Characterizing ionic liquids as reaction media through a chemical probe

Article abstract of DOI:10.1002/chem.200601078

The triplet N,N-dimethylaminophenyl cation, a highly reactive but chemospecific electrophile, has been used as a probe for characterizing the properties of reaction media for a series of imidazolium ILs. With the N-hexyl-N-methyl imidazolium derivatives (not with the W-butyl analogues), hydrogen transfer leading to the aniline was the main process. Trapping by iodide occurred with an inverse dependence on viscosity. Trapping by it nucleophiles exhibited a more complex behavior. This was explained by the effect of both the bulk viscosity and the structure of the IL cation on both steps of the reaction, namely, initial electrophilic attack and ensuing cation elimination or nucleophile addition. However, with an excellent nucleophile, such as thiophene, or when the latter step was intramolecular, as with 4-pentenol, the difference was obliterated and trapping became uniform. Incorporation of the probe into the IL cation (through insertion into the C-H bond a to the imidazolium ring) was demonstrated, while no addition to the anion tested (including bis(trifluoromethanesulfonimide)) took place.

Full text of DOI:10.1002/chem.200601078

DOI: 10.1002/chem.200601078
Characterizing Ionic Liquids as Reaction Media through a Chemical Probe
Valentina Dichiarante,[a] Cecilia Betti,[b] Maurizio Fagnoni,*[a] Angelamaria Maia,[c]
Dario Landini,[b] and Angelo Albini[a]
Abstract: The triplet N,N-dimethylami-
nophenyl cation, a highly reactive but
chemospecific electrophile, has been
used as a probe for characterizing the
nucleophiles exhibited a more complex
behavior. This was explained by the
effect of both the bulk viscosity and
the structure of the IL cation on both
steps of the reaction, namely, initial
electrophilic attack and ensuing cation
elimination or nucleophile addition.
However, with an excellent nucleo-
phile, such as thiophene, or when the
latter step was intramolecular, as with
4-pentenol, the difference was obliter-
ated and trapping became uniform. In-
corporation of the probe into the IL
properties of reaction media for
a
series of imidazolium ILs. With the N-
hexyl-N-methyl imidazolium deriva-
tives (not with the N-butyl analogues),
hydrogen transfer leading to the aniline
was the main process. Trapping by
iodide occurred with an inverse de-
pendence on viscosity. Trapping by p
À
cation (through insertion into the C H
bond a to the imidazolium ring) was
demonstrated, while no addition to the
anion tested (including bis(trifluorome-
thanesulfonimide)) took place.
À
Keywords: C C coupling · cations ·
ionic liquids
solvent effects
· photochemistry ·
Introduction
in synthetic applications, because they are easily prepared in
good to quantitative yields from relatively cheap and easily
available intermediates. They are chemically and thermally
stable and easy recyclable materials. In addition, an appro-
priate choice of both the N-substituents and the anion ena-
bles to tailor ILs for a very wide variety of synthetic applica-
tions.[2]
However, recent studies have demonstrated some limita-
tions to the general application of ILs in organic synthesis.
Thus, toxicity to crustacean Daphnia magna comparable to
that of common volatile organic solvents[3a] and an acute
toxicity to zebrafish[3b] have been demonstrated for some of
these salts.
Furthermore, the question of the chemical stability is of
evident importance for evaluating the viability of ILs as sol-
vent and should be tested by using a suitable probe. Indeed,
ILs are not chemically inert and both cations and anions
have been shown to react under particular conditions.[4] As
an example, hydrogen abstraction from imidazolium ILs by
benzophenone in the triplet state has been reported,[5]
though the activation energy of the process was higher than
with conventional solvents. As for the counterion, tosyl,
triflyl, and trifluoroacetyl anions competed as nucleophiles
in the fluorodediazoniation reaction of aromatic diazonium
salts.[6a] A phenyl cation in the singlet state was the inter-
mediate and even a non-nucleophilic anion such as [Tf2N]À
was trapped (more efficiently than BrÀ).[6b] Actually, phenyl
Room-temperature ionic liquids (ILs) are a class of novel
solvents that are attracting increasing interest as a potential
“green” alternative to conventional organic media. ILs have
a negligible vapor pressure[1] that, combined with excellent
thermal stability and easy preparation and recycling, makes
them viable ecosustainable solvents for stoichiometric and
catalytic processes. Ionic liquids solubilize inorganic, organ-
ic, and polymeric materials and hence have been utilized in
synthesis, catalysis, polymerization, industrial cleaning,
liquid/liquid extraction, and separation as documented by
the growing number of reviews and books published in
recent years.[2] In particular, imidazolium salts are popular
[a]V. Dichiarante, Dr. M. Fagnoni, Prof. A. Albini
Department of Organic Chemistry, University of Pavia
Via Taramelli 10, 27100 Pavia (Italy)
Fax : (+39)0382-987-323
E-mail: fagnoni@unipv.it
[b]C. Betti, Prof. D. Landini
Dipartimento di Chimica Organica e Industriale
Università degli studi di Milano, Via Venezian 21
20133 Milano (Italy)
[c]Prof. A. Maia
Istituto CNR-ISTM, Via Golgi 19
20133 Milano (Italy)
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FULL PAPER
cations exist in two spin states, singlet and triplet. The latter
states are easily accessible by photoheterolysis of electron-
donating substituted aryl chlorides[7] and exhibit a peculiar
reactivity. This includes addition both to p nucleophiles[7]
and to inorganic anions[8] (but, characteristically, not to neu-
tral n nucleophiles, including water), hydrogen abstraction
in a polar nonprotic solvent such as MeCN. Contrary to the
experiments in MeCN, the reactions in ILs were carried out
under stirring in order to avoid inhomogeneities in such
highly viscous medium. Viscosity is an important parameter
that is strongly affected by the presence of water or other
impurities[11] and was measured on the actually used sam-
ples. The viscosity values are reported in Table 1 and ranged
from 70 cP (for 2a) to 280 cP (2b and 2d), evidently much
higher than for MeCN (0.345 cP). The water content of the
ILs used in this work ranged from 700 to 1000 ppm.
À
from the medium, and intramolecular C H insertion reac-
tions.[9]
We reasoned that the high reactivity and selectivity of
these cations would make them a convenient probe for test-
ing the characteristics of ILs as solvents in organic reactions
by studying the effect on known trapping reactions as well
as any reaction with the ILs themselves.
Irradiation of aniline 1 (0.05m) in neat 2a–e for 16 h
caused a >90% consumption of the starting aniline in all of
the ILs considered and gave variable amounts of N,N-dime-
The photoreactivity of 4-chloro-N,N-dimethylaniline (1),
previously studied in organic solvents of different polarity,[10]
was thus explored in some ILs. Specifically, we chose five
methylalkylimidazolium salts (2a–e) as a homogeneous
series with similar polarity, polarizability, and hydrogen-
bond basicity and acidity by varying only the length of the
alkyl chain (butyl or hexyl) and the counterion (at any rate,
À
À
a
non-nucleophilic anion, namely
N(SO2CF3)2 , BF4 ,
G
À
À
ClO4 , PF6 ) and, accordingly, the viscosity of the medium.
thylaniline (3) and 2,4’-bis(N,N-dimethylamino)biphenyl (4,
through a “self-attack” reaction, see Scheme 1). In MeCN
the amounts of 3 and 4 were roughly equimolar, whereas in
ILs either reduction (in 2d,e) or self-attack (in 2a–c) largely
predominated.
The limited solubility of KI in ILs (ca. 0.02m) prevented
its use in equimolecular amount with respect to 1. Thus, the
reaction was carried out in a saturated solution, by addition
of 0.2m KI, sonication, and irradiation for 16 h.[12] This gave
Results
The photochemistry of chloroaniline (1) was tested both in
neat ILs and in the presence of selected nucleophiles,
namely, an inorganic anion (IÀ), an olefin (allyltrimethylsi-
lane, ATMS), a functionalized olefin (4-penten-1-ol), an aro-
matic (benzene), and an heteroaromatic (thiophene), and
the results were compared to the same reactions carried out
Table 1. Photolysis of 4-chloro-N,N-dimethylaniline (1; 0.05m) in ILs 2a–e in the presence of selected nucleophiles.
Additives
Products (% yield)[a,b]
[bmim][ClO4] [hmim]
(2c) [160][c] (2d) [280][c]
A
[Tf2N]
G
[PF6]
A
U
N
[ClO4]
A
[MsO]
MeCN
[0.345][c]
G
G
ACHTREUNG
none
reduction
self-attack
reduction
self-attack
trapping
reduction
self-attack
trapping
reduction
self-attack
trapping
reduction
self-attack
trapping
reduction
self-attack
trapping
reduction
self-attack
trapping
–
–
3[d] (<1)
4 (11)
3 (1)
3 (78)
–
3 (59)
–
3 (12)
–
3 (20)
4 (6)
5 (15)
3 (12, 14)
4 (0, tr.)
6 (0, 11)
3[d] (18, 16)
–
7 (28, 28), 8 (3, 3)
3 (11, 21)
4 (3, 0)
9 (19, 20)
3 (56)
3 (45)
4 (31)
3 (9)
4 (34)
3 (6)
4 (41)
5 (47)
3 (0, 3)
–
6 (1, 30)
3[d] (0, 5)
–
7 (4, 31) 8 (0, 4)
3 (0, 10)
4 (0, 5)
9 (3, 70)
3 (34)
4 (29)
3 (7)
–
5 (10)
3 (2)
4 (2)
KI (satd)[e]
–
–
5 (6)
5 (7)
5 (77)[f]
3 (5, 16)
ATMS (0.5m)
3 (0, 30)
4 (0, 2)
6 (0, 3)
3 (25)
3 (24, 35)
4 (5, 0)
6 (27, 10)
3 (15, 18)
–
7 (15, 30), 8 (18, 4)
3 (25, 23)
4 (4, 0)
9 (29, 22)
3 (53)
6 (81, 53)
3 (6, 17)
–
7 (47, 22), 8 (7, 5)
3 (20, 58)
4 (11, 0)
9 (44, 42)
3 (58)
4-penten-1-ol (0.5m)
benzene (1m)
3 (4)
–
7 (5)
3 (6)
4 (14)
–
7 (28), 8 (4)
3 (3, 14)
4 (10, 3)
9 (0, 42)
9 (29)
[g][g]
benzene (0.5m)
thiophene (1m)
–
–
–
–
–
–
–
–
9 (58)
3 (5)
4 (37)
10 (47)
9 (21)
3 (48)
4 (5)
10 (51)
9 (20)
3 (9)
4 (5)
10 (39)
9 (21)
3 (16)
4 (18)
10 (49)
3 (13)
4 (4)
10 (34)
4 (31)[h]
10 (54)
[a]GC and HPLC yields. [b]Values in italic are the yields in the presence of TEA 0.05 m. [c]The viscosity (in square brackets) is given in cP. [d]ESI-MS
spectra were recorded on the IL solution after the irradiation (see text). [e]See text. [f]70% in a 0.02 m KI solution at 30% aniline conversion. [g]Ex-
periment not carried out. [h]Yields corrected with respect to that reported in reference [14].
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1835

M. Fagnoni et al.
the latter case more aniline 3
was formed. It is worth noting
that the yield of biphenyl 9 in
ILs was only slightly decreased
when the amount of benzene
was reduced from 1m to 0.5m
(contrary to MeCN, for which
the yield halved from 44 to
21%), although reduction to 3
(34 to 56%) was by far the
main product under these con-
ditions.
With heteroaromatic thio-
phene, arylation occurred se-
lectively in 2[14] to form 10 with
a yield approximately constant
over the series of ILs consid-
ered, at least in the presence
of TEA. In 2a self-attack was
an important competitive path.
We also explored the inter-
action of the probe with ILs
themselves. As for the anion,
formation of N-(4-dimethyl-
aminophenyl)bis(trifluorome-
Scheme 1.
thanesulfonimide) (11) was
suspected in the experiments
in 2a, because there was a lit-
erature precedent (see above).
4-iodoaniline (5) in low-to-medium yield (5–47%) along
with compounds 3 and 4. The highest yield of 5 (47%) was
obtained in the low viscosity IL 2a (for which the yield of 4
remained high, however), although this was considerably
lower than that in MeCN, both with KI 0.2m (77%) and
0.02m (70% at 30% conversion, see Table 1).
Alkylaniline 6 was formed in the presence of allyltrime-
thysilane and involved elimination of the trimethylsilyl
cation. Since acidity was liberated in the reaction, the effect
of a buffering agent (triethylamine, TEA) was tested. TEA
improved the reaction mass balance (not satisfactory in the
absence of bases) for reaction in N-butyl ILs (2a, 2c), not in
N-hexyl ILs. Even in the best cases, the yield of 6 remained
far below (30%) that obtained in MeCN (81%).
Product 11 was synthesized (see Experimental Section) and
found not to be present in the irradiation mixture. As a
matter of fact, blank experiments showed that 11 was both
thermally and photochemically reactive in ILs, being in part
transformed into unidentified products. However, no such
products were detected in the irradiated solutions in 2a,
thus excluding the formation of 11.
The possible modification of the IL cations was explored
by carrying out an ESI-MS analysis on irradiated solutions
(after ethereal extraction) in representative cases in which
the mass balance was low (see Experimental Section for de-
tails). As shown in Figure 1 (top), besides the [bmim]+ peak
at 139 m/z, after irradiation in neat 2a an intense peak at
258 m/z corresponding to the cation incorporating the ami-
nophenyl moiety [+(Me2N-C6H4)-H]was present. This sug-
gested that an insertion reaction of the 4-aminophenyl
cation onto the [bmim]+ alkyl chain had taken place, as sup-
ported by the presence of the aggregates in the spectrum.
Thus, besides the [(bmim-ClO4)n-bmim]aggregates with
mass 378, 616 , 855, and 1093 m/z (n=1–4), further peaks
assigned to arylated derivatives were observed at 736 and
975 [(bmim-ClO4)n(bmim+-aminophenyl)], n=2–3. The
same 258 m/z peak was identified in the irradiation with
4-Penten-1-ol was then tested as a trap. Both in fluid sol-
vents and in ILs attack onto the double bond was followed
by intramolecular nucleophilic addition of the OH group
leading either to
a benzyl tetrahydrofuran (7, path a,
Scheme 1) or to a phenyl tetrahydropyran (8, path b).[13] The
former compound largely predominated (ca. 7:1 ratio). With
the N-butyl, but not the N-hexyl, ILs the presence of TEA
improved the otherwise poor mass balance. TEA also affect-
ed the 7/8 ratio in the case of 2d.
When turning to aromatics, we were delighted to find that
the arylation of benzene gave a better yield (70%) in 2a
than in MeCN, with a strong reduction in the yield of by-
products 3 and 4. Generally, the arylation of benzene gave
better yields with N-butyl than with N-hexyl ILs, since in
pentenol in [bmim]
hmim+-aminophenyl cation (286 m/z) along with the hetero-
aggregates (n=1–5) in the pentenol [hmim][MsO]photo-
lyzed solution (Figure 1, bottom). Furthermore, in Figure 1
(top and bottom) a small peak at 202 m/z was detected. This
A
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Ionic Liquids
FULL PAPER
Figure 1. ESI-MS spectrum of irradiated solutions of 1 (after ethereal extraction) in neat 2a (top), in 2a containing 0.5m pentenol (middle), and in 2e
containing 0.5m pentenol (bottom).
was reasonably attributed to the loss of the alkyl chain in
the arylated cation (258 (-C4H9) or 286 m/z (-C6H13)), sup-
porting that insertion involved (at least in part) the N-Me
imidazolium group. ESI-MS mass spectra[15] were also mea-
sured on these solutions and the fragmentation of the
[bmim]+- (139 and arylated 258 m/z) and [hmim]+-contain-
ing (167 and arylated 286 m/z) derivatives are shown in
Figure 2. The loss of an alkyl chain (a butyl and a hexyl
group, respectively) was apparent from these spectra.
Figure 2. Selected portion of ESI-MS mass spectrum of [bmim]contain-
ing cations (left; 139 and 258 m/z from Figure 1, top) and [hmim]con-
taining cations (right; 167 and 286 m/z from Figure 1, bottom).
Discussion
For the reasons mentioned in the introduction, ionic liquids
are regarded as “green solvents” and further it is hoped that
they can be designed in such a way as to direct chemical re-
actions (“designer media”). This makes all important that
suitable parameters are designed for characterizing ILs in
view of their application in organic synthesis. Polarity has
been extensively investigated by using various solvatochro-
mic dyes,[16a,b] or by exploiting the keto–enol tautomerism of
2-nitrocyclohexanone[16c] and the association of ILs with
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1837

M. Fagnoni et al.
water.[16d] Fluorescence and phosphorescence[17] from suita-
ble lumophores have also been studied in ILs and found to
be comparable to that of nonviscous solvents with a low
e,[17a] contrary to what may naively be expected. Other stud-
ies concerning the supramolecular structure of the
ILs[11b,18,19a,b] and the multiple solvation interaction[19c] have
recently been reported. Apart from the physical properties,
a key point is evidently to assess the compatibility of ILs
with charged intermediates involved in many organic reac-
tions. This has been addressed by using a chemical probe
and it has been recently found that some ILs are inert in sol-
volysis reactions of various triflates and mesylates via carbo-
cations[20a] as well as to Grignard reagents via carbanions.[20b]
For photochemical reactions, ILs have been rarely em-
ployed[21] and should be sensitive to the limited molecular
diffusion due to the high viscosity. This and the low oxygen
solubility lead to a longer lifetime of triplet excited state
with respect to common organic solvents[22] and should
make them more appealing for photochemical synthesis.
Most of the reported reactions involve photoinduced elec-
tron-transfer processes and ILs appear to be capable of sta-
bilizing the radical ions formed.[22] A couple of reports con-
cerning the photolysis of chloroaromatics in ILs have been
reported, for example, the reductive dechlorination of some
chlorophenols at 254 nm,[23] though at this wavelength ab-
sorption by the IL competed to some extent.
Scheme 2.
competing with any other process, see below). Hydrogen
abstraction by the phenyl cation is known to be efficient
in good hydrogen donors, such as alcohols.[7] ILs are
known to form an extended hydrogen-bond system in
the liquid state and their structure “adapts” to many spe-
cies due to the presence of hydrophobic regions.[11b] Fur-
thermore, ILs with alkyl side chains longer than, or
equal to, four carbon atoms showed an aggregation of
these chains in nonpolar domains.[19b] Apparently a
longer chain favored a supramolecular organization less
convenient for the migration of the intermediate, result-
ing in easier hydrogen transfer (Scheme 2). In parallel,
In the present work, a phenyl cation was used as a chemi-
cal probe and generated by photolysis of 4-chloro-N,N-dime-
thylaniline (1). The main results are discussed in the follow-
ing.
À
insertion into a C H bond to form aminophenylalkyl
imidazolium salts took place both with [bmim]+ and
with [hmim]+ salts. As reported above, this occurred at
least in part at the N-methyl position (see Scheme 2).
This suggested that the a-position was involved in both
hydrogen abstraction and insertion.[24] Returning to the
reaction with trap, another important point is that with
charged nucleophile such as iodide. The trapping was ef-
ficient even at the low concentration attainable in this
media and the difference between [bmim]+ and [hmim]+
was overwhelmed, resulting in a yield of 5 that showed a
simple dependence of the inverse of viscosity.
1) Photolysis of 1 was effective in ILs. Previous work had
shown that the efficiency of this process depended on
the polarity of the solvent. Thus, in this respect imidazo-
lium ILs behave as a medium polar or polar organic sol-
vent.
2) The chemistry occurring in ILs is the same as in common
organic solvents (see Scheme 1), supporting the fact that
the aminophenyl cation is again the intermediate and the
differences observed can be taken as indicative of how
ILs affects the chemistry of the cation. These differences
highlight, we believe, some key characteristics of ILs.
3) The bulk viscosity was determining. Thus, the preferred
reaction with p nucleophiles made the starting chloroani-
line an evident target in the absence of a purposely
added trap. This gave biphenyl derivative 4, the yield of
which showed an inverse dependence on the viscosity, a
parameter depending primarily on the IL anion. This fits
with the known decrease of kdiff (in ILs up to >20).
4) The course of the reaction was finely tuned by the IL
structure, which depended on the cation. Interaction
with the ILs molecules was revealed by two reactions,
À
5) The ILs effect depended on the mechanism of the reac-
tion. The p nucleophiles that we tested were in every
case effective traps when used at a high concentration
(0.5–1m); diminishing the concentration made trapping
less efficient, both in ILs and in MeCN, see the case of
benzene in Table 1. Previous studies in organic solvents
supported that addition to alkenes gave a phenonium ion
and that to aromatics a Wheland cation.[7] A successful
trapping depended both on the initial addition and the
ensuing step of the reaction, namely (TEA-assisted)
elimination. Desilylation from the initial adduct onto
ATMS was apparently most demanding. The yield of
allyl aniline 6 was consistently lower than in MeCN, and
decreased with increasing viscosity in accord, with the
idea that limits to molecular motion imposed by viscosity
were significant for a case in which a more complex se-
hydrogen transfer and insertion into
a C H bond
(Scheme 2). It is apparent that hydrogen abstraction was
much more effective with the N-hexyl-N-methylimidazo-
lium salts than for the N-butyl-N-methyl analogues,
indeed completely suppressing self-attack (and strongly
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Ionic Liquids
FULL PAPER
quence was involved. In the case of pentenol the last
molecular organization of this medium can have a key effect
on the products distribution, while maintaining, with a sensi-
ble arrangement, a good material balance. The effect is in
part due to the viscosity, and in part to the specific interac-
tion with the ILs, which depend on the specific structure.
This becomes apparent when complex conformation changes
are involved in the reaction, for example, in the reactions
step was an intramolecular nucleophilic attack, with clo-
sure to a five- (predominantly) or six-membered ring.[13]
Accordingly, the results in ILs were similar to those in
MeCN, particularly in the presence of TEA, with a simi-
lar 7/8 ratio. An exception was 2d, which, in the absence
of TEA, clearly forced a conformation favorable to six-
mode cyclization, evidencing that ILs do affect the con-
formation of the dissolved species. Trapping by aromatics
was more effective, a fact that may be related to a differ-
ent location in the ILs. TEA was again required (at least
in [bmim]+ salts), but deprotonation of the Wheland
cation was apparently an easier process, leaving only a
small dependence on viscosity in [bmim]+ ILs with ben-
zene and none with the better nucleophile thiophene.
Indeed, arylation of aromatics in ILs was appealing from
the synthetic point of view reaching in some cases a yield
close to, or even better than, that in MeCN.
À
with alkenes. Furthermore, insertion into a C H bond of the
imidazolium N-alkyl chain was observed for the first time
and the non-negligible hydrogen-donor ability of ILs cation
was confirmed. Thus, the use of a triplet phenyl cation as a
probe also evidences the limits of the use of ILs. The above
unprecedented insertion into ILs cation along with the re-
cently reported trapping of an intermediate by an IL anion
show that ILs are not necessarily innocent bystanders in or-
ganic reactions. These results are expected to help predic-
tion of the course of a reaction of known mechanism in a
given IL, and thus help to devise tailor-made ILs.
6) That IL anions act as nucleophiles depends likewise on
the medium. These anions have a key role on the reac-
tion, since the viscosity is mainly determined by them.
An excellent nucleophile such as thiophene obliterates
any difference, but with alkenes the lower the viscosity
was, the higher was the amount of the adduct, and the
low viscosity bis(trifluoromethanesulfonimide) derivative
2a was the only IL in the series comparable to MeCN in
these bimolecular reactions. However, the feared addi-
tion of this anion on the phenyl cation intermediate did
not take place, contrary to what observed when the
phenyl cation was generated thermally from diazonium
salts[6b,26] (in this case the unselective singlet cation is
produced, not the triplet).[27] Another “innocent” anion
Experimental Section
General: NMR spectra were recorded on a 300 MHz spectrometer. The
attributions were made on the basis of 1H and 13C NMR, chemical shifts
are reported in ppm downfield from TMS. The viscosity measurements
were performed at 258C by a rotational viscosimeter Rheotec RC20
(RheoTec, Germany), in constant shear stress mode, by using a measur-
ing cone CC25 DIN. The data were best fitted with the Rheo 2000 v2.7
software.
Positive ESI mass spectra were collected on a LCQ Advantage Finningan
mass spectrometer (I spray voltage: 3.44 kV; capillary voltage: 10.83 V)
at different capillary temperature T=280–1758C. The sample was dis-
solved in 80/20/0.2% vol of MeOH/H2O/CH3COOH at concentration of
1 mgmLÀ1
, and was introduced continuously at a flow rate of
À [6b]
was PF6 , but the high viscosity of 2b hampered an ef-
20 mLminÀ1. The water content of each IL was measured using a coulo-
metric Karl Fischer titrator (Metrohm 684 KF Coulometer). Samples
were prepared by dissolving 0.250–0.350 g of IL in anhydrous acetonitrile
in a 2 mL calibrated flask and duplicate determinations were performed
on each sample with results agreeing to within 5%.
ficient arylation reaction. As for the mesylate anion, its
coupling with phenyl cation may go undetected, since 4-
aminophenylmesylate undergo itself photoheterolysis[28]
leading back to the cation. At any rate, no mesylate was
The photochemical reactions were performed under stirring in quartz
tubes by using nitrogen-purged solution and a multilamp reactor fitted
with four 15-W phosphor-coated lamps (maximum of emission 310 nm)
for the irradiation. The yields of the end products were determined by
GC and HPLC techniques by comparison with authentic samples. The
detected. A virtually unexplored, but promising, anion in
À [29]
ILs chemistry is non-nucleophilic ClO4 .
It is worth
noting that in the case of [hmim][ClO4]the mass balance
G
after irradiation of the reaction is in most cases satisfac-
tory despite its high viscosity (280 cP). The reduction
path is highly favored in this case and 2d may find appli-
cation for the photoreduction of haloaromatics.
amount of
4 formed was determined by HPLC (AQUASIL C18
(250x4.6 mm) column, MeCN/water 60:40, flux 1 mL/min) with UV de-
tection at l=270 nm.
ILs 2a and 2b were commercially available, while 2c[30] and 2d[31] were
obtained by known procedures. In all cases, ILs were dried under
vacuum (60–708C at <0.001 mmHg) for 6 h before use.
Synthesis of 1-hexyl-3-methylimidazolium methanesulfonate [hmim]+
À
Conclusion
MeSO3
A
methyl imidazole (8.46 g, 0.103 mol) were placed in a 100 mL round-bot-
tomed flask, fitted with a reflux condenser and a CaCl2 drying tube. The
mixture was then heated at 908C under stirring for 20 h until the meth-
As it had been hoped, the use of a highly reactive, yet che-
moselective probe such as the 4-N,N-dimethylaminophenyl
cation was informative about the characteristics of ILs as re-
action media. These are indeed convenient media for carry-
ing out syntheses involving highly reactive intermediates,
provided that the concentration of the traps and of other re-
agents required are high enough to avoid undesired side re-
actions. Importantly, the choice of the IL and thus the supra-
anesulfonate disappeared (the progress of the reaction was monitored by
GC), dissolved in CH2Cl2 (80 mL), washed with a solution of saturated
Na2SO4 containing 5% methanesulfonic acid (2”5mL) and then with sa-
turated Na2SO4 (3”5mL) until neutrality. The organic phase was dried
over anhydrous Na2SO4, concentrated by using a rotary evaporator and
dried under vacuum (60–708C at <0.001 mmHg) for 6 h, obtaining
23.62 g (90% yield) of a very hygroscopic oil. 125 =1.112 gmLÀ1; 160
1.098 gmLÀ1; [a]D20 =1.4816; H NMR (CDCl3; 300 MHz): d=9.81 (s, 1H;
Chem. Eur. J. 2007, 13, 1834 – 1841
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1839

M. Fagnoni et al.
NCHN), 7.41 (A part of an AB system, d, J=1.53 Hz, 1H), 7.33 (B part
of an AB system, d, J=1.53 Hz, 1H), 4.19 (t, J=6.53 Hz, 2H; NCH2-),
3.99 (s, 3H; NCH3), 2.72 (s, 3H; OSO2CH3), 1.83–1.79 (m, 2H; CH2),
[4]See for example: J. Dupont, J. Spencer, Angew. Chem. 2004, 116,
5408–5409; Angew. Chem. Int. Ed. 2004, 43, 5296–5297.
[5]M. J. Muldoon, A. J. McLean, C. M. Gordon, I. R. Dunkin, Chem.
Commun. 2001, 2364–2365.
[6]a) K. K. Laali, V. J. Gettwert, J. Fluorine Chem. 2001, 107, 31–34;
b) R. Bini, C. Chiappe, E. Marmugi, D. Pieraccini, Chem. Commun.
2006, 897–899.
1.30–1.25 (m, 6H; (CH2)3), 0.81 ppm (t, 3H; CH3); MS
A
(100) [hmim]+, 429.6 (50) [(hmim)2CH3SO3]+.
Synthesis of N-(4-dimethylaminophenyl)bis(trifluoromethanesulfoni-
mide) (11): Compound 11 was obtained in 20% yield (light yellow solid)
after purification by column chromatography starting from 4-(dimethyl-
[7]a) M. Fagnoni, A. Albini, Acc. Chem. Res. 2005, 38, 713–721; b) M.
Fagnoni, Lett. Org. Chem. 2006, 3, 253–259.
amino)aniline by known procedure.[33] This sulfonimide is quite unstable
upon dissolution in organic solvents. M.p. 91–948C; 1H NMR (CDCl3):
d=3.00 (s, 6H), 6.60 (d, J=8.6 Hz, 2H), 7.10 ppm (d, J=8.6 Hz, 2H;
ArH); 13C NMR (CDCl3): d=40.3 (CH3), 112.5 (CH), 120.0 (q, J=
320 Hz), 121.2, 127.5 (CH), 150.2 ppm; IR (neat): n˜ =966, 1139, 1197,
1460, 1516, 2923 cmÀ1; elemental analysis calcd (%) for C18H22F6N4O4S2:
C 40.30, H 4.13; found: C 40.6, H 4.0.
[8]V. Dichiarante, M . Fagnoni, A. Albini, Chem. Commun. 2006,
3001–3003.
[9]E. Fasani, F. F. Barberis Negra, M. Mella, S. Monti, A. Albini, J.
Org. Chem. 1999, 64, 5388–5395.
[10]M. Freccero, M. Fagnoni, A. Albini, J. Am. Chem. Soc. 2003, 125,
13182–13190.
[11]a) K. R. Seddon, A. Stark, M. J. Torres, Pure Appl. Chem. 2000, 72,
2275–2287; b) M. Antonietti, D. Kuang, B. Smarsly, Y. Zhou,
Angew. Chem. 2004, 116, 5096–5100; Angew. Chem. Int. Ed. 2004,
43, 4988–4992.
[12]During this period the salt dissolved completely.
[13]Notice the formation of tetrahydropyrane 8 requires Wagner–Meer-
wein migration, a process previously observed in the phenyl cation
addition; see reference [7b].
[14]B. Guizzardi, M. Mella, M. Fagnoni, A. Albini, Tetrahedron 2000,
56, 9383–9389.
[15]For related ESI-MS mass experiments on imidazolium ILs, see: F. C.
Gozzo, L. S. Santos, R. Augusti, C. S. Consorti, J. Dupont, M. N.
Eberlin, Chem. Eur. J. 2004, 10, 6187–6193.
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Trans. 2 2001, 433–435; b) C. Reichardt, Green Chem. 2005, 7, 339–
351; c) G. Angelini, C. Chiappe, P. De Maria, A. Fontana, F. Gaspar-
rini, D. Pieraccini, M. Pierini, G. Siani, J. Org. Chem. 2005, 70,
8193–8196; d) T. Kçddermann, C. Wertz, A. Heintz, R. Ludwig,
Angew. Chem. 2006, 118, 3780–3785; Angew. Chem. Int. Ed. 2006,
45, 3697–3702.
[17]a) P. Bonhôte, A.-P. Dias, N. Papageorgiou, K. Kalyanasundaram,
M. Grätzel, Inorg. Chem. 1996, 35, 1168–1178; b) K. A. Fletcher, S.
Pandey, I. K. Storey, A. E. Hendricks, S. Pandey, Anal. Chim. Acta
2002, 453, 89–96.
Irradiation of 1 in ionic liquids 2a–e—general procedure: 4-Chloro-N,N-
dimethylaniline (1, 8 mg, 0.05 mmol, 0.05m) was placed in a quartz tube;
the ionic liquid (2a–e, 1 mL) and the nucleophile (from 0.2 to 1m) were
then added. The resulting mixture was sonicated for a few minutes to
allow the dissolution of the reagents and then flushed with nitrogen and
irradiated. The resulting mixture (16 h; consumption of 1 >90%) was
then extracted with diethyl ether (4”1 mL) and analyzed by GC and
HPLC technique. When 2a was used as the solvent, a cyclohexane/ethyl
acetate (4:1) mixture was used.
Compounds 7 and 8: Compounds 7 and 8 were obtained from the irradia-
tion of 1 in MeCN/water 1:5[34] and used as standard.
2-(4-N,N-dimethylbenzyl)tetrahydrofuran (7): Data for 7: oil; 1H NMR
(CDCl3): d=1.55–1.65 (m, 1H), 1.85–2.00 (m, 3H), 2.65–2.80 (dd, J=7,
13 Hz, 1H), 2.85–2.95 (dd, J=7, 13 Hz, 1H), 2.95 (s, 6H), 3.70–3.80 (m,
1H), 3.90–3.95 (m, 1H), 4.00–4.05 (quintet, J=7 Hz, 1H), 6.70–7.15 ppm
(AA’BB’, 4H); 13C NMR (CDCl3): d=25.5 (CH2), 30.7 (CH2), 38.7
(CH2), 40.8 (CH3), 67.8 (CH2), 80.3 (CH), 113.3 (CH), 113.5, 129.8 (CH),
149.1 ppm; IR (neat): n˜ =2956, 1616, 1524, 1061, 975, 803 cmÀ1; elemental
analysis calcd (%) for C13H19NO: C 76.06, H 9.33; found: C 75.1, H 9.1.
3-(4-N,N-dimethylphenyl)tetrahydropyran (8): Data for 8: oil; 1H NMR
(CDCl3): d=1.55–1.70 (m, 3H), 1.85–1.95 (m, 3H), 3.60–3.70 (m, 1H),
4.05–4.20 (m, 2H), 6.70–7.15 ppm (AA’BB’, 4H); 13C NMR (CDCl3): d=
24.0 (CH2), 25.9 (CH2), 33.5 (CH2), 40.8 (CH3), 68.9 (CH2), 79.9 (CH),
112.5 (CH), 114.5, 126.8 (CH), 149.1 ppm; IR (neat): n˜ =2957, 1621,
1522, 1059, 978, 799 cmÀ1; elemental analysis calcd (%) for C13H19NO: C
76.06, H 9.33; found: C 76.3, H 9.4.
[18]A. Mele, G. Romanò, M. Giannone, E. Ragg, G. Fronza, G. Raos, V.
Marcon, Angew. Chem. 2006, 118, 1141–1144; Angew. Chem. Int.
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Acknowledgements
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[21]“Photochemistry in Ionic Liquids”R. M. Pagni, C. M. Gordon, in
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CRC , Boca Raton, 2004.
Partial support of this work by Murst, Rome is gratefully acknowledged.
The authors greatly appreciate the contribution by Prof. P. Mustarelli for
the viscosity measurements and Dr. Anna Daghetti for the ESI-MS spec-
tra measurements.
[22]M. Alvaro, B. Ferrer, H. Garcia, M. Narayana, Chem. Phys. Lett.
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[1]Note, however, that dialkylimidazolium cations have been revealed
in the vapor phase upon heating ILs. See: H. Chen, Z. Ouyang, R.
Graham Cooks, Angew. Chem. 2006, 118, 3738–3742; Angew. Chem.
Int. Ed. 2006, 45, 3656–3660.
[2]a) Green Industrial Applications of Ionic Liquids (Eds.: R. D.
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[24] N-Dealkylation of a [bmim]+ ion has been previously reported to
occur upon addition of the bmim cation to a PdII complex to form
bis(methylimidazole) dichloropalladate. See: J. E. L. Dullius,
P. A. Z. Suarez, S. Einloft, R. F. de Souza, J. Dupont, Organometal-
lics 1998, 17, 815–819.
[25]It has been proposed that benzene and arenes were staggered
through p–p sandwich interactions between two bmim cations; see
reference [19a].
[26]D. J. Moody, N. A. Hamill (Avecia Pharmaceuticals), WO
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[27]This highlights again the carbene character at C of the triplet
1
phenyl cation, as opposed to the localized cation at C1 of the singlet,
1840
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Chem. Eur. J. 2007, 13, 1834 – 1841

Ionic Liquids
FULL PAPER
as formed, for example, from the photolysis of phenyl diazonium
salts.
[32]H. R. Williams, H. S. Mosher, J. Am. Chem. Soc. 1954, 76, 2984–
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[34]The yield of tetrahydrofurans or -pyrans of structures 7 and 8 from
the irradiation of 1 or other chloroaromatics dramatically depends
on the solvent used. This topic is currently under investigation.
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Received: July 26, 2006
Published online: November 20, 2006
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