593
Mes
concluded that the combination of all three components plays a
main role in forming the bisanthenes. Although the mechanistic
details of the reaction have not been clarified yet, strong
Brønsted acids may promote protonation of the anthracene ring
and electron transfer to DDQ, and arenium-type cations may be
generated as intermediate species.13,15,16 In comparison with the
previous synthetic methods of the meso-substituted bisanthenes
reported by Maulding,17a Wu,17b and Scott,17c it is noticeable that
our strategy can directly and cleanly afford bisanthene from the
corresponding bisanthryl in semi-gram scale.
a)
7
Mes
Mes
b) or c)
8
9
Mes
Mes
Mes
Our new method can also be applied to the synthesis
of larger homologs, teranthene 2a3a and quateranthene 3a3c
(Scheme 1). The MALDI-TOF mass spectra of the reaction
mixture supported the formation of 2a and 3a (Figures S1a and
S2a). Furthermore, the electronic absorption spectra of the
reaction product after purification were consistent with the
authentic spectra (Figures S1b and S2b).
The successful improvement of the synthesis of bisanthene
by using the improved Scholl reaction enables us to explore the
potential for building blocks toward polyperiacenes. The Diels-
Alder cycloaddition of bisanthene at the bay region has brought
about synthetic interest in building the skeleton toward large
³-conjugated systems such as ovalene,9c,18a cylindrical hydro-
carbons,17c,18b,18c and quinone-incorporated bisanthenes.18d In
order to extend the scope of the cycloaddition reactions of
bisanthene, we investigated the Diels-Alder cycloaddition of 1a
with several arynes, which can be generated in situ under mild
conditions as dienophiles.
Br
Br
d)
e)
f)
Mes
Mes
Mes
1a
10
11
Mes
Mes
Mes
Scheme 2. Diels-Alder reaction of 1a with several arynes.
Reagents and conditions: a) 3-amino-2-naphthoic acid, isoamyl
nitrite, toluene, reflux, 1 h, 39%; b) anthranilic acid, isopentyl
nitrite, toluene, reflux, 1 h, 75%; c) 2-(trimethylsilyl)phenyl
trifluoromethanesulfonate, CsF, THF, reflux, 1 day, 72%; d)
n-BuLi, 1,2,4,5-tetrabromobenzene, toluene, reflux, 1.5 h, 51%;
e) 1-bromonaphthalene, NaNH2, THF, reflux, 2 h, 49%; f)
9-bromophenanthrene, NaNH2, THF, reflux, 12 h, 40%.
Various arynes generated from several precursors19 are
subjected to the Diels-Alder cycloaddition of 1a followed by
dehydrogenation to afford ³-expanded bisanthenes in acceptable
yields (Scheme 2). Naphthalene-[b]-annulated 7 and benzene-
annulated 8 were obtained by gentle reflux of 1a in toluene with
naphthalene-2-diazonium-3-carboxylate and benzenediazonium-
2-carboxylate, respectively. 8 was also obtained by gentle reflux
of 1a in THF with 2-(trimethylsilyl)phenyl trifluoromethanesul-
fonate and cesium fluoride. Dibromobenzene-annulated 9 was
obtained by treating 1a and 1,2,4,5-tetrabromobenzene in
refluxed toluene with n-BuLi. Naphthalene-[a]-annulated 10
and phenanthrene-annulated 11 were synthesized by treatment of
1a and 1-bromonaphthalene or 9-bromophenathrene in refluxed
THF with excess NaNH2, respectively.
All reactions proceeded smoothly under mild conditions.
It should be notable that the mono-addition product can be
dominantly obtained even in the presence of the excess aryne
precursor, in contrast to previous reports affording a mixture of
mono- and double-addition products.18 It seems that the milder
reaction conditions, compared with the previous reactions
performed under refluxed nitrobenzene at 240 °C9c,18a,18d or in
a pressure vessel,17c,18b,18c would selectively lead to the mono-
addition products. From these results, it is proved that arynes
are useful dienophiles in the Diels-Alder cycloaddition with
bisanthene to precisely expand ³-systems in the lateral direction
under a mild condition.
Table 2. Optical and electrochemicala properties of 1a and
7-11
ox
ox
red
red
-
-
em
E
E
/V
E
E ¦
2,1/2
redoxE
/V
max
2,1/2
1,1/2
1,1/2
Compound
/nm /nm /V
/V
/V
1a
7
8
9
10
11
685 705 0.65
661 672 0.65
613 623 0.75
617 627 0.78
602 611 0.78
596 602 0.79
0.02 ¹1.66 ¹2.19b 1.68
0.08 ¹1.68 ¹2.07 1.76
0.14 ¹1.76 ¹2.29b 1.90
0.21 ¹1.70 ¹2.07b 1.90
0.17 ¹1.77 ¹2.30b 1.94
0.18 ¹1.80 ¹2.34b 1.98
aV vs. Fc/Fc+, in 0.1 M n-Bu4NClO4/CH2Cl2, scan rate
100 mV s¹1, +25 °C. Peak potentials.
b
shapes with vibrational fine structures, which can be assigned to
the HOMO-LUMO transition. Consistent with the reported
longest absorption bands of the laterally ³-expanded bisan-
thenes,18d 7-11, despite of larger size, also exhibited blue shifts
in comparison with 1a. The ³-annulation at the bay region
causes the increase in the HOMO-LUMO gap of 1a, which is
supported by the DFT calculation at the B3LYP/6-31G**
level.20 The DFT calculation suggests that the ³-annulation at
the bay region of bisanthene stabilizes the HOMO level and
destabilizes the LUMO level. The electronic perturbation given
by ³-annulations increases in order from naphthalene-[b]- to
benzene-, naphthalene-[a]-, phenanthrene-, ethylene-annulation
(Figure S5).12
The optical properties of compounds 1a and 7-11 recorded
in CH2Cl2 are summarized in Table 2. The electronic absorption
spectra of these compounds show two characteristic absorption
bands; the longest absorption band (p band: 550-700 nm);
the second longest absorption band (¢ band: 300-420 nm)
(Figures 1 and S812). The p bands of 1a and 7-11 have similar
Chem. Lett. 2013, 42, 592-594
© 2013 The Chemical Society of Japan
Konishi, Akihito
