Selective iodination of benzylic alcohols with KI/H2SO4 supported on natural kaolinitic clay under microwave irradiation

Article abstract of DOI:10.1007/s007060200013

Benzylic alcohols are selectively converted into their corresponding iodides using KI/H₂SO₄ supported on natural kaolinitic clay and microwaves.

Full text of DOI:10.1007/s007060200013

Monatshefte fuÈr Chemie 133, 345±348 ꢀ2002)
Selective Iodination of Benzylic Alcohols
with KI=H2SO4 Supported on Natural
Kaolinitic Clay Under Microwave Irradiation
Ã
Babasaheb P. Bandgar , Vibhav S. Sadavarte, and Sampada V. Bettigeri
School of Chemical Sciences, Organic Chemistry Research Laboratory,
Swami Ramanand Teerth Marathwada University, Nanded-431606, India
Summary. Benzylic alcohols are selectively converted into their correspondingiodides using
KI=H2SO4 supported on natural kaolinitic clay and microwaves.
Keywords. Iodination; Benzylic alcohols; Natural kaolinitic clay; Microwaves.
Introduction
Alkyl iodides or bromides are used for ionic or radical carbon±carbon coupling
reactions and can also act as intermediates in substitution reactions, eliminations,
and rearrangements. Therefore, the conversion of alcohols into alkyl halides is a
very important transformation in organic chemistry. Although various methods have
been reported to achieve this goal [1±4], some of these require expensive reagents,
longreaction times, and tedious work-up. Some methods involve mild conditions
and the use of iodo [5] and bromotrimethylsilanes [6], chlorotrimethylsilane-sodium
iodide [7], and hexamethyldisilazane-iodine [8] as reagents. More recently, reports
have been published regarding the selective conversion of allylic and benzylic and
other ꢀprimary, secondary, and tertiary) alcohols into halides [9±12].
Heterogenous organic reactions have proven useful in the laboratory as well as
in industry. These reactions are effected by reagents immobilized on porous solid
supports and have advantages over the conventional solution phase reactions
because of good dispersion of active reagent sites and easier work-up. The re-
cyclability of some of these solid supports renders these processes into truly eco-
friendly green protocols [13, 14]. A related development that had a profound
impact on heterogeneous reactions is the use of the microwave ꢀMW) irradiation
technique for the acceleration of organic reactions [15]. The combination of
supported reagents and microwave irradiation can be used to carry out a wide range
of reactions in short times and with high conversion and selectivity without the
need of solvents. This approach can prove bene®cial since the recovery of solvents
from conventional reaction systems always results in some losses. Recovery of
Ã
Correspondingauthor. E-mail: bandgar_bp@yahoo.com

346
B. P. Bandgar et al.
Scheme 1
both products and supported reagent or catalyst is generally possible by simple
®ltration and evaporation, leadingto an ef®cient and low-waste route to a range of
products [15].
We here report on the selective iodination of benzyl alcohols with conc. H2SO4
and KI supported on natural kaolinitic clay [16, 17] under pulsed microwave ir-
radiation ꢀScheme 1).
Results and Discussion
The iodination of various benzylic alcohols is summarized in Table 1. Chemo-
selective iodination of benzyl alcohols was carried out in the presence of primary,
secondary, and tertiary alcohols as well as phenols ꢀTable 2, entries 1±8). Attempts
to prepare ¯uorides, bromides, and chlorides failed when KF, KBr, or KCl and
conc. H2SO4 supported on natural kaolinitic clay under microwave irradiation were
used. It is also important to note that NaI and LiI in the presence of H2SO4
supported on natural clay are equally effective reagents for the iodination of benzyl
alcohols. Usingan alternative conventional heatingmode ꢀoil bath) at a tem-
perature of 100ꢀC, trace amounts of benzyl iodide were formed usingthis sup-
ported reagent and benzyl alcohol, thus indicates the importance of microwaves for
this transformation.
In conclusion, we have developed a simple method for the selective trans-
formation of benzylic alcohols into benzylic iodides usingKI and conc. H SO4
2
supported on natural kaolinitic clay under pulsed microwave irradiation. Fast
reactions, high yields, and an inexpensive catalyst are important features of this
method.
Table 1. Iodination of benzyl alcohols usingKI =H2SO4 supported on natural kaolinitic clay under
microwave irradiation
Substrate
Product
R
Reaction time ꢀmin)
Yielda ꢀ%)
1a
1b
1c
1d
1e
1f
2a
2b
2c
2d
2e
2f
H
5
5
4
6
4
2
7
92
92
90
91
93
93
74
4-Cl
2,4-Cl2
4-NO2
3,4-CH2O2
6-Cl,3,4-CH2O2
2-Cl,6-F
1g
2g
a
1
Yields of isolated products; products are characterized spectroscopically ꢀIR, H NMR) and by
comparison with authentic samples [17, 18]

Selective Iodination of Benzylic Alcohols
347
Table 2. Selective synthesis of benzyl iodides
Entry Hydroxylic compound
Products
t=min
Yield=%a
1
2
3
4
5
6
4-Nitrobenzyl alcohol
Octadecanol
4-Nitrobenzyl iodide
Octadecyl iodide
85
00
7
6
7
8
6
5
Benzyl alcohol
Cyclohexanol
Benzyl iodide
90
00
Cyclohexyl iodide
Benzyl alcohol
Benzyl iodide
91
00
t-Butyl alcohol
t-Butyl iodide
4-Nitrobenzyl alcohol
ꢀ-)-Menthol
4-Nitrobenzyl iodide
Menthyl iodide
85
00
4-Chlorobenzyl alcohol
4-Chlorobenzyl iodide
87
00
t-Butyl alcohol
t-Butyl iodide
4-Fluorobenzyl alcohol
4-Chlorophenol
4-Fluorobenzyl iodide
4-Chloro-iodobenzene
82
00
7
4
5
87
83
8
a
Yields of pure isolated product; products are characterized spectroscopically ꢀIR, 1H NMR) and by
comparison with authentic samples [17, 18]
Experimental
The kaolinitic clay was obtained from the Padappakara mine of Quilon District, Kerala, India and
was puri®ed, characterized [16, 17], and supplied by Dr. Lalithambika, RRL, Trivandrum. Solvents
were distilled before use. All chemicals were of analytical grade. IR spectra were recorded on a
Bomem MB 104 FT-IR spectrometer. Microwave oven: Kelvinator T-37 model ꢀ2450 MHz, 760 W).
General procedure
Natural kaolinitic clay ꢀ100 mg) was thoroughly mixed with one drop of conc. H2SO4 and 660 mgof
KI ꢀ4 mmol) and exposed to microwaves for 1 min. This reagent and 5 mmol of 3,4-methylenedioxy
benzyl alcohol were thoroughly mixed in a mortar and exposed to microwave irradiation for 10 to
15 s using100% power. The reaction mixture was cooled to room temperature ꢀ1 min) and irradiated
again for 10±15 s. After completion of the reaction ꢀTLC), 10 cm3 of diethyl ether were added to the
reaction mixture, and the catalyst was washed with 2 Â 10 cm3 diethyl ether. The solvent was re-
moved under reduced pressure to afford the product in good yield and in almost pure form. If nec-
essary, products were puri®ed by column chromatography using petroleum ether:ethyl acetate ˆ 9:1
as the eluent.
Acknowledgements
We thank Dr. Lalithambika, Regional Research Laboratory, Trivandrum, for a generous gift of
natural kaolinitic clay. Financial assistance from UGC, New Delhi, is gratefully acknowledged.
V. S. Sadavarte thanks CSIR, New Delhi, for a junior research fellowship.

348
B. P. Bandgar et al.: Selective Iodination of Benzylic Alcohols
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Received August 20, 2001. Accepted ꢀrevised)September 18, 2001

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