Synthesis of highly phenylene substituted p-phenylene oligomers from pyrylium salts

Article abstract of DOI:10.1039/b807382a

A transition metal free route to phenyl substituted oligophenylenes that tolerates halogens is described. The Royal Society of Chemistry.

Full text of DOI:10.1039/b807382a

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Synthesis of highly phenylene substituted p-phenylene oligomers
from pyrylium saltswz
Christian Mahler,a Ute Mu
¨
Chulsoon Moon, Gunther Brunklaus,* Herbert Zimmermann and Sigurd Hoger*
ller,a Walter M. Mu
¨
ller,a Volker Enkelmann,b
b
b
c
a
¨
Received (in College Park, MD, USA) 30th April 2008, Accepted 17th June 2008
First published as an Advance Article on the web 21st August 2008
DOI: 10.1039/b807382a
A transition metal free route to phenyl substituted oligophenyl-
enes that tolerates halogens is described.
products attractive substrates for metal catalyzed coupling
reactions.12,13
However, we are not aware of any report about the condensa-
tion of pyrylium salts 1 with sodium 1,4-phenylenediacetate 2 to
functionalized oligophenylenes 3 (Scheme 1). The reaction is
carried out by heating 2.2 equivalents of 1 and 1 equivalent of
2 in acetic anhydride to reflux for 3 h.y Yields are substrate
dependent and vary between 2.4 and 13.4% after purification
(Table 1).14
p-Conjugated oligomers and polymers are of considerable
interest for chemists, physicists and materials scientists, be-
cause of their promising optical and electronic properties.1
Moreover, their rigidity makes them attractive building blocks
for molecular composite materials.2 For example, poly-p-
phenylenes (PPPs) and oligo-p-phenylenes (OPPs) find versa-
tile applications as electronic conductors3 and light emitting
diodes (LEDs)4 as well as channel-forming molecules.5 Mono-
disperse oligomers are also important model compounds for
establishing structure–property relationships in the appropri-
ate long-chain conjugated polymers.6 Since unsubstituted
PPPs are hardly processible, side chains need to be attached
in order to process them from solution.7
The most dominant side reaction is the formation of
1,3,5-triarylbenzenes by reaction of 1 with the solvent. At-
tempts to increase the yield by longer reaction times, by
addition of 2 (0.5 equivalents) after 3 h and by conducting
the reaction under microwave irradiation were not successful.
Nevertheless, all compounds were easily isolated by column
chromatography and subsequent recrystallization. The com-
1
pounds were characterized by H NMR, 13C NMR and MS.
Generally, PPPs and OPPs are synthesized by metal-
catalyzed cross coupling reactions of the corresponding aryl
metal compound and an aryl halide.8 Despite considerable
progress in catalyst design, coupling of sterically demanding
reaction partners is still challenging.9 Moreover, the formation
of coupling products containing aryl halide groups requires
excess strategies or the intermediate introduction of protective
groups.
Among alternative methods for the preparation of aryl
substituted aromatics,10 condensation reactions of triarylpy-
rylium salts with sodium phenylacetate to 1,2,3,5-tetraaryl-
benzenes11 offer several advantages. Triarylpyrylium salts as
well as sodium arylacetates are either commercially available
or easily accessible with a wide variety of substituents. The
former can contain up to three different functionalized aryl
groups thus giving rise to products with complex substitution
patterns, even with halogen atoms. This makes their reaction
a
´
Kekule-Institut fur Organische Chemie und Biochemie der
Universitat Bonn, Gerhard-Domagk-Strasse 1, 53121 Bonn,
¨
¨
Germany. E-mail: hoeger@uni-bonn.de; Fax: (+49)228-73-5662;
Tel: (+49)228-73-3495
b MPI fur Polymerforschung, Ackermannweg 10, 55128 Mainz,
¨
Germany. E-mail: brunklaus@mpi-mainz.mpg.de;
Fax: (+49)6131-379-100; Tel: (+49)6131-379-240
c MPI fur medizinische Forschung, Jahnstr. 29, 69120 Heidelberg,
Germany
¨
w Dedicated to Professor Karl Heinz Dotz on the occasion of his 65th
¨
birthday.
z Electronic supplementary information (ESI) available: Full experi-
mental and analytical data of all new compounds. CCDC 686812. For
ESI and crystallographic data in CIF or other electronic format, see
DOI: 10.1039/b807382a
Scheme 1 Condensation of pyrylium salt with sodium 1,4-phenylene-
diacetate in acetic anhydride.
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Table 1 Synthesized oligo-p-phenylenes
Product
R1
R2
R3
Yield (%)
3a
3b
3c
3d
3e
3f
I
t-Bu
Br
Me
Me
t-Bu
t-Bu
4-Bromophenyl
I
13.4
9.8
10.0
2.4
5.1
6.1
6.9
OMe
I
t-Bu
Me
t-Bu
OMe
OMe
OMe
OMe
OMe
OMe
OMe
3g
In addition, 3b could be crystallized from dichloromethane
(Fig. 1).z
3b crystallizes in the monoclinic crystal system, where the
molecule is located on a center of symmetry. Owing to the
steric hindrance, the oligophenylene chain is not planar.
Rather, the substituted phenyl ring makes an angle of 57.11
and 40.11 with the central and terminal rings, respectively.
Notably, the ORTEP plot reveals fairly small thermal aniso-
tropies reflecting a stiff arrangement. From various discotic or
dendritic systems, however, it is well-known that different
segments in the molecule might exhibit large differences in
the mobility.15,16
Fig. 2 2H NMR spectrum of 3b-d4.
the 2H spectrum of 3b-d4 has a Pake pattern with a quadrupole
splitting of 128 kHz (Fig. 2). This also holds for elevated
temperatures (E 60 1C) indicating that the core is immobile on
the fast time scale (o10ꢁ4 s).
Molecular reorientations (e.g. phenyl flips) possibly occur-
2
ring within the sample are conveniently probed by H NMR
Although yields are at present limited, all products de-
scribed here are accessible in a synthetically useful scale since
pyrylium salts are easily available in large quantities. More-
over, functional group tolerance of this approach allows the
introduction of halogens. These compounds are not obtain-
able by transition metal chemistry, unless sophisticated pro-
tecting group chemistry is used. As described before,
unsymmetrical pyrylium salts (R1 a R3) in principle lead to
the formation of two atropisomers, if one or both substituents
(R1 and/or R3) are bulky enough to hinder free rotation of the
system along its main axis. These substances and their deri-
vatives are currently under investigation as well as optimizing
the reaction conditions to obtain higher product yields.
spectroscopy. Therefore, we selectively deuterated the central
phenyl ring of compound 3b (3b-d4) and performed tempera-
ture-dependent 2H NMR experiments. The 2H quadrupolar
splitting that is commonly observed in the case of rigid
aromatic C–D bonds is of the order of 128 kHz, while a
reduction in the splitting is observed in the event of more or
less defined rotational motions characterized by a dynamic
order parameter.17 In the case of 3b-d4, however, no motion of
the core phenyl ring could be detected, and, correspondingly,
Notes and references
y Preparation of 1b (R1 = R3 = OMe, R2 = Br): under an argon
atmosphere, BF3ꢂOEt2 (15 mL, 15 mmol) was added to a mixture of
1,5-bis(4-methoxyphenyl)-3-(4-bromophenyl)pentane-1,5-dione (5.0 g,
10.7 mmol) and chalcone (2.23 g, 10.7 mmol) without the use of
further solvent. The mixture was stirred at 100 1C for 2 h. After
cooling to room temperature, diethyl ether (30 mL) was added and the
red suspension was stirred for 12 h. The red precipitate was filtered off,
washed with cold diethyl ether and dried in vacuum to obtain 1b as a
red powder (4.4 g, 77%). dH (400 MHz, DMSO-d6) 8.80 (s, 2 H), 8.46
(d, J 9.01, 4 H), 8.41 (d, J 8.75, 2 H), 7.93 (d, J 8.69, 2 H), 7.25 (d, J
9.05, 4 H), 3.95 (s, 6 H); dC (400 MHz, DMSO-d6) 170.0, 165.8, 162.8,
133.6, 132.8, 132.3, 132.0, 130.0, 122.3, 116.4, 113.5, 57.0.
Preparation of 3b: a mixture of 1b (R1 = R3 = OMe, R2 = Br)
(15.1 g, 28.2 mmol), 2 (3.02 g, 12.8 mmol) and Ac2O (125 mL) was
stirred at 150 1C for 3 h. After cooling to room temperature, Ac2O was
removed in vacuum and the residue was digested in dichloromethane
and filtered over a short column of silica gel with dichloromethane as
eluent. The final purification was carried out by column chromato-
graphy using dichloromethane–petroleum ether (1 : 1) (Rf = 0.45 in
dichloromethane–petroleum ether 1 : 1) and subsequent precipitation
from dichloromethane by the addition of petroleum ether. 3b (660 mg,
9.8%) was obtained as a white solid. NMR: dH (500 MHz, CD2Cl2)
7.57 (s, 8 H), 7.53 (s, 4 H), 6.98 (d, J 8.81, 8H), 6.74 (d, J 8.81, 8H),
6.57 (s, 4 H), 3.78 (s, 12 H); dC (500 MHz, CD2Cl2) 158.4, 142.2, 139.5,
138.5, 137.9, 136.9, 134.3, 131.9, 131.1, 131.0, 128.7, 128.0, 121.6,
Fig. 1 ORTEP plot of the crystal structure of 3b, thermal ellipsoids
are presented at a 50% probability level.
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113.0, 55.2; UV (CH2Cl2): lmax = 260.5 nm; fluorescence (CH2Cl2):
l = 388 nm; MS (FAB): m/z = 964.3; mp: 299–302 1C
z Crystallographic data for 3b: C58H44Br2O4, M = 964.79, mono-
clinic, a = 9.3964(5), b = 9.7344(5), c = 23.5694(8) A, a = 90, b =
91.8388(12), g = 901, V = 2154.7(2) A3, T = 120 K, space group
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0.0439), R1 = 0.0439. The final wR(F2) was 0.0468 (all data). CCDC
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4818 | Chem. Commun., 2008, 4816–4818