Transimination reactions in [b-3C-im][NTf2] ionic liquid

Article abstract of DOI:10.1039/c0ob01094d

Compared to conventional molecular solvents, the ionic liquid [b-3C-im][NTf₂] was found to promote transimination reactions with up to ～100-fold rate enhancement. This rate effect observed at ambient temperature might be explained by the fact that the ionic liquid displays weak Lewis acidity with very low, if any, nucleophilicity and its imidazolium cation is expected to interact by associating with, and thus electrophilically activating, the CN bond of the starting imine, leading to increased stabilization of the polar, charged intermediate species and ultimately, rapid product formation. Moreover, the presence of 1 mol% Sc(OTf)₃ in [b-3C-im][NTf₂] further facilitates the transimination reactions studied.

Full text of DOI:10.1039/c0ob01094d

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PAPER
Transimination reactions in [b-3C-im][NTf ] ionic liquid†
2
Chao-Wen Chen, Ming-Chung Tseng, San-Kai Hsiao, Wen-Hao Chen and Yen-Ho Chu*
Received 27th November 2010, Accepted 24th March 2011
DOI: 10.1039/c0ob01094d
Compared to conventional molecular solvents, the ionic liquid [b-3C-im][NTf ] was found to promote
2
transimination reactions with up to ~100-fold rate enhancement. This rate effect observed at ambient
temperature might be explained by the fact that the ionic liquid displays weak Lewis acidity with very
low, if any, nucleophilicity and its imidazolium cation is expected to interact by associating with, and
thus electrophilically activating, the C N bond of the starting imine, leading to increased stabilization
of the polar, charged intermediate species and ultimately, rapid product formation. Moreover, the
presence of 1 mol% Sc(OTf)
3
in [b-3C-im][NTf
2
] further facilitates the transimination reactions studied.
6
7
Introduction
number of synthetic reactions. These include aziridination,
8
9
Michael reaction, Diels–Alder cycloaddition, acylative cleavage
of ethers, Heck reaction, and Friedel–Crafts reaction.
In our laboratory, we have a program to evaluate ionic
liquids as novel and stable media for synthetic and biochemical
We, and others, for example, have employed
ionic liquids as reaction media to afford syntheses of complex
As one of few reversible covalent bonds, imine formation and
exchange are fundamental and ubiquitous reactions in organic
10
11
12
1
chemistry, and have found unique applications in the chemos-
elective conjugation of biomolecules and the folding study of
supramolecular assemblies. Although Schiff’s base has been
exhaustively investigated and studied for decades, its impact in
chemical research has nevertheless been compromised, primarily
due to its slow kinetics of imine bond exchange that limit the rapid
response and sensitivity requirements for any new applications.
Herein we describe our development of a system that uses the
ionic liquid [b-3C-im][NTf
significantly accelerate the transimination reactions of imines 1
and 2 with various amines. This transimination reaction carried
4,5,13
applications.
2
6b,13d,13e
heterocycles.
With continuous development of novel ionic
] ionic
liquids for new applications, we exploited [b-3C-im][NTf
2
liquid as the solvent to greatly facilitate the transimination
reactions studied. This ionic liquid was selected primarily for
its superior chemical stability over the common [bmim][PF
]. In this work we report a more detailed study of
the exchange in Schiff bases 1 and 1a, and extend it to other imine
3
6
] and
4
2
], previously developed by us, to
[
bdmim][PF
4
6
systems.
5
out in an ionic liquid, to our knowledge, has not been explored.
Results and discussion
The major difference between molecular solvents and ionic liquids
is the weak electrostatic association of organic cations with
charge diffuse anions in ionic liquids. As known in the chemistry
literature, many organic reactions performed in molecular solvents
are catalyzed by Lewis acids and, recently, a number of these
reactions have been reported to be significantly accelerated when
Ionic liquids are low-melting molten salts composed entirely
of ions, and many of them are liquid at room temperature.
6
As neoteric solvents, ionic liquids carry numerous desirable
properties, such as a wide liquid range, thermal stability, excellent
solubility with many small molecules, attractive recyclability,
and negligible vapor pressure that are well suited for a myriad
of applications. Of them, ionic liquids with high polarity and
negligible vapor pressure have been recognized and employed as
environmentally benign media for a range of chemical processes
and have been found to outperform molecular solvents in a
14,15
carried out in pure ionic liquids.
been discussed and reported, this rate increase observed in pure
ionic liquids in some reactions was attributed to the inherent
Although other effects have
15
14
weak Lewis acidity of ionic liquids employed. Realizing that
metal triflates notably contributed to the transimination reactions
2c,3
in molecular solvents,
we therefore envisaged that, without
metal triflates, it should be possible to promote such reactions
in ionic liquids. The transimination reaction studied is illustrated
in Scheme 1.
Department of Chemistry and Biochemistry, National Chung Cheng Uni-
versity, Chia-Yi, Taiwan, ROC. E-mail: cheyhc@ccu.edu.tw; Fax: +886-5-
721040; Tel: +886-5-2428148
We first studied the influence of the [b-3C-im][NTf ] ionic liquid
2
2
†
in the transimination reaction of isopropylamine with the imine 1,
N-(4-nitrobenzylidene)-4-nitroaniline, used as a model substrate
(Scheme 2). Since transimination reactions can be sensitive to
Electronic supplementary information (ESI) available: Figures S1–S3,
1
13
complete H and C NMR spectra of all compounds 1, 1a–c, 2, and 2a–c.
See DOI: 10.1039/c0ob01094d
4
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These results indicate that, irrespective of their nature or position,
substituent groups present on aromatic rings react smoothly and
the resulting corresponding imines are obtained with high isolated
yields (80–89%, see Experimental section). Interestingly, sterically
hindered o-nitrobenzaldehyde gave its corresponding imine in high
yield, inferring that the nitro group around the reaction site does
not affect its reactivity in imine formation. NMR spectra of the
aldimines studied show that they all exist in a single form (likely the
trans isomer) with no other stereoisomeric cis form being detected
Scheme 1 A reversible transimination reaction.
(
ESI†).
When imine 1 (15 mM) was reacted with isopropylamine
15 mM) in deuterated dichloromethane (a common solvent used
(
for transimination reactions) at ambient temperature, a time of
53 h (9190 min) was necessary to complete the reaction (Fig. 1).
The time required at 50% conversion (t1/2) for the transimination
in CD Cl was approximately 2070 min. The progress of the
1
2
2
reaction could be readily monitored at the imine protons N CH
on both 1 and 1a by H NMR. It is also noted that the
Scheme 2 The transimination reaction of the imine 1 with isopropy-
1
lamine, catalyzed by [b-3C-im][NTf
2
] ionic liquid.
irreversible nature of the transimination reaction indicated the
non-equilibrium formation of the Schiff base 1a (Scheme 2). This
could be understood by the fact that the use of p-nitroaniline,
as an excellent leaving group, inhibits the reverse reaction of the
imine adduct 1a that was formed. To our delight, when carried
water, the hydrophobic [b-3C-im][NTf
for use in this study. The imine 1, composed of an aromatic
aldehyde and arylamine, was selected, rather than an aliphatic
one, largely due to its known thermodynamic stability. It is also
known in the literature that electron-withdrawing substituents,
2
] ionic liquid was chosen
16
out in [b-3C-im][NTf ] ionic liquid, this transimination reaction
2
17
such as nitro groups on aryl aldimines, retard hydrolysis. The
protocols developed by Chakraborti et al. and Andrade et al.
was greatly accelerated and completed in 7.5 h (450 min) with
a t1/2 of approximately 150 min (Fig. 1). A kinetic study, shown
in Fig. 1, clearly demonstrates the remarkable 14-fold difference
in reaction rate (t1/2) obtained in ionic liquid and in deuterated
dichloromethane. This result suggested that the transimination
18
19
were followed to prepare the imine substrates 1 and 2, and
the imine products 1a–c and 2a–c, respectively. All aldimine
compounds prepared (1, 1a–c, 2, and 2a–c) were readily purified
with diethyl ether and isolated in high purity, and gave satisfactory
reaction was greatly promoted if carried out in the [b-3C-im][NTf
ionic liquid.
2
]
1
13
spectroscopic data (for H and C NMR spectra, see ESI†).
Fig. 1 Kinetic measurements of the transimination reactions of N-(4- nitrobenzylidene)-4-nitroaniline 1 (15 mM) with isopropylamine (15 mM) in
[
b-3C-im][NTf
2
] ionic liquid and CD
2
Cl
2
solvent at ambient temperature. Inset: details of early conversions of the transimination reactions. The progress
1
of the transimination reactions could be readily monitored by H NMR.
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Solvent polarity can influence the outcome of chemical reac-
tions. Imidazolium ionic liquids are polar; the polarity of these
ionic liquids has previously been determined to be greater than
were quantitated by integration, and the data obtained were
plotted in Fig. 1. Not surprisingly, as shown in Fig. 1, Sc(OTf)
promotes the transimination reaction in both ionic liquid and
deuterated dichloromethane. With 1 mol% Sc(OTf) in [b-3C-
im][NTf ] solvent, 50% conversion of 1 to 1a was achieved shortly
3
common molecular solvents, such as CH
2
Cl
2
and acetonitrile,
3
similar to that of lower alcohols and dimethylsulfoxide (DMSO),
2
20,21
but clearly less than that of water.
In order to correlate solvent
after 95 min, while a time of 150 min was required without the
catalyst. A significant reduction in reaction time at 50% conversion
polarity with the transimination reaction efficiency, we decided
to investigate the solvent effect of the transimination reaction of
in CD
Sc(OTf)
or decomposition of imines 1 and 1a were observed when carried
out in [b-3C-im][NTf ] ionic liquid and CD Cl during the entire
2
Cl
2
(1360 vs. 2070 min at t1/2) was also obtained with 1 mol%
1
to 1a in two polar molecular solvents: the aprotic DMSO and
3
Lewis acid catalyst (Fig. 1). In our hands, no hydrolysis
21
the protic methanol. For reactions performed in DMSO-d at
6
1
ambient temperature, to our surprise, the H NMR result showed
that the imine adduct 1a was not observed at all and only its
polar aminal intermediate S1 was fortuitously accumulated and
obtained (Fig. S1, ESI†). This behavior was rather unexpected,
2
2
2
experiment. Moreover, under identical experimental condition,
the imine 1a and p-nitroaniline did not show any detectable
transimination reaction in both CD
2
Cl
2
and [b-3C-im][NTf ]
2
since other transimination reactions could be studied in DMSO-
within the experimental time (7 d and 820 min, respectively) at
ambient temperature, evidently indicating the irreversibility of the
transimination reaction of 1 to 1a, and that the p-nitroaniline
was an excellent leaving group. The reaction course of this Lewis
acid-catalyzed transimination may be explained on the basis of
Scheme 3. As shown in Scheme 3, the Lewis acid activates the
starting imine and accordingly facilitates the reaction of imine
exchange. In the mechanism, the Lewis acid may be the [b-3C-im]
3
d
6
. Under our experimental conditions (i.e., 15 mM each for 1
and isopropylamine), the polar DMSO promoted the nucleophilic
addition of isopropylamine to imine 1, appeared to stabilize and
“
lock in” its polar aminal species S1, and essentially entrapped
the amine reagent (Fig. S1, ESI†). As a result, this transimination
reaction proceeded incompletely in this polar aprotic solvent. At-
tempts to perform the same transimination reaction in methanol-
3,22
d
4
were also unsuccessful under our conditions (Fig. S2, ESI†).
imidazolium cation in ionic liquid or the Sc(III) cation in Sc(OTf) .
3
The striking feature of this reaction conducted in deuterated
methanol was that, in the presence of isopropylamine, methanol
effectively acted as a nucleophile and readily reacted with the imine
1
to afford a polar hemiaminal ether intermediate S2, stabilized
and trapped by the polar methanol solvent (Fig. S2, ESI†). This
nucleophilicity of methanol for the polar reaction is not totally
unexpected, and therefore it may be thought of as a reacting
solvent for transimination reactions. In our hands, although
DMSO, methanol and ionic liquids have similar solvent polarity
properties, only [b-3C-im][NTf ] ionic liquid greatly promotes and
2
smoothly proceeds to completion of the transimination reactions
studied.
When using [b-3C-im][Br] ionic liquid for the reaction of
imine 1 with isopropylamine, transimination proceeded 2.7 times
Scheme 3 A mechanistic hypothesis for the transimination reaction
catalyzed by a Lewis acid (LA). In this work, LA may be the [b-3C-im]
cation, loosely associated with its anion [NTf ], in ionic liquid, or Sc(OTf) .
2 3
faster than when carried out with [b-3C-im][NTf
2
] ionic liquid,
suggesting that the imidazolium cation in [b-3C-im][Br] is more
23
Lewis acidic than that of [b-3C-im][NTf ]. This rate increase
2
with [b-3C-im][Br] directly correlates with its significant downfield
shift of imidazolium ring H-4 and H-5 protons in the H NMR
data: 7.51 and 7.61 ppm for [b-3C-im][Br] vs. 7.19 and 7.25 ppm
Encouraged as we were by the results of the 1 → 1a process,
the substrate scope of this ionic liquid-promoted transimination
reaction was examined. As shown in Table 1, in all cases the
transiminations proceeded with complete reaction conversion
1
for [b-3C-im][NTf ], respectively, clearly indicating more electron
2
deshielding and deficiency, and, accordingly, increasing the Lewis
acidic nature of the imidazolium cation in [b-3C-im][Br] (Fig.
S3, ESI†). This result, along with the aforementioned results,
is entirely consistent with the Lewis acidity and high polarity
and increased reaction rates in [b-3C-im][NTf ] ionic liquid. The
2
sterically hindered isopropylamine may account for the relatively
slow conversion of imines 1 and 2 to imines 1a and 2a, respectively,
as compared to other amines in both ionic and molecular solvents
(entries 1 and 4, Table 1). In the cases of aliphatic benzylamine and
24
interpretations of [b-3C-im][NTf ] ionic liquid for high reaction
2
rates for polar transimination reactions.
aromatic aniline, transiminations in [b-3C-im][NTf
proceeded to completion in minutes, whereas hours of reaction
time were required for both reactions carried out in CD Cl (entries
2
] ionic liquid
Lewis acidic metal ions are known to catalyze transimina-
2c,3
tion reactions in molecular solvents. The Lewis acid catalyst
2
2
Sc(OTf)
3
was selected in this study primarily because of its
2, 3, 5, and 6, Table 1). Table 1 also shows that, irrespective of
the position of the nitro substituent group present on aromatic
ring, both starting imines (1 and 2) reacted smoothly and gave
the corresponding imines (1a–c and 2a–c), inferring that the nitro
group around the reaction site does not significantly affect its
reactivity in imine formation. It is worth noting that, for the
reaction of benzylamine with imine 1, the largest rate increase
3
reported effectiveness in the catalysis of transimination reactions.
We employed infrared spectroscopy (FT-IR) to monitor the
progress of the transimination reaction of 1 to 1a. This IR assay,
however, gave variable results, due mostly to the overlapped
-
1
C
N bands centered around 1630 cm in IR spectra. This
1
transimination reaction was then followed by H NMR, rates
4
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Table 1 Transimination reactions of imines 1 and 2 with isopropylamine, benzylamine, and aniline in the ionic liquid [b-3C-im][NTf
2
] and the molecular
a
solvent CD
2
Cl
2
at ambient temperature. Imines 1 and 2 were all 100% transiminated and converted to the corresponding imine adducts 1a–c and 2a–c,
respectively
b
Reaction time (min)
Entry
1
Starting imine
Amine
Product imine
[b-3C-im][NTf
450
2
]
2 2
CD Cl
Isopropylamine
9190
2
3
Benzylamine
Aniline
20
20
1900
1980
4
Isopropylamine
750
9260
5
6
Benzylamine
Aniline
50
60
1490
1900
a
b
Conditions: starting imine (15 mM), amine (15 mM), solvent (100 mL), rt. Time required to achieve 100% conversion in the transimination reaction
1
was monitored by H NMR.
(
~100-fold) in ionic liquid vs. CD
2
Cl
2
was observed (entry 2,
a Fargo MP-2D apparatus (Taiwan, ROC) and are uncorrected.
Solvents and reagents were obtained from commercial sources and
were used without further purification.
Table 1).
Conclusions
General procedure for transimination reactions in deuterated
dichloromethane. To a NMR tube containing the imine 1 or 2
As noted at the very outset, a number of organic reactions carried
out in ionic liquids proceed effectively when compared to those
performed in conventional molecular solvents. In this work, we
have developed an efficient platform for studying and facilitating
transimination reactions in ionic liquid at ambient temperature.
18,19
(
2.0 mg, 0.007 mmol)
and CD
2
2
Cl (500 mL) was added the
amine (0.007 mmol). The tube was then placed on a rotating
shaker and the resulting solution was allowed to react at ambient
1
temperature. The H NMR spectrum was collected periodically.
General procedure for Sc(OTf)
3
catalyzed transimination reac-
Our results suggested that the [b-3C-im][NTf ] ionic liquid is acting
2
tions in deuterated dichloromethane. To a NMR tube containing
as a polar, non-nucleophilic, but weakly Lewis acidic solvent, and
would therefore promote the transimination reactions studied.
This use of ionic liquid as reaction medium may potentially extend
the scope of transimination reactions to new applications.
18,19
the imine 1 or 2 (2.0 mg, 0.007 mmol)
and Sc(OTf)
3
(0.037 mg,
1
mol%) in CD Cl (500 mL) was added the amine (0.007 mmol).
2
2
The tube was then placed on a rotating shaker and the resulting
1
solution was allowed to react at ambient temperature. The H
NMR spectrum was collected periodically.
Experimental section
General procedure for transimination reactions in [b-3C-
Gerneral
im][NTf
2
] ionic liquid. To an eppendorf tube containing the imine
4
1
or 2 (2.0 mg, 0.007 mmol) in [b-3C-im][NTf
2
] (500 mL) was
Flash chromatography was performed on silica gel (230–400
mesh). TLC was carried out on aluminium-backed silica plates
precoated with silica (0.2 mm), which were developed using
standard visualizing agents, such as UV fluorescence and iodine.
Unless otherwise indicated, all reactions were carried out without
the aid of dry nitrogen or argon. NMR spectra were recorded on
added the amine (0.007 mmol). The tube was then placed on a
rotating shaker and the resulting solution was allowed to react at
ambient temperature. The reaction solution (40 mL) was pipetted
at regular intervals, ranging from every 10 min for the faster
reactions to every 30 min for the slower reaction, and then diluted
1
with CDCl
3
(360 mL). The H NMR was then recorded.
1
13
a Varian Gemini 200 at 200 MHz ( H) and 50 MHz ( C), both
in CDCl unless otherwise stated. Chemical shifts were quoted in
3
General procedure for Sc(OTf)
actions in [b-3C-im][NTf ] ionic liquid. To an eppendorf tube
containing the imine 1 or 2 (2.0 mg, 0.007 mmol) and Sc(OTf)
3
catalyzed transimination re-
parts per million (ppm). The FT-IR spectra were recorded in Jasco
FT-IR-460 Plus spectrometer. Melting points were determined on
2
3
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+
(
0.037 mg, 1 mol%) in [b-3C-im][NTf
2
] (500 mL) was added the
133.8, 146.0, 149.4, 156.7, 158.8; EI-HRMS m/z [M] calcd for
amine (0.007 mmol). The tube was then placed on a rotating
C
13
H
9
N
3
4
O 271.0593, found 271.0591.
shaker and the resulting solution was allowed to react at ambient
temperature. The reaction solution (40 mL) was pipetted every 15 s
1
and diluted with CDCl
3
(360 mL). The H NMR spectrum was
then collected.
N-(2-nitrobenzylidene)-isopropylamine (2a). Brown liquid; IR
-
1
1
(
KBr, cm ) 1637; H NMR (200 MHz, CDCl
3
) d 1.30 (6H, d, J =
6
8
.2 Hz, CH
.00–8.06 (2H, m, ArH), 8.72 (1H, s, CH N); C NMR (50 MHz,
) d 23.8, 61.7, 123.6, 128.6, 130.0, 131.5, 133.5, 148.5, 154.0;
3
), 3.63–3.70 (1H, m, CH), 7.27–7.71 (2H, m, ArH),
1
3
N-(4-nitrobenzylidene)-4-nitroaniline (1). Yellow solid, mp
CDCl
FAB-HRMS m/z [M+H] calcd for C10
93.0975.
3
◦
-1
1
1
97–198 C; IR (KBr, cm ) 1628; H NMR (200 MHz, CDCl
d 7.30 (2H, d, J = 9.2 Hz, ArH), 8.12 (2H, d, J = 9.0 Hz, ArH),
.22 (2H, d, J = 9.2 Hz, ArH), 8.38 (2H, d, J = 9.0 Hz, ArH), 8.54
3
)
+
H
13
N
2
2
O 193.0977, found
1
8
(
1
1
3
1H, s, CH N); C NMR (50 MHz, CDCl
29.9, 140.5, 146.1, 149.9, 156.6, 160.1; EI-HRMS m/z [M] calcd
271.0593, found 271.0593.
3
) d 121.3, 124.1, 125.5,
+
for C13
H
9
N
3
O
4
N-(2-nitrobenzylidene)-benzylamine (2b). Brown liquid; IR
-
1
1
(
KBr, cm ) 1637; H NMR (200 MHz, CDCl
NCH ), 7.25–7.31 (5H, m, ArH), 7.56–7.67 (2H, m, ArH), 8.01–
8.12 (2H, m, ArH), 8.84 (1H, s, CH N); C NMR (50 MHz,
CDCl ) d 64.8, 124.0, 126.7, 127.7, 128.2, 129.6, 130.7, 130.9,
133.2, 138.4, 148.7, 157.5; FAB-HRMS m/z [M+H] calcd for
241.0977, found 241.0979.
3
) d 4.89 (2H, s,
2
1
3
N-(4-nitrobenzylidene)-isopropylamine (1a). Brown solid, mp
◦
-1
1
5
2–53 C; IR (KBr, cm ) 1644; H NMR (200 MHz, CDCl
3
) d
3
+
1
.29 (6H, d, J = 6.4 Hz, CH ), 3.59–3.65 (1H, m, CH), 7.90 (2H,
3
d, J = 11.0 Hz, ArH), 8.27 (2H, d, J = 11.0 Hz, ArH), 8.38 (1H, s,
C
14
H
13
N
2
O
2
1
3
CH N); C NMR (50 MHz, CDCl
1
3
) d 23.8, 61.7, 123.6, 128.6,
+
41.9, 148.7, 156.0; EI-HRMS m/z [M] calcd for C10
H
12
N
2
O
2
192.0899, found 192.0902.
◦
N-(2-nitrobenzylidene)-aniline (2c). Red solid, mp 45–46 C;
-
1
1
IR (KBr, cm ) 1618; H NMR (200 MHz, CDCl
3
) d 7.27–7.38
(
7
3H, m, ArH), 7.40–7.50 (2H, m, ArH), 7.62–7.70 (1H, m, ArH),
.71–7.80 (1H, m, ArH), 8.10 (1H, d, J = 6.2 Hz, ArH), 8.32 (1H,
13
N-(4-nitrobenzylidene)-benzylamine (1b). Orange solid, mp
◦
-1
1
4
7–48 C; IR (KBr, cm ) 1644; H NMR (200 MHz, CDCl
), 7.26–7.38 (5H, m, ArH), 7.95 (2H, d, J =
.6 Hz, ArH), 8.28 (2H, d, J = 8.6 Hz, ArH), 8.47 (1H, s, CH N);
3
)
d, J = 6.2 Hz, ArH), 8.95 (1H, s, CH N); C NMR (50 MHz,
CDCl ) d 120.9, 124.1, 124.2, 126.6, 130.6, 130.8, 132.2, 134.0,
1
d 4.89 (2H, s, NCH
2
3
8
+
48.9, 157.0, 159.2; EI-HRMS m/z [M] calcd for C13
H
10
N
2
O
2
1
3
C NMR (50 MHz, CDCl
28.5, 138.3, 141.3, 148.4, 159.1; EI-HRMS m/z [M] calcd for
240.0899, found 240.0897.
3
) d 64.7, 123.4, 126.9, 127.6, 128.1,
2
26.0742, found 226.0748.
+
1
C
14
H
12
N
2
O
2
Acknowledgements
The National Science Council (Taiwan, ROC) is gratefully ac-
knowledged for the financial support of this work (97-2113-M-
N-(4-nitrobenzylidene)-aniline (1c). Light yellow solid, mp 89–
194-006-MY3). We thank reviewers for constructive comments
◦
-1
1
9
7
0
C; IR (KBr, cm ) 1625; H NMR (200 MHz, CDCl
3
) d
and valuable suggestions.
.24–7.38 (3H, m, ArH), 7.40–7.48 (2H, m, ArH), 8.08 (2H, d,
J = 8.8 Hz, ArH), 8.33 (2H, d, J = 8.8 Hz, ArH), 8.56 (1H, s,
Notes and references
1
3
CH N); C NMR (50 MHz, CDCl
29.2, 141.4, 149.8, 150.8, 157.2; EI-HRMS m/z [M] calcd for
226.0742, found 226.0741.
3
) d 120.9, 123.8, 127.3, 129.2,
+
1
1 For recent reviews, see: (a) J. B. Love, Chem. Commun., 2009, 3154–
3
2
165; (b) C. D. Meyer, C. S. Joiner and J. F. Stoddart, Chem. Soc. Rev.,
007, 36, 1705–1723.
C
13
H
10
N
2
O
2
2
(a) C. B. Minkenberg, L. Florusse, R. Eelkema, G. J. M. Koper and
J. H. Van Esch, J. Am. Chem. Soc., 2009, 131, 11274–11275; (b) N.
Giuseppone, J.-L. Schmitt and J.-M. Lehn, J. Am. Chem. Soc., 2006,
1
28, 16748–16763; (c) N. Giuseppone, J.-L. Schmitt and J.-M. Lehn,
Angew. Chem., Int. Ed., 2004, 43, 4902–4906.
3
N. Giuseppone, J.-L. Schmitt, E. Schwartz and J.-M. Lehn, J. Am.
Chem. Soc., 2005, 127, 5528–5539.
4 (a) H.-C. Kan, M.-C. Tseng and Y.-H. Chu, Tetrahedron, 2007, 63,
1
1
N-(2-nitrobenzylidene)-4-nitroaniline(2). Brown solid, mp
◦
-1
1
1
44–145 C; IR (KBr, cm ) 1617; H NMR (200 MHz, CDCl
3
) d
644–1653; (b) J.-Y. Cheng and Y.-H. Chu, Tetrahedron Lett., 2006, 47,
575–1579.
7
.34 (2H, d, J = 9.2 Hz, ArH), 7.67–7.84 (2H, m, ArH), 8.12 (1H,
1
3
m, ArH), 8.31 (3H, m, ArH), 8.94 (1H, s, CH N); C NMR
50 MHz, CDCl ) d 121.5, 124.7, 125.0, 129.8, 130.2, 132.1,
5
Our preliminary study of transimination reactions employed for
detecting small-molecule amine gases using a functionalized ionic
(
3
4
192 | Org. Biomol. Chem., 2011, 9, 4188–4193
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liquid by quartz crystal microbalance (QCM) appeared in the literature
last year: M.-C. Tseng and Y.-H. Chu, Chem. Commun., 2010, 46, 2983–
K. Hanlon, D. Fayne and P. McCormac, Tetrahedron Lett., 1997, 38,
3097–3100.
2
985.
15 (a) T. Zhao, Q. Zhou, X.-L. He, S.-D. Wei, L. Wang, J. M. N. van
Kasteren and Y.-Z. Wang, Green Chem., 2010, 12, 1062–1065; (b) A.
Wulf, K. Fumino and R. Ludwig, Angew. Chem., Int. Ed., 2010, 49,
449–453; (c) C. E. Rosella and J. B. Harper, Tetrahedron Lett., 2009,
50, 992–994; (d) H. M. Yau, A. G. Howe, J. M. Hook, A. K. Croft
and J. B. Harper, Org. Biomol. Chem., 2009, 7, 3572–3575; (e) H. M.
Yau, S. A. Barnes, J. M. Hook, T. G. A. Youngs, A. K. Croft and J. B.
Harper, Chem. Commun., 2008, 3576–3578; (f) B. Y. W. Man, J. M.
Hook and J. B. Harper, Tetrahedron Lett., 2005, 46, 7641–7645.
16 (a) C. Godoy-Alcantar, A. K. Yatsimirsky and J.-M. Lehn, J. Phys. Org.
Chem., 2005, 18, 979–985; (b) M. J. Makela and T. K. Korpela, Chem.
Soc. Rev., 1983, 12, 309–329.
6
(a) For recent reviews on ionic liquids, see: S. Sowmiah, V. Srinivasade-
sikan, M.-C. Tseng and Y.-H. Chu, Molecules, 2009, 14, 3780–3813;
(
b) M. A. P. Martins, C. P. Frizzo, D. N. Moreira, N. Zanatta and H. G.
Bonacorso, Chem. Rev., 2008, 108, 2015–2050; (c) N. V. Plechkova and
K. R. Seddon, Chem. Soc. Rev., 2008, 37, 123–150.
A. C. Mayer, A.-F. Salit and C. Bolm, Chem. Commun., 2008, 5975–
7
8
9
5
977.
J. S. Yadav, B. V. S. Reddy and G. Baishya, J. Org. Chem., 2003, 68,
7
098–7100.
C. E. Song, W. H. Shim, E. J. Roh, S.-g. Lee and J. H. Choi, Chem.
Commun., 2001, 1122–1123.
1
1
1
0 L. Green, I. Hemeon and R. D. Singer, Tetrahedron Lett., 2000, 41,
343–1346.
1 J. Mo, L. Xu and J. Xiao, J. Am. Chem. Soc., 2005, 127, 751–
60.
2 (a) M. J. Earle, P. B. McCormac and K. R. Seddon, Green Chem., 2000,
17 R. W. Layer, Chem. Rev., 1963, 63, 489–510.
18 A. K. Chakraborti, S. Bhagat and S. Rudrawar, Tetrahedron Lett., 2004,
45, 7641–7644.
1
7
19 C. K. Z. Andrade, S. C. S. Takada, L. M. Alves, J. P. Rodrigues, P. A. Z.
Suarez, R. F. Brandao and V. C. D. Soares, Synlett, 2004, 2135–2138.
20 (a) C. Reichardt, Green Chem., 2005, 7, 339–351; (b) S. N. V. K. Aki, J. F.
Brennecke and A. Samanta, Chem. Commun., 2001, 413–414; (c) M. J.
Muldoon, C. M. Gordon and I. R. Dunkin, J. Chem. Soc., Perkin Trans.
2, 2001, 433–435; (d) A. J. Carmichael and K. R. Seddon, J. Phys. Org.
Chem., 2000, 13, 591–595.
2
, 261–262; (b) J. A. Boon, J. A. Levisky, J. L. Pflug and J. S. Wilkes,
J. Org. Chem., 1986, 51, 480–483.
1
3 (a) M.-C. Tseng, M.-J. Tseng and Y.-H. Chu, Chem. Commun., 2009,
7
503–7505; (b) M.-C. Tseng, H.-C. Kan and Y.-H. Chu, Tetrahedron
Lett., 2007, 48, 9085–9089; (c) Y.-L. Lin, J.-Y. Cheng and Y.-H. Chu,
Tetrahedron, 2007, 63, 10949–10957; (d) M.-C. Tseng, Y.-M. Liang and
Y.-H. Chu, Tetrahedron Lett., 2005, 46, 6131–6136; (e) Y.-H. Yen and
Y.-H. Chu, Tetrahedron Lett., 2004, 45, 8137–8140; (f) J.-C. Hsu, Y.-H.
Yen and Y.-H. Chu, Tetrahedron Lett., 2004, 45, 4673–4676.
4 (a) M. Selva, M. Fabris, V. Lucchini, A. Perosa and M. Noe, Org.
Biomol. Chem., 2010, 8, 5187–5198; (b) C. Chiappe, M. Malvaldi and
C. S. Pomelli, Green Chem., 2010, 12, 1330–1339; (c) H. Yanai, H.
Ogura and T. Taguchi, Org. Biomol. Chem., 2009, 7, 3657–3659; (d) H.
Zhang, F. Xu, X. Zhou, G. Zhang and C. Wang, Green Chem., 2007, 9,
N
21 According to Reichardt’s normalized solvent polarity scale E with
T
N
N
T
20a
ET = 0.00 for tetramethylsilane and E = 1.00 for water, polarities
of the 1,2,3-trialkylimidazolium ionic liquids, likely including [b-3C-
2
im][NTf ], are in between DMSO (0.45) and methanol (0.75): range
0.50–0.56; mean value 0.53.
1
22 For examples of transimination reactions performed in methanol, see:
(a) F. Ragaini, M. Gasperini, E. Gallo and P. Macchi, Chem. Commun.,
2005, 1031–1033; (b) J. Toullec and R. Razafindralambo, J. Org. Chem.,
1987, 52, 1646–1647.
1
208–1211; (e) L. Crowhurst, N. L. Lancaster, J. M. P. Arlandis and T.
23 Alteration of anions in common nonhydroxyl ionic liquids has only
N
T
Welton, J. Am. Chem. Soc., 2004, 126, 11549–11555; (f) A. Aggarwal,
marginal influence on the solvent polarity scale E : see (a) ref. 20a;
N. L. Lancaster, A. R. Sethi and T. Welton, Green Chem., 2002, 4,
(b) S. Zhang, X. Qi, X. Ma, L. Lu and Y. Deng, J. Phys. Chem. B, 2010,
114, 3912–3920.
5
17–520; (g) J. R. Harjani, S. J. Nara and M. M. Salunkhe, Tetrahedron
Lett., 2002, 43, 1127–1130; (h) C. E. Song, W. H. Shim, E. J. Roh, S.-g.
Lee and J. H. Choi, Chem. Commun., 2001, 1122–1123; (i) C. E. Song
and E. J. Roh, Chem. Commun., 2000, 837; (j) C. E. Song, W. H. Shim,
E. J. Roh and J. H. Choi, Chem. Commun., 2000, 1695; (k) J. Howarth,
2
24 The pH values of both [b-3C-im][NTf ] and [b-3C-im][Br] ionic liquids
in water (0.1 M) were measured following the protocol developed by
X. Cui, and coworkers, ChemSusChem, 2010, 3, 1043–1047, and found
to be weakly acidic: 6.01 and 4.14, respectively.
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