Selective solvent-free oxidation of alcohols with potassium dichromate

Article abstract of DOI:10.1016/S0040-4039(02)02234-7

Selective oxidation of primary alcohols to the corresponding aldehydes by potassium dichromate at room temperature under solvent-free conditions are described. This new procedure can also oxidise secondary alcohols.

Full text of DOI:10.1016/S0040-4039(02)02234-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 8843–8844
Selective solvent-free oxidation of alcohols with potassium
dichromate
Ji-Dong Loua,* and Zhi-Nan Xub
aInstitute of Chemical and Biological Technology, New University of Lisbon, 2780 Oeiras, Portugal
bDepartment of Chemical Engineering and Bioengineering, Zhejiang University, Hangzhou, Zhejiang 310027, China
Received 25 September 2002; revised 3 October 2002; accepted 4 October 2002
Abstract—Selective oxidation of primary alcohols to the corresponding aldehydes by potassium dichromate at room temperature
under solvent-free conditions are described. This new procedure can also oxidise secondary alcohols. © 2002 Elsevier Science Ltd.
All rights reserved.
The oxidation of primary alcohols to the corresponding
aldehydes has been accomplished with large number of
hexavalent chromium compounds.1–4
tions. Most of existing K2Cr2O7 oxidations are per-
formed at temperatures above ambient.
We now report here that the oxidation of primary
alcohols with K2Cr2O7 at room temperature under sol-
vent-free conditions is a new system that offers a very
simple and efficient selective oxidation method for the
preparation of aldehydes. In the present method,
overoxidation and formation of an ester can be pre-
vented by the use of solvent-free conditions under
which the aldehyde produced is stable. On the other
hand, in comparison with the methods described above,
the advantages of the present method are that it does
not require the use of phase-transfer catalysis, the
preparation of supported reagents, the usage of expen-
sive polar aprotic solvents, and therefore it offers spe-
cial promise for the preparation of aldehydes by the
oxidation of alcohols into aldehydes. In addition, the
present method can also successfully oxidise secondary
alcohols to the corresponding ketones.
Potassium dichromate (K2Cr2O7), a readily available
and inexpensive reagent, has for some times been used
as an oxidant in this reaction. Unfortunately, the tradi-
tional K2Cr2O7 oxidation methods in this transforma-
tion are limited by performing in acidic aqueous
solution. Under the reaction condition, the oxidising
reagent can further oxidise the aldehyde to the acid,
since in an aqueous medium, the aldehyde product can
be hydrated to the geminal diol, which is further oxi-
dised. Therefore, to isolate the aldehyde, it should be
continuously removed as it is formed, usually by distil-
lation. This technique can be used successfully only
with relatively volatile aldehydes, and yields are moder-
ate. In addition, under acidic medium, the aldehyde
product and unreacted alcohol react to give a hemiac-
etal that is rapidly oxidised to an ester. Since oxidation
with K2Cr2O7 under anhydrous neutral media would
appear to be a way to circumvent the overoxidation
and the ester formation, in recent years the traditional
K2Cr2O7 oxidation has been modified. Improved
K2Cr2O7 oxidation methods, such as oxidation under
phase-transfer catalysis,5 oxidation with supported
reagents,6,7 and oxidation in polar aprotic media,8,9
have been developed that allow the reaction to be
conducted in organic media and under neutral condi-
In the present experiments, commercial K2Cr2O7 with
grinding to a fine power and a 1:1 molar ratio of
K2Cr2O7:substrate is employed. First, the oxidant is
carefully added to the substrate and the mixture is
stirred magnetically at room temperature until TLC
analysis indicates a completed reaction. In general, the
oxidations are complete within 5 h. The reaction mix-
ture is then worked up by dilution with
dichloromethane or diethyl ether and filtration. Distilla-
tion of the solvent gives a product that is of acceptable
purity for most purposes. For solid substrates, the best
method for doing this oxidation is first to dissolve them
with a minimum amount of dichloromethane or diethyl
Keywords: selective oxidation; alcohols; aldehydes; ketones; potas-
sium dichromate; solvent-free.
* Corresponding
lou@itqb.unl.pt
author.
Fax:
+351-21-4403609;
e-mail:
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02234-7

8844
J.-D. Lou, Z.-N. Xu / Tetrahedron Letters 43 (2002) 8843–8844
Table 1. Solvent-free potassium dichromate oxidations
ether at room temperature and then to perform the
reaction. The results, which are shown in Table 1, show
that the method is generally applicable to a wide range
of alcohols, and gives the corresponding products in
good yields.
and subsequent volumes in the series.
2. Freeman, F. In Organic Synthesis by Oxidation with
Metal Compounds; Mijs, W. J.; de Jonge, C. R. H. I.,
Eds.; Plenum Press: New York, 1986; pp. 68–81.
3. Haines, A. H. Methods for the Oxidation of Organic
Compounds, Alcohols, Alcohol Derivatives, Alkyl halides,
Nitroalkanes, Alkyl Azides, Carbonyl Compounds,
Hydroxyarenes and Aminoarenes; Academic Press: Lon-
don, 1988; pp. 17–41.
4. Ley, S. V.; Madin, A. In Comprehensive Organic Synthe-
sis; Trost, B. M.; Fleming, I., Eds.; Pergamon Press:
London, 1991; Vol. 7, pp. 251–289.
5. Hutchins, R. O.; Natale, N. R.; Cook, W. J.; Ohr, J.
Tetrahedron Lett. 1977, 18, 4167.
6. Santaniello, E.; Ferraboschi, P. Nouv. J. Chim. 1980, 4,
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8. Santaniello, E.; Ferraboschi, P.; Sozzani, P. Synthesis
1980, 646.
In conclusion, this solvent-free oxidation of alcohols
using K2Cr2O7 as an oxidant at room temperature is a
new and efficient method for the preparation of the
corresponding aldehydes and ketones.
Caution: K2Cr2O7 is a toxic agent. All chromium(VI)
reagents must be handled with care. The mutagenicity
of chromium(VI) compounds is well documented.10 We
have worked with quantities of 0.5–1 g of alcohols. Any
mixture of potassium dichromate and an organic sub-
stance must be viewed as potentially unstable. Special
care must always be exercised in adding K2Cr2O7 to
organic media.
9. Lou, J.-D.; Lu, L.-H.; Liu, W. Synth. Commun. 1997, 27,
3701.
References
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