N-Heterocyclic Olefin–Carbon Dioxide and –Sulfur Dioxide Adducts: Structures and Interesting Reactivity Patterns

Article abstract of DOI:10.1002/chem.201602973

Depending on the amount of methanol present in solution, CO₂adducts of N-heterocyclic carbenes (NHCs) and N-heterocyclic olefins (NHOs) have been found to be in fully reversible equilibrium with the corresponding methyl carbonate salts [EMIm][O

Full text of DOI:10.1002/chem.201602973

DOI: 10.1002/chem.201602973
Full Paper
&
Ionic Liquids
N-Heterocyclic Olefin–Carbon Dioxide and –Sulfur Dioxide
Adducts: Structures and Interesting Reactivity Patterns
Lars H. Finger, Jannick Guschlbauer, Klaus Harms, and Jçrg Sundermeyer*[a]
Abstract: Depending on the amount of methanol present in
solution, CO2 adducts of N-heterocyclic carbenes (NHCs) and
N-heterocyclic olefins (NHOs) have been found to be in fully
reversible equilibrium with the corresponding methyl car-
bonate salts [EMIm][OCO2Me] and [EMMIm][OCO2Me]. The
reactivity pattern of representative 1-ethyl-3-methyl-NHO–
CO2 adduct 4 has been investigated and compared with the
corresponding NHC–CO2 zwitterion: The protonation of 4
with HX led to the imidazolium salts [NHO–CO2H][X], which
underwent decarboxylation to [EMMIm][X] in the presence
of nucleophilic catalysts. NHO–CO2 zwitterion 4 can act as
an efficient carboxylating agent towards CH acids such as
acetonitrile. The [EMMIm] cyanoacetate and [EMMIm]2 cya-
nomalonate salts formed exemplify the first CÀC bond-form-
ing carboxylation reactions with NHO-activated CO2. The re-
action of the free NHO with dimethyl carbonate selectively
led to methoxycarbonylated NHO, which is a perfect precur-
sor for the synthesis of functionalized ILs [NHO–CO2Me][X].
The first NHO-SO2 adduct was synthesized and structurally
characterized; it showed a similar reactivity pattern, which
allowed the synthesis of imidazolium methyl sulfites upon
reaction with methanol.
um-carboxylate zwitterions.[1f,j] The latter are formed by the de-
protonation of the imidazolium cation and attack of the in situ
generated NHC on CO2, and have proven to be highly versatile
reagents themselves. Specifically, the imidazolium-2-carboxyl-
ates can be employed as proto-carbenes in the formation of
transition-metal complexes,[2] act as CO2 transfer reagents and
organocatalysts,[3] and can still be used as masked carbenes for
the synthesis of ILs by reaction with sufficiently acidic re-
agents.[1f,4] The specific conditions that allow the removal of
the CO2 moiety to generate a reactive carbene or the original
imidazolium cation have been thoroughly investigated.[2b,3a,b,5]
Owing to their fascinating properties, access to these NHC
inner salts has been studied extensively. Considering the some-
times ambiguous results, it has to be noted that the exact de-
pendence of the product on the reaction conditions is still not
fully understood. According to literature reports, imidazolium-
2-carboxylates can be prepared selectively by feeding gaseous
CO2 into a solution of the corresponding carbene,[6] directly
from 1-methylimidazole and dimethyl carbonate,[1f,4c] by con-
tinuous flow at 2008C over an Al2O3 catalyst,[7] and even at am-
bient temperature by introducing CO2 into imidazolium ace-
tate ILs.[8] Specifically, the first and last approaches allow exten-
sion to imidazolium-2-thio- and dithiocarboxylates.[9] The usual-
ly undesired 4- and 5-carboxylates, which are associated with
the so-called abnormal carbenes,[10] are typically formed at ele-
vated temperatures,[11] but have also been witnessed at tem-
peratures as low as 1208C.[12] Recent investigations have
shown that their formation is also dependent upon the partial
pressure of CO2 and the basicity of the reaction mixture.[13]
The introduction of a methyl group at the 2-position of the
imidazolium ring, which is also accompanied by a distinct ele-
vation of the melting point, is typically regarded as a protection
Introduction
Employing dimethyl carbonate as a mild and non-poisonous
methylation agent for nucleophilic cation precursors is by far
the most widely applicable way to synthesize ionic liquids (ILs)
in a sustainable metal- and halide-free fashion.[1] This route to-
wards methyl-onium methyl carbonates tolerates
a vast
number of starting nucleophiles, the only requirements being
the thermal stability of the reagent towards elevated tempera-
tures of around 1308C and the stability of the resulting IL cat-
ions towards the basic and nucleophilic, potentially weakly sol-
vated methyl carbonate anion and unselective cation carboxyl-
ations. In this respect, pentamethylguanidine is one of the few
nucleophiles not suitable: A hexamethylguanidinium cation is
attacked, whereas the sterically more protected methyl-pen-
taalkylguanidinium cations are inert towards nucleophilic
attack of methyl carbonate at 1308C.[1g] 1-Alkylimidazoles as
nucleophiles are at the borderline of usability, as the ring pro-
tons of the resulting N-heterocyclic cations are sufficiently
acidic to be engaged in follow-up reactions. This requires care-
ful elucidation of the procedure to find reaction conditions
that yield only the desired 1-alkyl-3-methylimidazolium methyl
carbonate salts.[1a] If their methyl carbonate anions are not sta-
bilized by solvating hydrogen bridges, for example, in metha-
nol solution, these salts tend to decompose to form imidazoli-
[a] Dr. L. H. Finger, J. Guschlbauer, Dr. K. Harms, Prof. Dr. J. Sundermeyer
Fachbereich Chemie and Materials Science Centre
Philipps-Universitꢀt Marburg, Hans-Meerwein-Strasse 4
35043 Marburg (Germany)
E-mail: JSU@staff.uni-marburg.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201602973.
Chem. Eur. J. 2016, 22, 1 – 13
1
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
against undesired side-reactions due to the acidic proton.
Recent investigations have shown that a much more sophisti-
cated substitution pattern is necessary to create imidazolium
cations that are stable towards strong nucleophiles and bases
such as the hydroxide ion over a period of time, even if they
are solvated.[14] It has also been proven that the formation of
contact ion pairs in solvents of medium polarity allows hydro-
gen/deuterium exchange to occur at the apical methyl
group.[15] However, starting from methyl carbonate salts, so far
carboxylate formation has not been observed at the apical
methyl group.[1e] However, it has already been noted that the
During our investigations, which were primarily concerned
with the preparation of organic salts with hydrochalcogenide
and trimethylsilylchalcogenolate anions,[1i,j] we noticed a selec-
tive and easy access to imidazolium-2-carboxylates and discov-
ered that 1,3-dialkyl-2-methylimidazolium methyl carbonate
salts form carboxylate zwitterions as well. As a consequence
we investigated whether these zwitterionic imidazolium-2-
methylenecarboxylates, formally NHO–CO2 adducts, to some
extent show comparable reactivity to the well-known imidazo-
lium-2-carboxylates, formally NHC–CO2 adducts, with regard to
the preparation of ionic liquids and in CÀC coupling reactions.
1
2-methyl group shows decreased or diffuse H NMR signals in
the presence of hydrogen or methyl carbonate anions,[16]
which can be attributed to hydrogen/deuterium exchange.
These results imply the intermediate formation of an N,N-
ketene diacetal or N-heterocyclic olefin (NHO),[17] for example,
7 in Scheme 1. The selective synthesis of such imidazolium-2-
methylenecarboxylates was achieved conveniently from the in
situ generated NHOs and CO2. These compounds have proven
to be efficient organocatalysts for the carboxylative cyclization
of CO2 and propargylic alcohols to yield a-alkylidene cyclic car-
bonates.[18] The procedure can be extended to COS and CS2 ad-
ducts;[19] the effect of the imidazolium substitution pattern on
the reactivity was investigated in detailed computational stud-
ies.[20] The NHOs, which are generated in situ and most likely
the active species in these catalytic cycles, have also been em-
ployed as highly basic starting materials for the synthesis of
ionic liquids[21] and as polymerization catalysts for propylene
oxide,[22] methyl methacrylate,[23] and lactones.[24] They are also
versatile organocatalysts for transesterification[25] and base-pro-
moted alkylation reactions.[26] Furthermore, they are interesting
ligands in main-group and transition-metal complexes.[27]
Results and Discussion
Scheme 1 gives an overview of the results described in this
manuscript. Details concerning the particular reactions can be
found in the corresponding sections indicated in the scheme.
1. Formation of imidazolium-2-carboxylate and imidazolium-
2-methylenecarboxylate
It has proven advantageous to prepare 1-alkyl-3-methylimid-
azolium methyl carbonate salts from the corresponding
alkylimidazole and dimethyl carbonate in the presence of
methanol, which not only accelerates the methylation but also
prevents the formation of undesired 4-carboxylates.[1a] As the
resulting solutions typically contained some excess dimethyl
carbonate and small amounts of colored impurities,[1d] we
chose to isolate the respective salts by evaporation of the
methanol solvent and recrystallization of the solid residue from
acetonitrile. During these preparations we noticed that in the
case of 1-ethyl-3-methylimidazolium (EMIm) methyl carbonate
Scheme 1. Overview of the results presented in this manuscript. EMIm=1-ethyl-3-methylimidazolium, EMMIm=1-ethyl-2,3-dimethylimidazolium, TFA=tri-
fluoroacetate, TFSI=bis(trifluoromethylsulfonyl)imide, DMC=dimethyl carbonate.
&
&
Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
2
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
ate zwitterion remained in the NMR sample even after several
hours, which allowed us to conclude that equilibrium condi-
tions are reached and only a larger excess will allow the full re-
generation of the methyl carbonate. With respect to green IL
synthesis protocols, this additional information is important as
zwitterion 2 does not show an equally high activity in proton-
induced decarboxylation reactions as the methyl carbonate
salt 1.[1f,4a] In our investigation of hydrochalcogenide ILs,[1k] we
also noticed that higher concentrated methanol solutions of
carboxylate 2 reacted significantly more slowly than dilute sol-
utions containing only the methyl carbonate salt. A mixture of
nonseparable products may be the undesirable consequence
in these IL syntheses.
Scheme 2. Known synthesis of EMIm methyl carbonate (1) and equilibrium
between 1 and NHC–CO2 adduct 2 at room temperature.
(1), even at ambient temperature only the respective 2-carbox-
ylate (2) was obtained (Scheme 2). In view of the high basicity
of the methyl carbonate anion and the instability of the form-
ing methyl carbonic acid, this has to be expected.
The driving force of this reaction is the removal of methanol
under vacuum, which slowly shifts the methyl carbonate/car-
boxylate equilibrium towards 2. The reaction proved to be per-
fectly reversible in NMR experiments. The process can be com-
pared with the removal of acetic acid in the gas stream of CO2
introduced into imidazolium acetate ILs.[8a]
The 2-methylated 1-ethyl-2,3-dimethylimidazolium (EMMIm)
cation in 3 showed an analogous highly selective reaction pat-
tern, although its equilibrium with zwitterion 4 was not as pro-
nounced as in the case of imidazolium cations bearing
a proton at the 2-position (Scheme 3). During the preparation
The NMR spectra in Figure 1 clearly show that the imidazoli-
um-carboxylate (2, marked by *) is not primarily formed during
the solvothermal synthesis, and that only minimum amounts
prevail in the reaction mixture along with the main product
imidazolium methyl carbonate (1, marked by ^). Spectrum A
also shows large amounts of methanol and dimethyl carbonate
and minor amounts of ethylimidazole that did not react. Only
if the solvent was removed did the carboxylate form to
a larger extent (B) until, after thorough drying and recrystalliza-
tion, the pure carboxylate was obtained (C). If an excess of
methanol (eight equivalents in this instance) was added to the
NMR sample of C, the carboxylate was very quickly trans-
formed back to the methyl carbonate. Despite the large excess
of eight equivalents of methanol, around 10% of the carboxyl-
Scheme 3. Preparation of EMMIm methyl carbonate (3) and partial equilibri-
um between 3 and NHO–CO2 adduct 4 at room temperature.
of 3 we noticed the formation of slightly varying amounts (5–
10%) of a side-product after work-up, which we later on identi-
fied as the corresponding NHO–CO2 adduct 1-ethyl-3-methyl-
imidazolium-2-methylenecarboxylate (4).
Analogously to the preceding synthesis, this side-product
was not present in the crude reaction mixture (Figure 2, A) but
was formed at ambient temperature upon evaporating all vola-
tile components and recrystallizing from a acetonitrile/diethyl
ether solvent mixture, as indicated by the small additional sig-
nals at 3.62 and 3.72 ppm (Figure 2, B). When the recrystallized
compound mixture was heated at 808C under a CO2 atmos-
phere (1 bar) or in a vacuum, a significant increase of the 2-
methylenecarboxylate species compared with the original
methyl carbonate salt was observed. However, this did not
allow convenient synthesis of the pure product. To gain access
to a pure reference sample of the carboxylate 4 (Figure 2, C),
the procedure of Wang et al.[18a] starting from NHO and CO2
was employed. Even with this pure standard 4 dissolved in
DMSO, immediate back-formation of the methyl carbonate salt
was observed upon addition of excess methanol (Figure 2, D).
These observations led to a partial reinterpretation of previ-
ous results, in which the formation of carboxylate impurities
was traced back solely to the elevated reaction tempera-
tures.[1e] The authors avoided the formation of carboxylate im-
purities by decreasing the reaction temperature to 708C while
extending the reaction time to 10 days. This might shift the re-
action path from thermodynamic to kinetic control. On at-
Figure 1. 1H NMR spectra (300.1 MHz, [D6]DMSO) of A) the reaction mixture
of Scheme 2 directly after the solvothermal synthesis, B) the residue of the
reaction mixture after removing most of the solvent, C) the isolated adduct
NHC–CO2 2 after recrystallization, and D) the sample of C with an excess of
methanol (1).
Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
3
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
Scheme 4. Attempted synthesis of 1-ethyl-2,3-dimethylimidazolium tert-bu-
tylthiolate (5) in acetonitrile.
acted with the CO2 released in the previous reaction step to
form zwitterionic carboxylate 4 (Scheme 4).
Figure 2. 1H NMR spectra (300.1 MHz, [D6]DMSO) of A) the reaction mixture
of Scheme 3 directly after the solvothermal synthesis in MeOH (1308C),
B) the residue of the reaction mixture after removing most of the solvent,
C) the isolated adduct EMMIm-CO2 4, synthesized according to reference
[18a] and D) the sample of C with an excess of methanol (3).
As a result, instead of the anticipated thiolate salt, the 2-
methylenecarboxylate 4 was isolated in a yield of 36% as the
pure crystallized compound. Compound 5 was later on synthe-
sized by treating the in situ prepared NHO with tert-butyl mer-
captan. The 2-methylimidazolium cation was stable towards
unsolvated tert-butylthiolate anion, but only in the absence of
CO2 as the NHO trapping agent. In contrast, the reaction of the
imidazolium methyl carbonate 3 with bis(trimethylsilyl) sulfide
led to the imidazolium trimethylsilylthiolate 8 in good yield
and acceptable purity. The only impurity was unreacted zwit-
terion 4 (ca. 5%), which was formed as a side-product in the
synthesis of the EMMIm methyl carbonate starting material.
Subsequently, we established a competition experiment
(Scheme 5). Salt 5 containing the stronger tert-butylthiolate
tempting to reproduce this low-temperature methylation by
DMC in a sealed glass ampoule we observed only a low con-
version to a variety of unidentified products. None of these
was the anticipated methyl carbonate salt. It is known that me-
thoxycarbonylation by DMC is preferred at temperatures of
around 908C. Only if a reaction mixture is heated to above
1208C under solvothermal conditions does DMC act as a meth-
ylation agent.[28] With the findings presented here, we are con-
vinced that the progressive removal of methanol induces two
effects working hand-in-hand: Less well solvated methyl car-
bonate anions act as a stronger base and the elimination of
the reaction product methanol shifts the equilibria of these
thermodynamically controlled (reversible) carboxylations quan-
titatively towards the formation of the adduct NHC–CO2 or
partly towards NHO–CO2 even at room temperature.
Scheme 5. Different reactivities of 5 and 8 towards CO2.
2. Effect of anion basicity on the reactivity of the EMMIm
cation towards CO2
base was carboxylated in acetonitrile at 208C to the adduct
NHO–CO2 4, whereas 8 containing the weaker TMS-thiolate
base was not carboxylated: After passing CO2 over a solution
of 5 for 30 min, a conversion to 4 of 32% could be observed
It was anticipated that increased anion basicity would allow
the formation of higher amounts of the NHO–CO2 adduct 4.
This is exemplified by our attempted synthesis of 1-ethyl-2,3-
dimethylimidazolium tert-butylthiolate (5) from the corre-
sponding methyl carbonate precursor. In contrast to H2S,[1i,k]
the weaker acid tBuSH was not deprotonated by 1-ethyl-2,3-di-
methylimidazolium methyl carbonate in MeOH to any amount
1
by H NMR monitoring.
The structures of all three imidazolium compounds in
Scheme 5 were elucidated by single-crystal X-ray structure de-
termination. Figure 3 depicts the structure of the tert-butylthio-
late salt 5, which is characterized as a hydrogen-bonded dimer.
A discussion of the structure of 5 and a comparison with the
analogous trimethylsilylthiolate salt 8 is presented in the Sup-
porting Information.
1
detectable by H NMR spectroscopy. Apparently, the solvated
anion tBuSÀ(MeOH) is more basic than MeOCO2À(MeOH). To
gain some extra driving force by SiÀO bond formation, we em-
ployed methyl carbonate as a nucleophilic desilylating agent[1j]
of tBuS-TMS (6) under aprotic conditions. As expected, TMS-
OMe was formed in acetonitrile, however, the basic alkylthio-
late anion formed initially apparently deprotonated the 2-
methylimidazolium cation to form the NHO 7, which rapidly re-
3. Decarboxylation behavior of the adduct NHO–CO2 4
Imidazolium-2-carboxylates have already proven valuable start-
ing materials for the synthesis of ionic liquids under protic con-
&
&
Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
4
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
Figure 3. Molecular structure of the hydrogen-bonded dimer of [EMMIm]
[tBuS] (5). Hydrogen-bond (donor–acceptor) distances are shown in ꢁ. The
disordered position of tBuSÀ and the THF solvate molecule of 5 are not
shown for clarity. Ellipsoids are shown at the 50% probability level. Symme-
try operation: I: Àx+1, Ày+2, Àz+2.
Figure 4. 1H NMR spectra (300.1 MHz, [D6]DMSO) of A) NHO–CO2 4, B) the re-
action mixture between NHO–CO2 4 and HTFSI after 4 days at ambient tem-
perature (10) and C) the same reaction mixture after a further 3 h at 808C
(11).
ditions.[4a,c] The strong solvent dependence of the decarboxyl-
ation was investigated by Denning and Falvey.[5b] For us, the
question arose as to whether the 2-methylenecarboxylate 4
can be analogously used for the synthesis of the correspond-
ing organic EMMIm salts upon reaction with Brønsted acids.
The observation that the formation of 4 is reversible upon the
addition of methanol in DMSO strongly pointed to this possi-
bility. Consequently, we treated 4 with bis(trifluoromethylsulfo-
nyl)imide (HTFSI), trifluoroacetic acid (HTFA), and NH4PF6 in
methanol and acetonitrile. From the reaction with NH4PF6 the
expected decarboxylation product [EMMIm][PF6] (9) was isolat-
ed in quantitative yield (Scheme 6).
mediate [NHO–CO2H][TFSI] (10). The small amount of the final
product was attributed to the presence of isomeric 4- and 5-
carboxylates, which were formed as minor side-products in the
preparation of 4 and apparently reacted with the strong acid
by immediate decarboxylation. The detection of the intermedi-
ate cation of 10 by ESI (+) mass spectrometry was not possible
(only the decarboxylated cation was observed), but the IR
spectra confirmed the presence of C=O and OÀH bonds. Anal-
ogous results were obtained from the reaction of 4 with tri-
fluoroacetic acid. Apparently, under specific conditions, the
CO2 elimination is kinetically hindered for the NHO–CO2
adduct. However, the decarboxylation occurred quickly upon
heating the reaction mixture at 60–808C (Figure 4, spec-
trum C). Overall, a temperature-dependent reaction of 4 with
strong acids has to be considered (Scheme 7).
Scheme 6. Synthesis of [EMMIm][PF6] (9) from NHO–CO2 adduct 4 and
NH4PF6.
However, the reaction of 4 (Figure 4, spectrum A) and bis(tri-
fluoromethylsulfonyl)imide (HTFSI) at ambient temperature, re-
gardless of the reaction time and the solvent (methanol or ace-
tonitrile), led to only a marginal amount of the anticipated
product (Figure 4, spectrum B).
Scheme 7. Reactions of NHO–CO2 adduct 4 with trifluoroacetic acid and bis-
(trifluoromethylsulfonyl)imide in methanol or acetonitrile.
Around 90% of the sample was a hitherto unknown cationic
species that shows all the signals of the preceding zwitterion,
This result is surprising in view of our previous observation
that 4 can be converted into EMMIm methyl carbonate (3) by
methanol in DMSO at ambient temperature. Weak acids seem
to promote a decarboxylation path, which is hampered by
a higher activation barrier for strong acids. It is presumed that
this reactivity difference can be correlated with the nucleophi-
licity of the particular conjugate base and that CO2 elimination
does not occur in a spontaneous unimolecular fashion but de-
1
but at different chemical shifts. In the H NMR spectra especial-
ly, the resonance signal of the CH2 group adjacent to the car-
boxylate moiety is shifted strongly downfield. Furthermore, an
additional signal appeared that had to be assigned to a proton
with a certain degree of dynamic motion due to its broadened
appearance and chemical shift at around 15 ppm. All the find-
ings indicate the formation of the stable carboxylic acid inter-
Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
5
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
mands the attack of a nucleophile at the carboxylate carbon
atom, similar to the Krapcho decarboxylation.[29] Accordingly,
better nucleophiles such as DMSO or NH3 present in equilibri-
found in almost identical percentages (ca. 10%), regardless of
the reaction temperature, reaction time, and initial NHO–CO2
concentration in acetonitrile. If 14a was the final product and
not in equilibrium with 14, the amount formed should vary
significantly for different reactant dilutions (the concentration
of 4 in acetonitrile was varied from 0.034 to 1.68 molLÀ1). This
equilibrium was also formed in DMSO solution. When pure
14a was dissolved in [D6]DMSO, the compound immediately
started to dissociate into 14 and the NHO–CO2 adduct 4.
When equimolar amounts of pure NHO–CO2 adduct 4 and iso-
lated cyanoacetate 14 were combined in [D6]DMSO, an analo-
gous product mixture was formed consisting of 4, 14, and 14a
in the approximate percentages 35, 48, and 17%, respectively
(see the Supporting Information for details).
+
um with NH4 will allow a faster reaction, whereas weak nucle-
ophiles such as TFA and TFSI anions under acidic conditions
lead to a significant increase of the activation barrier. This
theory has been substantiated by the observation that the ad-
dition of substoichiometric amounts (30 mol%) of nucleophilic
catalysts such as 4-dimethylaminopyridine leads to the forma-
tion of the anticipated imidazolium salts 11 and 13 at ambient
temperature. A related observation was made by Rogers and
co-workers, who noticed that 1,3-dimethylimidazolium-2-car-
boxylate reacted with almost all acids by decarboxylation to
form the corresponding 1,3-dimethylimidazolium salts, but
that upon reaction with nitric acid a carboxylic acid intermedi-
ate was formed as a very stable intermediate. This could be
transformed to the desired IL by either heating at 1408C or by
dissolving the substance in DMSO, which allowed the transfor-
mation at ambient temperature.[4a]
To conveniently separate the two salts the raw product was
triturated with dichloromethane, which selectively dissolved 14
but not the dianion-based salt 14a. The isolated yields were
64% for 14 and 10% for 14a. The structure of salt 14a was
characterized by single-crystal X-ray diffraction; its molecular
structure is depicted in Figure 5 and discussed in the Support-
ing Information.
These observations led us to conclude that under most cir-
cumstances the possible formation of the carboxylate will not
influence the preparation of ILs with Brønsted acid reagents.
However, care has to be taken if the methyl carbonate salt is
isolated prior to follow-up reactions and if the anion that is to
be introduced shows a low nucleophilicity. With these pre-
requisites it is mandatory to carry out the reaction at elevated
temperatures to avoid contamination of the final product with
carboxylate impurities.
4. NHO–CO2 adduct 4 as carboxylation agent for acetonitrile
During a preliminary exploration of the reaction of the CO2-
masked NHO 4 with CH acidic reaction partners, we noticed an
interesting CÀC coupling of CO2 and MeCN. Heating the
weakly soluble zwitterion 4 in acetonitrile yielded a readily
soluble orange oil that slowly solidifies upon thorough drying
at 5ꢂ10À3 mbar. The substance was identified as 1-ethyl-2,3-di-
methylimidazolium cyanoacetate (14) from its NMR and MS
spectra. Furthermore, a small amount of the cyanomalonate
salt 14a, which results from a second deprotonation and
attack at CO2, was present (Scheme 8). The cyanomalonate salt
14a appears to be formed in equilibrium with the main prod-
uct cyanoacetate 14 and the NHO–CO2 adduct 4, as it was
Figure 5. Molecular structure of [EMMIm]2[NC-C(CO2)2H] (14a). The second
cation molecule, the cation hydrogen atoms, and a disordered position of
the anion have been omitted for clarity. Ellipsoids are shown at the 50%
probability level. Selected bond distances [ꢁ] and angles [8]: C1ÀO1 1.327(3),
C1ÀO2 1.247(3), C1ÀC2 1.427(3), C2ÀC3 1.443(3), C2ÀC4 1.433(3), C4ÀN1
1.154(4), C3ÀO3 1.309(3), C3ÀO4 1.253(3), O1-H1…O3 2.432(2), O1-C1-O2
119.9(2), C1-C2-C3 124.1(2), O3-C3-O4 122.0(2), C2-C4-N1 179.2(2).
These reactions serve as the first examples of an N-heterocy-
clic olefin (NHO) acting as an organic mediator for CÀC bond-
coupling C-carboxylations of CH-acidic positions by NHO-acti-
vated CO2. A related CÀO bond formation has been described
by Wang and co-workers who employed NHO–CO2 adducts as
catalysts for the O-carboxylation of propargylic alcohols and
propylene oxides to obtain cyclic a-alkylidene and propylene
carbonates, respectively.[18a,19] The carboxylation of acetonitrile
was also observed with a 1,3-di-tert-butylimidazolium-2-carbox-
ylate.[30] In contrast to the NHO–CO2 adduct 4, the NHC–CO2
adduct 2 did not lead to such a selective carboxylation of ace-
tonitrile, even under solvothermal conditions at 1208C. These
observations are in accord with the interpretation that the tert-
Scheme 8. Formation of the imidazolium cyanoacetate 14 and cyanomalo-
nate 14a from 4 and acetonitrile.
&
&
Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
butyl-substituted carbene (van Ausdall)[30] and NHO (this work)
are sufficiently basic to assist the deprotonation of the CH acid
whereas NHC 16 is not basic enough. The same appears to be
true for the corresponding EMMIm methyl carbonate salt 3,
which did not lead to the carboxylation of acetonitrile at 808C.
The preliminary results presented here require a more detailed
investigation of NHO mediated—in the presence of strong in-
organic bases maybe even NHO catalyzed—carboxylations of
CH acidic compounds.
ed NHO 7, generated in situ from [EMMIm]Br and KH, very se-
lectively reacted with dimethyl carbonate between À208C and
ambient temperature to form the methoxycarbonylated NHO
15 (Scheme 9).
Surprisingly, NHC 16, generated in situ from [EMIm]Br and
KH, was also transformed into CÀC coupling product 15 upon
reaction with dimethyl carbonate under analogous conditions.
Compound 15 was found as a mixture with the NHC–CO2
adduct 2 in a ratio of 4:6 (Scheme 10).
5. Further CÀC coupling reactions of the NHO–CO2 adduct 4
We also treated the NHO–CO2 adduct 4 with dimethyl carbon-
ate hoping for either a selective O-methylation or, after decar-
boxylation, a C-methoxycarbonylation or C-methylation of the
NHO. The latter reaction would be related to known NHC
chemistry, precisely a methylation/deprotonation/methylation
at the 2-position of an NHC–CO2 adduct, which was observed
by Annese et al. and in sum led to a 2-ethylation by dimethyl
carbonate at high temperatures.[31] However, at temperatures
of 190 or 1508C, the reaction of 4 with dimethyl carbonate led
to a rather unselective decomposition of the reactants. Neither
by NMR spectroscopy nor by ESI mass spectrometry did we
find evidence for a C-methylation leading to a 2-ethylimidazoli-
um derivative. Upon further decreasing the reaction tempera-
ture a partly selective reaction could be achieved. The reaction
of 4 with dimethyl carbonate at 908C resulted in a mixture of
only three compounds. One of the major components of the
mixture was identified as the methoxycarbonylated NHO 15
(Scheme 9).
Scheme 10. Reactivity of NHC 16 with dimethyl carbonate (DMC) and a pro-
posed mechanism for the formation of 15 from NHC 16 and dimethyl car-
bonate.
To explain the formation of these two species, it was as-
sumed that the carbene attacks dimethyl carbonate preferen-
tially at the carbonyl carbon atom leading to the cationic 2-
methoxycarbonyl species 17 and a solvated methoxide anion.
Cationic species such as 17, which is an O-methylated NHC–
CO2 adduct, have been synthesized previously by treating
a lithiated alkylimidazole with methyl chloroformate and sub-
sequent methylation of the imidazole ring with methyl trif-
late.[32] Type 17 cations have also been obtained by methano-
lysis of a 2-chlorocarbonyl-substituted imidazolium cation.[6a]
Such a cationic methyl ester 17 would certainly be a better
methylating agent than neutral DMC. An SN2-type dimethyl-
ation would lead to 2 as the major product, which precipitates
from the reaction mixture (Scheme 10). The more nucleophilic
NHC 16 was methylated at the 2-position first by a CÀC cou-
pling reaction and the more basic methoxide anion formed
was then available to act as a sufficiently strong base to de-
protonate the generated 2-methylimidazolium cation 18 to
yield NHO 7. The methoxycarbonylation of 7 to 15 by DMC
has independently been proven with isolated 7 to be a highly
selective second CÀC coupling reaction (Scheme 9). If an alter-
native mechanism involving methylation of NHC 16 by DMC
occurred, a product ratio different to 4:6 (of 15:2) would have
been expected. Nevertheless, it should be pointed out that in
principle NHCs can also be methylated, for example, by MeI
(instead of DMC).[18a] With an excess of strong base a second
Scheme 9. Reactivity of NHO 7 and its CO2 adduct 4 with dimethyl carbon-
ate (DMC).
The relative amount of this component 15 and also the
number of side-reactions increased when the reaction temper-
ature was set to 1208C. Compound 15 may be formed in two
ways, for example, by methylation of the carboxylate moiety
and consequent deprotonation of the cation by the formed
methyl carbonate anion. However, as pointed out before, di-
methyl carbonate does not typically act as a methylation agent
below 1208C. The alternative is a temperature-induced decar-
boxylation of the NHO–CO2 zwitterion, the in situ formation of
the NHO, and consequent attack of this nucleophile at the cen-
tral carbon atom of DMC. During a nucleophilic substitution re-
action at the carbonyl group and in the presence of available
protons, methanolate may act as a leaving group deprotonat-
ing the imidazolium moiety to form 15. This scenario is sup-
ported by the observation that the corresponding unsubstitut-
Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
7
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
methylation or benzylation of the intermediate NHO with the
formation of 2-ethyl- or 2-ethylphenylimidazoles has also been
observed.[33] The NHO methoxycarbonylation product 15 de-
scribed here is thermally stable. It sublimes at 1208C/5ꢂ
10À3 mbar to yield colorless single crystals suitable for a molec-
ular X-ray structure determination (Figure 6).
show strong cross-peaks between the olefinic hydrogen atom
and the N-methyl group as well as between the olefinic hydro-
gen atom and the N-methylene group on the opposite side of
the imidazole moiety. This indicates that the double bond con-
figuration is not fixed but prone to isomerization in solution.
The presence of only one isomer in the crystalline state after
sublimation is probably a result of energetically more favorable
lattice packing.
In following experiments we investigated the reactions of 15
with acids. Compound 15 was protonated by trifluoroacetic
acid and even by the weak acid NH4PF6 at C8 with highly selec-
tive formation of functionalized ILs (Scheme 11).
Figure 6. Molecular structure of methoxycarbonyl-stabilized NHO 15. Hydro-
gen atoms and a second crystallographically independent molecule have
been omitted for clarity. Ellipsoids are shown at the 50% probability level.
Selected bond distances [ꢁ] and angles [8]: C2ÀN1 1.363(2), C2ÀN3 1.364(2),
C2ÀC8 1.407(2), C8ÀC9 1.411(2), C9ÀO1 1.384(2), C9ÀO2 1.232(2), O1ÀC10
1.441(2), N1-C2-N3 106.1(1), O1-C9-O2 120.0(1), N1-C2-C8-C9 31.6(3).
Scheme 11. Reaction of 15 with the acids HTFA and NH4PF6 to yield [NHO–
CO2Me][X].
This reaction behavior demonstrates that despite containing
one stabilizing methoxycarbonyl group, 15 still has the typical
basicity of an NHO, which can be used in highly efficient IL
syntheses.[21] In the light of the fact that NHO 7 and its COOR
stabilized derivative 15 exhibits the same reactivity pattern to-
wards electrophiles, and protons in particular, it is interesting
to note that the introduction of a COOR substituent at a classi-
cal olefin leads to contrasting reactivity patterns, for example,
ethylene and methyl acrylate, towards electrophiles and nucle-
ophiles.
The asymmetric unit consists of two crystallographically in-
dependent molecules that show almost identical bond lengths
and angles. For simplicity, the discussion of the structure will
detail only one of the molecules. In contrast to the carboxylate
4 and its literature-known congeners, the CO2 moiety is not
roughly perpendicular to the imidazolium ring but is rotated
with an interplanar angle of merely 328. This indicates partial
but not perfect p conjugation of the sp2 carbon at C8 with the
two electron-withdrawing substituents, namely the 2-imidazoli-
um cation and the methoxycarbonyl group. The two CÀC
bonds competing for carbon electron density, d(C2ÀC8)=
1.407(2) ꢁ and d(C8ÀC9)=1.411(2) ꢁ, are within 2s of the same
length and show a bond order between a single and double
bond. As expected, they are significantly shorter than the cor-
6. NHC– and NHO–SO2 adducts: zwitterionic sulfinates
We have learned that NHO–CO2 adducts are stable storage
forms of N-heterocyclic olefins. They can be used for the syn-
thesis of a wide range of 2-alkylimidazolium ionic liquids as
well as for interesting CÀC coupling reactions involving two of
the most prominent electrophiles of green chemistry, CO2 and
dimethyl carbonate. The question arises as to whether NHO 7
forms a similar adduct with the Lewis acid SO2 and whether
responding distances in the carboxylate
4
(d(C2ÀC8)=
1.474(2) ꢁ, d(C8ÀC9)=1.547(2) ꢁ). The conjugate electron de-
localization into the imidazolium cation of 15 leads to a short-
ening of both apical CÀN bonds (d(N1ÀC2)=1.363(2) ꢁ, d(C2À a NHO–SO2 adduct will show similar reactivity patterns to its
N3)=1.364(2) ꢁ), an elongation of the other two, and shorten-
ing of the C4ÀC5 distance compared with undisturbed aromat-
ic imidazolium cations such as in 4. Owing to the methyl sub-
stituent the charge delocalization by the carboxylate moiety is
asymmetric with d(C9ÀO1)=1.384(2) ꢁ and d(C9ÀO2)=
1.232(2) ꢁ, which contrasts with the very symmetric zwitterion
4 (see the Supporting Information for further bond lengths of
4). The bond distances and angles of 15 are in good agree-
ment with the related N,N’-diorgano-2-ethoxycarbonylmethyl-
enebenzimidazole structure.[34] In 15, similarly to the reference
structure, the olefinic hydrogen atom is oriented towards the
smaller N-methyl group, whereas the alkoxycarbonyl group is
oriented towards the larger N-ethyl substituent. The 2D H,H
NOESY NMR spectra of 15 in [D8]toluene at 25 and À508C
CO2 counterpart, for example, if treated with methanol. Ac-
cording to literature reports, aromatic NHCs such as 16 do not
form stable adducts with SO2.[35] Previous reports of an ada-
mantyl-substituted aromatic NHC–SO2 adduct in the form of
a planar sulfene[36] have been doubted.[37] Our experiments
showed, however, that NHO 7 as well as NHC 16 react selec-
tively with SO2 to form stable adducts. The NHO– and NHC–
SO2 adducts 21 and 22 precipitated as light-yellow solids and
were isolated in yields of 98 and 87%, respectively, when SO2
gas was passed into solutions of the Lewis bases in THF
(Scheme 12).
The molecular structures of sulfinate zwitterions 21 and 22
were validated by single-crystal X-ray structure analysis (Fig-
ures 7 and 8). Crystals of 21 were grown in an acetonitrile solu-
&
&
Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
8
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
significantly shorter than the CÀS distance in a cyclic diamino-
carbene–SO2 adduct (2.030(2) ꢁ).[35] It is, however, in perfect
agreement with the CÀS bond length in thiourea S,S-dioxide
(1.859(1) ꢁ).[41] In contrast to the diaminocarbene–SO2 adduct,
the sulfur atom in 22 is perfectly co-planar with the CN2 unit
of the NHC ring. The SO bonds in 21 (1.499(1) and 1.494(1) ꢁ)
and 22 (1.485(1) and 1.489(1) ꢁ) indicate negative charge de-
localization in the SO2 moiety. The SÀO bonds in the literature-
known diaminocarbene–SO2 adduct are significantly shorter
(both 1.469(1) ꢁ), which may be expected due to the equally
longer CÀS interaction. Nevertheless, the long CÀS bonds in 21
and 22 suggest a weakened bond that may be cleaved by ap-
propriate nucleophiles, thereby rendering these molecules suit-
able for further reactions. In a comparative experiment we elu-
cidated the relative stability of NHO–CO2 adduct 4 compared
with NHO–SO2 adduct 21. While 21 appeared to be stable to-
wards CO2 at 508C, the NHO–CO2 adduct reacted immediately
with SO2 at ambient temperature to form 21 (see the Support-
ing Information for details). However, both sulfinates 21 and
22 readily underwent cleavage in the presence of methanol at
ambient temperature to form the methyl sulfite salts 23 and
24 (Scheme 13).
Scheme 12. Synthesis of 1-ethyl-3-methylimidazolium-2-methylenesulfinate
(NHO–SO2, 21) and 1-ethyl-3-methylimidazolium-2-sulfinate (NHC–SO2, 22).
Figure 7. Molecular structure of NHO–SO2 adduct 21. Hydrogen atoms have
been omitted for clarity. Ellipsoids are shown at the 50% probability level.
Selected bond distances [ꢁ] and angles [8]: C2ÀN1 1.339(3), C2ÀN3 1.343(2),
C2ÀC8 1.470(3), C8ÀS1 1.874(2), S1ÀO1 1.499(1), S1ÀO2 1.494(1), N1-C2-N3
107.4 (2), O1-S1-O2 111.1(1).
Scheme 13. Cleavage of 21 with methanol to yield [EMMIm][MeOSO2] (23)
and the corresponding reaction of 22 to give [EMIm][MeOSO2] (24).
Figure 8. Molecular structure of NHC–SO2 adduct 22. Hydrogen atoms have
been omitted for clarity. Ellipsoids are shown at the 50% probability level.
Selected bond distances [ꢁ] and angles [8]: C2ÀN1 1.339(2), C2ÀN3 1.340(2),
C2ÀS1 1.859(1), S1ÀO1 1.485(1), S1ÀO2 1.489(1), N1-C2-N3 107.4 (1), O1-S1-
O2 111.3(1).
Up to now, imidazolium alkyl sulfites have been obtained in
patented processes by the methylation of N-alkylimidazoles
with dimethyl sulfite[42] or by the reaction of imidazolium
halide or carboxylate salts with a symmetrically substituted di-
alkyl sulfite.[43] The potential of the zwitterionic sulfinates 21
and 22 to act as precursors for other sulfite salts and sulfonyl-
ation reactions related to the corresponding carboxylation re-
actions is under continued investigation.
tion layered with diethyl ether, and 22 crystallized upon cool-
ing a room-temperature saturated solution of 22 in THF to
À208C.
In both structures the sulfur(IV) atom is coordinated in a pyr-
amidal fashion due to the remaining electron lone pair. The
C8ÀS1 bond in NHO–SO2 21 has a length of 1.874(2) ꢁ, which
is significantly longer than the sp3(C)ÀSO2 bond in N-[(1-phe-
nylethyl)ammonio]propanesulfinate (1.807 ꢁ)[38] as well as
those in the metal complexes of, for example, O-ethyl- or O-
methylsulfinato ligands (1.757–1.824 ꢁ).[39] Only in a propynesul-
finate ligand coordinated to tin through one of the oxygen
Conclusions
We have carefully investigated the conditions of the highly se-
lective and fully reversible formation of NHC–CO2 and NHO–
CO2 adducts in methanol from their imidazolium methyl car-
bonate precursors. In highly concentrated MeOH solution, in
atoms is a similar bond length (1.862 ꢁ) observed.[40] The C2À particular in the absence of methanol during the isolation of
S1 bond in NHC–SO2 22 has a length of 1.859(1) ꢁ, which is
[EMIm][OCO2Me] or [EMMIm][OCO2Me], the anion becomes
Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
9
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
less efficiently solvated, and thus a stronger base, such that
the fully reversible equilibria of these thermodynamically con-
trolled carboxylations are shifted nearly quantitatively towards
the NHC–CO2 adduct 2 or partly towards the NHO–CO2 adduct
4, even at room temperature. Following this synthetic strategy
the reaction patterns of the NHO–CO2 adduct 4 were investi-
gated and compared with the nowadays well-established
chemistry of the NHC–CO2 adduct 2.
Upon protonation, the NHO–CO2 adduct 4 can serve as
a masked NHO precursor for the preparation of ionic liquids
[EMMIm][X] based on 2-methylimidazolium cations. However,
its decarboxylation in methanol or acetonitrile is much more
dependent on the reaction conditions and partners than the
NHC–CO2 adducts: Its reaction with strong acids with weakly
nucleophilic anions (X=TFA, TFSI) primarily leads to imidazoli-
um salts [NHO–CO2H][X] with 2-CH2-COOH substituents at am-
bient temperature. Their decarboxylation to the corresponding
2-methylimidazolium salts [EMMIm][TFA] and [EMMIm][TFSI]
occurs at elevated temperatures or is catalyzed by nucleophiles
such as dimethylaminopyridine (DMAP). In contrast, the reac-
tion of the NHO–CO2 zwitterion 4 with the weak acid ammoni-
um hexafluorophosphate, which provides a sufficiently nucleo-
philic conjugate base NH3 as catalyst, leads to decarboxylation
and the quantitative formation of [EMMIm][PF6] at ambient
temperature.
Experimental Section
Methods and devices
Unless stated otherwise, all the synthetic steps were conducted by
using standard Schlenk techniques and freshly dried solvents. Ele-
mental analyses (C, H, N, S) were carried out by the service depart-
ment for routine analysis and mass spectra were recorded with
a vario MICRO cube (Elementar). Samples of air- or moisture-sensi-
tive compounds were weighed into tin capsules inside a nitrogen-
filled glovebox. Melting points were determined with a Bꢃchi Melt-
1
ing Point B540 apparatus. H, proton-decoupled 13C, and 19F NMR
spectra were recorded at 300 K in automation with a Bruker
1
Avance II 300 spectrometer. H and 13C NMR spectra were calibrat-
ed against residual proton and solvent signals, respectively
([D6]DMSO: dH =2.50 ppm, dC =39.52 ppm).[44] 19F NMR spectra
were referenced externally against CFCl3. 1H,13C HMBC and
29Si NMR spectra were recorded with a Bruker Avance III 500 or
Avance III HD 300 spectrometer. The latter spectra were calibrated
externally against Me4Si. IR spectra were recorded on a Bruker
APLPHA FT-IR spectrometer with Platinum ATR sampling (diamond
single crystal). High-resolution ESI mass spectra were acquired with
a LTQ-FT Ultra mass spectrometer (Thermo Fischer Scientific). The
resolution was set to 100000. High-resolution EI mass spectra were
acquired with a AccuTOF-GCv TOF mass spectrometer (JEOL).
Single-crystal structure determinations were performed on a Bruker
D8 QUEST diffractometer by the X-ray service department of the
Fachbereich Chemie, University of Marburg. Bruker software
(APEX2, SAINT) was used for data collection, cell refinement, and
data reduction.[45] The structures were solved with SIR97,[46]
SIR2011,[47] or SHELXS,[48] refined with SHELXL-2014,[49] and finally
validated by using the PLATON[50] software, all within the WinGX[51]
or ShelXle[52] software bundle. Absorption corrections were applied
beforehand within the APEX2 software (multi-scan).[53] Graphic rep-
resentations were created by using Diamond 4.[54] Hydrogen atoms
were constrained to the parent sites and are not shown except
when participating in hydrogen bonds.
Furthermore, we have demonstrated that the NHO–CO2
zwitterion 4 can act as an efficient carboxylating agent to-
wards CH acids such as acetonitrile: The decarboxylation of 4
at elevated temperatures generates an N-heterocyclic olefin in
situ. This base assists the deprotonation of the CH acidic posi-
tions of acetonitrile to form EMMIm cyanoacetate and EMMIm2
cyanomalonate salts, thereby exemplifying the first CÀC bond-
forming carboxylations with NHO-activated CO2. The reaction
of the NHO–CO2 zwitterion with dimethyl carbonate under ki-
netic control leads to methoxycarbonylation and the formation
of the methoxycarbonyl-stabilized NHO 15. NHO 15 is accessi-
ble with the highest selectivity from the corresponding NHO
and dimethyl carbonate.
CCDC 1485857 (4·DMSO), 1485858 (5·0.5THF), 1485859 (8),
1485860 (14a), 1485861 (15), 1485862 (21), and 1485863 (22) con-
tain the supplementary crystallographic data for this paper. These
data are provided free of charge by The Cambridge Crystallograph-
ic Data Centre.
With a view to the future, we have presented the synthesis
of the novel NHO–SO2 adduct 21 and corresponding NHC–SO2
22, which, by methanolysis, can be selectively transformed
into the corresponding methyl sulfite ILs, thereby demonstrat-
ing their potential to access a variety of organic salts. We have
shown that the CO2 and SO2 adducts of NHC and NHO are con-
venient storage forms of these carbon nucleophiles. The small
deviations of their reactivity patterns compared with the NHC
and NHO parent compounds can lead to undesired IL impuri-
ties, which are difficult to separate, in otherwise highly efficient
IL syntheses with these synthons. The results presented herein
allows these difficulties to be avoided in IL purification and
opens up opportunities for interesting new CO2 and SO2 reac-
tions mediated by NHCs and NHOs.
Starting materials
All solvents were dried according to common procedures[55] and
passed through columns of aluminium oxide, 3 ꢁ molecular sieves,
and R3-11G-catalyst (BASF) or stored over molecular sieves (3 or
4 ꢁ) until use. 1-Ethyl-2-methylimidazole was prepared according
to a published general procedure for the alkylation of imid-
azoles.[56] As the sodium salt of 2-methylimidazole reacted violently
with ethyl bromide if following the original specifications, the
sodium imidazolate was suspended in twice the recommended
volume of THF and ethyl bromide was then added dropwise at
08C before warming to room temperature.
Representative synthetic procedures
The procedures used for the synthesis of all compounds described
herein are supplied in the Supporting Information (S1–S56).
Reaction of NHO–CO2 4 with ammonium hexafluorophosphate:
NHO–CO2 4 (142 mg, 839 mmol, 1.00 equiv) was dissolved in meth-
anol (5 mL) and NH4PF6 (139 mg, 853 mmol, 1.02 equiv) was added
to the solution. The mixture was stirred at ambient temperature
&
&
Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
10
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
for 24 h and then evaporated to dryness in vacuo. [EMMIm][PF6]
[D6]DMSO): d=13.9 (1C; CH2CH3), 34.9 (1C; NCH3), 41.8 (1C;
CH2CH3), 48.4 (1C; OCH3), 52.6 (1C; CHCO2Me), 115.9 (1C; C-5),
118.1 (1C; C-4), 150.3 (1C; C-2), 165.8 (1C; CO2Me) ppm; HRMS (EI-
TOF): m/z: calcd for C9H14N2O2: 182.10553 [M]+; found: 182.10550;
elemental analysis calcd (%) for C9H14N2O2 (182.22 gmolÀ1): C 59.3,
H 7.7, N 15.4; found: C 59.35, H 7.8, N 15.6.
was isolated as a colorless solid in a yield of 201 mg (744 mmol,
1
3
89%). H NMR (300.1 MHz, [D6]DMSO): d=1.34 (t, JHH =7.3 Hz, 3H;
3
CH2CH3), 2.57 (s, 3H; CCH3), 3.74 (s, 3H; NCH3), 4.13 (q, JHH
7.3 Hz, 2H; CH2CH3), 7.59 (d, JHH =2.1 Hz, 1H; 4/5-H), 7.64 (d, JHH
=
=
3
3
2.1 Hz, 1H; 4/5-H) ppm; 13C NMR (75.5 MHz, [D6]DMSO): d=8.9
(1C; CCH3), 14.7 (1C; CH2CH3), 34.5 (1C; NCH3), 42.7 (1C; CH2CH3),
120.2 (1C; C-4/5), 122.3 (1C; C-4/5), 144.0 (1C; C-2) ppm; 19F NMR
(282.4 MHz, [D6]DMSO): d=À71.1 (d, 1JFP =711 Hz, 6F; PF6) ppm.
Synthesis of 1-ethyl-3-methylimidazolium-2-methylenesulfinate
inner salt (NHO–SO2, 21): 1-Ethyl-3-methyl-2-methylene-imidazo-
line (7; 2.30 g, 18.5 mmol, prepared and isolated in analogy to a lit-
erature procedure)[21] was dissolved in THF (50 mL) and the solu-
tion cooled to 08C. SO2 was introduced into the solution where-
upon a light-yellow solid precipitated immediately. The gas stream
was stopped after 1 min and the suspension was filtered. The filter
cake was washed with diethyl ether (20 mL) and dried under a fine
vacuum. NHO–SO2 (21) was isolated as a light-yellow powder in
a yield of 3.40 g (18.1 mmol, 98%). IR (neat): n˜max =3072 (w), 1578
(w), 1526 (m), 1452 (w), 1298 (w), 1256 (w), 1166 (m), 1068 (vs),
1019 (m), 990 (vs), 855 (w), 794 (m), 714 (m), 594 (w), 549 (w), 504
Synthesis of [NHO–CO2H][TFSI] (10): Bis(trifluoromethyl-sulfonyl)-
imide (HTFSI; 175 mg, 0.62 mmol, 1.1 equiv) was dissolved in ace-
tonitrile (5 mL) and added to NHO–CO2 4 (92 mg, 0.54 mmol,
1.0 equiv). The mixture was stirred at ambient temperature for
18 h and then evaporated to dryness in vacuo to leave a colorless
oil. The resulting residue consisted primarily (<80%) of the proton-
ated carboxylate salt [NHO–CO2H][TFSI] (10). Analogous results
were obtained on varying the reaction time between 2 h and 2 d
and upon changing the solvent for methanol. IR (neat): n˜max
=
1
3
(s), 449 (s) cmÀ1; H NMR (300.1 MHz, [D6]DMSO): d=1.34 (t, JHH
=
3450–3000 (br, w), 3149 (w), 1728 (w), 1540 (w), 1345 (m), 1327 (sh,
m), 1180 (vs), 1132 (s), 1050 (s), 825 (w), 792 (w), 762 (w), 740 (w),
7.3 Hz, 3H; CH2CH3), 3.71 (s, 2H; CH2SO2), 3.79 (s, 3H; NCH3), 4.15
3
3
1
608 (s), 569 (s), 510 (m) cmÀ1; H NMR (300.1 MHz, [D6]DMSO): d=
(q, JHH =7.3 Hz, 2H; CH2CH3), 7.56 (d, JHH =2.1 Hz, 1H; 4/5-H), 7.62
(d, 3JHH =2.1 Hz, 1H; 4/5-H) ppm; 13C NMR (75.5 MHz, [D6]DMSO):
d=15.2 (1C; CH2CH3), 35.1 (1C; NCH3), 42.8 (1C; CH2CH3), 54.9 (1C;
CH2SO2), 119.7 (1C; C-4/5), 122.1 (1C; C-4/5), 142.6 (1C; C-2) ppm;
elemental analysis calcd (%) for C7H12N2O2S1 (188.25 gmolÀ1): C
44.7, H 6.4, N 14.9, S 17.0; found: C 44.8, H 6.5, N 15.3, S 16.7.
3
3
1.33 (t, JHH =7.3 Hz, 3H; CH2CH3), 3.79 (s, 3H; NCH3), 4.19 (q, JHH
=
7.3 Hz, 2H; CH2CH3), 4.37 (s, 2H; CH2CO2H), 7.73 (d, 3JHH =1.7 Hz,
1H; 4/5-H), 7.79 (d, 3JHH =1.7 Hz, 1H; 4/5-H), 13.20 (brs, 1H;
CO2H) ppm; 13C NMR (75.5 MHz, [D6]DMSO): d=15.0 (1C; CH2CH3),
29.3 (1C; CCH2CO2H), 34.8 (1C; NCH3), 43.1 (1C; CH2CH3), 119.4 (q,
1JCF =322 Hz, 1C; CF3), 121.1 (1C; C-4/5), 123.3 (1C; C-4/5), 140.7
(1C; C-2), 167.5 (1C; CO2H) ppm; 19F NMR (282.4 MHz, [D6]DMSO):
d=À79.5 (3F; CF3) ppm.
Acknowledgements
If the isolated substance was redissolved in acetonitrile or metha-
nol and heated at 60–808C (30 min), or if the NMR sample in
[D6]DMSO was heated at 808C the substance quantitatively trans-
formed into [EMMIm][TFSI] (11). IR (neat): n˜max =3152 (w), 1591 (w),
1541 (w), 1346 (m), 1330 (sh, m), 1174 (vs), 1132 (s), 1051 (s), 789
We thank the Fond der Chemischen Industrie (doctoral fellow-
ship for L.H.F.) and the Deutsche Forschungsgemeinschaft
(GRK 1782, Functionalization of Semiconductors) for financial
support.
(w), 739 (w), 612 (m), 600 (m), 569 (m), 509 (m) cmÀ1
;
1H NMR
3
(300.1 MHz, [D6]DMSO): d=1.33 (t, JHH =7.3 Hz, 3H; CH2CH3), 2.58
3
(s, 3H; CCH3), 3.74 (s, 3H; NCH3), 4.13 (q, JHH =7.3 Hz, 2H; CH2CH3),
Keywords: carbenes · carbon dioxide fixation · carboxylation ·
heterocyclic olefins · ionic liquids
7.61 (d, 3JHH =1.9 Hz, 1H; 4/5-H), 7.65 (d, 3JHH =1.9 Hz, 1H; 4/5-
H) ppm; 13C NMR (75.5 MHz, [D6]DMSO): d=8.9 (1C; CCH3), 14.7
1
(1C; CH2CH3), 34.5 (1C; NCH3), 42.7 (1C; CH2CH3), 119.4 (q, JCF
=
[1] a) R. Kalb (PROIONIC), WO 2008052861, 2008; b) R. Kalb (PROIONIC),
WO 2008052860, 2008; c) N. J. Bridges, C. C. Hines, M. Smiglak, R. D.
Rogers, Chem. Eur. J. 2007, 13, 5207–5212; d) J. D. Holbrey, R. D.
Rogers, S. S. Shukla, C. D. Wilfred, Green Chem. 2010, 12, 407–413; e) M.
Smiglak, C. C. Hines, R. D. Rogers, Green Chem. 2010, 12, 491; f) M.
Aresta, I. Tkatchenko, I. Tommasi, in Ionic Liquids as Green Solvents,
Vol. 856 (Eds.: R. D. Rogers, K. R. Seddon), ACS, 2003, pp. 93–99; g) B.
Oelkers, J. Sundermeyer, Green Chem. 2011, 13, 608–618; h) L. H.
Finger, F. G. Schrçder, J. Sundermeyer, Z. Anorg. Allg. Chem. 2013, 639,
1140–1152; i) L. H. Finger, F. Wohde, E. I. Grigoryev, A. K. Hansmann, R.
Berger, B. Roling, J. Sundermeyer, Chem. Commun. 2015, 51, 16169–
16172; j) L. H. Finger, B. Scheibe, J. Sundermeyer, Inorg. Chem. 2015, 54,
9568–9575; k) L. H. Finger, J. Sundermeyer, Chem. Eur. J. 2016, 22,
4218–4230.
[2] a) A. M. Voutchkova, L. N. Appelhans, A. R. Chianese, R. H. Crabtree, J.
Am. Chem. Soc. 2005, 127, 17624–17625; b) A. M. Voutchkova, M. Feliz,
E. Clot, O. Eisenstein, R. H. Crabtree, J. Am. Chem. Soc. 2007, 129,
12834–12846; c) A. M. Voutchkova, R. H. Crabtree, R. D. Putman, K. H.
Kelley, T. B. Rachfuss, Inorg. Synth. 2010, 35, 88–91.
[3] a) I. Tommasi, F. Sorrentino, Tetrahedron Lett. 2005, 46, 2141–2145; b) H.
Zhou, W.-Z. Zhang, C.-H. Liu, J.-P. Qu, X.-B. Lu, J. Org. Chem. 2008, 73,
8039–8044; c) B. Bantu, G. Manohar Pawar, K. Wurst, U. Decker, A. M.
Schmidt, M. R. Buchmeiser, Eur. J. Inorg. Chem. 2009, 1970–1976; d) I.
Tommasi, F. Sorrentino, Tetrahedron Lett. 2009, 50, 104–107; e) L.-Q. Gu,
Y.-G. Zhang, J. Am. Chem. Soc. 2010, 132, 914–915; f) J. Sun, X. Yao, W.
Cheng, S. Zhang, Green Chem. 2014, 16, 3297–3304.
322 Hz, 1C; CF3), 120.2 (1C; C-4/5), 122.3 (1C; C-4/5), 144.0 (1C; C-
2) ppm; 19F NMR (282.4 MHz, [D6]DMSO): d=À78.9 (3F; CF3) ppm.
Synthesis of methyl (Z)-2-(1-ethyl-3-methyl-1,3-dihydro-2H-im-
idazol-2-ylidene)acetate (15): 1-Ethyl-2,3-dimethylimidazolium
bromide (935 mg, 4.56 mmol, 1.00 equiv), potassium hydride
(219 mg, 5.46 mmol, 1.20 equiv), and potassium tert-butanolate
(9 mg, 0.08 mmol, 2 mol%) were combined with THF (25 mL). The
suspension was stirred for 24 h and then filtered through Celite to
remove all solid components and the filter cake was washed with
THF (10 mL). Dimethyl carbonate (0.42 mL, 0.45 g, 5.0 mmol,
1.1 equiv) was added to the solution at À208C, whereupon the
mixture turned light yellow. The solution was stirred at À208C for
20 min and at ambient temperature for 1 h before all volatile com-
ponents were removed in vacuo. Compound 15 was obtained as
an off-white powder in a yield of 763 mg (4.19 mmol, 92%). The
NMR signals were assigned on the basis of HMQC and HMBC 2D
spectra. IR (neat): n˜max =3163 (w), 3122 (w), 2970 (w), 1629 (s), 1547
(vs), 1487 (m), 1434 (s), 1382 (m), 1326 (w), 1232 (m), 1147 (vs),
1056 (s), 1039 (s), 913 (s), 827 (m), 796 (w), 741 (s), 712 (m), 677
(m), 623 (s), 585 (s), 491 (w), 424 (w) cmÀ1 1H NMR (300.1 MHz,
;
[D6]DMSO): d=1.18 (t, 3JHH =7.2 Hz, 3H; CH2CH3), 3.36 (s, 3H;
3
OCH3), 3.37 (s, 3H; NCH3), 3.72 (s, 1H; CHCO2Me), 3.88 (q, JHH
=
7.2 Hz, 2H; CH2CH3), 6.91 (m, 2H; 4/5-H) ppm; 13C NMR (75.5 MHz,
Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
11
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
[4] a) M. Smiglak, J. D. Holbrey, S. T. Griffin, W. M. Reichert, R. P. Swatloski,
A. R. Katritzky, H. Yang, D. Zhang, K. Kirichenko, R. D. Rogers, Green
Chem. 2007, 9, 90–98; b) C. Rijksen, R. D. Rogers, J. Org. Chem. 2008,
73, 5582–5584; c) J. D. Holbrey, W. M. Reichert, I. Tkatchenko, E. Bouaji-
la, O. Walter, I. Tommasi, R. D. Rogers, Chem. Commun. 2003, 28–29.
[5] a) S. Seo, M. A. DeSilva, J. F. Brennecke, J. Phys. Chem. B 2014, 118,
14870–14879; b) D. M. Denning, D. E. Falvey, J. Org. Chem. 2014, 79,
4293–4299; c) B. R. Van Ausdall, J. L. Glass, K. M. Wiggins, A. M. Aarif, J.
Louie, J. Org. Chem. 2009, 74, 7935–7942; d) H. A. Duong, T. N. Tekavec,
A. M. Arif, J. Louie, Chem. Commun. 2004, 112–113.
b) A. El-Hellani, J. Monot, R. Guillot, C. Bour, V. Gandon, Inorg. Chem.
2013, 52, 506–514; c) K. Powers, C. Hering-Junghans, R. McDonald,
M. J. Ferguson, E. Rivard, Polyhedron 2016, 108, 8–14; d) D. A. Imbrich,
W. Frey, S. Naumann, M. R. Buchmeiser, Chem. Commun. 2016, 52,
6099–6102; e) N. R. Paisley, M. W. Lui, R. McDonald, M. J. Ferguson, E.
Rivard, Dalton Trans. 2016, 45, 9860–9870.
[28] P. Tundo, M. Selva, Acc. Chem. Res. 2002, 35, 706–716.
[29] a) A. P. Krapcho, G. A. Glynn, B. J. Grenon, Tetrahedron Lett. 1967, 8,
215–217; b) A. P. Krapcho, Synthesis 1982, 10, 805–822; c) A. P. Krapcho,
Synthesis 1982, 10, 893–914; d) A. Paul Krapcho, E. Ciganek, in Organic
Reactions, Vol. 81, Wiley, 2013, pp. 1–536.
[6] a) N. Kuhn, M. Steimann, G. Weyers, Z. Naturforsch. B 1999, 54, 427–
433; b) G. de Robillard, C. H. Devillers, D. Kunz, H. Cattey, E. Digard, J.
Andrieu, Org. Lett. 2013, 15, 4410–4413.
[30] B. R. Van Ausdall, N. F. Poth, V. A. Kincaid, A. M. Arif, J. Louie, J. Org.
Chem. 2011, 76, 8413–8420.
[7] D. Breuch, H. Lçwe, Green Process. Synth. 2012, 1, 261–267.
[8] a) G. Gurau, H. Rodrꢄguez, S. P. Kelley, P. Janiczek, R. S. Kalb, R. D. Rogers,
Angew. Chem. Int. Ed. 2011, 50, 12024–12026; Angew. Chem. 2011, 123,
12230–12232; b) M. Besnard, M. I. Cabaco, F. V. Chavez, N. Pinaud, P. J.
Sebastiao, J. A. P. Coutinho, Y. Danten, Chem. Commun. 2012, 48, 1245–
1247; c) M. Besnard, M. I. Cabaco, F. Vaca Chavez, N. Pinaud, P. J. Sebas-
tiao, J. A. P. Coutinho, J. Mascetti, Y. Danten, J. Phys. Chem. A 2012, 116,
4890–4901; d) M. I. CabaÅo, M. Besnard, Y. Danten, J. A. P. Coutinho, J.
Phys. Chem. A 2012, 116, 1605–1620.
[9] a) M. I. CabaÅo, M. Besnard, F. V. Chavez, N. Pinaud, P. J. Sebastiao,
J. A. P. Coutinho, J. Mascetti, Y. Danten, Chem. Commun. 2013, 49,
11083–11085; b) M. I. CabaÅo, M. Besnard, F. V. Chavez, N. Pinaud, P. J.
Sebastiao, J. A. P. Coutinho, Y. Danten, J. Chem. Phys. 2014, 140, 244307/
244301–244307/244308;c) L. Delaude, Eur. J. Inorg. Chem. 2009, 1681–
1699.
[10] a) H. Lebel, M. K. Janes, A. B. Charette, S. P. Nolan, J. Am. Chem. Soc.
2004, 126, 5046–5047; b) D. Bacciu, K. J. Cavell, I. A. Fallis, L.-l. Ooi,
Angew. Chem. Int. Ed. 2005, 44, 5282–5284; Angew. Chem. 2005, 117,
5416–5418; c) C. E. Ellul, M. F. Mahon, O. Saker, M. K. Whittlesey, Angew.
Chem. Int. Ed. 2007, 46, 6343–6345; Angew. Chem. 2007, 119, 6459–
6461; d) R. H. Crabtree, Coord. Chem. Rev. 2013, 257, 755–766.
[11] J. Fischer, W. Siegel, V. Bomm, M. Fischer, K. Mundinger (BASF), EP
0985668, 2001.
[31] C. Annese, L. D’Accolti, C. Fusco, I. Tommasi, Catal. Commun. 2014, 46,
94–97.
[32] C. Bakhtiar, E. H. Smith, J. Chem. Soc. Perkin Trans. 1 1994, 239–243.
[33] a) S. T. Handy, M. Okello, J. Org. Chem. 2005, 70, 1915–1918; b) B.
Huber, L. Rossrucker, J. Sundermeyer, B. Roling, Solid State Ionics 2013,
247–248, 15–21.
[34] R. C. F. Jones, J. N. Iley, M. Sanchis-Amat, X. Zhang, M. R. J. Elsegood, Tet-
rahedron Lett. 2009, 50, 3577–3579.
[35] M. K. Denk, K. Hatano, A. J. Lough, Eur. J. Inorg. Chem. 2003, 224–231.
[36] Y. G. Skrypnik, S. N. Lyashchuk, Sulfur Lett. 1994, 17, 287–294.
[37] a) J. F. King, K. M. Baines, M. R. Netherton, V. Dave, Can. J. Chem. 2001,
79, 461; b) A. Senning, Sulfur Lett. 1996, 20, 105; c) N. Kuhn, A. Al-
Sheikh, Coord. Chem. Rev. 2005, 249, 829–857.
[38] B. J. Wagner, J. T. Doi, W. K. Musker, J. Org. Chem. 1990, 55, 5940–5945.
[39] a) R. Eberhardt, M. Allmendinger, G. A. Luinstra, B. Rieger, Organometal-
lics 2003, 22, 211–214; b) M. F. Pilz, C. Limberg, B. B. Lazarov, K. C.
Hultzsch, B. Ziemer, Organometallics 2007, 26, 3668–3676; c) M. M. Ibra-
him, H. Vahrenkamp, Z. Anorg. Allg. Chem. 2006, 632, 1083–1085;
d) D. F. J. Piesik, S. Range, S. Harder, Organometallics 2008, 27, 6178–
6187; e) C. Mealli, M. Peruzzini, P. Stoppioni, J. Organomet. Chem. 1980,
192, 437–446.
[40] D. Ginderow, M. Huber, Acta Crystallogr. Sect. A 1973, 29, 560–563.
[41] Y. Wang, N. L. Chang, C. T. Pai, Inorg. Chem. 1990, 29, 3256–3259.
[42] a) L. Szarvas, K. Massonne (BASF AG), WO 2006021303, 2006; b) L. Szar-
vas, T. Rohde, L. Wittenbecher, K. Massonne (BASF AG), WO
2006021304, 2006.
[43] N. M. Ignatyev, U. Welz-Biermann, A. Kucheryna, H. Willner (Merck
Patent GMBH), EP 1902035, 2006.
[44] G. R. Fulmer, A. J. M. Miller, H. E. Gottlieb, N. H. Sherden, A. Nudelman,
B. M. Stoltz, K. I. Goldberg, J. E. Bercaw, Organometallics 2010, 29, 2176–
2179.
[12] A. Schmidt, A. Beutler, M. Albrecht, B. Snovydovych, F. J. Ramirez, Org.
Biomol. Chem. 2008, 6, 287–295.
[13] a) Z. Kelemen, B. Peter-Szabo, E. Szekely, O. Holloczki, D. S. Firaha, B.
Kirchner, J. Nagy, L. Nyulaszi, Chem. Eur. J. 2014, 20, 13002–13008; b) I.
Tommasi, F. Sorrentino, Tetrahedron Lett. 2006, 47, 6453–6456.
[14] K. M. Hugar, H. A. Kostalik, G. W. Coates, J. Am. Chem. Soc. 2015, 137,
8730–8737.
[15] M. Zanatta, A.-L. Girard, N. M. Simon, G. Ebeling, H. K. Stassen, P. R. Li-
votto, F. P. dos Santos, J. Dupont, Angew. Chem. Int. Ed. 2014, 53,
12817–12821; Angew. Chem. 2014, 126, 13031–13035.
[16] C. Maton, K. Van Hecke, C. V. Stevens, New J. Chem. 2015, 39, 461–468.
[17] R. D. Crocker, T. V. Nguyen, Chem. Eur. J. 2016, 22, 2208–2213.
[18] a) Y.-B. Wang, Y.-M. Wang, W.-Z. Zhang, X.-B. Lu, J. Am. Chem. Soc. 2013,
135, 11996–12003; b) W. Li, N. Yang, Y. Lyu, J. Org. Chem. 2016, 81,
5303–5313.
[45] Bruker, Bruker AXS Inc., Madison, Wisconsin, USA, 2012.
[46] A. Altomare, M. C. Burla, M. Camalli, G. Cascarano, C. Giacovazzo, A.
Guagliardi, A. G. G. Moliterni, G. Polidori, R. Spagna, J. Appl. Crystallogr.
1999, 32, 115–119.
[47] M. C. Burla, R. Caliandro, M. Camalli, B. Carrozzini, G. L. Cascarano, C.
Giacovazzo, M. Mallamo, A. Mazzone, G. Polidori, R. Spagna, J. Appl.
Crystallogr. 2012, 45, 357–361.
[19] Y.-B. Wang, D.-S. Sun, H. Zhou, W.-Z. Zhang, X.-B. Lu, Green Chem. 2015,
17, 4009–4015.
[20] L. Dong, J. Wen, W. Li, Org. Biomol. Chem. 2015, 13, 8533–8544.
[21] T. Linder, J. Sundermeyer, Chem. Commun. 2009, 2914–2916.
[22] S. Naumann, A. W. Thomas, A. P. Dove, Angew. Chem. Int. Ed. 2015, 54,
9550–9554; Angew. Chem. 2015, 127, 9686–9690.
[48] G. Sheldrick, Acta Crystallogr. Sect. A 2008, 64, 112–122.
[49] G. Sheldrick, Acta Crystallogr. Sect. A 2015, 71, 3–8.
[50] A. L. Spek, Acta Crystallogr. Sect. D 2009, 65, 148–155.
[51] L. Farrugia, J. Appl. Crystallogr. 2012, 45, 849–854.
[52] C. B. Hꢃbschle, G. M. Sheldrick, B. Dittrich, J. Appl. Crystallogr. 2011, 44,
1281–1284.
[23] Y.-B. Jia, Y.-B. Wang, W.-M. Ren, T. Xu, J. Wang, X.-B. Lu, Macromolecules
2014, 47, 1966–1972.
[24] S. Naumann, A. W. Thomas, A. P. Dove, ACS Macro Lett. 2016, 5, 134–
138.
[25] M. Blꢃmel, J.-M. Noy, D. Enders, M. H. Stenzel, T. V. Nguyen, Org. Lett.
2016, 18, 2208–2211.
[53] Bruker, Bruker AXS Inc., Madison, Wisconsin, USA, 2012.
[54] Crystal Impact, H. Putz & K. Brandenburg GbR, Kreuzherrenstr. 102,
53227 Bonn, Germany.
[55] W. L. Amarego, D. D. Perrin, Purification of Laboratory Chemicals, 4th
ed., Elsevier, Burlington, 1996.
[56] J. E. Bara, Ind. Eng. Chem. Res. 2011, 50, 13614–13619.
[26] M. Blꢃmel, R. D. Crocker, J. B. Harper, D. Enders, T. V. Nguyen, Chem.
Commun. 2016, 52, 7958–7961.
[27] a) A. Fꢃrstner, M. Alcarazo, R. Goddard, C. W. Lehmann, Angew. Chem.
Int. Ed. 2008, 47, 3210–3214; Angew. Chem. 2008, 120, 3254–3258;
Received: June 22, 2016
Published online on && &&, 0000
&
&
Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
12
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
FULL PAPER
&
Ionic Liquids
L. H. Finger, J. Guschlbauer, K. Harms,
J. Sundermeyer*
&& – &&
Heterocyclic stores: CO2 and SO2 ad-
ducts of N-heterocyclic carbenes (NHCs)
and N-heterocyclic olefins (NHOs) are
convenient storage forms of these
carbon nucleophiles (see figure). The
small deviations in their reactivity pat-
terns compared with the NHC and NHO
parent compounds open up opportuni-
ties for interesting new CO2 and SO2 re-
actions mediated by NHCs and NHOs.
N-Heterocyclic Olefin–Carbon Dioxide
and –Sulfur Dioxide Adducts:
Structures and Interesting Reactivity
Patterns
Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
13
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ