Microwave-assisted nucleophilic cleavage of 5,7-dimethyl-2-phenyl-1-oxo-1H- pyrazolo[1,2-a]pyrazol-4-ylium-3-olate to α-phenylmalonamides

Article abstract of DOI:10.1002/jhet.5570420716

Three α-phenylmalonamides have been prepared by the selective nucleophilic cleavage of 5,7-dimethyl-2-phenyl-1-oxo-1H-pyrazolo[1,2-a]pyrazol- 4-ylium-3-olate in solventless microwave syntheses. The three weak nucleophiles employed were aniline, p-chloroan

Full text of DOI:10.1002/jhet.5570420716

Nov-Dec 2005
1363
Microwave-assisted Nucleophilic Cleavage of 5,7-Dimethyl-2-phenyl-
1-oxo-1H-pyrazolo[1,2-a]pyrazol-4-ylium-3-olate to α-Phenylmalonamides
Daniel E. Lynch [a], Gillian E. Spicer [a], and Ian McClenaghan [b]
[a] School of Science and the Environment, Coventry University, Coventry CV1 5FB, UK
[b] Key Organics Ltd, Highfield Industrial Estate, Camelford, Cornwall PL32 9QZ, UK
Received April 12, 2005
Three α-phenylmalonamides have been prepared by the selective nucleophilic cleavage of 5,7-dimethyl-
2-phenyl-1-oxo-1H-pyrazolo[1,2-a]pyrazol-4-ylium-3-olate in solventless microwave syntheses. The three
weak nucleophiles employed were aniline, p-chloroaniline and m-toluidine. The α-phenylmalonamides of
these three aniline derivatives could not be prepared using the previously reported solvent syntheses via 3-
oxopyrazolo[1,2-a]pyrazol-8-ylium-1-olates. All products were characterised using, infrared spectroscopy,
1
H nmr and electrospray mass spectrometry. The single crystal X-ray structures of the starting pyrazolo-
[1,2-a]pyrazole and α-phenylmalon-m-toluidide are also reported.
J. Heterocyclic Chem., 42, 1363 (2005).
Introduction.
Friedrichsen [2] reported five derivatives of 3, which he
called 1-oxo-1H-pyrazolo[1,2-a]pyrazol-4-ium-3-olates;
prepared by the reaction of malonyldichlorides (1b) and
substituted pyrazoles. In 1982, Zvilichovsky and David
[3] formed more substituted derivatives of 3 by the con-
densation of 1,3-dicarbonyl compounds with 3,5-dihy-
droxy-4-phenylpyrazole and its analogues. It was in this
paper that compounds of 3 were found to decompose
under the influence of bases and amines and the kinetics of
decomposition with morpholine were studied. Two main
breakdown routes, and the subsequent formation of the
breakdown products, were dependant on several factors
In 1980 K.T. Potts et al. [1] investigated the reaction of
the strong bielectrophile, (chlorocarbonyl)phenylketene
(1a), with pyrazole (2a), which led to the formation of a
new type of heterocyclic zwitterion, anhydro-1-hydroxy-
3-oxo-2-phenylpyrazolo[1,2-a]pyrazolium hydroxide (3a)
(Figure 1). Compound (3a) could also be produced by the
reaction of malonaldehyde diethyl acetal and 3,5-dihy-
droxy-4-phenylpyrazole when the reaction was catalysed
by H SO . Phenyl-, methyl- and methylthio-substituted
2
4
pyrazoles and benzopyrazole derivatives gave substituted
derivatives of 3. A concurrent paper in 1980 by W.
Figure 1. Selected synthetic routes to 3-oxopyrazolo[1,2-a]pyrazol-8-ylium-1-olates.
Figure 2. Suggested breakdown pathway for the decomposition of 3 with nucleophiles.

1364
D. E. Lynch, G. E. Spicer, and I. McClenaghan
Vol. 42
including the presence of water in the reaction solution and
the substituents on 3. Continuing work by Potts et al. [4]
altered the naming scheme of 3 to 3-oxopyrazolo[1,2-
a]pyrazol-8-ylium-1-olate, similar to that of Friedrichsen.
In Potts et al.’s final major work on 3 [4d], decomposition
with nucleophiles (primarily morpholine, aniline or water)
was studied in more detail and a breakdown pathway sug-
gested (Figure 2). This current work covers the develop-
ment of methods to improve the general preparation of
malonamide derivatives from the nucleophilic cleavage of
3, thus microwave driven decomposition was investigated.
et al. [4d] found that 3b decomposed by nucleophilic
cleavage over several days at 25 ˚C, compared to a few
seconds for the unsubstituted derivative. For this current
s
tudy, three weak nucleophiles (aniline,
p-chloroaniline
and m-toluidine) where chosen because it was not possible
to prepare the three aniline derivatives, to the same level of
quality as the heterocyclic amines, using the boiling sol-
vent and precipitation methods previously employed by
Potts et al. [4d]. With the anilines, minimal product was
formed after 5 – 6 days of reflux in THF. In this current
study, all compounds were characterised using infrared
Figure 3. Chemical diagrams of the α-phenylmalonamides prepared in this study.
1
spectroscopy, H NMR and electrospray mass spectrome-
Vast reductions in both reaction times and thermal
decomposition products are the two main advantages of
microwave synthesis. In recent years interest into the suit-
ability of microwave irradiation for the synthesis of
organic compounds has greatly increased [5]. For exam-
ple, Veverkova et al. [5d] synthesised 9-sustituted acridine
derivatives in a 120 W microwave shortening the reaction
time from 20-40 hours for the conventional method to 3-8
minutes, with yields between 60 – 80 %. Aniyappan et al.
[6] synthesised derivatives of 1,4-dihydropyridines in a
domestic microwave with reaction times of 0.75 – 3 min-
utes (pulsed every 15 seconds) and produced very high
yields. Furthermore, Karale et al. [7] successfully pre-
pared derivatives of 3-methyl-4-[(chromon-3-yl)-methyl-
ene]-1-phenylprazolin-5-(4H)-one without solvent or the
required catalysts using a domestic microwave, taking
only 2 – 5 minutes and in good yields. Massicot et al. [8]
also prepared tartramides in a solvent-free synthesis using
microwave activation while selective peptide bond cleav-
age in acid solutions has been achieved by Wu et al. [9]
through the use of microwave techniques. Many
microwave syntheses have been performed using polar
solvents, such as alcohol, DMF and water as these medi-
ums efficiently transfer absorbed energy to the reaction.
Solvent free synthesis has the advantage of saving on
materials and time, as there is no solvent to remove after
completion of the reaction, thus increasing overall reaction
efficiency.
try. The single crystal X-ray structures of 3b and α-phenyl-
malon-m-toluidide 5c (Figure 3) are also reported.
Results and Discussion.
Preparation and Structure for 3b.
The synthesis of 3b was adopted from the method of
Kappe and Kos [10] where derivatives of 3 were prepared
by the reaction of substituted diethyl malonates with sub-
stituted pyrazoles in a 2:1 ratio. Compound 3b was pre-
pared from 1c and 2b with the loss of two molar equiva-
lence of ethanol. Diphenyl ether was used as the solvent
because of its high boiling point (bp 258 – 260 ˚C), which
took the reaction over 200 ˚C, instead of relying on the
boiling point of the malonate (bp 201 ˚C). The use of this
particular solvent meant that the reactant ratios could be
made more equivalent although the ethanol produced had
to be irreversibly removed because its presence in the reac-
tion solution lowered the boiling point of the solvent and
dramatically reduced the formation of product.
Furthermore, the ease of sublimation of 2b meant that
some of this reactant was lost in the open system although
use of a distillation head allowed most of the sublimed
material to be recollected. Yield using this procedure (as
opposed to that outlined in reference [10]) was also
improved by adding the thermally unstable malonate only
when the solvent containing 2b had begun to boil.
1
The infrared, H nmr and melting point data for 3b have
been previously reported by three separate groups
[2,4d,10] but the electrospray mass spectroscopy (es-ms)
results have not. In the es-ms spectrum a dominant molec-
Reported here are the results of how the use of
microwave synthesis was employed to significantly
improve the breakdown reaction rate of 3b. This specific
analogue was chosen because of its very low rate constant
in reactions with morpholine, a strong nucleophile. Potts
+
ular ion (M+H ) peak at m/z 241 is observed as well as
+
+
lesser (M+Na ) (m/z 263) and (2M+Na ) (m/z 503) peaks.

Nov-Dec 2005
Microwave-assisted Nucleophilic Cleavage
1365
Table 1
Es-ms experiments for 3b were conducted analogous to
Selected bonds distances (Å) for 3a [12] and 3b.
those done for a series of squaraine dyes where self-assem-
bled superstructures of doubly charged (xM+2Na ) ions of
up to x = 10 were formed [11]. Compounds such as 3 and
+
3a
3b
squaraines share similar molecular characteristics such as
C1–O1
C1–C2
C1–N8
C2–C3
C3–O3
C3–N4
N4–C5
N4–N8
C5–C6
C6–C7
C7–N8
1.215(3)
1.410(4)
1.491(3)
1.219(2)
1.409(2)
1.478(2)
1.422(2)
1.219(2)
1.481(2)
1.352(2)
1.352(2)
1.387(2)
1.389(2)
1.352(2)
-
resonant C–O moieties which are suspected to create a
+
coordination sphere around the Na ions. Experimentally,
1.411(3)[a]
1.216(3)[a]
1.492(4)[a]
1.336(4)[a]
1.340(3)
1.372(5)[a]
1.386(5)
1.335(4)
collection of the es-ms spectra required the use of dry
injection solvents. Langley et al. [11], observed that the
presence of water in the injection solvent hindered the self-
assembly process, leaving only a molecular ion peak and
possibly a 2M species.
The X-ray crystal structure of 3a has been previously
published [12] and the bond distances have generated sig-
nificant discussion about the resonant nature of these com-
pounds. The structure of 3a comprises an essentially flat
molecule with a 7.3º dihedral angle between the fused
pyrazole rings and the phenyl ring. The structure of 3a
resides on a two-fold axis thus only half of the molecule is
unique. The structure of 3b (Figure 4) comprises a full
molecule with a dihedral angle of 16.61(5)º. Table 1 lists
[a] generated distances.
being between expected pyrazole C–N bonds [av.
1.359(12) Å] [13] and C=N bonds [av. 1.331(14) Å] [13].
All bond lengths in both compounds are consistent with
the charge delocalisation over the molecule with the nega-
tive charge across the β-diketone enolate and phenyl ring
and the positive charge over the pyrazolium ring. This
charge delocalisation explains the shorter C–N bond
lengths in the pyrazolium ring, where the positive charge is
delocalised over the whole pyrazolium ring making the
bonds in this part of the molecule very strained and shorter.
In the other ring, where the negative charge is mostly delo-
calised through the O–C–C–C–O moiety, the C–N bonds
are correspondingly longer.
Nucleophilic Cleavage and Structure for 5.
The microwave preparation of the α-phenylmalon-
amides, one of the breakdown products of 3b, required the
heating of 3b and a nucleophile in a high-walled sealed ves-
sel in a microwave for 3 minutes. Most of the produced
pyrazole sublimed up the sides of the reaction vessel, hence
the use of the high walls, so extraction of the malonamide
using minimal THF, precipitation and subsequent washing
with a 2b miscible solvent (such as ethanol) yielded high
purity products. The microwave synthesis method devel-
oped for the aniline-based compounds in this study was
extremely efficient and gave very high yields for the three
malonanilides. One specific advantage in these reactions,
and the predominant reason for their high reaction effi-
ciency, is the production of a by-product with a high subli-
mation rate thus 2b could be separated from the malon-
amides as the reaction proceeded. The use of a sealed high-
walled vessel allowed the pyrazole to condense and crys-
tallise in an area of the container away from the malon-
amide. The optimal reaction time of 3 minutes was deter-
mined by carrying out small-scale reactions using 3b and
morpholine at different times from 30 seconds to 12 min-
utes (pulsed every 30 seconds) and then running a thin layer
chromatography plate using ethyl acetate as the solvent.
Figure 4. Molecular configuration and atom numbering scheme for 3b,
showing 50% probability ellipsoids.
selected bond distances for the structures of both 3a and
3b. The bond lengths obtained for 3b are highly consistent
with those for 3a, with the only molecular difference
between the two compounds being the inclusion of the two
-
methyl groups in 3b on C5 and C7. The resonant C–O
bonds in both compounds are slightly longer than the aver-
age C=O bond length of 1.200(12) Å [13], being between
1.215(3) - 1.219(2) Å. The N–N bond lengths for 3a and
3b are slightly shorter than the average N–N single bond of
1.366(19) Å [13]. There is no difference between the
C1–C2 and C2–C3 bond lengths in both compounds
[1.409(2) – 1.422(2) Å] and the similar bond lengths in
pyrazole [av. 1.412(16) Å] [13]. Bond lengths for C1–N8
and C3–N4 are close to C–N single bonds [av. 1.498(18)
Å] [13] but the C5–N4 and C7–N8 bonds are much shorter,

1366
D. E. Lynch, G. E. Spicer, and I. McClenaghan
Vol. 42
From these experiments it was found that up to 2.5 minutes,
3b was still present but after 3.5 minutes the reactants
started to char/decompose and the malonamide could not
be recovered. Visually, 3 minutes was when all of the red
colouration from 3b disappeared. No solvent was required
for any of the reactions in the microwave because the first
two nucleophiles used are all liquids at room temperature
and p-chloroaniline although a solid, has a melting point 69
– 72 °C, and quickly turned to liquid upon irradiation in the
microwave. Interestingly, when the morpholine
microwave experiments were scaled up the resultant α-
phenylmalonamide could not be obtained in high yield. In
summary, these results show that the stronger nucleophiles,
but weaker acids, are best suited to a solvent reflux reaction
whereas the weaker nucleophiles, but stronger acids, react
well using microwave techniques. The corresponding pK
a
values for each of the nucleophiles are 8.33 (morpholine),
4.63 (aniline), 4.15 (p-chloroaniline) and 4.73 (m-tolui-
dine) showing a significant difference between the hetero-
cyclic amine and the anilines. Further studies are planned
to investigate the performance in the microwave of nucle-
Figure 5. Molecular configuration and atom numbering scheme for 5c,
showing 50% probability ellipsoids.
ophiles with pK values between 5 – 8.
a
Two characteristic bands are commonly observed in the
infrared spectra of amides, these are the carbonyl stretching
-1
vibration between 1690 – 1650 cm and the N-H stretching
-1
absorption around 3300 cm . The frequency of the carbonyl
vibration is lower for amide C=O than for esters or ketones
-1
and can be reduced by as much as 20 – 60 cm [14,15]. The
carbonyl peaks for the secondary amides 5a – 5c have an
-1
expectedly higher range of 1679 – 1641 cm . The larger than
expected reduction in C=O position for these types of amides
may be due to the constraining effects of hydrogen-bonding
associations to the carbonyl oxygen atoms, observed in the
crystal structure of 5c. Interestingly, each of the carbonyl
-1
peaks are split with differences of 20 – 45 cm between the
separate peaks, which is not uncommon for amide C=O peaks
[14]. The infrared spectra of the secondary amides 5a – 5c
-1
also show N–H stretching frequencies 3310 – 3258 cm .
1
Standard H nmr spectra was obtained for all of the malon-
+
amides. In terms of es-ms analysis, self-assembly with Na
ions was also a possibility with the presence of the two car-
bonyl oxygens per malonamide thus each of the spectra were
run analogous to the Langley et al. [11] experiments. All
Figure 6. Molecular packing for 5f. Hydrogen-bonding interactions are
1
1
shown as dotted lines; N4–H—-O3 2.927(8) Å (x, -y + 1 / , z + / ) and
2
2
1
1
N12–H—-O1 2.891(8) Å (x, -y + 1 / , z + / ).
2
2
+
+
+
spectra show (M+H ), (M+Na ) and (2M+Na ) peaks except
+
for 5b and 5c that do not show (M+H ). However, the inter-
ously reported structures similar to the 5 series of molecules
are α-phenylmalonamide [18], α-phenylmalonpiperadide
and α-phenylmalonmorpholide [19].
esting aspect of the behaviour of these compounds in es-ms
analysis is the increased occurrence of the (xM+Na ) species,
+
which in all spectra are significantly stronger than the
+
(M+H ) species. The X-ray crystal structure of 5c is shown
in Figure 5 while a packing diagram of 5c showing the
EXPERIMENTAL
is given in Figure
N–H—-O hydrogen-bonding interactions
6. Direct syntheses and varying chemical data for 5b [16] and
5c [17] have been previously published but the only previ-
3,5-Dimethylpyrazole, diethyl phenylmalonate, diphenyl
ether, aniline, p-chloroaniline and m-toluidine were obtained

Nov-Dec 2005
Microwave-assisted Nucleophilic Cleavage
1367
Table 2
Crystallographic details for 3b and 5c.
pipetted into 100 mL of cool deionised water. Collection in
vacuo followed by washing with minimal amounts of cool
ethanol yielded a white powder, 0.58 g (84%), mp 196 – 198°; ir
1
3b
5c
(KBr): 3310 (NH), 3026 (CH), 1677, 1650 (CO); H nmr (400
MHz, d-DMSO, Me Si): δ 4.90 (s, 1 H), 7.04 - 7.65 (m, 15 H,
4
CCDC reference
Formula
204756
204759
+
ArH), 10.25 (s, 2 H, NH); es-ms: m/z 331 (94) (M+H ), 353
C
H
N O
C H N O
+
+
14 12
2
2
23 22 2 2
(100) (M+Na ), 683 (86) (2M+Na ).
Anal. Calcd. for C H N O : C, 76.33; H, 5.49; N, 8.48.
M
240.26
monoclinic
358.43
monoclinic
r
21 18
2 2
Crystal class
Space group
a (Å)
b (Å)
c (Å)
Found: C, 76.23; H, 5.48; N, 8.40.
P2 /c
P2 /c
1
1
Compound 5b was produced analogous to 5a using p-
chloroaniline (0.53 g, 4.17 mmol) and 3b (0.50 g, 2.08 mmol).
The final product was collected in vacuo as an off-white solid,
0.60 g (72%), mp 219 – 221°; ir (KBr): 3258 (NH), 3057 (CH),
7.3624(3)
11.4935(4)
13.9255(7)
94.287(1)
1175.08(9)
1.358
8.9449(6)
24.068(2)
9.1160(8)
109.095(5)
1854.6(3)
1.284
? (?)
3
1
V(Å )
1679, 1641 (CO); H nmr (400 MHz, d-DMSO, Me Si): δ 4.90
4
-3
D (g cm )
c
(s, 1 H), 7.28 - 7.68 (br m, 13 H, ArH), 10.36 (s, 2 H, NH); es-ms:
m/z 421 (100) (M+Na ), 423 (70) (M+Na ), 821 (59) (2M+Na ).
Anal. Calcd. for C H Cl N O : C, 63.31; H, 4.05; N, 7.04.
Z
4
0.93
4
0.82
+
+
+
-1
µ(Mo-K ) (mm )
α
21 16
2 2 2
T
, T
0.982, 0.991
red
0.20 x 0.10 x 0.10
8861
2689
0.063
1944
0.046
0.106
1.01
0.988, 0.998
colourless
0.15 x 0.05 x 0.02
9295
3532
0.316
877
0.092
0.147
0.88
min max
Found: C, 63.30; H, 4.02; N, 7.10.
Colour
Crystal size (mm)
Total data
Compound 5c was produced analogous to 5a using m-tolui-
dine (0.45 g, 4.17 mmol) and 3b (0.50 g, 2.08 mmol). The final
product was collected in vacuo as an off-white solid, 0.50 g
(68%), mp 180 – 182°; ir (KBr): 3260 (NH), 3063, 2920 (CH),
Unique data
R
int
1
1675, 1655 (CO); H nmr (400 MHz, d-DMSO, Me Si): δ 2.25
N [I>2.0σ(I)]
R1
wR2
S
A[a]
4
(s, 6 H, CH ), 4.90 (s, 1 H), 6.80 - 7.90 (m, 13 H, ArH), 10.15 (s,
2 H, NH); es-ms: m/z 381 (46) (M+Na ), 739 (100) (2M+Na ).
Anal. Calcd. for C H N O : C, 77.06; H, 6.19; N, 7.82.
3
+
+
23 22
2 2
0.0649
0.0436
Found: C, 77.08; H, 6.20; N, 7.78.
2
2
2 -1
2
2
[a] w = [σ (F ) + (AP) ] [where P = (F + 2F )/3]
X-ray Structure Analysis.
o
o
c
General crystallographic details for compounds 3b and 5c are
listed in Table 2. All crystals were grown from chloroform solu-
tions. Crystallographic data was collected on a Bruker Nonius
Kappa CCD area diffractometer using monochromatised Mo-Kα
X-ray radiation (λ = 0.71073Å) equipped with an Oxford
Cryosystems low temperature device (Southampton University).
Lattice parameters were calculated using 4997 3b and 6197 5c
reflections with 2.91º < θ < 27.48º. Intensity data were collected
at a temperature of 120 K using ϕ-ω scans to a maximum 2θ
value of 55º. Multi-scan absorption corrections were applied to
all data sets using the program SORTAV [20]. Structures were
solved by direct methods and refined using the SHELX-97 pack-
age [21]. All hydrogen atoms not involved in the strong hydro-
gen-bonding associations were included in the refinement at cal-
culated positions as riding models with C-H set to 0.95 Å (Ar-H),
from Key Organics Ltd. Aniline and m-toluidine were distilled
before use and p-chloroaniline was re-crystallised from ethanol.
Infrared spectra were recorded as pressed KBr discs on a Nicolet
1
205 FT-IR spectrometer. H NMR data were recorded on a
Bruker Spectrospin 400 NMR spectrometer. Electrospray mass
spectra were recorded in positive ion mode on a Micromass
Platform mass spectrometer (Southampton University).
5,7-Dimethyl-2-phenyl-1-oxo-1H-pyrazolo[1,2-a]pyrazol-4-
ylium-3-olate (3b).
Diethyl phenylmalonate (1c) (39.7 g, 0.18 mol) was dropwise
added to a boiling solution of 3,5-dimethylpyrazole (2b) (13.4 g,
0.14 mol) in diphenyl ether (80 mL) and boiled for a further 30
minutes. A distillation head was fitted to the top of the reaction
vessel to allow for the removal of ethanol as well as the recollec-
tion of 2b and any solvent vapour lost during the reaction. Upon
cooling the product precipitated and was collected in vacuo and
washed with light petroleum (40 – 60 °C), yielding intensely
0.98 Å (CH ) and 1.00 Å (CH). The NH hydrogen atoms in 5c
3
were located by difference methods and both positional and ther-
mal parameters were refined. For 5c, a high R value was the
int
result of weak high angle data.
Acknowledgements.
-
1
coloured red crystals, 5.93 g (18%); ir (KBr): 1672 (C–O ); H
The authors thank the School of Science and the Environment
(Coventry) for financial assistance, the EPSRC National
Crystallography Service (Southampton) and Dr G. J. Langley
(Southampton) for the collection of es-ms data.
nmr (400 MHz, d-DMSO, Me Si): δ 2.65 (s, 6 H, CH ), 6.65 (s, 1
4
3
H), 7.05 - 7.35 (m, 3 H, ArH), 7.95 - 8.00 (m, 2 H, ArH); es-ms:
+
+
+
m/z 241 (12) (M+H ), 263 (100) (M+Na ), 503 (28) (2M+Na ).
1
3
1
3
N ,N ,2-Triphenylmalonamide (5a), N ,N -Bis(4-chlorophenyl)-
1
3
2-phenylmalonamide (5b), 2-Phenyl-N ,N -di-m-tolylmalon-
amide (5c).
REFERENCES AND NOTES
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3b (0.50 g, 2.08 mmol) were heated in a sealed 10 mL glass sam-
ple vial in a 800 W Daewoo microwave for 3 minutes. Upon
cooling the crude product was dissolved in minimal THF and
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1368
D. E. Lynch, G. E. Spicer, and I. McClenaghan
Vol. 42
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