New tetrakis(4-aminophenyl)ethenes: Synthesis and electrochemical investigations

Article abstract of DOI:10.1016/j.tetlet.2005.10.071

Four novel functional conjugated tetraarylamines containing a tetraphenylenecore were synthesized via Ni-catalyzed aryl amination followed by McMurry olefination. Their electrochemical properties were studied by cyclic voltammetry, coulometry, and electro

Full text of DOI:10.1016/j.tetlet.2005.10.071

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 8793–8797
New tetrakis(4-aminophenyl)ethenes: synthesis and
electrochemical investigations
Jean-Franc¸ois Kuntz,a Raphae¨l Schneider,a,* Alain Walcariusb and Yves Forta,*
a
`
´
´
´
´
´
Synthese Organometallique et Reactivite, UMR 7565, Universite Henri Poincare, BP 239, 54506 Vandoeuvre les Nancy Cedex, France
bLaboratoire de Chimie Physique et Microbiologie pour l’Environnement, UMR 7564, CNRS-Universite´ Henri Poincare´,
54600 Villers les Nancy, France
Received 25 August 2005; revised 30 September 2005; accepted 10 October 2005
Abstract—Four novel functional conjugated tetraarylamines containing a tetraphenylenecore were synthesized via Ni-catalyzed
aryl amination followed by McMurry olefination. Their electrochemical properties were studied by cyclic voltammetry, coulometry,
and electrolysis monitored by UV–vis spectroscopy, and discussed with regard to those of the known tetrakis(dimethyl-
aminophenyl)ethane.
ꢀ 2005 Elsevier Ltd. All rights reserved.
X
Aromatic amines have been extensively used as excellent
hole-transport layers (HTL) in fabricating organic elec-
troluminescent (EL) devices,1 which have great potential
use in full color flat panel displays, as a result of their low
drive voltage, high efficiency, and high brightness. Effi-
cient hole transporting ability requires the facile oxida-
tion of aromatic amines to stable radical cations. The
ability to synthesize new arylamines allows to tune the
electronic structure of these materials; this is of interest
because matching the energy of the highest occupied
molecular orbitals of hole-transport materials with
respect to those of the anode and electron transport layer
strongly influences charge injection efficiencies and there-
fore the overall performance of organic devices.2
X
X
X
1: X = NR2; R = Me (1a), R = H (1b), R = Ar (1c)
2 : Cl
Figure 1.
of compounds 1 in constructing hole-transport materi-
als, very little is known about their synthesis and elec-
tronic properties. The oxidation/reduction of tetrakis
(aminophenyl)ethenes 1a,b has been investigated by
In the search for new potential charge transport materi-
als, we were interested in developing conjugated materi-
als of branched topologies and became attracted by the
two-dimensional conjugated unit 1 (Fig. 1) in which
three different conjugation modes are available: diago-
nal conjugation, lateral conjugation, and cross-conjuga-
tion. Several studies have demonstrated that conjugated
architectures based on tetraphenylethene core displayed
different properties from those of their one-dimensional
analogs.3 Surprisingly, despite the potential applications
Hunig and Fox.4,5 Cyclic voltammetry (CV) of 1a
¨
revealed two successive two-electron transfer processes
at ca. ꢀ0.30 and +0.67 V (vs Fc/Fc+). The electrochem-
ical features of 1c (R = C6H5 or m-MeC6H4) was also
briefly investigated by Plater.6
During recent years, we have directed our efforts toward
Ni- and Pd-mediated aryl amination chemistry with
the goal to develop new electron-donor compounds.7
We have recently reported the use of a Ni(0) catalyst
associated with a strong electron-donating and sterically
hindered N-heterocyclic carbene8 [N,N0-bis(2,6-diiso-
propylphenyl)dihydroimidazol-2-ylidene, SIPr] (Fig. 2)
to allow mild amination of aryl chlorides with several
classes of amines.9
Keywords: Diamination; Ni catalysis; N-Heterocyclic carbene; Tetra-
kis(4-aminophenyl)ethenes; McMurry olefination.
*
Corresponding authors. Tel.: +33 383 68 47 84; fax: +33 383 68 47 85
(R.S.); e-mail: Raphael.Schneider@sor.uhp-nancy.fr
0040-4039/$ - see front matter ꢀ 2005Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.10.071

8794
J.-F. Kuntz et al. / Tetrahedron Letters 46 (2005) 8793–8797
Table 1. Cross-coupling of 4,40-dichlorobenzophenone with aminesa
O
O
.. N
N
1 2
HNR R
Ni(0)/SIPr
R1
R1
NaOt-Bu
Cl
THF, 65ºC
N
N
R2
Cl
R2
4
Figure 2.
Entry
a
Amine
Time (h)
4
Yield (%)b
99c
0d,e
90d
We initially envisaged that Ni-catalyzed polyamination
of compound 2 could give an easy access to a variety
of new tetrakis(4-aminophenyl)ethenes. However, due to
the strong electron-donating properties of the 4-amino
substituent introduced in the course of the aryl amina-
tion reaction, the cross-coupling proceeded sluggishly
and afforded mixtures of mono- and di-amination prod-
ucts even after extended reaction times.
HN
O
b
c
12
6
H2N
Me
N
H
OMe
OMe
Me
d
3
93d
N
Herein, we report a new synthesis to tetrakis(4-amino-
phenyl)ethenes 3 based on a two-step process involving
(i) Ni(0)/SIPr catalyzed bis-amination of 4,40-dichloro-
benzophenone and (ii) McMurry olefination of 4,40-
diaminobenzophenones 4 produced (Scheme 1). This
synthetic route takes advantage of the ease with which
4,40-dichlorobenzophenone may be di-aminated in the
4 and 40 positions due to the activating effect of the car-
bonyl group.
H
a Reactions conditions: 5mol % Ni, t-BuONa, THF, 65 ꢁC.
b Yields of pure products after column chromatography.
c A Ni/SIPr ratio of 1/1 was used.
d A Ni/SIPr ratio of 1/2 was used.
e Imine was the only product observed after 12 h reaction.
Under these experimental conditions, we then investi-
gated direct cross-coupling reactions using primary
and secondary aromatic amines (Table 1). All sec-aro-
matic amine substrates underwent direct cross-coupling
reactions with 4,40-dichlorobenzophenone to afford
products 4 in excellent yields. The reaction times were
short (<6 h) and no side-products were detected. In con-
trast, p-anisidine (entry b) failed to give the di-amination
product and the imine, arising from attack of the pri-
mary amine on the carbonyl group, was the only prod-
uct isolated. Attempts to aminate the initially formed
imine by extending the reaction time and/or forcing
the reaction conditions by using an excess of p-anisidine
(up to 6 equiv) or increasing catalyst loading (10 mol %)
only gave a mixture of several unidentified products.
The synthesis of di[(4-methoxyanilino)phenyl]methanone
4b could however be achieved using a dioxolane protec-
tion for the carbonyl group of 4,40-dichlorobenzophe-
none (Scheme 2).
First, we evaluated the feasibility of direct cross-cou-
pling of 4,40-dichlorobenzophenone and amines using
Ni/SIPr catalyst and t-BuONa as the base. Ni(0)/SIPr/
t-BuONa was in situ generated from Ni(acac)2 and
SIPrÆHCl10 by reduction of Ni(acac)2 and deprotonation
of the azolium salt using t-BuONa activated sodium hy-
dride as previously described.9b A preliminary screening
of Ni/SIPr catalyst for coupling of 4,40-dichlorobenzo-
phenone with morpholine revealed that the reaction
required 5mol % of Ni/SIPr complex in refluxing THF
or dioxane. The optimum yield of 4a is obtained when
a ratio of aryl dichloride to amine of 1:3 was used. As
a control experiment, when the same reaction was
carried out in the absence of Ni/SIPr catalyst, very
little mono-aminated cross-coupled product (<7%) was
obtained even after heating for 15h in refluxing THF.
O
O
To achieve the targeted tetrakis(4-aminophenyl) ethenes
3, compounds 4 were subjected to McMurry olefination
using low valent titanium reagents (Table 2).11 Among
the various methodologies developed over the years to
effect reductive couplings of carbonyls, the combination
of TiCl4 with Zn was the best for the preparation of
compounds 3 and the reaction usually could be done
in 2–5h in refluxing dioxane. The use of TiCl 3–Li–
THF or TiCl3–LiAlH4 afforded the substrate 3a in lower
yields (respectively, 73% and 64%).
a
R1
R1
N
N
Cl
Cl
R2
R2
4
b
R2
N
R2
N
R1
R1
The electrochemical behavior of novel donors 3a–c
is close to that observed for the related 1a (Table 3).
Figure 3 depicts the CV curves recorded for 3a as a
representative example. Compounds 3a–c give rise to a
first electrochemically reversible oxidation signal (I),
involving the exchange of two electrons, during which
the respective dications are formed. The two-electron
R1
R1
N
R2
N
R2
3
Scheme 1. Reagents and conditions: (i) HNR1R2 (3 equiv), t-BuONa,
Ni/SIPr (5mol %), THF, 65 ꢁC; (b) TiCl4, Zn.

J.-F. Kuntz et al. / Tetrahedron Letters 46 (2005) 8793–8797
8795
Table 2. McMurry olefination of compounds 4a
O
O
O
R2
R2
a
R1
N
N
R1
O
Cl
Cl
Cl
Cl
TiCl4, Zn
R1
R1
Dioxane
b
N
R2
N
100ºC, 5 h
R2
R1
R1
N
R2
4
N
R2
O
O
O
3
c
Entry Product 3
Yield
(%)b
HN
NH
HN
NH
O
N
O
O
N
N
OMe
OMe
4b
OMe
OMe
5
a
81
Scheme 2. Reagents and conditions: (a) (CH2OH)2, APTS, xylene,
120 ꢁC, 97%; (b) p-anisidine (3 equiv), Ni/SIPr, t-BuONa, THF, 65 ꢁC,
7 h, 86%; (c) H2SO4, SiO2, acetone, rt, 2 h, 87%.
N
O
transfer process was confirmed by constant potential
coulometry performed at 0.8 V. It was followed by a
very small peak near 1 V (denoted III in Fig. 3), which
was also observed for the related 1a compound,4b but
its origin remains unexplained at this stage. Recording
CV curves at various scan rate revealed that the electron
transfer processed were diffusion controlled (peak cur-
rents directly proportional to the square root of scan
rate and cathodic-to-anodic current ratios equal to unity
independently on scan rate). This suggests that, under
the conditions applied, the dications are stable. The
corroborating evidence was obtained from the EPR
spectrum of the oxidized solutions. Oxidized 3a–c exhib-
ited a very strong EPR signal at 298 K in CHCl3. Using
2 equiv thianthrenium perchlorate12 (ThClO4) as oxi-
dant, the fluid solution showed a single broad-line
EPR spectrum with a spectral width of ca. 36 G. The
dications generated from compounds 3a–c are stable at
room temperature displaying a qualitative half-life of
ca. 8 days. These dications can be further oxidized by
extending the potential range in the anodic direction.
Interestingly, compounds 3a–c exhibit extremely large
potential gaps between the first and second oxidation
states (ca. 0.85–0.90 V). This second signal appearing
near 1.4–1.5V ( Fig. 3) is in fact constituted from two
successive one-electron transfers (IIa and IIb) occurring
at very close potential values (Table 3). They correspond
to the successive transformation of 32Æ2+ into transient
33Æ3+ and then to the fully oxidized 34Æ4+ product. This
behavior is similar as that of 1a4 and was observed for
all compounds 3a–c (Table 3). It is noteworthy, how-
ever, that the smaller sized compound 3a was character-
H
H
N
N
MeO
MeO
OMe
OMe
b
72
N
H
N
H
Me
N
Me
N
MeO
MeO
OMe
OMe
c
83
N
N
Me
Me
Me
N
Me
N
d
77
N
N
Me
Me
a Reactions were carried out using 5mmol 4, 5.5 mmol TiCl4, and
15mmol Zn in refluxing dioxane.
b Yields of pure products isolated after column chromatography.
Table 3. Electrochemical data of compounds 3a–d
Compound
3a
3b
3c
3d
EIox (V)
EIred (V)
0.58
0.55
1.44
1.38
1.50
1.44
0.61
0.58
1.46
1.40
1.52
1.46
0.62
0.58
1.48
1.42
1.53
1.47
0.61
0.55
ized by a higher diffusion coefficient (7 · 10ꢀ6 cm2 sꢀ1
)
than those of larger compounds 3b and 3c (4–5 ·
IIa
E
E
E
E
(V)
(V)
(V)
(V)
ox
10ꢀ6 cm2 sꢀ1).
IIa
red
IIb
ox
IIb
red
II
E
¼ 1:52
¼ 1:23
¼ 1:08
ox
II
red
E
The electrochemical behavior of compound 3d was sig-
nificantly different, displaying a single-electron oxida-
tion signal at 0.61 V (I) (Fig. 4, red curve). This was
confirmed by controlled potential coulometry at 0.8 V,
suggesting the generation of a rather stable mono(radi-
cal cation). Once again, this signal was diffusion
III
E
ox
controlled and the cathodic-to-anodic current ratio
was independent on the potential scan rate. Compound

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J.-F. Kuntz et al. / Tetrahedron Letters 46 (2005) 8793–8797
I
IIa,IIb
4.0x10-5
3.0x10-5
2.0x10-5
III
1.0x10-5
0.0
-1.0x10-5
-2.0x10-5
-3.0x10-5
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Potential (V)
Figure 3. Cyclic voltammograms of 3a in CH3CN/0.1 M Bu4NBF4; v = 100 mV sꢀ1; T = 293 K; c = 10ꢀ3 M. Full potential range is in blue and
restricted to the first signal is in red.
4.0x10-5
II
3.0x10-5
I
2.0x10-5
III
1.0x10-5
0.0
-1.0x10-5
-2.0x10-5
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Potential (V)
Figure 4. Cyclic voltammograms of 3d in CH3CN/0.1 M Bu4NBF4; v = 100 mV sꢀ1; T = 293 K; c = 10ꢀ3 M. Full potential range is in blue and
restricted to the first signal is in red.
3d was characterized by a diffusion coefficient of
4.3 · 10ꢀ6 cm2 sꢀ1
(2 equiv), in the dark, under nitrogen atmosphere,
according to the report of Effenberger et al. describing
the synthesis of dimeric r-complexes.14 After 10 min
of stirring, the reaction mixture was evaporated to dry-
ness and a solution of MeONa (2 equiv) in MeOH was
added. The presence of a benzidine moiety could clearly
.
This first reversible redox process was also followed by a
small irreversible signal at 1.08 V (peak III) and then by
a second main peak located at 1.52 V (peak II) giving on
scan reversal a small cathodic peak at 1.23 V (Fig. 4,
blue curve). This second peak is characterized by an irre-
versible one-electron transfer leading probably to unsta-
ble di(radical cation)s that underwent follow-up
reactions (e.g., dimerization13). It is likely that oligomer-
ization reactions between the generated radical cations
took place because 3d is unsubstituted at the para-posi-
tion relative to the nitrogen atom contrary to 3b,c for
whose dimerization through ortho-position is difficult.
This hypothesis was sustained by EPR experiments on
oxidized 3d. Using ThClO4 as oxidant, the fluid solution
showed a single broad-line EPR spectrum with a spec-
tral width of ca. 33 G. The dication 3d2Æ2+ was found
to be less stable than those generated from 3a–c (quali-
tative half-life of ca. 5h at 25 ꢁC).
1
be observed on the H NMR spectra of the crude reac-
tion mixture. This result argued strongly for the r-di-
meric nature of the materials obtained after oxidation
of compound 3d.
Interestingly, all compounds 3 exhibit an inferior
electron-donor ability in comparison with tetra-
kis(dimethyl-aminophenyl)ethene 1a. The shift in oxida-
tion potential can be rationalized in terms of amine
substitution. Indeed, dialkylamino groups exhibit the
most pronounced resonance effect and the strongest
donor character.15
The oxidation process of compounds 3 was also moni-
tored by UV–vis spectroscopy. Figure 5 shows the
UV–vis spectra recorded for 3a in CH2Cl2 solution;
the spectra of 3b,c show similar optical absorption fea-
tures. The neutral 3 were yellow in a CH2Cl2 solution
To provide further evidence for dimerization, we carried
out oxidation of compound 3d in CH2Cl2 with AgNO3

J.-F. Kuntz et al. / Tetrahedron Letters 46 (2005) 8793–8797
8797
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1.0
0.8
0.6
0.4
0.2
0.0
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400
600
800
Wavelength (nm)
Figure 5. UV–vis spectra of the stepwise electrochemical oxidation of
3a over Pt electrode.
4. (a) Hunig, S.; Kemmer, M.; Wenner, H.; Perepichka, I. F.;
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Ba¨uerle, P.; Emge, A.; Gescheid, G. Chem. Eur. J. 1999, 5,
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that exhibit two bands at ca. 290 and 360 nm. When the
solution of 3a was treated with ThClO4 or oxidized over
Pt electrode, the solution turned rapidly blue and three
broad bands appeared gradually in the lower energy re-
gion at ca. 490, 550, and 715 nm, in agreement to what
was reported for compound 1a.4b These peaks did not
decay with time confirming the formation of stable rad-
ical cations.
6. Plater, M. J.; Jackson, T. Tetrahedron 2003, 59, 4673.
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55, 12829; (c) Desmarets, C.; Schneider, R.; Fort, Y.
Tetrahedron Lett. 2000, 41, 2875; (d) Brenner, E.; Schnei-
der, R.; Fort, Y. Tetrahedon Lett. 2000, 41, 2881; (e)
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In summary, we have shown that Ni-catalyzed aryl
amination between 4,40-dichlorobenzophenone or its
dioxolane and amines followed by McMurry reduc-
tive homocoupling of 4,40-diaminobenzophenones 4
produced allows a rapid and efficient synthesis of new
tetrakis(4-aminophenyl)ethylene 3. Cyclic voltammetry
measurements reveal that compounds 3a–c exhibit good
donor properties and give thermally stable radical cat-
ions. The electrochemical behavior of 3d is more compli-
cated and it is likely that the oxidized species of para-
unsubstituted 3d caused follow-up reactions. Efforts
are underway to study the hole transporting ability of
new compounds 3 in electroluminescent devices.
9. (a) Gradel, B.; Brenner, E.; Schneider, R.; Fort, Y.
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10. SIPrÆHCl is commercially available from Strem Chemicals
Inc.
Supplementary data
Synthetic and characterization details for compounds
3a–d and 4a–d are available. Supplementary data associ-
ated with this article can be found, in the online version,
at doi:10.1016/j.tetlet.2005.10.071.
11. (a) McMurry, J. E. Chem. Rev. 1989, 89, 1513; (b) Lenoir,
D. Synthesis 1989, 883; (c) Furstner, A.; Bogdanovic, B.
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Caution! Thianthrenium perchlorate is a shock-sensitive
solid that should be handled only a small scale and with
due care.
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