Mohr's salt catalyzed oxidation of aldehydes with t-BuOOH

Article abstract of DOI:10.1002/aoc.1787

Various aromatic, aliphatic and conjugated aldehydes were transformed to the corresponding carboxylic acids with 70% t-BuOOH solution (water) in the presence of catalytic amounts (10 mol%) of Mohr's salt. This method possesses functional group compatibility, does not involve cumbersome work-up, exhibits chemoselectivity since other functional groups remain intact and proceeds under mild conditions. The resulting products are obtained in good yields within reasonable times. Copyright

Full text of DOI:10.1002/aoc.1787

Full Paper
Received: 20 August 2010
Revised: 17 December 2010
Accepted: 18 February 2011
Published online in Wiley Online Library: 5 May 2011
(wileyonlinelibrary.com) DOI 10.1002/aoc.1787
Mohr’s salt catalyzed oxidation of aldehydes
with t-BuOOH
Debashis Chakraborty , Chandrima Majumder and Payal Malik
∗
†
Various aromatic, aliphatic and conjugated aldehydes were transformed to the corresponding carboxylic acids with 70%
t-BuOOH solution (water) in the presence of catalytic amounts (10 mol%) of Mohr’s salt. This method possesses functional group
compatibility, does not involve cumbersome work-up, exhibits chemoselectivity since other functional groups remain intact
and proceeds under mild conditions. The resulting products are obtained in good yields within reasonable times. Copyright ꢀc
2
011 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: oxidation; aldehyde; Mohr’s salt; t-BuOOH; kinetics
Introduction
in the presence of different solvents, oxidants and 10 mol% Fe(II)
salts (Table 1).
The oxidation of aldehydes is of interest owing to their potential in
organic synthesis and industrial manufacturing, and is recognized
as one of the fundamental reactions.
The conversion of 2-methoxybenzaldehyde to 2-methoxy-
◦
benzoic acid is facile in dimethyl sulfoxide at 80 C in the
[
1–5]
The most popular
presence of 10 mol% Mohr’s salt and 5 equiv. 70% t-BuOOH
and widely used reagent for such a transformation is Jones’s
(water) as oxidant (Table 1, entry 6). Large-scale reactions must
reagent.[
6–11]
However, the reaction is stoichiometric and is
be avoided since t-BuOOH may spontaneously detonate at high
performed under highly acidic conditions. Substrates with acid-
sensitive functionalities may not tolerate such acidity. In addition,
the generation of Cr-based side products may be considered as a
[37,38]
temperatures.
At room temperature, the reaction is sluggish.
Oxidation with t-BuOOH (water) alone in DMSO was found to
be negligible (<5%). In the presence of 5 mol% Mohr’s salt and 5
equiv. 70% t-BuOOH (water) as the oxidant in DMSO, the reaction
required 5 h for completion with 80% isolated yield of the product.
In DMSO, with 10 mol% Mohr’s salt and 5 equiv. 70% t-BuOOH
[
12]
potential environmental hazard.
Other reagents that have been used successfully include
[
13]
[14]
oxone, calcium hypochlorite and 2-hydroperoxyhexafluoro-
-propanol.[15] Catalytic methods using metals have been devel-
2
(water) as the oxidant, the reaction went to completion in a shorter
oped using oxidation reactions.
Interesting methodologies for metal-mediated transformation
time. With 5 equiv. 5 M t-BuOOH (decane), the reaction was found
to complete in 3.5 h with 85% isolated yield. We did not use this
reagent further since the results are not as good as for the water
solution and it is much more expensive.
ofthealdehydefunctionalitytocarboxylicacidhavebeenreported
recently.[
16–27]
Theabovereagentsandthemethodsinvolvedhave
oneormorelimitationsthatincludetheuseof superstoichiometric
amounts of expensive compounds and employment of highly
basic or acidic reaction conditions. The search for catalytic
processes involving environmentally benign reagents remains
an attractive avenue in this area.
The reaction took 9 h when performed with 5 equiv. 30% H2O2
in DMSO and yielded 87% of product. The other solvents used for
optimization (Table 1, entries 1–9) were used under conditions
of reflux. DMSO yielded the best results (Table 1, entry 6). The
other Fe(II) salts (Table 1, entries 10 and 11) were found to be
inferior. Having determined the correct conditions for oxidation,
we continued our studies 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 aldehydes
were converted to the corresponding carboxylic acids in good
isolated yields in reasonable time (Table 2).
It is reported that, in the presence of O2, Cu(I) salts catalyze
[
28]
the oxidation of alcohols to aldehydes and ketones. Our recent
results highlight the oxidation of aldehydes to carboxylic acids
in the presence of catalytic amounts of AgNO3 and Bi2O3.[
Our continued interest in studying catalytically active and envi-
ronmentally benign processes led us to investigate the capability
of Fe(II) reagents to oxidized Mohr’s salt [(NH4)2(Fe)(SO4)2·6H2O];
a popular redox indicator was used as a stable Fe(II) source.
29,30]
∗
Correspondence to: Debashis Chakraborty, Department of Chemistry, Indian
Institute of Technology Madras, Chennai-600036, Tamil Nadu, India.
E-mail: dchakraborty@iitm.ac.in
Results and Discussion
Oxidation of Aldehydes
†
Summer intern from St. Stephens College, Delhi, India.
Initial attempts to optimize the reaction conditions for the
oxidation of aldehydes to the corresponding carboxylic acids
were done using 2-methoxybenzaldehyde as a suitable substrate
Department of Chemistry, Indian Institute of Technology Madras, Chennai-
600036, Tamil Nadu, India
Appl. Organometal. Chem. 2011, 25, 487–490
Copyright ꢀc 2011 John Wiley & Sons, Ltd.

D. Chakraborty, C. Majumder and P. Malik
Table 1. Optimization of the reaction conditions for the conversion of 2-methoxybenzaldehyde to 2-methoxybenzoic acid with different solvents,
5
equiv. 70% t-BuOOH (water) and 10 mol% Fe(II) salts
O
O
OH
Fe(II) salt, t-BuOOH
H
solvent
OMe
OMe
Entry
Catalyst
Solvent
Time (h)a
Yield (%)b
1
2
3
4
5
6
7
8
9
Mohr’s salt
Mohr’s salt
Mohr’s salt
Mohr’s salt
Mohr’s salt
Mohr’s salt
Mohr’s salt
Mohr’s salt
Mohr’s salt
FeCl2
EtOAc
MeCN
toluene
CH2Cl2
DMF
4
93
89
87
80
91
95
85
80
82
71
80
4
15
22
10
3
DMSO
THF
19
17
15
30
27
EtOH
CH3NO2
DMSO
DMSO
1
1
0
1
FeBr2
a
◦
Reactions performed at 80 C and monitored using TLC until all the aldehyde was found to have been consumed.
Isolated yield after column chromatography of the crude product with 2% standard deviation.
b
−
3
−1
−1
−2
−1
−1
It is pertinent to mention here that mild halogenic oxidants like
10 mol min and 6.9 × 10 L mol min , respectively,
hypochlorites,[
14,31,32]
chlorites
[33,34]
and NBS
[35,36]
are not suitable
(see Supporting Information for details).
for substrates with electron-rich aromatic rings, olefinic bonds and
secondary hydroxyl groups, since these functional groups also
react. Substitutions at different positions on the phenyl ring do Conclusions
not hinder the reaction, although the reaction time is affected.
Insummary,wehavedevelopedasimple,efficient,chemoselective
Our catalytic system is mild and shows sufficient selectivity
in carrying out the expected oxidation without affecting other
functionalities like phenol and amine (Table 2, entries 7 and 8).
Oxidation of α- and β-unsaturated derivatives (Table 2, entry 15)
resulted in the formation of the expected acid in very good yield.
and inexpensive catalytic method for the oxidation of aldehydes
to carboxylic acids with a common laboratory reagent such as
Mohr’s salt. It is noteworthy that this method does not use ligands
and other additives.
Kinetic Studies
Experimental Section
Kinetic studies of the oxidation with 3,4-dimethoxybenzaldehyde,
General Reagents and Equipment
2-nitrobenzaldehyde and crotonaldehyde were carried out next.
High-pressure liquid chromatography (HPLC) was used to deter-
mine the various starting materials and products present as a
function of time. The concentrations of reactant and product for
the oxidation of 3,4-dimethoxybenzaldehyde are shown in Fig. 1.
The concentration of the aldehyde decreased while that of the
carboxylic acid increased.
All the substrates used in this study, along with t-BuOOH, were
purchased from Aldrich and used as received. The solvents used
were purchased from Ranchem, India and purified using standard
1
13
methods. H and C spectra were recorded with a Bruker Avance
00 instrument. Chemical shifts (in ppm) were referenced to
4
residual solvent resonances and are reported as parts per million
relative to SiMe4. A 0.5 ml aliquot of CDCl3 was used for every
NMR spectral measurement. HPLC analysis was carried out using
a Waters HPLC instrument fitted with a Waters 515 pump and a
Waters 2487 dual λ absorbance detector. Suitable mehtods were
developed with different proportions of MeCN and alcohol.
We calculated the rate of such reactions. As an example, let
us consider the conversion of 3,4-dimethoxybenzaldehyde to 3,4-
dimethoxybenzoic acid. The Van’t Hoff differential method was
used to determine the order (n) and rate constant (k) (Fig. 2). From
Fig. 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 3,4-dimethoxybenzaldehyde.
With these data, log10(rate) vs 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, respectively. From Fig. 2, it
is clear that this reaction proceeds with second-order kinetics (n =
Typical Procedure for the Oxidation of Aldehyde to Carboxylic
Acid
Toastirredsolutionof[(NH4)2(Fe)(SO4)2·6H2O](39 mg,0.10 mmol)
and aldehyde (1 mmol) in 2.5 ml DMSO was added 70% t-BuOOH
(water; 0.90 ml, 5 mmol). The reaction mixture was heated to
−
1
−1
2
.17) and the rate constant k = 0.2 L mol min . For the other
◦
substrates, namely 2-nitrobenzaldehyde and crotonaldehyde, the
80 C. The progress of the reaction was monitored using TLC
order of the reaction n ≈ 2 with rate constants (k) 6.7 ×
until all aldehyde was found to have been consumed. The crude
wileyonlinelibrary.com/journal/aoc
Copyright ꢀc 2011 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 487–490

Mohr’s salt catalyzed oxidation of aldehydes with t-BuOOH
Table 2. Mohr’s salt-catalyzed oxidation of aldehydes to carboxylic acidsa
O
10 mol% Mohr's salt
equiv. 70% t-BuOOH (water)
O
5
R
H
R
OH
DMSO
Entry
1
Aldehyde
Acid
Time (h)b
4
Yield (%)c
93
CHO
COOH
2
3
3.5
3
90
95
MeO
CHO
MeO
COOH
CHO
OMe
COOH
OMe
4
5
6
5
90
92
91
CHO
COOH
MeO
MeO
MeO
MeO
MeO
MeO
2
CHO
COOH
1.5
OMe
OMe
MeO
MeO
CHO
MeO
MeO
COOH
7
8
9
4.5
6.5
5
90
88
91
93
HO
N
CHO
CHO
CHO
CHO
HO
N
COOH
COOH
COOH
COOH
Cl
Cl
Cl
Cl
10
11
12
7.5
Cl
Cl
3.5
5
89
90
CHO
NO2
COOH
NO2
CHO
COOH
O N
O N
2
2
1
3
4
6.5
3.5
90
92
O N
CHO
O N
2
COOH
2
1
O
CHO
O
COOH
15
3
93
CHO
COOH
Ph
Ph
1
6
7
CHO
CHO
COOH
COOH
7
4
87
90
1
a
◦
Reactions performed in DMSO with 10 mol% Mohr’s salt and 5 equiv. 70% t-BuOOH at 80 C.
Monitored using TLC until all the aldehyde was found to have been consumed. c Isolated yield after column chromatography of the crude with 2%
b
standard deviation.
Appl. Organometal. Chem. 2011, 25, 487–490
Copyright ꢀc 2011 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

D. Chakraborty, C. Majumder and P. Malik
3
,4-(OMe) C H COOH
the FIST program, sponsored by the Department of Science and
Technology, New Delhi is gratefully acknowledged.
2
6 3
3
,4-(OMe) C H CHO
2
6 3
0
0
0
0
0
.4
.3
.2
.1
.0
Supporting information
Supporting information may be found in the online version of this
article.
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[
This work was supported by Department of Science and
Technology and Council of Scientific and Industrial Research,
New Delhi. The services from the NMR facility purchased under
5
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Copyright ꢀc 2011 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 487–490