An alternative method for the highly selective iodination of alcohols using a CsI/BF3·Et2O system

Article abstract of DOI:10.1081/SCC-120021844

A variety of allylic and benzylic alcohols have been converted into the corresponding iodides using cesium iodide (CsI) in the presence of boron trifluoride etherate (BF₃·Et₂O) in acetonitrile under mild conditions.

Full text of DOI:10.1081/SCC-120021844

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An Alternative Method for the Highly Selective
Iodination of Alcohols Using a CsI/BF3·Et2O System
Safdar Hayat a , Atta-ur-Rahman a , Khalid Mohammed Khan a , M. Iqbal Choudhary a
Ghulam Murtaza Maharvi a , Zia-Ullah a & Ernst Bayer a
,
a International Center for Chemical Sciences, H. E. J. Research Institute of Chemistry,
University of Karachi, Karachi, Pakistan
Version of record first published: 20 Aug 2006.
To cite this article: Safdar Hayat, Atta-ur-Rahman, Khalid Mohammed Khan, M. Iqbal Choudhary, Ghulam Murtaza Maharvi,
Zia-Ullah & Ernst Bayer (2003): An Alternative Method for the Highly Selective Iodination of Alcohols Using a CsI/BF3·Et2O
System, Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 33:14,
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SYNTHETIC COMMUNICATIONSÕ
Vol. 33, No. 14, pp. 2531–2540, 2003
An Alternative Method for the Highly
Selective Iodination of Alcohols
Using a CsI/BF3?Et2O System
*
Safdar Hayat,Atta-ur-Rahman,Khalid Mohammed Khan,
M. Iqbal Choudhary,Ghulam Murtaza Maharvi,
#
Zia-Ullah,and Ernst Bayer
International Center for Chemical Sciences,
H. E. J. Research Institute of Chemistry,
University of Karachi, Karachi, Pakistan
ABSTRACT
A variety of allylic and benzylic alcohols have been converted into
the corresponding iodides using cesium iodide (CsI) in the presence
of boron trifluoride etherate (BF3ꢀEt2O) in acetonitrile under mild
conditions.
*Correspondence: Khalid Mohammed Khan, International Center for Chemical
Science, H. E. J. Research Institute of Chemistry, University of Karachi, Karachi,
75270, Pakistan; Fax: 92-21-9243190-91; E-mail: hassaan2@super.net.pk.
#This Communication is the result of joint research program with Late Prof. Dr.
Ernst Bayer, Institut fur Organische Chemie der Universitat Tubingen, Auf der
Morgenstelle 18, D-72076, Tubingen, Germany.
2531
DOI: 10.1081/SCC-120021844
Copyright & 2003 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com

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2532
Hayat et al.
Alkyl halids, especially iodides and bromides are generally used as
intermediates in substitution reactions, elimination reactions, and rear-
rangements. Consequently, the conversion of alcohols to halides is a
useful transformation in synthetic organic chemistry. A variety of meth-
ods have been appeared in the literature[1–4] for transformation of alcohols
into halides, however some of these involve expensive reagents, tedious
reaction work-up and long reaction times. Most of the methodologies
include: iodo[5]- and bromotrimethylsilanes,[6] chlorotrimethylsilane-
sodium iodide[7] and hexamethyldisilazane/iodide ion.[8] Moreover, some
reports[9–14] have been published regarding the selective conversion of
allylic and benzylic, primary, secondary, and tertiary alcohols into halides.
Very recently methanesulphonic acid/sodium iodide (CH3SO3H/ NaI) is
reported for the conversion of alkyl, allyl, and benzyl alcohols into
iodes.[15]
In this communication we describe the conversion of allylic and
benzylic alcohols into its corresponding iodides by using cesium iodides
(CsI) in the presence of BF3ꢀEt2O under mild conditions.
The present methodology deals with the conversion of alcohols into
iodines, using different allyl and benzyl alcohols as substrates. To estab-
lish the general effectiveness of our newly developed method, some
selected iodination reaction was carried out. This reagent appears to be
the most reactive and versatile. The reactions of allyl and benzyl alcohols
with a molar equivalent of cesium iodide in the presence of BF3ꢀEt2O
in acetonitrile under mild conditions cleanly gives the corresponding
iodides (Sch. 1).
The reaction was found to proceed smoothly at room tempera-
ture within 5–30 min. The products were isolated by aqueous work-up
followed by solvent extraction and were purified by column chro-
matography if necessary to afford pure alkyl iodide. The result and
spectroscopic data of the following compounds were collected in Tables
1 and 2.
The reaction was found to be highly selective for conversion of
allylic and benzylic alcohols into iodides in high yields (82–96%), whereas
saturated aliphatic alcohols did not yield iodides even after prolonged
stirring in acetonitrile (Entries 18–21). Iodination of benzyl alcohol and
substituted benzyl alcohols with electron-donating groups, e.g., -OH,
Csl=BF3ꢀ Et2O
RꢁOH ꢁꢁꢁꢁꢁꢁꢁꢁꢁ! RꢁI
1
CH3CN, 25ꢂC
2
Scheme 1.

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CsI/BF3?Et2O System
2533
Table 1. Conversion of alcohols to iodides using a CsI/BF3EEt2O system at 25ꢀC.
Entry Substrate
Product
Time (min) Yielda (%)
1
2
(1a)
(1b)
(2a)
(2b)
5
96
95
15
3
4
5
(1c)
(1d)
(1e)
(2c)
(2d)
(2e)
15
20
20
93
90
92
6
(1f)
(2f)
25
88
7
8
(1g)
(1h)
(2g)
(2h)
25
30
95
85
9
(1i)
(1j)
(2i)
(2j)
20
20
84
82
10
11
12
13
(1k)
(1l)
(2k)
(2l)
10
10
15
91
88
85
(1m)
(2m)
14
(1n)
(2n)
20
85
15
16
17
(1o)
(1p)
(1q)
(2o)
(2p)
(2q)
—
15
15
20
60
60
60
60
83
82
80
—
—
—
—
18
19
20
21
n-Octanol
Isoamyl alcohol (1s) No reaction
Cyclohexanol (1t) No reaction
n-Butyl alcohol (1u) No reaction
(1r) No reaction
—
—
—
aYields of isolated products.

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2534
Hayat et al.

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CsI/BF3?Et2O System
2535

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2536
Hayat et al.
-OCH3, -Cl, -O-CH2-O- proceeds very well and in high yields with
CsI/BF3ꢀEt2O in acetonitrile (Entries l–7). The electron-withdrawing
group, e.g., p-nitro benzyl alcohol (1h) also give very good yield 85%
at 30 min (Entry 8). The remarkable selectivity of this reaction allowed
only benzylic hydroxyl group to be iodinated without effecting the phe-
nolic OH group present in the same molecule (Entry 2). Similarly, sec-
ondary alcohols, such as diphenylcarbinoI (1i) and benzoin (1j) could
easily be substituted by an iodide group in high yields using this reagent
(Entries 9, 10).
Furthermore, the displacement of hydroxy groups with iodine from
furfuryl alcohol (1k), 2-hydroxymethyl-thiophene (1l), and 4-hydroxy-
methyl-pyridine (1m) are performed well by this reagent and produces
iodinated products in very good yields (Entries 11–13). The reaction of
allyl alcohols e.g., cinnamyl alcohol (1n), 2-buten-1-ol (1o), 2-propy-1-ol
(1p), and 1,4-dihydroxy-cis-2-butene (1q) with CsI/BF3ꢀEt2O also gave rise
to the desired corresponding iodides in excellent yields (Entries 14–17).
In conclusion, this method is an excellent for the rapid, simple,
mild, direct, highly regio-selective, and high yield conversion of allylic
and benzylic alcohols to their corresponding iodides on reaction with
CsI/BF3ꢀEt2O in acetonitrile. The present method will be a valuable
addition into the useful synthetic methodology.
EXPERIMENTAL
General
Melting points were determined with a BuchiSMP-20 apparatus and
are uncorrected. The ultraviolet spectra were measured in chloroform on
a Lambda 5 UV/VIS spectrometer (Perkin-Elmer). Infrared spectra
(KBr discs) were recorded on a Bruker FT-IR IFS 48 spectrometer. EI
mass spectral data were recorded with Varian MAT 711 (70 eV) spectro-
meter and data are tabulated as m/z. 3-Nitrobenzyl alcohol was used in
the matrix of mass spectra. 1H NMR and 13C NMR spectra were
recorded in CDCl3 and DMSO-d6 using Brucker AC 250 (250 MHz
and 62.9 MHz) spectrometer, respectively. Splitting patterns were as
follows: s, singlet; d, doublet; dd, double doublet; t, triplet; m, multiplet;
br, broad. Chemical shifts are reported in ꢀ (ppm) and coupling constants
are given in Hz. The progress of all reactions was monitored by TLC,
which was performed on 2.0 Â 5.0 cm aluminum sheets precoated with

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CsI/BF3?Et2O System
2537
silica gel 60F254 to a thickness of 0.25 mm (Merck). The chromatograms
were visualized under ultraviolet light (254–366 nm) or iodine vapors.
Materials
All allylic and benzylic alcohols, furfuryl alcohol, 2-hydroxymethyl-
thiophene and 4-hydroxymethyl-pyridine together with reagents CsI
and BF3ꢀEt2O are commercially available (Fluka, Aldrich). The
anhydrous acetonitrile was purchased from Merck and used without
purification.
General Procedure for Conversion of Benzylic
and Allylic Alcohols into Iodides
To a stirred solution of the alcohol (3.0 mmol) and cesium iodide
(3.0 mmol) in 10 mL dry acetonitrile was added a solution of BF3ꢀEt2O
(3.0 mmol) in 2 mL of acetonitrile during 15 min. The reaction mixture
was stirred at room temperature for a period as indicated in Table l. The
reaction mixture was poured into cooled water (20 mL), treated with
aqueous solution (15%) of Na2S2O3 and then extracted with ether
(3 Â 20 mL). The combined organic phases were washed with water
(20 mL), brine (20 mL) and then dried over anhydrous Na2SO4.
Evaporation of the solvent gave residue, which was purified by column
chromatography over silica gel using n-hexane:ethyl acetate (9:1) as an
eluent to afford pure alkyl iodides.
Benzyl iodide (2a). Yellow crystalline solid; yield: 0.63 g (96%); m.p.:
21–23ꢂC (lit.[16] m.p.: 24–25ꢂC); Rf ¼ 0.84 (hexane:ethyl acetate, 5:1); EI
MS: m/z (%) ¼ 218 [Mþ] (59), 141 (32), 127 (24), 91 (100), 77 (15), 65 (11),
53 (6), 43 (5). Anal. calcd. for C7H7I: C, 38.56; H, 3.24. Found: C, 38.47;
H, 3.32.
4-Hydroxybenzyl iodide (2b). Colorless crystalline solid; yield: 0.67 g
(95%); m.p.: 108–110ꢂC; Rf ¼ 0.52 (hexane:ethyl acetate, 5:1); EI MS: m/z
(%) ¼ 234 [Mþ] (46), 219 (18), 127 (42), 117 (51), 107 (100), 91 (31), 77
(16), 59 (8), 43 (72). Anal. calcd. for C7H7IO: C, 35.93; H, 3.01. Found:
C, 35.87; H, 2.99.
4-Chlorobenzyl iodide (2c). Colorless crystalline solid; yield: 0.70 g
(93%); m.p.: 58–60ꢂC (lit.[16] m.p.: 60–61ꢂC); Rf ¼ 0.84 (hexane:ethyl
acetate, 5:1); EI MS: m/z (%) ¼ 252 [Mþ] (57), 127 (72), 125 (100), 101
(6), 89 (34), 76 (7), 63 (11). Anal. calcd. for C7H7ClI: C, 33.30; H, 2.40.
Found: C, 33.41; H, 2.36.

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2538
Hayat et al.
4-Methoxybenzyl iodide (2d). Pale yellow solid; yield: 0.67 g (90%);
m.p.: 25–27ꢂC (lit.[17]m.p.: 27ꢂC); Rf ¼ 0.82 (hexane:ethyl acetate, 5:1); EI
MS: m/z (%) ¼ 248 [Mþ] (65), 217 (16), 142 (100), 127 (81), 121 (84), 106
(7), 91 (11), 77 (15), 63 (8), 43 (6). Anal. calcd. for C8H9IO: C, 38.74; H,
3,66. Found: C, 38.82; H, 3.59.
3,4-Dimethoxybenzyl iodide (2e). Yellow crystalline solid; yield: 0.77 g
(92%); m.p.: 40–42ꢂC (1it.[17] m.p.: 43ꢂC); Rf ¼ 0.48 (hexane:ethyl
acetate, 5:2); EI MS: m/z (%) ¼ 278 [Mþ] (35), 263 (41), 248 (32), 232
(19), 216 (23), 151 (100), 137 (14), 127 (17), 107 (8), 91 (6), 75 (8), 57 (11),
43 (7). Anal. calcd. for C9H11IO2: C, 38.87; H, 3.99. Found: C, 38.78;
H, 3.92.
3,4-Methylenedioxybenzyl iodide (2f ). Colorless crystalline solid;
yield: 0.69 g (88%); m.p.: 76–78ꢂC; Rf ¼ 0.62 (hexane:ethyl acetate, 5:1);
EI MS: m/z (%) ¼ 262 [Mþ] (23), 214 (6), 135 (100), 127 (11), 105 (15),
83 (7), 79 (4), 77 (21), 67 (6), 51 (21). Anal. calcd. for C8H7IO2: C, 36.67;
H, 2.69. Found: C, 36.72; H, 2.78.
4-(Iodomethyl)-benzyl iodide (2g). Colorless crystalline solid; yield:
1.02 g (95%); m.p.: 174–175ꢂC (lit.[18] m.p.: 176–177ꢂC); Rf ¼ 0.82
(hexane:ethyl acetate, 5:1); EI MS: m/z (%) ¼ 358 [Mþ] (62), 282 (8),
231 (24), 217 (31), 141 (43), 104 (100), 90 (19), 76 (11), 65 (14), 56 (7),
45 (16). Anal. calcd. for C8H8I2: C, 26.84; H, 2.25. Found: C, 26.77;
H, 2.18.
4-Nitrobenzyl iodide (2h). Yellow needle; yield: 0.67 g (85%); m.p.:
122–124ꢂC (lit.[19] m.p.: 125–127ꢂC); Rf ¼ 0.55 (hexane:ethyl acetate, 5:1);
EI MS: m/z (%) ¼ 263 [Mþ] (61), 149 (11), 136 (100), 127 (8), 106 (19), 89
(21), 76 (12), 63 (5), 51 (4). Anal. calcd. for C7H6NIO2: C, 31.96; H, 2.30;
N, 5.33. Found: C, 31.88; H, 2.39; N, 5.27.
Diphenylmethyl iodide (2i). Colorless crystalline solid; yield: 0.74 g
(84%); m.p.: 69–70ꢂC (lit.[19] m.p.: 72ꢂC); Rf ¼ 0.78 (hexane:ethyl acetate,
5:1); EI MS: m/z (%) ¼ 294 [Mþ] (26), 217 (43), 167 (15), 127 (8), 105
(100), 91 (7), 77 (51), 65 (7), 51 (14). Anal. calcd. for C13H11I: C, 53.09; H,
3.77. Found: C, 53.12; H, 3.73.
a-lodo-a-phenylacetophenone (2j). Colorless crystalline solid; yield:
0.79 g (82%); m.p.: 90–92ꢂC (lit.[20] m.p.: 92–93ꢂC); Rf ¼ 0.38 (hexa-
ne:ethyl acetate, 5:1); EI MS: m/z (%) ¼ 322 [Mþ] (44), 245 (19), 217
(21), 195 (100), 127 (12), 118 (25), 105 (31), 91 (16), 83 (8), 77 (15), 65
(5), 51 (9). Anal. calcd. for C14H11IO: C, 52.20; H, 3.44. Found: C, 52.27;
H, 3.38.
2-lodomethyl-furane (2k). Oily; yield: 0.57 g (91%); Rf ¼ 0.69 (hexa-
ne:ethyl acetate, 5:1); EI MS: m/z (%) ¼ 208 [Mþ] (29), 127 (41), 104 (4),
81 (100), 77 (6), 65 (11), 52 (6), 43 (15). Anal. calcd. for C5H5IO: C, 28.87;
H, 2.42. Found: C, 28.81; H, 2.48.

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CsI/BF3?Et2O System
2539
2-lodomethyl-thiopliene (2l). Oily; yield: 0.598 (88%); Rf ¼ 0.79
(hexane:ethyl acetate, 5:1); EI MS: m/z (%) ¼ 224 [Mþ] (48), 141 (11),
127 (32), 97 (100), 83 (23), 51 (19), 43 (8), 32 (6). Anal. calcd. for C5H5IS:
C, 26.80; H, 2.25. Found: C, 26.75; H, 2.31.
4-lodomethyl-pyridine (2m). Yellow needle; yield: 0.56 g (85%); m.p.:
61–63ꢂC; Rf ¼ 0.24 (hexane:ethyl acetate, 5:4); EI MS: m/z (%) ¼ 219
[Mþ] (23), 127 (11), 93 (100), 78 (4), 67 (14), 66 (36), 51 (13), 39 (23).
Anal. calcd. for C6H6IN: C, 32.90; H, 2.76; N, 6.39. Found: C, 32.86; H,
2.67; N, 6.44.
3-Iodo-1-phenyl-1-propene (2n). Colorless plates; yield: 0.62 g (85%);
m.p.: 54–56ꢂ(lit.[10] m.p. 56–57ꢂC); Rf ¼ 0.85 (hexane:ethyl acetate, 5:1);
EI MS: m/z (%) ¼ 244 [Mþ] (56), 167 (43), 127 (31), 117 (100), 106 (11),
91 (19), 77 (9), 57 (23), 43 (25). Anal. calcd. for C9H9I: C, 44.29; H, 3.72.
Found: C, 44.24; H, 3.78.
trans-1-Iodo-2-butene (2o). Oily; yield: 0.45 g (83%); Rf ¼ 0.53 (hex-
ane:ethyl acetate, 5:1); EI MS: m/z (%) ¼ 182 [Mþ] (36), 141 (11), 127
(15), 55 (100), 41 (23), 26 (11). Anal. calcd. for C4H7I: C, 26.40; H, 3.88.
Found: C, 26.46; H, 3.81.
3-Iodoprop-1-yne (2p). Oily; yield: 0.41 g (82%); Rf ¼ 0.48 (hexa-
ne:ethyl acetate, 5:1); EI MS: m/z (%) ¼ 166 [Mþ] (53), 141 (31), 127
(19), 39 (100), 25 (11). Anal. calcd. for C3H3I: C, 21.71; H, 1.82.
Found: C, 21.78; H, 1.88.
1,4-Diiodo-2-butene (2q). Oily; yield: 0.74 g (80%); Rf ¼ 0.64 (hexa-
ne:ethyl acetate, 5:1); EI MS: m/z (%) ¼ 308 [Mþ] (42), 282 (15), 181 (41),
167 (21), 141 (13), 54 (100), 40 (9), 26 (7). Anal. calcd. for C4H6I2: C,
15.60; H, 1.96 Found: C, 15.67; H, 1.89.
ACKNOWLEDGMENT
We express our gratitude to the Deutsche Akademische
Austauschdienst (DAAD, Bonn, Germany) for a sandwich-type scholar-
ship granted to Safdar Hayat.
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Received in the Netherlands September 20, 2002