Thermal stability of ionic liquid BMI(BF4) in the presence of nucleophiles

Article abstract of DOI:10.1016/j.tetlet.2004.07.050

We have found that a common ionic liquid-one containing a N-Na′-dialkylimidazolium cation-decomposes in the presence of nucleophiles at much lower temperatures than previously believed. The decomposition occurs through S_N2 attack of the nucleophile on the electrophilic alkyl groups attached to the imidazolium ring.

Full text of DOI:10.1016/j.tetlet.2004.07.050

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 6967–6969
Thermal stability of ionic liquid BMI(BF ) in
4
the presence of nucleophiles
b,
*
a
Anne G. Glenn and Paul B. Jones
a
Department of Chemistry, Guilford College, Greensboro, NC 27410, USA
Department of Chemistry, Wake Forest University, 115-A Salem Hall, Winston-Salem, NC 27109, USA
b
Received 7 June 2004; revised 9 July 2004; accepted 12 July 2004
Available online 7 August 2004
0
Abstract—We have found that a common ionic liquid––one containing a N-N -dialkylimidazolium cation––decomposes in the pres-
ence of nucleophiles at much lower temperatures than previously believed. The decomposition occurs through S 2 attack of the
N
nucleophile on the electrophilic alkyl groups attached to the imidazolium ring.
ꢀ
2004 Elsevier Ltd. All rights reserved.
Room temperature ionic liquids (RTILs) have been
increasingly studied as environmentally benign, or
ÔgreenÕ, solvents. Due to their very low vapor pressures
and limited miscibility with water and organic solvents,
these liquids are, conceivably, completely recyclable.
The most common RTILs are made up of a dialkylimi-
dazolium cation and a bulky, hydrophobic anion. These
solvents are relatively inert and tolerate a variety of
chemistries, as demonstrated by the large number of
on the electrophilic alkyl groups has been
6
documented. A recent report suggested ionic liquids
1
composed of imide anions may undergo alkyl transfer
from the cation to the anion via nucleophilic displace-
ment as one of several thermal decomposition path-
7
ways.
While studying another process using thiophenol (1) in
BMI(BF ) (1-butyl-3-methylimidazolium tetrafluorobo-
4
2
reactions that have been carried out in ionic solution.
Ionic liquids are generally very good solvents for nucleo-
rate) solvent at high temperature (>200ꢁC), we observed
the formation of thioanisole (2) and n-butyl phenyl sul-
fide with both microwave and conventional heating
(Scheme 1). The only possible source of these products
was nucleophilic attack by the thiophenol on the methyl
or butyl groups of the BMI cation. Because this result
could have a significant impact on use of ionic liquids
in microwave and/or high temperature applications or
as reaction media for nucleophilic substitution (or addi-
tion) reactions, we studied the reaction of BMI(BF4)
with various nucleophiles. We found that in some cases,
3
philic substitution reactions. Additionally, because the
salts have a strongly polar nature, they are effective as
solvents and co-solvents for reactions carried out under
microwave heating. Microwaves have been used to
great effect in a variety of reactions in ionic liquids,
4
5
including nucleophilic substitution reactions.
0
The decomposition at high temperature of N,N -dialkyl-
imidazolium halides by nucleophilic attack of the halide
SH
N
N
S
+
BF4
∆
+
N
N
1
BMI(BF4)
2
3
Scheme 1.
Keyword: Ionic liquid.
*
Corresponding author. Tel.: +1-336-7583708; fax: +1-336-7584656; e-mail: jonespb@wfu.edu
0
040-4039/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.07.050

6968
A. G. Glenn, P. B. Jones / Tetrahedron Letters 45 (2004) 6967–6969
a
Table 1. Reaction of nucleophiles with BMI(BF
4
) under microwave heating
Entry
Temp (ꢁC)/power (W)
Nuc
Nuc–Me
6%
39%
Nuc–Bu
Total substitution yield
1
2
3
4
5
6
7
8
9
1
1
1
225/20
PhSH
225/20
200/20
5%
NaSPh
%
1
7%
4%
46%
33%
29%
30%
21%
16%
27%
17%
15%
2%
175/20
150/20
125/20
4%
2%
34%
23%
1%
17%
225/20
225/15
PhCO Na
2
13%
6%
40% (32%)
23%
19%
200/15
175/20
4%
0
1
2
Trace
1%
Trace
2%
b
225/20
225/20
PhNH
PhONa
2
4%
2%
6%
2%
Yields were determined by GC and an internal standard (isolated yield in parentheses).
a
BMIBF
in a CEM Discover Labmate microwave (with stirring) at 15–20W for 10min. The reaction was diluted with 3mL DI H
· 2mL Et O and the Et O extracts washed with DI H O and dried (MgSO ). Mesitylene was added as an internal standard and the reaction
4
(1mL) and the desired nucleophile (0.7–1.0mmol) were placed in a microwave reaction tube and sealed under Ar. The reaction was heated
ꢂ
ꢃ
2
O, extracted with
3
2
2
2
4
analyzed by GC–MS. Products were identified in most cases by comparison of the MS of authentic samples.
b
In addition to methylation and butylation of aniline, some dimethylaniline was observed.
significant quantities of substitution products were
observed, even at temperatures as low as 125ꢁC. Our
results are described below and tabulated in Table 1.
N-Butylimidazole was detected in each reaction in which
significant substitution occurred. It proved difficult to
accurately measure yields of this compound as it tended
to remain in the ionic liquid layer upon extraction. Substi-
tution also occurred in reactions run in 1-butyl-3-ethyl-
imidazolium tetrafluoroborate, with attack on the
electrophilic ethyl group accounting for most of the sub-
stitution product.
Reaction yields were determined by GC versus an inter-
nal standard (and by isolated yield in certain cases).
Using this technique, the yield of thioanisole when thio-
phenol was heated to 225ꢁC in neat BMI(BF ) for
4
1
(
0min in a microwave was 6%. Butyl phenyl sulfide
1%) was also detected. The reaction darkened consider-
The relative reactivity of the BMI cation versus more
conventional electrophiles was also examined (Table
2). Our results suggest that the reaction of BMI as an
ably, which did not occur in the absence of thiophenol.
When the more reactive sodium thiophenolate was used
as the nucleophile, combined yields of substitution prod-
ucts (thioanisole and butyl phenyl sulfide) increased to
nearly 50%. In fact, some nucleophilic substitution oc-
curred on the butyl chain with each nucleophile studied.
+
electrophile is competitive with that of a typical alkyl
halide. The reaction of sodium thiophenoxide with do-
decyl bromide in BMI(BF ) at 200ꢁC gave a 25% yield
4
of the combined methyl and butyl substitution products
and a 20% yield of dodecyl phenyl sulfide. Under the
reaction conditions, the molar ratio of BMI(BF ) to do-
As expected, yields of substitution products decreased
with decreasing temperature. Sodium thiophenolate
gave significant product at temperatures as low as
4
decyl bromide was 4:1. Therefore, the BMI(BF ) reacted
4
at approximately one third the rate of the bromide.
When decyl chloride was included in the reaction at
the same molar ratio, a 34% combined yield of methyl
and butyl sulfides was obtained, along with a 19% yield
of decyl phenyl sulfide. This result showed that
1
25ꢁC. Sodium benzoate gave both methyl and butyl
benzoates in 40% combined yield (32% isolated yield)
at 225ꢁC and 17% combined yield at 200ꢁC. Aniline
and sodium phenoxide were significantly less reactive.
Aniline gave a combination of nucleophilic substitution
products in 6% combined yields at 225ꢁC. Under the
same conditions, sodium phenoxide only gave 2–3%
yields of the substitution products. All of the results in
Table 1 were obtained using microwave heating. Reac-
tions carried out using conventional heating techniques
gave similar results, but required much longer reaction
times (3h vs 10min).
BMI(BF ) reacted with thiophenolate at about half the
4
rate of decyl chloride.
0
The results obtained in our study indicated that N-N -
dialkylimidazolium cations, common in many ionic liq-
uids, were reactive with nucleophiles. The substitution
reaction of BMI(BF ) is especially facile when micro-
4
wave heating is used. While yields tended to be moderate
Table 2. Reaction of sodium thiophenoxide with alkyl halides in BMI(BF
4
) at 200ꢁC
a
Entry
Alkyl halide
PhSMe
PhSBu
PhS–alkyl
BMI/electrophile
1
2
n-C12
H
25Br
21Cl
21%
30%
4%
4%
20%
19%
1.25/1
1.8/1
n-C10H
a
+
This is the ratio of substitution products obtained from BMI to those obtained from the alkyl halide.

A. G. Glenn, P. B. Jones / Tetrahedron Letters 45 (2004) 6967–6969
6969
to low, depending on the nucleophile, this reactivity
References and notes
could interfere with recycling of ionic liquids––a point
that is repeatedly emphasized as these solvents are
increasingly utilized. RTILs are very efficient absorbers
of microwave energy, and as such have been suggested
as excellent solvents for microwave reactions, as well
as co-solvents to aid in the microwave heating of less
1. Welton, T. Chem. Rev. 1999, 99, 2071–2083, and references
cited therein.
2
. (a) Berthold, H.; Schotten, T.; Honig, H. Synthesis 2002,
1, 1607–1610; (b) Fraga-Dubreuil, J.; Bazureau, J. P.
1
Tetrahedron Lett. 2000, 41, 7351–7355; (c) Reynolds, J. L.;
Erdner, K. R.; Jones, P. B. Org. Lett. 2002, 4, 917–919; (d)
Handy, S. T.; Zhang, X. Org. Lett. 2001, 3, 233–236; (e)
Madeira Lau, R.; van Rantwijk, F.; Seddon, K.; Sheldon,
R. A. Org. Lett. 2000, 2, 4189–4191.
4
,5
microwave-absorbent solvents. In addition, ionic liq-
uids have been proposed for a variety of high tempera-
7
,8
ture applications.
Our results suggest that the
thermal stability of imidazolium based ionic liquids will
be significantly reduced in the presence of nucleophiles.
This data also implies that caution is advised when
attempting nucleophilic substitution or addition reac-
tions in ionic liquids, especially using microwave heating
or for extended times at high temperature, due to the
possibility of reaction of the nucleophile with the sol-
vent.
3. (a) Boovanahalli, S. K.; Kim, D. W.; Chi, D. Y. J. Org.
Chem. 2004, 69, 3340–3344; (b) Brinchi, L.; Germani, R.;
Savelli, G. Tetrahedron Lett. 2003, 44, 6583–6585; (c) Kim,
D. W.; Song, C. E.; Chi, D. Y. J. Org. Chem. 2003, 68,
4
281–4285.
4
. (a) Hoffmann, J.; Nuchter, M.; Ondruschka, B.; Wassersc-
heid, P. Green Chem. 2003, 5, 296–299; (b) Leadbeater, N.
E.; Torenius, H. M. J. Org. Chem. 2002, 67, 3145–3148.
. Leadbeater, N. E.; Torenius, H. M.; Tye, H. Tetrahedron
5
6
7
8
2
003, 59, 2253–2258.
. Chan, B. K. M.; Chang, N.-H.; Grimmett, M. R. Aust. J.
Chem. 1977, 30, 2005–2013.
Acknowledgements
. Baranyai, K. J.; Deacon, G. B.; MacFarlane, D. R.;
Pringle, J. M.; Scott, J. L. Aust. J. Chem. 2004, 57, 145–147.
. (a) Huddlestone, J. G.; Visser, A. E.; Reichert, W. M.;
Willauer, H. D.; Broker, G. A.; Rogers, R. D. Green Chem.
2001, 3, 156–164; (b) Ngo, H. L.; LeCompte, K.; Hargens,
L.; McEwen, A. B. Thermochim. Acta 2000, 357, 91–102.
The authors thank the Petroleum Research Fund (PRF
#
37882-G4) for financial assistance and Brittany Hayes
of CEM for helpful discussions. A.G.G. thanks Guil-
ford College for sabbatical leave support.