656 J. CHEM. RESEARCH (S), 1998
J. Chem. Research (S),
1998, 656±657$
Ammonium Chlorochromate Adsorbed on
Montmorillonite K-10: Selective Oxidation of
Alcohols under Solvent-free Conditions$
Majid M. Heravi,* Reza Kiakojoori and K. Tabar Hydar
Chemistry and Chemical Engineering Research Center of Iran, P.O. Box 14335-186, Tehran, Iran
A simple and selective oxidation of alcohols to carbonyl compounds on clay supported ammonium chlorochromate
under solventless conditions is expedited by microwave irradiation.
Prompted by stringent environment protection laws in
recent years, there has been increasing emphasis on the use
and design of environmentally friendly solid acid±base cata-
lysts to reduce the amount of toxic waste and by-products
arising from the chemical processes.1
The oxidation of alcoholic groups to carbonyl functional-
ities continues to receive attention from chemists in search
of newer and selective methods of oxidation.2
to dryness. The reaction is conducted by mixing ®nely
ground supported reagent with neat alcohols. In the absence
of the clay the reactions are slow and considerable amounts
of alcohols are recovered unchanged at room temperature
or even upon microwave irradiation for an extended period.
As an example the reaction of 5-methyl-2-nitrobenzyl
alcohol with ammonium chlorochromate results in the
formation of only 30% of 5-methyl-2-nitrobenzaldehyde,
whereas the yield increases to almost to quantitative in the
case of the clay-supported reagent. In a few cases, the reac-
tions are completed upon simple mixing; gentle warming by
microwaves accelerates some others (Table 1). The reactions
are relatively clean with no tar formation typical of many
CrO3 reactions.
Primary alcohols were oxidized to the corresponding alde-
hydes and an overoxidation of aldehydes to the correspond-
ing acid derivative was not observed even after prolonged
irradiation and with excess of supported chromium reagent.
On the other hand the oxidations of aryl-substituted unsatu-
rated alcohols were less than satisfactory. The carbon±
carbon double bond was partially cleaved under the above
conditions. Cinnamaldehyde for example was obtained in
only 61% yield along with 39% of benzaldehyde in the
oxidation of cinnamyl alcohol.
Chromium-based reagents have extensively been used in
organic synthesis.3 The utility of chromium(VI) reagents
in oxidative transformation is compromised due to their
inherent toxicity, cumbersome preparation and potential
danger (ignition or explosion) in terms of product isolation
and waste disposal. Introduction of reagents4 on solid sup-
ports has circumvented some of these problems and pro-
vided an attractive alternative in organic synthesis in view
of the selectivity and associated ease of manipulation.
Therefore it is not surprising that a large number of chro-
mium-based oxidants impregnated on solid supports have
been explored.5 These supported reagents in solvents oxidize
primary and secondary alcohols.6
Organic solvents are not only expensive, but are
often ¯ammable, toxic and environmentally hazardous.
Consequently, there is scope for the development of rapid
and solventless methods that have manipulative advantages
over heterogeneous reactions. In continuation of our investi-
gations on organic reactions in solventless systems,7,8 we
now report a facile and selective oxidation of alcohols to
carbonyl compounds using ammonium chlorochromate
adsorbed on montmorillonite K-10 under solvent-free
conditions that is accelerated in most cases by exposure to
microwaves.
In conclusion, oxidation with ammonium chlorochromate
supported on montmorillonite K-10 under solvent-free con-
ditions is a rapid, manipulatively simple and selective proto-
col which avoids the drastic conditions usually employed.
Experimental
All products are known compounds and their physical data were
essentially identical with those of authentic samples. Microwave
irradiations were carried out in a National oven, Model 5250 at
900 W.
The reagent is easily prepared by addition of a weighed
amount of montmorillonite K-10 to a solution of am-
monium chlorochromate9 in water and rotary evaporating
Table 1 Oxidation of alcohols to carbonyl compounds using ammonium chlorochromate adsorbed on
montmorillonite K-10
Entry
Alcohol
Product
Yield (%)a
1
2
Benzyl
Benzaldehyde
92 (85)
93 (85)
90 (81)
95 (83)
95 (82)
94 (88)
90 (81)
90 (80)
88 (72)
85 (75)
85 (74)
4-Methylbenzyl
5-Methyl-2-nitrobenzyl
4-Nitrobenzyl
Salicyl
a-Phenylethyl
Benzhydrol
Benzoin
Cyclohexanol
2-Methylcyclohexanol
(±)-Menthol
4-Methylbenzaldehyde
5-Methyl-2-nitrobenzaldehyde
4-Nitrobenzaldehyde
Salicylaldehyde
Acetophenone
Benzophenone
Benzil
Cyclohexanone
2-Methylcyclohexanone
Menthone
3b
4b
5b
6
7
8b
10b
11b
12b
aYields are based on GLC analysis; figures in parentheses refer to isolated yields. bReaction completed
under microwave irradiation.
*To receive any correspondence.
Preparation of Ammonium Chlorochromate/Montmorillonite K-10.
ÐTo a solution of chromium trioxide (40 g, 0.4 mol) in water
(100 mL) was added ammonium chloride (21.4 g, 0.4 mol) within
15 min at 40 8C. The mixture was cooled until a yellow-orange solid
$This is a Short Paper as de®ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1998, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).
Heravi, Majid M.
