Ammonium chlorochromate adsorbed on montmorillonite K-10: A new reagent for the oxidation of alcohols to the corresponding carbonyl compounds under non-aqueous conditions

Article abstract of DOI:10.1007/PL00010237

A variety of alcohols were oxidized to the corresponding carbonyl compounds by reaction of ammonium chlorochromate adsorbed on montmorillonite K-10 in dichloromethane at room temperature in excellent yields.

Full text of DOI:10.1007/PL00010237

Monatshefte f uÈ r Chemie 130, 581±583 (1999)
Ammonium Chlorochromate Adsorbed
on Montmorillonite K-10: A New Reagent
for the Oxidation of Alcohols to the
Corresponding Carbonyl Compounds
under Non-Aqueous Conditions
Ã
Majid M. Heravi , Reza Kiakojoori, and Kourosh Tabar-Hydar
Chemistry and Chemical Engineering Research Center of Iran, Tehran, Iran
Summary. A variety of alcohols were oxidized to the corresponding carbonyl compounds by
reaction of ammonium chlorochromate adsorbed on montmorillonite K-10 in dichloromethane at
room temperature in excellent yields.
Keywords. Ammonium chlorochromate; Montmorillonite; Oxidation; Alcohol; Carbonyl com-
pounds.
An Montmorillonit-K-10 adsorbiertes Ammoniumchlorochromat: Ein neues Reagens zur
Oxidation von Alkoholen zu Carbonylverbindungen unter nichtw aÈ ûrigen Bedingungen
Zusammenfassung. Eine Reihe von Alkoholen wurden mittels an Montmorillonit-K-10 adsorbierten
Ammoniumchlorochromats in Dichlormethan bei Raumtemperatur in ausgezeichneten Ausbeuten zu
den entsprechenden Carbonylverbindungen oxidiert.
Introduction
The oxidation of alcohols to carbonyl compounds is an important transformation in
organic synthesis [1]. Although a large number of reagents are known in the
literature [2], there still appears a need either to improve the existing oxidation
methods [3] or to introduce novel reagents [4] to permit better selectivity under
milder conditions and with easy work-up procedures. Among the various methods
available, chromium reagent based oxidations are very popular.
The concept of utilizing reagents adsorbed on inert inorganic supports for
organic synthesis has been reported and applied especially to chromium
compounds [6]. We have reported that bis(trimethylsilyl) chromate supported on
montmorillonite K-10 [7, 8] and 3-carboxypyridinium chlorochromate supported
on alumina [9, 10] oxidize alcohols to the corresponding carbonyl compounds.
à Corresponding author

582
M. M. Heravi et al.
Analogously, ammonium chlorochromate (ACC) adsorbed on montmorillonite
K-10 is a new reagent suitable for the oxidation of alcohols.
Results and Discussion
ꢀ
Heating chromium trioxide and ammonium chloride in water at 40 C results in a
nearly homogeneous solution [11]. ACC supported on montmorillonite was
conveniently prepared simply by adding montmorillonite K-10 into such a solution.
An orange free ¯owing solid was obtained after evaporation of the solvent.
The oxidation reactions were carried out by adding 1±2 equivalents of ACC
supported on montmorillonite K-10 to the stirred solution of an alcohol in
dichloromethane at room temperature. For example, benzyl alcohol was oxidized
to benzaldehyde in almost quantitative yield. Other benzylic alcohols were
similarly oxidized within 2 h at room temperature. Primary benzylic alcohols were
oxidized to the corresponding aldehydes, and over-oxidation to the corresponding
acids was not observed even after prolonged heating with an excess of supported
ACC.
Aliphatic primary alcohols were successfully oxidized to the corresponding
aldehydes, aliphatic secondary alcohols to the corresponding ketones. The results
are summarized in Table 1. The oxidation of aryl substituted unsaturated alcohols
proceeded less satisfactorily. Cinnamaldehyde, for example, was obtained in only
61% yield along with 39% of benzaldehyde demonstrating that carbon-carbon
double bonds are prone to cleavage with the reagent.
In conclusion, ammonium chlorochromate adsorbed on montmorillonite K-10
can be easily prepared and may serve as an excellent oxidant for the oxidation of
various types of alcohols. The products can be isolated by simple ®ltration of the
reaction mixture without aqueous work-up. The high reactivity and selectivity of
the supported reagent avoids the use of a larger excess of oxidant which often
causes over-oxidation.
Table 1. Oxidation of alcohols to carbonyl compounds by means of ammonium chlorochromate
supported montmorillonite K-10
Alcohol
Reaction
time (h)
Product
Yield
(%)
Benzyl
1
Benzaldehyde
97 (92)
95 (90)
88 (78)
92 (80)
90 (80)
85 (74)
92 (85)
88 (80)
82 (75)
92 (85)
88 (81)
85 (75)
4
5
4
-Methylbenzyl
1
4-Methylbenzaldehyde
2-Methyl-5-nitrobenzylaldehyde
4-Nitrobenzaldehyde
Salicylaldehyde
Octanal
-Methyl-2-nitrobenzyl
-Nitrobenzyl
2
1.5
2
Salicyl
1
2
-Octanol
2
-Ethylhexanol
2
2-Ethylhexanal
Cyclohexanol
-Methylcyclohexanol
1
Cyclohexanone
2
1
2-Methylcyclohexanone
(� )-Menthone
(
� )-Menthol
1.5
2
Benzhydrol
Benzoin
Benzophenone
2
Benzil
Yields determined by GLC; numbers in parentheses refer to yields of isolated products

Oxidation of Alcohols by Ammonium Chlorochromate
583
Experimental
Products were characterized by GLC and by direct comparison with authentic samples. Yields are
based on GLC analysis and isolated products. Montmorillonite K-10 was dried and simultaneously
activated in a microwave oven for 5 min at 900 W prior to use.
Preparation of ACC/montmorillonite K-10
To a solution of 5 g chromium trioxide (0.05 mol) in 10 ml H O 10.70 g NH Cl (0.1 mol) are added
2
4
ꢀ
ꢀ
within 15 min at 40 C. The mixture is cooled until a yellow-orange solid forms. Reheating to 40 C
ꢀ
gives a solution. Montmorillonite K-10 (20 g) is then added to the solution with stirring at 40 C.
ꢀ
After evaporation under reduced pressure, the orange solid is dried in vacuum for 1 h at 80 C. The
reagent can be kept for at least 3 months in air at room temperature without losing its activity.
Oxidation of alcohols
The above reagent (1.7 g, 2.6 mmol) was placed in a round bottomed ¯ask and covered with 10 ml
CH Cl . To the stirred mixture, 1.6 mmol of alcohol were added. After the indicated time (Table 1),
2
2
the solid was ®ltered and washed with three 20 ml portion of CH Cl . The combined ®ltrates were
2
2
evaporated, and the residue was passed through a small bed of silica gel to afford the corresponding
carbonyl compounds after removing the solvent.
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Received June 8, 1998. Accepted (revised) August 10, 1998