon
University, Kanda Surugadai, Chiyoda-ku, Tokyo 101-8308, Japan
c
Department of Bioscience and Engineering, Shibaura Institute of Technology, Fukasaku, Minuma-ku, Saitama 337-8570, Japan
Received: 17.01.2012; Accepted after revision: 22.03.2012
9
time (10–36 h). Yin et al. reported the oxidation of alco-
Abstract: A convenient method for the preparation of [4-hydroxy-
hols using 4-acetoamide TEMPO, FeCl , and NaNO . The
3
2
TEMPO + NaCl]/SiO was developed. And [4-hydroxy-TEMPO +
2
reaction was carried out under severe conditions (50 °C,
NaCl]/SiO with Fe(NO ) ·9H O was used for an aerobic oxidation
2
3
3
2
1
0
of alcohols to carbonyls under mild reaction conditions. Alcohols 0.4 MPa O ). Metal nitrates played a role as promoter of
2
were converted to the corresponding carbonyls in good to excellent reoxidation of TEMPO. Recently, procedures of transi-
1
1
yields. [4-Hydroxy-TEMPO + NaCl]/SiO can be easily separated tion-metal-free aerobic oxidation have been reported.
2
from the reaction mixture by filtration and reused at least six times.
As mentioned above, many methods for the oxidations of
alcohols using TEMPO derivatives have been reported.
However, in these methods, TEMPO derivatives have not
been reused. Recently, the oxidation using supported
TEMPO derivatives has been reported. For instance; (1)
Fey et al. have reported Anelli oxidation of alcohols using
Key words: alcohols, TEMPO, oxidation, supported catalyst, ke-
tones
The oxidation of alcohols to aldehydes or ketones is one
of the important reactions in organic synthesis. Chromic
acid oxidant has long been employed for the oxidation of
12
silica gel supported TEMPO as a catalyst; (2) PEG-sup-
ported TEMPO radicals were developed for an aerobic
1
alcohols. However, chromic acid has a strong toxicity. In
13
oxidation under Minisci’s conditions by Benaglia; (3)
the oxidation of alcohols using chromic acid as an oxi-
dant, excess amount of chromic acid is required against an
alcohol, and a large amount of toxic waste is generated.
Swern and Dess–Martin oxidation are also known as met-
Fall et al. have described ionic liquid supported TEMPO
14
for the oxidation of alcohols to aldehydes and ketones;
(
4) Karimi et al. have prepared magnetically separable
TEMPO, magnetic core-shell nanoparticle-supported
TEMPO, and have described the metal- and halogen-free
2
al-free oxidation. Swern oxidation, however, emits a bad
smell because dimethyl sulfide is generated during the re-
15
16
aerobic oxidation of alcohols in water, etc. The point,
supported TEMPO can be reused for subsequent reaction,
is significant progress. But many steps and long times
were taken for the preparation of this supported TEMPO.
3
action, and Dess–Martin oxidation uses an explosive re-
4
5
agent as an oxidant. In 1975, Cella et al. and Ganem
independently reported a novel method for the oxidation
of alcohols using 2,2,6,6-tetramethyl-piperidine-1-oxyl
In the course of our study using inorganic solid-supported
reagents, we found out that 4-hydroxy-TEMPO was easily
supported onto silica gel in nonpolar solvents such as tol-
uene, 1,2-dichloroethane (DCE), and chloroform. When
(
TEMPO). Since then, many studies on the oxidation of
alcohols using TEMPO have been reported. In the oxida-
tion using6 TEMPO, reoxidants such as NaClO,
,7
PhI(OAc)2, and oxygen are employed along with
SiO was added to the solution of 4-hydroxy-TEMPO in
2
TEMPO, because TEMPO should be reoxidized again and
again in order to oxidize alcohols into carbonyls. The ox-
idation using oxygen as a reoxidant is an ideal method for
the oxidation of alcohols, because; (1) oxygen exists a lot
toluene, which is orange, 4-hydroxy-TEMPO was ad-
sorbed on the surface of SiO , and the color of SiO
2
2
changed from white to orange (Figure 1).
in air; (2) only H O is yielded as a side product. Semmel-
2
hack et al. has reported an aerobic oxidation of alcohols
8
using TEMPO and CuCl. This method, however, is not
suitable for secondary alcohols. Wang et al. reported an
aerobic oxidation of alcohols using 4-hydroxy-TEMPO
and Fe(NO ) ·9H O. This method needs long reaction
3
3
2
Figure 1 Adsorption of 4-hydroxy-TEMPO to SiO2
SYNLETT 2012, 23, 1397–1401
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DOI: 10.1055/s-0031-1290980; Art ID: ST-2012-U0047-L
Georg Thieme Verlag Stuttgart · New York
©

1398
N. Tamura et al.
LETTER
This result interested us very much because this silica gel Table 2 Oxidation of Alcohols in the Presence of Various Nitrates
supported TEMPO (TEMPO–SiO ), which was easily
2
O
prepared, was expected as a catalyst for the oxidation of
alcohols in nonpolar solvent. In this paper, we describe
the aerobic oxidation of alcohols using TEMPO–SiO2.
4
-hydroxy-TEMPO/SiO2
nitrate
OH
H
MeO
r.t., 16 h, DCE, air (1 atm)
MeO
Various inorganic solids were tested in order to find out
the most effective adsorbent for 4-hydroxy-TEMPO (Ta-
ble 1). When various inorganic supports were added to the
solution of 4-hydroxy-TEMPO in toluene, 4-hydroxy-
TEMPO was adsorbed on the support, and the solution
changed from orange to colorless. Silica gel is the most ef-
fective adsorbent among the supports tested (Table 1, en-
try 1). Alumina and bentonite also adsorbed 4-hydroxy-
TEMPO, whereas celite and molecular sieves 4A did not
adsorb (Table 1, entries 2–5). When TEMPO was used in-
stead of 4-hydroxy-TEMPO, a large amount of silica gel
was needed to adsorb TEMPO completely (Table 1, entry
1a
2a
Entrya
Nitrate
none
Yield (%)b
1
2
3
4
trace
Fe(NO
Cu(NO ) ·3H O
)
·9H
O
17
3
3
2
15
3
2
2
Ce(NH
Co(NO ) ·6H O
)
(NO
)
3
4
2
3
6
5
trace
3
2
2
a
A mixture of 1a (2.0 mmol), 4-hydroxy-TEMPO/SiO
.0 g), and nitrate (5 mol%) was stirred in DCE (5 mL) at r.t. for 16 h.
Determined by GLC analysis using n-dodecane as internal standard.
(0.1 mmol/g,
2
1
6). This result suggests that the silanol on the surface of
b
SiO and the hydroxy group in TEMPO interacted strong-
2
ly with each other.
tries 2, 3, 5, and 7). The reaction using a reagent contain-
Table 1 Amount of 4-Hydroxy-TEMPO Adsorbed on Various Sup- ing FeCl was the fastest among them, 1a was converted
3
ports
completely in one hour, and 2a was yielded in 86% yield
Entrya
along with small amount of side products. Oxidation of
Support
TEMPO/support (mmol/g)
benzhydrol (1i) using FeCl , however, did not primarily
3
yield benzophenone (2i), and bis(diphenylmethyl)ether
1
2
3
4
5
SiO2
Al O
8.3
2.0
3.3
–
(
3i) was formed as a main product, because FeCl acts as
3
2
3
Lewis acid (Table 3, entry 8). When this reaction was car-
ried out in the presence of [4-hydroxy-TEMPO + NaCl]/SiO2
for seven hours, 2i was obtained quantitatively and 3i was
not formed (Table 3, entry 9).
bentonite
celite
MS-4A
SiO2
–
Table 3 An Effectiveness of Chloride in Supported Reagent on the
Oxidation of Alcohols
6b
1.1
a
[4-hydroxy-TEMPO + chloride]/SiO2
4
-Hydroxy-TEMPO (mmol) adsorbed on an inorganic support (1 g)
OH
O
Fe(NO3)3⋅9H2O
in toluene.
b
R1
R2
R1
R2
TEMPO.
r.t., DCE, air (1 atm)
1
2
1
a: R1 = 4-MeOC6H4, R2 = H
1i : R1 = Ph, R2 = Ph
Oxidation of 4-methoxybenzyl alcohol (1a) was em-
ployed as a model reaction in order to determine the opti-
mum reaction conditions (Table 2). A mixture of 1a (2
Entrya
Alcohol
Chloride
none
Time (h)
Yield (%)b
mmol) and 4-hydroxy-TEMPO/SiO (0.1 mmol, 1.0 g)
2
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1a
1i
16
1
17
86
93
94
93
27
96
24
99
was stirred in DCE (5 mL) at room temperature for 16
hours, but the reaction did not proceed (Table 2, entry 1).
When this reaction was carried out in the presence of var-
ious nitrates under the same conditions, the reaction was
slightly promoted. In the cases using Fe(NO ) ·9H O and
FeCl3
FeCl ·4H O
2
2
2
CoCl2
7
3
3
2
Cu(NO ) ·9H O, the oxidized product 2a was yielded in
3
2
2
CuCl ·2H O
3
2
2
17% and 15% yields (Table 2, entries 2 and 3).
MnCl ·4H O
7
2
2
Recently, Ma et al. reported that a catalytic amount of
chlorides such as RbCl, CsCl, and NaCl act as an acceler-
ator in the Fe(NO ) ·9H O/TEMPO-catalyzed aerobic ox-
NaCl
3
3
3
2
FeCl3
3
17
idation of alcohols. Various chlorides were supported
onto silica gel along with 4-hydroxy-TEMPO, and the
supported reagents were used as an oxidant (Table 3). The
supported reagents containing FeCl3, FeCl ·4H O,
9
1i
NaCl
7
a
A mixture of alcohol (2.0 mmol), [4-hydroxy-TEMPO + chlo-
ride]/SiO [(0.1 + 0.1) mmol/g, 1.0 g] and Fe(NO ) ·9H O (5 mol%)
2
2
2
3 3
2
CuCl ·2H O, and NaCl converted 1a into 2a in a short
2
2
was stirred in DCE (5 mL) at r.t.
b
time, and 2a was obtained in excellent yields (Table 3, en-
Determined by GLC analysis using n-dodecane as internal standard.
Synlett 2012, 23, 1397–1401
© Georg Thieme Verlag Stuttgart · New York

LETTER
A Reusable Catalyst for Aerobic Oxidation of Alcohols to Carbonyls
1399
The activity of 4-hydroxy-TEMPO/SiO was influenced
2
Table 5 An Effectiveness of Solvent on the Oxidation of 1a
by the conditions for preparing it (Table 4). [4-Hydroxy-
[
4-hydroxy-TEMPO + NaCl]/SiO2
TEMPO + NaCl]/SiO was prepared as follows: Silica gel
2
Fe(NO ) ⋅9H O
3
3
2
(
(
9.77 g) was added to the solution of 4-hydroxy-TEMPO
0.17 g) and NaCl (0.06 g) in water (50 mL), and the mix-
1a
2a
r.t., air (1 atm)
ture was stirred at room temperature for half an hour. The
water was removed by rotary evaporator under reduced
pressure. When the supported reagent prepared by remov-
ing water at 75 °C, and 20 mbar was used for the oxidation
of 1a, it took five hours to oxidize 1a completely (Table
Entrya
Yield (%)b
Solvent
Time (h)
1
2
3
4
5
6
toluene
DCE
4
2
3
5
5
5
6
92
97
4, entry 1).
CHCl
THF
3
98
41
Table 4 Effectiveness of Preparative Method of [4-Hydroxy-
TEMPO + NaCl]/SiO2
EtOAc
acetone
MeCN
96
[4-hydroxy-TEMPO + NaCl]/SiO2
trace
94
Fe(NO3)3⋅9H2O
1a
2a
7
r.t., DCE, air (1 atm)
a
Entrya
Conditionsb
c
d
A mixture of 1a (2.0 mmol), [4-hydroxy-TEMPO + NaCl]/SiO
(0.1+0.1) mmol/g, 1.0 g] and Fe(NO ) ·9H O (5 mol%) was stirred
2
W
a
t
e
r
(
%
)
T
i
m
e
(
h
)
Y
i
e
l
d
(
%
)
[
3
3
2
in solvent (5 mL) at r.t.
Determined by GLC analysis using n-dodecane as internal standard.
1
2
3
75 °C, 20 mbar
65 °C, 20 mbar
65 °C, 40 mbar
65 °C, 70 mbar
2.2
5
2
2
3
96
97
97
6
b
3.7
5.1
8.3
Table 6 Recyclability of [4-Hydroxy-TEMPO + NaCl]/SiO on the
Aerobic Oxidation of 1a
2
4
[
4-hydroxy-TEMPO + NaCl]/SiO2
a
A mixture of 1a (2.0 mmol), [4-hydroxy-TEMPO + NaCl]/SiO2
(0.1 + 0.1) mmol/g, 1.0 g] and Fe(NO ) ·9H O (5 mol%) was stirred
Fe(NO ) ⋅9H O
3 3
2
1
a
2a
[
3
3
2
r.t., 2 h, DCE, air (1 atm)
in DCE (5 mL) at r.t.
b
Evaporating conditions for the preparation of [4-hydroxy-TEMPO +
Cyclea
Yield (%)b
NaCl]/SiO .
2
c
Content of water in [4-hydroxy-TEMPO + NaCl]/SiO2.18
d
1
2
3
4
5
6
98
99
96
96
99
97
Determined by GLC analysis using n-dodecane as internal standard.
Reactions using the supported reagent prepared by remov-
ing water at 65 °C under 20 mbar and 40 mbar gave 2a
quantitatively in two hours (Table 4, entries 2 and 3). The
reaction rate rose according to the increase of water con-
tent in the supported reagent. However, when the reaction
was carried out using a supported reagent prepared by re-
moving water at 65 °C under 70 mbar, the reaction hardly
progressed (Table 4, entry 4).
a
A mixture of 1a (2.0 mmol), [4-hydroxy-TEMPO + NaCl]/SiO2
[(0.1 + 0.1) mmol/g, 1.0 g] and Fe(NO ) ·9H O (5 mol%) was stirred
in DCE (5 mL) at r.t. for 2 h.
3
3
2
The reaction also proceeded in various solvents. When
toluene was used as a solvent, 2a was obtained in 92%
yield in four hours. The yield of that using chloroform as
b
Determined by GLC analysis using n-dodecane as internal standard.
a solvent was 98% in three hours (Table 5, entry 3). THF, Table 7 shows the results of the oxidation of various alco-
EtOAc, and MeCN also gave 2a in moderate to excellent hols under optimized reaction conditions by this meth-
yield but 4-hydroxy-TEMPO was dissolved in the solvent. od. All alcohols employed were oxidized at room
Therefore supported reagent, [4-hydroxy-TEMPO + temperature to give the corresponding carbonyl com-
1
9
NaCl]/SiO , could not be reused.
pounds in good to excellent yields. Benzylic alcohols af-
forded the corresponding aldehydes quantitatively in a
short reaction time except for 4-nitrobenzyl alcohol (Ta-
ble 7, entries 1–8). 4-Nitrobenzyl alcohol required 6 hours
to obtain the oxidized product in good yields. In the case
of benzhydrols, it took 5–8 hours to obtain diaryl ketones
in high yields (Table 7, entries 9–11). In the reactions of
acyclic aliphatic alcohols and allylic alcohols, 8 mol% of
Fe(NO ) ·9H O and long reaction times were needed to
2
The possibility of recycling of [4-hydroxy-TEMPO +
NaCl]/SiO2 was examined (Table 6). [4-Hydroxy-
TEMPO + NaCl]/SiO was recovered from the reaction
2
mixture by filtration and used for the next reaction with-
out after-treatment. [4-Hydroxy-TEMPO + NaCl]/SiO2
was reused for the oxidation of 1a at least six times with-
out loss of catalytic activity.
3
3
2
©
Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 1397–1401

1400
N. Tamura et al.
LETTER
obtain the corresponding carbonyls in good yields (Table Table 7 Aerobic Oxidation of Alcohols Using [4-Hydroxy-TEMPO
+
NaCl]/SiO and Fe(NO ) ·9H O (continued)
2 3 3 2
7, entries 12, 13, and 15–17).
[
4-hydroxy-TEMPO + NaCl]/SiO2 (A)
Table 7 Aerobic Oxidation of Alcohols Using [4-Hydroxy-TEMPO
OH
Fe(NO ) ⋅9H O (B)
O
3
3
2
+
NaCl]/SiO and Fe(NO ) ·9H O
2 3 3 2
R1
R2
R1
R2
r.t., DCE, air (1 atm)
[
4-hydroxy-TEMPO + NaCl]/SiO2 (A)
1
2
OH
O
Fe(NO3)3⋅9H2O (B)
a
Entry Product
Catalyst A:B
mol%)
Time Yield
(h) (%)
R1
R2
R1
R2
r.t., DCE, air (1 atm)
b
(
1
2
a
Entry Product
Catalyst A:B
mol%)
Time Yield
(h) (%)
O
1
1
3
4
2m 5:8
4
93
b
(
O
O
O
2n 5:5
2o 5:8
4
4
96
91
1
H
2a 5:5
2
97
MeO
15
16
Ph
Ph
Ph
H
O
O
O
2
2b 5:5
2c 5:5
2
2
96
95
Ph
H
2p 5:8
2q 5:8
7
2
61
90
O
3
4
H
17
Ph
a
A mixture of alcohol (2.0 mmol), [4-hydroxy-TEMPO + NaCl]/SiO2
(0.1 + 0.1) mmol/g] and Fe(NO ) ·9H O was stirred in DCE (5 mL)
O
[
3
3
2
at r.t.
H
2d 5:5
2e 5:5
3
6
99
91
b
Determined by GLC analysis using n-dodecane as internal standard.
Cl
O
In conclusion, we found out that 4-hydroxy-TEMPO was
easily adsorbed on the surface of SiO in nonpolar solvent
such as DCE, toluene, and chloroform. The silica gel ad-
sorbed 4-hydroxy-TEMPO catalyzed the aerobic oxida-
tion of alcohols in the presence of Fe(NO ) ·9H O and
2
H
5
6
7
O2N
O
H
O
3 3
2
NaCl in a nonpolar solvent. The oxidation of various alco-
hols using [4-hydroxy-TEMPO + NaCl]/SiO2 and
Fe(NO ) ·9H O proceeded smoothly at room temperature,
2f 5:5
2g 5:5
2
3
95
98
3
3
2
and afforded the products in excellent yields. [4-Hydroxy-
TEMPO + NaCl]/SiO was able to be recovered by filtra-
tion and reused at least six times without loss of catalytic
activity. Further application of this supported reagent to
the other oxidation is now in progress.
2
H
PhO
O
8
9
2h 5:5
2i 5:5
2
5
97
96
References and Notes
Ph
Ph
O
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O
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2k 5:5
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8
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Synlett 2012, 23, 1397–1401
© Georg Thieme Verlag Stuttgart · New York

LETTER
A Reusable Catalyst for Aerobic Oxidation of Alcohols to Carbonyls
1401
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3
3
2
1
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mmol/g, 1.0 g] was stirred in DCE (5 mL) under air (1 atm)
at r.t., and then the used supported reagent were removed by
filtration. Product yield was determined by GLC analysis
using n-dodecane as internal standard.
(
(
©
Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 1397–1401

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