{[K.18-Crown-6]Br3}n: a unique tribromide-type and columnar nanotube-like structure for the oxidative coupling of thiols and bromination of some aromatic compounds

Article abstract of DOI:10.1016/j.tetlet.2007.09.050

Reaction of 18-crown-6 with KBr and then with bromine (Br₂) afforded a unique tribromide salt with a nanotube-like structure ({[K.18-crown-6]Br₃}_n). Oxidative coupling of thiols and bromination of some aromatic compounds is the first report on the chemistry of this reagent.

Full text of DOI:10.1016/j.tetlet.2007.09.050

Tetrahedron Letters 48 (2007) 7969–7973
{[K.18-Crown-6]Br3}n: a unique tribromide-type and columnar
nanotube-like structure for the oxidative coupling of thiols
and bromination of some aromatic compounds
Mohammad Ali Zolfigol,a,* Gholamabbas Chehardoli,a Sadegh Salehzadeh,a
Harry Adamsb and Michael D. Wardb
aFaculty of Chemistry, Bu-Ali Sina University, PO Box 4135, Hamedan 6517838683, Iran
bDepartment of Chemistry, University of Sheffield, Sheffield, S3 7HF, UK
Received 3 August 2007; revised 30 August 2007; accepted 7 September 2007
Available online 12 September 2007
Abstract—Reaction of 18-crown-6 with KBr and then with bromine (Br2) afforded a unique tribromide salt with a nanotube-like
structure ({[K.18-crown-6]Br3}n). Oxidative coupling of thiols and bromination of some aromatic compounds is the first report
on the chemistry of this reagent.
ꢀ 2007 Elsevier Ltd. All rights reserved.
1. Introduction
aromatic aldehydes11 and bromination of aromatic sub-
strates12 were reported. It would be extremely useful to
Crown ethers have attracted significant attention from
various field of science. Crown ether moieties are popu-
lar host compounds in host–guest chemistry and these
ligands have shown a remarkable ability to form strong
complexes with organic and inorganic cations1 or an-
ions,2 selectively. Among the crown ethers, 18-crown-6
forms a very stable complex with K+, which has the
maximum stability constant compared with other alkali
complexes involving metal cations.3
NO
O
N
O
O
O
O
O
O
O
O
-
NO+
O
H(NO3)2
N
NO
1
2
Organic tribromide reagents (OTBs) are preferable as
oxidants to molecular bromine, owing to the hazards
associated with elemental bromine. Several tribromides
have been reported, that is, tetramethylammonium
tribromide,4 phenyltrimethylammonium tribromide,5
cetyltrimethylammonium tribromide, tetrabutylammo-
nium tribromide,6 1,8-diazabicyclo[5,4,0]-tetrabutyl-
ammonium tribromide,7 pyridine hydrobromide
perbromide,8 hexamethylenetetramine-bromine,9 and
DABCO-bromine.10 Recently some ionic liquid tribro-
mides (IL-Br3À) for the preparation of bromoesters from
Figure 1.
Keywords: {[K.18-Crown-6]+Br3À}n; 18-Crown-6; Tribromide; Nano-
tube-like; Bromination; Columnar; Oxidative coupling; Thiols; Host–
guest chemistry.
*
Corresponding author. Tel./fax: +98 811 8257407; e-mail addresses:
zolfi@basu.ac.ir; mzolfigol@yahoo.com
Figure 2. Crystals of (3).
0040-4039/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.09.050

7970
M. A. Zolfigol et al. / Tetrahedron Letters 48 (2007) 7969–7973
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
KBr
Br2
K+
K+
O
Br-
-
Br3
EtOH, rt
EtOH, rt
O
n
3
Scheme 1.
develop further synthetic protocols for the synthesis of
OTBs.13
In continuation of our studies on the host–guest chem-
istry of crown ether 1,14 nitrosation of azacrown ethers
215 (Fig. 1), oxidative coupling of thiols,16 and bromin-
ation,17 we found that when Br2 was added to the com-
plex [K.18-crown-6]Br, the unique tribromide reagent 3
was formed easily and after recrystallization from aceto-
nitrile, nice red crystals were obtained (Fig. 2 and
Scheme 1).
Figure 4. View of X-ray structure of {[K.18-crown-6]Br3}n from
another perspective.
Crystallographic analysis confirmed the formulation of
these crystals as {[K.18-crown-6]Br3}n, in which the
[K.18-crown-6]+ cations are aligned in columnar18
stacks separated by bridging tribromide anions (Figs. 3
and 4). Each tribromide ligand spans two K+ ions,
although the coordination is far from linear, with Br–
Br–K angles of 83.2ꢁ. Thus the columnar stacks are infi-
nite one-dimensional polymeric chains of alternating
cations and anions. The coordination geometry in the
[K.18-crown-6]+ cation is unremarkable.
O
O
O
O
O
Ar-H
Thiol
K+
O
-
Ar-Br
Disulfide
Br3
CH3CN, rt
CH3CN, rt
n
Scheme 2.
Table 1. Rapid coupling of thiols using {[K.18-crown-6]Br3}n in
acetonitrile at room temperature under mild conditions
We have found that this reagent can be used for the fast
oxidative coupling of thiols, and bromination of aro-
matic compounds under mild conditions (Scheme 2
and Tables 1 and 2).
Entry Substrate
Product
Reagent/ Yield
substrate (%)
(mmol)
SH
SH
S
1
1.1
1.1
86a
2
2
100b
Me
S
Me
2
Cl
Cl
3
1.1
1.1
1.1
1.1
1.1
100b
SH
S
2
Cl
Cl
100b
S
Br
4
Br
SH
2
5
S
100b
SH
2
N
N
N
6
7
Sluggish
Sluggish
SH
S
2
N
N
S
N
S
S
SH
2
a Isolated yield.
b Conversion.
Figure 3. X-ray structure of {[K.18-crown-6]Br3}n.

M. A. Zolfigol et al. / Tetrahedron Letters 48 (2007) 7969–7973
7971
Table 2. Rapid bromination of aromatic compounds using {[K.18-crown-6]Br3}n in acetonitrile at room temperature under mild conditions
Entry
1
Substrate
Product
Reagent/substrate (mmol)
1
Yielda (%)
70
Br
NMe2
NMe2
NEt2
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
73
Br
Br
Br
NEt2
85
74
60
57
56
75
N
O
N
N
O
OH
OH
OH
N
OH
MeO
Br
OMe
MeO
Me
OMe
Me
NMe2
NMe2
O
Br
O
O
O
O
Br
Br
N
N
O
O
O
OH
Br
Cl
OH
OH
Br
Br
Cl
OH
1
1
37
10
—
Sluggish
OH
a Isolated yield.
To investigate the recyclability of the [K.18-crown-6]Br
complex, the residue after work-up was washed with eth-
anol and Br2 was added. The yellow solid obtained was
recrystallized from acetonitrile. {[K.18-Crown-6]Br3}n
was generated again and brominated the aromatic
amines successfully (Scheme 3). Also the oxidative
coupling of thiols was reproduced by recycled
{[K.18-crown-6]Br3}n as efficiently as with a fresh
sample.
O
O
O
O
Competitive reactions were designed. Mixtures of equal
amounts of aromatic substrates were subjected to bro-
mination in the presence of {[K.18-crown-6]Br3}n and
good chemoselectivity was observed (Scheme 4).
O
O
O
O
N
N
+ HBr
In conclusion, the ability of {[K.18-crown-6]Br3}n to
generate bromine in the reaction media will be useful
for organic functional group transformations.19
Br
O
O
O
O
O
K+
-
Br-
O
O
O
Br3
K+
n
O
2. Experimental
O
O
O
n
2.1. Preparation of {[K.18-crown-6]Br3}n
In a round-bottomed flask (100 mL) equipped with a
magnetic stirrer, KBr (0.95 g, 8 mmol) was added to a
solution of 18-crown-6 (1.32 g, 5 mmol) in ethanol
(30 mL) and the mixture stirred for 1 h, then filtered.
Bromine (0.8 g, 5 mmol) was added to the solution.
Br2
Scheme 3.

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M. A. Zolfigol et al. / Tetrahedron Letters 48 (2007) 7969–7973
N
Br
N
{[K.18-crown-6]Br3}n
+
+
N
H
CH3CN
N
H
Br
0%
100%
O
O
OMe
OMe
N
{[K.18-crown-6]Br3}n
CH3CN
N
+
+
Br
Br
0%
100%
O
O
O
O
O
O
O
N
O
OMe
OMe
N
{[K.18-crown-6]Br3}n
CH3CN
+
+
Br
Br
0%
100%
Scheme 4.
After filtration over a sintered glass funnel, a fine yellow
powder was obtained and recrystallized from CH3CN to
give red crystals [mp: 195–197 ꢁC].
I.R., Iran (Grant number: 32-2450, entitled ‘Synthesis
of various kryptofixe based nanotubes’), Center of
Excellence of Development in Chemical Methods
(CEDCM) of Bu-Ali Sina University, Hamedan, Iran,
and X-ray crystallography structure from the University
of Sheffield, Sheffield, S3 7HF, UK, is gratefully
acknowledged.
2.2. General procedure for reactions
{[K.18-Crown-6]Br3}n was added to a solution of the
substrate (1 mmol) in acetonitrile (4 mL) at room
temperature (see Tables 1 and 2 for molar ratios). After
immediate reaction, column chromatography on short
pad of silica gel with CH3CN afforded pure products
in 37–86% yields.
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˚
P21/n, a = 8.955(5), b = 8.054(5), c = 14.082(8) A,
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Financial support of this work by the Research Deputy
of Ministry of Science, Research and Technology of

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