Stabilization of (N-methyleneamino)imidoylketenes: Synthesis of dipyrazolo[1,2-a;1′,2′-d][1,2,4,5]tetrazines

Article abstract of DOI:10.1016/j.tet.2004.04.080

Thermolysis of substituted methyl 1-methyleneamino-4,5-dioxo-4,5-dihydro- 1H-pyrrole-2-carboxylates 2a,b led to substituted dimethyl 3,9-dioxo-1,5,7,11- tetrahydro-1H,7H-dipyrazolo[1,2-a;1′,2′-d][1,2,4,5]tetrazine-1, 7-dicarboxylates 4a,b and methyl 2,5-d

Full text of DOI:10.1016/j.tet.2004.04.080

Tetrahedron 60 (2004) 5319–5323
Stabilization of (N-methyleneamino)imidoylketenes:
synthesis of dipyrazolo[1,2-a;10,20-d][1,2,4,5]tetrazines
a
b
Natalia A. Lisowskaya,a Andrei N. Maslivets and Zainutdin G. Aliev
,
*
aDepartment of Organic Chemistry, Perm State University, 15 Bukireva Str., 614990 Perm, Russian Federation
bInstitute of Problems of Chemical Physics, Russian Academy of Science, 14 Institutskii Prosp, 142432 Chernogolovka,
Moscow Region, Russian Federation
Received 2 February 2004; revised 13 April 2004; accepted 29 April 2004
Abstract—Thermolysis of substituted methyl 1-methyleneamino-4,5-dioxo-4,5-dihydro-1H-pyrrole-2-carboxylates 2a,b led to substituted
dimethyl 3,9-dioxo-1,5,7,11-tetrahydro-1H,7H-dipyrazolo[1,2-a;10,20-d][1,2,4,5]tetrazine-1,7-dicarboxylates 4a,b and methyl 2,5-dihydro-
5-oxo-1H-pyrazole-3-carboxylates 5a,b as minor products. The structure of compound 4a was determined by X-ray crystallography. The
proposed mechanism of this conversion includes generation of (N-methyleneamino)imidoylketenes 6a,b and its intramolecular
transformation to azomethine imines—5-oxo-2,5-dihydropyrazole-1-methylium-2-ides 7a,b, which undergo dimerization in head-to-tail
manner yielding products 4a,b and partially hydrolyse to compounds 5a,b.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
2. Results and discussion
The chemistry of imidoylketenes was being extensively
investigated during the last few decades from preparative,
mechanistic and theoretical points of view.1 Furthermore,
these highly reactive molecules are of considerable current
interest because of their wide use as building blocks in
organic chemistry. The thermal cheletropic extrusion of CO
from pyrrol-2,3(1H)-diones is the convenient method for the
generation of these ketenes. The variation of substituents in
the pyrrole ring leads to the generation of new imidoyl-
ketenes and widens their potential for the new types of
transformation. As we and other authors have shown in the
previous works, imidoylketenes can be involved into
various types of intramolecular cyclization,1,2 dimerization3
and interaction with nucleophiles4 and dienophiles.5
Reaction of primary b-enaminoketones with oxalyl chloride
is the most common method of the pyrrole-2,3(1H)-diones7
synthesis. The corresponding methyl 3-acyl-1-diphenyl-
methyleneamino-4,5-dioxo-4,5-dihydro-1H-pyrrole-2-car-
boxylates (2a,b) were obtained from methyl ethers of
4-acyl-2-diphenylmethylenehydrazino-4-oxo-2-butenoic
acids8 (1a,b) and oxalyl chloride in 80, 60% yield (Scheme
1). The spectroscopic data of pyrrole-2,3(1H)-diones 2a,b
are in good agreement with the other similar systems.6,7,9
Unfortunately, the quantitative isolation and further appli-
cation of the deep red coloured pyrrole-2,3(1H)-diones
2a,b appeared to be difficult since they are highly sensi-
tive to moisture and easily hydrolyse to afford light
yellow methyl 3-acyl-1-diphenylmethyleneamino-2,4-di-
hydroxy-2,5-dihydro-5-oxo-1H-pyrrole-2-carboxylates
(3a,b), whose structure was confirmed by X-ray crystal-
lography.10
We report herein the synthesis and thermolysis of new
pyrrole-2,3(1H)-diones—methyl 3-acyl-1-diphenylmethyl-
eneamino-4,5-dioxo-4,5-dihydro-1H-pyrrole-2-carboxyl-
ates (2a,b) (Scheme 1). We also describe a new and unusual
type of stabilisation of (N-methyleneamino)imidoylketenes6
to produce the novel tetrazine derivatives, namely dimethyl
2,8-diacyl-3,9-dioxo-5,5,11,11-tetra0phenyl-1,5,7,11-tetra-
hydro-1H,7H-dipyrazolo[1,2-a;10,2 -d][1,2,4,5]tetrazine-
1,7-dicarboxylates (4a,b), whose structure was defined by
X-ray crystallography (Fig. 1).
We have noticed that the deep red solution of pyrrole-
2,3(1H)-diones 2a,b became deep blue at heating, this
colour gradually disappeared after cooling and some
colourless products were formed. The preparative thermo-
lysis of 2a,b (130–140 8C, p-xylene) resulted in substituted
dipyrazolo[1,2-a;10,20-d][1,2,4,5]tetrazines 4a,b as the main
products (80, 67%, respectively) and pyrazoles 5a,b (15,
25%, respectively) as the by-products (Scheme 1). The
structure of 4a was defined by X-ray analysis. The structure
Keywords: Pyrrole-2,3(1H)-diones; (N-Methyleneamino)imidoylketenes;
Azomethine imines; Dipyrazolo[1,2-a;10,20-d][1,2,4,5]tetrazines.
1
of 5a,b was established by elemental analysis, IR and H
NMR spectroscopic data.
*
Corresponding author. Tel./fax: þ7-3422-396367;
e-mail address: lisowskaya@mail.ru
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.04.080

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N. A. Lisowskaya et al. / Tetrahedron 60 (2004) 5319–5323
Scheme 1. Synthesis and thermolysis of 1-diphenylmethilenamino-2,3-dihydropyrrole-2,3-dione 2a,b.
Figure 1. Perspective view of the structure of 4a, showing the crystallographic labeling.
Crystals of 4a suitable for X-ray analysis were obtained
from a toluene solution. Figure 1 shows a perspective view
of the structure, which crystallizes in the monoclinic space
group P21/c. The molecule in the crystal lies in the centre of
the symmetry. The tetrazine has a chair-conformation. The
inflexion along N1· · ·N2a line is 48.58 and the deviation of
the C3 an angle 14.78. The torsion angle O2–C7–C3–C4 is
12.28. Hydrogen bonds and other shortened intermolecular
contacts were not found. The values of all bonds lengths in
this molecule are in nice agreement with the other known
data.11
˚
the C1 from the plane of four nitrogen atoms is 0.7 A. The
We propose the following mechanism of pyrrole-2,03(1H)-
diones 2a,b conversion into dipyrazolo[1,2-a;1 ,20-d]-
[1,2,4,5]tetrazines 4a,b and pyrazoles 5a,b. The thermal
cheletropic extrusion of CO from 2a,b affords the new
(N-methyleneamino)imidoylketenes 6a,b, which unexpect-
edly intermolecular cyclized into azomethine imines—
4-acyl-3-methoxycarbonyl-5-oxo-2,5-dihydropyrazole-1-
(diphenylmethylium)-2-ides 7a,b. These intermediates
undergo dimerization in head-to-tail manner yielding
nitrogen atoms have a pyramidal structure. The sums of the
valence angles around N1 and N2 are 347.5 and 348.68,
respectively. The pyrazole cycle with local double bond is
planar. The C3–C4 distance in the five-membered ring is
˚
1.368(4) A, that corresponds to the CvC double bond
length in the pyrazoles.11 The plane of the methoxycarbonyl
group at the C4 has an orthogonal position with respect to
the pyrazole ring, and the plane of the pivaloyl fragment at

N. A. Lisowskaya et al. / Tetrahedron 60 (2004) 5319–5323
5321
products 4a,b, while azomethine imines 7a,b are partially
hydrolysed under action of water traces in a solvent with
ejection of benzophenone molecule to produce the com-
pounds 5a,b.
by TLC on Silufol UV-254 plates. Solvents were dried
according to standard protocols.
X-ray crystallography. The unit cell parameters were
measured on a four-circle automatic detector KM-4
(KUMA DIFRACTION) with x-geometry (graphite mono-
chromatised Cu Ka radiation, v–2u scan mode, 2u#80.58).
The total number of data collected was 4249 [4119 of it
were independent with R(int.) 0.081]. No correction for
absorption was applied (m¼0.724 mm21). The structure
was determined by a direct method by program SIR9215
with the subsequent series of calculations of electronic
density maps. The hydrogen atoms positions were calcu-
lated from geometrical terms. The final anisotropic
specification LSM was completed by program SHELXL-
9716 at R1¼0.0519, wR2¼0.1346 on 1629 reflections with
I$2s(I) and R1¼0.1721; wR2¼0.1911 on all 4119 reflec-
tions, GOF¼1.002.
On the other hand, the dimerization of azomethine imines
7a,b is a reversible process, in our o0pinion. The colourless
solutions of dipyrazolo[1,2-a;10,2 -d][1,2,4,5]tetrazines
4a,b in completely anhydrous inert solvents (such as
p-xylene, toluene) become deep blue at heating (110–
140 8C) and again colourless after cooling. Moreover, the
analogous process is observed in mass spectrum of 4a,
where intense peak of the azomethine imine 7a (m/z 390) is
present.
An additional proof for the suggested mechanism is the
dimerization of stable pyrazolidin-3-ones—azomethine
imines, giving the centrally symmetric dipyrazolo[1,2-a;
10,20-d][1,2,4,5]tetraz0ine-1,7-diones and mirror-symmetric
dipyrazolo[1,2-a;10,2 -d][1,2,4,5]tetrazine-1,9-diones early
reported by Dorn and co-workers.12
Crystallographic data (excluding structural factors) for the
structure in this paper have been deposited with the
Cambridge Crystallographic Data Center as supplementary
publication number CCDC No 229731. Copies of the data
can be obtained, free of charge, on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK (fax: þ44-1223-
336033 or e-mail: deposit@ccdc.cam.ac.uk).
3. Conclusion
In summary, the results showed that the thermolysis of
methyl 3-acyl-1-diphenylmethyleneamino-4,5-dioxo-4,5-
dihydro-1H-pyrrole-2-carboxylates 2a,b led to unexpected
products 4a,b and 5a,b. We propose the mechanism of this
conversion, including (N-methyleneamino)imidoylketenes
6a,b generation, its further cyclization to azomethine imines
7a,b, which dimerize into final compounds 4a,b. A novel
pathway of imidoylketenes stabilization via their prelimi-
nary cyclization to azomethine imines is an unusual process
and it has never been observed before. Furthermore, the
pyrazoles 5a,b formation by hydrolysis of 7a,b indirectly
confirms the suggested mechanism.
4.1.1. Methyl 1-diphenylmethyleneamino-3-pivaloyl-4,5-
dihydro-4,5-dioxo-1H-pyrrole-2-carboxylate (2a). A solu-
tion of oxalyl chloride (0.25 mL, 28 mmol) in dry chloro-
form (1 mL) was added to a solution of 1a8 (1.00 g,
27 mmol) in dry chloroform (3 mL). The reaction mixture
was heated for 1.5 h, solvent (2 mL) was evaporated and
resulting solution was cooled. The solid was filtered off gave
the title compound 2a (0.92 g, 80%) as a deep red crystals,
mp 138–140 8C; [Found: C, 68.97; H, 5.21; N, 6.72.
C24H22N2O5 requires C, 68.89; H, 5.30; N, 6.69%]; nmax
1760 (O–CvO, C2vO); 1740 (C3vO); 1680 (C4–CvO);
dH (400 MHz, CDCl3) 7.10–7.62 (10H, m, 2Ph); 3.86 (3H,
s, OMe); 1.05 (9H, s, Me3C).
The insertion of methyleneamino moiety to the nitrogen
atom of pyrrole-2,3-diones has allowed us not only to obtain
an attractive heterocyclic system and to expand the
synthetic opportunities of five-membered 2,3-dioxohetero-
cycles, but also to offer a new synthetic way to arduous
unsaturated azomethine imines formation through the
intramolecular cyclization of (N-methyleneamino)imi-
doylketenes. Saturated azomethine imines have been
prepared by catalytic nucleophilic addition of substituted
pyrazolidin-3-ones to various aldehydes and ketones, as
reported by Dorn,12 Stanovnik,13 Sharpless14 and co-workers.
4.1.2. Methyl 1-diphenylmethyleneamino-2,4-di-
hydroxy-3-pivaloyl-2,5-dihydro-5-oxo-1H-pyrrole-2-
carboxylate (3a). The residue solution after synthesis
compound 2a was allowed to contact with air moisture for
24 h. The solid was filtered off and recrystallized from
chloroform–hexane (1:1) to give the title compound 3a
(0.17 g, 14%) as a light yellow crystals, mp 149–150 8C;
[Found: C, 66.15; H, 5.12; N, 6.47. C24H24N2O6 requires C,
66.05; H, 5.22; N, 6.42%]; nmax 3480, 3210 (OH); 1740
(O–CvO, C2vO); 1680 (C4–CvO); dH (300 MHz,
DMSO-d6) 7.12–7.84 (11H, m, 2PhþC5–OH); 3.84 (3H,
s, OMe); 1.20 (9H, s, Me3C).
4. Experimental
4.1. General
4.1.3. Methyl 1-diphenylmethyleneamino-3-p-toluoyl-
4,5-dihydro-4,5-dioxo-1H-pyrrole-2-carboxylate (2b).
This compound was prepared from 1b8 (1.00 g, 25 mmol)
and oxalyl chloride (0.22 mL, 26 mmol), according to
above described method for 2a. Yield compound 2b
(0.71 g, 60%) as a deep red crystals, mp 144–146 8C;
[Found: C, 71.83; H, 4.35; N, 6.24. C27H20N2O5 requires C,
71.67; H, 4.46; N, 6.19%]; nmax 1740 (O–CvO, C2vO);
1725 (C3vO); 1630 (C4–CvO); dH (400 MHz, CDCl3)
1
The H and 13C NMR spectra were recorded in DMSO-d6
and CDCl3 solutions with HMDS as the internal standard on
a Bruker AM-300 (300 MHz) and a Bruker DPX 400
(400 MHz) spectrometers. The IR spectra were recorded in
Nujol mulls on a UR-20 spectrometer. The mass spectrum
was recorded on a MX-1310 spectrometer (70 eV). The
melting points are uncorrected. Reactions were monitored

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N. A. Lisowskaya et al. / Tetrahedron 60 (2004) 5319–5323
7.00–7.79 (14H, m, 2PhþC6H4); 3.80 (3H, s, OMe); 2.29
25%) as a colourless solid, mp 210–212 8C; [Found: C,
59.89; H, 4.63; N, 10.80. C13H12N2O4 requires C, 60.00; H,
4.65; N, 10.76%]; nmax 3250 (NH); 1770 (O–CvO); 1670
(N–CvO); 1620 (C4–CvO); dH (300 MHz, DMSO-d6)
13.20 (1H, br s, NH); 10.35 (1H, br s, NH); 7.60 (2H, d,
J¼8.0 Hz, C6H4); 7.26 (2H, d, J¼8.0 Hz, C6H4); 3.60 (3H,
s, OMe); 2.40 (3H, s, Me).
(3H, s, Me).
4.1.4. Methyl 1-diphenylmethyleneamino-2,4-dihydroxy-
3-p-toluoyl-2,5-dihydro-5-oxo-1H-pyrrole-2-carboxylate
(3b). This compound was prepared according to above
described method for 3a. Purification crude precipitate from
chloroform–hexane (1:1) to give compound 3b (0.31 g,
25%) as a light yellow crystals, mp 164–166 8C; [Found: C,
68.98; H, 4.65; N, 6.00. C27H22N2O6 requires C, 68.93; H,
4.71; N, 5.95%]; nmax 3395, 3150 (OH); 1760 (O–CvO,
C2vO); 1620 (C4–CvO); dH (300 MHz, DMSO-d6) 7.22–
7.81 (15H, m, 2PhþC6H4þC5–OH); 3.87 (3H, s, OMe);
2.42 (3H, s, Me).
Acknowledgements
This work was carried out with the financial support of the
Russian Basic Research Fund (project No 04-03-33024).
We thank Dr. Elena V. Shklyaeva for helpful discussions
and Olga G. Stryapunina for experimental contributions.
4.1.5. Dimethyl 2,8-dipivaloyl-5,5,11,11-tetraphenyl-
1,5,7,11-tetrahydro-3,9-dioxo-1H,7H-dipyrazolo[1,2-a;
10,20-d][1,2,4,5]tetrazine-1,7-dicarboxylate (4a). A solu-
tion of 2a (1.00 g, 24 mmol) in dry p-xylene (6 mL) was
held at 138–140 8C for 0.5 h and then cooled. The solid was
isolated by filtration and then recrystallized from acetone
to give title compound 4a (0.75 g, 80%) as a colourless
crystals, mp 223–224 8C; [Found: C, 70.93; H, 5.51; N,
7.22. C46H44N4O8 requires C, 70.75; H, 5.68; N, 7.17%];
nmax 1760 (O–CvO); 1730, 1700 (N–C1(7)vO); 1670
(C2(8)–CvO); dH (400 MHz, DMSO-d6) 7.10–7.54 (20H,
m, 4Ph); 3.13 (6H, s, 2OMe); 1.07 (18H, s, 2Me3C); dC
(100.6 MHz, DMSO-d6) 203.7, 163.2, 160.5, 155.3, 130.6,
129.7, 126.6, 120.6, 114.7, 88.1, 56.8, 44.5, 26.7; MS (m/z,
%): 390 (M/2þ, 20), 333 (M/2þ2Me3C, 100).
References and notes
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4.1.6. Methyl 4-pivaloyl-2,5-dihydro-1H-pyrazole-3-carb-
oxylate (5a). The residue solution after synthesis compound
4a was allowed to contact with air moisture for 0.5 h. The
precipitate was filtered off and recrystallized from toluene to
give the title compound 5a (0.08 g, 15%) as a colourless
solid, mp 201–203 8C; [Found: C, 53.14; H, 6.18; N, 12.40.
C10H14N2O4 requires C, 53.09; H, 6.24; N, 12.38%]; nmax
3250 (NH); 1760 (O–CvO); 1720 (N–CvO); 1670 (C4–
CvO); dH (400 MHz, DMSO-d6) 13.52 (1H, br s, NH);
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Me3C); dC (100.6 MHz, DMSO-d6) 203.9, 163.5, 158.8,
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4.1.7. Dimethyl 5,5,11,11-tetraphenyl-2,8-ditoluoyl-1,5,7,
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ether to give compound 4b (0.57 g, 67%) as colourless
crystals, mp 207–209 8C. [Found: C, 72.93; H, 4.73; N,
6.84. C50H40N4O8 requires C, 72.80; H, 4.89; N, 6.79%];
nmax 1760 (O–CvO); 1690 (N–C1(7)vO); 1650 (C2(8)
–
CvO); 1600 (CvC); dH (400 MHz, DMSO-d6) 7.03–7.74
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