Tetrahedron Letters 51 (2010) 35213523
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Bi2O3-catalyzed oxidation of aldehydes with t-BuOOH
Payal Malik, Debashis Chakraborty *
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, Tamil Nadu, India
a r t i c l e
i n f o
Article history:
Received 31 March 2010
Revised 21 April 2010
Accepted 24 April 2010
Available online 6 May 2010
Keywords:
Oxidation
Aldehyde
Acid
Bi2O3
t-BuOOH
a b s t r a c t
A variety of aromatic, aliphatic and conjugated aldehydes were transformed to the corresponding carboxylic acids with 70% t-BuOOH solution (water) in the presence of catalytic amounts of Bi2O3. This method
possesses a wide range of capabilities, does not involve cumbersome work-up, exhibits chemoselectivity
and proceeds under mild conditions. The resulting products are obtained in good yields within reasonable
time. The overall method is green.
2010 Elsevier Ltd. All rights reserved.
1. Introduction
The oxidation of aldehydes has been of contemporary interest
due to diversied potentials in organic synthesis and industrial
manufacturing and recognized as one of the fundamental reaction.1 The most popular and widely used is Jones reagent for such
a transformation.2 However, the reaction is stoichiometric and is
performed under highly acidic conditions. Substrates having
acid-sensitive functionalities may not be able to tolerate such acidity. In addition, the generation of Cr-based side products may be
considered as a potential environmental hazard.3 Other reagents
that have been used successfully include Oxone,4 calcium
hypochlorite5 and 2-hydroperoxyhexauoro-2-propanol.6 Excellent catalytic methods using metals have been developed using
oxidation reactions. Interesting methodologies for metal-mediated
transformation of the aldehyde functionality to carboxylic acid
have been reported recently.7 The above-mentioned reagents and
methods involved have one or more limitations which include
the use of superstoichiometric amounts of expensive compounds,
employment of highly basic or acidic reaction conditions and high
temperature. The search for catalytic processes involving environmentally benign reagents shall always remain an attractive avenue
in this area. Similar catalytic transformation of the aldehydes to
carboxylic acids has been reported with SeO2.8 Our recent results
highlight the oxidation of aldehydes to carboxylic acid using 30%
H2O2 as the oxidant in the presence of catalytic amounts of
AgNO3.9 Our continued interest in studying catalytically active
environmentally benign processes compelled us to investigate
* Corresponding author. Tel.: +91 44 22574223; fax: +91 44 22574202.
E-mail address: dchakraborty@iitm.ac.in (D. Chakraborty).
0040-4039/$ - see front matter 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.04.101
the capability of Bi(III) reagents towards oxidation. It must be
noted that Bi2O3 is reported to catalyze the oxidation of a-hydroxy
ketones.10
2. Results and discussion
Initial attempts to optimize the reaction conditions for the
oxidation of aldehydes to the corresponding carboxylic acids were
done using 4-methoxybenzaldehyde as a suitable substrate in the
presence of different solvents, oxidants and 10 mol % of Bi(III) salts
(Table 1).
The conversion of 4-methoxybenzaldehyde to 4-methoxybenzoic acid is extremely facile in EtOAc under reux conditions in
the presence of 10 mol % Bi2O3 and 5 equiv 70% t-BuOOH (water)
as the oxidant (Table 1, entry 1). Oxidation with t-BuOOH (water)
alone in EtOAc was found to be negligible (<5%). In the presence of
5 mol % Bi2O3 and 5 equiv 70% t-BuOOH (water) as the oxidant in
EtOAc, the reaction required 32 h for completion with 77% isolated
yield of the product. In EtOAc, with 10 mol % Bi2O3 and 5 equiv 70%
t-BuOOH (water) as the oxidant, the reaction reached to completion in much shorter time. With 5 equiv 5 M t-BuOOH (decane),
the reaction was found complete in 2.5 h with 90% isolated yield.
The reaction took much longer time for completion (20 h) when performed with 5 equiv 30% H2O2 in EtOAc and yielded 82% of product.
Among the different solvents used for optimization (Table 1, entries
19), EtOAc yielded best results. The other Bi(III) salts (Table 1,
entries 1012) were found to be inferior as compared to Bi2O3.
Having realized the correct conditions for oxidation, we continued our quest with a variety of aromatic and aliphatic substrates
(Table 2). The scope of our catalytic system is applicable for a wide
range of aromatic, conjugated and aliphatic substrates. These
3522
P. Malik, D. Chakraborty / Tetrahedron Letters 51 (2010) 35213523
Table 1
Optimization of the reaction conditions for the conversion of 4-methoxybenzaldehyde to 4-methoxybenzoic acid with different solvents, 5 equiv 70% t-BuOOH (water)
and 10 mol % Bi(III) salts
Table 2
Bi2O3-catalyzed oxidation of aldehydes to carboxylic acidsa
10 mol% Bi2O3
O
5 equiv. 70% t-BuOOH (water)
OH
R
EtOAc
O
O
O
H
OH
Bi salt, t-BuOOH
solvent, reflux
Entry
MeO
Aldehyde
Timeb
(h)
Acid
MeO
a
b
Entry
Catalyst
Solvent
Timea (h)
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
11
12
Bi2O3
Bi2O3
Bi2O3
Bi2O3
Bi2O3
Bi2O3
Bi2O3
Bi2O3
Bi2O3
BiCl3
BiBr3
Bi(NO3)35H2O
EtOAc
MeCN
Toluene
CH2Cl2
DMF
DMSO
THF
EtOH
CH3NO2
EtOAc
EtOAc
EtOAc
2
15
20
18
4
5
32
17
10
30
27
36
97
90
80
84
87
85
70
80
82
71
80
79
COOH
MeO
CHO
MeO
COOH
OMe
COOH
CHO
90
97
93
6.5
90
90
92
MeO
MeO
MeO
CHO
MeO
COOH
MeO
MeO
OMe
6
2.5
OMe
COOH
CHO
Monitored using TLC until all the aldehyde was found consumed.
Isolated yield after column chromatography of the crude product.
aldehydes were converted to the corresponding carboxylic acids in
good isolated yields in reasonable time (Table 2). It is pertinent to
mention here that mild halogenic oxidants such as hypochlorites,5,11 chlorites12 and NBS13 are not suitable for substrates with
electron-rich aromatic rings, olenic bonds and secondary hydroxyl groups. Substitutions at different position on the phenyl ring
do not hinder the reaction, although the reaction time is affected.
The reaction is much faster with substrates containing electrondonating substituents in the aromatic ring (Table 2, entries 18
vs entries 913). Substitution of extremely electron-withdrawing
group like NO2 retards the oxidation reaction considerably (Table
2, entries 1113). Our catalytic system is mild and shows sufcient
selectivity in carrying out the expected oxidation without affecting
other functionalities such as phenol and amine (Table 2, entries 7
and 8). Oxidation of a, b unsaturated derivatives (Table 2, entry
15) resulted in the formation of the expected acid in very good
yield.
The kinetic studies of the oxidation with 4-methoxybenzaldehyde, 4-nitrobenzaldehyde, furan-2-carboxaldehyde and butyraldehyde were explored next. High-pressure liquid chromatography
(HPLC) was used to determine the various starting materials and
products present as a function of time. The concentration of reactant and product for the oxidation of 4-methoxybenzaldehyde is
shown in Figure 1.
The concentration of the aldehyde decreases steadily while that
of the carboxylic acid increases. We have calculated the rate of such
reactions. As an example let us consider the conversion of 4methoxybenzaldehyde to 4-methoxybenzoic acid. Vant Hoff differential method was used to determine the order (n) and rate constant
(k). From Figure 1, the rate of the reaction at different concentrations
can be estimated by evaluating the slope of the tangent at each point
on the curve corresponding to that of 4-methoxybenzaldehyde.
With these data, log10(rate) versus log10(concentration) is plotted.
The order (n) and rate constant (k) are given by the slope of the line
and its intercept on the log10(rate) axis. It is clear that this reaction
proceeds with second-order kinetics (n = 2.15) and the rate constant
k = 0.2134 L mol 1 min 1. For the other substrates namely 4-nitrobenzaldehyde, furan-2-carboxaldehyde and butyraldehyde, the
order of the reaction n 2 with rate constants (k) is 6.41 10 3
L mol 1 min 1, 1.76 10 2 L mol 1 min 1 and 6.75 10 2 L mol 1
min 1, respectively (see Supplementary data for details).
CHO
Yieldc
(%)
MeO
CHO
OMe
MeO
MeO
COOH
MeO
HO
CHO
HO
COOH
87
CHO
COOH
89
89
5.2
88
10
Cl
CHO
Cl
COOH
Cl
CHO
Cl
COOH
Cl
Cl
COOH
CHO
11
CHO
15
16
17
O 2N
Ph
40
85
41
82
10
91
90
85
90
O2N
O2N
14
84
COOH
12
13
42
NO 2
NO 2
CHO
CHO
CHO
CHO
CHO
O2N
O
Ph
COOH
COOH
COOH
COOH
COOH
a
Reactions performed in EtOAc with 10 mol % Bi2O3 and 5 equiv 70% t-BuOOH
under reux conditions.
b
Monitored using TLC until all the aldehyde was found consumed.
c
Isolated yield after column chromatography of the crude.
P. Malik, D. Chakraborty / Tetrahedron Letters 51 (2010) 35213523
References and notes
4-OMeC6H4COOH
Concentration (mol/L)
0.4
4-OMeC6H4CHO
0.3
0.2
0.1
0.0
0
20
40
60
80
3523
100
120
Time (min)
Figure 1. Concentration versus time in the oxidation of 4-methoxybenzaldehyde
with 10 mol % Bi2O3 and 5 equiv 70% t-BuOOH (water) in EtOAc under reux
conditions.
3. Conclusions
In summary, we have developed a simple, efcient, chemoselective and inexpensive catalytic method for the oxidation of aldehydes to carboxylic acids with a table top reagent such as
Bi2O3.14 It is noteworthy to mention that this method does not
use ligands and other additives.
Acknowledgements
This work was supported by Department of Science and
Technology and Council of Scientic and Industrial Research,
New Delhi. The services from the NMR facility purchased under
the FIST program, sponsored by the Department of Science and
Technology, New Delhi, are gratefully acknowledged.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.04.101.
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14. Typical experimental procedure: To a stirred suspension of Bi2O3 (68 mg,
0.10 mmol) and aldehyde (1 mmol) in 2.5 mL EtOAc was added 70% t-BuOOH
(water) (0.90 mL, 5 mmol). The reaction mixture was heated to reux. The
progress of the reaction was monitored using TLC until all aldehyde was found
consumed. The crude product was treated with saturated NaHCO3 solution.
This was extracted with ethyl acetate. Finally, the aqueous layer was acidied
using 2 N HCl and extracted with ethyl acetate. The organic layer was
concentrated and subjected to column chromatography. The spectral data of
the various carboxylic acids were found to be satisfactory in accordance with
the literature (see Supplementary data for details).
