Proton sponges condensed with pyridazine and pyrrole nuclei

Article abstract of DOI:10.1007/s10593-013-1242-0

A series of 5,6-bis(dimethylamino)acenaphthylene derivatives fused with pyridazine and pyrrole rings in positions 1 and 2 has been synthesized. The latter are of interest for constructing porphyrin-containing systems with highly basic fragments.

Full text of DOI:10.1007/s10593-013-1242-0

Chemistry of Heterocyclic Compounds, Vol. 49, No. 2, May, 2013 (Russian Original Vol. 49, No. 2, February, 2013)
PROTON SPONGES CONDENSED WITH PYRIDAZINE
AND PYRROLE NUCLEI
A. F. Pozharskii1*, M. A. Mekh1, and V. A. Ozeryanskii1
A series of 5,6-bis(dimethylamino)acenaphthylene derivatives fused with pyridazine and pyrrole rings in
positions 1 and 2 has been synthesized. The latter are of interest for constructing porphyrin-containing
systems with highly basic fragments.
Keywords: acenaphthylene, 8,9-diazafluoranthene, pyridazine, pyrrole, proton sponge, chlorination,
cycloaddition.
In the year 2000, we obtained 5,6-bis(dimethylamino)acenaphthylene (acenaphthylene proton sponge)
(1) for the first time [1]. It was shown that this compound had a highly reactive double bond, which was readily
reduced [1], underwent electrophilic substitution [2] and [4+2] cycloaddition with reversed electron demand [3].
In particular, diazafluoranthenes 4 and 5 were formed in high yield [3] by the action of sym-tetrazines 2 and 3
on compound 1.
[
_______
*To whom correspondence should be addressed, e-mail: apozharskii@sfedu.ru.
1Southern Federal University, 7 Zorge St., Rostov-on-the-Don 344090, Russia.
________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 277-283, February, 2013. Original
article submitted October, 26, 2012.
0009-3122/13/4902-0253©2013 Springer Science+Business Media New York
253

The mechanism of this conversion is known to involve elimination of a nitrogen molecule from the
[4+2] adduct and subsequent oxidation of the resulting dihydropyridazine either by air oxygen or by the initial
tetrazine [3, 4].
In the present work, we wanted to revisit the synthetic potential of compound 1. In particular, annelation
of a pyridazine ring to compound 1 with further transformation of the former into pyrrole made possible the
subsequent synthesis of porphyrin proton sponges (cf. with the analogous approach described in [5]).
First of all, we introduced the recently synthesized chlorine-containing acenaphthylenes 6 and 7 [6] into
a [4+2] cycloaddition reaction. It was assumed that the introduction of chlorine atoms into positions 2 and 7 of
the acenaphthylene system may lead to twisting of the NMe2 groups relative to the ring plane, decreasing their
+M effect, and thereby reducing the reactivity of the double bond. Pyridazines 8, 9 were obtained in high yield
by the action of an equivalent quantity of tetrazine 2 on compounds 6 and 7 in refluxing toluene for 15 h. Thus,
the reaction in this case required more forcing conditions than with compound 1, which reacted with
diphenyltetrazine (2) in refluxing benzene for 10 h [3].
It is remarkable that the dichloride 9 could also be obtained by treating the pyridazine 4 with
N-chlorosuccinimide (NCS). However, in comparison with the chlorination of substrate 1, the reaction in this
case proceeded with more difficulty (refluxing in chloroform for 8 h, yield 39%) and was accompanied by resin
formation. It was not possible to stop the conversion at the stage of monochloride 8, in contrast to the
chlorination of compound 1 where the monochloride was obtained in 80% yield [6].
We further found that the acenaphthylenes 6 and 7 reacted with tetrazine 3 and formed
diazafluoranthenes 10 and 11 in high yields. The reactions were carried out in dichloromethane at room
temperature with the participation of equimolar quantities of starting materials. The cycloaddition process was
rapid (<1 min) and was accompanied by vigorous evolution of nitrogen, which was used for qualitative
assessment of the substrate reactivity: 1 > 6 > 7. However, the subsequent aromatization of the resulting
(dihydro)pyridazine in the presence of air in chloroform medium required about 1 day.
Me2N
NMe2
R
Cl
1) CH2Cl2, 20°C
2) O2, CHCl3
+
6, 7
3
MeO2C
CO2Me
N
N
10 R = H (97%)
11 R = Cl (82%)
We established further that reduction of the pyridazine 5 with zinc in acetic acid (20°C, 48 h) gave the
pyrrole 12 in 57% yield (for the mechanism of similar reactions, see [5]).
254

Me2N
NMe2
Me2N
NMe2
[2H]
H2O
5
– H2O
CO2Me
H
MeO2C
MeO2C
CO2Me
N
NH
O
NH
NH2
Me2N
NMe2
Me2N
NMe2
[2H]
MeO2C
CO2Me
MeO2C
CO2Me
N
NH2
N
H
12
1
Compound 12 forms yellow crystals, readily soluble in organic solvents. In its H NMR spectrum the
NH proton was displayed as a singlet at 9.31 ppm. Such a small chemical shift clearly indicated against the
formation in solution of a bipolar ion with a chelated proton (cf. with the spectra of protonated acenaphthylene
sponge salts, where the chelated proton usually had δNH values of 15.5-18.5 ppm [6]). The pKa of pyrrole 12 in
DMSO, determined by the competitive transprotonation method, was equal to 12.5 (22°C, relative to 1,8-bis-
(dimethylamino)-2,7-dimethoxynaphthalene, pKa 11.5 in DMSO [7]). An X-ray structural investigation of
compound 12 revealed molecular pairing in the crystal lattice by intermolecular hydrogen bonds between the
pyrrole NH group and the carbonyl oxygen of the substituent. The main geometric parameters are given in
Fig. 1.
Fig. 1. Molecular structure of proton sponge 12, in which atoms are represented by thermal
vibration ellipsoids of 50% probability, showing the intermolecular H-bonding in crystals.
Key distances (Å) and angles (deg.): N(1)···N(2) 2.918, N(3)–H 0.89, O(3)···H(N(3)) 2.10,
N(3)–H···O(3) 153.
255

Compound 12 readily formed an anion on treatment with an equivalent amount of sodium hydride in
DMF. Its subsequent alkylation (MeI or EtI) gave the corresponding N-alkyl derivative 13 in good yield. The
pyrrole 12 was chlorinated with N-chlorosuccinimide in chloroform with the formation of the dichloro
derivative 14. The reaction was accompanied by strong tarring even at -15°C. The yield of compound 14 did not
exceed 11%.
Attempts to affect a synthesis of pyrrole 15 from pyridazine 4 by the action of zinc in acetic acid did not
1
succeed. The yellow fluorescent substance formed in the course of the reaction gave a very complex H NMR
spectrum and decomposed on adsorbents (Al2O3, SiO2) after a few minutes (eluents were chloroform or
toluene). Such behavior was apparently caused by the π-electron rich nature of compound 15. This view was
supported by our observation that the pyrrole 17, which contained no π-donating NMe2 groups, was relatively
stable. Pyrrole 17 is a pale-yellow fluorescent substance, which was obtained by us from the previously
described diazafluoranthene 16 in 14% yield [4, 8].
In conclusion it must be said that the 3,4-bis(dimethylamino)-8H-acenaphtho[1,2-c]pyrrole-7,9-
dicarboxylates obtained by us may serve as starting materials for preparing new porphyrin derivatives
containing proton sponge residues as substituents. The high basicity of such molecular ensembles may exert a
large influence on complex formation, proton dynamics, sensor properties, and many other features of these
compounds.
EXPERIMENTAL
The IR spectra were recorded on a Specord IR-71 instrument and the UV spectra on a Varian Cary 100
instrument (c = 5·10-5 M). The 1H NMR spectra were recorded on a Bruker DPX-250 instrument (250 MHz) at
20°C, internal standard was TMS. The mass spectra were recorded on a Finnigan MAT Incos 50 instrument
with a direct insertion probe, at 70 eV accelerating voltage. Elemental analysis was carried out on a PerkinElmer
2400 analyzer. Melting points were determined by the capillary method on a PTP instrument.
256

Acenaphthylenes 1, 6, and 7 were obtained by the procedures of [1, 6]. Synthesis of the tetrazines 2, 3
[9, 10] and pyridazines 4, 5 [3] was carried out by the indicated procedures.
2-Chloro-3,4-bis(dimethylamino)-7,10-diphenyl-8,9-diazafluoranthene (8). A mixture of chloro-
acenaphthylene 6 (0.109 g, 0.4 mmol) and diphenyltetrazine 2 (0.096 g, 0.4 mmol) in absolute toluene (10 ml)
was refluxed for 15 h. The solvent was removed, and the residue was chromatographed on Al2O3 (eluent –
chloroform), collecting the main red fraction of Rf 0.25. Yield 0.173 g (91%). Red crystals; mp 255-256°C
(C6H6). UV spectrum (MeCN), λmax, nm (log ε): 285 (4.41), 500 (4.23), longest wavelength of absorption at 600
1
nm. H NMR spectrum (CDCl3), δ, ppm: 2.92 (6H, s, 4-N(CH3)2); 3.09 (6H, s, 3-N(CH3)2); 6.90 (1H, d,
J5,6 = 8.4, H-5); 7.55-7.64 (7H, m, H-6, H Ph); 7.72 (1H, s, H-1); 7.89-7.94 (4H, m, H Ph). Found, %: C 75.58;
H 5.20; Cl 7.12. C30H25ClN4. Calculated, %: C 75.55; H 5.25; Cl 7.45.
2,5-Dichloro-3,4-bis(dimethylamino)-7,10-diphenyl-8,9-diazafluoranthene (9) was obtained analogously
from substrate 7 (0.120 g, 0.4 mmol) and tetrazine 2 (0.096 g, 0.4 mmol). Red fraction with Rf 0.25. Yield 0.196 g
(96%). Red crystals; mp 304-305°C (decomp., benzene). UV spectrum (EtOH), λmax, nm (log ε): 271 (4.49), 342
(3.79), 459 (4.05), longest wavelength of absorption at 650 nm. 1H NMR spectrum (CDCl3), δ, ppm: 3.05 (12H,
s, 2N(CH3)2); 7.61-7.64 (8H, m, H-1,6, H Ph); 7.88-7.91 (4H, m, H Ph). Found, %: C 70.02; H 4.71; Cl 13.50.
C30H24Cl2N4. Calculated, %: C 70.45; H 4.73; Cl 13.86.
Chlorination of 3,4-Bis(dimethylamino)-7,10-diphenyl-8,9-diazafluoranthene (4). A solution of
compound 4 (0.030 g, 0.06 mmol) and NCS (0.017 g, 0.12 mmol) in chloroform (10 ml) was refluxed for 8 h.
The mixture was treated with water (20 ml), the organic layer was evaporated, and the residue chromatographed
on Al2O3 (eluent was chloroform). The red fraction of Rf 0.25 gave compound 9 (0.020 g, 39%), the
physicochemical characteristics of which were in agreement with those given above.
Dimethyl 2-Chloro-3,4-bis(dimethylamino)-8,9-diazafluoranthene-7,10-dicarboxylate (10).
A
solution of tetrazine 3 (0.039 g, 0.2 mmol) in dichloromethane (4 ml) was added in portions to a solution of
compound 6 (0.055 g, 0.2 mmol) in dichloromethane (4 ml), with evolution of gas. After 5 min, the solvent was
evaporated, the residue was dissolved in chloroform (5 ml), and the obtained solution was allowed to slowly
evaporate in ambient conditions to complete the aromatization. The residue was recrystallized from toluene.
Yield 0.085 g (97%). Red crystals; mp 212-213°C (decomp.). UV spectrum (CHCl3), λmax, nm (log ε): 245
1
(4.95), 285 (sh, 4.56), 400 (4.30), 540 (4.51). H NMR spectrum (CDCl3), δ, ppm (J, Hz): 3.02 (6H, s,
4-N(CH3)2); 3.19 (6H, s, 3-N(CH3)2); 4.17 (3H, s) and 4.19 (3H, s, 2CO2CH3); 7.11 (1H, d, J5,6 = 8.2, H-5); 8.72
(1H, d, J5,6 = 8.2, H-6); 8.91 (1H, s, H-1). Found, %: C 60.00; H 4.70; Cl 8.12. C22H21ClN4O4. Calculated, %:
C 59.93; H 4.80; Cl 8.04.
Dimethyl 2,5-Dichloro-3,4-bis(dimethylamino)-8,9-diazafluoranthene-7,10-dicarboxylate (11) was
obtained analogously from substrate 7 (0.061 g, 0.2 mmol) and tetrazine 3 (0.039 g, 0.2 mmol), and was
recrystallized from toluene. Yield 0.078 g (82%). Dark-red crystals; mp 235-236°C (decomp.). IR spectrum
(nujol), , cm-1 1710 (CO). UV spectrum (CHCl3), λmax, nm (log ε): 247 (4.68), 280 (sh, 4.56), 400 (4.08), 535
1
(4.30). H NMR spectrum (CDCl3), δ, ppm; 3.15 (12H, s, 2N(CH3)2); 4.19 (6H, s, 2CO2CH3); 8.83 (2H, s,
H-1,6). Found, %: C 55.48; H 4.20; Cl 14.88. C22H20Cl2N4O4. Calculated, %: C 55.59; H 4.21; Cl 14.95.
Dimethyl 3,4-Bis(dimethylamino)-8H-acenaphtho[1,2-c]pyrrole-7,9-dicarboxylate (12). A suspension
of diazafluoranthene 5 (0.150 g, 0.37 mmol) and zinc dust (0.100 g, 1.54 mmol) in acetic acid (4 ml) was stirred
at room temperature for 48 h. The unreacted zinc was filtered off. The remainder, after dilution with water
(5 ml), was neutralized with concentrated aqueous ammonia to pH 7-8. The reaction products were extracted
with CHCl3 (34 ml), the extract was evaporated to minimum volume and chromatographed on Al2O3 (eluent
was CHCl3). The main yellow fraction had Rf 0.35. Yield 0.082 g (57%). Yellow crystals; mp 202-203°C
(MeOH–CHCl3, 16:1). IR spectrum (nujol),, cm-1: 3285 (NH), 1720 (C=O), 1565 (ring), 1260 (C–O–C).
UV spectrum (EtOH), λmax, nm (log ε): 255 (4.52), 278 (sh, 4.16), 352 (4.21), 404 (sh, 4.10), longest
wavelength of absorption at 500 nm. 1H NMR spectrum (CDCl3), δ, ppm (J, Hz): 2.91 (12H, s, 2N(CH3)2); 4.03
1
(6H, s, 2CO2CH3); 6.99 (2H, d, J = 7.7, H-2,5); 8.03 (2H, d, J = 7.7, H-1,6); 9.31 (1H, br. s, NH). H NMR
spectrum (DMSO-d6), δ, ppm (J, Hz): 2.84 (12H, s, 2N(CH3)2); 3.92 (6H, s, 2CO2CH3); 7.01 (2H, d, J = 7.9,
257

H-2,5); 7.97 (2H, d, J = 7.9, H-1,6); 12.1 (1H, br. s, NH). Mass spectrum, m/z (Irel, %): 393 [M]+ (76), 379 (24),
349 (39), 347 (33), 331 (74), 315 (18), 244 (45), 230 (25), 216 (28), 189 (30), 181 (22), 164 (37), 150 (52), 143
(41), 136 (90), 129 (57), 122 (84), 115 (32), 109 (23), 101 (26), 95 (18), 88 (21), 58 (100), 44 (94), 42 (68).
Found, %: C 67.10; H 6.00. C22H23N3O4. Calculated, %: C 67.17; H 5.89.
Dimethyl 3,4-Bis(dimethylamino)-8-methyl-8H-acenaphtho[1,2-c]pyrrole-7,9-dicarboxylate (13a).
Sodium hydride (0.003 g, 0.1 mmol, as 60% suspension in mineral oil) was added to a solution of pyrrole 12
(0.039 g, 0.1 mmol) in dry DMF (4 ml). The obtained mixture was stirred at 20°C for 20 min, after which
iodomethane (6 μl, 0.1 mmol) was added. The mixture was stirred for a further 20 min, water (4 ml) was added,
and the reaction products were extracted with toluene (14 ml). Toluene was removed from the extract, and the
residue was chromatographed on Al2O3 (eluent was chloroform). The main yellow fraction had Rf 0.90. Yield
1
0.023 g (56%). Yellow crystals; mp 115-116°C (MeOH). H NMR spectrum (CDCl3), δ, ppm (J, Hz): 2.90
(12H, s, 2N(CH3)2); 4.04 (6H, s, 2CO2CH3); 4.34 (3H, s, NCH3); 6.98 (2H, d, J = 7.9, H-2,5); 7.99 (2H, d,
1
J = 7.9, H-1,6). H NMR spectrum (DMSO-d6), δ, ppm (J, Hz): 2.84 (12H, s, 2N(CH3)2); 3.98 (6H, s,
2CO2CH3); 4.22 (3H, s, NCH3); 7.01 (2H, d, J = 7.9, H-2,5); 7.97 (2H, d, J = 7.9, H-1,6). Found, %: C 67.80;
H 6.00. C23H25N3O4. Calculated, %: C 67.81; H 6.14.
Dimethyl 8-Ethyl-3,4-bis(dimethylamino)-8H-acenaphtho[1,2-c]pyrrole-7,9-dicarboxylate (13b)
was obtained analogously from substrate 12 (0.039 g, 0.1 mmol), sodium hydride (0.003 g, 0.1 mmol, as 60%
suspension in mineral oil), and EtI (9.3 μl, 0.1 mmol). The yellow fraction had Rf 0.93. Yield 0.034 g (71%).
1
Yellow crystals; mp 181-182°C (MeOH–PhMe, 3:1). H NMR spectrum (CDCl3), δ, ppm (J, Hz): 1.43 (3H, t,
J = 6.9, CH2CH3); 2.90 (12H, s, 2N(CH3)2); 4.05 (6H, s, 2CO2CH3); 4.95 (2H, q, J = 6.9, CH2CH3); 6.98 (2H, d,
J = 7.9, H-2,5); 7.99 (2H, d, J = 7.9, H-1,6). Found, %: C 68.50; H 6.50. C24H27N3O4. Calculated, %: C 68.39; H
6.46.
Dimethyl
2,5-Dichloro-3,4-bis(dimethylamino)-8H-acenaphtho[1,2-c]pyrrole-7,9-dicarboxylate
(14). A solution of N-chlorosuccinimide (0.068 g, 0.50 mmol) in CHCl3 (10 ml) was added at -15oC with
vigorous stirring over 15 min to a solution of compound 12 (0.100 g, 0.25 mmol) in CHCl3 (10 ml). The
reaction mixture was washed with water (20 ml), the organic phase was evaporated to dryness, and the residue
was chromatographed on Al2O3 (eluent was ethyl acetate). The yellow fraction had Rf 0.85. Yield 0.013 g
(11%). Yellow crystals; mp 267-270°C (CHCl3). IR spectrum (nujol), , cm-1: 3290 (NH), 1700 (C=O), 1450
(ring), 1250 (C–O–C). UV spectrum (CHCl3), λmax, nm (log ε): 240 (4.82), 265 (sh, 4.80), 380 (4.30), longest
1
wavelength of absorption at 500 nm. H NMR spectrum (DMSO-d6), δ, ppm: 2.99 (12H, s, 2N(CH3)2); 3.96
(6H, s, 2CO2CH3); 7.89 (2H, s, H-1,6); 12.71 (1H, br. s, NH). Found, %: C 57.10; H 4.50. C22H21Cl2N3O4.
Calculated, %: C 57.14; H 4.58.
7,9-Diphenyl-8H-acenaphtho[1,2-c]pyrrole (17) was obtained analogously to compound 12 from
diazafluoranthene 16 (0.132 g, 0.37 mmol) and zinc (0.100 g, 1.54 mmol). The yellow main fraction had Rf
0.92. Yield 0.018 g (14%). Light-yellow crystals; mp 193-194°C (MeOH) (mp 223-224°C, CH2Cl2–hexane [8];
IR and NMR spectra have not been published). IR spectrum (nujol), , cm-1: 3410 (NH). UV spectrum (MeCN),
λ
max, nm (log ε): 249 (4.30), 270 (sh, 4.23), 340 (4.06), 405 (3.91), longest wavelength of absorption at 440 nm.
1H NMR spectrum (DMSO-d6), δ, ppm (J, Hz): 7.40 (2H, t, J = 7.6, H-2,5); 7.52-7.63 (6H, m, H Ph); 7.72 (2H,
d, J = 8.1, H-1,6): 7.07 (2H, d, J = 7.1, H-3,4); 7.94-7.98 (4H, m, H Ph); 11.43 (1H, br. s, NH).
X-Ray Structural Investigation of Acenaphthopyrrole 12 Monocrystal (yellow plates from MeCN)
was carried out at 100 K on a Bruker APEX II diffractometer (λ 0.71073 Å, MoKα radiation, ω scanning).
Crystals of compound 12, C22H23N3O4, M 393.43, were monoclinic, space group C2/c, a 40.151(9),
b 6.3252(13), c 15.390(3) Å, β 102.972(7)º, V 3808.8(14) Å3, Z 8, dcalc 1.372 g·cm-3, μ 0.096 mm-1, F(000)
1664. The structure was solved by the direct method and refined by the least squares method in a full-matrix
anisotropic approximation using the Bruker SHELXTL set of programs. The H(N) hydrogen atom in the
structure 12 was revealed by a Fourier difference synthesis and refined isotropically. The results of the
investigation have been deposited in the Cambridge Crystallographic Data Center (deposit number CCDC
907661).
258

The work was carried out with the financial support of the Russian Foundation for Basic Research
(project 11-03-00073).
The authors are grateful to Z. A. Starikova (Moscow, INEOC, Russian Academy of Sciences) for
carrying out the X-ray structural investigation.
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