Novel dihydropyrazines and their double ortho-annulation to hexaazapentacenes

Article abstract of DOI:10.5560/ZNB.2014-4002

The cycloacylation of oxalic amidines 7 with bis-imidoylchlorides 6 furnished the dihydropyrazine derivatives 8. Due to their vicinal amino-imino substructures, they provide good preconditions for a double intramolecular ring closure reaction. An alternative synthesis for hexaaza-pentacenes 1 was developed using potassium carbonate as the base and lead tetraacetate as the oxidizing agent.

Full text of DOI:10.5560/ZNB.2014-4002

Novel Dihydropyrazines and their Double ortho-Annulation to
Hexaazapentacenes
Rainer Strathausena,b, Rainer Beckerta, Jan Fleischhauera, Dirk Müllerc, and
Helmar Görlsd
a
Institut für Organische Chemie und Makromolekulare Chemie, Friedrich-Schiller-Universität
Jena, Humboldtstr. 10, D-07743 Jena, Germany
Present address: Biomolecular Photonics Group, Jena University Hospital, Nonnenplan 4,
b
D-07743 Jena, Germany
Klinikum Bad Berka, Department of Molecular Radiotherapy / Center for Molecular Imaging,
c
D-99437 Bad Berka, Germany
Institut für Anorganische und Analytische Chemie, Friedrich-Schiller-Universität Jena,
d
Lessingstr. 8, D-07743 Jena, Germany
Reprint requests to Prof. Dr. R. Beckert. E-mail: Rainer.Beckert@uni-jena.de
Z. Naturforsch. 2014, 69b, 641 – 649 / DOI: 10.5560/ZNB.2014-4002
Received January 8, 2014
Dedicated to Professor Peter Metz, Dresden, on the occasion of his 60th birthday
The cycloacylation of oxalic amidines 7 with bis-imidoylchlorides 6 furnished the dihydropyrazine
derivatives 8. Due to their vicinal amino-imino substructures, they provide good preconditions for
a double intramolecular ring closure reaction. An alternative synthesis for hexaaza-pentacenes 1 was
developed using potassium carbonate as the base and lead tetraacetate as the oxidizing agent.
Key words: Hexaazapentacenes, Dihydropyrazines, ortho-Annulation, Amidines, Fluorophores
Introduction
Acenes, particularly pentacene, have attracted sig-
transport materials, and there is a high need for re-
search activities in this field.
We recently reported syntheses of novel hexaaza-
nificant interest in organic electronic devices; how- pentacenes 1 and 2, via aminolysis reaction of
ever, they become increasingly unstable with increas- tetrachloropyrazine 3 with 2-aminophenyl(phenyl)-
ing size [1]. Whereas one approach to the stabiliza- amine [20] (Scheme 1). At the beginning, product 3 has
tion of higher acenes involves their substitution at se- only been available by a tedious chlorination of pyra-
lected positions, the other involves the exchange of CH zine-2,3-dicarboxylic acid or pyrazine itself, and an al-
atoms with heteroatoms, such as nitrogen. The result- ternative synthesis is desirable. In addition, pyrazines
ing aza-acenes [2 – 5] are of particular interest since such as 3 have a high oxidation potential and, there-
they possess different electronic and physical prop- fore, are able to oxidize vicinal diamines to undesirable
erties, and they also introduce new possibilities for by-products very easily.
chemical modification at the nitrogen atoms. Linear
The starting point for a new synthetic approach
systems, such as “pyrazinacenes” 1 [6], offer by far was the observation of highly fluorescent octaaza-
the most opportunities for derivatization and applica- hexacenes 5 as by-products during the derivatiza-
tions have been reported as solvatochromic dyes [7, 8], tion reactions of pyrazino[2,3-b]pyrazines 4 [21]
redox-switchable chromophores/fluorophores [9, 10] (Scheme 2). For the formation of 5, we first postu-
and liquid crystalline materials [11]. Their most impor- lated a cycloaddition-redox sequence, however, this re-
tant application, as easily processable organic deriva- action is based on a different reaction type: an ortho-
tives, is established in organic thin-film field effect annulation reaction primarily via radical cations. Con-
transistors (OTFTs) [12 – 18]. Recent reviews [19] em- sequently, the prototropic form 40, which possesses a
phasize that aza-acenes are underrepresented in charge vicinal amino-imino substructure, plays the key role in
© 2014 Verlag der Zeitschrift für Naturforschung, Tübingen · http://znaturforsch.com

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R. Strathausen et al. · Double ortho-Annulation
Scheme 1. Retrosynthetic approach to hexaazapentacenes of types 1 and 2.
Generally, benzannulation of aromatics and het-
eroaromatics forms an important basis for the syn-
theses of new molecules, mainly for material chem-
istry. Modern organic chemistry offers an arsenal
of methods using [4+2]-cycloaddition reactions of
o-quinodimethanes [26, 27] or appropriately struc-
tured heterocycles (isobenzofurans, isoquinolinium
systems) [28, 29], benzynes/pyridynes [30 – 32], and
3,3-benzannulation reactions [33].
We have previously demonstrated that other vici-
nal amino imines also allow an oxidative ring clo-
sure reaction. Applying 4H-imidazoles [34] and 3,4-
bis(arylamino)-substituted pyrroles as model com-
pounds [35], new ring-fused derivatives were synthe-
sized and characterized. Based on quantum-chemical
calculations [35], we showed that a multi-step pro-
Scheme 2. Pyrazino[2,3-b]pyrazines 4 and 40 as starting point
for a cascade reaction to give octaazahexacenes 5.
a double ortho-annulation reaction which finally leads cess with a radical cation as the key intermediate most
to the hexacyclic derivative 5. Similar annulations have likely takes place. The fact that during the formation of
been observed in previous work dealing with the re- 5 from 4, a double ortho-annulation process took place
activity of 2,3-diaminoquinoxalines [22]; however, the prompted us to re-examine this reaction. Derivatives 8
mechanism was interpreted as a rare example of a nu- (Scheme 3) provide ideal conditions to test this hypoth-
cleophilic aromatic substitution of hydrogen (NASH) esis and are easily accessible starting from common C2
promoted by complexation of ruthenium. This type of building blocks.
quinoxaline later served as the starting material for
the synthesis of novel bridged bis-benzimidazoles via Results and Discussion
oxidative cyclization [23]. On the other hand, deriva-
tives of quinoxaline were also obtained in the synthe-
As depicted in Scheme 3, derivatives 8 were syn-
sis of Ni-diazadiene complexes which contain addi- thesized by simple cycloacylation of amidines 7 with
tional diphenylphosphane groups. Since, in this case, bis-imidoyl chlorides 6. The only weakly electrophilic,
the prototropic formation of vicinal amino-imino sub- but selective C2 building blocks of type 6 have al-
structures is not possible, the authors postulated an in- ready been used frequently for syntheses which take
tramolecular [4+2]-cycloaddition reaction in which the place via cyclization-tautomerism sequences [36 – 39].
substituted benzene rings function as a dienophile in Accordingly, the dihydropyrazines 8 were successfully
the initial step [24, 25].
synthesized. The reaction was carried out by fast addi-

R. Strathausen et al. · Double ortho-Annulation
643
Scheme 3. Synthesis of dihydropyrazines 8.
tion of one equivalent of 6 to a dioxane solution con-
taining the deprotonated amidine 7 (base: NaHMDS).
The new products, which contain two linked oxami-
dine substructures, were isolated as orange crystalline
compounds in good yields. They form equilibria con-
sisting of different prototropic forms in common sol-
vents, as indicated by broad signals in their NMR spec-
tra. Starting from the 15N-labelled bis-imidoyl chloride
6e, a doublet (1J = 89 Hz, 15N–H) was detected be-
sides a singlet for the unlabelled NH in the NMR spec-
trum of its cyclization product 8e. This experimental
finding is a clear indication of the coexistence of at
least one other prototropic isomer 8e0. The structure
of the latter, being a donor-substituted diaza-quinone-
diimine, easily explains the strong absorption bands
(λmax = 430 – 450 nm, lg ε = 4.2) in the UV/Vis spec-
tra. We succeeded in obtaining single crystals suitable
for X-ray structure analysis. The structure determined
Fig. 1. Molecular structure of dihydropyrazine 8b in the crys-
tal. (Ellipsoids at the 50% probability level; H atoms as
spheres with arbitrary radius). Selected bond lengths in Å:
C9–N3 1.4230(19), N3–C1 1.3414(18), N1–C1 1.3132(17),
N1–C2 1.3777(18), C2–N2 1.2944(17), N2–C3 1.4069(19).
of the derivative 8b confirms the preference of the pro- between C1 and N3 (1.341 (2) Å), the bond length be-
totropic form 8b0 (Fig. 1).
tween N2 and C2 (1.294 (2) Å) is, therefore, signifi-
The molecule crystallizes in space group P21/m cantly reduced – a further indication for the preference
and has a twofold rotation axis perpendicular to the of the prototropic form 80 in Scheme 3.
pyrazine ring, which, together with the nitrogen atoms,
In the first oxidation test, derivative 8a was reacted
forms a plane. Both of the exocyclic imino substruc- under the conditions (CAN in the presence of K2CO3
tures exhibit Z-configuration in the crystal. The aro- in acetonitrile) used successfully for ortho-annulation
matic rings bonded to the nitrogen atoms N2 and N3 reactions already reported [34]. After a short time, the
are twisted out of the plane. The torsion angles be- formation of a strongly yellow-fluorescent substance
tween C2–C3–C8–N2 and between C1–N3–C9–C14 was detected by TLC. The chromatographic isolation
are 29.3◦ and 52.5◦, respectively; the smaller torsion yielded a black, amorphous substance; the MS data
angle is caused by the double bond. The proton on the (M+ m/e = 494) suggested the presence of structure
nitrogen atom N3 is connected via a hydrogen bond 1a (Ar = 4-tolyl). Optimization reactions for derivative
to a methoxy group of a neighboring molecule, thus 1a showed that potassium carbonate as a base and lead
forming a ladder-type structure. Compared to the bond tetraacetate as the oxidizing agent gave the best results

644
R. Strathausen et al. · Double ortho-Annulation
Scheme 4. Double oxidative ortho-annulation to give leuco forms of 1 and 2 which were immediately oxidized by air to give
regioisomeric hexaazapentacenes 1 and 2.
(Scheme 4, 1a: 24% yield). Traces of a strongly red- groups. Derivative 8d as a precursor gave better re-
fluorescent compound were always isolated as a by- sults. With a reaction time of 5 h, the derivative 1d
product, the MS data of which were similar to those of (yield up to 45%) was obtained and characterized. In-
compound 1a. The 1H NMR spectrum clearly showed terestingly, the sequence cyclization/oxidation (start-
two singlets for two different tolyl groups (δ = 2.11 ing from 6d and 7d via 8d) was successfully realized
and 2.43 ppm). However, no exact assignment was in a one-pot synthesis, and gave comparable yields.
possible in the region of aromatic protons; the integrals The hexaaza-pentacene 1d was obtained after purifi-
were consistent with the number of protons expected. cation by column chromatography as an amorphous,
The molecular structure of 2a was confirmed by X-ray black solid, which showed decomposition upon heat-
structure analysis. The poor quality of the crystals did ing above 400 ◦C. A molecular ion at m/e = 708 in its
not allow a satisfactory refinement. From these data, it mass spectrum confirmed the proposed structure. The
is evident that, in this case, a double ortho-annulation UV/Vis and the fluorescence spectra are almost identi-
also took place which led to the mesoionic regioisomer cal to that of derivative 1a and again show a very small
2a.
Because no single crystals could be obtained, the
Stokes shift of only 7 nm (Fig. 2).
Despite the testing of other oxidizing agents, the
further characterization of 1a proved to be problematic. yields for derivatives 1 could not be increased; instead,
Furthermore, all attempts to obtain suitable crystals by a series of highly fluorescent substances was formed
salt formation/N-methylation also failed. Suitable data as by-products. An interesting result was found when
could only be obtained from the UV/Vis fluorescence potassium ferricyanide was used as an oxidant. In this
spectra. The absorption spectrum revealed a well- case, a non-fluorescent iron complex was first formed,
structured pattern with a maximum at 518 nm, while which decomposed after some time to give the start-
the emission spectrum is nearly congruent with an in- ing material and small amounts of 1a and 2a. Further
tense emission at 525 nm. The very small Stokes shift studies with metal complexes as oxidizing agents are
of only 7 nm comprises a very small difference be- planned, and the results will be the subject of a sepa-
tween ground and excited state. The derivative 1c was rate publication.
isolated in very low yields starting from 8c and was
Based on our previous work and the findings of the
characterized by MS ([M]+, m/e = 662). All attempts present studies, we postulate the following mechanism
to involve derivative 8b in oxidative ortho-annulation (Scheme 5) for the formation of hexaaza-pentacenes of
led only to decomposition reactions with the formation types 1 and 2. First, the oxidation of the secondary
of inseparable mixtures of polymeric products, obvi- amine group in 8 takes place under the intermediary
ously due to the strong influence of the donor methoxy formation of the radical cation A. A relatively strong

R. Strathausen et al. · Double ortho-Annulation
645
Fig. 2. UV/Vis and fluorescence spectrum of 1d in THF (normalized).
Scheme 5. Proposed mechanism for the double ortho-annulation sequence.
acidity has been predicted for these types [40 – 42] and, supported by the following experimental facts. Gen-
consequently, deprotonation may result in the aminyl erally, anilines can easily be oxidized, and the radi-
radical B. Radical cation A as well as radical B are able cal cations thus formed tend toward a fast deproto-
to substitute the attached aromatic ring intramolecu- nation. It is noteworthy, however, that kinetic stud-
larly to give the leuco-semi form C. Due to the bi- ies revealed that cyclization reactions of aminyl rad-
functionality of 10, the same reaction subsequently can ical cations are much faster than those of the corre-
form Leuco-1 and Leuco-2, which are transformed by sponding neutral radicals [43, 44]. Despite the fact that
oxygen from air in a final oxidation step into 1 and some methods for the generation of aminyl radicals
2, respectively. The mechanism, in which the key in- are described in the literature [45 – 48], reactions of
termediates are radical cations/aminyl radicals A/B, is nitrogen-centered radicals are less well developed than

646
R. Strathausen et al. · Double ortho-Annulation
those of the analogous carbon species. In addition, only (400/100 MHz) spectrometers, using residual solvent peaks
as internal standards. Mass spectra were measured with
a Finnigan MAT SSQ 710 Fison Trio 200 (EI) instrument.
Elemental analyses were carried out with an automatic an-
alyzer LECO, CHNS-932. The UV spectra were measured
with a UNICAM UV500 spectrometer from Thermo Elec-
tron Corporation. For IR spectra a BIO-RAD FTS-25 in-
strument was used. Absorption spectra were recorded on
a LAMBDA 16 spectrophotometer (Perkin Elmer). Fluores-
cence emission and excitation spectra were measured using
an LS50B luminescence spectrometer (Perkin Elmer). Fluo-
rescence quantum yields were calculated relative to quinine
sulfate (purum; Fluka) in 0.1 N H2SO4 used as a standard
(φf = 0.55). The absorbance at the excitation wavelength
was kept below 0.05 for the samples and the reference. The
fluorescence lifetime was determined with a CD900 time-
correlating single photon counting spectrometer (Edinburgh
Instruments).
a few data for intramolecular cyclization reactions with
aminyl radicals/radical cations exist [43 – 45], among
which the Hoffmann-Löffler-Freytag reaction is the
oldest known reaction involving aminium cation rad-
icals [49]. Whereas aminyl radicals are nucleophilic,
aminium cation radicals and their metal complexes
render the nitrogen atom more electrophilic. Because
the interconversion of A to B is very fast, we are not
able to determine the key intermediate.
The oxidative cyclization of aryl-substituted
amidines, which have a similar substructure as 8, was
reported in 1996 [50, 51]. The authors suggest that
aminyl cations are formed via oxidation processes
and undergo a final cyclization reaction. Due to the
presence of the potassium carbonate base in our
experiments, however, the existence of aminyl radicals
cannot be excluded. In contrast, proton-mediated
redox processes on aromatic amines allow the syn-
thetic entry to phenazines. Thus, prolonged heating of
N-phenyl-o-phenylene diamine in 1 M HClO4 finally
resulted in tetraazapentacenes [52].
General procedure for the synthesis of dihydropyrazines 8
In a 100 mL two-necked flask 1 mmol of an oxalamidine 7
was dissolved in 5 mL of dioxane and heated to 80 ◦C. A 2 M
solution (1.5 mL) of NaHMDS in dioxane was added, and
the solution was stirred for 10 min followed by the addition
of 1 mmol of the appropriate bis-imidoyl chloride 6 in 5 mL
of dioxane. The mixture was stirred for 5 min at 80 ◦C. Then
50 mL of water was added to the hot reaction mixture. Af-
ter cooling to r. t. the mixture was filtered. The red solid was
washed with water, water-dioxane (1/1) and a small portion
of toluene-heptane (1/10). The product thus obtained was
used directly for further syntheses. An analytically pure sam-
ple was obtained by column chromatography (silica gel 60,
CHCl3).
Conclusion
A new and alternative entry to hexaaza-pentacenes
has been presented based on a double oxidative ortho-
annulation sequence. Applying this method, the deriva-
tives showing C2 symmetry of type 1 can be preferably
isolated, whereas the mesoionic regioisomers 2 were
formed only in traces. The formation of 1 is strongly
dependent on the nature of substituents on the aromatic
systems. Whereas derivatives bearing alkyl groups (1a,
1d) allow the isolation of hexaaza-pentacenes in mod-
erate yields, the reaction fails completely in the pres-
ence of methoxy groups. The new azaacenes exhibit
strong fluorescence with high quantum yields and very
small Stokes shifts.
2,5-Bis(4-tolylamino)-3,6-bis(4-tolylimino)-3,6-dihydro-
pyrazine (8a)
Orange solid; yield 65%. – M. p. 252 – 254 ◦C. – IR
(KBr): ν(cm−1) = 3293 (NH), 2901, 2865, 1628 (C=N). –
UV (CH2Cl2): λmax (lg ε) = 268 (4.4), 281 (4.5) 383 br
(4.4). – MS (EI): m/z (%) = 498 (30), 407 (8), 248 (38),
91 nm (100). – 1H NMR (250 MHz, CDCl3): δ = 9.48 (s,
2H, NH), 7.60 (d, J = 8.3 Hz, 8H, aryl), 7.17 (d, J = 8.2 Hz,
8H, aryl), 2.39 ppm (s, 12H, ar-CH3). – C32H30N6 (498.63):
calcd. C 77.08, H 6.06, N 16.85; found C 77.28, H 6.20, N
16.51.
Experimental Part
Unless otherwise indicated, all reagents were purchased
from commercial suppliers and used as received. Start-
ing materials that were not commercially available were
prepared according to literature procedures cited in the
text. Reactions were monitored by TLC using SiO2 (sil-
ica gel 60 F254) from Fluka. Melting points were mea-
2,5-Bis(4-methoxphenylamino)-3,6-bis(4-methoxyphenyl-
imino)-3,6-dihydropyrazine (8b)
Red crystals; yield 63%. – M. p. 240 – 242 ◦C. – MS
sured with a digital detector system KSPS 1000 from Krüss (EI): m/z (%) = 562 (100), 455 (6), 280 (30), 133 (84). –
and are uncorrected. The 1H and 13C NMR spectra were 1H NMR (250 MHz, CDCl3): δ = 9.46 (s, 2H, NH), 7.73
obtained on Bruker AC 250 (250/60 MHz) and DRC-400 (d, J = 9.0 Hz, 8H, aryl), 6.91 (d, J = 9.0 Hz, 8H, aryl),

R. Strathausen et al. · Double ortho-Annulation
647
3.86 ppm (s, 12H, CH3–O). – C32H30N6O4 (562.63): calcd. product was precipitated by adding 50 mL of water and fil-
C 68.31, H 5.37, N 14.94; found C 68.28, H 5.29, N 15.01.
tered off. The solid was washed with 100 mL of dioxane-
water (1/10), 10 mL of dioxane-water (1/1) and finally,
10 mL of diethyl ether. The azaacene was extracted with
methylene chloride, then the extract was evaporated in vacuo
and finally, the product was purified by column chromatog-
raphy (silica gel 60, CHCl3-acetone).
2,5-Bis(4-n-butylphenylamino)-3,6-bis(4-n-butylphenyl-
imino)-3,6-dihydropyrazine (8c)
Orange solid; yield 15%. – M. p. 103 – 106 ◦C. – UV
(CH2Cl2): λmax (lg ε) = 272 (4.4), 282 (4.4) 385 nm br
(4.2). – MS (EI): m/z (%) = 666 (42), 333 (14), 294 (46),
133 (100). – 1H NMR (250 MHz, CDCl3): δ = 9.39 (s, 2H,
NH), 9.22 (s, 2H, NH), 7.48 (m, 8H, aryl), 7.12 (m, 8H, aryl),
2.57 (t, J = 7.4 Hz, 4H, CH2), 1.53 (m, 4H, CH2), 1.30 (m,
4H, CH2), 0.88 ppm (t, J = 7.3 Hz, 6H, CH3). – 13C NMR
(63 MHz, CDCl3): δ = 157.4, 140.4, 139.9, 133.9, 130.0,
129.1, 128.6, 128.2, 122.5, 119.8, 35.2, 33.7, 22.3, 13.9 ppm.
5,12-Bis-(4-tolyl)-3,10-dimethyl-5,12-dihydro-
[5,6,7,12,13,14]hexaazapentacene (1a)
Black solid; yield 25%. – M. p. > 400 ◦C (dec.). – UV
(CH2Cl2): λmax (lg ε) = 272 (4.8), 403 (3.3), 428 (3.9),
454 (4.4), 486 (4.8), 520 nm (4.9). – Emission ( CHCl3):
λ = 528, 570, 617 nm, fluorescence quantum yield (φA) =
0.95(±0.05). – 1H NMR (250 MHz, CDCl3+ CF3COOD):
δ = 7.53 (m, 8H, aryl), 7.17 (d, J = 8.1 Hz, 4H, aryl), 6,9 (s,
1H, aryl), 6,78 (s, 1H, aryl), 5,05 (s, 2H, NH), 2.54 (s, 6H,
CH3), 2.11 ppm (s, 6H, CH3). – C32H26N6 (494.60): calcd.
C 77.71, H 5.30, N 16.99; found C 77.54, H 5.20, N 16.85.
2-(4-Bromophenyl)amino-3-(4-bromophenyl)imino-
5-(4-n-butylphenyl)amino-6-(4-n-butyl-phenyl)imino-
3,6-dihydropyrazine (8d)
Orange solid; yield 43%. – M. p. 148 – 150 ◦C. – UV
(CH2Cl2): λmax (lg ε) = 270 (4.4), 282 (4.4) 380 nm br (4.3).
– MS (EI): m/z (%) = 712 (16), 355 (6), 218 (100), 155 (40).
– 1H NMR (250 MHz, CDCl3): δ = 9.48 (s, 2H, NH), 9.40
(s, 2H, NH), 7.49 (m, 12H, aryl), 7.19 (d, J = 8.4 Hz, 4H,
aryl), 2.64 (t, J = 7.5 Hz, 4H, CH2), 1.65 (m, 4H, CH2),
1.40 (m, J = 7.2 Hz, 4H, CH2), 0.97 ppm (t, J = 7.3 Hz,
6H, CH3). – 13C NMR (63 MHz, CDCl3): δ = 140.4, 131.7,
128.7, 124.0, 122.3, 117.9, 35.2, 33.7, 22.3, 14.0 ppm. –
C36H36Br2N6 (712.53): calcd. C 60.68, H 5.09, Br 22.43,
N 11.79; found C 60.48, H 5.14, Br 22.20, N 11.88.
3-(n-Butyl)-5-(4-n-butylphenyl)-10-bromo-12-(4-bromo-
phenyl)-dihydro[5,6,7,12,13,14]hexaazapentacene (1d)
Black solid; yield 45%. – M. p. > 400 ◦C (dec.). – UV
(CH2Cl2): λmax (lg ε) = 272 (4.8), 431 (4.0), 458 (4.5),
488 (4.9), 524 nm (5.0). – Emission ( CHCl3): λ = 535, 576,
623 nm, fluorescence quantum yield (φA) = 0.95(±0.05). –
1H NMR (250 MHz, CDCl3): δ = 7.41 (m, 14H, aryl), 2.27
(m, 4H, CH2), 1.63 (m, 4H, CH2), 1.03 (t, J = 7.3 Hz, 6H,
CH3), 0.88 ppm (q, J = 7.3 Hz, 4H, CH2). – MS (EI): m/z
(%) = 708 (50), 665 (10), 398 (100), 171 (93), 132 (24). –
C36H32Br2N6 (708.50): calcd. C 61.03, H 4.55, Br 22.56, N
11.86; found C 60.95, H 4.46, Br 22.40, N 11.71.
2-(15N-Phenylamino)-3-(15N-phenylimino)-5-(4-t-butyl-
phenyl)amino-6-(4-t-butylphenyl)-imino-3,6-dihydro-
pyrazine (8e)
5,7-Bis-(4-tolyl)-3,9-dimethyl-5,12-dihydro[5,6,7,12,13,14]-
hexaazapentacene (2a)
Orange crystals; yield 42%. – M. p. 226 – 227 ◦C. – UV
(CH2Cl2): λmax (lg ε) = 267 (4.4), 278 (4.4) 374 br (4.3). –
MS (CI): m/z (%) = 557 (25), 484 (10), 353 (100), 337 (30),
297 (50). – 1H NMR (400 MHz, CDCl3): δ = 9.46 (d, 1J =
89 Hz, 15N–H), 9.45 (s, NH), 7.60 (m, 8H, aryl), 7.34 (m,
8H, aryl), 7.16 (t, J = 7.4 Hz, 2H, aryl), 1.34 ppm (s, 18H,
CH3). – 13C NMR (100 MHz, CDCl3): δ = 148.0, 128.6,
126.0, 125.4, 124.8, 122.0, 118.9, 34.4, 31.3 ppm.
In a 25 mL flask a mixture consisting of 100 mg
(0.2 mmol) of dihydropyrazine 8a, 111 mg (0.8 mmol)
of potassium carbonate and 263 mg (0.8 mmol) of
K3[Fe(CN)6] in 5 mL of DMSO was heated at 75 ◦C. After
5 h the reaction was complete and the crude product was
precipitated by addition of 10 mL of water. The precipitate
was filtered off and washed with 10 mL DMSO-water (1/3).
The product was purified by column chromatography (silica
gel 60, CHCl3-ethanol).
Transformation of derivatives 8a and 8d to
hexaazapentacenes 1a and 1d
Bluish-black crystals; yield 5%. – M. p. > 400 ◦C (dec.).
– UV (CH2Cl2): λmax (lg ε) = 375 (4.2), 409 (4.0), 525 (4.2),
In a 100 mL two-necked flask, a mixture consisting of 564 (4.1), 609 nm (4.5). – Emission ( CHCl3): λ = 644 nm,
1 mmol of a dihydropyrazine 8, 1.4 g (10 mmol) of potas- fluorescence quantum yield (φA) = 0.55(±0.05). – 1H NMR
sium carbonate and 3.6 g (12 mmol) of Pb(OAc)4 in 10 mL (250 MHz, CDCl3): δ = 7.11 (d, J = 8.0 Hz, 4H, aryl), 6.88
of dioxane was heated at 80 ◦C. The temperature was main- (d, J = 7.9 Hz, 8H, aryl), 6.16 (s, 2H, aryl), 2.43 (s, 6H,
tained for 5 h, and then unreacted Pb(OAc)4 was reduced CH3), 2.11 ppm (s, 6H, CH3). – MS (EI): m/z (%) = 494
to Pb(OAc)2 by adding 2 mL of ethylene glycol. The crude (66), 403 (10), 247 (14), 133 (35), 106 (100). – C36H32Br2N6

648
R. Strathausen et al. · Double ortho-Annulation
(708.50): calcd. C 61.03, H 4.55, Br 22.56, N 11.86; found C crystallographic data will not be deposited at The Cambridge
60.89, H 4.50, Br 22.52, N 11.78.
Crystallographic Data Centre.
XP (Siemens Analytical X-ray Instruments, Inc., Madi-
son, Wisconsin (USA)) was used for structure representa-
tions.
Crystal structure determinations
The intensity data for the compounds were collected
Crystal data of 8b: C32H30N6O4, M = 562.62 g mol−1
,
on a Nonius KappaCCD diffractometer using graphite- red-brown prisms, size 0.05 × 0.05 × 0.05 mm3, mono-
monochromatized MoKα radiation. Data were corrected clinic, space group P21/n, a = 6.3256(3), b = 10.8267(4),
for Lorentz and polarization but not for absorption ef- c = 19.9198(7) Å, β = 92.302(2)◦, V = 1363.11(9) Å3,
fects [53, 54]. The structures were solved by Direct Methods T = −140 ◦C, Z = 2, ρcalcd. = 1.37 g cm−3, µ(MoKα ) =
(SHELXS-97 [55]) and refined by full-matrix least-squares 1.9 cm−1, F(000) = 592 e, 9389 reflections in h(−7/8),
techniques against Fo2 (SHELXL-97 [55]). The hydrogen k(−14/13), l(−25/25), measured in the range 3.42◦ ≤ θ ≤
atoms of 8b were located by difference Fourier synthesis 27.45◦, completeness to θmax = 99.7%, 9389 measured re-
and refined isotropically. All non-disordered, non-hydrogen flections, 3112 independent reflections, Rint = 0.0574, 2189
atoms were refined anisotropically [55].
reflections with Fo > 4σ(Fo), 250 refined parameters, no re-
Crystals of 2a were extremely thin and of low quality, straints, R1obs = 0.0429, wR2obs = 0.1018, R1all = 0.0716,
resulting in a substandard data set (triclinic, space group wR2all = 0.1146, GoF = 1.024, largest difference peak and
P1, a = 8.7540(4), b = 12.0801(9), c = 14.6555(11) Å, hole 0.18/−0.23 e Å−3
.
¯
α = 84.264(3), β = 87.387(4), γ = 81.701(4)◦, V =
CCDC 974508 (8b) contains the supplementary crystallo-
1525.14(18) Å3, Z = 2). The structure refinement was suf- graphic data for this paper. These data can be obtained free
ficient, however, to confirm the connectivity and overall ge- of charge from The Cambridge Crystallographic Data Centre
ometry despite high final R values. For the latter reason the via www.ccdc.cam.ac.uk/data_request/cif.
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