Cobalt(II)-catalyzed oxidation of alcohols into carboxylic acids and ketones with hydrogen peroxide

Article abstract of DOI:10.1016/S0040-4039(03)01479-5

The oxidation of aliphatic and aromatic alcohols into the corresponding carboxylic acid analogues and ketones has been carried out using 30% H₂O₂ and cobalt(II) complex 1 in good to high yields. The reaction is safe, clean and functions in the absence of additives.

Full text of DOI:10.1016/S0040-4039(03)01479-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 6033–6035
Cobalt(II)-catalyzed oxidation of alcohols into carboxylic acids
and ketones with hydrogen peroxide
Subhabrata Das and T. Punniyamurthy*
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781039, India
Received 12 May 2003; revised 2 June 2003; accepted 13 June 2003
Abstract—The oxidation of aliphatic and aromatic alcohols into the corresponding carboxylic acid analogues and ketones has
been carried out using 30% H2O2 and cobalt(II) complex 1 in good to high yields. The reaction is safe, clean and functions in the
absence of additives.
© 2003 Elsevier Ltd. All rights reserved.
The selective oxidation of alcohols into carbonyl com-
pounds is one of the most important fundamental
reactions in organic chemistry both at the laboratory
and industrial level and numerous methods have been
developed using a variety of reagents for this purpose.1
Recent demand for eco-friendly chemical processes has
encouraged the development of several clean reactions2
and awaits further development of high yielding, clean
and economical methods for the oxidation of alcohols.
Herein, we wish to report the oxidation of aliphatic and
aromatic alcohols into carboxylic acids and ketones in
good to high yields with cobalt(II) complex3 1 in the
presence of H2O2 under atmospheric oxygen (Scheme
1).
Reaction of ethylenediamine with 2 equiv. of salicyl-
aldehyde followed by NaBH4 reduction in methanol at
ambient temperature provided salen–H4 which was fur-
ther reacted with Co(OAc)2 under a nitrogen atmo-
sphere to afford complex 1 as a green powder.3 In the
presence of a catalytic amount of 1, the oxidation of
4-chlorobenzyl alcohol was examined with H2O2 in
acetonitrile. We were pleased to find that the alcohol
was oxidized to a 1:6 mixture of the corresponding
aldehyde and carboxylic acid in 15% yield (17 h).
Alternatively, the reaction could be carried out under
heating (80°C) within 7 h to afford the carboxylic acid
in 74% yield (Table 1, entry 3 and Scheme 2).4
A
controlled experiment in the absence of 1 using the
same reaction conditions gave no reaction and the
starting alcohol was recovered. Co(OAc)2 and CoIIsalen
were also examined as the catalysts for this reaction,
however, these systems were less effective compared to
the protocol using 1. The reaction with CoIIsalen pro-
vided a 1:17 mixture of the corresponding aldehyde and
carboxylic acid in a 34% yield, while the Co(OAc)2
catalyzed process afforded only the aldehyde in 9%
yield (Scheme 2).
The oxidations of other alcohols were then examined
using the optimized reaction conditions (Table 1)4 and
aromatic alcohols, 4-bromo-, 2-nitro-, 3-nitro-, 4-nitro-,
4-methoxy- and 3,4,5-trimethoxybenzyl alcohol,
phenylethanol and diphenylmethanol, were oxidized to
the corresponding carboxylic acids and ketones in high
yields. The progress of the oxidation of phenylethanol
is shown in Figure 1. It is noteworthy to mention that
no oxidation was observed in the aromatic ring of
benzylic substrates. Similarly, aliphatic alcohols, decyl-
and cetyl alcohols, 2-methylcyclopentanol and cylohex-
Scheme 1.
Keywords: oxidation; alcohols; catalyst; hydrogen peroxide; cobalt(II)
complex.
* Corresponding author. Fax: +91-361-3690762; e-mail: tpunni@
iitg.ernet.in
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01479-5

6034
S. Das, T. Punniyamurthy / Tetrahedron Letters 44 (2003) 6033–6035
Table 1. Oxidation of alcohols to carboxylic acids and ketones with catalyst 1 and 30% H2O2

S. Das, T. Punniyamurthy / Tetrahedron Letters 44 (2003) 6033–6035
6035
trial Research (Sanction No. 01(1804)/02/EMR-II),
New Delhi.
References
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3. The reaction of salicylaldehyde (122 mg, 1 mmol) with
ethylenediamine (30 mg, 0.5 mmol) in methanol (5 ml)
afforded salen-H2 as a lemon yellow powder in 81% (217
mg), which was further reacted with NaBH4 (32 mg) in
methanol (5 ml) for 2 h at ambient temperature. Removal
of the solvent on a rotary evaporator followed by treat-
ment with water afforded salen-H4 as a colorless powder
in 72% (162 mg) yield. The salen-H4 (272 mg, 1 mmol) was
then reacted with Co(OAc)2 (195 mg, 1.1 mmol) in
methanol (10 ml) at 50°C for 2.5 h. Evaporation of the
solvent on a rotary evaporator gave a powder which was
passed through a short pad of silica gel using a mixture of
EtOAc and MeOH (15:5) to afford 1 as a green powder
(83%, 300 mg): UV–vis (CH3CN): umax 265, 387 nm; FAB+
MS: 329 (M+).
Scheme 2.
Figure 1. Progress of the oxidation of phenylethanol to ace-
tophenone.
Scheme 3.
anol, underwent oxidation to the corresponding car-
boxylic acids and ketones in moderate to good yields
(entries 9–12). The oxidation of cinnamyl alcohol was
also examined; however, it mainly afforded benzoic
acid due to cleavage of the carbonꢀcarbon double bond
(Scheme 3).
In conclusion, the oxidation of aromatic and aliphatic
alcohols to carboxylic acids and ketones has been
described using cobalt(II) complex 1 in the presence of
H2O2 in high yields. The reaction is simple, clean and
generates water as the only by-product.
4. A solution of the alcohol (3 mmol), complex 1 (5 mol%)
and 30% H2O2 (6.8 ml, 60 mmol) was stirred in acetonitrile
(5 ml) at ca. 80°C under atmospheric oxygen for the
appropriate time (see Table 1). The reaction mixture was
then cooled to ambient temperature and dimethyl sulfide
(50 ml) was added. After further stirring for 0.5 h at
ambient temperature, the aqueous acetonitrile was
removed by rotary evaporation under reduced pressure
and the residue was passed through a short pad of silica
gel using ethyl acetate and hexane to afford the analyti-
cally pure carboxylic acid.
Acknowledgements
This work was supported by the Department of Science
and Technology (Sanction No. SR/S1/OC-092002),
New Delhi and by the Council of Scientific and Indus-