A new crystal engineering approach for the synthesis of {[K.18-Crown-6]I3}n as a nanotube-like and recyclable catalyst for the chemoselective silylation of alcohols

Article abstract of DOI:10.1007/BF03249082

Reaction of 18-crown-6 with KI in ethanol solution followed by addition of iodine (I₂) afforded a unique triiodide salt with a nanotube-like structure ({[K.18-Crown-6]I₃}_n). It is shown that this reagent may be used for the chemoselective trimethylsilylation of alcohols. The synthesis of the crystalline reagent is a good example of crystal engineering. Reagent was recycled and reused.

Full text of DOI:10.1007/BF03249082

J. Iran. Chem. Soc., Vol. 8, No. 2, June 2011, pp. 484-494.
JOURNAL OF THE
Iranian
Chemical Society
A New Crystal Engineering Approach for the Synthesis of {[K.18-Crown-6]I3}n as a
Nanotube-Like and Recyclable Catalyst for the Chemoselective Silylation of Alcohols
M.A. Zolfigola,*, E. Kolvaria, N. Koukabia, S. Salehzadeha, G. Chehardolib, I.S. Tidmarshc and K. Niknamd
aFaculty of Chemistry, Bu-Ali Sina University, P. O. Box 4135, Hamedan 6517838683, Iran
bSchoolof Pharmacy, Hamedan University of Medical Sciences, Hamedan, Iran
cDepartment of Chemistry, University of Sheffield, Sheffield, S3 7HF, UK
dDepartment of Chemistry, Faculty of Sciences, Persian Gulf University, Bushehr 75169, Iran
(Received 10 July 2010, Accepted 4 October 2010)
Reaction of 18-crown-6 with KI in ethanol solution followed by addition of iodine (I2) afforded a unique triiodide salt with a
nanotube-like structure ({[K.18-Crown-6]I3}n). It is shown that this reagent may be used for the chemoselective
trimethylsilylation of alcohols. The synthesis of the crystalline reagent is a good example of crystal engineering. Reagent was
recycled and reused.
Keywords: {[K.18-Crown-6]I3}n, 18-Crown-6, Triiodide, Nanotube-like, Sylilation of alcohols, Host-guest chemistry
INTRODUCTION
reversible binding of smaller species for storage purposes [16],
crystal packing control over molecular structure [17], or to use
them for rational development of novel predictable bulk
structure with special properties [18]. Recently, nanotubes and
nanoribbons have been synthesized via supramolecular
assembly [19] or by self-assembly of suitable ligands [20].
Various organic triiodides (OTIs) [21-28] and polyiodides
(OPIs) [29-34] have been reported.
Following our studies of the host-guest chemistry of crown
ether 1 [35-39], nitrosation of azacrown ethers 2 [40-43] (Fig.
1) and the synthesis of the unique tribromide reagent 3 [44],
we now report the synthesis and application of a triiodide salt
with a nanotube-like structure ({[K.18-Crown-6]I3}n) 4.
Crown ethers are heterocycles that, in their simplest form,
are cyclic oligomers of dioxane and they have attracted
significant interest from various fields of science [1]. Crown
ether moieties are popular host compounds in host-gust
chemistry and corresponding ligands have shown a remarkable
ability to form strong complexes with organic and inorganic
cations [2-10] or anions [11-12], in a selective manner.
Among the crown ethers, 18-crown-6 forms a very stable
complex with K+ which has a maximum stability constant
when combined with other alkali complexes involving metal
cations [13-15].
The design of both organic and inorganic structures and
being able to organize them in a predicted manner to form
tubular structures is also an area of great active interest. The
idea is to assemble well-established paradigms of crystal
engineering and supramolecular chemistry nanospaces of
suitable dimension to carry out host-guest chemistry,
EXPERIMENTAL
General
Chemicals were purchased from Merck. Substrates,
Silylated products were characterized by comparison of their
1
spectral (IR, UV, H NMR and 13C NMR) and physical data
*Corresponding author. E-mail: zolfi@basu.ac.ir
with those of known samples.

Zolfigol et al.
NO
N
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
-
NO+
O
H(NO3)2
-
-
K+
O
K+
O
Br3
I3
n
n
N
NO
1
2
3
4
Fig. 1. Some microcycle structures.
Preparation of {[K.18-Crown-6]I3}n
C12H24I3KO6, M = 684.11, monoclinic, space group Cc, a =
8.381(11), b = 14.973(19), c = 17.322(2) Å, β = 98.612(2)˚,
V = 2149.2(5) Å3, Z = 4, Dc = 2.114 mg m-3, μ (Mo-Kα) =
4.576 mm-1, T = 150 K. Refinement of 199 parameters with
4408 unique reflections afforded a final R1 value [for selected
data with I > 2σ (I)] of 3.09% and a final wR2 value (all data)
of 5.01%.
In a round-bottomed flask (100 ml) equipped with a
magnetic stirrer, KI (1.32 g, 8 mmol) was added to a solution
of 18-crown-6 (1.32 g, 5 mmol) in ethanol (30 ml) and the
mixture was stirred for 1 h, then filtered. Iodine (1.27 g, 5
mmol) was added to the solution. After filtration over a
sintered glass funnel, a fine dark brown powder was obtained
and recrystallized from CH3CN to give dark brown crystals
[m.p.: 207-210 °C].
RESULTS AND DISCUSSION
General Procedure for the Trimethylsilylation of
Alcohols
The crystalline reagent with a nanotuble-like structure was
made through the packing of [K.18-Crown-6]+ cations- and
bridging symmetrical multi nuclear linear anions. We found
that the reaction of I2 with complex [K.18-Crown-6]I, easily
produces the triiodide reagent 4 (Scheme 1). Recrystallization
of the latter compound in acetonitrile gave brownish crystals,
suitable for X-ray crystallogeraphy (Fig. 2).
Crystallographic analysis confirmed the formulation of
these crystals as {[K.18-Crown-6]I3}n, in which the [K.18-
Crown-6]+ cations are aligned in columnar stacks separated by
bridging triiodide anions (Figs. 3, 4). Each triiodide ligand
interacts with two K+ ions, although the coordination is far
from linear, with I-I-K angles of 83.1˚. The latter bond angle is
very similar to the corresponding Br-Br-K angle, 83.2˚, in the
{[K.18-Crown-6]Br3}n [44]. Thus, the columnar stacks are
infinite one-dimensional polymeric chains of alternating
cations and anions. This is in close agreements with the
reported crystallographic data by Bock et al. [45]. The
coordination geometry in the [K.18-crown-6]+ cation is
unremarkable.
To a solution of the alcohol (1 mmol) in CH2Cl2 (4 ml),
HMDS (0.8 mmol) and {[K.18-Crown-6]I3}n 4 (0.1 mmol)
were added. The solution was vigorously stirred at room
temperature. After compellation the reaction, dichloromethane
was removed by water-bath distillation. The residue was
dissolved in n-hexane (5 ml) and filtered off. The filtrate was
washed with n-hexane (2 × 10 ml). n-Hexane was removed by
distillation to yield pure products.
General Procedure for Deprotection of Trimethylsilyl
Ethers Catalyzed by {[K.18-Crown-6]I3}n
{[K.18-Crown-6]I3}n (0.1 mmol) was added to a mixture
of TMS ethers (1 mmol) in CH2Cl2 (4 ml) moistened by three
drops of water and then the mixture was stirred at room
temperature for the specified time (see Table 5). The progress
of the reaction was monitored by TLC. On completion of the
reaction, column chromatography on short pad of silica gel
afforded pure products.
To the best of our knowledge, silylation of alcohols and
polyols is one of the simplest methods used for their protection
[46]. Trimethylsilylation is a classic way to produce volatile
derivatives of alcohols and polyols, as required for their vapor
Experimental Description of Structure
An orange crystal (0.40 × 0.20 × 0.10 mm3) was mounted
on a Bruker SMART-1000 diffractometer. Crystal data:
485

A New Crystal Engineering Approach for the Synthesis of {[K.18-Crown-6]I3}n
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
KI
I2
K+
O
K+
O
I-
-
I3
EtOH, r.t.
EtOH, r.t.
O
n
4
Scheme 1. Schematic synthesis of {[K.18-Crown-6]I3}n (4)
phase chromatography/mass spectrometry analysis [47].
Another application is conversion of trimethylsilyl ethers into
the corresponding ethers [48], so that the described procedure
due to its special ability has been selected by the American
Chemical Society (ACS) as heart-cut [49]. Generally, the
formation of trimethylsilyl ethers has been carried out by
treatment of alcohols with trimethylsilyl chloride or
trimethylsilyl triflate in the presence of base [50], Li2S [51],
and sometimes a nonionic super base catalyst [52]. However,
some of those methods have frequently suffered from
drawbacks, such as a lack of reactivity or the difficulty in
removal of amine salts derived from the reaction of by-
produced acid and co-bases during the silylation reaction.
1,1,1,3,3,3-Hexamethyldisilazane (HMDS) is
commercially available, and cheap reagent
a
stable,
for
Fig. 2. Crystals of (4).
trimethylsilylation of hydrogen-labile substrates [53], giving
ammonia as the only by-product.
On the other hand, silylation, using this silazan-type
reagent, is nearly neutral and does not need any special
precautions. However, the low silylating power of HMDS is
the main drawback for its application which needs forceful
conditions and long reaction times etc. A variety of catalysts
such as (CH3)3SiCl [54], silica chloride [55], ZnCl2 [56],
zirconium sulfophenyl phosphonate [57], LiClO4 [58], K-10
montmorilonite [59], H-β zeolite [60], tungestophosphoric
acid [61], poly(N-bromobenzene-1,3-disulfonamide) [62],
CuSO4.5H2O and Cu(OTf)2 [63], MgBr2.OEt2 [64], iodine
[65], Al(HSO4)3 [66], KBr [67], Al(OTf)3 [68], magnesium
triflate [69], silica supported perchloric acid [70], sulfonic
acid-functionalized ordered nanoporous silica [71],
trichloroisocyanuric acid [72] and 1,3-dibromo-5,5-
diethylbarbituric acid [73] have been reported for the silylation
of hydroxyl groups using HMDS [54-73]. However, in most of
these cases a long reaction time, drastic reaction conditions,
and the use of hazardous solvents or tedious work-up are
Fig. 3. X-Ray structure of {[K.18-Crown-6]I3}n (4).
486

Zolfigol et al.
the progress of reactions. As illustrated in the Table 1, we
needed. In addition, many of these reagents are moisture
sensitive or expensive.
have found that dichloromethane is the best solvent for the
above mentioned reactions due to fast reaction times and high
yields.
The lack of a facile and general synthetic methodology for
the silylation of alcohols, under essentially neutral conditions,
promoted us to develop an efficient procedure for the
protection of hydroxyl groups using {[K.18-Crown-6]I3}n as a
novel and recyclable catalyst for this purpose. Herein we wish
to report the first application of {[K.18-Crown-6]I3}n as an
excellent and recyclable catalyst for the chemoselective
trimethylsilylation of various alcohols without observation any
side products (Scheme 2).
The trimethylsilylation of hydroxyl groups is easily carried
out at room temperature under mild conditions. A good range
of alcohols were subjected to the trimethylsilylation in the
presence of a catalytic amount of {[K.18-Crown-6]I3}n as an
efficient catalyst and HMDS as a silylating system. The
corresponding trimethylsilyl ethers were obtained in good to
excellent yields under mild and homogeneous conditions
(Table 2 and Scheme 2).
Firstly, we investigated the effect of various solvents on
Table 1. Silylation of Benzyl Alcohol with HMDS in the Presence of {[K.18-Crown-6]I3}n
(4), as a Catalyst at Room Temperature in Various Solvents
Entry
Solventa
Substrate
Time
(h:min)
06:00
00:15
00:50
03:00
06:00
00:20
6:00
Yield
(%)b
65
100
95
100
85
100
92
HMDS
Catalyst
1
2
3
4
5
6
7
n-Hexane
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.1
0.1
0.1
0.1
0.1
0.1
0.1
Dichloromethane
Chloroform
Ethyl acetate
Diethyl ether
Acetonitrile
CCl4
aThe amount of solvent was chosen 2 ml. bGC yields.
Table 2. Silylation of Alcohols (1 mmol), with HMDS (0.8 mmol) in the Presence of (4), as a
Catalyst (0.1 mmol) at Room Temperature
Time
Yield
(%)a
Entry
1
Substrate
Product
(h:min)
CH2OH
CH2OTMS
00:16
00:12
100
CH2OTMS
CH2OH
2
3
100
H3CO
H3CO
CH2OTMS
CH2OTMS
CH2OH
00:20
100
Cl
Cl
Cl
CH2OH
4
5
00:37
00:10
100
100
Cl
Cl
Cl
OTMS
OH
487

A New Crystal Engineering Approach for the Synthesis of {[K.18-Crown-6]I3}n
Table 2. Continued
CH2CH2OH
CH2CH2OTMS
6
7
00:35
00:20
100
100
OTMS
OH
OH
OTMS
8
9
00:15
00:15
100
100
OH
OH
OH
OTMS
OTMS
10
11
12
01:30
00:15
00:16
100
100
100
OTMS
OTMS
OH
13
14
15
00:25
00:22
3:25
100
100
100
OH
OTMS
OTMS
OTMS
OH
OH
OMe
OH
OMe
OTMS
O
OH
O
OTMS
16
00:40
100
H
H
H
H
CH3
CH3
H3C
H3C
17
18
01:15
00:15
95b
H
H
CH3
CH3
H
H
H
H
OTMS
OH
OH
OTMS
OTMS
100
19
20
21
00:25
1:30
100
100
74b
OH
OTMS
OH
OH
OTMS
Me
09:00
Me
Me
Me
Me
Me
aGC Yield. bIsolated yield.
488

Zolfigol et al.
In order to investigate the selectivity of the catalyst,
To investigate the recyclability of the [K.18-Crown-6]I3,
the residue after work-up was washed with n-hexane and the
brown solid was obtained. {[K.18-Crown-6]I3}n was recycled
and reused for the silylation of alcohols successfully (Table 3).
various reactions have been carried out using different types of
alcohols and other functional groups (Table 4). The catalyst
shows good selectivity for 1° alcohols in the presence of 2°
O
O
O
O
K+
I3
O
0.1 eq
O
ROH
ROTMS
HMDS (0.8 eq), CH2Cl2, r.t.
Scheme 2. Silylation of alcohols in the presence of {[K.18-Crown-6]I3}n, (4), as catalyst
Table 3. Silylation of Alcohols (1 mmol), with HMDS (0.8 mmol) in the Presence of Recovered
{[K.18-Crown-6]I3}n, (4), as a Catalyst (0.1 mmol) at Room Temperature
Run 1
Run 2
Entry
Substrate
Product
Yield
Yield
(h:min)
(h:min)
(%)a
100
(%)a
100
CH2OTMS
CH2OTMS
OTMS
CH2OH
1
2
00:45
01:20
CH2OH
OH
00:25
100
00: 20
100
F
F
3
4
01:35
00:33
00:20
100
100
100
06:00
06:00
06:00
60
84
90
OH
OH
OTMS
OTMS
5
aGC Yield.
Table 4. Selective O-Trimethylsilylation of Alcohols in the Presence of HMDS and (4) as
Catalyst at Room Temperaturea
Product binary
Time
Yield
(%)b
Entry
1
Substrate
mixture
(h:min)
CH2OH
CH2OTMS
100
00:30
00:35
OTMS
OH
0
OH
OTMS
100
0
2
Me
Me
OH
Me
Me
OTMS
489

A New Crystal Engineering Approach for the Synthesis of {[K.18-Crown-6]I3}n
Table 4. Continued
OMe
OH
O
OH
3
2:00
-
Mix of Products
OH
OTMS
100
0
4
5
00:40
OTMS
Me
OH
Me
Me
Me
Me
Me
OH
OTMS
100
00:20
Me
Me
Me
OTMS
Me
OH
0
CH2OTMS
CH2OH
100
0
6
7
00:15
00:30
TMS
N
H
N
OH
OTMS
100
0
Me
Me
OH
Me
Me
OTMS
OH
OTMS
90
8
00:25
00:15
00:25
OH
OTMS
10
CH2OH
CH2OTMS
100
Cl
Cl
9
Me
Me
OH
Me
Me
0
OTMS
OTMS
OH
100
0
10
Me
Me
Me
Me
OTMS
OH
aMolar ratio, substrate 1/substrate 2/HMDS/catalyst: 1/1/0.8/0.1; bGC yields.
490

Zolfigol et al.
and 3° alcohols. The amines will mainly remain intact in the
In addition, we investigated the possibility of deprotection
of trimethylsilyl ethers using (4) as a catalyst in wet
dichloromethane (Scheme 4). We successfully converted
selected TMS ethers to the parent hydroxyl compounds in the
presence of catalytic amount of {[K.18-Crown-6]I3}n at room
temperature. The results are summarized in Table 5.
presence of alcohols under silylation conditions (Table 4,
entry 6). Interestingly we found that 2-thionaphthol is
converted to 2-naphyl disulfide under silylation condition in
the presence of benzyl alcohol, so no silylated product was
observed (Table 4, entry 11).
We also decided to use {[K.18-Crown-6]Br3}n, for the
silylation of alcohols but the reaction was not applicable for
this purpose due to the side product and oxidation of alcohols
(Scheme 3).
The following mechanism for the silylation reaction may
be suggested, according to the previously reported reagents
[44,65] and the obtained results (Scheme 5).
In conclusion, the mildness and recycle ability of {[K.18-
Scheme 3. Silylation of benzyl alcohol in the presence of {[K.18-Crown-6]Br3}n, (3), as a catalyst
O
O
O
O
O
K+
O
I3
0.1eq
ROH
ROTMS
CH2Cl2/ 3 drops of H2O, r.t.
Scheme 4. Desilylation of benzyloxytrimethylsilane in the presence of {[K.18-Crown-6]I3}n, (4), as catalyst
Table 5. Deprotection of Trimethylsilyl Ethers (1 mmol) in the Presence of Catalytic Amount of
(4) as Catalyst in CH2Cl2 (5 ml) Moistened by 3 Drops of Water at Room Temperature
Entry
1
Substrate
Product
Time (h:min)
Yield(%)a
CH2OTMS
CH2OH
00:20
00:40
100
100
2
3
OH
OTMS
CH2OTMS
CH2OH
00:25
100
Cl
Cl
OTMS
OTMS
OH
4
00:35
00:18
100
100
OH
5
aGC Yield.
491

A New Crystal Engineering Approach for the Synthesis of {[K.18-Crown-6]I3}n
H
N
n
Me3Si
n[K.18-Crown-6]I
SiMe3
I
I
NH
{[K.18-Crown-6]I3}n
Me3Si
n
SiMe3
n
R
OH
n[K.18-Crown-6]I
n
n
H3N I2
R
OTMS
n
Me3Si NH2I
I
n
R
OTMS
R
OH
n
Scheme 5. A reasonable mechanism for the silylation reaction with [K.18-Crown-6]I3
Crown-6]I3}n is the major advantage of the described nano-
tube like reagent making it an attractive and useful reagent for
organic functional group transformations [77-79].
[5]
[6]
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ACKNOWLEDGMENTS
Financial support of this work by the Research Deputy of
Ministry of Science, Research and Technology of I.R. Iran,
Center of Excellence in Development of Chemical Methods
(CEDCM) of Bu-Ali Sina University, Hamedan, Iran, and
Professor Michael D. Ward for supporting the X-ray
crystallography structure from the University of Sheffield,
Sheffield, S3 7HF, UK, is gratefully acknowledged.
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