N-heterocyclic carbenes of 5-haloindazoles generated by decarboxylation of 5-haloindazolium-3-carboxylates

Article abstract of DOI:10.1002/ejoc.200700192

Syntheses and properties of 5-fluoro-, chloro- bromo-, and iodo-substituted 11,2-dimethylindazohum-3-carboxylates as new representatives of pseudo-cross-conjugated heterocyclic mesomeric betaines are described, and results of an X-ray single crystal analy

Full text of DOI:10.1002/ejoc.200700192

FULL PAPER
DOI: 10.1002/ejoc.200700192
N-Heterocyclic Carbenes of 5-Haloindazoles Generated by Decarboxylation of
5
-Haloindazolium-3-carboxylates
[a]
[a]
[a]
[b]
Andreas Schmidt,* Bohdan Snovydovych, Tobias Habeck, Petra Dröttboom,
[c]
[c]
Mimoza Gjikaj, and Arnold Adam
Keywords: Thermogravimetric analysis / Betaines / Carbenes / Differential scanning calorimetry
Syntheses and properties of 5-fluoro-, chloro- bromo-, and
iodo-substituted 1,2-dimethylindazolium-3-carboxylates as
new representatives of pseudo-cross-conjugated heterocyclic
mesomeric betaines are described, and results of an X-ray
single crystal analysis of the nonhalogenated parent com-
pound are presented. These betaines decarboxylate on heat-
ing to yield 5-halo-1,2-dimethylindazol-3-ylidenes, which
can be trapped by protons, sulfur, and 2,4-dichlorophenyl
isocyanate, respectively. The decarboxylation is studied by
electrospray-ionization mass spectrometry, NMR spec-
troscopy, thermogravimetric analysis, and differential scan-
ning calorimetry.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
tBuOK or the dimethyl sulfoxide anion,[2] or from imid-
Introduction
[
22]
azole–thiones with potassium in boiling THF. 1,2,4-Tria-
zol-5-ylidenes were obtained by the thermal elimination of
After first examinations of N-heterocyclic carbenes
NHC) by Wanzlick,[ Arduengo published the synthesis,
1]
(
[
4]
5
-methoxytriazoles in vacuo. The thermal extrusion of
isolation, and X-ray analysis of a stable imidazol-2-ylid-
leaving groups from heterocyclic mesomeric betaines is usu-
ally observed for an architecture that is depicted in Fig-
ure 1. A dipole partial structure of type I, in which the
charges are delocalized within a common π-electron system,
undergoes cleavage of the union bond (“u”) to form a heter-
ocumulene and nucleophilic carbene II. Mesomeric betaines
that possess masked dipole I are referred to as pseudo-
ene[ and derivatives in 1991 and 1992, respectively.
2]
[3]
[
4]
[5]
Meanwhile 1,2,4-triazol-5-ylidenes, thiazol-2-ylidenes,
saturated systems such as imidazolin-2-ylidene, and acy-
clic carbenes such as diaminocarbenes and aminooxy- as
[
6]
[7]
[
8]
well as aminothiocarbenes and aminoarylcarbenes were
[
9]
developed. N-Heterocyclic carbenes serve are versatile li-
gands in numerous transition-metal-catalyzed reactions.
cross-conjugated
PCCMB). They form one of the four distinct categories of
this class of compounds, which display profound differences
heterocyclic
mesomeric
betaines
[
10]
[11]
Thus, Suzuki couplings,
ring enlargements,
reductive
(
cyclizations, living polymerizations,[13] intramolecular in-
[
12]
[
14]
sertions,
formations of 1,4-diketones, and α-amino-
[
23,24]
in their chemistry.
[
15]
ketones are widely applied reactions. Recently, polymeric
[
16]
[17]
carbene–metal complexes, oxidative esterifications, cy-
[
18]
anosilylations,
and the formation of spiro-γ-butyrolac-
[
19]
tones under NHC catalysis were described. Some review
articles appeared.[
20]
The synthesis of known nucleophilic carbenes starts
either from the corresponding salts, which can be deproton-
ated with potassium tert-butoxide,[ sodium, or potassium
hydride in the presence of catalytic amounts of either
21]
[
a] Clausthal University of Technology, Institute of Organic Chem-
istry,
Leibnizstrasse 6, 38678 Clausthal-Zellerfeld, Germany
Fax: +49-5323-722858
E-mail: schmidt@ioc.tu-clausthal.de
[
[
b] Clausthal University of Technology, Institute of Polymer Mate-
rials and Plastics Engineering,
Agricolastrasse 6, 38678 Clausthal-Zellerfeld, Germany
c] Clausthal University of Technology, Institute of Inorganic
Chemistry,
Figure 1. Architecture of pseudo-cross-conjugated systems and
their conversion into N-heterocyclic carbenes.
Paul-Ernst-Strasse 4, 38678 Clausthal-Zellerfeld, Germany
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Known pseudo-cross-conjugated heterocyclic mesomeric
betaines, which form N-heterocyclic carbenes by in situ de-
Eur. J. Org. Chem. 2007, 4909–4916
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4909

A. Schmidt, B. Snovydovych, T. Habeck, P. Dröttboom, M. Gjikaj, A. Adam
FULL PAPER
carboxylation, are presented in Figure 2. The alkaloid Ho-
1
2
3
[25]
marine (III, R = R = R = H),
and quinolinium (III,
1
2
3
R = H, R –R = –CH=CH–CH=CH–), as well as isochin-
1
2
3
olinium derivatives (III, R –R = –CH=CH–CH=CH–, R
[
26]
[27]
=
H),
imidazolium-2-carboxylates IV,
pyrazolium-3-
[
28]
carboxylates V,
as well as indazolium-3-carboxylates V
1
2
[29]
(
R –R = –CH=CH–CH=CH–)
were found to undergo
thermal fragmentation to CO and the corresponding heta-
2
rene ylidenes. Extrusions of isocyanates and isothiocyanates
have been reported as well.[ The reverse process, namely,
the formation of PCCMB by the reaction of NHC with
heterocumulenes, has also been described.[
29]
30]
Figure 2. PCCMB which form NHC on decarboxylation.
Scheme 1. Synthesis of 5-haloindazolecarboxylic acids.
In continuation of our interest in heterocyclic mesomeric
[
31]
betaines
and their relationship to N-heterocyclic carb-
we describe here the syntheses of 5-fluoro-,
[
29]
enes,
chloro-, bromo-, and iodo-substituted 1,2-indazolium-3-
carboxylates, their decarboxylations to NHC, and some
trapping experiments. In addition, we present results of
thermogravimetric analyses and differential scanning calo-
rimetry.
Results and Discussion
Syntheses and Characterizations
5-Fluoro- and 5-chloroindazole-3-carboxylic acids (2a,b)
were synthesized starting from 1H-indole-2,3-diones 1a,b
by diazonium salts that were reduced in situ by tin(II)chlo-
ride to the target indazoles[ (Scheme 1). 5-Bromoindazol-
32]
3-carboxylic acid 2c was prepared by bromination of inda-
zole-3-carboxylic acid 3. In contrast to the information
given in the literature,[ aqueous acetic acid as a solvent
was essential for the success of this reaction. 5-Iodo deriva-
tive 2d was obtained on diazotization of 5-aminoindazole-
33]
3
-carboxylic acid and subsequent substitution by iodide.
The 5-halogen substituted indazol-2,3-carboxylic acids
Scheme 2. Formation of 1,2-dimethylindazolium-3-carboxylates as
new representatives of PCCMB.
were first subjected to esterification with methanol to give
a–d (Scheme 2). Methylation of 5a–d to form 6a–d was
5
accomplished with dimethyl sulfate in xylene in the pres-
ence of catalytic amounts of nitrobenzene. Final saponifica-
tion yielded pseudo-cross-conjugated mesomeric betaines
a concentrated solution in n-propanol. The molecular draw-
[
34]
ing is presented in Figure 3.
In the elemental cell, this
betaine is essentially planar (τmin = 0.7°, τmax = 1.8°) except
for the carboxylate group which adopts an angle of τ(O11–
C10–C3–N2) = 21.5° with respect to the indazole ring. The
bond length of the union bond u depicted in Figure 1 was
determined to be 151.96 pm (C3–C10), which is consider-
7a–d in one step from 5a–d. The ester cleavage of 7e was
performed with diluted sulfuric acid, which is an alternative
[
29]
approach.
X-ray Single Crystal Analysis of 1,2-Dimethylindazolium-3-
carboxylate (7e)
2
2
ably longer than a C(sp )–C(sp ) single bond (147 pm). In
the single crystal, the betaine binds three equivalents of
Single crystals of the pseudo-cross-conjugated meso- water of crystallization. The 1,2-dimethylindazolium-3-car-
meric betaine 7e were obtained by the slow evaporation of boxylate units form layers, which alternate with layers of
4910
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© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2007, 4909–4916

N-Heterocyclic Carbenes of 5-Haloindazoles
water molecules. They are linked by hydrogen bonds, with conditions. They can be detected, however, in high resolu-
O(w)···O(carboxylate) bond lengths from 278.0 to 290.7 pm tion electrospray ionization mass spectrometry (HRESIMS)
and O(w)···O(w) bond lengths from 276.1 to 282.2 pm.
as sodium adducts under very mild conditions (Table 2). In
addition, spraying samples of 7a–e dissolved in methanol
from the same solvent at zero volt fragmentor voltage gave
prominent peaks of the proton-trapped carbenes, namely,
indazolium salts 9a–e.
Figure 3. Molecular drawing of 7e.
In view of these results, a charge-separation between the
positive and the negative partial structure of the mesomeric
betaine is therefore maintained by a diminished conjugation
as a consequence of torsion angle as well as bond length. In
accordance with the definition of pseudo-cross-conjugated
mesomeric betaines, the negative partial structure is joined
to the positive through an unstarred position of the isocon-
jugated equivalent, the propenyl anion (VI, Figure 4). This
position is an inactive position of the HOMO of the mole-
cule (VII, Figure 4). This nodal position thus serves as an
Scheme 3. Conversion of PCCMB into NHC.
Table 2. High resolution electrospray ionization mass spectroscopic
detection of N-heterocyclic carbenes 8a–e as Na+ adducts and of
indazolium cations 9a–e.
“isolator” between the charge-separated partial structures
of the mesomeric betaine.
Carbene/Cation Calcd.
Found
Formula
8a
8b
8c
8d
8e
9a
9b
9c
9d
9e
187.0647
203.0352
246.9847
294.9709
169.0742
165.0823
181.0527
225.0022
272.9883
147.0917
187.0646
203.0353
246.9847
294.9708
169.0738
165.0753
181.0550
225.0024
272.9875
147.0912
C
9
C
9
C
9
C
9
C
9
C
9
C
9
C
9
C
9
C
9
H
H
H
H
H
H
H
H
H
H
9
9
9
9
FN
ClN
BrN
2
Na
Na
Na
Na
Na
2
2
IN
2
10
N
2
10FN
10ClN
10BrN
10IN
2
2
2
2
11 2
N
Figure 4. Characteristic features of pseudo-cross-conjugation.
A similar behavior was observed in [D ]DMSO solution.
6
Indazolium-3-carboxylates 7a–e decarboxylate on warming
to give N-heterocyclic carbenes, which are in turn inter-
cepted by protons. A series of VTNMR experiments re-
Properties
The UV absorption maxima of 7a–e are dependent on
the substitution pattern. Each betaine displays three intense
peaks between λmax = 224 and 328 nm which shift batho-
chromically on changing the halogen from fluorine to
iodine (Table 1).
vealed that the ratio of betaines to salts in [D ]DMSO is
6
dependent on several competing and interplaying param-
eters such as the concentration of the solution, the halogen
atom attached to C-5, the amount of water of crystalli-
zation, temperature, and time. Thus, a solution of 7a con-
tained 35% of indazolium salt 9a after 10.0 min at 25 °C.
Under analogous conditions, 7b contained 25% of 9b and
Table 1. UV absorption maxima of the betaines in MeOH [nm].
Betaine
Peak 1
Peak 2
Peak 3
7c contained 17% of 9c. After 60.0 min at 80 °C, however,
7a
7b
7c
7d
7e
228
230
230
238
224
264
268
270
270
270
310
314
322
328
310
5-chloroindazolium-3-carboxylate 7b was completely con-
verted into the 5-chloroindazolium salt 9b, whereas the 5-
fluoroindazolium and the 5-bromoindazolium salts were
present in 78 and 58%, respectively. In either case, yet un-
identified decomposition products were seen in the spectra.
To determine the amount of absorbed water of crystalli-
On heating, the color of the betaines changes to red while
carbon dioxide is extruded and indazol-3-ylidenes 8a–e are zation and the decarboxylation temperature in the solid
formed (Scheme 3). Carbenes 8a–e decompose under these state we performed thermogravimetric analyses (TGA) and
Eur. J. Org. Chem. 2007, 4909–4916
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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A. Schmidt, B. Snovydovych, T. Habeck, P. Dröttboom, M. Gjikaj, A. Adam
FULL PAPER
differential scanning calorimetry (DSC). The former men- Table 3. Thermogravimetric data of betaines 7a–e.
tioned type of testing enables the determination of changes
Temp. range Weight loss
Species
evolved
Calcd. weight
loss [%]
in weight in relation to change in temperature. Figure 5 dis-
plays the TGA thermograms, the weight loss as a function
of temperature, of 5-halogen-substituted indazolium-3-car-
[
°C]
40–80
00–170
[%]
7
a
1.0
26.1
0.6
H
2
O
0
21.1
0
1
CO
2
boxylates 7a–d. The species evolved at temperatures up to 7b 48–94
4–153
40–92
2–154
2
H O
9
20.5
6.7
CO
2
19.6
6.3[
16.4
0
100 °C is the water of crystallization that is present in 7c
a]
7
7
c
2
H O
CO
2
H O
2
CO2
H O
2
CO
2
and 7e (cf. Figure 6) even after a 5 h period of drying in
vacuo. In either case one equivalent of water of crystalli-
zation is evolved. During the thermogravimetric analyses,
9
12.3
1.0
d
33–71
71–191
12.7
8.9
13.9
8.7[
23.1
a]
the decarboxylation of 7a–e can be detected between 71 and 7e 33–116
116–152
19.6
191 °C, depending on the substitution pattern. Details are
presented in Table 3. The lowest temperatures for the decar- [a] calcd. for 1 equiv. of water of crystallization.
boxylation were determined for the chlorine and bromine
derivatives 7b and 7c, respectively. By contrast, substitution
of the 5-position by iodine in 7d requires up to 191 °C for
The decarboxylation can be observed at 139.5 °C by a
strongly exothermic process. This value is in accord with
the weight loss in the TGA. Analogous curves were ob-
tained on examination of derivatives 7a–d, respectively (see
Supporting Information). Details, including the measured
enthalpies of transition, are given in Table 4.
the thermal elimination of CO from the solid sample.
2
Table 4. Differential Scanning Calorimetric data of 7a–e.
–
1
Heat flow [°C]
Enthalpy of transition [Jg ]
7
7
7
a
b
c
147.1
138.4
90.9
121.7
329.8
–132.6
119.6
71.3
–196.6
201.0
138.9
7
7
d
e
167.3
98.6
139.5
Figure 5. Thermogravimetric analyses of 7a–d.
Chemical Properties
The decarboxylation temperature in the solid state differs
from the temperature in solution. Thus, N-heterocyclic
carbenes 8a–d, generated in situ from 7a–d in toluene at
reflux temperature, can be trapped by sulfur, which form
thiones 10a–d in very high yields (Scheme 4). 2,4-Dichlo-
rophenyl isocyanate as a trapping reagent results in the for-
mation of indazolium-3-amidates 11a–d from 8a–d, which
Figure 6. Combined TGA (–) and DSC (---) thermograph of 7e.
Differential scanning calorimetry (DSC) is a thermoana-
lytical technique in which the difference in the amount of
heat required to increase the temperature of a sample and
reference are measured as a function of temperature. The
DSC thermogram of unsubstituted betaine 7e is presented
in Figure 6, in combination with the TGA thermograph as
a comparison. The endothermic process, detected as the
maximum of the curve at 98.6 °C, displays the loss of water
Scheme 4. Trapping experiments with in situ generated N-heterocy-
of crystallization and corresponds well to the TGA curve. clic carbene.
4912
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Eur. J. Org. Chem. 2007, 4909–4916

N-Heterocyclic Carbenes of 5-Haloindazoles
are new representatives of the class of pseudo-cross-conju- 129.2, 123.8, 123.3, 115.1, 113.3 ppm. IR (KBr): ν˜ = 3170, 2937,
–
1
1
689, 1472, 1250 cm .
gated heterocyclic mesomeric betaines (PCCMB). The ex-
trusion of heterocumulenes from PCCMB with the forma- 5-Iodoindazole-3-carboxylic Acid (2d): 5-Aminoindazole-3-carbox-
tion of NHC is not restricted to carbon dioxide. Thus, ami- ylic acid (6.1 g, 34.5 mmol) was suspended in sulfuric acid (60%,
dates 11a–d react with sulfur to give thiones 10a–d, obvi- 25 mL), heated at reflux, and cooled rapidly to 0 °C. With intense
stirring, sodium nitrite (7.5 g, 108 mmol) was added in small por-
tions within 45 min at 0 °C, and the mixture was stirred for an
additional 30 min. Then, the excess nitrite was destroyed by urea
and the solution was adjusted to pH 3–4 with sodium carbonate.
ously via carbenes 8a–d, formed by extrusion of 2,4-dichlo-
rophenyl isocyanate (Scheme 4). This observation is in
agreement with the aforementioned architecture of pseudo-
cross-conjugated mesomeric betaines (cf. Figure 1).
Potassium iodide (15 g, 90 mmol) was then added, and the solution
was stirred for 24 h. The precipitate was then filtered off and recrys-
tallized from glacial acetic acid. Yield: 6.65 g (67%), m.p. 274–
1
Conclusions
276 °C. H NMR (200 MHz, [D ]DMSO): δ = 13.96 (br. s, 1 H),
6
1
8
3.16 (br. s, 1 H), 8.45 (dd, J = 1.66, 0.64 Hz, 1 H), 7.70 (dd, J =
.7, 1.66 Hz, 1 H), 7.53 (dd, J = 8.7, 0.64 Hz, 1 H) ppm. C NMR
ylates, which decarboxylate to indazole-3-ylidenes. These (50 MHz, [D ]DMSO): δ = 163.3, 140.0, 134.9, 134.4, 129.6, 124.6,
undergo typical trapping reactions. The decarboxylation in 113.3, 87.2 ppm. IR (KBr): ν˜ = 3243, 1688 cm .
We present 5-halogen-substituted indazolium-3-carbox-
13
6
–
1
the solid state can be observed by thermogravimetric analy-
ses (TGA) and differential scanning calorimetry (DSC). In
Methyl 5-Fluoroindazole-3-carboxylate (5a): 5-Fluoroindazole-3-
carboxylic acid (2a; 968 mg, 5.4 mmol) was heated at reflux for
solution, additional parameters such as concentration, 5 h in methanol (70 mL) and concentrated sulfuric acid (4 mL).
water of crystallization, and others also influence the for- Methanol was then distilled off in vacuo. The remaining oil was
mation of N-heterocyclic carbenes.
3
treated with water (100 mL) and neutralized with NaHCO . The
ester was extracted with dichloromethane and purified by column
chromatography (silica gel, ethyl acetate/petroleum ether, 1:2).
1
Yield: 660 mg (63%), m.p. 208 °C. H NMR (200 MHz, [D
6
]-
Experimental Section
DMSO): δ = 14.10 (s, 1 H), 7.73 (m, 2 H), 7.38 (m, 1 H), 3.94 (s,
1
3
[
32]
5-chloro-,[35] and 5-bromoindazole-3-carboxylic
3 H) ppm. C NMR (50 MHz, [D ]DMSO): δ = 162.2, 158.5 (d,
5
acid
-Fluoro-,
6
[
32]
1
J = 235 Hz), 137.9, 135.0 (d, J = 5.8 Hz), 122.3 (d, J = 11.1 Hz),
were prepared according to literature procedures. The H
and 13C NMR spectra were recorded with Bruker ARX-400 and
116.3 (d, J = 27.6 Hz), 113.0 (d, J = 9.9 Hz), 104.8 (d, J = 24.7 Hz),
–
1
5
=
1
1.6 ppm. IR (KBr): ν˜ = 3283, 1727, 1481, 1224 cm . MS: m/z (%)
194 (100). HRESIMS: calcd for C FN 195.0564; found
95.0565.
DPX-200 spectrometers and were taken in [D
CDCl at 200 and 400 MHz. The chemical shifts are reported in
ppm relative to internal tetramethylsilane (δ = 0.00 ppm) or HDO
δ = 4.65 ppm). Multiplicities are described by using the following
6 2
]DMSO, D O, or
9
H
8
2 2
O
3
(
Methyl 5-Chloroindazole-3-carboxylate (5b): A sample of 5-chloro-
indazole-3-carboxylic acid (2b; 3 g, 15.2 mmol) in methanol
abbreviations: s = singlet, d = doublet, t = triplet, m = multiplet,
br. = broad. Peak assignments were accomplished by results of
HMBC-, HSQC-NMR, and HH-NOESY measurements. Spectro-
scopic data are presented here unless they are described in the lit-
erature. FTIR spectra were obtained with a Bruker Vektor 22 in
the range of 400 to 4000 cm–1 (2.5% pellets in KBr). The GC–MS
spectra (EI) were recorded either with a GC Hewlett–Packard 5980,
Serie II/MS Hewlett Packard 5989 B, or a Varian GC3900 with
SAT2100T mass spectrometer. Under EI conditions, the betaines
decarboxylate so that the corresponding indazolium salts gave the
base peaks. The ESI mass spectra were measured with an Agilent
LCMSD Series HP1100 with APIES. Samples were sprayed from
methanol at a fragmentor voltage of 0 V, unless otherwise noted.
HRSEIMS data refer to 35Cl and Br peaks. All samples were
dried for 5 h in vacuo at room temp. prior to thermogravimetric
analyses and differential scanning calorimetry. The TGA were per-
formed with a TGA 2950 and DSC examinations on DSC 2920 TA
(
300 mL) and concentrated sulfuric acid (15 mL) was heated at re-
flux for 5 h. Methanol was then distilled off in vacuo, and the resi-
due was treated with water (400 mL) and neutralized with
NaHCO . The ester was extracted with dichloromethane and puri-
3
fied by column chromatography (silica gel, ethyl acetate/petroleum
1
ether, 1:2). Yield: 1.8 g (43%), m.p. 212 °C. H NMR (200 MHz,
6
[D ]DMSO): δ = 14.14 (s, 1 H), 8.05 (s, 1 H), 7.72 (d, J = 8.9 Hz,
13
1
H), 7.47 (d, J = 8.9 Hz, 1 H), 3.95 (s, 3 H) ppm. C NMR
(
1
50 MHz, [D ]DMSO): δ = 162.2, 139.3, 134.6, 127.5, 127.0, 122.9,
6
–1
19.8, 113.0, 51.7 ppm. IR (KBr): ν˜ = 3266, 1687, 1492, 1254 cm .
ClN
(210.62): calcd. C 51.32,
MS: m/z (%) = 210 (100). HRESIMS: calcd for C
11.0269; found 211.0252. C ClN
9
H
8
2 2
O
2
9
H
7
2 2
O
79
H 3.35, N 13.30; found C 51.38, H 2.97, N 13.21.
Methyl 5-Bromoindazole-3-carboxylate (5c): 5-Bromoindazole-3-
carboxylic acid (2c; 0.6 g, 2.5 mmol) was dissolved in methanol
(
50 mL) and concentrated sulfuric acid (4 mL), and the solution
–1
Instruments. The DSC scan rate was 5 Kmin . Nitrogen purge
gas was used at a flow rate of 24 mLmin . Melting points were
determined with a Boëtius melting apparatus and are not corrected.
was heated at reflux for 5 h. Methanol was then distilled off in
vacuo, and the residue was treated with cold water (100 mL) and
–1
3
neutralized with NaHCO . The ester precipitated and was filtered
1
1
5-Fluoroindazole-3-carboxylic Acid (2a): H NMR (200 MHz, [D
6
]-
off. Yield: 0.60 g (96%), m.p. 208–210 °C. H NMR (200 MHz,
DMSO): δ = 13.94 (s, 1 H), 13.07 (s, 1 H), 7.70 (m, 2 H), 7.35 (m,
[D ]DMSO): δ = 14.16 (s, 1 H), 8.20 (s, 1 H), 7.60 (m, 2 H), 3.94
6
H) ppm. 13C NMR (50 MHz, CDCl
): δ = 163.4, 158.3 (d, J =
13
1
2
3
(s, 3 H) ppm. C NMR (50 MHz, [D ]DMSO): δ = 162.2, 139.6,
6
35 Hz), 139.1, 138.0 (d, J = 2 Hz), 122.5 (d, J = 10.7 Hz), 116.0 134.9, 129.5, 123.5, 123.0, 115.6, 113.3, 51.7 ppm. IR (KBr): ν˜ =
–
1
(d, J = 27.6 Hz), 112.9, 104.9 (d, J = 24.3 Hz) ppm.
3429, 3183, 2950, 1732, 1479, 1232 cm . MS: m/z (%) = 254 (100).
HRESIMS: calcd for C BrN 254.9764; found 255.0952.
-Bromoindazole-3-carboxylic Acid (2c): 1H NMR (200 MHz,
9
H
8
2 2
O
5
[D
6
]DMSO): δ = 13.36 (br. s, 2 H), 8.23 (dd, J = 1.8, 0.64 Hz, 1
Methyl 5-Iodoindazole-3-carboxylate (5d): 5-Iodoindazole-3-carb-
H), 7.67 (dd, J = 8.82, 0.64 Hz, 1 H), 7.56 (dd, J = 8.82, 1.8 Hz, 1
oxylic acid (3.56 g, 12.4 mmol) was suspended in methanol
H) ppm. 13C NMR (50 MHz, [D
]DMSO): δ = 163.4, 139.8, 135.5, (300 mL) and sulfuric acid (24 mL) and heated at reflux for 6 h.
6
Eur. J. Org. Chem. 2007, 4909–4916
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4913

A. Schmidt, B. Snovydovych, T. Habeck, P. Dröttboom, M. Gjikaj, A. Adam
FULL PAPER
The solution was concentrated in vacuo, poured into cold water
150 mL), and neutralized with aqueous sodium carbonate solu-
tion. The resulting precipitate was then filtered off. Yield: 3.58 g
5-Iodo-1,2-dimethylindazolium-3-carboxylate (7d): A boiling solu-
tion of methyl 5-iodoindazole-3-carboxylate (3.46 g, 12 mmol) and
nitrobenzene (0.1 mL) in xylene (80 mL) was treated with dimethyl
sulfate (34 mL, 35 mmol), and the mixture was heated over a period
of 1 h. Then, the solvent was distilled off. The resulting brown oil
was treated with KOH (1  in methanol, 150 mL), and the solution
was stirred overnight. Then, the solution was neutralized with con-
centrated HCl and evaporated to dryness, and the residue was chro-
(
1
(96%), m.p. 232–235 °C (dec.). H NMR (200 MHz, [D
6
]DMSO):
δ = 8.43 (dd, J = 1.66, 0.64 Hz, 1 H), 7.71 (dd, J = 8.82, 1.66 Hz,
1
3
1
H), 7.55 (dd, J = 8.82, 0.64 Hz, 1 H), 3.94 (s, 3 H) ppm.
C
6
NMR (50 MHz, [D ]DMSO): δ = 162.3, 139.9, 134.6, 133.9, 129.3,
1
1
C
24.3, 113.5, 87.7, 51.7 ppm. IR (KBr): ν˜ = 3217, 1689, 1483,
–1
244 cm . MS: m/z (%) = 302 (100). HRESIMS: calcd for
IN 302.9625; found 302.9560.
matographed (silica gel, MeOH). Yield: 1.05 g (28%), m.p. 156–
1
9
H
8
2
O
2
159 °C (dec.). H NMR (200 MHz, D
2
O): δ = 8.55 (dd, J = 1.66,
0
.76 Hz, 1 H), 8.03 (dd, J = 9.2, 1.66 Hz, 1 H), 7.55 (dd, J = 9.2,
5
-Fluoro-1,2-dimethylindazolium-3-carboxylate (7a): A solution of
1
0
CD
.76 Hz, 1 H), 4.56 (s, 3 H), 4.27 (s, 3 H) ppm. H NMR (400 MHz,
OD): δ = 8.81 (dd, J = 1.6, 0.8 Hz, 1 H), 8.00 (dd, J = 9.0,
.6 Hz, 1 H), 7.62 (dd, J = 9.0, 0.8 Hz, 1 H), 4.63 (s, 3 H), 4.21 (s,
methyl 5-fluoroindazole-3-carboxylate (5a; 400 mg, 2.06 mmol)
and nitrobenzene (0.1 mL) in xylene (10 mL) was heated at reflux.
Dimethyl sulfate (0.6 mL, 6.3 mmol) was then added. Heating was
continued for one additional hour, before the solvent was distilled
off in vacuo. The resulting brown oil was treated with KOH (1 
in methanol, 60 mL) and stirred overnight. The solution was then
neutralized with concentrated HCl, and the solvents were evapo-
3
1
3
1
3
13
H) ppm. C NMR (100 MHz, CD
3
OD): δ = 159.8, 141.1, 138.7,
37.8, 133.3, 121.9, 111.9, 88.6, 34.9, 32.2 ppm. IR (KBr): ν˜ = 3425,
007, 1647, 1320, 740 cm . MS: m/z (%) = 273 (100) [M – CO +
2
–1
+
+
H] , 317 (Ͻ1) [M + H] . HRESIMS: calcd for C10
16.9787; found 316.9786.
2 2
H10IN O
3
rated to dryness. The resulting residue was chromatographed (silica
1
5-Fluoro-1,2-dihydro-1,2-dimethylindazole-3-thione (10a): 5-Fluoro-
,2-dimethylindazolium-3-carboxylate (7a; 84 mg, 4 mmol) and
gel, MeOH). Yield: 294 mg (67%), m.p. 126–129 °C (dec.).
NMR (400 MHz, [D
.67 (s, 3 H), 4.23 (s, 3 H) ppm. 13C NMR (100 MHz, [D
δ = 160.2 (d, J = 239 Hz), 157.6, 140.3 (d, J = 7.3 Hz), 136.7, 123.1
H
1
6
]DMSO): δ = 8.05 (m, 2 H), 7.76 (m, 1 H),
]DMSO):
sulfur (150 mg) in toluene (10 mL) was heated at reflux for 1 h.
Then, the solvent was distilled off, and the residue was chromato-
graphed (silica gel, EtOAc/petroleum ether, 1:1). Yield: 78 mg
4
6
(d, J = 28.4 Hz), 120.1 (d, J = 12.4 Hz), 113.7 (d, J = 9.5 Hz), 108.7
1
(
99%), m.p. 158 °C. H NMR (200 MHz, CDCl
3
): δ = 7.67 (m, 1
(d, J = 25.5 Hz), 35.7, 33.7 ppm. IR (KBr): ν˜ = 3021, 1652, 1466,
–
1
+
H), 7.33 (m, 1 H), 7.19 (m, 1 H), 3.97 (s, 3 H), 3.61 (s, 3 H) ppm.
1
241 cm . MS: m/z (%) = 165 (100) [M – CO
2
+ H] , 209 (Ͻ1)
1
3
+
3
C NMR (50 MHz, CDCl ): δ = 170.9 (d, J = 9.89 Hz), 158.8 (d,
[
M + H] . HRESIMS: calcd for C10
H10FN
2
O
2
209.00726; found
J = 240.2 Hz), 141.1, 127.5 (d, J = 9.4 Hz), 120.9 (d, J = 26.8 Hz),
11.3 (d, J = 8.7 Hz), 110.1 (d, J = 24.3 Hz), 35.8, 32.6 ppm. IR
2
09.0724.
1
5-Chloro-1,2-dimethylindazolium-3-carboxylate (7b): A sample of
–1
(
KBr): ν˜ = 3045, 1584 cm . MS: m/z (%) = 196. HRESIMS: calcd
methyl 5-chloroindazole-3-carboxylate (5b; 1.2 g, 5.6 mmol) was
heated in nitromethane (35 mL) at reflux, and then treated with
dimethyl sulfate (1.6 mL, 16.8 mmol). Heating at reflux was contin-
ued for 4 h, and then the solvent was distilled off in vacuo. To the
resulting brown oil was added KOH (1  in methanol, 100 mL).
After stirring for 24 h, the KOH was neutralized with concentrated
HCl. After evaporation to dryness, the betaine was extracted with
ethanol, evaporated to dryness, and chromatographed (MeOH,
for C S 197.0549; found 197.0547.
9
H10FN
2
5
-Chloro-1,2-dihydro-1,2-dimethylindazole-3-thione (10b): Under an
inert atmosphere, 5-chloro-1,2-dimethylindazolium-3-carboxylate
7b; 70 mg, 0.333 mmol) and sulfur (150 mg) were heated at reflux
(
in toluene (10 mL) for 1 h. Then, the solvent was distilled off, and
the residue was chromatographed (silica gel, EtOAc/petroleum
ether, 1:1). Yield: 70 mg (99%), m.p. 156 °C. H NMR (200 MHz,
1
). Yield: 865 mg (69%), m.p. 127–131 °C (dec.). 1H NMR
200 MHz, [D ]DMSO): δ = 8.42 (dd, J = 2.06, 0.78 Hz, 1 H), 8.03
dd, J = 9.2, 0.78 Hz, 1 H), 7.83 (dd, J = 9.2, 2.06 Hz, 1 H), 4.67
CDCl
3
): δ = 8.00 (dd, J = 2.04, 0.64 Hz, 1 H), 7.50 (dd, J = 8.7,
SiO
2
2
3
.0 Hz, 1 H), 7.16 (d, J = 8.7, 0.6 Hz, 1 H), 3.97 (s, 3 H), 3.63 (s,
H) ppm. C NMR (50 MHz, CDCl ): δ = 170.3, 142.5, 132.5,
3
(
(
(
6
1
3
13
128.4, 127.7, 124.6, 111.0, 35.5, 32.5 ppm. IR (KBr): ν˜ = 3038,
s, 3 H), 4.22 (s, 3 H) ppm. C NMR (50 MHz, [D
6
]DMSO): δ =
–
1
1
469, 1169, 798 cm . MS: m/z = 212. HRESIMS: calcd for
1
56.9, 139.4, 137.5, 132.5, 128.7, 123.5, 120.2, 112.8, 35.1,
–1
C
9
H
10ClN S 213.0253; found 213.0258. C ClN S (212.70):
2
9
H
9
2
33.1 ppm. IR (KBr): ν˜ = 3046, 1650, 1329, 739 cm . MS: m/z (%)
+
+
calcd. C 50.82, H 4.26, N 13.17, S 15.08; found C 50.65, H 4.58,
N 13.14, S 15.18.
=
181 (100) [M – CO
10ClN
2
+ H] , 225 (Ͻ1) [M + H] . HRESIMS:
calcd for C10
H
2
O
2
225.0431; found: 225.0431.
5
-Bromo-1,2-dihydro-1,2-dimethylindazole-3-thione (10c): 5-Bromo-
5
-Bromo-1,2-dimethyl-1H-indazolium-3-carboxylate (7c): To a solu-
tion of methyl 5-bromoindazole-3-carboxylate (5c; 523 mg;
.06 mmol) and nitrobenzene (0.1 mL) in xylene (10 mL)was added
1,2-dimethylindazolium-3-carboxylate (4c; 107.6 mg, 0.4 mmol)
and sulfur (200 mg) were heated at reflux in toluene (10 mL) for
h. The solvent was then distilled off, and the residue was chro-
matographed (silica gel, EtOAc). Yield: 83 mg (81%), m.p. 177 °C.
2
1
dimethyl sulfate (0.25 mL 2.64 mmol) at reflux temperature. Heat-
ing was continued for an additional hour. Then, the solvent was
distilled off in vacuo. To the remaining brownish oil was added
KOH (1  in methanol, 60 mL), and the mixture was stirred for
1
H NMR (200 MHz, CDCl
3
): δ = 8.14 (m, 1 H), 7.63 (dd, J = 8.7,
1
3
1
1
.8 Hz, 1 H), 7.11 (dd, J = 8.7, 1.8 Hz, 1 H), 3.96 (s, 3 H), 3.63 (s,
1
3
H) ppm. C NMR (50 MHz, CDCl
3
): δ = 170.1, 142.7, 135.0,
24 h, neutralized with concentrated HCl, and the solvents evapo-
28.2, 127.8, 115.5, 111.3, 35.3, 32.4 ppm. IR (KBr): ν˜ = 3341,
620, 1468, 1223 cm . MS: m/z = 256. HRESIMS: calcd for
10BrN S 256.9748; found 256.9740. C BrN S (257.15):
rated to dryness. The residue was chromatographed (silica gel,
MeOH). Yield: 243 mg (44%), m.p. 131–135 °C, the compound
darkens and decomposes at 143–144 °C. 1H NMR (200 MHz,
–
1
C
9
H
2
9
H
9
2
calcd. C 42.04, H 3.53, N 10.89, S 12.47; found C 42.20, H 3.74,
N 10.54, S 12.13.
[
4
D
6
]DMSO): δ = 8.61 (dd, J = 1.66, 1.02 Hz, 1 H), 7.96 (m, 2 H),
.67 (s, 3 H), 4.22 (s, 3 H) ppm. 13C NMR (50 MHz, [D
]DMSO):
δ = 157.4, 139.7, 138.1, 135.4, 127.3, 121.4, 117.2, 113.5, 35.6,
6
5-Iodo-1,2-dihydro-1,2-dimethylindazole-3-thione (10d): Under an
inert atmosphere, 5-iodo-1,2-dimethylindazolium-3-carboxylate
+ H] , 269 (Ͻ1) [M + H] . HRESIMS: calcd (7d; 158 mg, 0.5 mmol) and sulfur (250 mg) were heated at reflux
O 268.9920; found 268.9922. in toluene (10 mL) over a period of 1 h. Then, the solvent was
2 2
–1
3
2
3.6 ppm. IR (KBr): ν˜ = 3040, 1649, 1327 cm . MS: m/z (%) =
+
+
25 (100) [M – CO
2
for C10
H10BrN
4914
www.eurjoc.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2007, 4909–4916

N-Heterocyclic Carbenes of 5-Haloindazoles
distilled off, and the residue was chromatographed (silica gel,
measured because of insufficient solubility. IR (KBr): ν˜ = 3441,
–
1
EtOAc/petroleum ether, 1:1). Yield: 100 mg (65%), m.p. 224–
1585, 1456, 1340 cm . HRESIMS: calcd for C16
2 3
H13Cl IN O
1
2
0
0
26 °C. H NMR (200 MHz, [D
6
]DMSO): δ = 8.05 (dd, J = 1.78,
459.9475; found 459.9480. C16 12Cl IN O·0.5H O (469.10): calcd.
H
2
3
2
.52 Hz, 1 H), 7.87 (dd, J = 8.7, 1.78 Hz, 1 H), 7.50 (dd, J = 8.7,
C 40.97, H 2.79, N 8.96; found C 41.05, H 2.25, N 8.92.
.52 Hz, 1 H), 3.93 (s, 3 H), 3.79 (s, 3 H) ppm. 13C NMR (50 MHz,
General Procedure for the Preparation of Thiones 10a–d Starting
from Amidates 11a–d: To a suspension of amidate 11a–d (0.2 mmol)
in toluene (5 mL) was added sulfur (52 mg, 1.6 mmol), and the
mixture was then heated at reflux for 3 h. The solvent was then
distilled off in vacuo, and the residue was chromatographed (silica
gel, EtOAc/petroleum ether, 1:2). Yields: 10–20%. All spectroscopic
data are in agreement to those described above.
[
3
6
D ]DMSO): δ = 165.4, 141.3, 139.3, 132.0, 127.2, 112.9, 84.6, 34.2,
–1
1.8 ppm. IR (KBr): ν˜ = 3441, 1611, 1321 cm . MS: m/z = 304.
S 304.9609; found 304.9585.
S (304.15): calcd. C 35.54, H 2.98, N 9.21, S 10.54; found
HRESIMS: calcd for C
IN
9 2
H10IN
C
9
H
9
2
C 35.61, H 3.12, N 9.14, S 10.74.
N-(2,4-Dichlorophenyl)-5-fluoro-1,2-dimethylindazolium-3-amidate
(
2
11a): A sample of mesomeric betaine (7a; 156 mg, 0.75 mmol) and
,4-dichlorophenyl isocyanate (141 mg, 0.75 mmol) were suspended
Supporting Information (see also the footnote on the first page of
this article): Drawing of the elemental cell of 7e, the TGA/DSC
curves of 7a–d, and the structure refinement data of 7e.
in toluene (12 mL)and heated at reflux for 1 h. After cooling, the
precipitate was filtered off and washed with toluene (2ϫ5 mL) and
1
ethanol (1 mL). Yield: 100 mg (38%), m.p. 211–212 °C. H NMR
Acknowledgments
6
(400 MHz, [D ]DMSO): δ = 8.75 (dd, J = 9.38, 2.5 Hz, 1 H), 8.38
(
d, J = 8.8 Hz, 1 H), 8.06 (dd, J = 9.38, 4.1 Hz, 1 H), 7.77 (m, 1
The Deutsche Forschungsgemeinschaft (DFG) is gratefully ac-
knowledged for financial support. Dr. Gerald Dräger (University
of Hannover, Germany) is gratefully acknowledged for measuring
the HRESIMS spectra.
H), 7.41 (d, J = 2.5 Hz, 1 H), 7.17 (dd, J = 8.8, 2.5 Hz, 1 H), 4.86
(
1
1
13
s, 3 H), 4.25 (s, 3 H) ppm. C NMR (100 MHz, [D
6
]DMSO): δ =
59.0 (d, J = 238.4 Hz), 158.4, 147.6, 141.7 (d, J = 6.6 Hz), 136.8,
28.8, 128.4, 126.8, 126.1, 123.4, 123.0 (d, J = 28.4 Hz), 119.9 (d,
J = 12.4 Hz), 113.5 (d, J = 9.5 Hz), 110.2 (d, J = 26.2 Hz), 36.2,
–1
[1] H.-W. Wanzlick, Angew. Chem. 1962, 74, 129; Angew. Chem.
Int. Ed. Engl. 1962, 1, 75; H.-W. Wanzlick, H.-J. Schönherr,
Angew. Chem. 1968, 80, 154; Angew. Chem. Int. Ed. Engl. 1968,
7, 141; K. Öfele, J. Organomet. Chem. 1968, 12, P42.
33.7 ppm. IR (KBr): ν˜ = 3441, 1593, 1460, 1343 cm . HRESIMS:
calcd for C16 FN O 352.0420; found 352.0421.
H13Cl
2
3
5
-Chloro-N-(2,4-dichlorophenyl)-1,2-dimethylindazolium-3-amidate
(11b): Betaine 7b (105 mg, 0.5 mmol) and 2,4-dichlorophenyl isocy-
[2] A. J. Arduengo III, R. L. Harlow, M. Kline, J. Am. Chem. Soc.
1
991, 113, 361.
anate (94 mg, 0.5 mmol) were suspended in toluene (10 mL) and
heated at reflux for 1 h. After cooling, the precipitate was filtered
[
[
3] A. J. Arduengo III, H. V. R. Dias, R. L. Harlow, M. Kline, J.
Am. Chem. Soc. 1992, 114, 5530.
1
off and washed with toluene (2ϫ5 mL). Yield: 130 mg (73%). H
4] D. Enders, K. Breuer, G. Raabe, J. Runsink, J. H. Teles, J.-P.
Melder, K. Ebel, S. Brode, Angew. Chem. 1995, 107, 1119; An-
gew. Chem. Int. Ed. Engl. 1995, 34, 2021.
5] A. J. Arduengo III, J. R. Goerlich, W. J. Marshall, Liebigs
Ann./Recueil 1997, 113, 365.
NMR (200 MHz, [D
H), 8.38 (d, J = 8.82 Hz, 1 H), 8.03 (dd, J = 9.2, 0.64 Hz, 1 H),
.83 (dd, J = 9.2, 2.1 Hz, 1 H), 7.41 (d, J = 2.56 Hz, 1 H), 7.18
6
]DMSO): δ = 9.16 (dd, J = 2.1, 0.64 Hz, 1
7
[
1
3
(dd, J = 8.82, 2.56 Hz, 1 H), 4.86 (s, 3 H), 4.24 (s, 3 H) ppm.
C
NMR not measured because of insufficient solubility. IR (KBr):
[6] A. J. Arduengo III, J. R. Goerlich, W. J. Marshall, J. Am.
Chem. Soc. 1995, 117, 11027.
–1
ν˜ = 3010, 1564, 1460 cm . HRESIMS: calcd for C16
68.0119; found 368.0124. C16 O (368.65): calcd. C 52.13,
H 3.28, N 11.40; found C 51.81, H 2.853, N 11.33.
3 3
H13Cl N O
[
7] R. W. Alder, P. R. Allen, M. Murray, A. G. Orpen, Angew.
Chem. 1996, 108, 1211; Angew. Chem. Int. Ed. Engl. 1996, 35,
3
3 3
H12Cl N
1221.
5
(
2
-Bromo-N-(2,4-dichlorophenyl)-1,2-dimethylindazolium-3-amidate
11c): A sample of mesomeric betaine 7c (135 mg, 0.5 mmol) and
,4-dichlorophenyl isocyanate (94 mg, 0.5 mmol) were suspended
[8] R. W. Alder, C. P. Butts, A. G. Orpen, J. Am. Chem. Soc. 1998,
120, 11526.
[
9] S. Solé, H. Gornitzka, W. W. Schoeller, D. Bourrissou, G. Ber-
trand, Science 2001, 292, 1901.
in toluene (10 mL) and heated at reflux for 1 h. After cooling, the
[
[
10] F. Zeng, Z. Yu, J. Org. Chem. 2006, 71, 5274.
11] A. Sánchez Pelegrí, M. R. J. Elsegood, V. McKee, G. W.
Weaver, Org. Lett. 2006, 8, 3049; A. W. Waltman, T. Ritter,
R. H. Grubbs, Organometallics 2006, 25, 4238.
12] I. G. Jung, J. Seo, S. I. Lee, S. Y. Choi, Y. K. Chung, Organome-
tallics 2006, 25, 4240.
precipitate was filtered off and washed with toluene (2ϫ5 mL) and
1
ethanol (1 mL). Yield: 170 mg (82.3 %), m.p. 221 °C. H NMR
(200 MHz, [D
6
]DMSO): δ = 8.75 (dd, J = 9.38, 2.52 Hz, 1 H), 8.38
(d, J = 8.8 Hz, 1 H), 8.06 (dd, J = 9.38, 4.1 Hz, 1 H), 7.77 (m, 1
[
H), 7.41 (d, J = 2.52 Hz, 1 H), 7.17 (dd, J = 8.8, 2.56 Hz, 1 H),
.86 (s, 3 H), 4.25 (s, 3 H) ppm. 13C NMR not measured because of
4
[13] A. P. Dove, R. C. Pratt, B. G. G. Lohmeijer, D. A. Culkin, E. C.
Hagberg, G. W. Nyce, R. M. Waymouth, J. L. Hedrick, Poly-
mer 2006, 47, 4018.
–
1
insufficient solubility. IR (KBr): ν˜ = 3440, 1587, 1458, 1342 cm .
HRESIMS: calcd for C16 13BrCl O 441.9619; found 441.9622.
12BrCl O·0.5H O (422.10): calcd. C 45.53, H 3.10, N 9.95;
H
2 3
N
[
14] G. C. Lloyd-Jones, R. W. Alder, G. J. J. Owen-Smith, Chem.
Eur. J. 2006, 12, 5361.
C
16
H
2
N
3
2
found C 45.28, H 2.61, N 10.20.
[15] A. E. Mattson, A. R. Bharadwaj, A. M. Zuhl, K. A. Scheidt,
J. Org. Chem. 2006, 71, 5715.
N-(2,4-Dichlorophenyl)-5-iodo-1,2-dimethylindazolium-3-amidate
(
11d): A sample of mesomeric betaine 7d (159 mg, 0.5 mmol) and [16] A. J. Boydston, C. W. Bielawski, Dalton Trans. 2006, 4073.
[
[
[
[
17] Y. Kageyama, S. Murata, J. Org. Chem. 2005, 70, 3140; K.
2,4-dichlorophenyl isocyanate (94 mg, 0.5 mmol) were suspended
Zeitler, Org. Lett. 2006, 8, 637.
in toluene (10 mL) and heated at reflux for 1 h. After cooling, the
precipitate was filtered off and washed with toluene (2ϫ5 mL) and
methanol (10 mL). Yield: 107 mg (46%), m.p. 220 °C. 1H NMR
18] J. J. Song, F. Gallou, J. T. Reeves, Z. Tan, N. K. Yee, C. H. Sen-
anayake, J. Org. Chem. 2006, 71, 1273.
19] V. Nair, S. Vellalath, M. Poonoth, R. Mohan, E. Suresh, Org.
Lett. 2006, 8, 507.
20] D. Bourissou, O. Guerret, F. P. Gabbai, G. Bertrand, Chem.
Rev. 2000, 100, 39; N. M. Scott, S. P. Nolan, Eur. J. Inorg.
Chem. 2005, 1815.
6
(200 MHz, [D ]DMSO): δ = 9.61 (dd, J = 1.14 Hz, 1 H), 8.40 (d,
J = 8.82 Hz, 1 H), 8.04 (dd, J = 8.96, 1.6 Hz, 1 H), 7.81 (d, J =
8
2
.96 Hz, 1 H), 7.41 (d, J = 2.48 Hz, 1 H), 7.17 (dd, J = 8.82,
.48 Hz, 1 H), 4.85 (s, 3 H), 4.23 (s, 3 H) ppm. 13C NMR not
Eur. J. Org. Chem. 2007, 4909–4916
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4915

A. Schmidt, B. Snovydovych, T. Habeck, P. Dröttboom, M. Gjikaj, A. Adam
FULL PAPER
[
21] H. W. Wanzlick, H. J. Schönherr, Justus Liebigs Ann. Chem.
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Received: March 5, 2007
2002, 796; N. Kuhn, M. Steimann, G. Weyers, Z. Naturforsch.,
Published Online: August 10, 2007
4916
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