Ethylenebis(N-methylimidazolium) chlorochromate (EBMICC): A new selective and mild reagent for oxidation of alcohols, hydroquinones and trimethylsilyl ethers

Article abstract of DOI:10.1002/jccs.200800036

The ethylenebis(N-methylimidazolium) chlorochromate was prepared by addition of N-methylimidazole to 1,2-dibromoethane to form the corresponding dibromide salt and subsequent treatment of this salt with CrO₃ in 6N HCl solution. It is a stable yellow-orange solid which selectively oxidized benzylic and allylic alcohols, hydroquinones and trimethylsilyl ethers in refluxing acetonitrile. Oxidation of alcohols was also examined under solvent-free conditions and showed much better yields of the corresponding carbonyls in a very short reaction time when compared with the conventional method.

Full text of DOI:10.1002/jccs.200800036

Journal of the Chinese Chemical Society, 2008, 55, 239-243
239
Ethylenebis(N-methylimidazolium) Chlorochromate (EBMICC): A New
Selective and Mild Reagent for Oxidation of Alcohols, Hydroquinones and
Trimethylsilyl Ethers
Rahman Hosseinzadeh,* Mahmood Tajbakhsh* and Hamid Khaledi
Department of Chemistry, Mazandaran University, Babolsar, Iran
The ethylenebis(N-methylimidazolium) chlorochromate was prepared by addition of N-methylimida-
zole to 1,2-dibromoethane to form the corresponding dibromide salt and subsequent treatment of this salt
with CrO3 in 6N HCl solution. It is a stable yellow-orange solid which selectively oxidized benzylic and
allylic alcohols, hydroquinones and trimethylsilyl ethers in refluxing acetonitrile. Oxidation of alcohols
was also examined under solvent-free conditions and showed much better yields of the corresponding car-
bonyls in a very short reaction time when compared with the conventional method.
Keywords: Ethylenebis(N-methylimidazolium) chlorochromate; Oxidation; Alcohols;
Trimethylsilyl ethers; Solvent free; Microwave.
INTRODUCTION
Since the appearance of Collins reagent in 1968,1 the
technology. In solid state reactions, work-up is consider-
ably simplified, cost is reduced, increased amounts of re-
agents can be used in the same equipment, and reactivities
and sometimes selectivities are enhanced without dilution.
Organic solid state reactions are usually carried out by
keeping a mixture of finely powdered reactant and reagent
at room temperature. In some cases, these reactions are ac-
celerated by heating, shaking, and grinding of the reaction
mixture using a mortar and pestle and irradiation with ul-
trasound or microwave.11 The microwave enhanced or-
ganic reactions have gained popularity over the usual ho-
mogeneous and heterogeneous reactions, as they can be
conducted rapidly and produce pure products in quantita-
tive yields without the use of solvents.12,13
development of new chromium(VI) oxidizing agents for
the effective and selective oxidation of organic substrates,
in particular alcohols, has attracted a great deal of contin-
ued interest in organic synthesis. Up to now several effi-
cient chromium(VI) reagents such as pyridinium chloro-
chromate,2 pyridinium dichromate,3 2,2-bipyridinium chlo-
rochromate,4 2,6-dicarboxypyridinium chlorochromate,5
2,6-dicarboxypyridinium fluorochromate,6 tetrabutylphos-
phonium dichromate,7 1-methylimidazolium chlorochro-
mate8 and 1-(benzoylamino)-3-methylimidazolium chloro-
chromate9 have been introduced to improve the selectivity,
the mildness and the effectiveness of the oxidant species,
especially in the oxidation of complex and highly sensitive
compounds. Although many chromium(VI) reagents are
available for the oxidation of organic substrates, they have
certain limitations such as instability, the need of an excess
amount of the reagent, and poor selectivity to substrates.
Therefore there still exists a need for highly efficient and
mild oxidizing agents.
RESULTS AND DISCUSSION
As a part of our continuous efforts to develop new ef-
ficient oxidizing agents,5,6 we have synthesized ethylene-
bis(N-methylimidazolium) chlorochromate (EBMICC) and
studied its oxidizing properties for alcohols, hydroquin-
ones and trimethylsilyl ethers under different reaction con-
ditions.
Recently an area of intense synthetic endeavor has
emphasized the use and design of reagents without any sol-
vent.10 Avoiding organic solvents during the reactions in
organic synthesis leads to clean, efficient, and economical
This compound is easily and cheaply prepared from
N-methylimidazole and 1,2-dibromoethane followed by
treatment with CrO3 in the presence of HCl in high yield. It
* Corresponding author. Fax: +98-1125242002; E-mail: r.hosseinzadeh@umz.ac.ir

240 J. Chin. Chem. Soc., Vol. 55, No. 1, 2008
Hosseinzadeh et al.
nol with this reagent in refluxing acetonitrile for 20 h led to
recovery of unchanged starting material (Table 1, entries
20-22).
ClCrO3
N
N
N
N
We have also conducted these oxidation reactions un-
der solvent-free conditions and microwave irradiation. We
have found that by keeping a mixture of finely powdered
reactant and reagent at 80 °C, the reaction was completed in
10-45 min. The results in Table 1 indicate that under sol-
vent-free conditions, all reactions gave better yields in
comparison to conventional conditions. Also, that aliphatic
alcohols remained unchanged attests to the selectivity of
this reagent under solvent-free conditions (Table 1, entries
18 and 19). When we carried out the oxidation reaction un-
der microwave irradiation, oxidation of allylic and ben-
zylic alcohols occurred in an even much shorter time (4-7
min) in excellent yields (Table 1).
ClCrO3
EBMICC
is soluble in DMF, acetone and acetonitrile, and insoluble
in methylenechloride, n-hexane and diethyl ether. It shows
a pH value of 2.6 (0.01 N aqueous solution) that attests to
its less acidic character in comparison to PCC and N-meth-
yl piperidinium chlorochromate (The pH of 0.01 N solu-
tions of PCC and N-methyl piperidinium chlorochromate
were found to be 1.75 and 1.85, respectively).14 One more
advantage of EBMICC is that it is much less hygroscopic
and can be used after one year of its preparation without
any decomposition which is not the case with PCC and sim-
ilar Cr(VI) reagents. The chromium content of the reagent
was determined by atomic absorption and the experimental
and calculated results are in very good agreement.
Conversion of the hydroxyl function to trimethylsilyl
ether is one of the most useful and convenient methods for
the protection of this functional group and oxidation of
trimethylsilyl ethers to the corresponding carbonyl com-
pounds has received considerable attention during recent
years.15-18 We have studied the treatment of a variety of
trimethylsilyl ethers such as benzylic, allylic and aliphatic,
with one equivalent of EBMICC in refluxing acetonitrile.
The results presented in the Table 2 show that under reac-
tion conditions benzylic and allylic trimethylsilyl ethers
converted to the corresponding aldehydes or ketones in
high yields (Table 2, entries 1-6), but in the case of aliphatic
saturated trimethylsilyl ethers only deprotection was ob-
served and corresponding alcohols remained unchanged
even after 15 h (Table 2, entries 7 and 8).
A wide variety of alcohols such as benzylic, allylic
and aliphatic were treated with one equivalent of EBMICC
in refluxing CH3CN. As shown in Table 1, benzylic alco-
hols were oxidized to the corresponding aldehydes or ke-
tones in 3-6 h (Table 1, entries 1-13). The oxidation of al-
lylic alcohols were somewhat slower. Thus cinnamyl alco-
hol required 10 h for completion of the reaction (Table 1,
entry 14). It is evident from Table 1 that with the exception
of 2-nitro benzyl alcohol (Table 1, entry 9), the reaction for
all benzylic and allylic alcohols gave good yields of the
products. Furthermore, hydroquinones in acetonitrile at
room temperature were oxidized very fast and gave excel-
lent yields of the corresponding benzoquinones (Table 1,
entries 15 and 16). Triphenylphosphine was also oxidized
under the same reaction conditions and gave excellent yield
of triphenylphosphine oxide in 4 minutes (Table 1, entry
17). The selectivity of this reagent was indicated by the oxi-
dation of benzylic and allylic alcohols and hydroquinones
in the presence of aliphatic primary and secondary alco-
hols. For example, treatment of 1-heptanol or cyclohexanol
in refluxing CH3CN with EBMICC for 15 h led to recovery
of unchanged alcohols (Table 1, entries 18 and 19).
In conclusion, we have shown that EBMICC is an ef-
ficient and inexpensive reagent for selective oxidation of
benzylic and allylic alcohols, hydroquinones and trimeth-
ylsilyl ethers. Tetrahydropyranyl ether, oxime and phenolic
groups survive in the procedure of oxidation in refluxing
acetonitrile. Allylic and benzylic alcohols under solvent-
free conditions are oxidized in a much shorter reaction time
than in refluxing acetonitrile. Under microwave irradiation
alcohols are oxidized in 4-7 min in excellent yields. Over-
oxidation of products to the corresponding carboxylic ac-
ids was not observed at all. Interestingly, the a,b-unsatu-
rated alcohols and trimethylsillyl ethers underwent oxida-
tion very efficiently without affecting the olefinic bond and
the reaction is essentially chemoselective. Furthermore,
functional groups such as chloro, methoxy and nitro were
also inert to this reagent and no byproduct formation was
It is noteworthy that acid-labile groups such as tetra-
hydropyranyl ethers and oximes and also phenolic groups
survive under reaction conditions. Thus treatment of 1-hep-
tyl tetrahydropyranyl ether, propiophenone oxime or phe-

EBMICC: A Mild Oxidant for Organic Compounds
J. Chin. Chem. Soc., Vol. 55, No. 1, 2008 241
Table 1. Oxidation of alcohols and hydroquinones with EBMICC
Yield/%a (Time)
Entry
Substrate
Product
Solution
Solvent free
MWb
CHO
OH
1
2
3
4
85 (3.5 h)
88 (10 min) 93 (4 min)
90 (10 min) 94 (4 min)
91 (25 min) 93 (6 min)
92 (10 min) 95 (5 min)
CHO
OH
87 (3 h)
82 (6 h)
90 (3.5 h)
Cl
Cl
CHO
Cl
OH
Cl
CHO
OH
Br
Br
CHO
OH
5
6
7
85 (4 h)
89 (3.5 h)
80 (6 h)
87 (15 min) 94 (5.5 min)
91 (20 min) 96 (5 min)
85 (25 min) 94 (7 min)
Br
Br
CHO
OH
H3CO
H3CO
CHO
OH
OCH3
OCH3
OH
CHO
O
8
87 (4.5 h)
92 (20 min) 94 (5 min)
O
CHO
NO2
OH
9
27 (6 h)
72 (45 min) 92 (6 min)
91 (20 min) 93 (5 min)
NO2
OH
O
10
89 (4.5 h)
CH3
CH3
O
OH
11
12
13
83 (5 h)
90 (6 h)
91 (4 h)
89 (25 min) 94 (5 min)
93 (25 min) 93 (7 min)
O
O
O
OH
OH
O
OCH3
OCH3
94 (15 min) 95 (4.5 min)
91 (35 min) 93 (7 min)
O
O
OH
O
14
15
90 (10 h)
HO
HO
OH
t-Bu
OH
O
O
O
t-Bu
O
95 (4 min)
¾
¾
¾
¾
16
94 (4 min)

242 J. Chin. Chem. Soc., Vol. 55, No. 1, 2008
Hosseinzadeh et al.
17
18
Ph3P
Ph3P=O
94 (4 min)
N.R (15 h)
¾
¾
¾
OH
¾
N.R (60 min)
OH
19
¾
N.R (15 h)
N.R (60 min)
¾
OTHP
20
21
¾
¾
N.R (20 h)
N.R (20 h)
¾
¾
¾
¾
NOH
OH
22
N.R (20 h)
¾
¾
¾
a Yield refers to isolated products; all products were identified by comparing IR, NMR, and TLC with those
of authentic samples. b Activation by domestic microwave oven (operating at 2.45 GHz, 900 W). The final
temperature of the reactions after irradiation depending on the reaction time varies between 120-160 °C.
Table 2. Deprotection of trimethylsilyl ethers with EBMICC
Entry
1
Substrate
Product
Time (h)
4.5
Yield% a
87
CHO
OTMS
CHO
OTMS
2
3
4
5
91
89
Cl
Cl
CHO
OTMS
OTMS
OCH3
OCH3
CHO
4
5
5.5
7
85
83
Br
Br
O
OTMS
OTMS
O
6
7
8
11
1
88
92
90
OTMS
OH
OTMS
OH
1
a Yield refers to isolated products; all products were identified by comparing IR, NMR, and
TLC with those of authentic samples.
observed.
and 1,2-dibromoethane (1.9 mL, 22 mmol) was stirred in
DMF (40 mL) at 120 oC for 2 h. After cooling the mixture,
the white solid formed was filtered, washed with diethyl
ether and dried under vacuum. Then it was dissolved in 40
mL of 6N HCl and slowly was added to a stirred solution of
CrO3 (4.4 g, 44 mmol) in 6N HCl (40 mL). The reaction
EXPERIMENTAL SECTION
Preparation of ethylenebis(N-methylimidazolium)
chlorochromate (EBMICC)
o
A mixture of N-methylimidazole (4.0 g, 48.7 mmol)
mixture was cooled to 0 C and the yellow-orange solid

EBMICC: A Mild Oxidant for Organic Compounds
J. Chin. Chem. Soc., Vol. 55, No. 1, 2008 243
formed was collected by filtration, washed with cold water
and dried under vacuum to give EBMICC (7.0 g, 69%).
m.p. 99-101 oC. 1H NMR (300.13 MHz, DMSO-d6): d 3.75
(s, 6H, CH3), 4.60 (s, 4H, CH2), 7.43 (s, 2H, imi-H), 7.49 (s,
2H, imi-H), 8.78 (s, 2H, NCHN). 13C NMR (75.48 MHz,
DMSO-d6): d 36.4 (CH3), 48.9 (CH2), 122.8 (imi-C), 124.4
(imi-C), 137.6 (NCHN). IR (KBr) 945, 897, 740 cm-1.
Anal. Calcd. for C10H16Cl2Cr2N4O6: C, 25.93; H, 3.48; Cr,
22.45. Found: C, 25.97; H, 3.54; Cr, 22.12.
time given in Table 1 and worked up as above to get the cor-
responding aldehyde or ketone.
ACKNOWLEDGMENTS
Financial support from the research council of
Mazandaran University is gratefully acknowledged.
Received February 16, 2007.
General procedure for oxidation of alcohols, hydro-
quinones, triphenylphosphine and trimethylsilyl
ethers in acetonitrile
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To a solution of an alcohol or trimethylsilyl ether (10
mmol) in acetonitrile (150 mL) was added EBMICC (10
mmol) and the reaction mixture was refluxed for the time
specified in Tables 1 and 2. The solvent was evaporated and
diethyl ether was added to the residue. The supernatant was
decanted and the insoluble residue was washed three times
with diethyl ether. The combined ether extracts were con-
centrated under reduced pressure and the crude product
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crude material was purified on a silica gel column. When a
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mmol) and EBMICC (1 mmol) was irradiated in a closed-
vessel in an microwave oven (operating at 900 W) for the
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