Efficient and selective iodination of benzylic alcohols using NaI/bronsted ionic liquid system at room temperature

Article abstract of DOI:10.1080/00397911003629481

A simple, chemoselective, and effective procedure for the conversion of benzylic and allylic alcohols into corresponding iodides using NaI in the presence of a catalytic amount of 3-methylimidazolium hydrogensulfate ([Hmim]+[image omitted]) at room temperature is reported.

Full text of DOI:10.1080/00397911003629481

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Efficient and Selective Iodination
of Benzylic Alcohols Using NaI/
Brønsted Ionic Liquid System at Room
Temperature
a
a
a
Abdol R. Hajipour , Fatemeh Rafiee & Arnold Ruoho
a
Pharmaceutical Research Laboratory, Department of Chemistry,
Isfahan University of Technology, Isfahan, Iran
Published online: 02 Feb 2011.
To cite this article: Abdol R. Hajipour , Fatemeh Rafiee & Arnold Ruoho (2011) Efficient and Selective
Iodination of Benzylic Alcohols Using NaI/Brønsted Ionic Liquid System at Room Temperature,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic
Chemistry, 41:4, 603-611, DOI: 10.1080/00397911003629481
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Synthetic Communications , 41: 603–611, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911003629481
EFFICIENT AND SELECTIVE IODINATION OF BENZYLIC
ALCOHOLS USING NaI/BRØNSTED IONIC LIQUID
SYSTEM AT ROOM TEMPERATURE
Abdol R. Hajipour, Fatemeh Rafiee, and Arnold Ruoho
Pharmaceutical Research Laboratory, Department of Chemistry,
Isfahan University of Technology, Isfahan, Iran
Abstract A simple, chemoselective, and effective procedure for the conversion of benzylic
and allylic alcohols into corresponding iodides using NaI in the presence of a catalytic
þ
ꢀ
amount of 3-methylimidazolium hydrogensulfate ([Hmim] HSO ) at room temperature
is reported.
4
Keywords Alkyl iodide; benzylic alcohols; Brønsted acidic ionic liquid; iodination; ionic
liquid; 3-methylimidazolium hydrogensulfate
INTRODUCTION
Development of efficient and selective methods for the preparation of
halogen-containing compounds, which have extensive applications in organic
[1]
syntheses, is a worthwhile goal. These compounds react with nucleophiles such
as amines or alkoxides to give the corresponding substituted products and can be
[
2]
lithiated to introduce electrophiles via a halogen–lithium exchange reaction. Alkyl
iodides or bromides are widely used for ionic and radical carbon–carbon coupling
reactions and also act as intermediates in substitution, elimination, and rearrange-
ment reactions. Although alkyl iodides are less stable than the corresponding
chloride or bromide, they are more reactive than the other halides, and in some cases
iodides are the only reactive halides. The most common precursors to alkyl halides
are alcohols, and their conversion into iodides is one of the most frequently used
[
3,4]
functional-group transformation reactions.
To perform iodination of hydroxyl groups, several methods have been
[
5]
described using a variety of reagent systems such as BF –Et O=NaI, P I ,
[6]
3
2
2 4
Received October 30, 2009.
Address correspondence to Abdol R. Hajipour, Pharmaceutical Research Laboratory, Department
of Chemistry, Isfahan University of Technology, Isfahan 84156, Iran. E-mail: haji@cc.iut.ac.ir
6
03

604
A. R. HAJIPOUR, F. RAFIEE, AND A. RUOHO
[8] [9]
P =I , Cl SO–dimethylformamide (DMF) = KI, MgI2, PPh = diethyl acetylene-
[
7]
4
2
2
3
[
10]
[11]
dicarboxylate (DEAD) = LiI,
methylphosphoramide (HMPA),
dicyanobenzoquinone (DDQ) = R N X ,
petroleum ether,
triphenylphosphine = I = ImH,
conditions using microwave irradiation.
Ionic liquids have attracted extensive research interest in recent years as
environmentally benign solvents because of their favorable properties such as non-
ClSiMe = NaI,
R PI –Et O, or C H = hexa-
3
3
2
[13]
2
6
6
[12]
CeCl ꢁ 7H O = NaI,
PPh = 2,3-dichloro-5,6-
3
3
2
þ
ꢀ [14]
[15]
[16]
KI=BF Et O,
2
ZrCl =NaI,
4
polymer-supported
I =
2
4
3
[17]
[18]
[19]
NaI=amberlyst 15,
silphos=I2,
and triphenylphosphine = I under solvent-free
[20]
2
2
[21]
[
22,23]
flammability, negligible vapor pressure, reusability, and high thermal stability.
They have also been referred to as ‘‘designer solvents’’ because their physical and
chemical properties could be adjusted by a careful choice of cation and anion. Apart
from this, they exhibit acidic properties. Combining these unique properties, they are
emerging as green reaction media (catalyst þ solvent). The use of ionic liquids as
reaction medium may offer a convenient solution to both the solvent emission and
[24–26]
catalytic recycling problems.
Brønsted acidic ionic liquids have been successfully applied to a variety of
[
27]
reactions including esterification of carboxylic acids,
[
protection of aldehydes
[34,35]
28–31]
[32]
[33]
and ketons,
[
deprotection,
cleavage of ethers,
[37]
oxidation,
alcohols, acetylation of alcohols, and synthesis of organic compounds.
iodination
[38]
36]
RESULTS AND DISCUSSION
In continuation of our efforts to develop an efficient method in organic
[
39–42]
synthesis,
containing nitrogen-based organic cation 3-methylimidazole and anion HSO
(
in this article we report the synthesis of Brønsted acidic ionic liquids
ꢀ
4
[Hmim] HSO ) in excellent yields and its application as a green catalyst and reac-
þ
ꢀ
4
tion medium for iodination of benzylic alcohols with NaI. The combination of
-methylimidazole with sulfuric acid in a 1:1 molar ratio produced the 3-methylimi-
1
dazolinium hydrogensulfate ionic liquid at room temperature. This Brønsted acidic
ionic liquid as catalyst afforded good to excellent yields for conversion of benzylic
and allylic alcohols to the corresponding iodo derivatives (Scheme 1).
A variety of benzylic and allylic alcohols were smoothly converted to the
corresponding iodides using 3-methylimidazolinium hydrogensulfate = NaI system
at room temperature. The reactions are highly selective for the conversion of
benzylic and allylic alcohols into the corresponding benzylic and allylic iodides
(Table 1).
Scheme 1. Conversion of benzylic and allylic alcohols to the corresponding iodides.

IODINATION OF BENZYLIC ALCOHOLS
605
þ
ꢀ
a
Table 1. Conversion of alcohols to iodides using NaI=[Hmim] HSO at room temperature
4
Entry
1
Substrate
Product
Time (min)
Yield (%)
7
94
2
6
93
3
4
8
5
94
92
5
5
89
6
7
10
12
90
90
8
9
14
10
88
93
1
0
1
8
6
90
1
92
(
Continued )

606
A. R. HAJIPOUR, F. RAFIEE, AND A. RUOHO
Table 1. Continued
Entry
Substrate
Product
Time (min)
35
Yield (%)
56
1
2
3
1
7
8
89
85
1
1
4
5
10
82
1
6
7
8
82
0
1
30
1
1
8
9
30
30
0
0
Yields refer to pure isolated products, and products were characterized by comparison of their physical
1
and spectral data (IR, H NMR, MS) with those of authentic samples.
In the case of phenol and saturated alcohols, the conversion into the correspond-
ing iodides was not carried out even after a prolonged time. We investigated various
benzylic alcohols with both electron-donating and electron-withdrawing groups on
the aryl ring. It was observed that the reaction is slightly accelerated by an
electron-donating group and the substitution of the electron-withdrawing group onto
the aromatic ring retards the transformation. Also, this reagent is suitable for the con-
version of allylic alcohols and secondary benzylic alcohols into their corresponding
products. To evaluate the chemoselectivity of the present method, we studied competi-
tive reactions for the iodination of benzylic alcohols in the presence of phenol and
saturated alcohols. The remarkable selectivity of these reactions allowed only benzylic
hydroxyl groups to be iodinated without affecting OH phenols or aliphatic alcohols.

IODINATION OF BENZYLIC ALCOHOLS
607
þ
ꢀ
a
Table 2. NaI=[Hmim] HSO4 used for selective iodination of different alcohols
1
2
3
4
5
a
The progress of reactions was determined by GC and TLC analysis.
We also studied the iodination of benzyl alcohol versus 4-methoxybenzyl alcohol
and 4-nitrobenzyl alcohol in the presence of benzyl alcohol. These reactions proceeded
þ
ꢀ
with high selectivity in the presence of ([Hmim] HSO ), showing the importance of
electronic effects upon these reactions using this technique (Table 2).
4
EXPERIMENTAL
All reagents were purchased from Merck and Aldrich and used without further
purification. All yields refer to isolated products after purification. Products were
characterized by comparison with authentic samples and by spectroscopic data
1
1
(IR, H NMR, and mass spectrometric (MS) analysis). H NMR spectra were
recorded at 300 MHz in CDCl , unless otherwise stated, relative to tetramethylsilane
3
(TMS) (0.00 ppm). Gas chromatographic (GC) analysis was run with a Shimadzu
GC-14A instrument.
Preparation of 3-Methylimidazolium Hydrogensulfate
þ
ꢀ
[Hmim] HSO )
4
[34]
(
1
-Methylimidazole (0.82 g, 0.01 mol) was placed in a two-necked flask with
ꢂ
magnetic stirrer and was cooled to 0 C. Then 10 mL of acetonitrile was added to
the reaction mixture, and sulfuric acid (0.98 g, 0.01 mol) was added slowly under
stirring. The mixture was stirred for 30 min, and the acetonitrile was removed by
[
34]
simple decanting to afford the ionic liquid in quantitative yields.

608
A. R. HAJIPOUR, F. RAFIEE, AND A. RUOHO
Iodination of 4-Chlorobenzyl Alcohol (Typical Procedure)
In a mortar, a mixture of 4-chlorobenzyl alcohol (1 mmol, 0.142 g), NaI
1 mmol, 0.15 g), and 3-methylimidazolinium hydrogensulfate (0.5 mmol, 0.09 g)
(
was ground with a pestle. The completion of the reaction was monitored by thin
layer chromatography (TLC) or GC. After disappearance of the alcohol (TLC
(
(
EtOAc–cyclohexane, 25:75), the reaction mixture was diluted with dichloromethane
25 mL) and filtered to removed the solids. The organic layer was washed with an
aqueous solution of Na S O (10%, 10 mL) and then H O (2 ꢃ 10 mL), and the
2
2
3
2
organic layer was dried over anhydrous Na SO . The solvent was evaporated under
2
4
reduced pressure, and the residue was purified by short column chromatography
(
(
EtOAc–cyclohexane, 25:75). 4-Chlorobenzyl iodide was obtained in 92% yield
0.231 g) as a crystalline white solid.
CONCLUSION
In conclusion, we have developed a simple, direct, and highly chemoselective
process for the iodination of benzylic and allylic alcohols using NaI in the presence
þ
ꢀ
of a catalytic amount of 3-methylimidazolium hydrogensulfate ([Hmim] HSO ).
4
The mild reaction conditions, short reaction times, and good to excellent yields
are the advantages of this method.
ACKNOWLEDGMENTS
We gratefully acknowledge the funding support received for this project from
the Isfahan University of Technology (IUT), Iran (A. R. H.), and Grant GM 33138
(A. E. R.) from the National Institutes of Health. Further financial support from the
Center of Excellency in Sensor and Green Chemistry Research (IUT) is gratefully
acknowledged.
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4
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periodate: A mild and efficient oxidation for the cleavage of oxime double bonds under
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and iodine as mild reagents for the iodination of activated aromatic compounds. Org.
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