An acidity scale of 1,3-dialkylimidazolium salts in dimethyl sulfoxide solution

Article abstract of DOI:10.1021/jo070973i

(Chemical Equation Presented) Equilibrium acidities of 16 1,3-dialkylimidazolium-type ionic liquid (IL) molecules (1-16) were systematically measured by the overlapping indicator method at 25°C in dimethyl sulfoxide (DMSO) solution. The pK_a values were observed to range from 23.4 for IL 12 to 19.7 for IL 6 (Tables 1 and 2), responding mainly to structural variations on the cation moiety. Excellent agreement between the spectrophotometrically determined pK_a and that derived from NMR titration for 1,3,4,5-tetramethylimidazolium bis(trifluoromethanesulfonyl)imide (12) and the close match of the obtained pK values with the reported data in literature provide credence to the acidity measurements of the present work. The substituent effects at the imidazolium ring and the effects of counterions on the acidities of ionic liquids are discussed.

Full text of DOI:10.1021/jo070973i

An Acidity Scale of 1,3-Dialkylimidazolium Salts
in Dimethyl Sulfoxide Solution
SCHEME 1. Formation of N-Heterocyclic Carbene by
Deprotonation of Imidazolium Ionic Liquid
Yuan Chu, Hui Deng, and Jin-Pei Cheng*
Department of Chemistry, State Key Laboratory of
Elemento-organic Chemistry, Nankai UniVersity,
Tianjin 300071, China
6
7
were normally used as “inert solvents” under acidic, neutral,
chengjp@nankai.edu.cn
8
and basic conditions. However, recent investigations started
to accumulate increasing examples of the “non-innocent” nature9
of the imidazolium ILs and therefore suggest an immediate
demand for more fundamental studies on the stability of this
type of new reaction media under harsh conditions.
ReceiVed June 6, 2007
The instability of the imidazolium ILs stems mainly from
the relatively high acidity of the C2 hydrogen in the cation
1
7
moiety. This property of the imidazolium cations may, on the
other hand, lead to formation of the N-heterocyclic carbenes
1
0
(
Scheme 1) that have recently been found to promote a wide
11
range of applications in organometallic catalysis and a number
of important reactions such as benzoin condensation and acyl
transfer. However, the base-sensitive C2 hydrogen may also
cause unexpected side reactions and thus defines a borderline-
window for imidazolium ILs to be used as reaction media. The
promising prospects of the RTILs and N-heterocyclic carbenes
in modern organic synthesis thus raise two important funda-
mental questions for deeper understanding of their properties:
12
1
3
Equilibrium acidities of 16 1,3-dialkylimidazolium-type ionic
liquid (IL) molecules (1-16) were systematically measured
by the overlapping indicator method at 25 °C in dimethyl
14
sulfoxide (DMSO) solution. The pK
to range from 23.4 for IL 12 to 19.7 for IL 6 (Tables 1 and
), responding mainly to structural variations on the cation
moiety. Excellent agreement between the spectrophotometri-
cally determined pK and that derived from NMR titration
for 1,3,4,5-tetramethylimidazolium bis(trifluoromethane-
sulfonyl)imide (12) and the close match of the obtained pK
values with the reported data in literature provide credence
to the acidity measurements of the present work. The
substituent effects at the imidazolium ring and the effects
of counterions on the acidities of ionic liquids are discussed.
a
values were observed
2
(
1) How acidic is the C2-hydrogen of imidazolium IL, or in
other words, how stable is its conjugate base N-heterocyclic
carbene under basic conditions? (2) How does the substituent
variation at N- and C(4,5)-positions of the imidazolium cation
ring affect the C-H acidity (or the N-heterocyclic carbene
stability)?
a
There have been only a few reports on the acidities of
imidazolium salts (or basicity of imidazol-2-yl carbenes) in the
literature up to the present.
15,16
Alder et al. measured the pKa
(
(
6) Corma, A.; Garc ´ı a, H. Chem. ReV. 2003, 103, 4307-4366.
7) (a) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39,
In recent years, the room temperature ionic liquids (RTILs)
have become a fascinating new class of “greener” alternatives
to volatile organic solvents that are characterized by their
favorable properties such as liquidity over a wide range of
temperatures, negligible vapor pressure, low flammability, good
3
772-3789. (b) Gorden, C. M. Appl. Catal., A 2001, 222, 101-117. (c)
Dupont, J.; Souza, R. F.; Suarez, P. A. Chem. ReV. 2002, 102, 3667-3692.
(
d) Sheldon, R. Chem. Commun. 2001, 2399-2407.
(8) (a) Morrison, D. W.; Forbes, D. C.; Davis, J. M. J. Tetrahedron Lett.
2
001, 42, 6053-6055. (b) Kmentova, I.; Gotov, E.; Solcainova, E.; Toma,
1
thermal stability, high conductivity, and so on. Among the
S. Green Chem. 2002, 4, 103-106. (c) Forment ´ı n, P.; Garc ´ı a, H.; Leyva,
common RTILs developed, the imidazolium-type ionic liquids
derived from 1,3-dialkylimidazolium cations in association with
weakly coordinating anions represent the most popular and have
A. J. Mol. Catal. A 2004, 214, 137-142.
(
9) Dupont, J.; Spencer, J. Angew. Chem., Int. Ed. 2004, 43, 5296-
5
297.
(10) Arduengo, A. J., III Acc. Chem. Res. 1999, 32 (11), 913-921.
2
recently found many applications in electrochemistry, polymer
(11) (a) Bourissou, D.; Guerret, O.; Gabbaie, F. P.; Bertrand, G. Chem.
3
4
5
Rev. 2000, 100, 39-91. (b) Herrmann, W. A. Angew. Chem., Int. Ed. 2002,
chemistry, organic and inorganic synthesis, and separation
4
3
1, 1290-1309. (c) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001,
4, 18-29.
and catalytic6 processes. In these cases, the imidazolium ILs
,7
(12) (a) Teles, J. H.; Melder, J. P.; Ebel, K.; Schneider, R.; Gehrer, E.;
*
To whom correspondence should be addressed. Phone: (+86)22-2349-9174.
Fax: (+86)22-2350-2458.
1) (a) Welton, T. Chem. ReV. 1999, 99, 2071-2083. (b) Forsyth, S. A.;
Pringle, J. M.; MacFarlane, D. R. Aust. J. Chem. 2004, 57, 113-119.
2) (a) Buzzeo, M. C.; Evans, R. G.; Compton, R. G. ChemPhysChem
004, 5, 1106-1120. (b) Yoshizawa, M.; Narita, A.; Ohno, H. Aust. J.
Harder, W.; Brode, S.; Enders, D.; Breuer, K.; Raabe, G. HelV. Chim. Acta
1996, 79, 61-83. (b) Miyashita, A.; Suzuki, Y.; Kobayashi, M.; Kuriyama,
N.; Higashino, T. Heterocycles 1996, 43, 509-512.
(13) (a) Grasa, G. A.; Kissling, R. M.; Nolan, S. P. Org. Lett. 2002, 4,
3583-3586. (b) Nyce, G. W.; Lamboy, J. A.; Connor, E. F.; Waymouth,
R. M.; Hedrick, J. L. Org. Lett. 2002, 4, 3587-3590.
(14) Aggarwal, V. K.; Emme, I.; Mereu, A. Chem. Commun. 2002,
1612-1613.
(
(
2
Chem. 2004, 57, 139-144.
(
3) Kubisa, P. Progr. Polym. Sci. 2004, 29, 3-12.
(4) Wasserscheid, P.; Welton, T., Eds. Ionic Liquids in Synthesis; Wiley-
(15) Alder, R. W.; Allen, P. R.; Williams, S. J. J. J. Chem. Soc., Chem.
Commun. 1995, 1267-1268.
VCH: Weinheim, Germany, 2003.
5) Zhao, H.; Xia, S. Q.; Ma, P. S. J. Chem. Technol. Biotechnol. 2005,
0, 1089-1096.
(
(16) Kim, Y.-J.; Streitwieser, A. J. Am. Chem. Soc. 2002, 124, 5757-
5761.
8
10.1021/jo070973i CCC: $37.00 © 2007 American Chemical Society
7
790
J. Org. Chem. 2007, 72, 7790-7793
Published on Web 08/29/2007

of 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene as 24.0 in
DMSO-d6 by the NMR method.15 Kim and Streitwieser reported
TABLE 1. Equilibrium Acidity Measurements for
1,3-Dialkylimidazolium Ionic Liquids in DMSO at 25 °C
a DMSO pKa for a slightly different species 1,3-di-tert-
butylimidazol-2-ylidene as 22.7.16 Recently, Amyes and co-
workers estimated a few aqueous-phase kinetic acidities for
17
simple imidazolium cations by the deuterium exchange method.
Yates et al. predicted the basicity of some nucleophilic carbenes
by theoretical computation.18
Despite this important progress, fundamental acidity studies
of imidazolium cations are, nevertheless, very rare and still
remain in urgent need. Besides, most of the previous attention
was placed on the imidazolium ion itself and the acidity data
were not derived from a normalized medium. It is therefore
important to investigate the stability of the commonly used 1,3-
dialkylimidazolium ionic liquids under basic conditions and to
make the acidity measurements in a single solvent for the
purpose of meaningful comparison. This has stimulated us to
carry out more inclusive acidity measurements and a structural
study of the popular ILs in DMSO solution. Here we wish to
report the equilibrium acidities of a family of 16 1,3-dialkyl-
imidazolium-type ILs in DMSO at 25 °C and the structural
effects on the stability of their conjugate bases, i.e., the
corresponding N-heterocyclic carbenes.
The overlapping indicator method used in this work measures
the differences of the equilibrium acidity (pKa) of an “unknown”
acid relative to that of an “indicator” acid (whose pKa is known)
by monitoring the changes of UV/vis absorption of either the
a
The counterion is bis(trifluoromethanesulfonyl)imide, (CF3SO2)2N-.
b
The indicators (HIn) are FH, fluorene; 2,3-BenzoFH, 2,3-benzofluorene;
and CNAH, 4-chloro-2-nitroaniline. c Measured in DMSO by overlapping
19 d
indicator method as reported previously.
The assigned pKa value is the
indicator or the unknown acid during titrations of one acid to
average of pKHA values. e Number of the separate experimental measure-
the other under standard conditions.19 The “absolute” equilib-
ments.
rium acidity of the unknown acid can then be calculated
according to the following equations:
TABLE 2. Equilibrium Acidity Measurements for
-n-Butyl-3-methylimidazolium Ionic Liquids with Different
Counterions in DMSO at 25 °C
1
Keq
-
-
HA + In y z HIn + A
(1)
pKad
no.
counteriona
(CF3SO2)2N- FH (22.6)
HIn (pKHIn)b
pKHAc
(assigned) runse
pKHA ) pKHIn - logK )
eq
-
-
3
22.05 ( 0.03
22.0
4
pKHIn - log([HIn][A ]/[In ][HA]) (2)
2,3-BenzoFH 22.01 ( 0.04
(23.1)
13 Cl-
4
5
6
FH (22.6)
FH (22.6)
FH (22.6)
FH (22.6)
22.01 ( 0.05
22.09 ( 0.01
22.12 ( 0.005
22.08 ( 0.02
22.0
22.1
22.1
22.1
2
3
3
2
First, the pKa range of the 1,3-dialkylimidazolium ionic liquids
-
1
1
1
Br
in DMSO was bracketed to be between 17.9 and 25.6 by using
-
BF4
1
9
two indicator anions with pKHA of 17.9 (9-phenylfluorene)
-
PF6
and 25.6 (1,3,3-triphenylpropene),20 respectively. It was then
narrowed down to be around 20-22, using an indicator anion
with pKHA ) 22.6 (fluorene).19 For acidity measurement of
individual ILs, two carefully selected indicators were used for
each ionic liquid. To ensure the most reliable results, the
indicators were selected in such a way that their pKHA values
should be close enough (<2 pK units) to the presumed value
of the ionic liquid of interest. The equilibrium acidities
determined for 16 1,3-dialkylimidazolium-type ionic liquids (and
analogs) in DMSO under standard conditions along with the
indicators used in each measurement are summarized in Tables
a
The cation is 1-n-butyl-3-methylimidazolium. b The indicators (HIn)
are FH, fluorene; and 2,3-BenzoFH, 2,3-benzofluorene. c Measured in
19
d
DMSO by the overlapping indicator method as reported previously.
The
assigned pK value is the average of pK values. e Number of separate
a
HA
experimental measurements.
Effect of Substituents on Equilibrium Acidities. Table 1
shows that the pKa of ionic liquid 1 in DMSO is 22.0, which is
close enough to the previous estimate of the same IL in aqueous
1
7
solution (23.0) from a kinetic study. The small difference of
the acidities in water and in dimethyl sulfoxide is understandable
because water is known to solvate (i.e., to stabilize) cations
better through hydrogen bonding than nonhydroxylic solvents
such as DMSO. In comparison, the solvation difference for the
deprotonated product, i.e., the neutral 1,3-dimethylimidazol-2-
yl carbene, should be relatively small. The stronger solvated
IL 1 in water has to overcome a greater energetic barrier during
deprotonation than it does in DMSO and thus resulted in a higher
pKa value. Similar phenomena were also observed previously
1
and 2.
(
17) Amyes, T. L.; Diver, S. T.; Richard, J. P.; Rivas, F. M.; Toth, K. J.
Am. Chem. Soc. 2004, 126, 4366-4374.
18) Magill, A. M.; Cavell, K. J.; Yates, B. F. J. Am. Chem. Soc. 2004,
26, 8717-8724.
19) Matthews, W. S.; Bares, J. E.; Bartmess, J. E.; Bordwell, F. G.;
(
1
(
Cornforth, F. J.; Drucker, G. E.; Margolin, Z.; McCallum, R. J.; McCallum,
G. J.; Vanier, N. R. J. Am. Chem. Soc. 1975, 97, 7006-7014.
(20) pKa values for standard hydrocarbon acids in DMSO are taken
from: (a) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456-463. (b) Izutsu,
K., Ed. Acid-Base Dissociation Constants in Dipolar Aprotic SolVents;
Blackwell Scientific Publications: Oxford, UK, 1990.
2
1
for the thiazolium cations and for the secondary and tertiary
amines.
2
2
J. Org. Chem, Vol. 72, No. 20, 2007 7791

1
6
SCHEME 2. Example for Determination of Acid-Base Equilibrium Constant by H NMR Spectroscopy in DMSO-d Solution
Replacement of 1-N-methyl in 1 with ethyl, n-butyl, or n-octyl
to give 2, 3, and 4) did not cause appreciable change on
well with a separately measured value of 23.6 of this work by
H NMR titration (see text below). The 0.2 pK unit deviation
1
(
equilibrium acidity (∆pK e 0.1), suggesting that the short or
sterically flexible alkyl chains have minimal influence on the
solvation of imidazolium cation. On the other hand, substitution
of a tert-butyl group to the 1-N position of 1 (to give 5) causes
a 0.6 pK unit decrease in acidity. This can be attributed to the
so-called “steric inhibition of solvation” by tert-butyl in the
deprotonation transition state. This steric inhibition effect can
also be found in compound 11 where a second tert-butyl group
is attached to the 3-N position. The effect appears to be additive
and causes altogether a 1.2 pK decrease in acidity (pKa ) 23.25).
It is noted that this pKa for IL 11 is close to but somewhat greater
than the value of 22.7 from an equilibrium acidity study of the
same compound in the literature.23
of the acidity data by the two different methods is reasonable
because the NMR pKa was determined in a more concentrated
solution and the high concentration is known to lead to a pK
1
6
measurement greater than normal. Our pKa value of 23.4 for
12 is also reasonably close to the reported value of 24.0 for an
analogous compound, 1,3-diisopropyl-4,5-dimethylimidazol-2-
ylidene, measured in DMSO by the NMR method.15 The 0.6
unit difference may be attributed mainly to the greater electro-
donating effect and the “steric inhibition of solvation” effect of
the bulky N-isopropyl groups, and partially to the concentration
effect on the NMR acidity measurement as mentioned previ-
ously.
Effect of Counterions. The influence of counterions on the
acidity of ionic liquids has not been examined previously in
the literature. We therefore measured the equilibrium acidities
of 1-n-butyl-3-methylimidazolium cation with various counter-
anions (ILs 3, 13-16) as an example of such a study (see data
in Table 2). The derived pKa values of the cation with five
different anions are all found to be around 22.1, indicating that
the effect of counteranions is essentially negligible. This is not
entirely unexpected, however, because previous computational
On the other hand, substitution of 1-N-methyl in 1 with phenyl
(to give 6) brought about 2.3 pK units increase in equilibrium
acidity (pKa ) 19.7). The large increase in acidity may be
explained by considering a combination of the electron-
withdrawing inductive effect of phenyl and the delocalization
effect of the phenyl π-system that pulls the p-electron pair at N
atoms away from the imidazolium ring and thus makes it less
stable.
24
Table 1 also shows that replacement of 1-N-methyl in 1 with
a benzyl or p-substituted benzyl (to give 7-9) causes an increase
in acidity by about 0.4 to 0.75 pK unit. The substitution effect
is rather small because the aromatic moiety is further apart, but
nevertheless, it still reflects a normal type inductive effect of
the benzyls. In the same line, the 1.5 units decrease in the pK
value of 1 by replacing one of the hydrogen atoms at 1-methyl
with methoxy (to give 10, pKa ) 20.5) should be easily
understandable because of the strong electron-pulling property
of the oxygen atom.
study on the intermolecular interaction energies in the ion pairs
of the 1-n-butyl-3-methylimidazolium ionic liquids showed that
the interaction energies (mainly electrostatic in nature) differ
only by a few kilocalories per mole in the absence of solvent
and the strong solvation effect in DMSO in the present study
may well level off the small differences of interaction energies
to the point as observed. The pKa value of 1,3-dialkylimidazo-
lium-type ionic liquids is thus ultimately dependent on the
structure of the cations.
Examination of Acidity Constant of Imidazolium Ionic
Liquid in DMSO-d6 by the NMR Method and Stability of
the Corresponding N-Heterocyclic Carbenes in Dilute DMSO
Solution. A number of excellent literature works have demon-
strated that reversible acid-base equilibrium between 1,3-
In comparison to the rather weak substituent effects of the
alkyl substitution at the 1,3-N positions of imidazolium ILs (1-
1
1), further substitution of the hydrogen atoms at C4 and C5
positions with methyl (to give 12) caused a quite appreciable
acid-weakening effect on the C2-hydrogen by as large as 1.4
pK units (relative to 1), due primarily to the cation-stabilizing
electron-donating nature of the methyl. This pKa value of 12
dialkylimidazolium ionic liquid and its conjugate base (imidazol-
5,25
2
-ylidene) can be established in DMSO solution.1
In this
work, we examined the acid-base equilibrium of IL 12 with
indicator anion 9-tert-butylfluorenide in DMSO (Scheme 2) by
(23.4) derived by the overlapping indicator method agrees very
1
H NMR titration and obtained the acidity constant of IL 12
for the purpose of comparison with the spectrophotometrically
(
21) Bordwell, F. G.; Satish, A. V.; Jordan, F.; Rios, C. B.; Chung, A.
C. J. Am. Chem. Soc. 1990, 112, 792-797.
22) Kolthoff, I. M.; Chantooni, M. K., Jr.; Bhowmik, S. J. Am. Chem.
Soc. 1968, 90, 23-28.
23) The pKa of this cation was previously measured as 22.7 in DMSO
determined pKa data.
(
The indicator anion 9-tert-butylfluorenide (pKHA ) 24.3)20
was generated by addition of a DMSO-d6 solution of potassium
dimsyl (about 0.5 equiv) to a DMSO-d6 solution of 9-tert-
butylfluorene. Successive additions of a DMSO-d6 solution of
(
1
6
from a reaction of neutral carbene with an indicator acid. This value is
.55 pK unit lower than the pKa of 23.25 of the present work by the
0
overlapping indicator method, using an indicator anion and imidazolium
cation as the starting acid-base pair. The 0.55 pK difference still seems to
be too large even if the difference in method is taken into consideration.
However, as pointed out in ref 16, their pKa of 22.7 was somewhat
underestimated due to ion pair effect. If the ion pair effect was calibrated,
the free-ion pKa would have been corrected to be 23.04, which should be
reasonably within the uncertainty range to the pKa value of the present
measurement.
1
2 to the resulting indicator anion solution in small aliquots
(24) (a) Tsuzuki, S.; Tokuda, H.; Hayamizu, K.; Watanabe, M. J. Phys.
Chem. B 2005, 109, 16474-16481. (b) Turner, E. A.; Pye, C. C.; Singer,
R. D. J. Phys. Chem. A 2003, 107, 2277-2288.
(25) Filipponi, S.; Jones, J. N.; Johnson, J. A.; Cowley, A. H.; Grepioni,
F.; Braga, D. Chem. Commun. 2003, 2716-2717.
7
792 J. Org. Chem., Vol. 72, No. 20, 2007

sor imidazolium salts27 found in the literature were minimal
under our experimental conditions.
To sum up, the equilibrium acidities of 16 1,3-dialkylimida-
zolium-type ionic liquids in DMSO solution have been suc-
cessfully determined in this work that provide a scaled measure
for both the stability of the imidazolium-type ionic liquids under
basic conditions and the stability of their conjugate base
N-heterocyclic carbenes. Substituent effect on acidity was
examined, and it demonstrated that the ring substitution has quite
a significant influence on acid-base equilibria with a pKa span
of 19.7-23.4. In contrast, the influence of simple counterions
on acidity was shown to be minimal. The observed variations
of pKa with the ring substitution are understandable in terms of
the electronic effects and steric inhibition of solvation effect of
the substituents.
Experimental Section
FIGURE 1. Upfield 1H NMR spectra of (a) a mixture of 9-tert-
butylfluorenide ion and 9-tert-butylfluorene; (b) the mixture from a +
Ionic liquids (IL 1-16) were synthesized according to the
literature and were dried at 70 °C for 3 h under reduced pressure
before used. Indicator compounds were either purchased from a
0
9
.25 equiv of 12 (to total molar amount of 9-t-butylfluorenide ion and
-t-butylfluorene); (c) the mixture from a + 0.50 equiv of 12; and (d)
commercial supplier or synthesized in this laboratory. DMSO-d
6
the mixture from a + 0.75 equiv 12.
1
solutions of IL 12 and 9-tert-butylfluorenide ion used in H NMR
titration were prepared in the NMR tubes directly in the glovebox.
Determination of pK . The procedures for purification and
a
preparation of the DMSO solvent, the potassium dimsyl base, the
solutions of the indicator, and ionic liquids, as well as the
a
overlapping indicator method for pK determination as described
gave mixtures of the four species in Scheme 2 in varying
concentrations under equilibrium conditions. The changes of
NMR spectrum after each titration are demonstrated in Figure
1
where the signals at δ 3.43, 1.99, 0.91, and 0.84 ppm represent
the averages of the N-methyl and C(4,5)-methyl protons in IL
2 and in its conjugate base, and the t-Bu protons in 9-tert-
in details by Bordwell and co-workers,19 were closely followed in
the present work. The concentrations of the indicator and ionic
liquid solutions were about 50 mmol/L. The concentration of the
potassium dimsyl solution was determined internally as a part of
the manipulation during each run. Details of the experimental
operation are described in the Supporting Information.
1
butylfluorene and in 9-tert-butylfluorenide, respectively. The
1
H NMR signals of the t-Bu in 9-tert-butylfluorene (δ 0.91)
and in its conjugate anion (δ 0.84) were eventually used to
evaluate the concentrations of these four species in the
established equilibrium. The equilibrium acidity of 12 thus
derived from eqs 1 and 2 on the basis of NMR titration is 23.6
Acknowledgment. Financial support from the National
Natural Science Foundation of China (Grant Nos. 20332020,
(
0.1, which agrees very well with the pKa of the same
2
0472038, 20421202, and 20672060) is gratefully acknowl-
compound determined by the overlapping indicator method (see
above).
edged.
Note Added after ASAP Publication. There were errors in
The acidity data of RTILs obtained in the present work may,
on the other hand, be regarded as a direct measure of the stability
of the corresponding N-heterocyclic carbenes that are derived
from deprotonation of the 1,3-dialkylimidazolium salts. The
results indicate that these N-heterocyclic carbenes (imidazol-
the structures in the Abstract graphic and Schemes 1 and 2 in
the version published ASAP August 29, 2007; the corrected
version was published ASAP September 6, 2007.
Supporting Information Available: Preparative procedures,
2
-ylidenes) are quite stable in dilute DMSO solution at least
1
13
characterization data, and copies of H and C NMR spectra for
,3-dialkylimidazolium ionic liquids (IL 1-16), and details of
during the time scales of the NMR and UV titrations. There
1
1
were no observable changes in the H NMR spectra of the acid-
experimental operation and absorption spectra of the model titration
process. This material is available free of charge via the Internet at
http://pubs.acs.org.
base mixture solutions after standing about half an hour at rt,
indicating that dimerization26 and side reaction with the precur-
JO070973I
(26) (a) Arduengo, A. J., III; Dias, H. V. R.; Harlow, R. L.; Kline, M.
J. Am. Chem. Soc. 1992, 114, 5530-5534. (b) Alder, R. W.; Blake, M. E.;
Chaker, L.; Harvey, J. N.; Paolini, F.; Sch u¨ tz, J. Angew. Chem., Int. Ed.
(27) Arduengo, A. J., III; Gamper, S. F.; Tamm, M.; Calabrese, J. C.;
2
004, 43, 5896-5911.
Davidson, F.; Craig, H. A. J. Am. Chem. Soc. 1995, 117, 572-573.
J. Org. Chem, Vol. 72, No. 20, 2007 7793