A task-specific ionic liquid [bmim]SCN for the conversion of alkyl halides to alkyl thiocyanates at room temperature

Article abstract of DOI:10.1016/j.tetlet.2005.01.040

A new task-specific ionic liquid (TSIL), 1-n-butyl-3- methylimidazolium thiocyanate ([bmim]SCN), has been prepared and used for the first time as the medium as well as reactant for the synthesis of alkyl thiocyanates from the corresponding alkyl halides by thiocyanate-halide exchange at room temperature. The alkyl thiocyanate products can be easily isolated from the reaction mixture by simple extraction and the ionic liquid 1-n-butyl-3-methylimidazolium halide may be reused for the synthesis of the ionic liquid [bmim]SCN and recycled for further use.

Full text of DOI:10.1016/j.tetlet.2005.01.040

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 1489–1491
A task-specific ionic liquid [bmim]SCN for the conversion of
alkyl halides to alkyl thiocyanates at room temperature
*
Ahmed Kamal and Gagan Chouhan
Biotransformation Laboratory, Division of Organic Chemistry, Indian Institute of Chemical Technology, Hyderabad 500 007, India
Received 3 November 2004; revised 21 December 2004; accepted 7 January 2005
Available online 25 January 2005
Abstract—A new ‘‘task-specific’’ ionic liquid (TSIL), 1-n-butyl-3-methylimidazolium thiocyanate ([bmim]SCN), has been prepared
and used for the first time as the medium as well as reactant for the synthesis of alkyl thiocyanates from the corresponding alkyl
halides by thiocyanate-halide exchange at room temperature. The alkyl thiocyanate products can be easily isolated from the reaction
mixture by simple extraction and the ionic liquid 1-n-butyl-3-methylimidazolium halide may be reused for the synthesis of the ionic
liquid [bmim]SCN and recycled for further use.
ꢀ
2005 Published by Elsevier Ltd.
Room temperature ionic liquids (ILs) are attracting con-
siderable attention as solvents for multiphasic catalysis
because they can be tuned for specific applications.
anato carbonyl compounds are intermediates for a
9
preferred synthetic route to several types of thiazoles.
1
Thiocyanation is generally carried out via nucleophilic
substitution using thiocyanate anions. The low nucleo-
philicity of the NCS-ion requires rather harsh reaction
conditions. Metal thiocyanates and organic halides or
sulfonates are generally used to introduce the thiocya-
ILs based on imidazolium cations have been extensively
studied with a variety of structural modifications leading
to differences in the physical and chemical properties of
2
the liquid. Moreover, most attention has been focused
on modification of the cation, particularly by incorpo-
rating hydroxyl, nitrile and carboxylic acid functional-
1
0
nate functionality into an organic molecule. However,
the thiocyanate group is not that stable when heated or
under acidic conditions. Chromatography on silica gel
or prolonged heating over 50 ꢁC can cause intramolecu-
lar rearrangement to the thermodynamically favored
3
,4
ities. Not much attention has been directed toward
the preparation of ILs with useful anions. A functional-
ized anion could notonly influence ht e physical and
chemical properties of the IL but also may function as
1
1
isothiocyanate isomers.
have been obtained from alcohols, silyl ethers or
Furthermore, thiocyanates
12
5
13
a reactant. There are very few examples where ILs have
been employed as reaction media and as the nucleophile
1
4
amines using Ph P(SCN) . However, many drawbacks
3 2
6
in an organic transformation. The application of task
specific ionic liquids (TSILs) enhances the versatility of
have been observed for these thiocyanation methodolo-
1
5
gies. This led us to the preparation of the [bmim]SCN
ionic liquid 1.
7
classical ionic liquids. Herein, we describe a synthesis
of a TSIL containing an imidazolium cation combined
8
with a thiocyanato anion. Furthermore, this TSIL has
been applied as the reagent and medium for the nucleo-
philic substitution reactions of alkyl halides.
The ionic liquid, 1-n-butyl-3-methyl imidazolium thiocya-
nate [bmim]SCN 1 was prepared by the anion exchange
16
of 1-n-butyl-3-methyl imidazolium chloride
KSCN in acetone for 48 h at room temperature (Scheme
1).
with
1
7
Alkyl thiocyanates are important synthetic precursors
for the preparation of sulphur-containing organic com-
pounds. This functional group can be used as a masked
mercapto group or as a precursor for sulphur-contain-
ing heterocyclic compounds. Additionally, a-thiocy-
It was characterized by IR and NMR spectroscopy to-
gether with FAB mass spectrometry. The IR spectrum
of [bmim]SCN 1 exhibited the characteristic absorption
À1
of the thiocyanate functionality at 2070 cm . Further-
more, the FAB mass spectrum of 1 is differentfrom
the corresponding halide salts and exhibits a peak at
m/z 336 (10%) corresponding to (C H N ) SCN. The
*
Corresponding author. Tel.: +91 40 27193157; fax: +91 40
7193189; e-mail: ahmedkamal@iict.res.in
2
8
15 2 2
0
040-4039/$ - see front matter ꢀ 2005 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2005.01.040

1490
A. Kamal, G. Chouhan /Tetrahedron Letters 46 (2005) 1489–1491
acetone
Cl- + KSCN
N
N
SCN- + KCl
N
N
48 h, r.t.
1
Scheme 1.
halide salts [bmim]X (X=Cl, Br) show peaks at m/z 357
4%) and m/z 313 (15%), respectively, for (C H N ) Br
and (C H N ) Cl. Ionic liquid 1 has been investigated
to observe that in this case transformation to the corre-
sponding thiocyanate took place with only 20–30% con-
version even after 2 h of reaction time. This indicated
that the nucleophilicity of the SCN anion is much higher
in [bmim]SCN compared to KSCN in [bmim]BF4.
(
8
15 2 2
8
15 2 2
for the nucleophilic substitution of a variety of alkyl ha-
lides giving alkyl thiocyanates. In a typical experiment,
a-chloroacetophenone was reacted with 1.2 equiv of
the ionic liquid [bmim]SCN 1 and itwas observed (by
TLC) that the a-chloroacetophenone was converted
quantitatively into the corresponding a-thiocyanato car-
bonyl compound after about 10 min at room tempera-
The application of this TSIL for the transformation of
different alkyl halides to their alkyl thiocyanates has
been examined and the results are summarized in Table
1 (Scheme 2).
1
8
ture. The product was extracted by ether from the
ionic liquid and was pure. Itdid notrequire any column
chromatography thus avoiding the possibility of rear-
rangement. Significantly, the ionic liquid [bmim]Cl
formed after the reaction, on treatment with KSCN in
acetone regenerated this TSIL, [bmim]SCN, thus allow-
ing recycling of the TSIL. A comparison with 1 was car-
In conclusion, we have demonstrated the synthesis of a
new task-specific ionic liquid [bmim]SCN and its appli-
cation as a reaction medium as well as reactant for the
conversion of alkyl halides to their alkyl thiocyanates
in high yields at room temperature. More importantly
there is no formation of any side-products in this pro-
cess and there is no need for column chromatography
for the purification of the products.
ried outby re ai nc gt
[
a-chloroacetophenone in
bmim]BF with 1.5 equiv of KSCN. It was interesting
4
Table 1. Thiocyanation of alkyl halides using the ionic liquid [bmim]SCN
a
b
Entry
Halide 2
Product 3
Time (min)
10
Yield (%)
O
O
Cl
SCN
a
95
O
O
Cl
Cl
SCN
SCN
b
c
d
e
f
9
93
96
90
93
90
Cl
Br
Cl
Br
O
O
10
15
8
O
O
Cl
SCN
MeO
O
MeO
O
Br
Cl
SCN
SCN
15
S
S
O
O
Br
Br
SCN
SCN
g
h
i
60
95
80
75
24 h
24 h
HO
HO
HO
HO
O
Br
SCN
O
a
1
15
All products were characterized by H NMR, IR, and mass spectroscopy.
Isolated yields after extraction with ether.
b

A. Kamal, G. Chouhan /Tetrahedron Letters 46 (2005) 1489–1491
1491
O
O
or R2 X + [bmim]SCN
r.t.
SCN or R2 SCN
+ [bmim]X
X
R1
R1
acetone
KSCN
1
R = C H -, p-BrC H , p-ClC H ,
6
5
6
4
6 4
p-MeOC H , 2-thienyl
6
4
R2 = C H CH , HOC H , HOOC-C H
6
5
2
2
4
3 6
X = Cl, Br
Scheme 2.
Acknowledgments
Kiesowski, J.; Michalski, J.; Pakulski, M.; Skowronska,
A. Tetrahedron 1983, 39, 4175.
3. Iranpoor, N.; Firouzabadi, H.; Shaterian, H. Synlett 2000,
1
1
1
One of the authors G.C. is thankful to CSIR (New
Delhi) for the award of senior research fellowship.
1
0, 65.
4. Molina, P.; Alajarin, M.; Ferao, A.; Lindon, M. J.;
Fresneda, P. M.; Vilaplana, M. J. Synthesis 1982, 472.
5. (a) Renard, P. Y.; Schwebel, H.; Vayron, P.; Leclerc, E.;
Dais, S.; Mioskowski, C. Tetrahedron Lett. 2001, 42,
References and notes
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17. The ionic liquid 1-n-butyl-3-methylimidazolium chloride
[bmim]Cl (3 g) was dissolved in acetone (25 mL) and to
this was added KSCN (2 equiv). The mixture was allowed
to stir at room temperature for 48 h. The resulting
suspension was filtered and the filtrate was subjected to
a vacuum to remove volatile material. The residue was
dissolved in dichloromethane and again filtered.
The filtrate was dried using anhydrous sodium sulfate.
Finally, upon concentration under vacuum at 70 ꢁC 1-n-
butyl-3-methylimidazolium thiocyanate was obtained as a
2
3
4
5
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. Branco, L. C.; Rosa, J. N.; Moura Ramos, J. J.; Afonos,
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À1
1
red colored liquid. IR (cm ): 2070 (-SCN); H NMR
(200 MHz, CDCl ): d = 0.98 (t, 3H, J = 7.4 Hz), 1.25–
3
1.40 (m, 2H), 1.80–2.00 (m, 2H), 3.95 (s, 3H), 4.20 (t,
2H, J = 7.4 Hz), 7.50 (S, 1H), 7.55 (S, 1H), 8.75 (s, 1H);
FABMS (+ve): m/z = 139 (M-SCN) (100%), 336
2
002, 200; (f) Levillain, J.; Dubant, G.; Abrunhosa, I.;
8 15 2 2
(C H N ) SCN (10%).
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18. Typical experimental procedure:
A
mixture of
1
6
7
8
(1.2 mmol) and a-chloroacetophenone (1 mmol) was
placed in a flask and stirred at room temperature. After
completion of the reaction as indicated by TLC, ether
(3 · 4 mL) was added to the reaction mixture with
vigorous stirring for 5 min. The mixture was allowed to
stand for a further 5 min and the clear supernatant liquid
was decanted. The combined ether layers were dried over
sodium sulphate and concentrated under reduced pressure
to give a-thiocyanatoacetophenone as an oil. The remain-
ing ionic liquid was dissolved in acetone and reacted with
KSCN (2 equiv) at room temperature for 48 h to afford
[bmim]SCN ionic liquid 1 which was used in subsequent
runs.
1
2, 3475.
. Metzer, J. B. In Comprehensive Heterocyclic Chemistry;
9
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