Wet carbon-based solid acid/potassium bromate as an efficient heterogeneous reagent for oxidation of alcohols under mild conditions

Article abstract of DOI:10.1080/00397910701861230

Wet carbon-based solid acid and potassium bromate were used as new reagent for oxidation of alcohols to their corresponding aldehyde or ketone derivatives in dichloromethane with good yields. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910701861230

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Wet Carbon‐Based Solid Acid/Potassium
Bromate as an Efficient Heterogeneous
Reagent for Oxidation of Alcohols under Mild
Conditions
Abbas Zali a & Arash Shokrolahi a
a Chemistry Department , Malek‐ashtar University of Technology ,
Shahin‐shahr, Iran
Published online: 12 Mar 2008.
To cite this article: Abbas Zali & Arash Shokrolahi (2008) Wet Carbon‐Based Solid Acid/Potassium
Bromate as an Efficient Heterogeneous Reagent for Oxidation of Alcohols under Mild Conditions, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 38:7,
1064-1069, DOI: 10.1080/00397910701861230
To link to this article: http://dx.doi.org/10.1080/00397910701861230
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Synthetic Communicationsw, 38: 1064–1069, 2008
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701861230
Wet Carbon-Based Solid Acid/Potassium
Bromate as an Efficient Heterogeneous
Reagent for Oxidation of Alcohols under
Mild Conditions
Abbas Zali and Arash Shokrolahi
Chemistry Department, Malek-ashtar University of Technology,
Shahin-shahr, Iran
Abstract: Wet carbon-based solid acid and potassium bromate were used as new
reagent for oxidation of alcohols to their corresponding aldehyde or ketone derivatives
in dichloromethane with good yields.
Keywords: Alcohols, carbon-based solid acid, oxidation, potassium bromate
INTRODUCTION
Replacement of conventional, toxic, polluting Bronsted and Lewis acid catalysts
with environmentally benign, reusable, solid heterogeneous catalysts such as
acidic zeolites, sulphated zerconia, and acidic groups containing resins is an
area of current interest. The use of solid acid catalysts instead of liquids has
many advantages such as reduced equipment corrosion, ease of product separ-
ation, recycling of the catalyst, and environmental acceptability.[1–5]
An ideal solid material for the applications considered here should have
high stability and numerous strong protonic acid sites. It is essential for the
solid acid to maintain strong acidity even in water because water participates
in many industrially important acid-catalyzed reactions. While organic acid/
inorganic solid oxide hybrids and strong acidic cation-exchangeable resins,
Received in the U.K. April 4, 2007
Address correspondence to Abbas Zali, Chemistry Department, Malek-ashtar
University of Technology, Shahin-shahr, P. O. Box 83145-115, Iran. E-mail:
abbaszali@gmail.com or abaszali@mut-es.ac.ir
1064

Wet Carbon-Based Solid Acid/Potassium Bromate
1065
including perfluorosulfonated ionomers (for example, nafion), have been
studied extensively as promising approaches for the construction of desired
solid acids or proton conductors,[6] such materials are expensive, and the
acid activities are still much lower than those of sulfuric acid.[7]
Michikazu Hara and coworkers reported the synthesis of a carbon-based
solid acid with a high density of sulfonic acid groups (SO3H). Such carbon-
based solid acids can be readily prepared by heating aromatic compounds
such as naphthalene in sulfuric acid at 473–573 K. In this synthesis, the
sulfonation of the aromatic compounds is the first stage of the reaction.
The resulting sulfonated aromatic compounds are incompletely carbonized,
which results in the formation of a solid with a nominal sample composition
of CH0.35O0.35S0.14. The resulting black powder is insoluble in the solvents
such as water, methanol, ethanol, benzene, and hexane even at boiling
temperatures.[8]
Metal oxides or their mineral salts were used for oxidation of organic
functionalities.[9] Classic reagents of this type are manganese dioxide,
potassium permanganate, chromium trioxide, potassium chromate, and
potassium dichromate.[10] These are all frequently used reagents, in the labora-
tory or in industry, and yet they are beset with multiple liabilities. For satisfac-
tory and reproducible results, these oxidants demand rigorous control of the
experimental conditions. The other drawbacks of such oxidants and their
use in multistage organic synthesis, in spite of their power, are their lack of
selectivity, strong protic and aqueous conditions, low yields of the products,
and tedious workup. For instance, overoxidation of aldehydes to carboxylic
acids is often an unavoidable side reaction.
Furthermore, the elevated reflux temperatures required by some oxidation
procedures favor inopportune secondary reactions. Likewise, the presence of
strong acids or bases, which are required adjuncts as catalysts for some
reactions, often leads to detrimental side reactions. As an example, the
oxidation of primary alcohols to aldehydes by a chromium(VI) salt in
sulfuric acid is often accompanied by the formation of an hemiacetal
between the resulting aldehyde and the alcohol substrate, followed by the
ready oxidation of this intermediate to an ester.[11]
Therefore, we decided to develop a new reagent or reagent system to
overcome these limitations. On the other hand, any reduction in the amount
of sulfuric acid needed and/or any simplification in handling procedures is
required for risk reduction, economic advantage, and environment protec-
tion.[12] In addition, there is current research and general interest in hetero-
geneous systems because of the importance such systems have in industry
and in developing technologies.[13] Carbon-based solid acids are excellent
candidates for replacing sulfuric acid in organic reactions[8,14] because the
heterogeneous reagent systems have many advantages such as simple exper-
imental procedures, mild reaction conditions, and minimization of chemical
wastes as compared to their liquid-phase counterparts.[15,16] The goal of this
work was two-fold: (a) to overcome the limitations and drawbacks of the

1066
A. Zali and A. Shokrolahi
reported methods, such as tedious workup, strongly acidic media, and safety
problems (storage, handling, use, and presence of toxic transition-metal
cations such as Crþ3, Hgþ2, and Cuþ2 within the molecular structure of the
reagents), and to use a novel combination of reagents for (b) high-yielding
production of carbonyl compounds. These goals encouraged us to seek a com-
pletely heterogeneous system for the oxidation of various alcohols, and we
have investigated a number of different reaction conditions based upon the
in situ generation of HBrO3 by wet carbon-based solid acid and potassium
bromate.
In this article we report a simple and convenient method for the effective
conversion of alcohols to their corresponding aldehyde or ketone derivatives
under mild and heterogeneous conditions.
Different types of alcohols were subjected to oxidation reactions in the
presence of wet carbon-based solid acid and KBrO3 in dichloromethane.
The oxidation reactions were performed under mild and completely hetero-
geneous conditions with good to excellent yields (Table 1). The oxidation
of primary alcohols gives only aldehydes (Scheme 1).
The present oxidation reaction can be readily carried out by placing wet
carbon-based solid acid and KBrO3 in dichloromethane in a reaction vessel
and efficiently stirring the resulting heterogeneous mixture. The highly pure
aldehyde or ketone derivative can be obtained by simple filtration and evap-
oration of the solvent. The used substrates, their products, reaction times,
and yield of reactions are tabulated in the Table 1.
Table 1. Oxidation of various alcohols to aldehydes and ketones by wet carbon-based
solid acid and KBrO3 in CH2Cl2 under heterogeneous conditions
Time Yielda
Conditions (min) (%)
Entry
Substrate
Product
1
2
3
Benzyl alcohol
Cyclohexanol
m-Methoxybenzyl
alcohol
Benzaldehyde
Cyclohexanone
m-Methoxybenzaldehyde Reflux
Reflux
Rt
45
20
80
90
95
65
4
5
Hydroquinone
1-Hexanol
Benzoin
p-Benzoquinone
Hexanal
Benzil
Rt
20
45
80
26
45
50
75
45
97
82
70
95
87
90
73
90
0
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
6
7
p-Chlorobenzylalcohol p-Chlorobenzaldehyde
2-Pentanone
Butanal
8
2-Pentanol
1-Butanol
9
10
11
Benzhydrol
Benzyl alcohol þ
cyclohexanol
Benophenone
Benzaldehyde þ
cyclohexanone
aIsolated yields.

Wet Carbon-Based Solid Acid/Potassium Bromate
1067
Scheme 1.
To show the chemoselectivity of the method, we have carried out the
successful oxidation of benzyl alcohol in the presence of cyclohexanol
(entry 11).
In conclusion, the low cost and the availability of the reagents, easy and
clean workup, and good to high yields make this method attractive for large-
scale operations. This procedure is very simple, and contamination by side
products from overoxidation is avoided. We believe that the present method-
ology will be an important addition to existing methodologies (Figs. 1 and 2).
EXPERIMENTAL
Reagents
Chemicals were purchased from Fluka, Merck, and Aldrich chemical
companies. The products were characterized by comparison of their spectral
1
(IR, H NMR), thin-layer chromatographic (TLC), and physical data with
the authentic samples. Carbon-based solid acid was prepared according to
the reported procedure.[8]
Figure 1. The proton NMR of the crude reaction mixture (Entry1).

1068
A. Zali and A. Shokrolahi
Figure 2. The proton NMR of the crude reaction mixture (Entry 2).
Typical Procedure
A mixture of substrate (2 mmol), potassium bromate (0.167 g, 1 mmol), wet
carbon-based solid acid (70% w/w, 0.26 g), and 5 mL dichloromethane was
stirred at specified times and conditions (Table 1). The reaction was
monitored by TLC. The mixture was filtered and washed with Na2S2O3
solution (10% 2 ꢀ 20 mL); the organic phase was collected. Anhydrous
Na2SO4 (2 g) was added to the solution and filtered after 15 min. Dichloro-
methane was removed under reduced pressure, and a suitable method of puri-
fication was employed to afford the pure product. The yield of obtained
products was in the range of 63–95%.
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