A surprising formation of novel bridged bis-benzimidazoles by oxidation of diaminoquinoxalines

Article abstract of DOI:10.1515/znb-2012-0411

A synthesis of novel hexacyclic bis-benzimidazoles 6 starting from 2,3-diarylamino-quinoxalines via an oxidative cyclization cascade is described. These very stable and high-melting derivatives are featured by their strong fluorescence in the blue region of the visible spectrum. The cyclization reaction between 2,3-dichloroquinoxaline and 1,2-phenylenediamine did not lead to derivatives of type 6. In this case, only fluoflavine 7 was isolated quantitatively.

Full text of DOI:10.1515/znb-2012-0411

A Surprising Formation of Novel Bridged Bis-Benzimidazoles
by Oxidation of Diaminoquinoxalines
Jens Preßlera, Rainer Beckerta, Sven Raud, Roberto Menzela, Eckhard Bircknerb,
Wolfgang Gu¨nthera, and Helmar Go¨rlsc
a Institut fu¨r Organische und Makromolekulare Chemie, Friedrich-Schiller-Universita¨t Jena,
Humboldtstraße 10, D-07743 Jena, Germany
b Institut fu¨r Physikalische Chemie, Friedrich-Schiller-Universita¨t Jena, Lessingstraße 10,
D-07743 Jena, Germany
c Institut fu¨r Anorganische und Analytische Chemie, Friedrich-Schiller-Universita¨t Jena,
Lessingstraße 8 – 10, D-07743 Jena, Germany
d Institut fu¨r Anorganische Chemie I, Universita¨t Ulm, Albert-Einstein-Allee 11, D-89081 Ulm,
Germany
Reprint requests to Prof. Dr. R. Beckert. E-mail: Rainer.Beckert@uni-jena.de
Z. Naturforsch. 2012, 67b, 367 – 372; received December 8, 2011
A synthesis of novel hexacyclic bis-benzimidazoles 6 starting from 2,3-diarylamino-quinoxalines
via an oxidative cyclization cascade is described. These very stable and high-melting derivatives
are featured by their strong fluorescence in the blue region of the visible spectrum. The cyclization
reaction between 2,3-dichloroquinoxaline and 1,2-phenylenediamine did not lead to derivatives of
type 6. In this case, only fluoflavine 7 was isolated quantitatively.
Key words: Bis-Benzimidazoles, ortho-Annulation, Quinoxalines, Fluorophores
Introduction
ations [6]. Very recently, a Japanese patent described
the use of bis-benzimidazo[1,2-a:2ꢀ,1ꢀ-c]quinoxalines
a well- 6 as electroluminescent materials [7].
2,2ꢀ-Bis-benzimidazoles (1) represent
established class of heterocycles and are of interest
Generally, bridged derivatives of type 1 have gained
as ligands for the formation of functional metal increased interest for the construction of supramolec-
complexes [1] as building blocks for supramolec- ular structures, as well as for functional metal com-
ular/polymeric architectures [2], for metal-organic plexes. Continuing our work focused on the oxida-
frameworks [3], and as biologically active deriva- tive ortho-annulation of vicinal amino-imino structural
tives [4] (Scheme 1). In addition, bridged derivatives units [8], we were interested in the easily available
of bis-benzimidazole have often been applied in 2,3-bis(arylamino)quinoxalines2 [9]. Using optimized
order to study the formation and dimerization of conditions, this methodology allows the simultaneous
nucleophilic carbenes [5]. In 2009, Baldridge and condensation of two ring systems to give ring-fused N-
Siegel reported the synthesis of a series of 1,1ꢀ-bridged arylsubstituted quinoxalines 3 (Scheme 1). In princi-
2,2ꢀ-bis-benzimidazoles for supramolecular applic- ple, based on these facts a new synthetic strategy could
Scheme 1. General for-
mulae for bridged 2,2ꢀ-
bis-benzimidazoles 1 and
2,3- diarylamino- quinox-
alines 2; the oxidative or-
tho-annulation approach
to give ring-fused quinox-
alines 3 and derivatives of
octaaza-hexacenes 4a, b.
c
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368
J. Preßler et al. · Bridged Bis-Benzimidazoles
Scheme 2. Two possible prod-
ucts of oxidative annulation
of 2,3-bis(arylamino)quinoxal-
ines 2: fluoflavines
5 and
hexacyclic 2,2-bis-(benzimid-
azoles) 6.
be developed for oligoaza-acenes which are important
building blocks for organic electronics. This has been
successfully implemented in the past for derivatives
of octaaza-hexacene 4a and their leuco-forms 4b [10]
(Scheme 1), however with other mechanistic interpre-
tations.
Results and Discussion
Based on our previous work we employed the oxida-
tive ortho-annulation method for the syntheses of new
types of aza-acenes. Several oxidizing reagents com-
bined with potassium/sodium carbonate (the addition
of a base resulted in shorter reaction times and higher H atoms as spheres with arbitrary radii).
yields) have been previously employed. In these exper-
Fig. 1. Molecular structure of bis-benzimidazole 6a in the
crystal (displacement ellipsoids at the 50 % probability level,
to the characteristic signals for aryl protons only one
iments, the combination of bis(trifluoroacetoxy)iod-
benzene and sodium carbonate (substrate: oxidizing
reagent = 1 : 1.2 – 1.5) proved to be the best for the
transformation of quinoxalines of type 2. Upon com-
pletion of the reaction, TLC indicated that the re-
sulting mixture consisted of a broad spectrum of
highly fluorescent compounds. We succeeded in iso-
lating one product, which was exhibited by a strong
blue fluorescence, by employing fractional precipita-
tion/recrystallization. The product was well soluble in
hot DMSO and pyridine and formed yellow solutions;
its solubility in other common solvents was rather low.
All attempts to isolate by-products and/or intermedi-
ates were fruitless; the mass spectra only indicated sub-
stances with low molecular mass, most likely resulting
from oxidative degradation processes.
methyl group was detected at δ = 2.64 ppm, and no
signal for NH was present. A single-crystal X-ray anal-
ysis allowed the unambiguous structural assignment of
these compounds, as shown in Fig. 1. Hence, the cy-
clization products 6 have the structure of bis-benzimid-
azo[1,2-a:2ꢀ,1ꢀ-c]quinoxaline.
As depicted in Fig. 1, the hexacyclic compound 6a
has approximate C2V symmetry in the solid state. The
C2V symmetry is weakly distorted in the crystal state
as evidenced by the non-zero value of the dihedral
angle N1–C1–C2–N2. Furthermore, pyridine is incor-
porated as a solvent molecule in the crystal struc-
ture of 6a. The comparison with the planar molecu-
lar structure of 2,2ꢀ-bis(benzimidazole) [11] revealed
that the central bond C1–C2 in 6a is slightly short-
Based on previous experimental findings, tetracyclic ened (143.0(4) vs. 143.5(5) pm). Due to the o-phenyl-
compounds of type 5 (Scheme 2) were expected. How- ene bridging of the nitrogen atoms N3 and N4, the an-
ever, the first indications for the formation of new and gle between N1–C1–C2 in 6a is expanded to 126.7(3)◦
unexpected products were provided by the NMR spec- compared to 122(1)◦ in 2,2ꢀ-bis(benzimidazole). The
tra. Thus, the 1H NMR spectrum of the oxidation prod- bonds N1–C1 and N2–C2 (130.4(4) pm), respectively,
uct from 2a revealed a symmetric structure. In addition have clearly a double bond character. It is noteworthy
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J. Preßler et al. · Bridged Bis-Benzimidazoles
369
Fig. 2. Normalized ab-
sorption, fluorescence
excitation (measured
at 384 nm) and flu-
orescence
emission
(excitation at 341 nm)
spectra of compound 6a
measured in dioxane.
Scheme 3. The cyclization reaction of
2,3-dichloroquinoxaline with o-phenyl-
enediamine to give fluoflavine 7.
that the strong blue fluorescence of 6a in solution at and 1,2-phenylenediamine, however, its structure as-
λ = 384 nm has a quantum yield of close to 100 % and signment was only based on elemental analysis and IR
a fluorescence lifetime of 1.74 ns (Fig. 2). The small and MS data.
Stokes shift and the mirror symmetry indicate only
small differences in the geometry between the ground
and the first excited electronic states. A quantum yield
of 60 % was determined for compound 6b. Addition-
Furthermore, we were unsuccessful in the synthesis
of 6 using this protocol. In all attempts, only 5,12-di-
hydrochinoxalino[2,3-b]quinoxaline “fluoflavine” (7)
was isolated (Scheme 3), whereby the mass spectra of
ally, due to their 1,4-diazadiene subunit, derivatives 6 its dihydrochloride showed the same isotopic peaks as
bis-benzimidazo[1,2-a:2ꢀ,1ꢀ-c]quinoxaline 6 (R = H).
possess good prerequisites for the complexation of cat-
alytically active metals. For instance, new ruthenium
complexes were synthesized which show some unusual MS data. To our knowledge, therefore, the described
Derivative 7 was further characterized by its NMR and
spectral properties. These results will be the object of a
separate publication. The features reported above make
the new bis-benzimidazoles 6 interesting for the pur-
fluoflavines 7 are the result of the reaction reported
in lit. [12], and the bis-benzimidazoles of type 6 were
formed in trace amounts at best, and failed to exper-
pose of application as a sensor, label and scintillation imentally verify the typical fluorescence of 6 in this
counter.
From a literature search for the molecular structure
case.
We postulate the following mechanism (Scheme 4)
of 6a, a note was found on the synthesis of a struc- for the formation of the bis-benzimidazo[1,2-a:2ꢀ,1ꢀ-
turally homologous derivative [12]. Thus, the synthe- c]quinoxalines 6. Firstly, the oxidation takes place with
sis is described starting from 2,3-dichloroquinoxaline the intermediate formation of the radical cation [2ꢀ]·+
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370
J. Preßler et al. · Bridged Bis-Benzimidazoles
Experimental Section
Unless otherwise indicated, all reagents were purchased
from commercial suppliers and were used as received. Start-
ing materials that were not commercially available were pre-
pared according to literature procedures, cited in the text. Re-
actions were monitored by TLC using SiO2 (silica gel 60
F254) from Fluka. Melting points were measured with a
digital detector system KSPS 1000 from Kru¨ss and are un-
corrected. The 1H and 13C NMR spectra were obtained
on a Bruker AC 250 (250/60 MHz) or Bruker DRC-400
(400/100 MHz) spectrometer, 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 an 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
a 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 flu-
orescence lifetime was determined with a CD900 time cor-
relating single photon counting spectrometer (Edinburgh In-
struments).
Scheme 4. Mechanistic interpretation of the formation of
derivatives of type 6.
derived from a secondary amine. A relatively strong
acidity has been predicted for these radical cations and,
consequently, deprotonation may result in the aminyl
radical [2ꢀ]·. The latter can be regarded as a delocal-
ized 1,3-diazaallyl radical, which is able to attack in-
tramolecularly two different aromatic rings. Accord-
ing to Way a, the angular annulation under forma-
tion of a ring-fused imidazole takes place; the same
oxidation-cyclization sequence including the second
amidine substructure finally leads to the isolated prod-
ucts of type 6. One the other hand, the attack of the
aromatic ring of the second amidine (Way b) should
be possible, thus forming ring-fused pyrazine systems
of type 5. Although different, highly fluorescent prod-
ucts were detected by TLC, and it was not possible to
isolate the substituted fluoflavines. With a view to the
cyclization reaction calculations using the recently de-
veloped B2PLYP-D hybrid functional, with the def2-
TZVP-basis set, and the B3LYP/6-311+G(d,p) geome-
tries for the energy determinations [8a] clearly favor
the radical mechanism, which shows the lowest activa-
tion barrier.
4,4ꢀ-Dimethyl-bis-benzimidazo[1,2-a:2ꢀ,1ꢀ-c]quinoxaline
(6a)
In a stirred suspension of sodium carbonate (1.56 g,
14.7 mmol) in trichloromethane (150 mL), 2,3-bis(4-tolyl-
amino)quinoxaline (2a) (0.5 g, 1.47 mmol) was dissolved.
To this mixture bis(trifluoroacetoxy)iodobenzene (3.0 g,
6.97 mmol) was added within 3 min. The progress of the
oxidation reaction was monitored by irradiation of the re-
action mixture with UV light (350 nm). Immediately after
the reaction mixture developed a deep blue fluorescence, iso-
propyl alcohol (100 mL) was added, and the reaction mix-
ture was stirred for 5 additional minutes. Subsequently, the
solvent was removed in vacuo. To the resulting dark-red so-
lution, sodium carbonate was added until the development
of gases ceased. The mixture was filtered off and after re-
moving the isopropyl alcohol, the red filtrate was concen-
trated to dryness under fine vacuum conditions. Dissolving
the red residue in ethyl acetate (20 mL) under warming re-
sulted in a colorless precipitate overnight. The precipitate
was filtered off and washed with ethyl acetate. Recrystalliza-
tion from pyridine yielded colorless crystals of 6a (53 mg,
11 % yield); M. p. 320 ◦C (decomp.). – IR (KBr) (intensity):
ν (cm−1) = 3311 (w), 3181 (w), 3064 (w), 3020 (w), 2909
Conclusions
In summary, we report a new formation reaction of
highly fluorescent, hexacyclic bis-benzimidazoles of
type 6. These highly symmetric heterocycles are not
only new examples for bis-benzimidazoles but also
interesting 1,4-diazadienes, capable of forming metal
complexes. Evaluation of the scope of this ring-fusion
method, as well as the inclusion of groups for subse-
quent coupling reactions, are the subject of current re-
search and will be reported in due course.
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J. Preßler et al. · Bridged Bis-Benzimidazoles
371
(w), 1638 (m), 1597 (m), 1560 (m), 1499 (s), 1485 (s), 1447 color of the solution turned to red. After the temperature
(m), 1378 (s), 1330 (s), 1290 (s), 1284 (m), 1216 (m). – UV reached 165 ◦C, a yellow-orange precipitate was formed.
(DMSO): λmax(lgε) = 306 (3.9), 331 (4.4), 344 (4.5), 364 After 1 h reaction time, the mixture was cooled to r. t. and
(4.5). – Emission (dioxane), (%): 365 nm (85), 385 (100), was poured into crushed ice. The slurry was filtered off, and
405 (60). – Fluorescence quantum yield (φA): 0.96; fluores- the precipitate was dried. Recrystallization from methanol
cence lifetime: 1.74 ns. – MS (EI): m/z (%) = 336 (85), resulted in a yellow amorphous powder. (Yield 2.35 g,
333 (8), 306 (2), 283 (2), 257 (2), 232 (2), 204 (12), 167 10 mmol, 100 %). – 1H NMR (250 MHz, [D6]DMSO): δ =
(30), 147 (8), 116 (3), 97 (10), 77 (12), 69 (65), 50 (20), 6.57 (m, 4H), 7.01 (m, 4H), 9.91 (s, 2H). – MS (EI): m/z
1
44 (100). – H NMR (400 MHz, [D6]DMSO): δ = 2.64 (s, (%) = 234 (100), 233 (39), 206 (5), 192 (2), 179 (2), 129
12H), 7.34 (d, J = 8.4 Hz, 2H), 7.68 – 7.71 (m, 2H), 7.89 (1), 117 (8), 102 (4), 76 (1), 51 (1), 28 (4). – C14H10N4
(d, J = 8.4 Hz, 2H), 8.47 (s, 2H), 8.79 – 8.82 (m, 2H). – (234.26): calcd. C 71.28, H 4.30, N 23.92; found C 71.37,
13C NMR (22.5 MHz, [D6]DMSO): δ = 22.0, 114.4, 117.3,
120.7, 126.2, 126.3, 126.6, 131.7, 135.1, 143. – C22H16N4
H 4.22, N 24.05.
(336.40): calcd. C 78.55, H 4.79, N 16.66; found C 78.74,
H 4.70, N 16.51.
Crystal structure determination
The intensity data of 6a were collected on a Nonius
KappaCCD diffractometer using graphite-monochromatized
MoKα radiation. Data were corrected for Lorentz and po-
larization effects but not for absorption [13, 14]. The struc-
ture was solved by Direct Methods (SHELXS-97 [15]) and
4,4ꢀ-Dichloro-bis-benzimidazo[1,2-a:2ꢀ,1ꢀ-c]quinoxaline
(6b)
Compound 6b was obtained by the same procedure by us-
ing 2,3-bis(4-chlorophenylamino)quinoxaline 2b as starting
material as colorless crystals (Yield 13 %); M. p. 350 ◦C (de-
comp.). – IR (KBr) (intensity): ν (cm−1) = 3462 (m), 3128
(w), 3081 (w), 1782 (w), 1639 (m), 1596 (m), 1563 (m),
1497 (s), 1478 (s), 1435 (s), 1377 (s), 1327 (m), 1288 (m). –
UV (DMSO): λmax(lgε) = 307 (3.5), 328 (3.9), 344 (4.1),
363 (4.1) nm. – Emission (dioxane) (%): 389 nm (100), 410
(60). – MS (FAB in nba): m/z (%) = 379 (15), 377 (18), 364
(3), 329 (10), 290 (30), 258 (20), 242 (22), 226 (20), 212
2
refined by full-matrix least-squares techniques against Fo
(SHELXL-97 [15]). All hydrogen atoms were included at cal-
culated positions with fixed thermal parameters. All non-
hydrogen atoms were refined anisotropically. The software
XP (Siemens Analytical X-ray Instruments, Inc.) was used
for structure representations.
Crystal structure data for 6a: C22H16N4, 2 (C5H5N), Mr =
494.59, colorless prism, size 0.04 × 0.04 × 0.03 mm3, tri-
¯
clinic, space group P1, a = 10.2821(9), b = 11.5712(9), c ◦=
˚
1
11.8386(9) A, α = 78.252(5), β = 79.965(4), γ = 65.744(4) ,
(15). – H NMR (400 MHz, [D6]DMSO): δ = 7.61 (d, J =
3
V = 1250.77(17) A , T = −140 ◦C, Z = 2, ρcalcd.
=
˚
8.6 Hz, 2H), 7.67 – 7.71 (m, 2H), 8.03 (d, J = 8.6 Hz, 2H),
8.78 – 8.81 (m, 4H). – 13C NMR (100 MHz, [D6]DMSO):
δ = 114.7, 117.7, 122.5, 125.8, 126, 127, 130.1, 132.1, 140.7,
143.8. – C20H10Cl2N4 (377.23): calcd. C 63.68, H 2.67,
N 14.85, Cl 18.80; found C 63.56, H 2.70, N 14.71, Cl 18.89.
1.31 g cm−3, µ (MoKα ) = 2.9 cm−1, F(000) = 520 e,
8918 reflections in h (−11/13), k (−14/15), l ( 15), mea-
sured in the range 1.95◦ ≤ θ ≤ 27.50◦, completeness 98.6 %,
5678 independent reflections, Rint = 0.0364, 3164 reflec-
tions with Fo ≥ 4σ(Fo), 345 refined parameters, 0 restraints,
R1obs = 0.0907, wR2
= 0.2289, R1all = 0.1597, wR2
=
all
obs
5,12-Dihydroquinoxalino[2,3-b]quinoxaline, “fluoflavine”
(7)
0.2766, GOOF = −13.032, largest difference peak / hole:
˚
0.493 / −0.434 e A
.
CCDC 809050 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data request/cif.
2,3-dichloroquinoxaline (2.0 g, 10 mmol) and 1,2-phen-
ylenediamine (2.17 g, 20 mmol) were dissolved in dry
dimethylformamide (200 mL). The colorless solution was
heated under reflux to 200 ◦C whereby at 50 ◦C the
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