The introduction of OH and COOH groups into 4H-imidazoles: Water-soluble functional dyes and quinomethides

Article abstract of DOI:10.1055/s-2008-1067249

Water-soluble 4H-imidazoles, which represent redox- as well as pH-switchable functional dyes, are synthesized via a novel procedure. In a smooth reaction, phthalic anhydride reacts with oxalic acid amidine 5 yielding 4H-imidazole 1a. Analogously, but in lower yields, naphthalene-1,8-dicarboxylic anhydride and 2-sulfobenzoic anhydride can be transformed into 4H-imidazoles 1b,c. 4-Hydroxybenzoic acids do not form the expected 4H-imidazoles, which possess hydroxyaryl substructures. However, in the course of the cyclization-long-range prototropism sequence, the new quinomethides 7a-c were obtained. This sequence could be adapted successfully for the synthesis of corresponding thioxo derivatives 7d. The water-soluble 4H-imidazoles 1a-c as well as quinomethides of type 7 proved to be multifunctional and switchable dyes. They show acidochromism and in addition, can be transformed into fluorescent leuco forms, which reoxidize when exposed to air. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2008-1067249

PAPER
2957
The Introduction of OH and COOH Groups into 4H-Imidazoles:
Water-Soluble Functional Dyes and Quinomethides
I
ntroduction of OH and
a
C
OOH Groups int
t
o
4
H
-Im
i
idazo
n
a
Institute of Organic and Macromolecular Chemistry, Friedrich Schiller University, Humboldtstraße 10, 07743 Jena, Germany
Fax +49(3641)948212; E-mail: c6bera@uni-jena.de
b
Institute of Physiology II, Friedrich Schiller University, Kollegienstraße 9, 07743 Jena, Germany
Received 17 April 2008; revised 29 May 2008
Ar2
NH
Abstract: Water-soluble 4H-imidazoles, which represent redox- as
well as pH-switchable functional dyes, are synthesized via a novel
procedure. In a smooth reaction, phthalic anhydride reacts with ox-
alic acid amidine 5 yielding 4H-imidazole 1a. Analogously, but in
lower yields, naphthalene-1,8-dicarboxylic anhydride and 2-sul-
fobenzoic anhydride can be transformed into 4H-imidazoles 1b,c.
4-Hydroxybenzoic acids do not form the expected 4H-imidazoles,
which possess hydroxyaryl substructures. However, in the course of
the cyclization–long-range prototropism sequence, the new qui-
nomethides 7a–c were obtained. This sequence could be adapted
successfully for the synthesis of corresponding thioxo derivatives
7d. The water-soluble 4H-imidazoles 1a–c as well as quino-
methides of type 7 proved to be multifunctional and switchable
dyes. They show acidochromism and in addition, can be trans-
formed into fluorescent leuco forms, which reoxidize when exposed
to air.
N
Ar1
N
H
NH
Ar2
4
O2
reduction
_
H2O2
Ar2
N
Ar2
NH
Ar2
N
+
N
N
+
N
H
Ar1
–
Ar1
Ar1
_
H
+
N
N
N
NH
Ar2
NH
Ar2
N
Ar2
2
3
1
Ar2
N
NH2
Key words: heterocycles, carboxylic acids, quinomethides, func-
tional dyes, tautomerism
Ar1 COCl
+
N
Ar2
H2N
5
Scheme 1 4H-Imidazoles; acidochromic and redox switchable
functional dyes
During the last few years there has been an exciting vari-
ety of developments in new technologies using functional
dyes. In addition to their features as functional materials,
modern chromophores and fluorophores are mainly used
in medicinal diagnostics, biology, and biochemistry as
marker substances and sensors for biological active mole-
cules. Therefore, the water solubility of these derivatives
is a major requirement.
electron-rich olefins that immediately reoxidize in the
presence of oxygen.4 When the reduction is carried out in
aqueous solution, e.g. by sodium dithionite, exactly one
equivalent of hydrogen peroxide forms upon reoxidation.
This redox reaction of 4H-imidazoles is connected with a
drastic color alteration making them attractive for applica-
tions in molecular switches, markers, and others. The wa-
ter solubility of these derivatives can only be realized thus
far by the use of co-solvents, therefore we are interested in
the synthesis of such species that display water solubility.
4H-Imidazoles 1, developed in our group, represent a new
class of functional dyes; these cyclic merocyanines are
pH-sensitive and can be transformed by protonation/
deprotonation into chromophores that absorb at longer
wavelengths (cyanines 2/azaoxonoles 3, Scheme 1).1 Re-
cently, palladium complexes that contain merocyanine-
like 4H-imidazoles as ligands have been synthesized.
These complexes can be regarded as new functional dyes
as well as deeply colored and redox active metallacycles
that display catalytic activity.2 However, the most impor-
tant feature is their reversible redox behavior. They can
easily be reduced giving derivatives of 4,5-diaminoimida-
zole 4 (Scheme 1) as leuco forms.3,4
Based on earlier findings, functional groups (e.g., halo,
CF3, or COOR) can easily be integrated into the peripheral
aryl substituents of their five-ring system.5 Due to the high
reactivity of XH groups in the synthesis of ethers or esters,
in connection with characteristic features such as tauto-
merism and absorption/chemisorption on surfaces, our ad-
ditional aim was the introduction of OH/SH groups.
4H-Imidazoles can be synthesized by different synthetic
processes.5–9 In our case, the cycloacylation of oxala-
midines 5, which tolerate a broad variability of substitu-
ents in the 2-position, was the method of choice.9 Instead
of acyl chlorides, carboxylic anhydrides were used. First-
ly, phthalic anhydride was tested as model substance
(Scheme 2). Upon addition of the carboxylic acid deriva-
tive to a solution of the deprotonated oxalamidine 5a, the
The latter, 4,5-diaminoimidazoles 4, are not only highly
substituted heteroaromatics, but also cyclic versions of
SYNTHESIS 2008, No. 18, pp 2957–2962
x
x.
x
x
.
2
0
0
8
Advanced online publication: 04.09.2008
DOI: 10.1055/s-2008-1067249; Art ID: T06208SS
© Georg Thieme Verlag Stuttgart · New York

2958
M. Matschke et al.
PAPER
NH2
NAr
ish fluorescent 1H-imidazoles 4 (Scheme 1), which
quickly reoxidize when exposed to air. As depicted in
Figure 1 the well-separated graphs of both species in the
UV/Vis-spectra allow their quantification. In case of re-
duction/reoxidation of derivative 1a/4a, an isosbestic
point at 431 nm was obtained.
ArN
H2N
5a Ar = 4-Tol
1. THF, n-BuLi
2. anhydride
Whereas 4H-imidazoles 1a–c reacted with aqueous alkali
hydroxides to give deep purple carboxylates/sulfonates,
upon addition of hydrochloric acid fast hydrolytic decom-
position (pH <5.5) with decolorization takes place.
Ar
Ar
N
Ar
N
SO3H
COOH
N
N
N
N
N
N
NH
NH
Ar
N
3,5-Di-tert-butyl-4-hydroxybenzoic acid and 4-hydroxy-
benzoic acid were chosen as starting materials for the syn-
thesis of 4-hydroxyaryl-substituted 4H-imidazoles
because of their commercial availability. Both acids can
be transformed nearly quantitatively into the correspond-
ing acid chlorides 6a,b upon treatment with thionyl chlo-
ride/oxalyl chloride,15 which were then cyclized with
oxalamidines 5a,b according to known procedures7 to
give deep red products in moderate yields. Elemental
analyses and MS/NMR data confirmed the structure of
quinomethides16 7 for the new products obtained. As
shown in Scheme 3, based on the existence of 4-OH
groups the assignment of different tautomeric forms is
possible. The well-resolved 1H NMR spectra measured at
different temperatures show that only one of three species
is predominant in solution. The existence of tautomeric
form 7¢¢ could be excluded by IR spectroscopy. Instead of
characteristic broad absorptions in the frequency range of
OH groups, a strong absorption band for the carbonyl
group at about 1730 cm–1 was detected.
COOH
NH
Ar
Ar
1a (25%)
1b (9%)
1c (12%)
Scheme 2 Synthesis of carboxy/sulfo-functionalized 4H-imida-
zoles
color of the reaction mixture immediately turned from
yellow to orange-red. The workup was optimized as fol-
lows: the separation of the formed lithium salts was real-
ized by washing the dissolved (CH2Cl2 or CHCl3) product
with water, subsequently the 4H-imidazole was purified
by column chromatography. While the organic layer
mostly contained unreacted starting material 5a and N-
tolylphthalimide as a byproduct, the 4H-imidazole 1a was
extracted from the aqueous layer with dichloromethane by
addition of a small amount of acetic acid.
Other anhydrides were tested in the same manner, where-
by only naphthalene-1,8-dicarboxylic anhydride and 2-
sulfobenzoic anhydride gave the corresponding 4H-im-
idazoles 1b and 1c in appreciable yields. 3-Nitronaphtha-
lene-1,8-dicarboxylic anhydride and benzene-1,2,4,5-
tetracarboxylic dianhydride proved to be reactive; howev-
er, inseparable mixtures of isomers were obtained. Naph-
thalene-1,4,5,8- and perylene-3,4,9,10-tetracarboxylic
dianhydrides gave no results due to their low solubility;
the corresponding 4H-imidazoles could only be detected
in trace amounts by TLC. In addition, as in the case of ma-
leic anhydride, the formation of cyclic imides is favored
here.10–14
The spectroscopic evidence was supported by the fact that
derivative 7 showed no tendency to form highly fluores-
cent boracycles17 upon treatment with boron compounds,
e.g. BF3·OEt2. Furthermore, all attempts to establish pro-
totropic form 7¢ by using cyclization reactions (trialkyl
orthoformate or triphosgene) were unsuccessful. This ob-
servation is in accordance with results obtained for vinyl-
ogous tetraazafulvalenes, where these cyclization
reactions completely failed.18 All these spectral and chem-
ical findings clearly underline the existence of a structure
in accord with 7 (Scheme 3).
All 4H-imidazoles 1a–c are highly soluble in water show-
ing strong hydrophilicity, which makes these derivatives
insoluble even in dried organic solvents. Reduction with
aqueous sodium dithionite solution yields yellow-green-
For a better understanding of this type of prototropism
(‘long-range-prototropism’), 4-mercaptobenzoic acid was
integrated into this cyclization reaction. Thus, starting
Figure 1 Reduction of 1a and reoxidation of 4a (Ar1 = 4-Tol, Ar = 2-HO2CC6H4)
Synthesis 2008, No. 18, 2957–2962 © Thieme Stuttgart · New York

PAPER
Introduction of OH and COOH Groups into 4H-Imidazoles
2959
Ar
R
R
R
R
NH
N
5a or 5b
THF, n-BuLi
7a: X = O, R = H, Ar = 4-Tol (41%)
7b: X = O, R = t-Bu, Ar = 4-Tol (37%)
X
HX
COCl
N
H
7c: X = O, R = t-Bu, Ar = 4-t-BuC6H4 (34%)
7d: X = S, R = H, Ar = 4-Tol (23%)
7e (from 7b): X = O, R = t-Bu, Ar = 4-Me2NC6H4 (20%)
N
Ar
6a: X = O, R = H
6b: X = O, R = t-Bu
6c: X = S, R = H
5a: Ar = 4-Tol
5b: Ar = 4-t-BuC6H4
Ar
Ar
Ar
Ar
R
R
OH
O
NH
NH
NH
NH
N
N
N
N
N
N
X
HX
N
N
H
NH
Ar
N
N
N
1d
1d'
R
R
7''
7'
Ar
Ar
Ar
Scheme 3 Synthesis of quinomethides 7
Ar
Ar
t-Bu
t-Bu
N
N
red.
N
N
N
N
.
7b,c
O
O
.
ox.
N
N
t-Bu
t-Bu
Ar
Ar
8
ox.
red.
deprotonation
protonation
Ar
t-Bu
Na2S2O4
H2O
NH
N
HO
7b,c
N
H
O2
NH
t-Bu
Ar
9b,c
Scheme 4 Reduction of quinomethides 7
from the chloride 6c, a deeply colored product was isolat- however only one was detected for the C=O/C=S group.
ed with spectral data in accordance with structure 7d.
The absorption maxima of the deep red colored solutions
of derivatives 7a–d lie in the range of ~520 nm with ex-
tinction coefficients higher than 4.0. Compared to the 4H-
imidazoles prepared earlier,1,7,8 tautomerization into qui-
nomethides of type 7 causes a bathochromic shift of the
longest wavelength absorption band. While the chro-
mophoric system of 4H-imidazoles 1 is based on a diaza-
pentamethine–merocyanine, that of quinomethides 7a–e
was extended to a diazaheptamethine–merocyanine.
In addition to the derivatives described above, the former-
ly synthesized 4H-imidazole 1d7 was included in the fol-
lowing considerations. In contrast to its 4-substituted
derivative, this 4H-imidazole does not show any tendency
to tautomerize into ortho-quinomethide 1d¢.
The different behavior of such derivatives with respect to
tautomerization processes shows that exclusively XH
substituents in the 4-position effected the transformation
into quinomethides of type 7. In the course of the proton
shift, the electronic nature partially changes, meaning that
the aryl subunit was oxidized, while the 4H-imidazole nu-
clei was reduced.19 Similar transformations have been ob-
served on substituted benzothiazoles of 7-hydroxy-1,6-
methano[10]annulenes.20
Generally, derivatives 7 show reactivity comparable to
their parent compounds. Thus, derivative 7b can be con-
verted into the deep purple 7e (X = O, R = t-Bu, Ar = 4-
Me2NC6H4) by transamination7,8 with 4-(dimethyl)-1,2-
phenylenediamine in the presence of catalytic amounts of
p-toluenesulfonic acid.
Being amphoteric heterocycles, derivatives of type 7 react
with acids as well as with bases to yield salts. The differ-
ent colors of their solutions are caused by alteration of the
chromophoric system. Whereas deprotonation of 7b with
butyllithium gives deep purple (aza)oxonoles of type 3
(l = 539 nm), the transition from merocyanine to cyanine
2 in case of protonation resulted in a bathochromic shift of
70–100 nm. The possible application of 4H-imidazoles as
The spectroscopic properties of derivatives 7 slightly vary
from those of ‘normal’ aryl-substituted 4H-imidazoles.
Despite the non-equivalence of the arylamino/arylimino
substituents resulting in different signal sets, only two
doublets for the aromatic protons were detected. This
characteristic feature is explained by very fast intermolec-
ular proton-exchange processes. According to structure 7
(Scheme 3), the IR spectra show two NH absorptions,
Synthesis 2008, No. 18, 2957–2962 © Thieme Stuttgart · New York

2960
M. Matschke et al.
PAPER
IR (ATR): 1707, 3262 (br), 3428 cm–1.
1H NMR (250 MHz, THF-d8/D2O): d = 2.17 (s, 6 H), 6.98 (d,
quantitative pH-sensor systems has already been dis-
cussed.1 The reduction of the quinomethides 7 proceeds in
a similar manner as for 4H-imidazoles (Scheme 4).3 Com- 3J = 8.3 Hz, 4 H), 7.20–7.35 (m, 2 H), 7.68–7.80 (m, 6 H).
13C NMR (62 MHz, THF-d8/D2O): d = 18.1, 119.3, 126.9, 127.1,
pared with earlier data, very high semiquinomethide 8 for-
mation constants (KSEM = 3.6 × 1013 to 3.8 × 1014) were
ascertained. Most likely, this increasing rate is caused by
the localization of the radical electron at the oxygen atom.
In case of 8b, better stabilization by steric effects can ad-
ditionally be discussed.1,21,22 However, the kinetic stabili-
ty of the radical 8b derived from 7b is very low, so that no
signals could be detected by ESR measurements. The re-
duction of 7b,c with an aqueous solution of sodium
dithionite yields yellow, greenish fluorescent and oxygen-
sensitive 1H-imidazoles 9b,c in a single step, which
quickly reoxidize upon contact with air.
127.6, 129.5, 134.5, 135.2, 135.6, 145.5, 147.7, 163.6, 168.4, 174.7.
MS (DEI): m/z (%) = 91 (58), 106 (100), 133 (46), 149 (29), 167
(32), 237 (41), 266 (76), 351 (7), 396 (68) [M]+, 397 (25) [M + H]+.
UV/Vis (THF–H2O, 1:1): lmax (log e) = 448 (4.3), 482 nm (4.1).
Anal. Calcd for C24H20N4O2: C, 72.71; H, 5.08; N, 14.13. Found: C,
72.82; H, 5.15; N, 14.10.
8-[5-(4-Tolylamino)-4-(4-tolylimino)-4H-imidazol-2-yl]-1-
naphthoic Acid (1b)
Red powder; yield: 9%.
IR (ATR): 1698, 3326 (br), 3679 cm–1.
1H NMR (250 MHz, THF-d8/D2O): d = 2.28 (s, 6 H), 6.80 (d,
A novel synthetic route to water-soluble 4H-imidazoles
1a–c starting from aromatic carboxylic acid anhydrides
was developed. These derivatives are fully reversible two-
step redox systems, evidenced by UV/Vis and electro-
chemical measurements. Therefore they might be applica-
ble as functional dyes for the quantification of
biochemical redox processes. A further interesting feature
is their long wavelength absorption (lmax = 480 to 530
nm) connected with high extinction coefficients (log e
>4).
3J = 8.2 Hz, 4 H), 6.91 (d, 3J = 8.2 Hz, 1 H), 7.09 (d, 3J = 8.2 Hz, 4
3
3
H), 7.28 (d, J = 8.2 Hz, 1 H), 7.39 (t, J = 8.3 Hz, 1 H), 7.53 (t,
3J = 8.2 Hz, 1 H), 7.65 (d, 3J = 8.3 Hz, 1 H), 7.89 (d, 3J = 8.3 Hz, 1
H), 8.04 (s, 1 H).
13C NMR (62 MHz, THF-d8/D2O): d = 18.1, 121.8, 126.2, 126.7,
127.1, 127.6, 128.0, 129.1, 129.5, 134.4, 134.9, 135.0, 135.6, 145.5,
147.7, 163.2, 167.6, 176.7.
MS (FAB in nba): m/z (%) = 235 (100), 259 (29), 265 (44), 313
(69), 349 (48), 466 (10) [M]+.
UV/Vis (THF–H2O, 1:1): lmax (log e) = 430 (4.2), 500 nm (4.1).
Starting from benzoic acids which possess XH groups in
the 4-position novel quinomethides 7 were synthesized. In
addition to their halochromism, they proved to be revers-
ible two-step redox systems with high KSEM values.
Anal. Calcd for C28H22N4O2: C, 75.32; H, 4.97; N, 12.55. Found: C,
75.43; H, 5.05; N, 12.60.
2-[5-(4-Tolylamino)-4-(4-tolylimino)-4H-imidazol-2-yl]ben-
zenesulfonic Acid (1c)
Red powder; yield: 12%.
IR (ATR): 1606, 3306 (br), 3428 cm–1.
All reactions were monitored by TLC, carried out on 0.25 mm
1
Merck silica gel plates (60F254) using UV light. H and 13C NMR
spectra were recorded with a Bruker DRX 400 or Bruker AC 250
spectrometer. Melting points are measured with a Galen TM 3 ap-
paratus and are uncorrected. UV-VIS spectra were recorded on a
Perkin-Elmer Lambda 19 spectrophotometer. MS spectra were tak-
en from measurements on a Finnigan MAT SAQ 710 mass spec-
trometer. Electrochemical measurements were carried out with a
Metrohm 663VA Stand using Hg or Pt electrodes and Bu4NPF6 as
conductive salt.
1H NMR (250 MHz, acetone-d6/D2O): d = 2.17 (s, 6 H), 6.56 (d,
3J = 8.0 Hz, 1 H), 6.81 (d, 3J = 8.0 Hz, 1 H), 7.34 (d, 3J = 8.5 Hz, 4
H), 7.4–7.5 (m, 2 H), 7.71 (d, 3J = 8.0 Hz, 4 H).
13C NMR (62 MHz, acetone-d6/D2O): d = 22.3, 116.0, 120.2, 124.4,
126.9, 127.1, 127.4, 129.1, 129.5, 130.6, 138.5, 139.2, 146.4, 176.3.
MS (FAB in nba): m/z (%) = 207 (45), 233 (50), 248 (51), 267
(100), 293 (44), 333 (67), 347 (25), 359 (19), 376 (29), 401 (21),
432 (12) [M]+.
Carboxy/Sulfo-Substituted 4H-Imidazoles 1; General Proce-
dure
UV/Vis (THF–H2O, 1:1): lmax (log e) = 462 nm (4.0).
Anal. Calcd for C23H20N4O3S: C, 63.87; H, 4.66; N, 12.95; S, 7.41
Found: C, 63.90; H, 4.72; N, 13.04; S, 7.34.
Bis(tolyl)oxalamidine 5 (266 mg, 1.0 mmol) dissolved in anhyd
THF (20 mL) was deprotonated with 1.6 M BuLi in THF (1.3 mL,
2.1 mmol) at r.t. to obtain a yellow-orange soln of amidinate. After
addition of the corresponding acid anhydride (0.7 mmol), the mix-
ture immediately turned orange-red. The mixture was refluxed for
2–4 h until completion, then cooled to r.t., evaporated to dryness,
and the residue was dissolved in CH2Cl2. The organic layer was
washed with H2O to extract the crude product and inorganic salts.
The 4H-imidazoles 1 were isolated by extraction of the aqueous lay-
er with CH2Cl2 with the addition of AcOH (1–2 drops). The organic
layers were dried (anhyd Na2SO4) and filtered and the solvent was
evaporated under reduced pressure to give the orange-red 4H-imi-
dazoles 1.
Quinomethides 7; General Procedure
The corresponding oxalamidine 5 (1.0 mmol) was dissolved in an-
hyd THF (30 mL) under an argon atmosphere. To this soln 1.0 M
BuLi in n-hexane (2.0 mL) was added and the mixture was stirred
at r.t. for 10 min. Then, the corresponding acid chloride 6 (1.0
mmol) was added dropwise to the mixture, which was finally heated
under reflux for 3 h. The solvent was removed in vacuo and the res-
idue was purified by column chromatography (silica gel, toluene–
acetone, 10:1).
4-[4-(4-Tolylamino)-5-(4-tolylimino)-1,5-dihydro-2H-imidazol-
2-ylidene]cyclohexa-2,5-dienone (7a)
Dark red solid; yield: 41%; mp > 250 °C (dec.).
2-[5-(4-Tolylamino)-4-(4-tolylimino)-4H-imidazol-2-yl]benzoic
Acid (1a)
Orange-red powder; yield: 25%; mp > 200 °C (dec.).
IR (ATR): 1729, 3303, 3426 cm–1.
Synthesis 2008, No. 18, 2957–2962 © Thieme Stuttgart · New York

PAPER
Introduction of OH and COOH Groups into 4H-Imidazoles
2961
1H NMR (250 MHz, THF-d8): d = 2.41 (s, 6 H), 7.31 (d, 3J = 8.3 Hz,
4 H), 7.39 (d, 3J = 8.5 Hz, 1 H), 7.91 (d, 3J = 8.3 Hz, 4 H), 8.10 (d,
3J = 8.5 Hz, 2 H), 8.64 (d, 3J = 8.5 Hz, 1 H).
2,6-Di-tert-butyl-4-(4-{[4-(dimethylamino)phenyl]amino}-5-
{[4-(dimethylamino)phenyl]imino}-1,5-dihydro-2H-imidazol-
2-ylidene)cyclohexa-2,5-dienone (7e)
Purple solid; yield: 20%.
IR (ATR): 1730, 3621 cm–1.
MS (DEI): m/z (%) = 91 (22), 107 (46), 133 (15), 205 (44), 233
(100), 252 (29), 267 (12), 309 (32), 366 (10) [M]+.
UV/Vis (THF): lmax (log e) = 450 (4.1), 485 (4.3), 515 nm (4.1).
CV: ERED1 = –1.01 V, ERED2 = –1.25 V.
1H NMR (250 MHz, THF-d8): d = 1.44 (s, 18 H), 3.00 (s, 12 H),
6.79 (d, 3J = 8.5 Hz, 4 H), 8.09 (d, 3J = 8.5 Hz, 4 H), 8.15 (s, 2 H).
MS (DEI): m/z (%) = 104 (41), 136 (58), 145 (35), 166 (33), 205
(100), 233 (83), 338 (6), 368 (5), 469 (25), 538 (28) [M]+, 539 (41)
[M + H]+.
Anal. Calcd for C23H20N4O: C, 74.98; H, 5.47; N, 15.21 Found: C,
75.03; H, 5.55; N, 15.40.
2,6-Di-tert-butyl-4-[4-(4-tolylamino)-5-(4-tolylimino)-1,5-dihy-
dro-2H-imidazol-2-ylidene]cyclohexa-2,5-dienone (7b)
Dark red solid; yield: 37%; mp >280 °C (dec.).
IR (ATR): 1728, 3292, 3427 cm–1.
UV/Vis (THF): lmax (log e) = 266 (4.2), 312 (4.1), 484 (4.1), 569
nm (4.2).
Anal. Calcd for C33H42N6O: C, 73.57; H, 7.86; N, 15.60; Found: C,
73.66; H, 7.78; N, 15.72.
1H NMR (400 MHz, CDCl3): d = 1.46 (s, 18 H), 2.36 (s, 6 H), 7.00
(s, 2 H), 7.18 (d, 3J = 8.5 Hz, 4 H), 7.52 (d, 3J = 8.5 Hz, 4 H), 8.43
(br s, 1 H).
13C NMR (100 MHz, THF-d8): d = 20.9, 30.2, 34.4, 120.5, 124.2,
125.3, 129.5, 134.5, 135.7, 136.2, 143.8, 151.5, 157.1, 166.6, 182.5.
Reduction of Derivatives 7; General Procedure
A mixture of 7 (1.0 mmol), THF (50 mL) and 0.06 M Na2S2O4 soln
(10 mL) was stirred at r.t. for 15 min. During the reduction the color
of the soln changed from deeply red to yellow. The solvent was re-
moved in vacuo. Leuco forms 9 were obtained as yellowish solids,
which immediately reoxidize when exposed to air. Therefore, no ac-
curate MS and NMR spectra could be obtained.
MS (DEI): m/z (%) = 91 (22), 107 (46), 133 (15), 205 (44), 233
(100), 267 (12), 339 (32), 480 (6) [M]+.
UV/Vis (THF): lmax (log e) = 455 (4.1), 490 (4.2), 524 nm (4.1).
CV: ERED1 = –0.93 V, ERED2 = –1.73 V.
2,6-Di-tert-butyl-4-[4,5-bis(4-tolylamino)-1H-imidazol-2-
yl]phenol (9b)
Yellow solid; yield: 100%.
Anal. Calcd for C31H36N4O: C, 77.47; H, 7.55; N, 11.66 Found: C,
77.53; H, 7.65; N, 11.75.
UV/Vis (THF): lmax (log e) = 405 nm (4.1).
Fluorescence (THF, 407 nm): lmax,em = 473 nm.
2,6-Di-tert-butyl-4-{4-[(4-tert-butylphenyl)amino]-5-[(4-tert-
butylphenyl)imino]-1,5-dihydro-2H-imidazol-2-ylidene}cyclo-
hexa-2,5-dienone (7c)
Dark red powder; yield: 34%; mp >300 °C (dec.).
IR (ATR): 1735, 3340, 3484 cm–1.
2,6-Di-tert-butyl-4-{4,5-bis[(4-tert-butylphenyl)amino]-1H-imi-
dazol-2-yl}phenol (9c)
Yellow solid; yield: 100%.
UV/Vis (THF): lmax (log e) = 408 nm (4.1).
Fluorescence (THF, 409 nm): lmax,em = 470 nm.
1H NMR (250 MHz, THF-d8): d = 1.44 (s, 18 H), 1.51 (s, 18 H),
6.99 (s, 2 H), 7.19 (d, 3J = 8.2 Hz, 4 H), 7.56 (d, 3J = 8.3 Hz, 4 H).
13C NMR (62 MHz, THF-d8): d = 30.2, 31.5, 32.9, 34.5, 120.6,
124.2, 125.5, 129.7, 134.4, 135.7, 136.0, 143.8, 151.3, 157.6, 167.0,
182.4.
Acknowledgment
We thank Deutsche Forschungsgesellschaft (DFG) and the
European Union (SNIB, MTKD-CT-2005-029554) for the financial
support.
MS (DEI): m/z (%) = 91 (65), 106 (71), 134 (97), 159 (95), 175
(87), 233 (100), 276 (75), 293 (92), 332 (83), 352 (90), 381 (44),
507 (10), 564 (15) [M]+.
UV/Vis (THF): lmax (log e) = 280 (4.3), 491 (4.1), 520 nm (4.1).
CV: ERED1 = –0.96 V, ERED2 = –1.82 V.
References
(1) Matschke, M.; Beckert, R. Molecules 2007, 12, 723.
(2) Blumhoff, J.; Beckert, R.; Walther, D.; Rau, S.; Rudolph,
M.; Görls, H.; Plass, W. Eur. J. Inorg. Chem. 2007, 3, 481.
(3) Matschke, M.; Käpplinger, C.; Weiß, D.; Beckert, R.
Tetrahedron Lett. 2005, 46, 8249.
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Anal. Calcd for C37H48N4O: C, 78.68; H, 8.57; N, 9.92 Found: C,
78.58; H, 8.65; N, 9.90.
4-[4-(4-Tolylamino)-5-(4-tolylimino)-1,5-dihydro-2H-imidazol-
2-ylidene]cyclohexa-2,5-dienethione (7d)
Dark red solid; yield: 23%.
IR (film): 1630, 3447 cm–1.
1H NMR (250 MHz, CDCl3): d = 2.39 (s, 6 H), 7.31 (d, 3J = 8.5 Hz,
4 H), 7.52 (d, 3J = 8.0 Hz, 2 H), 7.88 (d, 3J = 8.5 Hz, 4 H), 7.99 (d,
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Günther, W.; Görls, H.; Fabian, J. J. Prakt. Chem./
Chem.-Ztg. 1997, 339, 729.
13C NMR (62 MHz, CDCl3): d = 21.5, 117.7, 125.3, 128.3, 129.1,
137.8, 138.4, 145.0, 163.4, 173.5, 187.6.
(9) Müller, D.; Beckert, R.; Görls, H. Synthesis 2001, 601.
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1983, 39, 2073.
MS (DEI): m/z (%) = 77 (35), 91 (25), 106 (83), 131 (61), 136
(100), 243 (16), 296 (53), 369 (26), 383 (32), 384 (14) [M]+.
UV/Vis (THF): lmax (log e) = 416 (3.9), 485 (4.2), 523 nm (4.1).
Anal. Calcd for C23H20N4S: C, 71.85; H, 5.24; N, 14.57; S, 8.34
Found: C, 71.78; H, 5.18; N, 14.60; S, 8.42.
Synthesis 2008, No. 18, 2957–2962 © Thieme Stuttgart · New York

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