Practical synthesis of bis-substituted tetrazines with two pendant 2-pyrrolyl or 2-thienyl groups, precursors of new conjugated polymers

Article abstract of DOI:10.1016/S0040-4020(03)00709-9

Novel linear oligoheterocycles based on substituted tetrazines are described. The desired compounds have been accomplished by a variation of the original Pinner [Ann. Chem., 297 (1897) 221] synthesis in which the aromatic nitrile reacted with hydrazine in

Full text of DOI:10.1016/S0040-4020(03)00709-9

Tetrahedron 59 (2003) 4761–4766
Practical synthesis of bis-substituted tetrazines with two pendant
2-pyrrolyl or 2-thienyl groups, precursors of new
conjugated polymers
a
a
b
Jadwiga Sol⁄oducho,a Jacek Doskocz, Joanna Cabaj and Szczepan Roszak
,
*
aDepartment of Chemistry, Institute of Organic Chemistry, Biochemistry and Biotechnology, Wrocl⁄aw, University of Technology,
´
Wybrzez˙e Wyspianskiego 27, 50-370 Wrocl⁄aw, Poland
bDepartment of Chemistry, Institute of Physical and Theoretical Chemistry, Wrocl⁄aw, University of Technology,
´
Wybrzez˙e Wyspianskiego 27, 50-370 Wrocl⁄aw, Poland
Received 5 February 2003; revised 7 April 2003; accepted 1 May 2003
Abstract—Novel linear oligoheterocycles based on substituted tetrazines are described. The desired compounds have been accomplished by
a variation of the original Pinner [Ann. Chem., 297 (1897) 221] synthesis in which the aromatic nitrile reacted with hydrazine in an aqueous
solution to give bis(pyrrolyl)tetrazines or bis(phenyl)tetrazines. The bis(phenyl)tetrazines reacted with 3,4-ethylenedioxy-2-(trimethyl-
tin)thiophene or 2-(trimethyltin)thiophene in the presence of Pd(PPh3)2Cl2 or Pd(PPh3)4 as catalyst to give the desired compounds. Quantum-
chemical calculations were performed to assess the usefulness of the synthesized compounds for electropolymerization. Studies have
indicated qualitative difference between bis-pyrrole tetrazine and bis-phenyl tetrazines regarding the electronic density rearrangement due to
the loss of an electron. q 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction
newly synthesized compounds are performed with the aim
of assessing the usefulness of newly synthesized moieties
for polymerization.
During the past decades, the design and synthesis of novel
conjugated polymers as low-cost materials with high
potential for electronic and optoelectronic applications
have received considerable attention.2 A few years ago,
we reported on the synthesis of a 1,4-bis(pyrrol-2-
yl)arylenes.3–5 This new synthetic route opens up a number
of opportunities for the synthesis of derivatized, potentially
soluble and processable pyrrole containing polymers. With
our ongoing interest in precursors of conducting polymers
we report on the synthesis of novel symmetric bis-pyrrole-,
and also bis-thiophene or bis-dioxanethiophene, derivatives
based on tetrazines. By contrast, the interest in thiophene6 or
3,4-ethylenedioxythiophene7 is more recent. As is known
from the literature, the resulting polymers exhibit high
electroconductivity and improved electrochemical and
thermal stability. We reasoned that the tetrazines with
pyrrole, thiophene or 3,4-ethylenedioxythiophene rings
would be good candidates for polymerization.
2. Results and discussion
2.1. Synthesis of 3,6-bis-(pyrrol-2-yl)-1,2,4,5-tetrazine
Herein we present our synthesis of 3,6-bis(pyrrolyl)-1,2,4,5-
tetrazine. The synthesis was carried out as outlined in
Scheme 1. The 2-pyrrolealdehyde (1) was reacted with
hydroxylamine hydrochloride and sodium acetate to give
2-pyrrolealdoxime.8 Treatment of compound 2 with an
excess of acetic anhydride produced the corresponding
nitrile (3).8 2-Pyrrolenitrile was reacted with tetrazine to
give new 3,6-bis(pyrrolyl)-1,2-dihydro-1,2,4,5-tetrazine 4
(in a procedure similar to that used for the preparation of
3,6-bis(phenyl)-1,2-dihydro-1,2,4,5-tetrazine).9 Compound
4 after reaction with isoamyl nitrite, oxidized to the
corresponding fully aromatic compound 5 (according to a
literature procedure for the preparation of 3,6-bis-diphenyl-
1,2,4,5-tetrazine).10
It has been established5 that the rearrangement of the
electronic density distribution correlates with parameters
important for the experimental electropolymerization.
Theoretical studies of the structures and properties of
In this paper we also report a synthetic study of bis-
thiophene- and bis-(dioxanethiophene)-tetrazines. This
study has been inspired by a literature review6,7 and our
previous experiences5 concerning conjugated oligomers
Keywords: precursors; tetrazines; bis-dioxanethiophene; bis-thiophene.
*
Corresponding author. Tel.: þ48-71-320-24-27; fax: þ48-71-328-40-64;
e-mail: soloducho@kchf.ch.pwr.wroc.pl
0040–4020/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4020(03)00709-9

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J. Sol⁄oducho et al. / Tetrahedron 59 (2003) 4761–4766
Scheme 1.
based on various combinations of thiophene and
3,4-ethylenedioxythiophene.
hydrogen peroxide (6%) following a literature procedure.11
3,6-Bis-(4-aminophenyl)-tetrazine (9) was isolated after
purification by column chromatography. Under the con-
ditions shown in Scheme 2, diamine 9 was converted by a
Sandmeyer reaction (in a procedure similar to that used for
the iodination of 2-bromo-3,6-dimethoxyaniline)12 into 3,6-
(bis-p-iodophenyl)-tetrazine 10.
2.2. Synthesis of 3,6-bis-(4-iodophenyl)-1,2,4,5-tetrazine
4-Aminobenzonitrile (7) prepared in 70% yield from
4-nitrobenzonitrile (6) as previously reported11 (Scheme 2)
was cyclized by heating with anhydrous hydrazine (98%)11
into the previously unreported 3,6-bis-(4-aminophenyl)-1,2-
dihydrotetrazine (8) in 65% yield. Compound (8) was
oxidized to the corresponding tetrazine 9 by treatment with
2.3. Synthesis of 3,6-bis-(4-bromophenyl)-1,2,4,5-
tetrazine
An analogous Sandmeyer reaction12 was used to prepare
bromide 11 (as was used to synthesize tetrazine 10). 3,6-Bis-
(4-aminophenyl)-1,2,4,5-tetrazine (9) was reacted with
NaNO2/HBr/CuBr in methanolic acetic acid at 08C to give
11 in 80% yield (Scheme 2).
2.4. Synthesis of 3,6-bis-(4-phenyl-4-thiophene)-1,2,4,5-
tetrazine
The synthesis of bis(thienyl)tetrazines was accomplished by
the reaction of 3,6-bis(4-halogenophenyl)tetrazines with
2-metallated thiophene (12).13
Compound 13 was synthesized through the coupling
reaction between 12 and 3,6-bis(4-iodophenyl)- or 3,6-
bis(4-bromophenyl)-1,2,4,5-tetrazine (10,11, Scheme 3)
according to the literature procedure for the preparation of
bis(thienyl)fluorene derivatives.14
2.5. Synthesis of 3,6-bis{(4-phenyl-4-[3,4-ethylene-
dioxythiophene])}-1,2,4,5-tetrazine
The synthesis was carried out as outlined in Scheme 3 where
a Stille coupling was employed to couple 3,4-ethylene-
dioxy-2-(trimethyltin)thiophene7 (14, Scheme 3) with 3,6-
bis(4-bromophenyl)- or 3,6-bis(4-iodophenyl)-1,2,4,5-tetra-
zine (Scheme 3) to give 3,6-bis{(4-phenyl-4-1,4-dioxane-
[2,3-c]-thiophene)}-1,2,4,5-tetrazine (15).
2.6. Molecular modelling results
The properties of polymers are related to properties of
monomers. It has been shown5 that the electronic charge
distribution as well as an ionization potential correlate with
experimental parameters describing the polymerization
process. The optimization of tetrazine derivatives
Scheme 2.

J. Sol⁄oducho et al. / Tetrahedron 59 (2003) 4761–4766
4763
Scheme 3.
(5,10,11,13,15) leads to structures with the planar con-
jugated three ring fragment (Fig. 1). All the molecules
studied possess Cn symmetry resulting in the lack of a dipole
moment. The thiophene rings in 3,6-bis-(4-phenyl-4-
thiophene)-1,2,4,5-tetrazine (13) and and 3,6-bis(4-[3,4-
ethylenedioxythiophene])-4,40-bis(phenyl)-1,2,4,5-tetrazine
(15) derivatives are out of plane by 248 and 178,
respectively. Substitution has little impact on the central
tetrazine fragment regarding structural changes as well as
electronic charge distributions.
The loss of an electron due to ionization leads to a change in
elctronic density distribution, which is qualitatively differ-
ent in 3,6-bis(pyrrol-2-yl)-1,2,4,5-tetrazine (5) and phenyl
containing (13,15) moieties. The ionization potential of bis-
pyrrolyl-tetrazine amounts to 8.06 eV and the main change
of electronic density takes place on the tetrazine ring. The
calculated electronic population loss from this fragment is
0.49 electron. The population change on the possible site of
polarization (Fig. 1) only amounts to 0.04 electron. The
derivatives of bis-phenyl-tetrazine are ionized at lower
energies (6.86 and 6.41 eV for 13 and 15, respectively). The
electronic population change due to the ionization is below
0.1 electron on the tetrazine ring. The main loss of density is
observed on thiophene in 3,6-bis-(4-phenyl-4-thiophene)-
1,2,4,5-tetrazine (13, 0.260 electron) and on dioxane-
thiophene in 3,6-bis(4-[3,4-ethylenedioxythiophene])-4,40-
bis(phenyl)-1,2,4,5-tetrazine (15, 0.283 electron) rings.
More importantly, the potential sites of the polymerization
(Fig. 1) are characterized by a large electronic population
change of 0.49 electron in 13 and 0.49 electron in 15.
3. Conclusions
We have successfully developed a new strategy for
preparation of 3,6-bis(pyrrolyl)-bis(thiophene)- or bis-
(dioxanethiophene)-tetrazines: a potential bifunctional
building block for the synthesis of linear oligoheterocycles.
The desired compounds have been accomplished by a
variation of the original Pinner synthesis1 to give bis(pyr-
rolyl)tetrazines or diphenyltetrazines. The diphenyltetra-
zines reacted with 3,4-ethylenedioxy-2-(trimethyltin)-
thiophene in the presence of Pd(PPh3)3Cl2 or Pd(PPh3)4 as
catalysts to give the designed compounds. This method-
ology may also be applicable for the synthesis1 of other
tetrazine derivatives. Further work is in progress in order to
Figure 1. The optimized structure of (a) 3,6-bis-(pyrrol-2-yl)-1,2,4,5-
tetrazine (5), (b) 3,6-bis-(4-phenyl-4-thiophene)-1,2,4,5-tetrazine (13), and
(c) 3,6-bis-{(4-phenyl-4-[3,4-ethylenedioxythiophene])}-1,2,4,5-tetrazine
(15). The asterisk ( p ) indicates the potential sites of the polymerization.

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J. Sol⁄oducho et al. / Tetrahedron 59 (2003) 4761–4766
understand the role of tetrazine in these reactions and also in
polymer results.
4.1.4. 3,6-Bis-(pyrrol-2-yl)-1,2,4,5-tetrazine (5). To a
solution of isoamyl nitrite (5 g in 95% ethanol,100 mL)
was added 3,6-bis(pyrrol-2-yl)-1,2-dihydro-1,2,4,5-tetra-
zine (6 g, 28.0 mmol) (4). The mixture was refluxed for
4 h, cooled to room temperature, and filtered The product
was recrystallized from toluene to yield 85% (red
The structures and properties of synthesized molecules were
determined theoretically. All examined molecules possess
the planar three-ring fragment with negligibly different
structural parameters. The main difference between bis-
pyrrole tetrazine and substituted bis-phenyl tetrazines
concerns the change in charge distribution due to ionization.
The main change in bis-pyrrole is observed on the central
pyrazine ring with the little impact on the potential
polarization sites. The situation in substituted bis-phenyl
moieties is qualitatively different. The electron loss due to
the ionization takes place on thiophene in 3,6-bis-(4-phenyl-
4-thiophene)-1,2,4,5-tetrazine (15) and on dioxane-thio-
phene in 3,6-bis-(4-[3,4-ethylenedioxythiophene])-4,40-
bis(phenyl)-1,2,4,5-tetrazine (19) rings. More important,
the concentration on the electron loss is observed on atoms
being potential centers for the polarization.
1
product, 5.05 g), mp 2858C. H NMR d (DMSO-d6) 6.12
(s, 2H, pyrrole H), 6.55 (s, 2H, pyrrole H), 6.68 (s, pyrrole
H), 11.14, (s, 2H, NH); 13C NMR d (DMSO-d6)
108.7, 109.3, 118.39, 120.5, 148.5. Anal. calcd for
C10H8N6: C, 76.89; H, 5.16; N, 17.94. Found: C, 76.71;
H, 5.03; N, 17.90.
4.1.5. 3,6-Bis-(4-aminophenyl)-1,2-dihydro-1,2,4,5-tetra-
zine (8). A solution of 4-nitrobenzonitrile (6) (2.0 g,
13.5 mmol) and ammonium sulfide in ethanol (20 mL)
was heated on a steam bath for 24 h. The precipitate was
removed by filtration, and the solvent concentrated under
reduced pressure. The residue was heated with a small
amount of water and filtered.
Anhydrous hydrazine (98%, 5 mL) was added to crude
4-aminobenzonitrile (7) (1.0 g, 8.4 mmol). The solution was
heated on a steam bath for 18 h. After cooling, the resulting
orange-yellow precipitate was filtered, washed with water
and immediately used in the next step without purification.
4. Experimental
4.1. General comments
1
Melting points are uncorrected. All NMR spectra were
performed using a Varian VXR-300 spectrometer operating
at 300 MHz (1H) and 75 MHz (13C) in DMSO and CDCl3.
Chromatography column was carried out on Merck Keisel
gel 60 (5386) silica gel. THF was used immediately after
distilling from a solution containing benzophenone/sodium.
Other starting materials, reagents and solvents were used as
received from suppliers.
Yield 51% (2.29 g, red product), mp 2618C; H NMR d
(DMSO-d6) 5.47 (s, 4H, NH2), 6.53 (d, 4H, J¼8.3 Hz, arom.
H), 7.47 (d, 4H, J¼8.3 Hz, arom. H), 8.51 (s, 2H, NH); 13C
NMR d (DMSO-d6) 113.0, 117.3, 126.9, 148.6, 150.4.
HRMS calcd for C14H14N6 267. 1371 (M2þ1). Found:
267.1371.
4.1.6. 3,6-Bis-(4-aminophenyl)-1,2,4,5-tetrazine (9). A
solution of 3,6-bis-(4-aminophenyl)-1,2-dihydro-1,2,4,5-
tetrazine (8) (0.5 g, 1.8 mmol) and aqueous hydrogen
peroxide (6%, 50 mL) was warmed at 608C for 2 h, to
yield a red solid, which was collected by filtration and air-
dried. The crude product was purified by column chroma-
tography on silica gel using acetone and hexane (2:1) as
eluent. Yield 60% (0.3 g), brown-red product), mp 288–
4.1.1. 2-Pyrrolaldoxime (2). A solution of hydroxylamine
hydrochloride (7.5 g, 0.11 mol) and sodium trihydrate (15 g,
0.11 mol) in 40 mL water was shaken vigorously with
2-pyrrolaldehyde 1 (9.0 g, 0.095 mol) for 10 min, then
refrigerated overnight. The crude product was recrystallized
from benzene. Yield 70% (7.29 g), mp 164–1668C (lit.
164–164.58C).
1
2898C; H NMR d (DMSO-d6) 6.01 (s, 4H, NH2), 6.73 (d,
4H, J¼8.7 Hz, arom. H), 8.15 (d, 4H, J¼8.7 Hz, arom. H);
13C NMR d (DMSO-d6) 113.8, 118.3, 128.5, 152.8, 162.3;
HRMS calcd for C14H12N6 265.1201 [Mþ1]. Found:
265.1207.
4.1.2. 2-Pyrrolenitrile (3). 2-Pyrrolealdoxime (9.2 g,
0.084 mol) in acetic anhydride was warmed slowly and
then refluxed gently for 30 min. The cooled reaction
products were poured into 200 mL water, extracted three
times 20 mL portion of either, and dried over sodium
sulfate. After removal of the residue they were vacuum-
distilled and the fraction bp 89–918C was collected. Yield
75% (5.76 g).
4.1.7.
3,6-Bis-(4-iodophenyl)-1,2,4,5-tetrazine
(10).
Sodium nitrate (0.10 g, 4.8 mmol) in water (0.8 mL) was
added during 5 min to a solution of 3,6-bis-(4-amino-
phenyl)-1,2,4,5-tetrazine (0.1 g, 0.37 mmol) in concentrated
hydrochloric acid (1 mL) and ice (1.0 g) at 08C while
stirring. The mixture was stirred at 08C for a further 20 min,
then was placed into a jacketed addition funnel at 08C, and
was added over 20 min to a stirred solution of potassium
iodide (2.45 g, 0.015 mol) in water (3 mL) at rt. The mixture
was left to stand at rt overnight and then extracted with ether
(2£30 mL). The combined organic extracts were washed
successively with 10% aqueous NaOH (30 mL), 5%
NaHCO3 (30 mL), and H2O (50 mL) and dried over
anhydrous MgSO4. Concentration in vacuo and flash
chromatography of the residue (hexane/ethyl acetate, 2:1)
gave 3,6-bis-(4-iodophenyl)-1,2,4,5-tetrazine (5). Yield
4.1.3. 3,6-Bis-(pyrrol-2-yl)-1,2-dihydro-1,2,4,5-tetrazine
(4). Anhydrous hydrazine (98%, 5 mL) was added to
pyrrolyl-2-nitrile (3) (1.0 g, 8.4 mmol). The solution was
heated on a steam bath for 18 h. After cooling, the resulting
orange-yellow precipitate was filtered, washed with water
and immediately used in the next step without purification.
1
Red plates, yield (1.55 g, 65%), mp 2058C. H NMR d
(DMSO-d6) 6.11 (s, 2H, pyrrole H), 6.53 (s, 2H, pyrrole H),
6.67 (s, 2H, pyrrole H), 8.50 (s, 2H, –NH–NH), 11.11 (s,
2H, NH); 13C NMR d (DMSO-d6) 108.7, 109.3, 118.39,
120.5, 148.5.

J. Sol⁄oducho et al. / Tetrahedron 59 (2003) 4761–4766
4765
1
70% (0.13 g, pink prism), mp 2788C; H NMR d (DMSO-
69.8, 122.7, 126.6, 128.3, 132.5, 164.0. Anal. calcd for
C26H18N4O4S2:C,60.71;H,3.53;N,10.90.Found,C,60.3;H,
3.48; N, 10.6.
d6) 7.84 (d, 4H, J¼8.4 Hz, arom. H), 7.91 (d, 4H, J¼8.1 Hz,
arom. H); 13C NMR d (DMSO-d6) 96.3, 105.9, 127.9, 137.7.
Anal. calcd for C14H8N4J2: C, 35.59; H, 1.66; N, 11.52.
Found: C, 35.58; H, 2.06.
4.2. Theoretical methods
4.1.8. 3,6-Bis-(4-bromophenyl)-1,2,4,5-tetrazine (11).
Sodium nitrate (3.14 g, 0.045 mol) in water (9.1 mL) was
added with stirring over 10 min to a cold (08C) solution of
3,6-bis-(4-aminophenyl)-1,2,4,5-tetrazine (0.5 g, 0.019 mol)
in mixture of concentrated sulfuric acid (4.5 mL), methanol
(2.3 mL), and water (6.7 mL). The mixture was stirred at 08C
for further 30 min and then added over 45 min to a stirred,
warm (608C) solution of copper(I) bromide (0.4 g, 2.8 mmol),
hydrobromic acid (48%, 1.2 mL), and water (6.7 mL). At the
end of the addition the mixture was refluxed for 1 h, cooled,
and filtered. The precipitate was washed with water (15 mL),
and air-dried. Purification by flash chromatography (hexane/
ethyl acetate, 2:1) gave 3,6-bis-(4-bromophenyl)-1,2,4,5-
tetrazine (6). Yield 80% (0.60 g, red needles), mp 1988C; 1H
NMR d (DMSO-d6) 7.69 (d, 4H, J¼8.3 Hz, arom. H), 8.00 (d,
4H, J¼8.5 Hz, arom. H); 13C NMR d 122.7, 126.6, 128.3,
132.5, 164.0. Anal. calcd for C14H8N4Br2: C, 42.88; H, 2.05.
Found; C, 42.88; H, 2.15.
The calculations presented here were performed apply-
ing the density functional theory (DFT) method. The
DFT approach utilized Backe’s three-parameter func-
tional15 with the local correlation part of Vosco et al.16
and the nonlocal part of Lee et al.17 (abberivated as
B3LYP). The calculations were performed in the
standard 6-31Gp atomic basis set.18 The geometry of
molecules were optimized and later used for calcu-
lations of vertical ionization energies. The electronic
density distribution is based on the Mulliken population
scheme utilizing the DFT/B3LYP function. The results
reported here were obtained using the GAUSSIAN-98
code.19
Acknowledgements
4.1.9. 2-(Trimethyltin)-thiophene (12). Compound 12 was
prepared according the standard procedure.13 Yield 70%.
1HNMR (CDCl3): 0.35 (s, 9H, CH3); 6.05 (s, 1H, CH); 6.20
(s, 1H, CH), 6.45 (s, 1H, CH).
The authors are indebted to Prof. A. R. Katritzky for
valuable discussion of manuscript. This work was facilitated
in part by Wroclaw University of Technology Grant and
Wroclaw University of Technology CMZiN Grant no.
331977.
4.1.10. 3,6-Bis-(4-phenyl-4-thiophene)-1,2,4,5-tetrazine
(13). Compound 12 (0.52 g, 2.1 mmol). 3,6-Bis-(4-bromo-
phenyl)-1,2,4,5-tetrazine (0.4 g, 1.0 mmol), and Pd(PPh3)4
(0.6 g, 0.8 mmol) were added to THF (100 mL). The
mixture was stirred at 658C overnight under nitrogen
atmosphere. The THF was evaporated to give solid. The
residue was purified by column chromatography (eluent:
hexane/ethyl acetate, 2:1). Yield 85% (0.34 g, yellow-green
plates), mp 199–2018C. 1HNMR (CDCl3): 6.15 (s, 2H, CH,
thiophene); 6.25 (s, 2H, CH, thiophene), 6.35 (s, 2H, CH),
7.15 (d, 4H, J¼7.05 Hz, arom. H), 7.45 (d, 4H, J¼7.42 Hz,
arom.H); 13C NMR d 60.5, 61.4, 67.2, 69.7, 122.7, 126.6,
128.3, 132.5, 164.0. Anal. calcd for C22H14N4S2: C, 63.33;
H, 3.54; N, 14.07. Found, C, 63.1; H, 3.45; N, 14.00.
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thiophene); 7.05 (d, 4H, J¼7.05 Hz, arom. H), 7.34 (d, 4H,
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