1460
Wavelength / 102 nm
a large lower field shift for the olefinic proton of 1b was
observed at 8.44 ppm in the H NMR spectrum. This unusual
1
5.0 4.5 4.0
12
3.5
3.0
2.5
behavior may be understood in terms of the deshielding effects
of the two coplanar pyrenes in the trans-conformation.14 The
absorption spectrum of 1b is shown in Figure 1 along with that
of dialkylpyrene 2¤. A strong broad absorption was observed
around 450-320 nm, Band-I, along with Band-II (310-280 nm)
and Band-III (270-240 nm). Vibrational structures characteristic
of pyrene were not observed in 1b. This behavior could be
explained in terms of the free rotation around the pyrene-ethene
single bonds in its ground state.15 As is clear in the figure, all
bands of 1b were shifted to lower energies suggesting a narrow
intramolecular HOMO-LUMO gap. Density functional theory
(DFT) and time-dependent DFT calculations revealed that 1) the
energy levels of HOMO (º112) and LUMO (º113) were ¹4.94
and ¹1.99 eV, respectively, which were much higher and lower
than those of pyrene; 2) Band-I was assigned to an allowed
HOMO (º112) to LUMO (º113) transition, that is, S1 ← S0, with
an oscillator strength of 1.33 in vacuum (SI#3). The lower
oxidation potential of 1b, estimated from electrochemical
studies to be 0.44 and 0.83 V vs. Fc/Fc+ in PhCN for 1b and
2c, respectively, was consistent with the results of the theoretical
studies.
1b
2'
10
Band-III
8
6
4
2
0
Band-I
Band-II
2.0
2.5
3.0
3.5
4.0
4.5
Wavenumber / 104 cm-1
Figure 1. Absorption spectra of dipyrenylethene 1b (solid
line) and 2¤ (dotted line) in hexane.
the reaction required that bromine be added as slowly as possible
at low temperature. Conducting the reaction at temperatures
above ¹40 °C and/or quickly adding bromine induced the
formation of significant amounts of 1,3-dibromide. Replacement
of bromine with NBS addressed this problem. Treatment of one
equivalent of NBS at room temperature selectively afforded
desired monobromopyrene 3 selectively. Introduction of a vinyl
group to the pyrene was achieved by the Suzuki-Miyaura
coupling in the presence of excess amounts of vinylboronic acid
pinacol ester I. Heavy viscous oil 4 was obtained in an
acceptable yield of 77% as unstable material toward heat and
acid as usually observed for vinyl compounds such as styrene.
The metathesis reaction was carried out using the Grubbs 2nd
generation catalyst II. The reaction proceeded smoothly to give
desired olefin 1b in 57% yield as yellow needles (corrected yield
of 1b considering the recovered yield of 4: 96%). It is worth
noting that 1b was stable toward typical operations, including
column chromatography and recrystallization from boiling
solvent, such as heptane (bp 98 °C), under ambient conditions.
This stability sharply contrasted with the instability of 1a, the
parent molecule, which required special care and the avoidance
of light and heat.8-11 Presumably, the introduction of two bulky
tert-alkyls kinetically stabilized the solid and concentrated
solution states. Reflecting the bulkiness of the substituents, the
1H NMR and absorption spectra of 1b did not depend on the
concentration or solvent, suggesting that the tert-alkyl groups
prevented aggregation due to intermolecular interactions among
the large ³-systems of 1b.
1,2-Di(pyren-1-yl)ethylene 1b was synthesized and charac-
terized with the goal of creating new CHCs. Along with the
successful olefin metathesis, the authors serendipitously found a
cascade reaction involving Suzuki-Miyaura and Mizoroki-Heck
couplings. The yield was not high (38%); however, it was
acceptable. Because this one-pot reaction avoids multistep
syntheses and the handling of unstable vinyl intermediates, the
reaction potentially provides a simple and convenient approach
to the synthesis of DArEs from easily available bromoaryls. The
excited state chemistry of 1b and a systematic synthetic study of
DArEs using the cascade reaction will be described elsewhere.
This work was supported in part by Grants-in-Aid for
Scientific Research from the Ministry of Education, Culture,
Sports, Science and Technology. We appreciate the helpful
discussion with Professor Masahiko Hada. We also appreciate
the technical assistance for elemental analysis provided by Mr.
Toshihiko Sakurai, Tokyo Metropolitan University (TMU), and
EI mass spectrometry provided by Dr. Kazunori Hirabayashi
(TMU).
This paper is in celebration of the 2010 Nobel Prize
awarded to Professors Richard F. Heck, Akira Suzuki, and
Ei-ichi Negishi.
References and Notes
1
J. C. Fetzer, Large (C>=24) Polycyclic Aromatic Hydro-
carbons: Chemistry and Analysis, Wiley Interscience, New
York, 2000, Vol. 158.
A careful workup of the synthesis of 4 revealed the
formation of trace amounts of 1b. Presumably, the initially
formed Suzuki-Miyaura coupling product 4 then reacted with
the organopalladium intermediate generated from remaining 3
via the Mizoroki-Heck reaction.13 Therefore, treatment with 0.5
equiv of boronic acid ester I produced 4 in 50% theoretical
yield. Then, 4 reacted with unreacted 3 to afford 1b in one-pot.
As expected, 1b was obtained as the main product (38%).
All spectroscopic studies supported the formation of 1b (see
Supporting Information #1-#3, SI16 It should be emphasized that
2
3
M. D. Watson, A. Fechtenkötter, K. Müllen, Chem. Rev.
4
5
6
G. V. Ponomarev, V. V. Borovkov, K.-i. Sugiura, Y. Sakata,
Chem. Lett. 2011, 40, 1459-1461
© 2011 The Chemical Society of Japan
Yamashita, Ken-Ichi
