Bis-4H-imidazoles-tetraazafulvalenes-2,2′-biimidazoles: three variations of one redox system

Article abstract of DOI:10.1016/j.tet.2006.06.055

The reversibility of the two-electron reduction of tetraazafulvalenes 1 could be confirmed by employing cyclovoltammetric measurements. However, attempts to oxidize these systems electrochemically, as well as by oxidizing agents, failed. In contrast, the

Full text of DOI:10.1016/j.tet.2006.06.055

Tetrahedron 62 (2006) 8586–8590
Bis-4H-imidazoles–tetraazafulvalenes–2,20-biimidazoles: three
variations of one redox system
*
€
M. Matschke, C. Kapplinger and R. Beckert
Institute of Organic and Makromolecular Chemistry, Friedrich-Schiller University, Humboldt Street 10, D-07743 Jena, Germany
Received 27 March 2006; revised 7 June 2006; accepted 15 June 2006
Available online 17 July 2006
Abstract—The reversibility of the two-electron reduction of tetraazafulvalenes 1 could be confirmed by employing cyclovoltammetric
measurements. However, attempts to oxidize these systems electrochemically, as well as by oxidizing agents, failed. In contrast, the bis-
vinylogous derivatives 2 proved to be multi-step redox systems showing two-reversible reduction as well as oxidation waves. The chemically
initiated oxidation of 2 yielded the bis-4H-imidazoles 6. In the presence of an excess of the oxidizing agent they dimerized and formed the
deep blue colored derivative 7a. Treatment of the phenylogous systems 8 with sodium dithionite provided a new entry to quinomethides of
type 9, which can be stabilized towards oxygen by cyclization reaction to give the pigment-like bis-urea 11. Derivative 9 represents the SEM
form of this four-step redox system and thus can finally be reduced to yield the tetraaminosubstituted biimidazoles 12. Based on these findings,
the close correlation among bis-4H-imidazoles (OX), tetraazafulvalenes (SEM), and tetraaminosubstituted biimidazoles (RED) could be
demonstrated. Due to the fact that tetraazafulvalenes constitute stable closed-shell SEM systems, their intense UV–vis absorptions can
now be explained and related to their redox behavior.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
groups, further derivatization can be realized by a large
variety of alkylation/acylation, cyclization, and cross-coupling
procedures.5–8 Their long wavelength absorptions in the
UV–vis spectra together with high extinction coefficients
(log 3>4) make them promising candidates for the construc-
tion of functional dyes.9
Electrochromic behavior is mainly based on two-step redox
€
systems for which Hunig and others have developed a new
principle.1,2 In the field of advanced materials, the so-called
violenes,3 which are the radical ions SEM+/ꢀ, always repre-
sent the species with the longest wavelength absorptions and
highest extinction coefficients within the redox system.
Their color intensity is closely related to the thermodynamic
stability for which the semiquinone formation constant
(KSEM) is the most commonly applied value. Recently, the
synthesis and electrochemical characterization of boratetra-
azapentalenes derived from 4H-imidazoles 1 was reported.4
The radical anions of this new class of electrophores show
unusual high values of KSEM (up to 1015). Since radical reac-
tions often have low activation barriers, these open-shell spe-
cies tend to decompose more rapidly than closed-shell
systems. We therefore focused our work on the development
of redox systems in which the radical SEM state is replaced
by closed-shell moieties.
In principle, the electron-poor heterocyclic part of 2 enables
them to be the electronic counterpart of tetrathiafulvalenes
and in addition, offers the ability for the synthesis of
charge-transfer complexes with such donor molecules.10
On the other hand, tetraazafulvalenes can easily be reduced
to yield 4,40,5,50-tetraaminosubstituted 2,20-biimidazoles 6.
These electron-rich heterocycles can also be regarded as
leuco-forms of 2 and immediately reoxidize in the presence
of air oxygen to its starting material (Scheme 1). The green-
ish fluorescent derivatives 6 can be stabilized under strictly
anaerobic conditions by acylation reactions with Boc2O or
CF3CH2SO2Cl.11
Tetraazafulvalenes 2 and their bis-vinylogous derivative 3
represent a novel class of cross-conjugated heterocyclic
compounds for which different syntheses have been devel-
oped in our laboratories. Due to the secondary arylamino
2. Results and discussion
Due to its inherent merocyanine type system, 3 is supposed
to behave as a multi-step redox system according to Scheme 1.
We could recently11 demonstrate that tetraazafulvalenes 3
behave as electrophores, which can easily be switched
between oxidized and reduced form. In the first step, most
probably the radical anion would be generated. The second
Keywords: bis-4H-Imidazoles; Tetraazafulvalenes; Biimidazoles; Four-
electron redox system; Quinomethides.
* Corresponding author. Tel.: +49 3641 948230; fax: +49 3641 948212;
e-mail: c6bera@uni-jena.de
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.06.055

M. Matschke et al. / Tetrahedron 62 (2006) 8586–8590
8587
-
-
e
e
NAr
NHAr
NHAr
NHAr
NHAr
H+
H+
N
N
N
.
Ar
Ar
Ar
-
-
-e
-e
N
N
N
H
-H+
-H+
NHAr
1 (OX)
SEM
RED
(open-shell-system)
Ar
N
Ar
NH
Ar
NH
-
-
Ar
HN
2e
2e
Ar
HN
Ar
HN
2H+
N
N
2H+
N
N
N
H
N
N
N
N
N
-
-
NH
Ar
NH
Ar
NH
Ar
-2e
-2e
n
n
n
-2H+
N
-2H+
H
N
N
Ar
HN
Ar
HN
Ar
4,5
OX
2,3
SEM
(closed-shell-system)
6,7
RED
Scheme 1. Compounds 2, 4, and 6: n¼0; 3, 5, and 7: n¼1.
electron transfer step leads to the formation of the dianion,
which is immediately protonated by water to yield the
2,20-biimidazoles of type 7. Now, both consecutive single-
electron transfer processes could be recorded by electro-
chemical measurements. Employing difference pulse
polarographic measurements, two peaks can be clearly
ascribed to two different single-electron transfer steps. The
quasi-reversibility of the reduction was confirmed by cyclo-
voltammetric measurements (DERED,OX1,2>0.059 V). The
redox potentials and semiquinone formation constants
KSEM of selected tetraazafulvalenes are listed in Table 1.
Compared to other acceptor molecules such as tetracyano-
quinodimethane (TCNQ) or N,N0-dicyanoquinodiimines
(DCNQI)12 with KSEM values in the range of 107–1011, the
tetraazafulvalenes show rather small parameters (102–1011).
The oxidation reaction of 2 provides the possibility of ob-
taining derivatives of type 4, in which two 4H-imidazoles
are directly connected via a single bond. The cyclovolt-
ammogram of 2b (Ar¼4-Tol) showed two-irreversible oxida-
tion waves at positive potentials. These findings were in
agreement with the results obtained by using a series of
oxidation reagents. All attempts to chemically oxidize 2
yielded polymeric material and only small amounts of iso-
cyanides and derivatives of parabanic acid. The latter re-
sulted from a direct oxidative cleavage of the central
double bond of 2. The independent synthesis of 4 starting
from oxalyl chloride and oxalic amidines13 was unsuccess-
ful and proved to be an additional evidence for the instability
of the bis-4H-imidazoles 4.
In further experiments we therefore studied the redox behav-
ior of the easily accessible tetraazafulvadienes 3. Such vinyl-
ogous derivatives should show a similar redox chemistry as
their parent compound 2. The cyclic voltammograms, as
well as the difference pulse polarographic measurements
The data also demonstrates the small influence of the
substituted aryl groups, the bonding mode of the two imi-
dazoles (n¼0,1), as well as the nature of an additional ring
fusion on the reduction potential.
Table 1. Reduction potentials E1RED, ER2 ED, and KSEM of 2 and 3
Ar
Ar
N
N
X
N
N
X
X
X
N
N
n
N
N
Ar
Ar
2,3
Compound
n
Ar
X–X
E1RED (V)
E2RED (V)
KSEM
2a
2b
2c
2d
2e
2f
0
0
0
0
0
0
4-t-C4H9–C6H4
4-CH3–C6H4
4-I–C6H4
4-Br–C6H4
4-Cl–C6H4
4-t-C4H9–C6H4
H, H
H, H
H, H
H, H
H, H
ꢀ0.669
ꢀ0.775
ꢀ0.648
ꢀ0.762
ꢀ0.861
ꢀ0.570
ꢀ0.786
ꢀ1.050
ꢀ0.781
ꢀ0.961
ꢀ1.133
ꢀ1.251
9.6E+001
1.5E+005
1.8E+002
2.4E+003
4.1E+004
3.5E+011
–CH]CH–
2g
0
3-CF3–C6H4
ꢀ1.375
ꢀ1.625
1.7E+004
CN CN
>CH(OEt)
H, H
2h
3a
3b
0
1
1
4-t-C4H9–C6H4
4-t-C4H9–C6H4
4-Br–C6H4
ꢀ0.762
ꢀ0.654
ꢀ0.693
ꢀ1.050
ꢀ1.043
ꢀ0.825
7.6E+004
3.9E+006
1.7E+002
H, H

8588
M. Matschke et al. / Tetrahedron 62 (2006) 8586–8590
of 3a and 3b, reveal two completely reversible reduction
waves, which correspond to two single-electron transfer
steps. This reduction of 3 yielded their leuco-forms 7, which
could also be obtained by ultrasound irradiation of 3 in
THF in the presence of small amounts of aqueous sodium
dithionite. Employing metallic lithium, this chemically in-
duced reduction of derivatives 3 was already used for the
preparation of stilbenoid biimidazoles.5 However, in deriva-
tives 3, two-reversible oxidation waves (3a: E1OX¼0.566 V,
EO2 X¼0.879 V; 3b: E1OX¼0.664 V, EO2 X¼0.908 V) promise
an entry to stable oxidation products of type 5 (Scheme 1),
which are not accessible via tetraazafulvalenes 2.
did not show such oxidative dimerization reactions
(Scheme 2).
We have now expanded our research to consider the redox
chemistry of bifunctional, ‘phenylogous’ 4H-imidazoles in
order to combine the experimental findings of 4H-imi-
dazoles4 with those of tetraazafulvalenes. These molecules
can easily be obtained via two different pathways13,15 and
are stable crystalline compounds. Due to their low solubil-
ities in solvents used for CV measurements, only chemically
induced redox reactions were studied. Thus, upon treatment
of bis-4H-imidazoles 9 with sodium dithionite, the color of
the solution turned from red to blue. Purification of the reac-
tion mixture by flash chromatography under argon yielded
deep blue stable substances, but upon exposure to air a fast
reoxidation to the starting material was observed. The spec-
tral data (NMR and MS) of 10 was in agreement with the
structural assignment presented in Scheme 3. The UV–vis
spectrum of 10 exhibited a strong absorption at long wave-
lengths (lmax (log 3): 10a: 634 (4.4); 10b: 633 (4.2)), which
is characteristic for such quinoid compounds.16–19 Since
no radical species could be detected by ESR, the quinoid
structure of 10 could be achieved by electron coupling of
both parts of the peripheric reduced heterocycles. Derivative
10 constitutes the first tetraaminosubstituted quinomethide
and can be regarded as a hybrid between tetraazafulvalenes
and quinodimethides. The cyclization of 10 with triphosgene
formed the deep purple bis-urea 12, which showed pigment-
like properties and was stable towards oxygen. The reduction
The chemical initiated oxidation of 3 has also been
succeeded by using several oxidizing agents (DDQ, Cu2+/
pyridine, iodosobenzene bis-trifluoroacetate, 1,4-benzoqui-
none, cerium(IV) ammonium nitrate). In all cases studied
the oxidation products were purified by column chromato-
graphy and identified as bis-4H-imidazoles 5. These deep
red compounds are stable up to 50 ꢁC; at higher temperatures
and in the presence of an excess of oxidizing agent they were
transformed into deep blue compounds. By means of MS
and NMR spectroscopy, derivative 8a (Ar¼4-t-BuPh) was
characterized as a dimerization product derived from the
corresponding bis-4H-imidazole 5a. This somewhat surpris-
ing dimerization probably took place via cationic radical
intermediates14 formed by oxidation of the exocyclic me-
thine groups of 5 (R¼H). Derivatives in which the methine
hydrogens are replaced by methyl groups (R¼CH3)14
Ar
H
N
N
Ar
ArN
NHAr
Ar
ArN
HN
NHAr
H
N
N
N
N
Ar
N
N
N
N
(a)
N
(b)
H
R
R
N
R
H
Ar
N
R
N
R = H
(c)
R = H
H
N
N
N
N
NH
N
N
Ar
Ar
N
N
ArHN
NAr
ArHN
NAr
H
Ar
3a
5a
8a
R = H, Me
Scheme 2. Compounds 3a, 5a, and 8a: Ar¼4-t-C4H9–Ph; Reaction conditions: (a) 1 equiv DDQ, 5 min, rt, yield 45%; (b) 1 equiv DDQ, 18 h, rt, yield 50% and
(c) 0.06 M Na2S2O4, ultrasound irradiation, 15 min, rt, yield 92%.
ArN
NAr
N
N
N
N
ArHN
NHAr
9
Ox
Ox
Red
Ar
Ar
ArHN
ArHN
NHAr
NHAr
N
N
N
N
N
N
N
N
N
N
(CCl3O)2CO
O
O
N
N
10
12
Ar
Ar
Red
PG
PG
PG
H
N
ArHN
ArHN
NHAr
NHAr
ArN
NAr
N
N
N
Boc2O
TFA
N
N
N
H
N
ArN
NAr
11
13a
PG
PG
PG
Scheme 3. Compounds 9–12a: Ar¼4-t-C4H9–Ph; b: Ar¼4-nC6H13–Ph; 13a: Ar¼4-t-C4H9–Ph, PG¼Boc.

M. Matschke et al. / Tetrahedron 62 (2006) 8586–8590
8589
of 10 with an excess of sodium dithionite finally lead to phe-
nylogous 2,20-biimidazoles of type 11 (Scheme 3).
(250 MHz, THF): d 7.06 (d, J¼8.8 Hz, 8H), 6.53 (d,
J¼8.8 Hz, 8H); UV–vis (THF) lmax: 405 nm; Fluores-
cence (THF, 340 nm) lmax,em: 534 nm; C32H24Br4N8
(840.2).
These derivatives possess two-electron-rich 4,5-diamino-
imidazoles and are therefore only stable if air is strictly ex-
cluded. The stabilization of 11a can be realized via acylation
reaction with Boc2O to yield the greenish fluorescent deriva-
tive 13a. The deprotection of this conserved leuco-form with
trifluoroacetic acid reproduced the bis-imidazole 11a, which
reoxidized immediately in the presence of air to revert back
to 9a. No formation of stable charge-transfer complexes was
observed with para benzoquinone. In a smooth reaction, only
oxidation took place and yielded the starting material 9.
3.3. General procedure for the oxidation of tetraaza-
fulvalenes of type (3)
A mixture of 3 (0.1 mmol), 30 ml of THF, and 0.1 mmol
DDQ was stirred at rt for about 10 min. The solvent was
removed in vacuo and the crude product 5 was purified by
column chromatography (SiO2, toluene/acetone 9:1).
1
5a: Yield: 45%, red solid, mp 260 ꢁC (decomp.); H NMR
(250 MHz, THF): d 1.37 (s, 36H), 7.49 (m, 16H), 8.06 (s,
2H); 13C NMR (250 MHz, THF): d 28.8, 32.4, 117.6,
123.2, 123.4, 123.6, 126.0, 126.8, 134.6, 135.6, 141.3,
147.8; MS (DCI with H2O): m/e (%): 745 (M)+ (1), 613
(4), 508 (50), 466 (46), 389 (10), 353 (12), 307 (30), 251
(6), 176 (14), 160 (66), 150 (100), 134 (44), 94 (30);
UV–vis (THF) lmax (log 3): 451 (4.2), 479 (4.2), 533
(4.2) nm; CV: E1OX¼0.566 V, E2OX¼0.879 V; C48H56N8
(745.04).
3. Experimental
3.1. General
All reactions were monitored by TLC, carried out on
0.25 mm Merck silica gel plates (60F254) using UV light.
1H and 13C NMR spectra were recorded with a Bruker
DRX 400 or Bruker AC 250 spectrometer. Melting points
are measured with a Galen TM3 apparatus and are uncor-
rected. UV–vis spectra were recorded on a Perkin–Elmer
Lambda 19 spectrophotometer. MS spectra were taken
from measurements on a Finnigan MAT SAQ 710 mass spec-
trometer. Elemental analyses were carried out in-house with
an automatic analyzer LECO CHNS 932. The tetraazafulva-
lenes 26,8 and 314,20,21 were prepared according to literature
procedures. Electrochemical measurements were carried
out with a Metrohm 663VA Stand using mercury or platinum
electrodes and tetrabutylammoniumhexafluorophosphate as
conductive salt.
5b: Yield: 42%, red solid, mp 258 ꢁC; 1H NMR (250 MHz,
THF): d 6.91 (s, 2H), 7.08–7.20 (m, 16H); 13C NMR
(250 MHz, THF): d 117.5, 122.3, 123.1, 124.9, 125.0,
127.9, 128.7, 131.4, 137.2, 137.4, 147.0, 147.7, 151.7; MS
(CI): m/e (%): 837 (M+1)+ (4), 803 (35), 558 (50), 471
(60), 429 (66), 413 (100), 349 (38), 309 (40), 291 (61), 223
(18), 81 (24); UV–vis (THF) lmax (log 3): 231 (4.1), 288
(4.2), 526 (4.3) nm; CV: E1OX¼0.664 V, EO2 X¼0.908 V;
C32H20N8Br4 (836.34).
3.4. Procedure for the preparation of the dimer (8a)
3.1.1. (syn, anti)-5,50-Diethoxy-4,6,40,60-tetrakis(4-tert-
butylphenyl)-5,6,50,60-tetrahydro-4H,40H-[2,20]bis(imi-
dazo[4,5]imidazolylidene) (2h). Yield: 52%, metallic green
crystals, mp>300 ꢁC; 1H NMR (250 MHz, THF): d 1.01 (t,
J¼6.9 Hz, 6H), 1.36 (s, 36H), 3.35 (d, J¼7.8 Hz, 4H), 7.48
(d, J¼8.7 Hz, 8H), 8.04 (d, J¼8.7 Hz, 8H); 13C NMR
(250 MHz, THF): d 11.5, 28.5, 31.8, 53.2, 100.5, 114.6,
123.5, 132.5, 143.9, 155.0, 157.8; MS (DCI with H2O):
m/e (%): 833 (M+1)+ (100), 786 (21), 775 (11), 728 (31),
671 (10), 480 (98), 428 (25), 333 (48), 186 (13), 147 (61),
133 (42), 57 (28); UV–vis (THF) lmax (log 3): 431 (4.4),
458 (4.7), 490 (4.8) nm; Fluorescence (THF, 340 nm):
lmax,em: 509, 562 nm; Anal. calcd for C52H64N8O2: C,
74.97; H, 7.74; N, 13.45. Found: C, 74.90; H, 7.79; N, 13.49.
Compound 5a (0.05 mmol) was dissolved in 20 ml of THF.
After addition of 0.05 mmol DDQ, the reaction mixture was
stirred at rt for 18 h. After removing the solvent in vacuo, the
dimer 8a was purified by column chromatography (SiO2,
toluene/acetone 9:1).
8a: Yield: 50%, dark blue solid, mp 254 ꢁC; 1H NMR
(250 MHz, THF): d 1.21 (s, 18H), 1.29 (s, 18H), 1.32 (s,
18H), 1.36 (s, 18H), 6.83 (d, J¼8.5 Hz, 4H), 7.06 (d,
J¼8.5 Hz, 4H), 7.12 (d, J¼8.5 Hz, 4H), 7.17 (d, J¼8.5 Hz,
4H), 7.28 (d, J¼8.5 Hz, 4H), 7.36 (d, J¼8.5 Hz, 4H), 7.40
(d, J¼8.5 Hz, 4H), 7.85 (d, J¼8.5 Hz, 4H), 8.44 (s, 2H),
8.64 (s, 2H); 13C NMR (250 MHz, THF): d 30.5, 33.5,
117.7, 118.2, 119.3, 125.2, 135.4, 137.1, 145.3, 148.4,
150.4, 157.2; MS (DCI with H2O): m/e (%): 1490 (M)+
(80), 1342 (100), 1130 (10), 985 (4), 894 (12), 833 (30),
721 (8), 508 (14), 231 (8), 217 (8), 131 (8); UV–vis (THF)
lmax (log 3): 228 (4.9), 286 (4.7), 529 (4.5), 571 (4.7), 623
(4.6) nm; C96H114N16 (1490.08).
3.2. Reduction of the tetraazafulvalenes of type (3)11
3.2.1. 2-[2-[4,5-Bis-(4-tert-butylphenylamino)-1H-imida-
zol-2-yl]-vinyl]-N4,N5-bis-(4-tert-butylphenyl)-1H-imi-
dazol-4,5-diamine (7a). Yield: 94%, brownish solid; H
1
NMR (250 MHz, THF): d 1.19 (s, 36H), 6.55 (d,
J¼8.7 Hz, 8H), 7.01 (d, J¼8.7 Hz, 8H); UV–vis (THF)
3.5. Reduction of bis-4H-imidazoles (9) to quino-
methides (10)
lmax:
515 nm; C48H60N8 (748.5).
387 nm; Fluorescence (THF, 340 nm) lmax,em
:
Derivative 9 (0.1 mmol) dissolved in THF was reduced with
a 0.06 M sodium dithionite solution (1 equiv). The deep blue
colored quinomethides 10 were obtained after removing the
solvent in vacuo.
3.2.2. 2-[2-[4,5-Bis-(4-bromophenylamino)-1H-imidazol-
2-yl]-vinyl]-N4,N5-bis-(4-bromophenyl)-1H-imidazol-
4,5-diamine (7b). Yield: 92%, brownish solid; H NMR
1

8590
M. Matschke et al. / Tetrahedron 62 (2006) 8586–8590
10a: Yield: 94%, dark blue solid; 1H NMR (400 MHz, THF):
d 1.27 (s, 36H), 6.84 (d, J¼8.8 Hz, 8H), 7.31 (d, J¼8.8 Hz,
8H), 8.06 (s, 4H); HRMS (DEI): m/z calcd for C52H60N8
(M)+: 796.4941, found: 796.5030; UV–vis (THF) lmax
(log 3): 634 (4.4) nm.
4H); UV–vis (THF) lmax (log 3): 317 (4.3), 375 (4.4) nm;
C60H78N8 (910.60).
3.8. Derivative (13a) was prepared according to the
literature procedure11
10b: Yield: 95%, dark blue solid; 1H NMR (250 MHz,
THF): d 0.78 (t, J¼8.0 Hz, 12H), 1.20 (m, 24H), 1.45 (m,
16H), 6.55 (d, J¼8.5 Hz, 8H), 6.77 (d, J¼8.5 Hz, 8H),
7.97 (s, 4H); HRMS (DEI): m/z calcd for C60H76N8 (M)+:
908.6035, found: 908.6040; UV–vis (THF) lmax (log 3):
633 (4.0) nm.
13a: Yield: 15%, brownish solid; 1H NMR (250 MHz, THF):
d 1.17 (s, 18H), 1.22 (s, 36H), 1.29 (s, 36H), 7.24 (d,
J¼8.8 Hz, 8H), 7.70 (m, 12H); MS (micro-ESI, acetone/
methanol): m/e (%): 1399 (M)+ (36), 1299 (12), 1199 (14),
1100 (8), 1001 (6), 901 (8), 800 (4), 775 (8), 623 (10), 471
(20), 413 (100), 301 (10); IR (KBr): 2966, 2871, 1758,
1717, 1611, 1515, 1477, 1392, 1367, 1325, 1302, 1145,
1107, 836 cmꢀ1; UV–vis (THF) lmax: 271, 296, 335 nm.
3.6. Cyclization of the quinomethides (10) with triphos-
gene to bis-urea derivatives (12)
Acknowledgements
Compound 10 (0.1 mmol) and 0.25 mmol of triphosgene
were dissolved under argon in 20 ml of dry toluene. The re-
action mixture was then heated under reflux for 5 h. After
completion of the reaction the cyclic bis-urea derivatives
were separated by addition of a mixture of water/acetone
(1:1) as deep purple pigment-like solids, which were filtered
off and dried in vacuo.
This work was supported by Deutsche Forschungsgemein-
schaft (DFG), SFB 436. Degussa AG and Clariant GmbH
are acknowledged for support by chemicals.
References and notes
12a: Yield: 30%, purple, metallic-shining microcrystals,
1. H
€unig, S.; Kemmerer, M.; Wenner, H.; Perepichka, I.; B€auerle,
P.; Emge, A.; Gescheid, G. Chem.—Eur. J. 1999, 7, 1969.
1
mp 280 ꢁC (decomp.); H NMR (250 MHz, THF): d 1.29
€
(s, 36H), 7.05 (s, 4H), 7.42 (d, J¼8.5 Hz, 8H), 7.54 (d,
J¼8.5 Hz, 8H); MS (DCI with H2O): m/e (%): 849 (M)+
(22), 793 (2), 424 (6), 379 (10), 323 (2), 176 (100), 159
(26), 123 (28), 109 (22), 93 (15); IR (KBr): 2963, 2906,
2868, 1754, 1624, 1517, 1437, 1406, 1266, 1107, 1088,
1034, 978, 877, 831, 802, 693, 667 cmꢀ1; UV–vis (THF)
lmax: 475, 556 nm; C54H56N8O2 (848.45).
2. H€unig, S.; Kemmerer, M.; Wenner, H.; Perepichka, I.; Bauerle,
P.; Barbosa, F.; Emge, A.; Gescheid, G.; Peters, K. Chem.—Eur.
J. 2000, 14, 2618.
€
3. Deuchert, K.; Hunig, S. Angew. Chem. 1978, 90, 927.
4. Gebauer, T.; Beckert, R.; Weiß, D.; Knop, K.; Kapplinger, C.;
€
€
Gorls, H. Chem. Commun. 2004, 1860.
€
5. Kapplinger, C.; Beckert, R.; Imhof, W. J. Prakt. Chem./
Chem.-Ztg. 1998, 340, 323.
€
€
€
6. Kapplinger, C.; Beckert, R.; Gunther, W.; Gorls, H. Liebigs
12b: Yield: 34%, purple, metallic-shining microcrystals,
1
mp 300 ꢁC (decomp.); H NMR (400 MHz, THF): d 0.92
Ann. Recl. 1997, 617.
€
8. Kapplinger, C.; Beckert, R. Synlett 2000, 1679.
(t, J¼8.0 Hz, 12H), 1.36 (m, 32H), 2.70 (t, J¼8.0 Hz, 8H),
7.38 (d, J¼8.5 Hz, 8H), 8.05 (d, J¼8.5 Hz, 8H), 8.15 (s,
4H); MS (DCI with H2O): m/e (%): 961 (M+1)+ (4), 745
(12), 584 (16), 407 (12), 309 (8), 204 (100), 178 (59), 132
(27), 106 (13); IR (KBr): 2922, 2854, 1751, 1621, 1512,
1484, 1403, 975 cmꢀ1; UV–vis (THF) lmax (log 3): 246
(4.4), 560 (4.8) nm; C62H72N8O2 (960.58).
7. Kapplinger, C.; Beckert, R. Synthesis 2002, 13, 1843.
€
€
€
9. Kuhn, C.; Beckert, R.; Grummt, U.-W.; Kapplinger, C.;
Birckner, E. Z. Naturforsch. 2004, 59b, 406.
10. Prasanna, S.; Radhakrishnan, T. P. Synth. Met. 1996, 78, 127.
11. Matschke, M.; Kapplinger, C.; Weiß, D.; Beckert, R.
Tetrahedron Lett. 2005, 46, 8249.
€
12. Aum€uller, A.; H€unig, S. Liebigs Ann. Chem. 1986, 165.
€
€
13. Muller, D.; Beckert, R.; Gorls, H. Synthesis 2001, 4, 601.
3.7. Reduction of the quinomethides (10) to biimidazoles
(11)
€
€
€
14. Muller, D.; Beckert, R.; Weston, J.; Gunther, W.; Gorls, H.;
Friedrich, M. Eur. J. Org. Chem. 2001, 4551.
€
15. Atzrodt, J.; Brandenburg, J.; Kapplinger, C.; Beckert, R.;
Derivative 10 (0.1 mmol), dissolved in 20 ml of THF was re-
duced with 5 ml of 0.06 M solution of sodium dithionite.
The reaction mixture was irradiated with ultrasound for
10 min at rt. The biimidazoles 11 were obtained as yellow
solids by removing the solvent in vacuo.
€
€
Gunther, W.; Gorls, H.; Fabian, J. J. Prakt. Chem./Chem.-Ztg.
1997, 339, 729.
€
16. Mayer, U.; Baumgartel, H.; Zimmermann, H. Angew. Chem.
1966, 78, 303.
17. Mayer, U.; Baumgartel, H.; Zimmermann, H. Tetrahedron Lett.
€
1
11a: Yield: 96%, yellow solid; H NMR (250 MHz, THF):
1966, 42, 5221.
18. Sakaino, Y.; Kakisawa, H.; Kusumi, T.; Maeda, K. J. Org.
Chem. 1979, 44, 1241.
19. Gompper, R.; Mehrer, M.; Polborn, K. Tetrahedron Lett. 1993,
40, 6379.
€
20. Brandenburg, J.; Kapplinger, C.; Beckert, R. Synthesis 1996,
1302.
€
21. Kapplinger, C.; Beckert, R. Synlett 2001, 1188.
d 1.24 (s, 36H), 6.82 (d, J¼8.3 Hz, 8H), 7.29 (d, J¼
8.3 Hz, 8H), 8.04 (s, 4H); UV–vis (THF) lmax (log 3): 317
(4.4), 370 (4.5) nm; C52H62N8 (798.48).
1
11b: Yield: 97%, yellow solid; H NMR (250 MHz, THF):
d 0.79 (t, J¼8.0 Hz, 12H), 1.20 (m, 24H), 1.45 (m, 16H),
6.51 (d, J¼8.5 Hz, 8H), 7.09 (d, J¼8.5 Hz, 8H), 7.863 (s,