Published on the web September 18, 2013
1499
Nonplanar and Dynamic Structures of 1,8-Anthrylene-Ethenylene Cyclic Dimers
Masataka Inoue, Tetsuo Iwanaga, and Shinji Toyota*
Department of Chemistry, Faculty of Science, Okayama University of Science,
1-1 Ridaicho, Kita-ku, Okayama 700-0005
(Received August 19, 2013; CL-130765; E-mail: stoyo@chem.ous.ac.jp)
iPr
R
The title cyclic compounds, synthesized by cross-coupling
reactions, have nonplanar frameworks, as revealed by X-ray
analysis and DFT calculations. The barrier to exchange between
the nonplanar structures of the mesityl derivative was deter-
¹1
mined to be 34 kJ mol by a dynamic NMR method.
a: R = H
iPr
In the molecular design of π-conjugated oligomers, several
arene units are alternately connected to unsaturated linkers
to extend the π system. These compounds are attractive for
applications in material chemistry because of their interesting
electrical and optical properties.1 As examples of such com-
pounds, we have studied anthracene-acetylene cyclic oligomers
by adopting 1,8-anthrylene units and ethynylene (or butadiynyl-
ene) linkers.2-4 Their structures, dynamic behavior, and spectro-
scopic properties are influenced by the number of anthracene
units, as revealed by the experimental and theoretical results for
dimers to dodecamers. Dimers 15 and 2,6 the smallest analogs,
have planar rigid frameworks and show characteristic electronic
properties (Figure 1). This structural feature has been applied to
the generation of new stereoisomers by introducing intraannular
substituents.7 1,2-Ethenylene (or vinylene, hereafter ethenylene)
linkers are also commonly used in the π-conjugated system,1
and anthrylene-ethenylene oligomers and polymers have been
studied by many researchers for potential use of functional
materials.8 In contrast to linear ethynylene linkers, ethenylene
linkers have a nonlinear zigzag shape, and the conformation
about single bonds considerably influences the chain shape and
the extent of conjugation. As regards anthracene cyclic dimers,
ethenylene derivative 3a9 was reported as a [14]annulene analog
by Akiyama and Nakagawa in 1971.10 Although the authors
focused on the nonplanar structure, no further details were
disclosed because of the low solubility of the compound. Hence,
we synthesized soluble substituted derivatives 3b with a phenyl
group and 3c with a mesityl group to solve the structural
problem. We expected that the mesityl group in 3c would enable
conformational analysis, as in the case of anthracene-acetylene
macrocyclic oligomers.11
In the original procedure, compound 3a was synthesized by
the Wittig reaction.10 We adopted cross-coupling reactions to
connect the aromatic and ethenylene moieties according to the
protocol reported by Itami et al. (Scheme 1).12 Ethenyl groups
were introduced by the Mizoroki-Heck reaction of 1,8-di-
iodoanthracene (4a)13 and ethenylboronate 5 to give 6 in 86%
yield. Cyclization of 6 and 4a by the Suzuki-Miyaura coupling
under high-dilution conditions (ca. 1 © 10¹4 mol L¹1) afforded
3a in 32% yield. Compound 6 was similarly cyclized with
substituted 1,8-diiodoanthracenes 4b and 4c, and cyclic products
3b and 3c were obtained in 30% and 33% yields, respectively.
In each cyclization reaction, the mass spectrum of the crude
product showed peaks due to the corresponding cyclic tetramer
b: R = Ph
c: R = Mes
1
2
3
Figure 1. 1,8-Anthrylene cyclic dimers with various linkers.
I
I
O
O
B
O
O
O
O
B
B
I
I
R
4
5
3
[Pd(PPh3)4]
Cs2CO3
toluene, H2O
[Pd(Pt-Bu3)2]
i-Pr2NH
toluene
a: R = H
b: R = Ph
c: R = Mes
4a
6
Scheme 1. Synthesis of cyclic dimers 3.
and other oligomers. Compounds 3b and 3c were much more
soluble in organic solvents than was 3a. Even though the yields
were not high, this synthetic route offered rapid access to the
cyclic compounds. The compounds were obtained as yellow
crystals and reasonably characterized by NMR spectroscopy and
mass spectrometry.14 In the 1H NMR spectra, alkene proton
signals were observed at ¤ ca. 7.8 as a singlet for 3a and 3c15
and as an AB quartet with J = 16.8 Hz for 3b. This large
coupling constant supports the trans junction at the ethenylene
moieties. Hereafter, we mainly describe the results of mesityl
derivative 3c.
The electronic spectra of 3c were measured in chloroform
(Figure 2). A broad absorption band was observed at 417 nm
in the p-band region of the UV-vis spectrum. The wavelength
was red-shifted by 19 nm compared with that of 1,8-diethenyl-
anthracene (398 nm).10,16 The TDDFT calculations of 3a
supported that this absorption was assignable to the transition
from the HOMO to the LUMO, and orbitals were delocalized
over the molecules at those levels (see Supporting Information).
The fluorescence spectrum gave an intense and broad emission
band at 491 nm (Φf = 0.71). The large Stokes shift (70 nm) was
attributable to the large structural change after excitation.
The X-ray structure of 3c is shown in Figure 3. The
macrocyclic framework is nonplanar, and the two double bond
moieties are parallel and twisted by ca. 40° from the coplanar
conformation to the same side of each anthracene unit (syn
form). Accordingly, the two anthracene planes are also nearly
parallel in a stair-like orientation. The mesityl-phenyl group
is almost perpendicular to the attaching anthracene unit. The
Chem. Lett. 2013, 42, 1499-1501
© 2013 The Chemical Society of Japan
Inoue, Masataka
