4-Benzamido-TEMPO catalyzed oxidation of a broad range of alcohols to the carbonyl compounds with NaBrO3 under mild conditions

Article abstract of DOI:10.1002/cjoc.201400141

4-Benzamido-TEMPO catalyzed oxidation system for conversion of a wide range of alcohols to the aldehydes or ketones with NaBrO₃ under room temperature conditions has been developed. The credible, operationally convenient and economical, and condition mild oxidation protocol is particularly of interest in laboratory and in fine chemicals manufacture. 4-Benzamido-TEMPO catalyzed oxidation system for conversion of a wide range of alcohols to the aldehydes or ketones with NaBrO₃ as oxidant under room temperature conditions has been developed. Copyright

Full text of DOI:10.1002/cjoc.201400141

FULL PAPER
DOI: 10.1002/cjoc.201400141
4
-Benzamido-TEMPO Catalyzed Oxidation of a Broad Range
of Alcohols to the Carbonyl Compounds with NaBrO3
under Mild Conditions
Jiaxuan Shen, Jiangkai Sun, Shuangshuang Qin, Changhu Chu, and Renhua Liu*
School of Pharmacy, East China University of Science and Technology, Shanghai 200237, China
4-Benzamido-TEMPO catalyzed oxidation system for conversion of a wide range of alcohols to the aldehydes
or ketones with NaBrO under room temperature conditions has been developed. The credible, operationally con-
3
venient and economical, and condition mild oxidation protocol is particularly of interest in laboratory and in fine
chemicals manufacture.
Keywords alcohols, oxidation, carbonyl compounds, sodium bromated, 4-benzamido-TEMPO
Introduction
Sodium bromate is a commercially available and in-
expensive compound, and has been extensively used in
organic synthesis. It can be used as an effective bromo-
The selective oxidation of alcohols into their corre-
sponding aldehydes or ketones is a very important func-
tional group transformation in organic synthesis. In
recent years, major efforts in the transformation have
been devoted to the discovery and development of en-
vironmentally benign methods using O or H O as the
[4]
[
1]
hydroxylation reagent for alkenes and alkynes. It can
also oxidize sulfides to the sulfoxides, thiols to the di-
[
5]
sulfides and hydroquinones to the quinones. As the
bromine of sodium bromate is a positive five-valence
element, the oxidation of 1 mol of alcohols to the alde-
hydes or ketones only requires 1/3 mol sodium bromate
in theory. In addition, the oxidative systems with
2
2
2
[2]
terminal oxidant. Although catalytic oxidation with O
or H O is certainly of importance, there are also im-
2
2
2
portant advantages in the use of less toxic or nontoxic
and easily available other oxidants. This is true espe-
cially for smaller scale operations such as in fine
chemicals manufacture, where the credible, operation-
ally convenient and economical, as well as condition
NaBrO oxidant are free of transition metals, therefore,
3
the oxidative systems are intrinsically devoid of com-
plications caused by transition metal oxidants. Sodium
bromate had been previously employed in the presence
of transition metals as catalysts to oxidize alcohols to
[
3]
mild oxidation methods are particularly of importance.
[
6]
As such, one key criterion is the use of minimum quan-
tities or total nonuse of auxiliary additives. Herein, we
reported a 4-benzamido-TEMPO catalyst system for
oxidation of a broad range of alcohols to the aldehydes
or ketones with sodium bromate oxidant under mild
conditions (Scheme 1).
aldehydes. In addition, transition metal-free catalytic
oxidations of alcohols with sodium bromate as oxidant
[7,5c]
or co-catalyst were also reported.
However, a major
drawback to these reported methods is their inability to
oxidize primary aliphatic alcohols to the aldehydes.
We and other groups have recently reported a series
of TEMPO-based catalyst systems for aerobic oxidation
Scheme 1 Oxidation of alcohols with 4-benzamido-TEMPO/
HCl/NaBrO3
[8]
of alcohols to the aldehydes or ketones. Based on our
[8e,8f]
previous work,
we find the ability of 4-benzamido-
OH
O
TEMPO to catalytically oxidize primary aliphatic alco-
hols to the aldehydes with high effectiveness. We rea-
soned that 4-benzamido-TEMPO in combination with
sodium bromate would be a competent catalytic system
for oxidation of a wide range of alcohols to the carbonyl
compounds.
4
-benzamido-TEMPO
R1
R2
R1
R2
HCl, NaBrO , DCM/H O, r.t.
3
2
O
N
H
N O
4
-benzamido-TEMPO=
*
E-mail: liurh@ecust.edu.cn; Tel.: 0086-021-64250627; Fax: 0086-021-6425-0627
Received March 10, 2014; accepted April 15, 2014; published online May 22, 2014.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201400141 or from the author.
Chin. J. Chem. 2014, 32, 405—409
© 2014 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
405

Shen et al.
FULL PAPER
Results and Discussion
The use amount of 4-benzamido-TEMPO greatly
influenced the selectivity of the reaction. Decrease of
the use amount of 4-benzamido-TEMPO to 1 mol% led
to the generation of 28% of ester (Table 1, Entry13). All
The investigation began with 1-octyl alcohol (1
mmol) as a representative substrate, 4-benzamido-
TEMPO as the catalyst, sodium bromate as the oxidant
at room temperature for optimization of the reaction
conditions (Table 1). As can be seen in Table 1, the
catalytic system is applicable to a wide range of
solvents with regard to the conversion (Table 1, Entries
the HCl, NaBrO
3
and 4-benzamido-TEMPO are
required, and if any additive is absent the reaction will
not proceed (Table 1, Entries 14－16). As expected,
3
only 33 mol% of NaBrO was required for totall
conversion of the substrate. Therefore, the optimal
reaction conditions were considered as 2 mol% HCl, 35
1
－10). The use of a mixture of H
and DMF, H O and dioxane as the solvents can provide
high conversion. Fortunately, when a mixture of DCM
and H O was used we obtained 69% of conversion and
3% of selectivity (Table 1, Entry 3). However, low
conversion and selectivity were observed when DCM
and H O were used as the solvent, separately (Table 1,
2 2
O and AcOH, H O
2
mol% NaBrO
3
, 2 mol% 4-benzamido-TEMPO and
DCM/H O as the solvent at room temperature.
2
2
Having established the optimal conditions, we next
examined the range of alcohols, and the results are
summarized in Table 2. Several features of this simple
yet robust catalytic system are worth noting. First, the
catalytic system results in fairly efficient conversion of
a broad spectrum of alcohols to the corresponding al-
dehydes or ketones under very mild conditions. The
reaction tolerates both simple aliphatic alcohols (Table
9
2
Entries 9－10). We were pleased to find that 100% of
conversion and 96% of selectivity were reached when 2
mol% of HCl were extra added to the catalytic system
under standard reaction conditions (Table 1, Entry 11).
a
Table 1 Optimization of reaction conditions
2
, Entries 14 and 17－20) and more hindered deriva-
O
tives (Entries 11－12 and 21) as well as protecting
groups (Entry 15). Most of the substrates employed
were quantitatively converted into the corresponding
aldehydes or ketones with 99% selectivity. The benzylic
alcohols, including pyridine methanol, were converted
into the corresponding benzaldehydes or ketones in very
high isolated yields (Table 2, Entries 1－10). The elec-
tronic properties of the substituents on the benzene ring
seldom interfered the high yields of reactions. Of par-
ticular interest is the oxidation of aliphatic primary al-
cohols under these conditions. Although several effec-
tive methods for oxidation of primary alcohols have
been reported, the overoxidation and/or generation of
esters has been a recurring problem toward oxidation of
aliphatic primary alcohols. The present protocol for
oxidation of aliphatic primary alcohols showed that
carboxylic acid and esters generated less than 5% (Table
2, Entries 15－20). It is interesting to note that the al-
cohols, which remain difficult recalcitrant substrates in
many anerobic and aerobic oxidation protocols, were
smoothly oxidized to the ketones in more than 90% iso-
lated yields (Table 2, Entries 21 and 22).
O
O
OH
A
B
OH 4-benzamido-TEMPO
NaBrO , HCl, r.t.
3
O
C
b
Select. /%
b
Entry HCl/mol% Solvent
Time/h Conv. /%
(
A/B/C)
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
5
5
5
5
5
5
5
5
5
5
2
1
2
0
2
2
2
CH
3
CN/H
2
O
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
16
85
69
12
86
50
70
68
57
15
100
0
75/6/19
78/4/18
93/0/7
DCE/H
2
O
DCM/H O
2
PhMe/H O
50/0/50
40/41/19
40/50/10
30/64/6
29/59/12
14/0/86
0/0/100
96/0/4
2
AcOH/H O
2
THF/H O
2
DMF/H O
2
Dioxane/H
2
O
c
c
DCM
1
1
1
1
1
1
1
1
2
H O
DCM/H O
2
Conclusions
DCM/H O
0/0/0
2
d
In conclusion, we have developed a highly efficient
catalyst system for oxidation of a wide range of alcohols
by using 4-benzamido-TEMPO as the catalyst and
DCM/H O
2
90
0
72/0/28
0/0/0
2
DCM/H O
e
f
DCM/H O
2
2
2
0
0/0/0
NaBrO as the terminal oxidant under room temperature.
3
DCM/H O
0
0/0/0
This inexpensive and convenient catalytic system could
be broadly used in laboratory and in fine chemicals
manufacture.
g
DCM/H O
92
97/0/3
a
The reactions were carried out with alcohols (1 mmol),
-benzamido-TEMPO (2 mol%), NaBrO (0.35 mmol, H O 0.5
mL), solvent (1.5 mL, 2∶1) at room temperature. GC yield.
4
3
2
b
Experimental
c
d
NaBrO
-Benzamido-TEMPO (0 mol%). NaBrO
0.3 mmol, H O 0.5 mL).
3
was added in solid. 4-Benzamido-TEMPO (1 mol%).
e
f
g
General experimental methods
4
3 3
(0 mmol). NaBrO
(
All the chemical reagents were obtained from com-
2
406
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© 2014 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2014, 32, 405—409

4-Benzamido-TEMPO Catalyzed Oxidation of a Broad Range of Alcohols to the Carbonyl Compounds
a
3
Table 2 Scope of the 4-benzamido-TEMPO/HCl/NaBrO system
b
b
Entry
Alcohol
CH OH
2-MeC CH
4-NO C H CH OH 4-NO C H CHO
Product
CHO
2-MeC
Time/h
Conv. /%
Yield /%
1
2
3
4
5
6
7
8
C
6
H
5
2
C
6
H
5
1
1
1
1
1
1
1
1
99
99
99
99
99
99
99
99
99
99
6
H
4
2
OH
6 4
H CHO
2
6
4
2
2
6
4
99 (99)
99 (96)
99
2-ClC H CH OH
6 4
2-ClC H CHO
6
4
2
4-FC H CH OH
6 4
4-FC H CHO
6
4
2
4-BrC H CH OH
4-BrC H CHO
99
6
4
2
6
4
4-IC H CH OH
4-IC H CHO
99
6
4
2
6
4
4-MeOC H CH OH 4-MeOC H CHO
99
6
4
2
6
4
OH
O
c
9
1
99
99 (93)
N
N
OH
O
1
0
1
2
99
99 (93)
H C
H C
3
3
H C
CH3
OH
H C
CH3
3
3
1
1.5
99
99 (92)
O
12
13
14
4
1.5
2
99
99
99
99 (93)
99 (94)
99
OH
OH
O
H3CO
OH
H3CO
O
O
OH
O
N
N
1
5
4
99
99 (93)
O
O
OH
O
16
17
18
1
2
5
99
99
95
98 (91)
96 (90)
96
OH
O
OH
O
1
2
9
0
OH
2
4
99
99
97
95
O
tetradecan-1-ol
tetradecanal
OH
O
d
CH3
CH3
2
1
9
8
99
97
99 (92)
H C
H C
3
3
CH3
CH3
OH
O
O
d
O
2
2
99 (90)
O
O
a
The reactions were carried out with alcohols (1 mmol), 4-benzamido-TEMPO (2 mol%), NaBrO
3
(0.35 mmol, H
2
O 0.5mL), HCl (2
b
c
d
mol%) at room temperature. Detected by GC. Isolated yields are in parentheses. 1 equiv. AcOH was added. 4-Benzamido-TEMPO
10 mol%), NaBrO (1 mmol, H O 0.5mL) were used.
(
3
2
Chin. J. Chem. 2014, 32, 405—409
© 2014 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
407

Shen et al.
FULL PAPER
mercial sources and used without further purification.
CDCl
3
) δ: 3.61－3.55 (m, 1H), 3.37 (s, 3H), 2.56－2.48
1
13
H NMR (400 MHz) and C NMR (100 MHz) spectra
were obtained on a Bruker DRX-400 NMR using CDCl
(m, 2H), 2.25－2.19 (m, 2H), 2.09－2.01 (m, 2H),
1.94－1.85 (m, 2H).
1-Benzoylpyrrolidine-2-carbaldehyde
(400 MHz, CDCl ) δ: 9.68 (s, 1H), 7.59－7.57 (m, 2H),
7.45－7.40 (m, 3H), 4.66 (t, J＝6.8 Hz, 1H), 3.66－
3.53 (m, 2H), 2.24－2.16 (m, 1H), 2.07－2.00 (m, 1H),
1.96－1.89 (m, 2H); C NMR (100 Hz, CDCl ) δ:
3
1
as the solution with TMS as an internal reference. Col-
umn chromatography was performed with 300－400
mesh silica gel. GC analysis was measured on Agilent
H NMR
3
7
0
890N. General GC conditions: HP-5 column, 30 m×
.32 mm (id); FID detector; injection: 0.2 μL; carrier
1
3
3
2
gas: N ; carrier gas rate: 1.2 mL/min; area normalization.
1
2
99.3, 170.1, 135.6, 130.4, 128.2, 127.2, 65.0, 50.0,
6.2, 25.3.
The reactions were detected under the conditions: col-
umn temperature: 75 ℃ for 2 min, rising to 300 ℃ at
a rate of 10 ℃/min.
1
2
-Phenylacetaldehyde
H NMR (400 MHz,
CDCl ) δ: 9.75 (s, 1H), 7.39－7.21 (m, 5H), 3.68 (d,
J＝2 Hz, 2H); C NMR (100 Hz, CDCl ) δ: 199.4,
3
1
3
3
Procedure for the catalytic oxidation of alcohols
1
31.8, 129.6, 129.0, 127.4, 50.5.
A mixture of alcohols (1 mmol), 4-benzamido-
TEMPO (0.02 mmol, 5.5 mg) and HCl (3.7%, 0.02
mmol, 20 mg) in DCM (1 mL) were powerfully stirred
at room temperature, aq. sodium bromate (0.35 mmol,
1
Octanal H NMR (400 MHz, CDCl ) δ: 9.75 (s,
3
1
1
H), 2.42－2.41 (m, 2H), 1.63－1.61 (m, 2H), 1.29－
.27 (m, 8H), 0.87 (t, J＝7.2 Hz, 3H); C NMR (100
13
Hz, CDCl ) δ: 202.9, 43.8, 31.5, 29.0, 28.9, 22.5, 22.0,
3
0
.5 mL) was added once. The mixture was continuously
1
4.0.
2S,5R)-2-Isopropyl-5-methylcyclohexanone
NMR (400 MHz, CDCl ) δ: 2.30 (d, J＝12.8 Hz, 1H),
and powerfully stirred for a certain period of time, the
conversion and selectivity of the product were detected
by GC. After completion of the reaction, H O (10 mL)
2
and DCM (10 mL) were added, the organic layer was
separated and the aqueous layer extracted with di-
chloromethane (10 mL×2). The combined organic ex-
tracts were dried over anhydrous sodium sulfate, filtered,
and concentrated under reduced pressure. The residue
was purified on silica gel column chromatography (pe-
troleum/EtOAc＝10/1) to give the corresponding alde-
1
(
H
3
2
6
3
3
.13－1.78 (m, 6H), 1.35－1.28 (m, 2H), 0.96 (d, J＝
.4 Hz, 3H), 0.89 (d, J＝6.8 Hz, 3H), 0.83 (d, J＝6.8 Hz,
H); C NMR (100 Hz, CDCl ) δ: 212.2, 55.7, 50.7,
1
3
3
5.3, 33.8, 27.7, 25.8, 22.1, 21.1, 18.6.
1
Ethyl 2-oxopropanoate
CDCl ) δ: 4.32－4.26 (q, J＝7.2 Hz, 2H), 2.44 (d, J＝
H NMR (400 MHz,
3
0
.8 Hz, 3H), 1.34 (t, J＝7.2 Hz, 3H).
hydes or ketones.
1
4
-Nitrobenzaldehyde H NMR (400 MHz, CDCl
3
)
Acknowledgement
δ: 10.15 (s, 1H), 8.39 (d, J＝7.6 Hz, 2H), 8.07 (d, J＝
13
We are grateful to the financial support from the Na-
tional Natural Science Foundation of China (No.
21072055) and the National High Technology Research
and Development Program of China (863 Program) (No.
8
1
3
.4 Hz, 2H); C NMR (100 Hz, CDCl ) δ: 190.2, 151.1,
40.0, 130.4, 124.2.
2
1
-Chlorobenzaldehyde
CDCl
J＝8.4 Hz, 1H), 7.44 (d, J＝8 Hz, 1H), 7.37 (t, J＝8 Hz,
1
1
H NMR (400 MHz,
3
) δ: 10.47 (s, 1H), 7.91 (d, J＝8 Hz, 1H), 7.51 (t,
2
008AA06Z306).
1
3
H); C NMR (100 Hz, CDCl
3
) δ: 189.8, 137.9, 135.1,
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