Barium dichromate [BaCr2O7], a mild reagent for oxidation of alcohols to their corresponding carbonyls in non-aqueous polar aprotic media

Article abstract of DOI:10.1016/j.tetlet.2004.10.010

Barium dichromate is used as a mild oxidizing agent for the selective conversion of primary and secondary alcohols to their corresponding aldehydes and ketones, respectively. Over-oxidation does not occur and primary alcohols undergo oxidation to the aldehyde. Primary and secondary benzylic alcohols are oxidized faster and more efficiently. Barium dichromate is used as a mild oxidizing agent for the selective conversion of primary and secondary alcohols to their corresponding aldehydes and ketones, respectively. Over-oxidation does not occur and primary alcohols undergo oxidation to the aldehyde. Primary and secondary benzylic alcohols are oxidized faster and more efficiently.

Full text of DOI:10.1016/j.tetlet.2004.10.010

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 8823–8824
Barium dichromate [BaCr O ], a mild reagent for oxidation of
2
7
alcohols to their corresponding carbonyls in non-aqueous polar
aprotic media
*
Enayatollah Mottaghinejad, E. Shaafi and Z. Ghasemzadeh
Department of Chemistry, Azad University of Iran-Rey Branch, PO Box 19395-5858 Tehran, Iran
Received 5 July 2004; revised 22 September 2004; accepted 1 October 2004
Abstract—Barium dichromate is used as a mild oxidizing agent for the selective conversion of primary and secondary alcohols to
their corresponding aldehydes and ketones, respectively. Over-oxidation does not occur and primary alcohols undergo oxidation to
the aldehyde. Primary and secondary benzylic alcohols are oxidized faster and more efficiently.
Ó 2004 Elsevier Ltd. All rights reserved.
Chromium-based oxidations are the most widely used of
all oxidation reactions. These methods have been devel-
oped and modified to improve selectivities. However,
these methods are associated with several drawbacks.
reported as a mild and efficient oxidizing agent for
selective deoximation of benzylic oximes to their corre-
sponding carbonyl compounds and also for oxidation
of different types of primary and secondary hydroxy
9
–11
compounds in the same solvent.
Over-oxidation, removal of the products from toxic
chromium contaminants, non-selectivity, use of strongly
acidic media and other problems are encountered with
these methods.
Oxidation reactions were attempted with aliphatic and
benzylic alcohols in boiling acetonitrile. During our
investigations we found that barium permanganate is
slightly soluble in polar aprotic solvents especially in
acetonitrile, and this results in a mild oxidation poten-
tial, different from other manganese oxyanions.
For instance, sodium dichromate in aqueous sulfuric
1
acid is limited by over-oxidation. Two-phase oxidations
have been developed for the oxidation of acid-sensitive
2
12
reactants and products. Phase transfer conditions have
been used for substances with limited solubility in aque-
We now report that barium dichromate is a suitable
oxidizing agent for the conversion of alcohols to their
corresponding carbonyl compounds in boiling acetonit-
rile or dioxane. Acetonitrile is a suitable solvent because
of its lower boiling point and higher polarity than diox-
ane. Non-polar solvents are not suitable for these con-
versions because of the insolubility of barium
dichromate in such solvents.
3
ous media. Cr(VI)/aq H SO /acetone converts satu-
2
4
4
rated primary alcohols to the corresponding acids.
Other well-known chromium-based oxidants such as
5
pyridinium chlorochromate (PCC), pyridinium fluoro-
6
chromate (PFC),
7
bipyridinium chlorochromate
8
(BPCC) and pyridinium dichromate (PDC), also have
advantages and disadvantages.
Primary alcohols undergo oxidation to produce alde-
hydes in moderate to good yields (63–85%) (Table 1).
In this work we utilized barium dichromate as an oxi-
dant in acetonitrile, a polar non-aqueous aprotic
solvent. Recently the use of barium permanganate was
Secondary alcohols gave ketones in good to excellent
yields (72–98%) (Table 2).
Products were purified by chromatograpic methods and
the structures verified with spectroscopic methods and
by comparing their physical properties with those of
known compounds.
Keywords: Barium dichromate; Oxidation; Oxidizing agent; Non-
aqueous media.
*
Corresponding author. E-mail: e.mottaghi@math.com
0
040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.10.010

8824
E. Mottaghinejad et al. / Tetrahedron Letters 45 (2004) 8823–8824
Table 1.
removed and the residue was purified using petroleum
ether (bp 60–80°C) as eluent on a silica gel column.
Evaporation of the solvent gave pure cyclohexanone
BaCr O , MeCN
2
7
RCH OH
2
RCHO
63-85%
(106mg, 72%).
reflux, 2.5-4 h,
Oxidant / Reactant: 2/1
Oxidation of benzhydrol to benzophenone as a typical pro-
cedure: A mixture of barium dichromate (354mg,
Entry
R
Yield (%)
A
B
C
D
E
Me–
Et–
63
65
85
63
75
3
(
mmol) and benzhydrol (184mg, 1mmol) in acetonitrile
10mL) was stirred for 0.75h under reflux. The progress
Ph–
of reaction was followed by TLC. The reaction mixture
was filtered, the solvent was removed and the residue
was purified using n-hexane/Et O (80:20) as eluent on
2
PhCH –
PhCH@CH–
2
a silica gel column. Evaporation of the solvent gave pure
benzophenone (178mg, 98%).
Table 2.
Acknowledgements
BaCr O , MeCN
2
7
1
2
1
2
R R CH-OH
R R C=O
72-98%
reflux, 0.75-4 h,
Oxidant / Reactant: 2/1
E.M., thanks Dr. H. Firouzabai and Dr. N. Iranpoor
for their guidance in the course of study. We also thank
Azad University of Iran-Rey Branch Research Council.
1
R , R
2
Entry
Yield (%)
F
G
H
I
Me, Me
Me, Et
85
87
72
92
98
96
90
References and notes
2 5
–(CH ) –
Me, Ph
Ph, Ph
Ph, p-C
1
2
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7
2
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3
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2
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2
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1
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1
1
0. Firouzabadi, H.; Mottaghinejad, E.; Sedighi, M. Synth.
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3
mmol) and cyclohexanol (150mg, 1.5mmol) in aceto-
nitrile (12mL) was stirred for 4h under reflux. The pro-
gress of reaction was followed by gas chromatography.
The reaction mixture was filtered, the solvent was