Download CaCl2.2H2O assisted oxidation of alcohols with (NH4)2Cr2O7

Indian Journal of Chemistry
Vol. 44B, November 2005, pp. 2383-2386
Note
CaCl2.2H2O assisted oxidation of alcohols
with (NH4)2Cr2O7
F Shirini*a, M A Zolfigol b & A Safaria
a
Department of Chemistry, Faculty of Science, Guilan University,
Rasht, Iran
b
Department of Chemistry, College of Science,
Bu-Ali Sina University, Hamadan, Iran
E-mail: [email protected]
Received 15 September 2004; accepted (revised) 19 April 2005
CaCl2.2H2O has been found to be an efficient reagent for the
acceleration of the oxidation of alcohols to their corresponding
carbonyl compounds with (NH4)2Cr2O7 in solution and under
solvent free conditions.
Keywords: Calcium chloride oxidation, alcohols, carbonyl
compounds
IPC: Int.Cl.7 C 07 C 39/00
The oxidation of alcohols to their corresponding
carbonyl compounds1,2 is one of the most important
reactions in organic synthesis since the products are
quite useful synthetic intermediates especially for the
construction of carbon skeletons. There are many
useful methods for this transformation of which
manganese dioxide3, activated dimethylsulfoxides4-5,
n-butylphenylphosphonium dichromate7, pyridinium
chlorochromate8,9, ferric dichromate10, polyvinylpyridine-supported ferric dichromate11 and hypervalent iodines12-15 are examples . Although these
methods are useful in organic synthesis, each of them
suffers from one or more of the following
disadvantages; lack of selectivity, long reaction time,
strong protic and aqueous conditions, tedious work-up,
producing bad-smelling co-products and explosivity
on impact or on heating. Thus, a milder, more
selective and inexpensive reagent is still in demand.
In continuation of our ongoing research program
directed towards the development of new oxidizing
agents and systems16-18, herein we wish to report a
convenient, efficient and simple method for the
oxidation of alcohols to their corresponding carbonyl
compounds in solution and under solvent free
conditions.
Results and Discussion
Oxidation of different types of alcohols was
investigated in the absence of solvent by ammonium
dichromate in the presence of CaCl2.2H2O (Table I,
Scheme I). The reaction is simply performed by
stirring a mixture of alcohol, (NH4)2Cr2O7 and
CaCl2.2H2O in an oil-bath (60°C) for the appropriate
time (Table I). Alcohols were oxidized efficiently
and the corresponding carbonyl compounds were
isolated in good to high yields. No traces of
carboxylic acids or other by-products were detected in
all the cases studied. The only isolated products were
the expected carbonyl compounds and, in the cases
where the reaction had not gone to completion, the
starting material was recovered.
A or B
R1R 2CHOH
R1,R2 = Aryl, alkyl
R1R 2CO
A: (NH4)2Cr2O7/CaCl2.2H2O, solvent free, 60 OC
B: (NH4)2Cr2O7/CaCl2.2H2O, CH3CN, reflux
Scheme I
In order to compare the obtained results with those
obtained in solution, we studied the oxidation reaction
in refluxing CH3CN. As shown in the Table I, in
most cases, there are appreciable differences between
the results obtained in solution and those in solvent
free conditions. In conclusion, by omitting the
solvent, in addition to ease of the work-up procedure,
the reaction time was reduced and the need for solvent
is avoided. This method is not suitable for the
oxidation of allylic alcohols (Table I, entry 16).
It must be emphasized that the oxidation did not
proceed using (NH4)2Cr2O7 alone, even after
prolonged heating. Although the mechanism of the
reaction is not clear, but the presence of the water in
the structure of CaCl2.2H2O is very important so that
the rate of the reaction is diminished when CaCl2 is
used instead of CaCl2.2H2O (Table I, entry 17). On
the basis of these results we thought that the effect of
CaCl2.8H2O on the acceleration of the reaction may
only be related to the presence of H2O, considering
that the presence of the other part of the molecule is
not important. Lack of the progress of the reaction in
wet acetonitrile disproved this hypothesis.
INDIAN J. CHEM., SEC B, NOVEMBER 2005
2384
Table I ⎯ Oxidation of alcohols to their corresponding carbonyl compoundsa
Entry
Substrate
1
2
Product
CH2OH
CHO
CH2OH
CHO
Cl
3
CH2OH
Cl
CHO
CH2OH
CHO
Br
5
CH2OH
6
CHO
CH2OH
CHO
CH2OH
O2N
CHO
t-Bu
CH2OH
t-Bu
CHO
CH2OH
CHO
Me
10
11
PhO
CH2OH
PhO
CHO
MeO
CH2OH
MeO
CHO
OH
N
N
OH
OH
CHO
O
16
OH
a
90
0.12
99
2.2
90
0.08
92
4
85
0.12
95
3.75
82
0.17
85
1.67
90
1
60
3.75
70
0.42
75
1.5
90
0.17
95
3.3
95
0.08
92
1.5
80
0.83
92
1.5
85
0.25
80
3.3
90
0.42
95
2.5
70
1.67
75
1
50
0.5
95
3
70
1.8
95
1
⎯c
1
⎯c
2
30 d
2
40 d
CHO
14
17
3.2
O
OH
15
95
Me
12
13
0.17
O2N
O2N
9
92
NO2
O2N
8
1.75
Br
NO2
7
Solvent free oxidation
Time/hr
Yield (%) b
Cl
Cl
4
Oxidation in CH3CN
Time/hr
Yield (%)b
CH2OH
CHO
CHO
Products were characterized by their physical constants, comparison with authentic samples and IR and NMR
spectroscopy; b Isolated yield; c Mixture of products; d Oxidation in the presence of CaCl2.
NOTES
2385
Table II ⎯ Comparison of some of the results obtained by the oxidation of alcohols using (NH4)2Cr2O7 in the
presence of CaCl2.2H2O under solvent free conditions at 60°C, with some of those reported by K2Cr2O7
(ref.19), MnO2 (ref. 20) and 4-aminopyridinium chlorochromate supported on silica gel (ref. 21).
Entry
Substrate
(NH4)2Cr2O7
1
2
CH2OH
MeO
CH2OH
3
Time (hr) (Yield %)
K2Cr2O7
MnO2
4-Aminopyridinium
chlorochromate
(0.17)(95)
(4)(80)
(48)(77)
(2)(97)
(0.25)(80)
(4)(90)
(48)(83)
(1.5)(100)
(0.42)(95)
(3)(95)
(24)(87)
(3)(95)
OH
In ordet to show the efficiency of the present
method, some of our results have compared with
some of those reported in the literature19-21 (Table II).
In summery, the CaCl2.2H2O was found to be an
efficient reagent for the promotion of the oxidation of
alcohols to their corresponding carbonyl compounds
using (NH4)2Cr2O7. This new procedure is very
simple and cleanly affords the desired products in
good to high yields.
(NH4)2Cr2O7 (0.756 g, 3 mmoles) and CaCl2.2H2O
(0.147 g, 1 mmole) in CH3CN (3 mL) was stirred
magnetically under reflux condition for the specified
time (Table I). The progress of the reaction was
monitored by TLC. After completion of the reaction
the mixture was filtered and the solid residue was
washed with CH3CN (5 mL). Evaporation of the
solvent followed by column chromatograghy on silica
gel gave the corresponding carbonyl compounds in
good to high yield.
Experimental Section
General. Chemicals were purchased from Merck,
Fluka, BDH and Aldrich Chemical Companies.
Products were separated and purified by different
chromatography techniques, and were identified by
the comparison of their m.p., b.p., IR, NMR and
refractive index with those reported for the authentic
samples. All yields refer to the isolated products. The
purity determination of the substrates and reaction
monitoring were accomplished by TLC on silica gel
polygram SILG/UV 254 plates. Column chromatography was carried out on Merck kisselgel 60H.
General procedure for the oxidation of alcohols
under solvent free conditions. A mixture of alcohol
(1 mmole), (NH4)2Cr2O7 (0.756 g, 3 mmoles) and
CaCl2.2H2O (0.147 g, 1 mmole) was heated in an oilbath (60°C) for the specified time (Table I). The
progress of the reaction was monitored by TLC. After
completion of the reaction the mixture was triturated
with CH3CN (5 mL) and filtered. Evaporation of the
solvent followed by column chromatograghy on silica
gel gave the corresponding carbonyl compounds in
good to high yield.
General procedure for the oxidation of alcohols
in CH3CN. A suspension of alcohol (1 mmole),
Acknowledgement
Authors gratefully acknowledge the financial
support from Research Council of Guilan University.
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