Polyazaacenes - On the way to stable, fluorescent and redox-active derivatives

Article abstract of DOI:10.1002/ejoc.200600803

A new synthetic method for the preparation of polyazaacenes is described starting from two different nucleophilic building blocks. Disubstituted oxalic amidines 1 can be cyclized under mild conditions with 2,3-dichloro-5,6- dicyanopyrazine (3) to yield 5,6-dihydropyrazino[2,3-b]pyrazines 4a-c. By employing higher temperatures and 2 equiv. of 3, octaazanaphthacene 6 can be isolated. Similarly, pyrazino[2,3-b]pyrazines 2 and bielectrophile 3 yielded novel dodecaazahexacenes 8 in addition to semicyclized derivative 9. When tetraazafulvalene 10d was heated in the presence of an amine in xylene in the presence of oxygen, octaazahexacene 13 was isolated as the main product. Instead of pyrazino[2,3-5]pyrazines 2, this highly fluorescent polyazaacene was formed from a cascade reaction which involves a dyotropic rearrangement, an intramolecular Diels-Alder reaction and a multistep redox reaction. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejoc.200600803

FULL PAPER
DOI: 10.1002/ejoc.200600803
Polyazaacenes – On the Way to Stable, Fluorescent and Redox-Active
Derivatives
Frances Stöckner,[a] Rainer Beckert,*[a] Dieter Gleich,[a] Eckhard Birckner,[b]
Wolfgang Günther,[a] Helmar Görls,[c] and Gavin Vaughan[d]
Dedicated to Prof. Dr. Manfred Christl, Würzburg, on the occasion of his 65th birthday
Keywords: Fused ring systems / Polycycles / Nitrogen heterocycles / Rearrangement / Redox chemistry
A new synthetic method for the preparation of polyazaacenes
is described starting from two different nucleophilic building
blocks. Disubstituted oxalic amidines 1 can be cyclized under
mild conditions with 2,3-dichloro-5,6-dicyanopyrazine (3) to
yield 5,6-dihydropyrazino[2,3-b]pyrazines 4a–c. By em-
lene 10d was heated in the presence of an amine in xylene
in the presence of oxygen, octaazahexacene 13 was isolated
as the main product. Instead of pyrazino[2,3-b]pyrazines 2,
this highly fluorescent polyazaacene was formed from a cas-
cade reaction which involves a dyotropic rearrangement, an
ploying higher temperatures and 2 equiv. of 3, octaazanaph- intramolecular Diels–Alder reaction and a multistep redox re-
thacene 6 can be isolated. Similarly, pyrazino[2,3-b]pyrazines
2 and bielectrophile 3 yielded novel dodecaazahexacenes 8
action.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
in addition to semicyclized derivative 9. When tetraazafulva- Germany, 2007)
tution results in higher stabilities towards oxidation and in
Introduction
addition, introduces a new pattern of polarity into the sys-
tem. As a synthetic consequence, acenes which possess aza
nitrogen atoms should be accessible for modifications such
as protonation, alkylation and formation of N-oxides. On
the other hand, azaacenes are electronically “umpoled” sys-
tems bearing acceptor properties. Furthermore, the rigid
system of such polyazaheterocycles often shows intense
fluorescence, which makes them applicable as marker mole-
cules.[1] Because they are representative derivatives, substi-
tuted quinoxalino[2,3-b]phenazines[2] and their regioiso-
mers[3] are well-known; however, nitrogen-rich azaacenes
are rather rare. Only a few literature reports exist that deal
with the synthesis of systems possessing more than two lin-
ear condensed pyrazine rings including their hydro deriva-
tives such as fluorubine,[4] 5,10-dihydropyrazino[2,3-b:2Ј,3Ј-
e]-pyrazines,[5,6] decaazapentacenes[7] and octaazadihydro-
hexacenes.[8]
Disubstituted oxalic amidines 1,[9] which were already
used as building blocks for five-membered heterocyclic ring
systems,[10,11] should also be applicable to six-membered
rings. Moreover, we demonstrate that the dyotropic re-
arrangement of 1,4,5,8-tetraazafulvalenes allows easy ac-
cess to tetrasubstituted pyrazino[2,3-b]pyrazines 2.[12] Such
previously unknown compounds should be tested for ring
fusion reactions through all four nucleophilic amino func-
tions (Scheme 1).
Acenes are derived from the parent compound anthra-
cene by linear annulation with further benzo rings. In con-
trast to anthracene itself, the homologues exhibit an in-
crease in their reactivity and parallely, a crucial shift in the
longest wavelength absorption. Whereas tetracene is still
relatively stable, the deeply coloured pentacene, hexacene
and heptacene are air- and light sensitive. This behaviour
can easily be explained by a decrease in the aromaticity
which is accompanied by an increase in the polyene charac-
ter of the compound, which intensifies the colour. The sta-
bilization of higher acenes is possible by substitution with
aryl substructures (rubrene) or by angular ring fusion. An-
other approach used to stabilize these systems exists by the
replacement of CH groups by an aza nitrogen. This substi-
[a] Institut für Organische und Makromolekulare Chemie, Fried-
rich-Schiller-Universität,
Humboldtstraße 10, 07743 Jena, Germany
Fax: +49-3641-948-212
E-mail: c6bera@uni-jena.de
[b] Institut für Physikalische Chemie, Friedrich-Schiller-Uni-
versität,
Lessingstraße 10, 07743 Jena, Germany
[c] Institut für Anorganische und Analytische Chemie, Friedrich-
Schiller-Universität,
August-Bebel-Strasse 2, 07743 Jena, Germany
[d] European Synchroton Radiation Facility (ESRF) BP220,
38043 Grenoble Cedex, France
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R. Beckert et al.
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only one prototropic isomer could be detected in which the
hydrogen is located at the exocyclic arylamino group
(Scheme 2, Figure 1).
Scheme 1. Building blocks for polyazaacenes based on bis(amidine)
s 1 and pyrazino[2,3-b]pyrazines 2.
Results and Discussion
First, 2,3-dichloro-5,6-dicyanopyrazine (3) was employed
as a bielectrophilic partner in order to synthesize polyaza-
acenes. This easily accessible pyrazine constitutes a valuable
building block for a series of azaheterocycles.[13] Some de-
rivatives obtained in this manner have strong electron-
accepting properties and thus can be used for the construc-
tion of air-stable n-type transistors in organic FET de-
vices.[14]
Scheme 2. Synthesis of ring-fused pyrazines 4–6.
In the first step, amidines 1a–c were deprotonated by
means of nBuLi at –78 °C. Upon addition of cyclization
partner 3, 5,6-dihydropyrazino[2,3-b]pyrazines 4a–c were
formed in yields up to 70% in a smooth reaction. The struc-
tures of bicyclic products 4 were assigned by elemental
analyses, mass spectra and NMR spectroscopy. In addition,
X-ray crystal structure analysis was performed from a single
crystal of derivative 4a. The structure in Figure 1 clearly
shows that the cyclization reaction occurred by attack at
two different nitrogen atoms in amidine 1. The bicyclic pyr-
azinopyrazine has an almost planar geometry, only the tolyl
substituent at the ring nitrogen is twisted out of plane. In
the solid state as well as in solution (NMR experiments),
Because of their vicinal amino substructure, compounds
4 can be further converted into azaheterocycles, as exem-
plified by hexaazacyclopentanaphthalenes 5a,b. Thus, sim-
ple heating of derivatives 4a,b in orthoformate yielded yel-
low fluorescent tricycles 5a,b. No twofold cyclization reac-
tion of derivatives 1 could be observed upon the employ-
ment of an excess of bielectrophile 3. However, the desired
twofold ring fusion products of type 6 could be synthesized
by reacting molten 4b with 3 at 200 °C under an atmosphere
of argon. After purification by flash chromatography, new
C2-symmetrical octaazanaphthacene 6 was isolated in a
yield of 50% as yellow crystals. Despite its rigidity, this de-
rivative is relatively soluble in different solvents. In acetoni-
trile, it displays fluorescence with a maximum at λ = 487 nm
(φFl = 0.55) and a small Stokes shift of 5 nm.
Because of their chelating substructure, derivatives of
type 4 are a good choice for the construction of metal com-
plexes. For example, the complexation reaction of 4a with
NiCl2 resulted in the formation of square planar NiII com-
plex 7. From a red single crystal, the crystal structure of 7
was obtained and the result is depicted in Figure 2
(Scheme 3).
However, similarly to the reaction of molten 1 with 3,
new polyazaacenes of type 8 could be prepared starting
from pyrazino[2,3-b]pyrazines 2a. Their NMR spectra sug-
gest a high molecular symmetry by single signal sets. This
novel dodecaazahexacene displays fluorescence at λ =
522 nm (excitation at λ = 517 nm) in solution with quantum
yields of 35% and a small Stokes shift. As a byproduct,
semicyclic derivative 9 was isolated. By means of 2D NMR
experiments, the predominance of the prototropic isomer
was confirmed in which the two NH-protons are located at
exocyclic aryl–nitrogen and pyrazine ring nitrogen atoms.
In contrast to fully cyclized derivative 8, this azacycle shows
Figure 1. Solid state (X-ray) structure of compound 4a, selected
bond lengths [Å]: N2–C3 1.393, C4–N6 1.354, C3–N5 1.278, N3–
C4 1.301, N2–C2 1.362, N3–C5 1.344, C2–C5 1.452, C3–C4 1.508. no fluorescence (Scheme 4).
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Polyazaacenes – Towards Stable, Fluorescent and Redox-Active Derivatives
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pyrazines 2a–c were easily formed, 2d could be isolated only
under strict exclusion of oxygen. However, when this re-
arrangement was carried out in the presence of oxygen, tet-
raazafulvalene 10d was transformed into a new product
which could be detected by TLC by way of its strong red
fluorescence. The structure of this deep blue compound was
assigned by HRMS, NMR and UV/Vis data as well as
chemical transformations. Hence, it possesses the structure
of an aryl-substituted derivative of octaazahexacene 13
(Scheme 5).
As a first step of the mechanism, we postulate the dy-
otropic rearrangement of tetraazafulvalene 10d into pyrazi-
nopyrazine 2d. The latter compound should also exist as
prototropic isomer 2dЈ possessing an exocyclic s-cis 1,4-di-
azadiene unit which fulfills the requirements for a subse-
quent reaction cascade. DFT calculations predict a relative
free energy of only +37 kJ/mol for tautomer 2dЈ compared
to 2d (see Experimental Section, Computational Methods).
As the key step of the reaction, an intramolecular [4+2]
cycloaddition takes place followed by rearomatization to
give tetrahydrotetraazanaphthalene 12. In this reaction, the
preformed 1,4-diazadiene acts as the (dihetero)diene and
the benzene ring as the dienophile. Generally, thermally in-
duced [4+2] cycloaddition reactions which yield a cy-
clohexa-1,3-diene originating from benzene are extremely
rare. Only in one case has such a reaction been described
with the use of a specially designed organic ring system
which reacts in an unselective reaction at high tempera-
tures.[15] In other cases, a fast rearrangement of the cy-
clohexa-1,3-diene intermediate resulted in the formation of
an aromatic ring.[16] Very recently,[17] an example of an in-
tramolecular metal-mediated case was described for the first
time. Thus, a twofold cycloaddition–rearomatization cas-
cade sequence may lead to the formation of a system con-
sisting of four fused pyrazines with two peripheral benzo
rings and two electron rich tetraaminoethene substructures
12. Finally, a multistep oxidation by oxygen takes place
forming the stable quinoid system of 13. This is in agree-
ment with experimental findings for other tetraaminoeth-
enes derived from imidazoles[18] and 2,2Ј-biimidazoles.[19]
Furthermore, these observations were proposed by earlier
works of Armand and Badger[20,21] who reported structures
and stabilities of ring-fused pyrazines and their 1,4-de-
hydroderivates connected with their redox behaviour.
As a byproduct, deep purple compound 14 could be iso-
lated. Its structure has been solved by 2D NMR experi-
ments and HRMS. Because of the fact that 14 yields 13 by
neat conversion by heating in the presence of oxygen, it can
be regarded as a semicyclized precursor of 13. Further evi-
dence for the structure of 13 results from its reduction with
sodium dithionite in a THF/water mixture. Immediately,
the intense red fluorescence turned yellow, which originated
from the formation of reduced form 15. Derivative 14
Figure 2. Solid state (X-ray) structure of compound 7, selected
bond lengths [Å]: Ni–N2 2.091, Ni–N1 2.021, N2–C2 1.312, N1–
C1 1.276, C1–N3 1.387, C2–N4 1.352, C1–C2 1.492, N4–C4 1.332,
N3–C3 1.386; selected bond angles [°]: N1–Ni–N1A 180.00, N1–
Ni–N2 79.12, N1–Ni–N2A 100.88, N1A–Ni–N2 100.88, N1A–Ni–
N2A 79.12, N2–Ni–N2A 180.00, N1–C1–C2 118.52, N1–C1–N3
124.12, N2–C2–N4 125.02, N2–C2–C1 112.79, N4–C2–C1 122.19.
Scheme 3. Complexation reaction of 4a with NiCl2.
Scheme 4. Cyclization reaction of pyrazino[2,3-b]pyrazines 2 with
2,3-dichloro-5,6-dicyanopyrazine 3.
The dyotropic rearrangement of 1,4,5,8-tetraazafulval- shows the same redox behaviour; hence, reduced aromatic
enes 10a–c (ArЈ = 3-CF3C6H4, 4-IC6H4, 4-EtOOCC6H4) [12] structure 16 is describable. Exposure to air caused the re-
(which forms the basis for the synthesis of compounds 2) generation of the fluorescence. This colour change is fully
could now be optimized. Here, an unexpected reaction was reversible and can be repeated several times. Cyclic voltam-
observed: whereas strong yellow fluorescent pyrazino- metric measurements revealed two reversible one-electron
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R. Beckert et al.
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Scheme 5. Mechanism proposed for the formation of octaazahexacenes of type 13.
reduction potentials at –0.5 V, –1.1 V for 13 and –0.3 V, addition of base. In 1890, Fischer and Hepp[3] reported sim-
–1.0 V for 14. ilar spectral properties for fluorinidine, which possess an
A zwitterionic structure as in syn-derivate 17 can be ex- analogous quinone-like substructure (Scheme 6).
cluded because of the fact that derivatives of type 13 show
no solvatochromism.[22–24] The emission spectrum of deriv-
ative 13 is symmetrical to its absorption spectrum and
shows only a small Stokes shift of about 4 nm.
Experimental Section
General Information: Reagents were purchased from Aldrich, Acros
or Fluka and used without further purification. Solvents were puri-
fied by distillation with the appropriate drying agent. Reactions
were performed under a dry atmosphere of argon by using standard
Schlenk techniques. The solvents used (THF, xylene) were dried by
distillation over sodium and benzophenone. Liquid chromatog-
raphy was performed with columns of silica gel 60 Å, Merck,
0.040–0.063 mesh or alumina 90 Å, Merck, 0.063–0.200 mesh, ac-
tivity I. TLC for reaction monitoring was performed with plates of
thickness 0.2 mm, Roth, Polygram SIL G/UV254 or Polygram
ALOX N/UV254. NMR experiments were performed with a
Bruker AC 250 und Bruker AC 400. Chemical shifts are reported
as δ values relative to TMS, defined as δ = 0.00 ppm (1H NMR)
or δ = 0.0 ppm (13C NMR). Mass spectra were performed with a
Quadrupol SSQ 710 Finnigan MAT or Finnigan MAT 900 XL
TRAP instrument. Elemental analyses were measured with a
LECO, CHNS-932 analysis instrument. Melting points were deter-
mined with a Galen III (Boetius System, Cambridge Instruments)
and are uncorrected. Absorption spectra were recorded with a Per-
kin–Elmer Lambda 19 spectrophotometer. Fluorescence quantum
yields were calculated according to the procedure outlined by De-
mas and Crosby[25] against quinine sulfate in 0.1  sulfuric acid as
standard with an LS 50 luminescence spectrometer (Perkin–Elmer).
Cyclic voltammetry (CV) experiments were conducted at 298 K by
using a Metrohm 663 VA 25-mL cell [10 mL solution of 3 mg of
each compound dissolved in 0.3 g Bu4NBF4 (99%) in CH2Cl2
(99.9%)] equipped with a glassy carbon working electrode, a plati-
num counter electrode, and a silver wire, which worked as a pseu-
Upon protonation with perchloric acid, a bathochromic
shift of 50 nm was observed, which is reversible upon the
Scheme 6. Redox behaviour of systems 13 and 14, zwitterionic
structure for syn-derivative 17.
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Polyazaacenes – Towards Stable, Fluorescent and Redox-Active Derivatives
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doreference electrode. All potentials reported in this work were
measured against the ferrocenium/ferrocene (Fc+/Fc) redox couple.
All of the electrolyte solutions were deoxygenated by bubbling ar-
gon gas through the solution for at least 10 min.
6-Imino-5-(4-tolyl)-7-(4-tolylamino)-5,6-dihydropyrazino[2,3-b]-
pyrazine-2,3-dicarbonitrile (4a): Following the general procedure,
the reaction with 1a gave 1.2 g of a yellow solid, 60% yield. M.p.
1
Ͼ 250 °C. H NMR (250 MHz, [D8]THF): δ = 9.88 (s, 1 H, NH),
3
7.89 (d, J = 7.5 Hz, 2 H, tolyl), 7.68 (s, 1 H, NH), 7.37 (d, 3J =
Crystal Structure Determination: The intensity data for the com-
pounds were collected with a Nonius Kappa CCD diffractometer
by using graphite-monochromated Mo-Kα radiation. For com-
pound 7, measurements were carried out at beamline ID11 at the
European Synchrotron Radiation Facility (ESRF). Data were col-
lected with a Bruker “Smart” CCD-camera system at fixed 2θ,
while the sample was rotated over 0.1° intervals during 2 s expo-
10 Hz, 2 H, tolyl), 7.02 (m, 4 H, tolyl), 2.42 (s, 3 H, CH3); 2.24 (s,
3 H, CH3) ppm. 13C NMR (62 MHz, [D8]THF): δ = 151.7, 145.2,
144.7, 141.4, 140.3, 135.1, 135.0, 131.2, 130.8, 129.3, 128.4, 126.0,
123.6, 121.4, 114.0, 20.4, 20.1 ppm. UV/Vis (CH3CN): λmax (logε)
= 226 (4.4), 278 (4.1), 392 (4.5) nm. DEI-MS: m/z (%) = 392 (55)
[M]+, 391 (100), 377 (100), 349 (5), 301 (10), 286 (5), 181 (15), 131
(10), 116 (10), 91 (80), 65 (55), 39 (30), 28 (55). C22H16N8 (392.42):
calcd. C 67.34, H 4.11, N 28.55; found C 67.68, H 4.59 N 28.39.
sures with the use of monochromated radiation from
λ =
0.42468 Å. Data were corrected for Lorentz and polarization ef-
fects, but not for absorption effects.[26–28] The structures were
solved by direct methods (SHELXS[29]) and refined by full-matrix
5-(4-tert-Butylphenyl)-7-(4-tert-butylphenylamino)-6-imino-5,6-dihy-
dropyrazino[2,3-b]pyrazine-2,3-dicarbonitrile (4b): Following the ge-
neral procedure, the reaction with 1b gave 1.67 g of a yellow solid,
2
least-squares techniques against Fo (SHELXL-97[30]). All hydro-
1
70% yield. M.p. Ͼ 250 °C. H NMR (250 MHz, [D6]DMSO): δ =
gen atoms were included at calculated positions with fixed thermal
parameters. All non-hydrogen atoms were refined anisotropi-
cally:[31] XP (SIEMENS Analytical X-ray Instruments, Inc.) was
used for structure representations.
10.56 (s, 1 H, NH), 7.89 (d, 3J = 7.5 Hz, 2 H, phenyl), 7.82 (s, 1
3
3
H, NH), 7.71 (d, J = 10 Hz, 2 H, phenyl), 7.47 (d, J = 10 Hz, 2
H, phenyl), 7.34 (d, 3J = 7.5 Hz, 2 H, phenyl), 1.38 (s, 9 H,
C(CH3)3), 1.31 (s, 9 H, C(CH3)3) ppm. 13C NMR (62 MHz, [D8]-
THF): δ = 152.4, 149.2, 148.0, 146.1, 145.0, 144.8, 128.3, 127.4,
125.8, 125.2, 124.7, 122.8, 122.5, 121.1, 114.8, 114.5, 34.6, 34.2,
31.3, 31.1 ppm. UV/Vis (CH3CN): λmax (logε) = 393 (4.3), 280
(4.1), 225(4.3) nm. DEI-MS: m/z (%) = 476 (20) [M]+, 419 (100),
195 (10), 115 (20), 57(20). C28H28N8 (476.58): calcd. C 70.57, H
5.92, N 23,51; found C 71.98, H 6.88, N 22.15.
Crystal Data for 4a:[31] C22H16N8·C2H6OS, FW = 548.68 gmol–1,
orange prism, size 0.03ϫ0.03ϫ0.02 mm3, orthorhombic, space
group Pna21, a = 18.2722(6), b = 5.7074(3), c = 26.1666(9) Å, V =
2728.8(2) Å3, T = –90 °C, Z = 4, ρcalcd. = 1.336 gcm–3, µ (Mo-Kα)
= 2.35 cm–1, F(000) = 1152, 11461 reflections in h(–20/23), k(–6/7),
l(–33/33), measured in the range 2.23°ՅΘՅ27.46°, completeness
Θmax = 99.6%, 5557 independent reflections, Rint = 0.0689, 3711
6-Imino-5-(2,4,6-trimethylphenyl)-7-(2,4,6-trimethylphenylamino)-
5,6-dihydropyrazino[2,3-b]pyrazine-2,3-dicarbonitrile (4c): Following
the general procedure, the reaction with 1c gave 1.57 g of a yellow
solid, 70 % yield. M.p. Ͼ 250 °C. 1H NMR (250 MHz, [D6]-
DMSO): δ = 9.03 (s, 1 H, NH), 7.58 (s, 1 H, NH), 7.20 (s, 2 H,
phenyl), 7.05 (s, 2 H, phenyl), 2.44 (s, 3 H, CH3), 2.37 (s, 3 H,
CH3), 2.25 (s, 6 H, CH3), 1.97 (s, 6 H, CH3) ppm. 13C NMR
(62 MHz, [D8]THF): δ = 152.7, 147.8, 147.4, 146.8, 139.5, 136.9,
134.6, 133.1, 129.2, 128.7, 128.1, 127.6, 127.1, 126.0, 116.8, 115.5,
113.2, 20.6, 20.4, 18.0, 17.4 ppm. UV/Vis (acetone): λmax (logε) =
386 (4.4) nm. DEI-MS: m/z (%) = 450 (10) [M+H]+, 434(100),
421(20), 412(18). C26H24N8 (448.53): calcd. C 69.62, H 5.39, N
24.98; found C 70.03, H 5.65, N 24.47.
reflections with Fo Ͼ 4σ(Fo), 343 parameters, 1 restraints, R1obs
=
0.0927, wR2obs = 0.2350, R1all = 0.1375, wR2all = 0.2715, GOOF =
1.043, Flack-parameter 0.24(19), largest difference peak and hole:
2.041/–0.529 e Å–3.
Crystal Data for 7:[31] C48H40N16NiO2S2, FW = 995.79 gmol–1,
orange prism, size 0.02ϫ0.02ϫ0.01 mm3, monoclinic, space group
P21/c, a = 9.0737(9), b = 11.5249(10), c = 23.164(3) Å, β =
97.005(4)°, V = 2404.3(4) Å3, T = 100 °C, Z = 2, ρcalcd.
=
1.376 gcm–3, µ (λ = 0.42468 Å) = 2.93 cm–1, F(000) = 1032, 24079
reflections in h(–11/11), k(–14/14), l(–23/28), measured in the range
1.61°ՅΘՅ15.39°, completeness Θmax = 84.9%, 4186 independent
reflections, Rint = 0.0403, 3948 reflections with Fo Ͼ 4σ(Fo), 313
parameters, 0 restraints, R1obs = 0.0541, wR2obs = 0.1471, R1all
=
General Procedure for the Synthesis of Derivatives of Type 5: A
mixture of 5,6-dihydropyrazino[2,3-b]pyrazine 4 (25 mmol) and tri-
methylorthoformate (20 mL) in the presence of a small amount of
p-TsOH was heated under reflux for 50 h under an atmosphere of
argon. The reaction was monitored by TLC (silica, toluene/ethyl
acetate, 10:1). After completeness of the reaction, the mixture was
evaporated to dryness in vacuo, and the crude product was purified
by column chromatography (silica, toluene/ethyl acetate, 19:1).
0.0565, wR2all = 0.1488, GOOF = 1.096, largest difference peak
and hole: 0.696/-0.789 e Å–3
Computational Methods: Tetraazanaphthalenes 2d and 2dЈ were cal-
culated with ArЈ = phenyl. For all DFT Calculations (program
Gaussian03[32]) the 6-311G(d,p) standard basis set[33] was used. Ge-
ometry optimisations with default values for grid and convergence
parameters were made using the hybrid functional B3LYP.[34] The
optimised structures were verified to be true minima by analytical
determination of harmonic frequencies.
2-Methoxy-1,4-bis(4-tolyl)-2,4-dihydro-1H-1,3,4,5,8,9-hexaazacy-
clopenta[b]naphthalene-6,7-dicarbonitrile (5a): Following the gene-
ral procedure, the reaction with 4a gave 0.76 g of a yellow solid,
General Procedure for the Synthesis of Pyrazino[2,3-b]pyrazines of
Type 4: nBuLi (5 mL, 0.01 mol, 2  solution in n-hexane) was
added dropwise to a solution of bis(amidine) 1 (0.005 mol) in
freshly dried THF (30 mL) at –78 °C. After stirring the reaction
mixture for 20 min, 2,3-dichloro-5,6-dicyanopyrazine 3 (1.0 g,
5 mmol) was added, and the mixture was stirred for one hour while
slowly warming to room temperature to give an brown–orange
solution. The solution was evaporated to dryness in vacuo. Then,
chloroform was added and the resulting solution was washed sev-
eral times with water. After drying the solution with Mg2SO4, it
was evaporated and the residue was purified by column chromatog-
raphy (silica, toluene/ethyl acetate, 19:1).
1
70% yield. M.p. 237 °C (dec.). H NMR (250 MHz, [D6]DMSO):
3
δ = 7.87 (d, J = 8.5 Hz, 2 H, tolyl), 7.45 (m, 6 H, tolyl), 3.16 (s, 3
H, OCH3), 2.47 (s, 3 H, CH3-tolyl), 2.42 (s, 3 H, CH3-tolyl) ppm.
13C NMR (62 MHz, [D6]DMSO): δ = 153.4, 151.6, 147.0, 142.3,
139.1, 136.3, 132.7, 131.3, 130.1, 129.7, 128.8, 128.1, 127.8, 125.2,
123.2, 121.3, 114.5, 114.3, 105.2, 49.9, 20.8, 20.6 ppm. UV/Vis
(CH3CN): λmax (logε) = 384 (4.5), 275 (4.2), 225 (4.4) nm. Fluores-
cence (CH3CN): λmax = 486, 508, 514 nm. DEI-MS: m/z (%) = 434
(40) [M]+, 419 (60), 387 (10), 313 (5), 284 (5), 201 (10), 131 (10),
116(10), 91 (100), 65 (60). C24H18N8O (434.46): calcd. C 66.35, H
4.18, N 25.79; found C 65.75, H 4.18, N 24.98.
Eur. J. Org. Chem. 2007, 1237–1243
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1241

R. Beckert et al.
1,4-Bis(4-tert-butylphenyl)-2-methoxy-2,4-dihydro-1H-1,3,4,5,8,9- N,NЈ,NЈЈ,NЈЈЈ-Tetrakis(4-carboxyethylphenyl)pyrazino[2,3-b]-
FULL PAPER
hexaazacyclopenta-[b]naphthalene-6,7-dicarbonitrile (5b): Following
the general procedure, the reaction with 4b gave 0.65 g of a yellow
pyrazine-2,3,6,7-tetraamine (2c): Following the general procedure,
the reaction of 10c gave 1.57 g of a orange solid, 20% yield. M.p.
1
solid, 50% yield. M.p. Ͼ 250 °C. 1H NMR (400 MHz, [D6]DMSO, 240 °C (dec.). H NMR (250 MHz, [D8]THF): δ = 10.33 (s, 4 H,
3
3
65 °C): δ = 7.82 (d, J = 8.8 Hz, 2 H, aryl), 7.64 (d, J = 8.4 Hz, 2
NH), 7.93–7.82 (m, 16 H), 4.25 [q, J = 7.0 Hz, 8 H, CH2], 1.28 (t,
3
H, aryl), 7.58 (d, 3J = 8.8 Hz, 2 H, aryl), 7.43 (d, J = 8.8 Hz, 2 H,
J = 7.5 Hz, 12 H, CH3) ppm. 13C NMR (63 MHz, [D6]DMSO): δ
aryl), 7.33 (s, 1 H, CH), 3.15 (s, 3 H, OCH3), 1.38 (s, 9 H, = 165.2, 158.7, 141.8, 130.0, 125.6, 120.0, 60.5, 14.1 ppm. UV/Vis
C(CH3)3), 1.34 (s, 9 H, C(CH3)3) ppm. 13C NMR (100 MHz, [D6]- (CH3CN): λmax (logε) = 249 (3.8), 355 (3.9), 458 (3.6), 485 (3.7)
DMSO): δ = 154.2, 152.5, 152.1, 150.1, 147.6, 142.7, 133.2, 131.8,
128.1, 126.9, 126.5, 125.9, 125.7, 123.8, 121.9, 115.0, 114.8, 106.0,
50.5, 35.1, 34.9 31.6 ppm. UV/Vis (CH3CN): λmax (log ε) = 385
nm. Fluorescence (CH3CN): λmax = 500 nm. Φem = 0.69. DEI-MS:
m/z = 784 [M]+, 755[M–C2H5]+, 165 [NHC6H4COOCH2CH3]+.
ESI-HRMS: calcd. for C42H40N8O8 784.2969; found 784.2941.
(4.5), 277 (4.2), 225 (4.4) nm. Fluorescence (CH3CN): λmax
=
N,NЈ,NЈЈ,NЈЈЈ-Tetrakis(4-tert-butylphenyl)pyrazino[2,3-b]pyrazine-
2,3,6,7-tetraamine (2d): Following the general procedure under an
atmosphere of argon, the reaction of 10d gave 1.8 g of a orange
solid, 25% yield. M.p. 190 °C (dec.). 1H NMR (250 MHz, [D8]-
491 nm. Φem = 0.18. DEI-MS: m/z (%) = 518 (15) [M]+, 487 (25),
461 (100), 429 (7), 244 (25), 118.2 (30), 115 (30), 91 (40), 57 (20).
C30H30N8O (518.62): calcd. C 69.48, H 5.83, N 21.61; found C
69.10, H 5.87, N 20.98.
3
THF): δ = 8.09 (s, 4 H, NH), 7.68 (d, J = 7.1 Hz, 8 H), 7.27 (d,
3J = 8.7 Hz, 8 H), 1.27 (s, 36 H, C(CH3)3) ppm. 13C NMR
(63 MHz, [D6]DMSO): δ = 145.0, 141.6, 139.5, 135.7, 126.0, 120.1,
34.8, 31.9 ppm. UV/Vis (acetone): λmax (logε) = 449 (4.4), 473 (4.4)
nm. DEI-MS: m/z = 721 [M]+, 706, 689. ESI-HRMS: calcd. for
C46H57N8 721.4706; found 721.471.
5,11-Bis(4-tert-butylphenyl)-5,11-dihydro-1,4,5,6,7,10,11,12-octa-
azatetracene-2,3,8,9-tetracarbonitrile 6b: 5,6-Dihydropyrazino-
[2,3-b]pyrazine 4b (1 mmol) and 2,3-Dichloro-5,6-dicyanopyrazine-
( 3, 1.0 g, 5 mmol) were heated for 15 min at 200 °C under an atmo-
sphere of argon. After cooling to room temperature, the crude
product was purified by column chromatography (alumina, 3 was
eluted by toluene and product 6b by toluene/ethyl acetate, 15:1).
Derivative 6b was obtained as a yellow solid, 0.18 g (30% yield).
3,11-Di-tert-butyl-5,13-bis(4-tert-butylphenyl)-5,13-dihydro-
5,6,7,8,13,14,15,16-octaazahexacene (13): Tetraazafulvalene 10d
(7.68 g, 0.01 mol) was heated in the presence of oxygen at the same
conditions as described for the synthesis of 2. After 20 h, the reac-
tion was stopped. Product 13, which displays an intense red fluores-
cence, and precursor derivative 14 (deep purple, no fluorescent)
were obtained by purification by column chromatography (alu-
mina, toluene/acetone). Derivative 13 was obtained as a blue, green
metallic shining solid, 3.56 g (50% yield). M.p. Ͼ 250 °C. 1H NMR
1
3
M.p. Ͼ 250 °C. H NMR (250 MHz, [D8]THF): δ = 7.68 (d, J =
3
8.7 Hz, 4 H, aryl), 7.65 (d, J = 8.7 Hz, 4 H, aryl), 1.41 (s, 18 H,
C(CH3)3) ppm. UV/Vis (CH3CN): λmax (logε) = 482 (4.9), 452
(4.8), 425 (4.9), 403 (3.9), 330 (3.9), 258 (4.7), 232 (4.4) nm. Fluo-
rescence (CH3CN): λmax = 487, 520 nm. Φem = 0.55. DEI-MS:
m/z (%): 602 (15) [M]+, 587 (30), 275 (15), 219 (100), 91 (90). ESI-
HRMS: calcd. for C34H27N12 603.2481; found 603.2501.
3
3
(400 MHz, [D8]THF): δ = 7.75 (d, J = 8.0 Hz, 4 H), 7.67 (d, J =
4
3
8.0 Hz, 2 H), 7.46, 7.44 (dd, J = 4.0 Hz, J = 8.0 Hz, 2 H), 7.38
General Procedure for the Rearrangement of 10 to Pyrazino[2,3- (d, 3J = 8.0 Hz, 4 H), 6.70 (d, 4J = 4.0 Hz, 2 H), 1.31 (s, 36 H,
b]pyrazines of Type 2: The appropriate tetraazafulvalene 10a–c
(0.01 mol) was heated in xylene (100 mL) at 130 °C in the presence
of diisobutylamine (1 mL) for 5–24 h. The completeness of the re-
action was monitored by TLC (alumina, toluene/acetone/methanol,
20:20:1). The mixture was evaporated to dryness in vacuo and the
crude product was purified by column chromatography (alumina,
toluene/acetone). The rearrangement of 10d was performed under
an atmosphere of argon.
C(CH3)3) ppm. UV/Vis (CH3CN): λmax (logε) = 271 (4.3), 482
(3.4), 482 (3.9), 515 (3.9), 554 (4.4), 599 (4.7) nm. Fluorescence
(CH3CN): λmax = 603 nm. Φem = 0.88. DEI-MS: m/z = 716
[M+1]+. ESI-HRMS: calcd. for C46H50N8 715.422; found 715.424.
8-tert-Butyl-6-(4-tert-butylphenyl)-3-(4-tert-butylphenylimino)-3,6-
dihydro-1,4,5,6,11,12-hexaazanaphthacen-2-yl]-(4-tert-butylphenyl)-
amine (14): The semicyclized product was obtained as a purple so-
1
lid. M.p. Ͼ 250 °C. H NMR (250 MHz, [D8]THF): δ = 9.82 (s, 1
3
3
H), 7.99 (d, J = 8.7 Hz, 2 H), 7.90 (d, J = 8.7 Hz, 1 H), 7.64 (d,
N,NЈ,NЈЈ,NЈЈЈ-Tetrakis(3-trifluoromethylphenyl)pyrazino[2,3-b]-
pyrazine-2,3,6,7-tetraamine (2a): Following the general procedure,
the reaction of 10a gave 1.54 g of a orange solid, 20% yield. M.p.
170 °C (dec.). 1H NMR (250 MHz, [D8]THF): δ = 8.59 (s, 4 H,
3J = 8.6 Hz, 2 H), 7.57 (dd, J = 1.9 Hz, J = 8.7 Hz, 1 H), 7.45
4
3
(d, 3J = 8.6 Hz, 2 H), 7.35 (dd, 4J = 1.6 Hz, 3J = 8.6 Hz, 2 H), 7.18
3
(d, J = 8.6 Hz, 2 H), 6.73 (s, 1 H), 1.82 (s, 9 H, C(CH3)3), 1.61 (s,
9 H, C(CH3)3), 1.36 (s, 9 H, C(CH3)3), 1.08 (s, 9 H, C(CH3)3) ppm.
13C NMR (63 MHz, [D6]DMSO): δ = 169.1, 155.1, 154.2, 153,4,
152.9, 152.2, 148.3, 147.8, 146.7, 145.9, 144.6, 144.0, 136.74, 135.5,
133.9, 131.5, 130,2, 127.5, 127.4, 126.3, 124.9, 124.6, 124.3, 124.1,
120.6, 112.2 ppm. UV/Vis (THF): λmax (logε) = 278 (3.9), 538 (3.9),
601 (3.6) nm. DCI-MS: m/z = 717 [M]+. ESI-HRMS: calcd. for
C46H53N8 718.4401; found 717.4407.
3
NH), 8.29 (m, 8 H), 7.55 (m, 4 H), 7.32 (d, JH,H = 7.8 Hz, 4 H)
ppm. 13C NMR (62 MHz, [D8]THF): δ = 142.0, 140.6, 134.8, 131.3,
131.3, 130.2, 127.0, 122.7, 118.4, 116.1 ppm. UV/Vis (THF): λmax
(logε) = 256 (4.3), 329 (4.4), 440 (4.3), 467 (4.3) nm. Fluorescence
(CH3CN): λmax = 486 nm. Φem = 0.80. MS (FAB): m/z = 768
[M]+, 426, 400, 210. ESI-HRMS: calcd. for C34H20F12N8 768.1619;
found 768.1563.
5,8,13,16-Tetrakis(3-trifluoromethylphenyl)-5,8,13,16-tetrahydro-
1,4,5,6,7,8,9,12,13,14,15,16-dodecaazahexacene-2,3,10,11-tetra-
carbonitrile (8): 2a (0.004 g, 0.52 mmol) and 2,3-dichloro-5,6-di-
cyanopyrazine (3, 0.02 g, 0.1 mmol) were heated for 30 minutes at
N,NЈ,NЈЈ,NЈЈЈ-Tetrakis(4-iodophenyl)pyrazino[2,3-b]pyrazine-
2,3,6,7-tetraamine (2b): Following the general procedure, the reac-
tion of 10b gave 1.0 g of a orange solid, 10% yield. M.p. Ͼ 250 °C.
1H NMR (250 MHz, [D8]THF): δ = 8.24 (s, 4 H, NH), 7.64 (m, 16 200 °C under am atmosphere of argon. After cooling to room tem-
H) ppm. 13C NMR (63 MHz, [D8]DMSO): δ = 142.6, 142.1, 139.5, perature, the crude product was purified. Starting material 3 was
139.2, 136.4, 123.34 ppm. UV/Vis (CH3CN): λmax (log ε) = 270 eluted by toluene by column chromatography (silica). The products
(4.1), 334 (4.3), 450 (4.2), 475 (4.2) nm. Fluorescence (CH3CN):
λmax = 501 nm. Φem = 0.50. DEI-MS: m/z = 999 [M – 1]+, 873
[M–I]+.
were then eluted by TLC (silica, toluene/ethyl acetate, 2:1). Deriva-
tive 8 was obtained as an orange solid, 0.001 g (20% yield). M.p.
Ͼ 250 °C. 1H NMR (400 MHz, [D8]THF): δ = 7.8 (d, 3J = 8.0 Hz,
1242
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© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2007, 1237–1243

Polyazaacenes – Towards Stable, Fluorescent and Redox-Active Derivatives
FULL PAPER
3
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4 H), 7.70 (m, 4 H), 7.59 (s, 4 H), 7.55 (d, J = 8.0 Hz, 4 H) ppm.
UV/Vis (CH3CN): λmax = 264, 451, 482, 502, 517 nm. Fluorescence
(CH3CN): λmax = 522 nm. Φem = 0.35. DEI-MS: m/z = 1020 [M]+.
[17]
5,12-Bis(3-trifluoromethylphenyl)-9-(3-trifluoromethylphenyl-
amino)-8-(3-trifluoromethylphenylimino)-5,7,8,12-tetrahydro-
1,4,5,6,7,10,11,12-octaazanaphthacene-2,3-dicarbonitrile (9): This
[18]
[19]
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derivative was isolated as an orange solid, 0.0014 g (30% yield).
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1
M.p. Ͼ 280 °C. H NMR (400 MHz, [D8]THF): δ = 8.75 (s, 1 H,
3
ring-NH), 8.15 (s, 1 H, exocyclic NH), 7.97 (d, J = 8.4 Hz, 2 H),
3
3
7.87 (d, J = 8.0 Hz, 2 H), 7.77 (m, 8 H), 7.43 (t, 2 H), 7.33 (d, J
= 7.6 Hz, 2 H) ppm. UV/Vis (THF): λmax (logε) = 293 (4.3), 470
(4.3), 499 (4.4) nm. DCI-MS: m/z = 895 [M]+. ESI-HRMS: calcd.
for C40H18F12N12 895.168; found 895.166.
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Received: September 15, 2006
Published Online: November 29, 2006
Eur. J. Org. Chem. 2007, 1237–1243
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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