Silica Chloride (SiO2-Cl), a New Heterogeneous Reagent, for the Selective and Efficient Conversion of Benzylic Alcohols to Their Corresponding Chlorides and Iodides

Article abstract of DOI:10.1081/SCC-120025175

Structurally different benzylic alcohols were efficiently converted to their corresponding chlorides by silica chloride (SiO₂-Cl) in CHCl₃ at room temperature. Silica chloride is also able to convert benzylic alcohols to their iodides in the presence of NaI in a mixture of CH₃CN/CHCl₃ in excellent yields.

Full text of DOI:10.1081/SCC-120025175

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Silica Chloride (SiO2-Cl), a New Heterogeneous
Reagent, for the Selective and Efficient Conversion of
Benzylic Alcohols to Their Corresponding Chlorides and
Iodides
Habib Firouzabadi a , Naser Iranpoor a , Babak Karimi a & Hassan Hazarkhani a
a Department of Chemistry , College of Sciences , Shiraz University , Shiraz, Iran
Published online: 16 Aug 2006.
To cite this article: Habib Firouzabadi , Naser Iranpoor , Babak Karimi & Hassan Hazarkhani (2003) Silica Chloride (SiO2-Cl), a
New Heterogeneous Reagent, for the Selective and Efficient Conversion of Benzylic Alcohols to Their Corresponding Chlorides
and Iodides, Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
33:21, 3671-3677, DOI: 10.1081/SCC-120025175
To link to this article: http://dx.doi.org/10.1081/SCC-120025175
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SYNTHETIC COMMUNICATIONSÕ
Vol. 33, No. 21, pp. 3671–3677, 2003
Silica Chloride (SiO2-Cl), a New Heterogeneous
Reagent, for the Selective and Efficient
Conversion of Benzylic Alcohols to Their
Corresponding Chlorides and Iodides
*
*
Habib Firouzabadi, Naser Iranpoor, Babak Karimi,
and Hassan Hazarkhani
Department of Chemistry, College of Sciences,
Shiraz University, Shiraz, Iran
ABSTRACT
Structurally different benzylic alcohols were efficiently converted to
their corresponding chlorides by silica chloride (SiO2-Cl) in CHCl3 at
room temperature. Silica chloride is also able to convert benzylic
alcohols to their iodides in the presence of NaI in a mixture of
CH3CN/CHCl3 in excellent yields.
Key Words: Silica chloride; Benzylic alcohols; Benzylic chlorides;
Benzylic iodides.
*Correspondence: Habib Firouzabadi and Naser Iranpoor, Department of
Chemistry, College of Sciences, Shiraz University, Shiraz 71454, Iran; Fax:
þþ98-711-2286008; E-mail: firouzabadi@chem.susc.ac.ir; iranpoor@chem.
susc.ac.ir.
3671
DOI: 10.1081/SCC-120025175
Copyright & 2003 by Marcel Dekker, Inc.
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3672
Firouzabadi et al.
INTRODUCTION
Generation of halides from corresponding alcohols[1,2] is an important
transformation in organic synthesis. The important role of halides in the
carbon–carbon bond formation by organometallic and free radical
reactions as well as nucleophilic substitution has been well documented.[3]
One of the most commonly used reagent for this purpose is thionyl chloride
that can be applied alone or in combination with bases. However, the
reactions are invariably slow and often require reflux conditions.[4]
Recently a modified procedure is reported in which thionyl chloride has
been used in the presence of benzotriazole at room temperature for this
purpose.[5] It is also reported that iodine in petroleum ether at 60–80ꢀC can
directly convert alcohols to their corresponding iodides with rather low
yields.[6] It has also been established that alcohols can be converted to the
corresponding iodides or bromides by the reaction of TMSI or TMSBr
respectively.[19,20] However, similar reaction does not occur with the less
reactive TMSCl alone, although, it has been shown that the transforma-
tion can be accomplished with TMSCl when a catalytic amount of SeO2,[2]
BiCl3,[7] or DMSO[8] has been employed. In recent years, we have paid
attention to the preparation of the high capacity silica chloride and to
explore its synthetic utilities as a catalyst and as a reagent in organic
reactions.[9–13]
RESULTS AND DISCUSSIONS
In continuation of this study we have found that in contrast to
TMSCl alone, silica chloride can be simply applied specifically for the
conversion of aryl alcohols to their corresponding chlorides (Sch. 1).
Compounds under this study were primary, secondary, allylic,
propargylic, and benzylic alcohols. Among different solvents such as;
CHCl3, CH3CN, THF, and CH2Cl2, chloroform turned out to be a
suitable solvent for this transformation. Our preliminary observations
showed that, except benzyl alcohol (Table 1, Entry 1), chlorination of
OH
Cl
SiO2
CHCl3, rt
Cl
1
2
1
2
R
R
R
R
R1, R2 = Aryl or R1, R2 = Activated aryl and H
Scheme 1.

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Conversion of Benzylic Alcohols
3673
Table 1. Chlorination of alcohols using silica chloride (SiO2-Cl).
Yielda,b
(%)
Entry
R1
R
Time (h)
1
2
3
4
5
Ph
H
24
5
27c,[14]
88[15]
93[1]
p-MeC6H4
p-MeOC6H4
Ph
H
H
0.75
4
Et
Ph
90[6]
Ph
5
89[15]
6
4.5
5
89d,[16]
7
87d,[17]
8
9
PhCH2CH2CH2
CH3 (CH2)5
CH3 (CH2)4
CH3 (CH2)4
H
Me
24
24
24
24
NR
NR
20c,[15]
23c,[18]
10
11
HCꢁC
H2C¼CH
aIsolated yields.
b1H NMR and 13C NMR of some of the halides are given in the
Ref.[13]
cGC yields.
dLess than 10% of the reaction mixture was composed of the
corresponding benzylic alkenes.
substituted benzyl alcohols with electron-donating groups such as –CH3
and –OCH3 proceeded very well in high yields at room temperature in the
presence of this reagent (Table 1, Entries 2, 3). Conversion of a variety of
secondary benzylic alcohols to the corresponding chlorides was also
achieved in good to excellent yields under similar reaction conditions
(Table 1, Entries 4–7). Moreover, we have found that primary and
secondary aliphatic alcohols remained intact in the presence of this
reagent in CHCl3 at room temperature (Table 1, Entries 8, 9). However,
we also noticed that the presented procedure was not satisfactory for
the efficient conversion of allylic and propargylic alcohols to their
corresponding chlorides (Table 1, Entries 10,11).
The earlier works of Jung[19] and Olah[20] showed the effective use of
TMSI and TMSBr for the conversion of alcohols to the corresponding
halides. Moreover, it has been shown that chlorosilanes in combination
with iodide and bromide anions convert alcohols to iodides and bromides
via in situ generation of iodo- and bromosilanes respectively. These

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3674
Firouzabadi et al.
OH
I
SiO
Cl(NaI)
2
2
1
R
1
2
R
R
R
CHCl /CH CN, rt
3
3
1
2
R = aryl, alkyl R = aryl, alkyl, H
Scheme 2.
results prompted us to study the possibility of the in situ formation of
inorganic solid analogs of iodo- and bromosilanes by the reaction of
silica chloride with iodide and bromide anions. We first examined in
situ generation of silica iodide by the reaction of silica chloride with a
solution of molar excess of NaI in a CH3CN/CHCl3 solvent system. After
120 min, the reaction of benzyl alcohol with the resulting silica iodide was
studied. We have observed that, in contrast to silica chloride, in this case,
the reaction proceeded well to afford benzyl iodide in 89% isolated yield
(Sch. 2, Table 2, Entry 1).
This result encouraged us to investigate the ability of the in situ
generated silica iodide for the conversion of the other structurally differ-
ent alcohols to their iodides. We have found that different types of pri-
mary benzylic alcohols with electron-donating and electron-withdrawing
substituents and also secondary benzylic alcohols were converted to the
corresponding iodides in good to excellent yields (Table 2, Entries 2–6).
Surprisingly, iodination of the alcohol (Table 2, Entry 7) was accompa-
nied with the elimination reaction and the corresponding alkene was
obtained in 92% yield. Interestingly, allylic and propargylic alcohols
were converted to the corresponding iodides in moderate to good yields
upon treatment with the in situ generated silica iodide (Table 2, Entries 8,
9). However, this method was not suitable for the conversion of primary
and secondary aliphatic alcohols to their iodides (Table 2, Entries 10, 11).
Unfortunately, our efforts for the preparation of alkyl and benzyl bro-
mides using in situ generated silica bromide (silica chloride treated with
an excess of LiBr) were not satisfactory, and in most cases, a mixture of
the chloride and bromide was formed in the reaction mixture.
EXPERIMENTAL
General. Silica chloride (SiO2-Cl) was prepared according of the
reported procedure.[8] IR spectra were recorded on a Perkin-Elmer 781

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Conversion of Benzylic Alcohols
3675
Table 2. Iodination of alcohols using silica chloride (SiO2-Cl)/NaI in a
mixture of CHCl3/CH3CN.
Time Yielda,b,ref
Entry
R1
R2
(h)
(%)
1
2
3
4
5
6
Ph
H
2
89[5]
p-MeC6H4
p-MeOC6H4
p-ClC6H4
Ph
H
1.3
1
91[17]
90[17]
88[17]
89[17]
86[17]
H
H
3
Et
Ph
1.5
2
Ph
7
1.5
92d,[15]
8
9
9-(Hydroxymethyl) anthracene
1,4-Benzenedimethanol
Cinnamyl alcohol
CH3 (CH2)4
4
5
2
2
2
2
2
80[21]
88[15]
89b,[22]
NRc
NRc
NRc
NRc
10
11
12
13
14
HCꢁC
H2C¼CH
H
CH3(CH2)4
PhCH2CH2CH2
CH3 (CH2)5 Me
Me
aIsolated yields.
bReactions were performed at room temperature with the ratio of sub-
strates: silica chloride: NaI 1mmol/1g/3 mmol.
c4 mmol NaI was used.
dGC yields.
spectrophotometer.1H NMR and 13C NMR spectra were run on a Bruker
Avance DPX 250 MHz instrument. Mass spectra were run on a
Shimadzu GC MS-QP 1000 EX. Products are known and they were
identified by the comparison of their spectra and physical constants
with authentic samples.
Conversion of 4-methoxybenzyl alcohol to 4-methoxybenzyl chloride, a
typical procedure. To a solution of 4-methoxybenzyl alcohol (0.69 g,
5 mmol) in CHCl3 (25 mL), silica-chloride (5 g) was added and the result-
ing mixture was stirred at room temperature for 0.75 h. After completion
of the reaction (TLC, n-hexane/EtOAc, 5/1), the mixture was filtered
through a silica gel pad (3 cm thick). Evaporation of the solvent afforded
liquid 4-methoxybenzyl chloride in 93% yield (Table1, entry 3).
Conversion of benzyl alcohol to benzyl iodide. To a solution of sodium
iodide (2.25 g, 15 mmol) in CHCl3/CH3CN (15/10 mL), silica chloride
(5 g) was added and stirred for 5 min. To the resulting mixture, benzyl

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3676
Firouzabadi et al.
alcohol (0.54 g, 5 mmol) was added and the resulting mixture was
stirred at room temperature for 2 h. After completion of the reaction
(TLC, n-hexane/EtOAc, 5/1), Na2S2O3 powder (0.5 g) was added to the
mixture and the solvent was evaporated under reduced pressure. The
resulting solid mixture was added on a silica gel pad (5 cm thick) and
was washed with petroleum ether (200 mL). After filteration, the solvent
was evaporated to dryness to afford pure benzyl iodide in 89% yield as a
low melting yellow crystalline compound (Table 2, Entry 1).
ACKNOWLEDGMENT
The authors are thankful to The Shiraz University Research Council
for the partial support of this work.
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Received in the UK February 11, 2002

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