Anionic N-heterocyclic carbenes by decarboxylation of sydnone-4-carboxylates

Article abstract of DOI:10.1016/j.tet.2018.03.019

Unstable N-heterocyclic carbenes can be masked and stabilized as pseudo-cross-conjugated hetarenium-carboxylates which decarboxylate on warming. This study deals with the decarboxylation of carboxylates of mesoionic compounds to generate anionic N-heteroc

Full text of DOI:10.1016/j.tet.2018.03.019

Tetrahedron xxx (2018) 1e8
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Anionic N-heterocyclic carbenes by decarboxylation of sydnone-4-
carboxylates
a
a
a
b
c
ꢀ
€
Ana-Luiza Lücke , Sascha Wiechmann , Tyll Freese , Martin Nieger , Tamas Foldes ,
c
d
d
a, *
ꢀ
Imre Papai , Mimoza Gjikaj , Arnold Adam , Andreas Schmidt
a Clausthal University of Technology, Institute of Organic Chemistry, Leibnizstraße 6, D-38678 Clausthal-Zellerfeld, Germany
b University of Helsinki, Department of Chemistry, Laboratory of Inorganic Chemistry, P.O. Box 55, FIN-00014 University of Helsinki, Finland
c
ꢀ
€
Research Centre for Natural Sciences, Hungarian Academy of Sciences, Magyar Tudosok Korútja 2, H-1117 Budapest, XI, Hungary
d Clausthal University of Technology, Institute of Inorganic and Analytical Chemistry, Paul-Ernst-Straße 4, D-38678 Clausthal-Zellerfeld, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Unstable N-heterocyclic carbenes can be masked and stabilized as pseudo-cross-conjugated hetarenium-
carboxylates which decarboxylate on warming. This study deals with the decarboxylation of carboxylates
of mesoionic compounds to generate anionic N-heterocyclic carbenes. Lithium sydnone-4-carboxylates
were therefore prepared via 4-bromosydnones by halogen-lithium exchange with nBuLi and subse-
quent treatment with carbon dioxide. Protonation gave the corresponding sydnone-4-carboxylic acids.
Thermogravimetric measurements in addition to temperature dependent IR spectroscopy proved the
decarboxylation of lithium sydnone-4-carboxylates and formation of the corresponding sydnone anions
which can be represented as anionic N-heterocyclic carbenes. In DMSO-d6 solution, water favors the
decarboxylation. Calculations have been performed to elucidate the mechanism of the decarboxylation in
the absence and presence of water.
Received 15 January 2018
Received in revised form
2 March 2018
Accepted 8 March 2018
Available online xxx
Keywords:
Mesoionic compound
N-heterocyclic carbene
Sydnone
© 2018 Elsevier Ltd. All rights reserved.
Decarboxylation
IR spectroscopy
1. Introduction
hybridized carbon atoms interrupt the conjugation between car-
bene and anionic moieties.8
In the past years anionic N-heterocyclic carbenes (anionic NHC)
have risen from laboratory curiosities to an innovative emerging
field. Consequently, recent results concerning chemistry, properties
and applications have been summarized in a first comprehensive
review.1 Anionic N-heterocyclic carbenes can be tripolar zwitter-
In order to generate conjugated anionic N-heterocyclic car-
benes, it has been shown that mesomeric betaines are suitable
starting materials. Application of Ramsden's betaine classification
theory9 leads to several subclasses of conjugated anionic N-het-
erocyclic carbenes. A review summarizes knowledge which has
been gained to date.10 Thus, the deprotonation of mesoionic com-
pounds such as imidazolium olate 3 (X ¼ O)11 or imidazolium
ionic or anionic
carbene 1 bearing a borate at N1 is a zwitterionic anionic N-het-
erocyclic carbene.2 Its negative charge is not in
-conjugation with
p-conjugated. For example, the N-heterocyclic
p
aminide 3 (X ¼ N)12 give
p-conjugated anionic N-heterocyclic car-
the heterocycle (Scheme 1). Borate ligands can also be joined to the
4-position of imidazole-2-ylidene,3 to the 2-position of imidazole-
4-ylidene,4 or to two N1 positions of two imidazole-2-ylidenes
which form a B-bridged biscarbene.5
benes such as 4. Other examples have been described.13 In these
systems, the HOMO is joined to the diaminocarbene partial struc-
ture through an active atomic orbital coefficient. Cross-conjugated
anionic N-heterocyclic carbenes such as 6 have been prepared from
Kappe's betaine 5 by deprotonation.14 In carbene 6 the dia-
minocarbene moiety is joined to the anionic backbone through
phase inversions (nodal positions; inactive positions) of the cor-
Aluminates,6 gallates,7 and other species also belong to this type
of N-heterocyclic carbenes. Carbenes with one or more coordi-
nating tethered anionic ligands such as 2 can also be termed
zwitterionic anionic N-heterocyclic carbenes, because the sp3-
responding HOMO which cause a p-electronic charge-separation in
the ground state.15 Thus, systems such as 6 have to be distinguished
from the former mentioned species (Scheme 2).
The anions 8(I) e 8(III) of sydnones 7 (1,2,3-oxadiazolium-5-
olates16) delocalize their negative charge within the
p-conjugated
* Corresponding author.
E-mail address: schmidt@ioc.tu-clausthal.de (A. Schmidt).
https://doi.org/10.1016/j.tet.2018.03.019
0040-4020/© 2018 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Lücke A-L, et al., Anionic N-heterocyclic carbenes by decarboxylation of sydnone-4-carboxylates, Tetrahedron
(2018), https://doi.org/10.1016/j.tet.2018.03.019

2
A.-L. Lücke et al. / Tetrahedron xxx (2018) 1e8
2. Results and discussion
2.1. Syntheses and characterizations of sydnone-4-carboxylates
Lithium sydnone-4-carboxylates 10a,b were prepared by
bromination at the C4 positions of the sydnones 7a,b based on
literature procedures to yield 9a26 and 9b.27 This was followed by
bromine-lithium exchange and treatment of the resulting species
with carbon dioxide. The formed sydnone-4-carboxylate 10a has
already been described in the literature before.28 Finally, the
lithium-carboxylates were protonated to give the carboxylic acids
Scheme 1. Examples of zwitterionic anionic N-heterocyclic carbenes.
11a,b. In contrast to the generally applied procedures,19 29
hydro-
,
chloric acid was replaced by hydrogen tetrafluoroborate as pro-
tonating agent in order to obtain better yields (Scheme 4).
We performed single crystal X-ray analyses on the isolated
compounds 7a and 7b. We found, that the sydnone moiety and the
aryl ring at position N3 are twisted by 104.7ꢀ (7a)30 and 178.9ꢀ
(7b)17 from the molecular plane. In addition, single crystals of N-
phenylsydnone-4-carboxylic acid 11a were grown through slow
evaporation of a concentrated solution in ethanol. The monoclinic
crystals comprised neither water of crystallization nor solvent
molecules (Fig. 1).
In the crystallized form, the N-phenylsydnone-4-carboxylic acid
11a forms dimers which are connected via two relatively stable31
homo-intermolecular hydrogen bonds32 (Fig. 2).
Scheme 2. Anionic N-heterocyclic carbenes of mesomeric betaines.
According to the single crystal analysis, the torsion angle be-
tween the sydnone ring and the N3-aryl ring (C12eC10eN3eN2) is
116.0ꢀ and the dihedral angle between sydnone and carboxylate
group (C5eC4eC7eO9) was measured to be 174.4ꢀ.The bond be-
tween the carboxylic acid moiety and the sydnone ring (1.44 Å)
appears to be longer than in aliphatic carboxylic acids. The carbonyl
double bond C7¼O has a length of 1.23 Å. The bond length between
the carbonyl carbon atom and the hydroxyl group is shortened
(1.34 Å), caused by the dimeric structure.33
system (Scheme 3).17 They can be represented as abnormal anionic
N-heterocyclic carbenes 8(I), anionic normal N-heterocyclic car-
benes 8(II) and as electron sextet structures 8(III). Thus, sydnone
anions, which proved to be quite stable as lithium adducts under
exclusion of moisture,17 are elements of the intersection of the
compound classes of mesomeric betaines and anionic N-hetero-
cyclic carbenes. They have been applied in synthetic chemistry18
and as ligands of catalysts for Suzuki-Miyaura reactions under
various conditions.19 Sydnone imine carbenes have been described
recently.20 According to a very recently introduced quantitative
analysis of factors influencing the ease of formation of oxa- and
thia-N-heterocyclic carbenes, N-phenyl-sydnone has a CREF (car-
bene relative energy of formation) index value of 0.572 which is
between the values of 3 (Ar ¼ Ph; 0.600) and 5 (Ar ¼ Me; 0.547).21
In continuation of our studies we became interested in sydnone-
4-carboxylates, because a number of hetarenium-carboxylates can
behave as masked N-heterocyclic carbenes. Examples are carbox-
ylates of pyridin-2-ylidene derivatives,22 imidazole-2-ylidenes,23
pyrazolium-3-ylidenes,24 and indazolium-3-ylidenes.25 They were
proven to be versatile starting materials for their in-situ generation
by thermal decarboxylation which is a valuable alternative to
deprotonations. Based on these reported developments and our
finding that sydnone-4-carboxylates are effective additives for
Suzuki-Miyaura reactions in acid,19 we investigated the decarbox-
ylation and the thermal behavior of sydnone-4-carboxylates in the
solid state, in solution, and under ESI mass spectrometric condi-
tions. The mechanism of the decomposition was also examined via
DFT computations.
2.1.1. Classifications
Sydnones (I) are conjugated mesomeric betaines (CMB) con-
structed of the 1,3-dipole azomethin-imine (II) and carbon dioxide
(Scheme 5). Their isoconjugated equivalents are even, non-
alternant hydrocarbon dianions III. Mesoionic compounds of type
A such as sydnones are constructed of atoms a-f which contribute
electrons to the system as indicated by the superscripts (IV). Syd-
none-4-carboxylates are anionic tripoles, therefore they are ex-
ceptions to the definition of mesomeric betaines in the classical
sense.34 Nevertheless they can be described as hybrids of the
conjugated mesomeric betaines sydnone and pseudo-cross-con-
jugated mesomeric betaines (PCCMB), because they possess the
Scheme 3. Selected canonical forms of sydnone anions.
Scheme 4. Synthesis of sydnone-4-carboxylic acids.
Please cite this article in press as: Lücke A-L, et al., Anionic N-heterocyclic carbenes by decarboxylation of sydnone-4-carboxylates, Tetrahedron
(2018), https://doi.org/10.1016/j.tet.2018.03.019

A.-L. Lücke et al. / Tetrahedron xxx (2018) 1e8
3
Fig. 1. Unit cell of single crystals of N-phenylsydnone-4-carboxylic acid 11a.
Fig. 3. TGA measurements of 10a and 10b.
sydnone-4-carboxylate 10b which can be attributed to the loss of
approximately 2.3 mol of water of crystallization. Considering that
no decarboxylation expected to take place under these conditions,
the release of approximately 0.3 mol of CO2 in the temperature
range between 83 ꢀC and 198 ꢀC can be estimated.
To prove the formation of CO2 on heating the solids 10a,b we
performed vibrational spectroscopy in a wide temperature range.
The decarboxylation was initiated by heating the KBr pellet on a
Linkam table. The IR spectra of 10a and 10b are shown in Figs. 4 and
5, respectively.
Fig. 2. Molecular drawing of N-phenylsydnone-4-carboxylic acid 11a in the unit cell.
The absorbance bands at 2340-2400 cmꢁ1 and 659 cmꢁ1, which
increase on heating the lithium sydnone-4-carboxylate 10a were
identified as deformation vibrations of the released CO2. We
considered that this carbon dioxide was trapped within the KBr
pellet and thus also could undergo recarboxylations of the sydnone
carbene. Parallel to the CO2 formation, the absorption bands at
1750 cmꢁ1 and 1605 cmꢁ1, attributable to the asymmetric stretch-
ing vibration of the carboxylate group,35 decreased and a new ab-
sorption band at 1646 cmꢁ1 is formed. As the C (4)eH out-of-plane
vibration absorbance band at 930 cmꢁ1 as well as the vibration
absorbance band at 3100 cmꢁ1 of the sydnone 7a could not be
observed, the reprotonation under the applied measurement con-
ditions appears to be negligible. In Fig. 5 we compared the room
temperature IR spectra of 7a and 10a to the IR spectra of 10a at
200 ꢀC. The CO2 bands, which are observable in the spectra
Scheme 5. Architecture of sydnones, sydnone-4-carboxylates and lithium sydnone-4-
carboxylates.
characteristic structural motif of 2-iminio-carboxylate VI. As
mentioned before, this is also present in other masked N-hetero-
cyclic carbenes.22e25 In the carboxylic acids and their lithium salts
(VII) the general type of conjugation does not change as they
represent a vinylogous prolongation of the p-conjugated system of
sydnones through a starred position of the isoconjugated hydro-
carbon equivalent (VIII).
2.2. Decarboxylations
In order to study the decarboxylation of sydnone carboxylates in
the solid state, we first performed thermogravimetric analysis
(TGA) measurements of the lithium sydnone-4-carboxylates 10a
and 10b (Fig. 3). In the case of 10a, a slight weight loss could be
observed between approximately 50 ꢀC up to 205 ꢀC which can be
attributed to the release of approximately 0.2 mol of CO2 under
these conditions. At higher temperatures (above 205 ꢀC) the com-
pound decomposed. In contrast, a weight loss in the range between
50 ꢀC and 83 ꢀC can be observed on measuring the lithium
Fig. 4. IR spectra of 10a at various temperatures, 25 ꢀCe200 ꢀC.
Please cite this article in press as: Lücke A-L, et al., Anionic N-heterocyclic carbenes by decarboxylation of sydnone-4-carboxylates, Tetrahedron
(2018), https://doi.org/10.1016/j.tet.2018.03.019

4
A.-L. Lücke et al. / Tetrahedron xxx (2018) 1e8
the sydnone rings fragmented similarly as described for sydnones,
as peaks at m/z ¼ 131.1345 and m/z ¼ 173.0356 were detectable,
respectively.36
Next, we focused our investigations on decarboxylations in so-
lution. Surprisingly, no decarboxylation was observed by 1H NMR
spectroscopy at increasing temperatures when anhydrous DMSO-
d6 was used as solvent. The spectra of lithium sydnone-4-
carboxylate 10a in wet DMSO-d6 are shown in Fig. 7. At 50 ꢀC
compound 10a decarboxylated within a couple of hours to the
corresponding sydnone 7a, with a notable induction period at the
beginning. Analogously, we observed the formation of 7b from
lithium sydnone-4-carboxylate 10b under these conditions.
To gain insight into the mechanism of the decarboxylation, we
performed DFT computations on the investigated system (for
computational details, see Supporting Information). First, we
calculated several structures of the contact ion pair complexes
(CIPC) of lithium sydnone-4-carboxylate 10a (Fig. 8.). We found that
the anion is not planar due to steric interaction between the phenyl
ring and the carboxyl group; in the most stable structure (10a-I),
the torsion of the phenyl ring is ꢁ52.7ꢀ. The results suggest that the
energetically favored position of the lithium cation is between the
carboxylate oxygen and the exocyclic oxygen atoms. This is in
accordance with IR spectroscopic observations, where red shifts of
exocyclic CO stretching can be explained by the interaction of the
oxygen with the lithium.37
Fig. 5. IR spectra of 7a and 10a at 25 ꢀC and 10a at 200 ꢀC after heating for 5 min.
measured at 25 ꢀC, are due to CO2 from the atmosphere as the
measurements were performed under air. After more than 3 h at
200 ꢀC the sample decomposed.
The lithium sydnone-4-carboxylate 10b behaved similarly
(Fig. 6). In the range of 1580e1800 cmꢁ1 the absorbance bands of
the carboxylate group at 1750 cmꢁ1 and 1600 cmꢁ1 disappeared on
warming, while the absorption band at 1650 cmꢁ1 increased.
Neither the characteristic C (4)eH out-of-plane vibration of the
sydnone 7b at 930 cmꢁ1 nor the vibration band at 3100 cmꢁ1 were
visible.
Low resolution electrospray ionization mass spectra were
measured of methanolic solutions of the samples which were
sprayed from acetonitrile in the anion detection mode. They dis-
played the [2 x 8a - Li]- peak at m/z (%) ¼ 329.0 (50%), and the peaks
of the non-associated sydnone anion [8b]-, of [2 ꢂ 10b -Li]-, and of
[5 ꢂ 10b - Li]- at m/z ¼ 203.1 (100%), 501.0 (55%), and 1263.2 (15%),
respectively, so that in both cases the anionic N-heterocyclic car-
bene is detectable. This is also confirmed by high resolution mass
spectrometry. The sydnone anion 8a was identified by its peak at m/
z ¼ 161.0359 (calcd 161.0357), and the existence of 8b under these
conditions was verified by its peak at m/z ¼ 203.0814 (calcd
203.0814). In the absence of lithium, the sydnone-4-carboxylates
fragmented even at 0 V fragmentor voltage. HRESI-MSMS mea-
surements reveal that the first step of the fragmentation, in both
cases, is the decarboxylation to the sydnone anions 8a,b. After that,
At this position, the positively charged lithium induces electron
transfer from the electron rich C (4) atom towards the carboxylate
and strengthens the CeC bond between them.38
The free energy barrier height of the decarboxylation of the CIPC
is estimated to be 32.4 kcal/mol, and this process is predicted to be
highly endergonic (27.5 kcal/mol).39 However, the observed decar-
boxylation occurs in wet DMSO solvent media, which can allow the
dissociation of the CIPC. We thus investigated the decarboxylation
of the free anion as well. The influence of the presence of water on
the energetics was evaluated through optimized structures and
transition states with one or two coordinated water molecules.
In the absence of water molecules, the electronic energy of the
anion increases gradually with the CeC distance and no local
maximum could be identified. Nevertheless the calculated reaction
free energy is significantly lower (14.4 kcal/mol), which suggests a
more feasible pathway for the decomposition. In the presence of
one coordinated water molecule, a transition state could be found;
activation free energy on this reaction route is only 27.7 kcal/mol
(Fig. 9.). According to our computations additional water coordi-
nation is not beneficial to the reaction, because the barrier in the
presence of two water molecules is slightly larger (28.4 kcal/mol).
As the decarboxylation requires the formation of an electron
rich carbene, the stabilization of this state might facilitate the CeC
Fig. 7. Decarboxylation/reprotonation of 10a at 50 ꢀ
C in DMSO-d6, observed at
Fig. 6. IR spectra of 10 b at various temperatures, 20 ꢀCe150 ꢀC.
400 MHz and a measuring temperature of 25 ꢀC.
Please cite this article in press as: Lücke A-L, et al., Anionic N-heterocyclic carbenes by decarboxylation of sydnone-4-carboxylates, Tetrahedron
(2018), https://doi.org/10.1016/j.tet.2018.03.019

A.-L. Lücke et al. / Tetrahedron xxx (2018) 1e8
5
Fig. 8. Investigated contact ion pair structures of 10a. Selected interatomic distances
(in Å units) and torsion angles are shown in blue. Relative free energy values in kcal/
mol units are collected in brackets.
Fig. 11. TGA measurements of 11a and 11b.
175 ꢀC (11a) and 180 ꢀC (11b). The small temperature range in
which the decompositions were taking place is also striking. 11b
decomposes very rapidly above 203 ꢀC. The temperature is in the
range of those which also shows the carboxylate 10b, Fig. 4. How-
ever, the decomposition of the acid proceeds in a smaller temper-
ature range. In comparison with the corresponding lithium
sydnone carboxylates, the sydnone carboxylic acids are more
volatile.
3. Conclusion
Anionic sydnone carbenes are formed by decarboxylation of
sydnone-4-carboxylic acids in the solid state, in solution, and in the
gas phase as evidenced by temperature-dependent IR spectroscopy,
NMR spectroscopy in wet DMSO-d6, and by ESI mass spectrometry,
respectively. In solution, the presence of water facilitates the
decarboxylation. This is in contrast to the carboxylates of other
mesomeric betaines in which water is considered as a stabilizer.
Computational analysis indicated that the lithium ion stabilizes the
molecule and that one molecule of water is necessary to initiate the
decarboxylation. The decarboxylation of sydnone-4-carboxylates
supplement our knowledge concerning pseudo-cross-conjugated
partial structures of mesomeric betaines as masked N-heterocy-
clic carbenes.
Fig. 9. Water assisted decarboxylation routes investigated computationally.
bond cleavage. This is fully consistent with the obtained transition
state structures, where stabilizing interaction can be observed be-
tween an OH and the C (4) atom (Fig. 10).
These computational results suggest that the wet DMSO plays
double role in the decomposition process in solution: (a) it allows
the dissociation of the salt (b) and stabilizes the formed carbene via
coordinated water molecules.
To complete our studies we also performed TGA measurements
of the sydnone-4-carboxylic acids 11a,b (Fig. 11.). The decomposi-
tion of the sydnone carboxylic acids 11a,b occurs under milder
conditions in comparison to the sydnone-4-carboxylates, i.e. at
4. Experimental section
4.1. General considerations
The reactions were carried out under an atmosphere of nitrogen
in oven-dried glassware. Nuclear magnetic resonance (NMR)
spectra were obtained with a Bruker Avance 400 and Bruker Avance
III 600 MHz. 1H NMR spectra were recorded at 400 MHz or
600 MHz. 13C NMR spectra were recorded at 100 MHz or 150 MHz,
with the solvent peak or tetramethylsilane used as the internal
reference. Multiplicities are described by using the following ab-
breviations: s ¼ singlet, d ¼ doublet, t ¼ triplet, q ¼ quartet, and
m ¼ multiplet, and the signal orientations in DEPT experiments
were described as follows: o ¼ no signal; þ ¼ up (CH, CH3);
- ¼ down (CH2). FT-IR spectra were obtained on a Bruker FT-IR-
Raman-spectrometer,
Vertex
70 V,
in
the
range
of
400e4000 cmꢁ1. A Linkam-table was connected to the IR device
which used to heat the KBr-pellets. ATR-IR spectra were obtained
on a Bruker Alpha in the range of 400e4000 cmꢁ1. The mass spectra
were measured with a Varian 320 MS Triple Quad GC/MS/MS with a
Varian 450-GC. The electrospray ionization mass spectra (ESIMS)
were measured with an Agilent LCMSD series HP 1100 with APIES.
Fig. 10. Calculated decomposition transition states with the assistance of one water
molecule (left) and two water molecules (right). Relative Gibbs free energy values
(with respect to separated anion and water state) in kcal/mol units are shown in
brackets. Selected C-C and CeHO distances are shown in Å units.
Please cite this article in press as: Lücke A-L, et al., Anionic N-heterocyclic carbenes by decarboxylation of sydnone-4-carboxylates, Tetrahedron
(2018), https://doi.org/10.1016/j.tet.2018.03.019

6
A.-L. Lücke et al. / Tetrahedron xxx (2018) 1e8
The compound samples were sprayed from MeCN at 4000 V
capillary voltage and fragmentor voltages of 30 V, unless otherwise
noted. The HR-MS spectra were obtained with a Bruker Impact II or
with a Bruker Daltonik Tesla-Fourier transform-ion cyclotron
resonance mass spectrometer with electrospray ionization, or a
Waters Micromass LCT with direct inlet. Melting points are un-
corrected and were determined in an apparatus according to Dr.
Tottoli (Büchi). Yields are not optimized. The TGA measurements
were taken with a Q5000 IR from TA Instruments at a heating rate
of 10 ꢀC/min and nitrogen as the purge gas. All density-functional
theory (DFT)-calculations were carried out by using Gaussian09
software.
(C-4), 129.6 (HC-3’þHC-50), 133.9 (C-1’þC-2’þC-60), 142.4 (C-40),
165.1 (C-5) ppm. IR (ATR):
n
¼ 2957, 2919, 1754, 1739, 1729, 1703,
1605, 1482, 1454, 1435, 1413, 1382, 1336, 1306, 1294, 1210, 1186,
1134, 1038, 1011, 975, 890, 853, 735, 714, 667, 595, 577, 538, 508,
497, 489, 465, 449, 428 cmꢁ1. MS (ESI, 30 V): m/z (%) ¼ 589.0 (100)
[2 M þ Na]þ. HRESI-MS: calc. 304.9896 Da [MþNa]þ, meas.
304.9902 Da [MþNa]þ.
4.1.3. Lithium N-phenylsydnone-4-carboxylate 10a
A sample of 0.10 g (0.4 mmol) of 4-bromo-N-phenylsydnone 9a
was suspended in 5 mL of diethyl ether, cooled to ꢁ50 ꢀC and stir-
red at that temperature for approximately 30 min. Then, 0.2 mL
(0.12 g) of nBuLi (23% in cyclohexane) was added dropwise care-
fully, and stirring was continued for 30 min. Finally, solid carbon
dioxide was added rapidly. The ether was removed at rt with a
continuous nitrogen flow and the resulting product was dissolved
in distilled water. Then, the aqueous phase was washed subse-
quently with toluene, petroleum ether, ethyl acetate and
dichloromethane, evaporated in vacuo at maximum 30 ꢀC to dry-
ness. Toluene was added and distilled off in vacuo. Finally, the
product was dried in vacuo for 6 h without heating. Yield: 0.88 g
(>99%), dec > 220 ꢀC (decarboxylation). 1H NMR (400 MHz,
X-Ray structure analysis of 11a, M ¼ 206.16 g molꢁ1: A suitable
single crystal of 11a was selected under a polarization microscope
and mounted in a glass capillary (d ¼ 0.3 mm). The crystal structure
was determined by X-ray diffraction analysis using graphite mon-
ochromated Mo-Ka radiation (0.71073 Å) [T ¼ 123 (2) K], whereas
the scattering intensities were collected with a single crystal
diffractometer (STOE IPDS II). The crystal structure was solved by
Direct Methods using SHELXS-97 and refined using alternating
cycles of least squares refinements against F2 (SHELXL-97). All non-
H atoms were located in Difference Fourier maps and were refined
with anisotropic displacement parameters. The H positions were
determined by a final Difference Fourier Synthesis.40
DMSO,d6):
d
¼ 7.57e7.60 (m, 2H, 2‘-HC þ6‘-HC), 7.64e7.66 (m, 3H,
3‘-HCþ4‘-HCþ5‘-HC)ppm. 13C NMR (100 MHz, DMSO,d6):
d
¼ 107.1
11a crystallized in the monoclinic space group P21/c (no. 14),
lattice parameters a ¼ 10.242 (2) Å, b ¼ 6.233 (1) Å, c ¼ 27.783 (5) Å,
(C-4), 125.5 (HC-3’þHC-50 or HC-2’þHC-60), 128.7 (HC-2’þHC-60 or
HC-3’þHC-50), 131.2 (HC-40), 135.4 (C-10), 157.0 (C-7), 168.7 (C-5)
b
¼ 94.65 (2)ꢀ, V ¼ 1767.8 (6) Å3, Z ¼ 8, dcalc. ¼1.549 g cmꢁ3, F
ppm. IR (KBr):
n
¼ 3094, 3068, 3051, 1839, 1723, 1633, 1519, 1494,
(000) ¼ 848 using 3270 independent reflections and 319 parame-
1473, 1441, 1422, 1388, 1345, 1322, 1237, 1187, 1172, 1116, 1073, 1027,
1013, 1003, 979, 919, 850, 817, 797, 777, 765, 719, 692, 673, 661, 646,
608, 523, 491, 458 cmꢁ1. MS (ESI, 100 V): m/z (%) ¼ 329.0 (50) [2M-
Li-2CO2]. HRESI-MS: calc. 161.0351 Da [M-LieCO2]-, meas.
161.0355 Da [M-LieCO2].
ters. R1 ¼0.0719, wR2 ¼ 0.1388 [I > 2
s(I)], goodness of fit on
F2 ¼ 1.059, residual electron density 0.241 and ꢁ0.253 e Åꢁ3
.
Further details of the crystal structure investigations have been
deposited with the Cambridge Crystallographic Data Center, CCDC
1815512. Copies of this information may be obtained free of charge
from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK
(Fax: þ44 (1223)-336 033; e-mail: fileserv@ccdc.ac.uk or http://
www.ccdc.cam.ac.uk).
4.1.4. Lithium N-mesitylsydnone-4-carboxylate 10 b
The product was prepared under analogous reaction conditions.
A sample of 0.20 g (0.4 mmol) of 4-bromo-N-mesitylsydnone in
15 mL of diethylether (abs.) with 0.3 mL (0.20 g, 0.6 mmol) of nBuLi
(23% in hexane) gave 0.17 g (95%). Dec. > 244 ꢀC. 1H NMR (400 MHz,
4.1.1. 4-Bromo-N-phenylsydnone 9a
A sample of 2.50 g (15.4 mmol) of N-phenylsydnone were
treated with 2.70 g (32.9 mmol) of anhydrous sodium acetate and
then with 9 mL of glacial acetic acid. Then, 2.46 g (0.79 mL,
15.4 mmol) bromine was dissolved in 9 mL of glacial acetic acid and
added dropwise under vigorous stirring to the mixture of sydnone
and NaOAc in AcOH. After stirring for 1 h at rt the reaction mixture
was poured on 400 mL of ice and water, and then the crude product
was extracted with dichloromethane. The organic phase was
separated, washed subsequently with 10% of sodium thiosulfate in
water, 30 mL of brine, and finally with 10% of NaHCO3 in water. The
organic phase was dried over MgSO4, evaporated and the crude
product was recrystallized from ethanol. Yield: 3.32 g (89%). 1H
DMSO,d6):
d
¼ 2.00 (s, 6H, 1‘‘-H3Cþ3‘‘-H3C), 2.32 (s, 3H, 2‘‘-H3C),
7.07 (s, 2H, 3‘-HCþ5‘-HC) ppm. 13C NMR (100 MHz, DMSO,d6):
d
¼ 16.3 (H3C-1‘‘þH3C-3‘‘), 20.6 (H3C-2‘‘), 107.3 (C-4), 128.6 (HC-
3‘þHC-5‘), 132.5 (C-1‘), 133.1 (C-2‘þC-6‘), 140.0 (C-4‘), 156.8 (C-7),
168.3 (C-5) ppm. IR (ATR):
¼ 2923, 1805, 1739, 1602, 1456, 1440,
n
1393, 1338, 1307, 1240, 1191, 1145, 1052, 889, 860, 851, 825, 792, 751,
727, 684, 634, 599, 556, 476, 444, 422, 417, 413 cmꢁ1. MS (ESI, 50 V):
m/z (%) ¼ 211.1 (23) [M-CO2þH]þ, 261.1 (100) [MþLi]þ, 509.0 (10)
[2 M þ H]þ, 769.2 (43) [3 M þ Li]þ, 858.8 (33) [Mþ3 (M ꢁ CO2)þH]þ.
MS (ESI, 30 V): m/z (%) ¼ 203.1 (100) [M-LieCO2], 501.0 (55)
[2M ꢁ Li]-, 1263.2 (15) [5M ꢁ Li]-. HRESI-MS: calc. 271.0689 Da [M-
Li þ H þ Na]þ, meas. 271.0691 Da [M-Li þ H þ Na]þ, calc.
203.0821 Da [M-LieCO2]-, meas. 203.0818 Da [M-LieCO2].
NMR (600 MHz, DMSO-d6):
d
¼ 7.61e7.70 (m, 4H, 2‘-HCþ3‘-HCþ4‘-
HCþ5‘-HC), 7.71e7.75 (m, 1H, 4‘-HC). 13C NMR (150 MHz, DMSO-
d6):
d
¼ 85.2 (C-4, 1JCC ¼ 99.6 Hz), 129.6 (HC-2‘þHC-6‘), 130.0 (HC-
4.1.5. Lithium N-phenylsydnone-4-carboxylic acid 11a
1
3‘þHC-5‘), 134.11 (HC-4‘), 142.7 (C-1‘), 165.8 (C-5, JCC ¼ 85.6 Hz)
A sample of 0.22 g (1.0 mmol) of lithium N-phenyl-sydnone-4-
carboxylate was dissolved in distilled water and cooled to 0 ꢀC in
an ice bath. HBF4 (50% in water) was added dropwise until pH 3.
After stirring for 5 min, the solid was filtered off. The crude product
was recrystallized from ethanol. Yield: 0.11 g (54%). Mp. 202 ꢀC
ppm. 15N NMR (61 MHz, DMSO,d6):
ppm.26
d
¼ ꢁ35.4 (N-2), ꢁ100.7 (N-3)
4.1.2. 4-Bromo-N-mesitylsydnone 9 b
This compound was prepared in analogy to 9a. A sample of 1.0 g
(4.9 mmol) of N-mesitylsydnone, 0.40 g (4.9 mmol) of NaOAc,
0.78 g (0.25 mL, 4.9 mmol) of bromine, and 5.8 mL of glacial acetic
acid gave 1.15 g (83%) of 4-bromo-N-mesitylsydnone, mp.123 ꢀC. 1H
(decarboxylation). 1H NMR (400 MHz, DMSO,d6):
d
¼ 3.44 (br s, 1H,
OH), 7.62e7.66 (m, 2H, 2‘-HCþ6‘-HC or 3‘-HCþ5‘-HC), 7.69e7.73 (m,
1H, 4‘-HC), 7.76e7.78 (m, 2H, 3‘-HCþ5‘-HC or 2‘-HCþ6‘-HC) ppm.
13C NMR (100 MHz, DMSO,d6):
d
¼ 100.6 (C-4), 125.6 (HC-2’þHC-60
NMR (400 MHz, CDCl3):
d
¼ 2.08 (s, 6H, 1‘‘-H3Cþ3‘‘-H3C), 2.36 (s,
or HC-3’þHC-50), 129.1 (HC-3’þHC-50 or HC-2’þHC-60), 132.0 (C-4‘),
3H, 2‘‘-H3C), 7.23e7.24 (m, 2H, 3‘-HCþ5‘-HC) ppm. 13C NMR
135.2 (C-1‘), 157.6 (C-7), 164.4 (C-5) ppm. IR (ATR):
n
¼ 2899, 2861,
(100 MHz, CDCl3):
d
¼ 16.1 (H3C-1’’þH3C-300), 20.7 (H3C-200), 86.7
2840, 2765, 2718, 2693, 2653, 2616, 2589, 2554, 2539, 2503, 2483,
Please cite this article in press as: Lücke A-L, et al., Anionic N-heterocyclic carbenes by decarboxylation of sydnone-4-carboxylates, Tetrahedron
(2018), https://doi.org/10.1016/j.tet.2018.03.019

A.-L. Lücke et al. / Tetrahedron xxx (2018) 1e8
7
ꢀ
11. (a) Benhamou L, Vujkovic N, Cesar V, Gornitzka H, Lugan N, Lavigne G. Or-
ganometallics. 2010;29:2616e2630;
2460, 2445, 2432, 1805, 1769, 1670, 1654, 1649, 1611, 1483, 1459,
1414, 1401, 1366, 1325, 1307, 1210, 1177, 1160, 1059, 1026, 1001, 990,
977, 921, 903, 879, 842, 766, 758, 740, 727, 714, 685, 669, 663, 629,
ꢀ
(b) Benhamou L, Cesar V, Gornitzka H, Lugan N, Lavigne G. Chem Commun (J
Chem Soc Sect D). 2009:4720e4722;
(c) Biju AT, Hirano K, Frohlich R, Glorius F. Chem Asian J. 2009;4:1786e1789.
607, 592, 518, 479, 437 cmꢁ1
.
€
12. Danopoulos AA, Yu K, Monakhov P, Braunstein. Chem Eur J. 2013;19:450e455.
13. (a) Pidlypnyi N, Wolf S, Liu M, Rissanen K, Nieger M, Schmidt A. Tetrahedron.
2014;70:8672e8680;
4.1.6. Lithium N-mesitylsydnon-4-carboxylic acid 11 b
(b) Pidlypnyi N, Namyslo JC, Drafz MHH, Nieger M, Schmidt A. J Org Chem.
2013;78:1070e1078;
(c) Pidlypnyi N, Uhrner F, Nieger M, et al. Eur
A sample of 0.8 g (3.0 mmol) of lithium N-phenyl-sydnone-4-
carboxylate was dissolved in distilled water and cooled to 0 ꢀC in an
ice bath. HBF4 (50% in water) was added dropwise until pH 3. After
stirring for 5 min the solid was filtered off. The crude product was
recrystallized from ethanol. Yield: 0.73 g (97%). Mp. 203 ꢀC
J Org Chem. 2013;2013:
7739e7748.
ꢀ
14. (a) Lavigne G, Cesar V, Lugan N. Chem Eur J. 2010;16:11432e11442;
ꢀ
(b) Cesar V, Lugan N, Lavigne G. J Am Chem Soc. 2008;130:11286e11287.
15. (a) Potts KT, Murphy PM, Kuehnling WR. J Org Chem. 1988:2889e2898;
(b) Potts KT, Murphy PM, DeLuca MR, Kuehnling WR. J Org Chem. 1988:
2898e2910.
(decarboxylation). 1H NMR (400 MHz, MeOD,d4):
d
¼ 2.11 (s, 6H, 1‘‘-
H3Cþ3‘‘-H3C), 2.39 (s, 3H, 2‘‘-H3C), 7.13 (s, 2H, 3‘-HCþ5‘-HC) ppm.
13C NMR (100 MHz, MeOD,d4):
d
¼ 16.7 (H3C-1’’þH3C-300), 21.2
16. (a) Brown AW, Harrity JPA. Tetrahedron. 2017;73:3160e3172;
ꢀ
(b) Brown AW, Comas-Barcelo J, Harrity JPA. Chem Eur J. 2017;23:5228e5231;
(c) Wezeman T, Comas-Barcelo J, Nieger M, Harrity JPA, Brase S. Org Biomol
Chem. 2017;15:1575e1579;
(d) Brown DL, Harrity JP. Tetrahedron. 2010;66:553e568;
(e) Gilchrist TL. In: Storr RC, Gilchrist TL, eds. In Science of Synthesis. vol. 13.
Stuttgart, Germany: Thieme Verlag; 2004:109e125;
(f) Chandrasekhar R, Nanjan MJ. Mini Rev Med Chem. 2012;12:1359e1365;
(g) Wu C, Fang Y, Larock RC, Shi F. Org Lett. 2010;12:2234e2237;
(h) Bhosalea SK, Deshpandeb SR, Waghc RD. J Chem Pharmaceut Res. 2015;7:
1333e1343;
(H3C-200), 101.8 (C-4), 130.3 (HC-3’þHC-50), 133.3 (C-10), 134.9 (C-
2’þC-60), 143.5 (C-40), 159.0 (C-7), 166.9 (C-5) ppm. MS (ESI, 30 V):
m/z (%) ¼ 271.0 (100) [MþNa]þ, 293.0 (23) [M-Hþ2Na]þ, 519.2 (15)
ꢀ
€
[2 M þ Na]þ, 541.2 (35) [2M-Hþ2Na]þ. IR (ATR):
n
¼ 2986, 2960,
2916, 2849, 2711, 2662, 2600, 2567, 2514, 1806, 1687, 1683, 1660,
1605, 1471, 1389, 1390, 1366, 1324, 1305, 1211, 1185, 1139, 1046, 980,
915, 893, 862, 846, 803, 761, 735, 715, 704, 680, 625, 598, 554, 480,
464, 446, 434 cmꢁ1. HRESI-MS: calc. 271.0695 Da [MþNa]þ, mears.
271.0695 Da [MþNa]þ.
(i) Asundaria ST, Pannecouque C, de Clercq E, Patel KC. Pharm Chem J. 2014;48:
260e268.
17. Wiechmann S, Freese T, Drafz MHH, et al. Chem Commun (J Chem Soc Sect D).
2014;50:11822e11824.
18. (a) Lebedev SN, Cherepanov IA, Kalinin VN. Russ. Chem. Bull., Int. Ed. 2002;51:
899e900;
Acknowledgement
(b) Morozova LN, Isaeva LS, Petrovskii PV, Kratsov DN, Min SF, Kalinin VV.
J Organomet Chem. 1990;381:281e284;
€
Dr. Gerald Drager, University of Hannover (Germany), is grate-
(c) Kalinin VN, She FM, Khandozhko VN, Petrovskii PV. Russ. Chem. Bull. Int. Ed.
2001;50:525e530.
19. (a) Lücke A-L, Wiechmann S, Freese T, Schmidt A. Synlett. 2017;28:1990e1993;
(b) Lücke A-L, Wiechmann S, Freese T, Guan Z, Schmidt A. Z Naturforsch.
2016;71B:643e650.
fully acknowledged for measuring the HR-ESI-MS spectra.
We owe special thanks to Karin Bode (Institute of Inorganic and
Analytical Chemistry, Clausthal University of Technology) and Petra
€
Drottboom (Institute of Polymer Sciences and Plastics Engineering,
20. Freese T, Lücke A-L, Schmidt CAS, et al. Tetrahedron. 2017;73:5350e5357.
21. Ramsden CA, Oziminski WP. J Org Chem. 2017;82:12485e12491.
22. (a) Breslow R. J Am Chem Soc. 1958;80:3719e3726;
(b) Ratts KW, Howe RK, Phillips WG. J Am Chem Soc. 1969;91:6115e6121;
(c) Quast H, Schmitt E. Liebigs Ann Chem. 1970;732:43e63;
(d) Katritzky AR, Faid-Allah HM. Synthesis. 1983:149e150;
(e) Lavorato D, Terlouw JK, Dargel TK, Koch W, McGibbon GA, Schwarz H. J Am
Chem Soc. 1996;118:11898e11904;
Clausthal University of Technology) for measuring the IR and TGA/
DSC spectra, respectively.
The Deutsche Forschungsgemeinschaft (DFG) is gratefully
acknowledged for financial support.
Appendix A. Supplementary data
(f) Katritzky AR, Cozens AJ, Ossana A, Rubio O, Dabbas N. J. Chem. Soc., Perkin
Trans. 1985;1:2167e2172.
Supplementary data related to this article can be found at
https://doi.org/10.1016/j.tet.2018.03.019.
ꢂ
23. (a) Fevre M, Pinaud J, Leteneur A, et al. J Am Chem Soc. 2012;134:6776e6784;
(b) Kolychev EL, Bannenberg T, Freytag M, Daniliuc CG, Jones PG, Tamm M.
Chem Eur J. 2012;18:16938e16946;
ꢀ
(c) Olszewski TK, Jaskolska DE. Heteroat Chem. 2012;23:605e609;
References
(d) Le Gall T, Baltatu S, Collins SK. Synthesis. 2011:3687e3691;
(e) Rahimi A, Namyslo JC, Drafz MHH, et al. J Org Chem. 2011;76:7316e7325;
(f) Sauvage X, Zaragoza G, Demonceau A, Delaude L. Adv Synth Catal. 2010;352:
1934e1948;
(g) Li J, Peng J, Zhang G, Bai Y, Lai G, Li X. New J Chem. 2010;34:1330e1334;
(h) Bissinger P, Braunschweig H, Kupfer T, Radacki K. Organometallics. 2010;29:
3987e3990;
(i) Sauvage X, Demonceau A, Delaude L. Adv Synth Catal. 2009;351:2031e2038;
(j) Tommasi L, Sorrentino F. Tetrahedron Lett. 2009;50:104e107;
(k) Schmidt A, Beutler A, Albrecht M, Ramírez FJ. Org Biomol Chem. 2008;6:
287e295.
1. Nasr A, Winkler A, Tamm M. Coord Chem Rev. 2016;316:68e124.
2. Wacker A, Pritzkow H, Siebert W. Eur J Inorg Chem. 1998:843e849.
3. (a) Chianse AR, Kovacevic A, Zeglis BM, Faller JW, Crabtree RH. Organometallics.
2004;23:2461e2468;
(b) Gründemann S, Kovacevic A, Albrecht M, Faller JW, Crabtree RH. J Am Chem
Soc. 2002;124:10473e10481;
(c) Wang Y, Abraham MY, Gilliard RJ, Wei P, Smith JC, Robinson GH. Organo-
metallics. 2012;31:791e793;
(d) Holschumacher D, Bannenberg T, Hrib CG, Jones PG, Tamm M. Angew Chem
Int Ed. 2008;47:7428e7432. Angew. Chem. 2008, 120, 7538 e 7542;
(e) Kronig S, Theuergarten E, Daniliuc CG, Jones PG, Tamm M. Angew Chem Int
Ed. 2012;51:3240e3244. Angew. Chem. 2012, 124, 3294 e 3298.
24. (a) Schmidt A, Münster N, Dreger A. Angew Chem. 2010;122:2851e2854.
Angew. Chem. Int. Ed. 2010, 49, 2790 e 2793;
(b) Schmidt A, Dreger A. Curr Org Chem. 2011;15:2897e2970;
(c) Schmidt A, Habeck T. Lett Org Chem. 2005;2:37e39.
25. (a) Guan Z, Gjikaj M, Schmidt A. Heterocycles. 2014;10:2356e2367;
(b) Guan Z, Wiechmann S, Drafz M, Hübner E, Schmidt A. Org Biomol Chem.
2013;11:3558e3567;
ꢁ
4. (a) Jana A, Azhakar R, Tavcar G, Roesky HW, Objartel I, Stalke D. Eur J Inorg
Chem. 2011:3686e3689;
(b) Pranckevicius C, Stephan DW. Chem Eur J. 2014;20:6597e6602.
€
€
5. (a) Frankel R, Kniczek J, Ponikwar W, Noth H, Polborn K, Fehlhammer WP. Inorg
Chim Acta. 2001;312:23e39;
(c) Schmidt A, Snovydovych B, Hemmen S. Eur
4313e4319;
J Org Chem. 2008;25:
(b) Nieto I, Bontchev RP, Smith JM. Eur J Inorg Chem. 2008;2008:2476e2480.
6. (a) Schmitt A-L, Schnee G, Weiter R, Dagorne S. Chem Commun (J Chem Soc Sect
D). 2010;46:2480e2482;
(b) Wang Y, Xie Y, Abraham MY, et al. J Am Chem Soc. 2010;132:14370e14372.
7. Chen M, Wang Y, Gilliard RJ, Wei P, Schwartz NA, Robinson GH. Dalton Trans.
2014;43:14211e14214.
(d) Schmidt A, Merkel L, Eisfeld W. Eur J Org Chem. 2005:2124e2130.
26. a) Earl JC. Recl Trav Chim Pays-Bas. 1956;75:1080e1082;
b) Kato H, Nakahara K, Ohta M. Nippon Kagaku Zasshi. 1956;77:1304e1306.
27. Turnbull K. J Heterocycl Chem. 1985;22:965e968.
28. Kato H, Ohta M. Bull Chem Soc Jpn. 1959;32:282e284.
29. Lavorato D, Terlouw JK, Dargel TK, Koch W, McGibbon GA, Schwarz H. J Am
Chem Soc. 1996;118:11898e11904.
8. Lui B, Zhao J, Luo C, Lu F, Tao S, Tong Q. J Mater Chem C. 2016;4:2003e2010.
9. (a) Ramsden CA. Tetrahedron. 2013;69:4146e4159;
30. Nespurek S, Hasek J, Sorm M, Huml K. J. Mol. Sturct. 1982;82:95e105.
31. (a) Schmidt A, Wiechmann S, Otto in Scriven CF, Ramsden (Hrsg). Adv Heter-
ocycl Chem. 2016;119:143e172;
(b) Ramsden CA, Oziminski WP. Tetrahedron. 2014;70:7158e7165;
(c) Oziminski WP, Ramsden CA. Tetrahedron. 2015;71:7191e7198.
10. Schmidt A, Wiechmann S, Freese T. ARKIVOC. 2013;i:424e469.
Please cite this article in press as: Lücke A-L, et al., Anionic N-heterocyclic carbenes by decarboxylation of sydnone-4-carboxylates, Tetrahedron
(2018), https://doi.org/10.1016/j.tet.2018.03.019

8
A.-L. Lücke et al. / Tetrahedron xxx (2018) 1e8
ꢀ
(b) Schmidt A, Snovydovych B, Casado J, Quirante JJ, Lopez Navarrete JT,
Boca Raton: CRC Press LLC; 1999;
Ramírez FJ. Phys Chem Chem Phys. 2009;11:341e348.
(b) Palacios EG, Monhemius AJ. Hydrometallurgy. 2001;62:135e143.
36. Dougherty RC, Foltz RL, Kier LB. Tetrahedron. 1970;26:1989e2005.
37. Ezernitskaya MG, Lokshin BV, Kazimirchuk EI, Khandozhko VK, Kalinin VN.
Mikrochim. Acta, Suppl. 1997;14:381e382.
32. (a) Desiraju GR. Acc Chem Res. 1991;24:290e296;
(b) Desiraju GR. Acc Chem Res. 1996;29:441e449;
(c) Steiner T. Crystallogr Rev. 1996;6:1e51;
(d) Steiner T. Chem Commun (J Chem Soc Sect D). 1997:727e734;
(e) Desiraju GR, Steiner T. The Weak Hydrogen Bond in Structural Chemistry and
Biology. Oxford University Press; 1999.
38. We performed potential energy scan calculations for the decarboxylations
varying the distance between the carbene and carboxylate carbon atoms. These
calculations showed significant stabilization when the cation was present (for
details see Supporting Information).
€
33. Vollhardt KPC, Schore NE, Butenschon H. Organische Chemie. 5. Edition.
Weinheim: Wiley-VCH; 2011:936.
39. For details see the Supporting Information.
34. Ollis WD, Stanforth SP, Ramsden CA. Tetrahedron. 1985;41:2239e2329.
35. (a) Smith BC. Infrared Spectral Interpretation: A Systematic Approach. 1. Edition.
40. Sheldrick GM. SHELXS-97, Programs for Crystal Structure Determination. Uni-
€
versity of Gottingen; 1997.
Please cite this article in press as: Lücke A-L, et al., Anionic N-heterocyclic carbenes by decarboxylation of sydnone-4-carboxylates, Tetrahedron
(2018), https://doi.org/10.1016/j.tet.2018.03.019