N-Aminopyrroledione-hydrazonoketene-pyrazolium oxide-pyrazolone rearrangements and pyrazolone tautomerism

Article abstract of DOI:10.1039/b304070d

Flash vacuum thermolysis (FVT) of 1-(dimethylamino)pyrrole-2,3-diones 5 causes extrusion of CO with formation of transient hydrazonoketenes 7. The transient ketenes 7 are observable in the form of weak bands at 2130 (7a) or 2115 cm^-1 (7b) in the Ar matrix IR spectra resulting from either FVT or photolysis of either 5 or 1,1-dimethylpyrazolium-5-oxides 8, and these absorptions are in excellent agreement with B3LYP/6-31G* frequency calculations. Under FVT conditions the ketenes 7 cyclize to pyrazolium oxides 8, which undergo 1,4-migration of a methyl group to yield 1,4-dimethyl-3-phenylpyrazole-5(4H)-one 9a and 1,4,4-trimethyl-3-phenylpyrazole-5(4H)-one 9b. All three tautomers of 9a have been characterized, viz. the CH form 9a (most stable form in the gas phase, the solid state and solvents of low polarity), the OH form 9a′ (metastable solid at room temperature) and the NH form 9a″ (stable in aprotic dipolar solvents). The isomeric 1,4-dimethyl-5-phenylpyrazole-3(2H)-one 12 tautomerizes to the 3-hydroxypyrazole 12′. The crystal structure of the hydrochloride 14 of 9a′/9a″ is reported, representing the first structurally characterised example of a protonated 5-hydroxypyrazole.

Full text of DOI:10.1039/b304070d

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N-Aminopyrroledione–hydrazonoketene–pyrazolium
oxide–pyrazolone rearrangements and pyrazolone tautomerism†
Sieglinde Ebner,a,b Bianca Wallfisch,a,b John Andraos,a Ilyas Aitbaev,b Michael Kiselewsky,a
Paul V. Bernhardt,a Gert Kollenzb and Curt Wentrup*a
a Department of Chemistry, School of Molecular and Microbial Sciences,
The University of Queensland, Brisbane, Qld 4072, Australia. E-mail: wentrup@uq.edu.au
b Institut für Chemie, Karl-Franzens-Universität Graz, A-8010 Graz, Austria
Received 16th April 2003, Accepted 30th May 2003
First published as an Advance Article on the web 11th June 2003
Flash vacuum thermolysis (FVT) of 1-(dimethylamino)pyrrole-2,3-diones 5 causes extrusion of CO with formation
of transient hydrazonoketenes 7. The transient ketenes 7 are observable in the form of weak bands at 2130 (7a) or
2115 cmϪ1 (7b) in the Ar matrix IR spectra resulting from either FVT or photolysis of either 5 or 1,1-dimethyl-
pyrazolium-5-oxides 8, and these absorptions are in excellent agreement with B3LYP/6-31G* frequency calculations.
Under FVT conditions the ketenes 7 cyclize to pyrazolium oxides 8, which undergo 1,4-migration of a methyl group
to yield 1,4-dimethyl-3-phenylpyrazole-5(4H )-one 9a and 1,4,4-trimethyl-3-phenylpyrazole-5(4H )-one 9b. All three
tautomers of 9a have been characterized, viz. the CH form 9a (most stable form in the gas phase, the solid state and
solvents of low polarity), the OH form 9aЈ (metastable solid at room temperature) and the NH form 9aЉ (stable in
aprotic dipolar solvents). The isomeric 1,4-dimethyl-5-phenylpyrazole-3(2H )-one 12 tautomerizes to the
3-hydroxypyrazole 12Ј. The crystal structure of the hydrochloride 14 of 9aЈ/9aЉ is reported, representing the first
structurally characterised example of a protonated 5-hydroxypyrazole.
dimethylhydrazine.11 Subsequently, Chuche and co-workers8
Introduction
used flash vacuum thermolysis (FVT) of N,N-dimethyl-3-
The peculiar structures of the first mesoionic (mesomeric zwit-
hydrazinopropenoates to generate transient N-(dimethylamino)-
terionic) compounds to be characterized, the sydnones1 (1,2,3-
oxadiazolium-5-olates) 1a, revealed long endocyclic O–CO
imidoylketenes, which cyclized to 1,1-dimethylpyrazolium-5-
oxides. These compounds were isolable using reaction temper-
bonds (1.41 A) and large C4C5O angles (136Њ), which led to the
atures below 400 ЊC, but at higher temperatures they isomerised
suggestion of bond–no bond mesomerism with the ketene
to 4,4-disubstituted pyrazole-5(4H )-ones, formally by 1,4-shifts
of a methyl group from N1 to C4. 1,5-Alkyl shifts to give
1,2-dimethylpyrazolin-5-ones (antipyrine-type compounds)
were not observed, nor were the presumed transient hydra-
resonance forms 1b.1,2 Similar structures have also been reported
for the münchnones (oxazolium-5-olates) 2,3 pyrazolium oxides
3 (e.g. N–CO = 1.55–1.57 Å; C4C5O = 140Њ),4,5,6 and chromium
complexes of pyrrolium-2-oxides (N-CO = 1.59 Å; C3C2O =
zonoketenes detectable.
138.6Њ).7 Pyrazolium and pyrrolium oxides can be formed by
cyclization of transient hydrazono- or imino-substituted ketene
intermediates,6,7,8 and thus they can be regarded as cyclic
ketene–amine zwitterions (see 4).9 However, in spite of much
Results and discussion
We have prepared N-aminopyrrole-2,3-diones 5 by conden-
sation of N,N-dimethylhydrazones of aromatic ketones with
oxalyl chloride.12 As with other pyrrole-2,3-dione derivatives,13
FVT of 5 is expected to result in CO extrusion and formation
of hydrazonoketenes 7. In fact, FVT of the deeply red com-
pound 5a at 400 ЊC gave a product mixture which was separated
by dry-column chromatography into the orange pyrazolium
oxide 8a and its rearrangement product, 1,4-dimethyl-3-
phenylpyrazole-5(4H )-one 9a (1 : 4)(Scheme 1). FVT of the
hydrazone 6 at 600 ЊC (Chuche’s method8) also resulted in a
mixture of 8a and 9a (ca. 1 : 1) together with unchanged start-
ing material 6 as established by GC-MS analysis. Moreover,
compound 9a was also obtained by FVT of 8a at 500 ЊC. The
1
IR and H NMR spectra demonstrated that only traces of the
discussion and some circumstantial evidence, ketene valence
isomers of these five-membered heterocycles have never been
rigorously identified.2 We have reported direct observation of
such ring—chain valence isomers in another 5-membered
mesoion series, viz. pyrrolo[1,2-a]pyridinium olates and
(2-pyridyl)carbonylketenes.10
known14 antipyrine-type isomer 10a were formed in these
experiments. A plausible reason can be found in the thermo-
chemistry: the calculated energy of 10a is ca. 12 kcal molϪ1
above that of 9a at the B3LYP/6-31ϩG* level of theory.
The possible intermediates formed in the FVT reactions were
investigated by matrix isolation IR spectroscopy. FVT of 5a at
400 ЊC with isolation of the product in Ar matrix at ca. 10 K
allowed the observation of a new band of weak intensity at
2130 cmϪ1 together with a strong band due to carbon monoxide
(2139 cmϪ1) and unchanged starting material 5a. FVT at 700 ЊC
resulted in formation of the rearrangement product 9a (1732
cmϪ1). The mesoion 8a was barely detectable in these experi-
1,1-Dimethylpyrazolium-5-oxides of type
8 were first
obtained from the reaction of ethyl benzoylacetate with N,N-
† Electronic supplementary information (ESI) available: calculated and
observed IR spectra of 8a and X-ray structure, packing diagram, bond
lengths and angles for compound 14. See http://www.rsc.org/suppdata/
ob/b3/b304070d
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 5 5 0 – 2 5 5 5
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
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Scheme 1
ments, and FVT of 8a itself (IR (Ar) 1771 cmϪ1) at 500–665 ЊC
the negative charge.2,10 This is also true of the pyrazolium oxide
resulted in the formation of 9a (1732 cmϪ1).
8a (69 ppm for the vinylic CH carbon, C4) and other pyrazo-
lium6,8,17 and pyrrolium18 oxides. The IR spectra of both
six-membered (e.g. pyridinylium and oxazinylium olates2) and
five-membered (e.g. münchnones2 and pyrrolopyridinylium
olates10) mesoionic compounds are very peculiar. Due to their
highly polar character, the IR spectra in KBr and in Ar matrix
are very different, with shifts in the range 30–60 cmϪ1 towards
higher wavenumbers in the matrices.2 In theoretical calculations
of the IR spectra it is necessary to use diffuse functions on
heavy atoms (6-31ϩG* basis set)2 or to incorporate a simulated
solvent field with a high dielectric constant (e.g. B3LYP/6-31G*
calculation employing a self consistent reaction field (SCRF)
with ε up to 40).2 The use of diffuse functions is most effective.
Photolysis of either 5a or 8a isolated in Ar matrices at 7 K
also caused the development of a weak band at 2130 cmϪ1 in the
IR spectra. It may be assumed that the absorptions at 2130
cmϪ1 are due to the transient hydrazonoketene 7a (see below).
In the photolysis of 8a the amount of ketene doubled between
90 min and 4.5 h of irradiation, and this peak disappeared
again after 7.5 h of irradiation. At this time the starting
material (1771 cmϪ1) had disappeared and been fully converted
to 9a (1732 cmϪ1).
The 4-methylpyrroledione analog 5b underwent similar ther-
molysis and photolysis reactions. However, 5b sublimes very
poorly, and so only low yields of FVT products and matrix-
isolated materials can be obtained. The use of N2 as a carrier
gas and mixing of the starting material with finely powdered Cu
improved the sublimation of this substance at 70 ЊC. The opti-
mal FVT temperature was 400 ЊC. Even so, the amount of
material subliming and undergoing FVT was only ca. 5%. The
main product was identified as trimethylpyrazolone 9b.
FVT of 5b with Ar matrix isolation of the product at 7 K
gave largely unchanged starting material, but at temperatures
above 400 ЊC a new and weak peak at 2115 cmϪ1 appeared
together with CO (2139 cmϪ1) in the IR spectrum. Photolysis of
this matrix, containing largely unchanged 5b, resulted in an
increase in the 2115 cmϪ1 peak, which may be ascribed to
hydrazonoketene 7b. The IR absorptions ascribed to 7a (2130
cmϪ1) and 7b (2115 cmϪ1) are in excellent agreement with the
calculated values, 2133 and 2117 cmϪ1, respectively. All other
calculated vibrations are weak to very weak, and therefore it is
In the case of 8a the C᎐O stretching vibration is found at 1733
᎐
(neat film)11b or 1730 and 1710 (KBr)11a cmϪ1; we find 1735vs
and 1710s cmϪ1 in KBr, and 1771vs cmϪ1 as the only strong
band in the Ar matrix. As is often the case,2 the B3LYP/6-31G*
calculation predicts a C᎐O stretching vibration 37 cmϪ1 too
᎐
high for the gas phase molecule (1808 cmϪ1). The 6-31ϩG*
basis set brings the predicted value (1769 cmϪ1) into excel-
lent agreement with experiment (1771 cmϪ1) (see Fig. S1 in the
electronic supplementary information†). We have used the same
scaling factor (0.9613) for frequencies with the expanded
6-31ϩG* basis set as for the 6-31G* basis set; this may slightly
underestimate the resulting frequencies. The high value of the
C᎐O stretching frequency as well as the C4 chemical shift indi-
᎐
cate that the enolate mesomer of 8 is not the dominant reson-
ance structure. There is a significant degree of negative charge
at C4 (cf. resonance structure 3a), and a ketene-type ‘no-bond’
mesomer may contribute to the ground state of 8 as suggested
by the X-ray data4,5 (see structure 3c). This is an example of the
“structure-correlation principle” whereby the structure of a
compound may presage the transition state for its formation or
destruction, especially when the activation barrier is low.19 In
agreement with this, the calculated structure of 8a features a
not possible to identify any but the very strong C᎐C᎐O stretch-
᎐ ᎐
ing bands in the experimental spectra. The good agreement and
the fact that these bands are obtained both thermally and
photolytically and from two different precursors (5 and 8)
support their assignment to ketenes 7.
A byproduct absorbing at 2259 cmϪ1 appeared in several of
the FVT reactions of both 5a and 5b, particularly in the case of
5b, and especially at high temperatures (e.g. 700 ЊC). The carrier
of the 2259 cmϪ1 peak cannot be dimethylamino isocyanate
(Me N–N=C᎐O), which absorbs at 2215 cmϪ1 in an Ar matrix.15
very long CO–N bond (1.64 Å), a normal C᎐O bond (1.21 Å), a
᎐
wide CCO angle (142Њ), an acute OCN angle (117Њ), and dis-
tances N–N = 1.45 Å, N–C(Ph) = 1.33 Å, C(Ph)–CH = 1.42 Å,
and CH–CO = 1.39Å.
᎐
2
Attendant bands at 3517, 3506 and 770 cmϪ1 identify the
substance as isocyanic acid, HNCO.16
Tautomerism in pyrazolones
Mesoionic compounds usually exhibit 13C NMR resonances
at rather high field for the carbon atom that can formally carry
The calculated relative energies (B3LYP/6-31ϩG*; gas phase)
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 5 5 0 – 2 5 5 5
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of the isomeric molecules of interest (see Scheme 1) are as fol-
lows: 7a (46.2), 8a (25.4), 9a (0.0), 9aЈ (7.9), 9aЉ (4.7), 10a (11.6),
12 (7.5), 12Ј (5.5 kcal molϪ1). These data predict that for 9a
the most stable form will be the CH-keto form, whereas the
regioisomer 12 will exist preferably in the OH form 12Ј. Our
experimental data support this contention (see below). The
structures and composition of tautomerizable pyrazolones–
hydroxypyrazoles have been fraught with controversy and con-
fusion.20,21 In some cases, the hydroxy form predominates in the
gas phase, in the solid state, and in nonpolar and dipolar apro-
tic solvents. Depending on substitution, the NH (keto) and CH
forms may also participate in the equilibria. The proper identi-
fication of pyrazolone 9a in the reactions above necessitated an
unravelling of its tautomeric behaviour.
indefinitely stable in DMSO-d6 solution, presumably due to
H-bonding.
When the compound is dissolved in CDCl3 that has been
freed of any DCl contaminant by treatment with dry K2CO3,
the NMR spectrum obtained is that of 9a only. However,
CDCl3 usually contains varying amounts of DCl, and in such
solutions, varying ratios of 9a and 9aЉ can be observed. Com-
pound 9aЉ can acquire aromaticity by existing in a zwitterionic
mesomeric form 13, which helps explain the low wavenumbers
of the carbonyl groups in such compounds (≤ 1700 cmϪ1). It has
long been known that antipyrine (1,5-dimethyl-2-phenyl-
pyrazol-3(2H )-one) and related compounds have very high
dipole moments (of the order of 5.5 D), which imply large
contributions by zwitterionic (mesoionic) resonance structures
such as 13.24 Protonation of 13 on oxygen (or of 9aЈ on N2)
would generate a pyrazolium salt. In fact, 9aЉ crystallizes from
chloroform solution containing HCl (DCl) to form a salt 14.
We repeated the original synthesis22 of 9a by reaction of
β-ketoester 11 with methylhydrazine, separated the two prod-
1
ucts 9a and 12, and examined their structures by H and 13C
NMR and IR spectroscopy. The IR spectra of 12 in Ar and Xe
matrices demonstrate that this compound exists in the OH form
12Ј (3615m, 1533s, 1517vs cmϪ1). The crystalline solid in KBr
also exists exclusively in the OH form 12Ј (3200–2100s very
broad hydrogen bonded OH; 1536vs, 1524vs cmϪ1), and no
carbonyl group is discernible in the IR spectrum. The experi-
mental IR spectrum of 12Ј is in excellent agreement with the
B3LYP/6-31G* calculated spectra. The IR spectra of CCl4 and
CHCl3 solutions are virtually identical with the one in KBr, and
by far the major tautomer in these solvents must therefore be
the hydroxy form 12Ј, but the IR spectrum of the solutions in
CHCl3 and DMSO show an additional, very weak band near
1700 cmϪ1 which may be ascribed to a minor amount of the
keto tautomer 12. The 1H and 13C NMR spectra are essentially
those of the OH tautomer 12Ј. Adembri et al.22 and Katritzky
et al.23 had assigned the NH structure 12 to this compound on
the basis of the 1H and 13C NMR spectra, respectively.
The single crystal X-ray structure of hydrochloride 14‡ is
shown in the electronic supplementary information (Fig. S2†).
The sites of protonation were established unequivocally during
refinement as being O14 and N2. Delocalisation (aromaticity)
in the five-membered ring is apparent with bond lengths inter-
mediate of single and double bond order (C–C (1.379(6) and
1.391(6) Å), C–N (1.326(5) Å and 1.346(6) Å) and N–N
(1.352(5) Å). The C5–O14 and C4–C5 bond lengths (1.329(5)
and 1.379(6) Å, respectively) are consistent with a hydroxy-
pyrazole.25,26,27 By contrast, pyrazol-5-ones in their keto form
typically exhibit C–O and C–C bond lengths less than 1.28 and
greater than 1.42 Å, respectively, with a wide range of distances
reported according to the nature of substituents on the
five-membered ring.28,29
A feature of the structure is an H-bonded chain extending
along the direction of the b axis. The Cl ؒ ؒ ؒ H-bonds involving
the NH (H2 ؒ ؒ ؒ Cl1 2.33(6) Å, N2 ؒ ؒ ؒ Cl1 3.048(4) Å, N2-
H2 ؒ ؒ ؒ Cl1 170(6)Њ) and OH groups (H14 ؒ ؒ ؒ Cl1Ј 1.96(8) Å,
O14 ؒ ؒ ؒ Cl1Ј 2.914(4) Å, O14–H14 ؒ ؒ ؒ Cl1Ј 162(6)Њ) are of
comparable strength. The phenyl and pyrazole rings stack in a
centrosymmetric dimeric array (Fig. S3†).
The isomer 9a is more complicated. In fact, all three tauto-
meric forms, 9a, 9aЉ, and 9aЉ can be obtained selectively. To
purify the compound, 12Ј is best removed by recrystallization,
and 9a is then purified by chromatography on SiO2. The solid
compound so obtained, after removal of most of the solvent
(ethyl acetate–methanol), is the enol form 9aЈ, which shows a
strong, broad signal for a hydrogen bonded OH group at 3200–
2000 cmϪ1 but no C᎐O group in the IR (ATR and KBr).
᎐
Because this compound tautomerises to 9a on dissolution, an
NMR spectrum cannot be obtained in solution. However, an
excellent solid state 13C NMR spectrum of 9aЈ was obtained,
and this shows good agreement with the calculated spectrum
(B3LYP/6-31G*-GIAO).
After removing the solvent completely in vacuo, the com-
pound isomerises to the pure keto form 9a (C᎐O group at 1732
᎐
cmϪ1 in the Ar matrix IR spectrum; 175 ppm in the 13C NMR
spectrum). When dissolving the compound in CCl4, it largely
isomerises to 9a, but a small amount of the enol form 9aЈ is
detectable in the initial IR spectrum. After 6 hours at room
temperature, the compound has isomerised completely to the
keto form 9a. Sublimation or matrix isolation of the compound
by vaporisation and co-condensation with Ar at 10 K affords
only the keto form 9a, which is calculated to be the lower energy
isomer. Therefore, this is also the tautomer obtained in all FVT
experiments.
There are several examples of hydroxypyrazoles and pyrazol-
5-ones with a substitution pattern similar to 14 in the
Cambridge Structural Database.30 However, 14 represents the
first structurally characterized example of
a 5-hydroxy-
pyrazolium salt.
When the compound is dissolved in DMSO-d6, in contrast,
the NH tautomer 9aЉ is obtained. The IR spectrum and the 1H
and 13C chemical shifts identify this tautomer as the NH form
9aЉ, and they are in good agreement with the calculated spectra.
The NH proton in 9aЉ appears as a broad peak at ca. 10 ppm in
DMSO-d6 and with a very variable chemical shift in CDCl3.
Computational details
Unless otherwise noted, all calculations were carried out at the
B3LYP/6-31G* level of theory using the Gaussian 98 program
package31 and a scaling factor32 of 0.9613 for frequencies.
Energies of optimised geometries were calculated at both the
B3LYP/6-31G* and B3LYP/6-31ϩG* levels. The differences in
The C᎐O group appears at 1696 cmϪ1 in the IR and at 149–150
᎐
ppm in the 13C NMR spectrum. The reported 1H and 13C NMR
spectra reveal that Adembri et al.22 and Katritzky et al.23 also
observed 9aЉ, in DMSO-d6 and CD3CN, respectively, but the
former assigned the CH structure 9a to it. Compound 9aЉ is
‡ CCDC reference number 207791. See http://www.rsc.org/suppdata/
ob/b3/b304070d/ for crystallographic data in .cif or other electronic
format.
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 5 5 0 – 2 5 5 5
2552

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energies obtained from the two basis sets were very minor, and
only those from the B3LYP/6-31ϩG* calculations are reported.
Zero point vibrational energy corrections are included. The
data for the ketenes 7 refer to the s-Z conformers shown in
Scheme 1 with syn configuration around the imine link (NMe2
syn to the phenyl group). The s-Z and s-E conformers of
imidoylketenes are usually very close in energy, and, depending
on substituents, either conformer may be of lower energy.33
Calculations on α-oxoketenes and imidoylketenes have been
published previously.34 For the vibrational data reported below,
relative intensities are given in parentheses or as absolute values
in km molϪ1 where so indicated. NMR chemical shifts in ppm
relative to TMS were calculated using B3LYP/6-31G*-GIAO.
Peaks are listed in the numerical order of the corresponding
atom numbers, and the following numbering is used: pyrazole
ring: N1, N2, CO(3), C(Me)(4), C(Ph)(5); phenyl ring: atoms
6–11 (shifts for atoms 10–11 not listed); methyl groups: atoms
12 (on N) and 13 (on C). Hydrogen atoms are numbered as the
heavy atoms they are attached to, OH is number 14, and the
phenyl group hydrogens are not listed.
12: 13C NMR 159.2, 109.3, 153.0, 125.2, 123.5, 121.8, 122.4,
39.8, 10.4. 1H NMR 5.3, 2.6, 1.8.
12Ј: 13C NMR 150.8, 95.4, 138.0, 125.8, 123.8, 121.7, 120.8,
35.1, 9.4. 1H NMR 3.3, 1.8, 4.1.
Experimental section
General methods for flash vacuum thermolysis (FVT), matrix
isolation and photolysis have been reported previously.10,35 Ar
matrix isolation experiments were done at 7–10 K. In pre-
parative FVT experiments samples were sublimed into the
thermolysis tube at 65–70 ЊC at a vacuum of ca. 2 × 10Ϫ4 mbar,
and samples of 5 were sometimes mixed with Cu powder (1 : 1)
to improve heat conductivity and sublimability. The unfiltered
light from a 1000 W high pressure Hg/Xe lamp was used for
the irradiations. GC-MS was recorded on a Hewlett-Packard
instrument 5970 equipped with a BP5 capillary column (30 ×
1.25 mm, phase thickness 0.25 mm); detector temperature
280 ЊC; column temperature programmed from 100 to 250 ЊC at
16Њ per minute. Melting points are uncorrected. IR spectra were
obtained in Ar matrix, KBr, ATR (attenuated total reflexion)
1
Relative energies. (B3LYP/6-31ϩG*; kcal molϪ1): 7a (46.2),
8a (25.4), 9a (0.0), 9aЈ (7.9), 9aЉ (4.7), 10a (11.6), 12 (7.5), 12Ј
(5.5). Absolute energy of 9a: Ϫ610.905379 Hartree.
of solids, or in solution as indicated. H NMR spectra were
recorded at 400 or 200 MHz, and 13C NMR spectra at 100 or
50 MHz. Chemical shifts are on the δ scale.
Crystallography
Infrared spectra. 7a (s-Z-syn conformer shown): 2875 (16),
2133 (100; abs. int. 981), 1561 (12), 1381 (10), 1194 (3), 1046 (4),
Cell constants were determined by least-squares fits to the
setting parameters of 21 independent reflections measured on
an Enraf-Nonius CAD4 four-circle diffractometer employing
graphite-monochromated Mo Kα radiation (0.71073 Å) and
operating in the ω-θ scan mode. Data reduction was performed
with the WinGX package.36 The structure was solved by direct
methods with SHELXS and refined by full-matrix least-squares
analysis with SHELXL-97.37 All non-H atoms were refined
with anisotropic thermal parameters. The H atoms attached to
the N– and O-atoms were located from difference maps then
refined isotropically. All other H-atoms were included in esti-
mated positions using a riding model. A drawing of the mole-
cule was produced with ORTEP38 and the packing diagram was
created with PLUTON.39
974 (7), 685 (4), 644 (3), 535 (2) cmϪ1
.
7b (s-Z-syn conformer shown): 2874 (17), 2117 (100; abs. int.
846), 1559 (2), 1474 (2), 1346 (3), 1255 (5), 1201 (2), 998 (4), 763
(2), 685 (3) cmϪ1
.
8a: 3092 (1), 3081 (4), 3070(1), 3066 (1), 2970 (3), 2965 (1),
1808 (100), 1497 (2), 1480 (17), 1473 (2), 1462 (2), 1446 (1), 1433
(16), 1415 (2), 1406 (15), 1221 (2), 1183 (1), 1169 (1), 1127 (2),
987 (1), 955 (3), 913 (1), 811 (1), 716 (7), 683 (5), 655 (2), 650 (1),
615 (1), 595 (1), 572 (3), 615 (1), 511 (4) cmϪ1
.
8a (B3LYP/6-31ϩG*): 3090 (1), 3079 (3), 3062 (1), 2968 (2),
2963 (1), 1769 (100), 1491 (2), 1470 (15), 1468 (1), 1458 (1), 1441
(1), 1429 (12), 1400 (17), 1216 (12), 1181 (2), 1167 (1), 1130 (2),
952 (2), 911 (1), 808 (1), 716 (6), 679 (6), 654 (2), 616 (2), 605 (1),
572 (2), 521 (4) cmϪ1
.
9a: 2940 (14), 1743 (100), 1479 (60), 1226 (22), 1098 (15), 970
1,1-Dimethylamino-5-phenylpyrrole-2,3-dione 5a
(9), 754 (7), 682 (6), 549 (6) cmϪ1
.
A solution of 1.9 g (15 mmol) of oxalyl chloride in 2 mL of
acetonitrile was added dropwise to a mixture of freshly pre-
pared acetophenone N,N-dimethylhydrazone40 (2.43 g; 15
mmol) and dry triethylamine (3.04 g; 30 mmol) in 20 mL of
acetonitrile under N2. The reaction is exothermic, and the
addition rate should be such that no overheating occurs. The
colour of the mixture changed from yellow to dark red, and a
precipitate was formed. The mixture was stirred at 20 ЊC for 5 h,
the precipitate was filtered, and the dark red solution was evap-
orated in vacuo. The resulting red oil was taken up in dry ether,
and the solution was filtered and again evaporated in vacuo.
Addition of 0.3 mL of dry ether and 1 mL of ether–hexane 1 : 1
caused crystallization with the aid of scratching. Yield: 0.65 g
(20%); mp 81–82 ЊC; GC-MS Rt = 10.0 min; m/z 216 (4%), 188
(100), 173 (30), 145 (30), 116 (20), 102 (32), 86 (20), 57 (33),
43 (65); 1H NMR (CDCl3) 7.73–7.47 (m, 5H), 5.52 (s, 1H), 2.86
(s, 6H); 13C NMR (CDCl3) 181.3 (s), 171.4 (s), 150.0 (s), 132.6
(s), 129.0 (d), 128.5 (d), 127.8 (d), 99.5 (s), 44.0 (q). IR (Ar
matrix, 8 K) 1764s, 1728vs cmϪ1. Anal. Calcd for C12H12N2O2:
C, 66.66; H, 5.59; N, 12.95. Found: C, 66.46; H, 5.85; N,
13.16%.
9aЈ: 3616 (43), 3079 (34), 2944 (42), 2902 (43), 1583 (50),
1564 (75), 1539 (64), 1441 (21), 1374 (100), 1290 (20), 1270 (22),
1231 (30), 1154 (61), 1003 (43), 691 (27), 220 (42), 196 (44)
cmϪ1
.
9aЉ: 3329 (1), 3098 (1), 3089 (4), 3082 (4), 3071 (1), 2983 (7),
2977 (3), 2925 (13), 2920 (6), 1714 (100), 1619 (4), 1486 (2), 1457
(2), 1450 (2), 1436 (2), 1394 (7), 1359 (18), 1247 (9), 1226 (3),
1206 (2), 1158 (3), 1119 (2), 1050 (7), 980 (3), 807 (4), 796 (20),
755 (4), 734 (6), 706 (10), 686 (4), 617 (3), 595 (3), 286 (4) cmϪ1
.
9b: 3084 (8), 3011 (4), 2998 (5), 2941 (17), 1738 (100), 1478
(7), 1389 (4), 1379 (14), 1303 (6), 1287 (6), 1230 (26), 1033 (12),
1023 (5) 948 (6), 710 (7), 681 (7), 548 (8) cmϪ1
.
10: 3085 (3), 3009 (3), 2983 (6), 2926 (10), 2923 (8), 1723
(100), 1562 (4), 1481 (3), 1465 (3), 1348 (13), 1317 (11), 1228 (5),
1161 (5), 1121 (3), 788 (4), 753 (7), 687 (3) cmϪ1
.
12: 3435 (4), 1732 (100), 1609 (5), 1170 (10), 1134 (6), 994 (4),
750 (5), 736 (9), 509 (24) cmϪ1
.
12Ј: 3581 (26), 3082 (13), 2943 (28), 2922 (19), 1563 (11), 1516
(100), 1500 (23), 1484 (15), 1468 (12), 1299 (24), 1198 (30), 1158
(60), 1051 (14), 749 (15), 414 (29), 400 (21) cmϪ1
.
NMR spectra. 9a: 13C-NMR 163.9, 41.8, 151.6, 125.3, 120.0,
121.5, 122.2, 30.1, 17.1. 1H NMR 3.0, 3.2, 1.3.
9aЈ: 13C NMR 141.0, 87.6, 142.2, 128.8, 119.5, 120.5, 119.6,
34.4, 11.1. 1H NMR 3.5, 1.9, 3.8.
1,1-Dimethylamino-4-methyl-5-phenylpyrrole-2,3-dione 5b
This compound was prepared analogously to 5a from propi-
ophenone N,N-dimethylhydrazone. The dark red product was
recrystallized from toluene–hexane to yield 1.86 g (54%); mp
146–148 ЊC; GC-MS Rt = 10.0 min; m/z 230 (10), 202 (100), 187
9aЉ: 13C NMR 158.3, 107.8, 145.1, 125.1, 122.6, 121.9, 122.4,
31.4, 10.7. 1H NMR 4.5, 3.0, 1.9.
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 5 5 0 – 2 5 5 5
2553

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(34), 159 (70), 130 (34), 115 (34), 103 (20), 77 (31), 58 (28), 43
(70); 1H NMR 7.51–7.42 (m, 5H), 2.80 (s, 6H), 1.74 (s, 3H); IR
(Ar matrix, 8 K) 1756s, 1717vs cmϪ1. Anal. Calcd for
C13H14N2O2: C, 67.81; H, 6.12; N, 12.53. Found: C, 67.41; H,
6.22; N, 11.98%.
amount of 9a coexists besides (protonated) 9aЉ: IR (CDCl3/
DCl) 1696s cmϪ1; 1H NMR (CDCl3/DCl): 1.88 (s, 3H), 3.43 (s,
3H), 7.3–7.6 (5H), 9.0 (br, 1H, exchanges with D2O, variable
shift); 13C NMR (CDCl3/DCl): 148.76, 142.17, 133.47, 130.48,
128.81, 128.32, 101.76, 34.81, 8.25.
Pyrazolium salt 14. Compound 9aЉ crystallizes from chloro-
form (deuterochloroform) containing HCl (DCl) to afford a
hydrochloride 14, mp 194–196 ЊC, the structure of which was
determined by X-ray crystallography. Anal. Calcd for C11H13-
N2Cl: C, 58.79; H, 5.84; N, 12.47. Found: C, 58.16;, H, 5.48; N,
12.39%.
1,1-Dimethyl-3-phenylpyrazolium-5-oxide 8a
This compound was prepared from 6 according to the liter-
ature.11 GC-MS Rt = 9.5 min; m/z 188 (100%), 173 (26), 159
(16), 145 (23), 116 (13), 102 (29), 89 (10), 86 (19), 77 (39),
51 (30), 43 (62); IR (KBr) 1735vs, 1710s, 1458m, 700m cmϪ1
;
IR (Ar, 10 K) 2951w, 1771vs, 1490m, 1450m (center of several
bands), 1416m (center of several bands), 1254w, 1237w,
1199w, 1177w, 1147w, 1059w, 977w, 930w, 738m, 694m, 675w,
Crystal data‡. C11H13ClN2O, M = 224.68, triclinic, space
¯
group P1, a = 7.348(2), b = 8.642(2), c = 9.832(3) Å, α =
646w-m, 588m, 554w cmϪ1
;
1H NMR (CDCl3) 7.83–
103.70(2), β = 109.61(2), γ = 97.53(2)Њ, U = 556.1(3) Å3, Z = 2,
Dc = 1.342 g cmϪ3, µ = 3.18 cmϪ1, 2115 reflections measured,
1948 unique (Rint = 0.0423), R1 = 0.0692 (for 1101 observed
data, I > 2σ), wR2 = 0.2244 (all data). Crystallographic data for
compound 14 (in CIF format) have been deposited with the
Cambridge Crystallographic Data Centre CCDC reference
number 207791.
7.77 (m, 2H), 7.55–7.35 (m, 3H), 4.76 (s, 1H), 3.05 (s, 6H); 13
C
NMR (CDCl3) 178.8, 178.7, 131.5, 131.3, 128.6, 127.0, 68.9 (d),
47.9 (q).
1,4-Dimethyl-3-phenylpyrazol-5(4H)-one 9a, 1,4-Dimethyl-
3-phenyl-5-hydroxypyrazole 9aЈ and 1,4-Dimethyl-3-phenyl-
pyrazol-5(2H)-one 9aЉ
1,4-Dimethyl-5-phenylpyrazol-3(2H)-one 12 and 1,4-Dimethyl-
5-phenyl-3-hydroxypyrazole 12Ј
Pyrazolone 9a was prepared as the minor product of the
reaction between methylhydrazine and 11 as described in the
literature.22 The compound was separated from 12Ј by recrystal-
lization of 12Ј from benzene as described in the original prepar-
ation;22 however, due to its toxicity, the use of benzene is
discouraged. Flash chromatography on SiO2 using gradient
elution, starting with pure ethyl acetate and ending with ethyl
acetate–methanol (ratio 4 : 1) followed by rapid and incomplete
evaporation of the solvent gives the OH form 9aЈ: IR (KBr)
3200–2000s v.br, 1586s, 1565s, 1540s, 1463m, 1449m, 1378m,
1293m, 1267m, 1251m, 1184m, 1047w, 771m, 697s cmϪ1; IR
(ATR) 3200–2100 v.br, 1584m, 1564m, 1537m, 696s; 13C NMR
(solid state) 152.4, 148.9, 131.3, 128.8 (several aromatic carb-
ons), 91.7, 32.8, 5.6.
This isomer is obtained as the major product of the reaction of
11 with methylhydrazine and purified by recrystallization from
benzene (toluene).22 The compound exists largely as the OH
form 12Ј: IR (Ar matrix, 7 K) 3615m, 1533s, 1517vs, 1328m,
1165s, 1014m 764m, 700m-s cmϪ1; IR (KBr) 3200–2100s v.br,
1536vs, 1524vs cmϪ1; IR (CCl4) 3200–2100s v.br, 1537vs, 1522s
cmϪ1; IR (CHCl3) 3200–2100 v.br, 1697vw, 1533s, 1520s cmϪ1
;
IR (DMSO) 1709w, 1528s, 1509vs cmϪ1; H NMR (CDCl3) 8
(br s, 1H; very variable chemical shift between 6 and 10 ppm),
7.47–7.40 (m, 3H), 7.34–7.24 (m, 2H), 3.61 (s, 3H), 1.91 (s, 3H);
1H NMR (DMSO-d6) 9.51 (s, 1H), 7.53–7.34 (m, 5H), 3.48 (s,
3H), 1.76 (s, 3H); 13C NMR (CDCl3) 160.0, 142.8, 130.0, 129.5,
128.6, 128.3, 98.8, 36.0, 6.85; GC-MS (12 and/or 12Ј) Rt = 8.8
min (broad); m/z 188 (100), 187 (90), 171 (3), 159 (6), 143 (10),
130 (7), 116 (8), 115 (33), 111 (28), 103 (5), 91 (8), 63 (7), 51 (10),
43 (7).
1
When the solvent is completely removed from 9aЈ and the
solid is dried in vacuo overnight, it tautomerises to the CH form
9a. When the melting point of 9aЈ is determined, the compound
tautomerises to 9a, mp 128–130 ЊC (lit.22 128–129 ЊC; lit23 130–
132 ЊC).
FVT of 5a
Sublimation of the compound from the chromatography
or generation by FVT of 5a or 8a with isolation in Ar matrix at
ca. 10 K affords only the CH form 9a: IR (Ar, 8 K) 1732s,
N-Aminopyrroledione 5a was subjected to FVT at 400 ЊC with
a sublimation temperature of 65 ЊC. The products were separ-
ated by dry-column chromatography on SiO2 into 8a and 9a
(1 : 4), which were identified by comparison with the NMR and
IR data reported above. In addition, traces of 10a were detected
1475w, 1378w, 1347m, 1239m, 765w, 694m, 560w cmϪ1 1H
;
NMR (CDCl3) 1.46 (d, 3H), 3.39 (s, 3H), 3.54 (q, 1H), 7.40
(m, 2H), 7. 62 (m, 2H), 7.95 (m, 1H); 13C NMR (CDCl3)
175.68, 159.20, 129.75, 128.65, 127.17, 126.05, 42.74, 31.31,
14.49; GC-MS Rt = 8.5 min; m/z 188 (90), 173 (10), 159 (10),145
(40), 130 (22), 130 (21), 117 (100), 115 (69), 103 (95), 91 (35), 77
(69), 51 (65), 43 (40).
1
by its characteristic H NMR signal at 5.6 ppm due to the
methine proton.14
FVT of 6
Dissolution of the solid from the chromatography in CCl4
(not well soluble) also affords largely 9a: IR (CCl4) 1714s (a
small amount of the enol 9aЈ is detectable in the initial IR
spectrum but has disappeared after 6 h at room temperature);
1H NMR (CCl4) 1.39 (d, 3H), 3.29 (s, 3H), 3.33 (q, 1H). 13C
NMR (CCl4): 174.21, 158.84, 131.00, 128.24, 128.08, 127.24,
126.66, 41.89, 31.06, 14.50.
Dissolution of the solid from chromatography in DMSO-d6
affords 9aЉ: 1H NMR (DMSO-d6) 2.01 (s, 3H), 3.56 (s, 3H), 7.3–
7.6 (5H), 10.0 (br s, 1H, variable shift); 13C NMR (DMSO-d6)
149.94, 146.16, 135.03, 128.23, 126.64, 126.38, 93.15, 33.49,
8.43. In DMSO-d6 solution there is no tautomerization to
9a; only the NH tautomer 9aЉ is present. The calculated 13C
NMR spectra for 9a and 9aЉ are in good agreement with the
experimental data.
The hydrazone 6 was subjected to FVT at 600 ЊC, and the
products were analysed by GC-MS as a 1 : 1 mixture of 8a and
9a together with unchanged starting material 6.
FVT of 8a
Mesoion 8a was subjected to FVT at 500 ЊC with a sublimation
temperature of 70 ЊC. 9a was isolated by dry-column chromato-
1
graphy and identified by GC-MS, IR and H NMR spectro-
scopy.
FVT of 5b
N-Aminopyrroledione 5b was mixed with finely powdered Cu
and subjected to FVT at 400 ЊC with a sublimation temperature
of 70 ЊC. N2 was used as a carrier gas at ca. 10Ϫ3 mbar. Due to
the involatility of 5b only ca. 5% of the material underwent
FVT. The main product was separated by thick-layer plate
chromatography and identified as 1,4,4-trimethyl-3-phenyl-
A solution in pure CDCl3 that has been treated with dry
K2CO3 shows only 9a: 1H NMR (CDCl3) 1.46 (d, 3H), 3.39 (s,
3H), 3.55 (q, 1H). In CDCl3 containing traces of DCl a lesser
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 5 5 0 – 2 5 5 5
2554

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Tetrahedron Lett., 1995, 36, 6547; (d ) H. Briehl, A. Lukosch and
pyrazol-5(4H )-one 9b by its GC-MS behaviour and by
1
comparison of its H NMR spectrum with literature data.8,41
C. Wentrup, J. Org. Chem., 1984, 49, 2772; (e) A. N. Maslivets,
O. P. Krasnykh, L. I. Smirnova and Y. S. Andreichikov, J. Org.
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15 A. Lukosch, PhD Thesis 1985, Diploma Thesis 1981, Universität
Marburg, Germany.
GC-MS Rt 8.14 min; m/z 202 (100), 187 (22), 159 (30), 144 (20),
131 (80), 115 (45), 104 (30), 91 (46), 77 (36), 51 (31), 43 (32); IR
(Ar, 8 K) 1728 cmϪ1; 1H NMR (CDCl3) 7.85 (m., 2H), 7.65 (m,
3H), 3.30 (s, 3H), 1.38 (s, 6H).
Acknowledgements
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We thank the Karl-Franzens Universität Graz for a Gandolph-
Dölter-Stipendium as well as a Förderungsstipendium der
Naturwissenschaftlichen Fakultät for B. W. and the Studien-
beihilfenbehörde/Stipendienstelle Graz for an Ausland-
stipendium for S. E., enabling their stays in Brisbane, The
University of Queensland for a postdoctoral fellowship for
J. A., and the Australian Research Council for support.
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O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 5 5 0 – 2 5 5 5
2555