Fused bispentacenequinone and its unexpected Michael addition

Article abstract of DOI:10.1021/ol101720e

Fused bispentacenequinone 2 was synthesized by photocyclization of bispentacenequinone 1. Unusual regioselective Michael addition was observed for 2 when excess aryl Grignard reagent was used. Subsequent acidification and oxidation in air gave diaryl-subs

Full text of DOI:10.1021/ol101720e

ORGANIC
LETTERS
2010
Vol. 12, No. 17
3946-3949
Fused Bispentacenequinone and Its
Unexpected Michael Addition
Xiaojie Zhang, Jinling Li, Hemi Qu, Chunyan Chi, and Jishan Wu*
Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3,
Singapore 117543
chmwuj@nus.edu.sg
Received July 23, 2010
ABSTRACT
Fused bispentacenequinone 2 was synthesized by photocyclization of bispentacenequinone 1. Unusual regioselective Michael addition was
observed for 2 when excess aryl Grignard reagent was used. Subsequent acidification and oxidation in air gave diaryl-substituted
bispentacenequinone 3. Tetra-aryl-substituted fused bispentacenequinone 4 was obtained from 3 after the second Michael addition followed
by oxidation in air.
Polycyclic aromatic hydrocarbons represent a vast library
of molecular geometries with properties exploitable for
organic opto-electronic materials.1 Acenes, linearly fused
aromatic hydrocarbons, have been the subject of extensive
studies owing to their applications for electronic devices such
as organic field-effect transistors (OFETs)2 and organic light-
emitting diodes (OLEDs).3 Our group recently studied the
peri-fused acenes, which usually exhibit low band gap and
can be used as building blocks for near-infrared (NIR) dyes.4
Syntheses of higher order acenes (ring number n g 5) and
peri-fused acenes are challenging because without appropri-
ate substituents they are usually unstable under ambient
conditions and insoluble in normal organic solvents. In recent
years, many research works have emphasized the chemical
modification of acenes to increase solubility and stability and
to improve solid-state ordering.5 Synthesis of soluble and
stable peri-fused acenes is of interest to challenge synthetic
chemistry and to exploit new materials with NIR absorption/
emission.
One of the most prominent methods for the synthesis of
soluble and stable acenes was the nucleophilic addition of
organometallic reagent to the corresponding acenequinones
(Figure 1) followed by reduction of the as-formed diol to
afford the desired substituted acenes. Anthony and co-
workers first synthesized the silylethynyl derivatives of
pentacene from pentacenequinone by this method.6 Later,
they also reported the synthesis of functionalized silylethynyl
acenes from dithienoacenequinone, hexacenequinone, and
(1) (a) Clar, E. Polycyclic Hydrocarbons; Academic Press: London, New
York, 1964; Vols. 1 and 2. (b) Ronald, G. H. Polycyclic Aromatic
Hydrocarbons; Wiley-VCH: New York, 1997.
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M.; J. van Wees, B.; Palstra, Thomas, T. M. AdV. Mater. 2007, 19, 688.
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Neckers, D. C. J. Org. Chem. 2007, 72, 6584. (c) Lehnherr, D.; Mcdonald,
R.; Tykwinski, R. R. Org. Lett. 2008, 10, 4163. (d) Li, Y.; Wu, Y.; Liu, P.;
Prostran, Z.; Gardner, S.; Ong, B. Chem. Mater. 2007, 19, 418. (e) Kaur,
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(6) Anthony, J. E.; Brooks, J. S.; Eaton, D. L.; Parkin, S. R. J. Am.
Chem. Soc. 2001, 123, 9482.
10.1021/ol101720e  2010 American Chemical Society
Published on Web 08/12/2010

previous work.9 Oxidative photocyclization was conducted
in benzene in the presence of I2 and propylene oxide.
Interestingly, only the fused bispentacenequinone 2 was
formed with formation of two new C-C bonds. The product
2 precipitated from the benzene solution, and subsequent
purification by filtration and column chromatography gave
the pure compound 2 as a yellow powder in 59% yield.
Extension of the reaction time does not provide further
cyclized product such as the fully fused peripentacene-
quinone. Compound 2 can be regarded as a tetrabenzobisan-
thenequinone, and it can possibly be used as a building block
to prepare tetrabenzobisanthene derivatives with expected
long wavelength absorption. Thus, the nucleophilic reaction
of compound 2 with excess Grignard reagent of 1-bromo-
3,5-di-tert-butylbenzene in anhydrous THF was tested.
Surprisingly, a diaryl-substituted fused bispentacenequinone
3 was obtained in 50% yield after standard acidification
workup and column chromatography in air, which means
the aryl Grignard reagent attacked onto the benzene rings
instead of the carbonyl groups in 2! The strucutre of 3 was
unambiguously confirmed by NMR spectroscopy, mass
spectrometry (see Supporting Information), and single-crystal
analysis (vide infra). Such an unusual addition reaction is
similar to the Michael 1,4-addition of R,ꢀ-unsaturated
ketone.12 Considering that there are similar R,ꢀ-unsaturated
CdC bonds in the four benzene rings near the -CdO units
in 2, 1,4-conjugation addition could take place to give the
intermediate 3a after treatment with aqueous acid. Compound
3a is supposed to be very unstable, and it can quickly
undergo oxidative dehydrogenation (aromatization) in air.
As a result, the disubstituted fused bispentacenequinone 3
is obtained. It is also worth noting that although a large
excess of aryl Grignard reagent was used, there were no
derivatives with more than two aryl substituents detected.
This is reasonable since after addition of two aryl anions
the intermediate consists of two negative charges, and
addition of third negative charge will be very difficult due
to Coulombic repulsion. It is also understandable that the
two aryl units in 3 selectively adopt a trans alignment because
the Coulombic repulsion is minimized in this configuration.
In addition, compound 3 has the same framework as that of
2. Thus, further Michael addition is possible if this mecha-
nism is true. In fact, treatment of 3 with excess aryl Grignard
reagent followed by acidification in air gave the tetraaryl-
substituted fused bispentacenequinone 4 in 36% yield
presumably via intermediate 4a. The FT-IR spectra of 2-4
(see the Supporting Information) confirmed the existence of
the carbonyl group as the typical intense CdO stretching
vibration band was observed at 1660 cm-1 for 2, 1667 cm-1
for 3, and 1672 cm-1 for 4.
Figure 1. Structures of acenequinones and fused acenequinones.
heptacenequinone.7 Soluble and stable acenes (n ) 7, 9) were
successfully synthesized by Miller and co-workers from their
corresponding acenequinones.8 The peri-fused acenes in
principle can also be prepared from the corresponding
quinones. In fact, we recently reported the synthesis of
bisanthenequinone and bispentacenequinone (1) (Figure 1),
which were successfully used for the preparation of soluble
and stable bisanthene-based NIR dyes4,9 and cruciform 6,6′-
dipentacenyl.10
In parallel to these works, we have been working on the
synthesis of soluble and stable higher order peri-fused acenes
by using the corresponding peri-fused acenequinone, which
could be prepared by photocyclization of the singly linked
quinone.11 In our previous work, we have prepared bisan-
thenequinone from the bisanthracenequinone using a pho-
tooxidative cyclization reaction.10 Thus, we also expected
the occurrence of such photooxidative ring-closing reactions
in our bispentacenequinone 1 to give the fused aromatic
systems such as 2 and/or peripentacenequinone (Figure 1),
which can probably be used for the synthesis of fused
bispentacene derivatives. In this work, we report the pho-
tocyclization reaction of 1 and an unusual Michael addition
reaction of the obtained fused bispentacenequinone 2 during
our attempts to prepare fused bispentacenes.
As shown in Scheme 1, we first tested the photocyclization
of the bispentacenequinone 1, which was reported in our
(7) (a) Payne, M. M.; Odom, S. A.; Parkin, S. R.; Anthony, J. E. Org.
Lett. 2004, 6, 3325. (b) Payne, M. M.; Parkin, S. R.; Anthony, J. E. J. Am.
Chem. Soc. 2005, 127, 8028.
Considering that the 1,2-addition of the carbonyl group
usually takes place when aryllithium and Grignard reagent
are used, the observed 1,4-conjugation addition is very
unusual. Compared with the singly linked bispentacene-
quinone 1, the carbonyl groups in the fused bispentacen-
(8) (a) Kaur, I.; Stein, N. N.; Kopreski, R. P.; Miller, G. P. J. Am. Chem.
Soc. 2009, 131, 3424. (b) Kaur, I.; Jazdzyk, M.; Stein, N. N.; Prusevich,
P.; Miller, G. P. J. Am. Chem. Soc. 2010, 132, 1261.
(9) Zhang, X.; Jiang, X.; Luo, J.; Chi, C.; Chen, H.; Wu, J. Chem.sEur.
J. 2010, 16, 464
.
(10) Zhang, K.; Huang, K.; Li, J.; Luo, J.; Chi, C.; Wu, J. Org. Lett.
2009, 11, 4854.
(12) (a) Tokoroyama, T. Eur. J. Org. Chem. 2010, 2009. (b) Ikawa,
M.; Stahmann, M. A.; Link, K. P. J. Am. Chem. Soc. 1944, 66, 902. (c)
Lippert, A. R.; Kaeobamrung, J.; Bode, J. W. J. Am. Chem. Soc. 2006,
128, 14738. (d) Allen, C. F. H.; Bell, A. J. Am. Chem. Soc. 1942, 64, 1253.
(11) (a) Yang, C.; Harvey, R. G. J. Org. Chem. 1993, 58, 4155. (b)
Mallory, F. B.; Butler, K. E.; Evans, A. C. Tetrahedron Lett. 1996, 37,
7173. (c) Broene, R. D.; Diederich, F. Tetrahedron Lett. 1991, 32, 5227.
Org. Lett., Vol. 12, No. 17, 2010
3947

Scheme 1. Synthetic Route toward Compounds 2-4
quinone 2 show lower reactivity presumably due to extended
conjugation of the -CdO units with the fused π-system and
activation of the R,ꢀ-unsaturated CdC bond induced by
structural distortion.
To further understand the possible reasons for this unusual
reaction, single crystals of 3 suitable for X-ray diffraction
analysis were successfully grown by slow diffusion of
methanol into a solution of 3 in chloroform, and the single-
crystal structure is shown in Figure 2.13 The two parallel
concentrated in pairs of carbon atoms that act as pivot points
(highlighted with gray circles in Figure 2a). A comparison
was made between some of the bond lengths in this fused
bispentacenequinone 3 and those from the anthracenequinone
crystal structure.14 One of the most interesting features is
that the Cb-Cc and Cd-Ce bonds (highlighted with red color
in Figure 2a) are shorter in 3 (1.36 Å) relative to the
anthracenequinone (1.40 Å). A major contribution from the
radialene resonance would explain these unusual bond
lengths. The bond lengths for all of these unusual bonds are
quite close to an individual CdC bond, and thus, R,ꢀ-
unsaturated carbonyl structures (highlighted with red color
in Figure 2b) are actually constructed. Thus, the low
reactivity of the -CdO and the existence of R,ꢀ-unsaturated
ketones would explain the unexpected Michael addition
reactions of 2 and 3. In addition, we also found that no further
reaction occurred between 4 and 1-hexynelithium reagent
presumably due to steric hindrance around the carbonyl
center.
Compounds 2-4 are soluble in chlorinated solvents such
as chloroform and chlorobenzene, and their UV-vis absorp-
tion spectra in chloroform are shown in Figure 3a. Solutions
of 2-4 in chloroform display a light yellow color, and well-
resolved absorption bands between 400 and 510 nm with
the peak at 493 nm (log ε ) 4.43, 4.34 and 4.39, respectively;
ε, molar extinction coefficient in M-1 cm-1) were observed.
Additional bands in the UV range were also found. Com-
pounds 2-4 show similar absorption bands, which indicates
that there is only weak electronic coupling between the
bispentacenequinone framework and the 3,5-di-tert-butylphe-
nyl units. A similar optical band gap around 2.44 eV was
deduced for compounds 2-4 from the low energy absorption
onset. The UV-vis-NIR absorption spectra of 2 and 3
recorded in concentrated sulfuric acid are shown in Figure
3b. Solutions of 2 and 3 in protonated form displayed a deep
green color and well-resolved absorption bands between 500
Figure 2. Single-crystal structure of 3. Hydrogen atoms are removed
for clarity. (a) Face-on view with marked pivot points (gray circles).
(b) Top view along the c axis. (c) Molecular structure of 3 with
labeled R,ꢀ-unsaturated ketone subunits.
pentacene subunits contort into a intersecting conformation
due to steric congestion. The bending of the core is visualized
most clearly in Figure 2c. Essentially all of the bending is
(13) Single-crystal data for 3: C72H60O2; triclinic; space group P-1; a )
10.293(2) Å, b ) 15.245(4) Å, c ) 21.665(5) Å; R ) 87.020(6)°, ꢀ )
86.029(6)°, γ ) 79.479(6)°; V ) 3331.6(13) Å3; Z ) 2; F ) 1.311 Mg/m3;
R1 ) 0.0985, wR2 ) 0.2767.
(14) Wolstenholme, D. J.; Cameron, T. S. J. Phy. Chem. A 2006, 110,
8970.
3948
Org. Lett., Vol. 12, No. 17, 2010

Figure 4. Cyclic voltammograms of 2-4 in hot chlorobenzene (70
°C) under nitrogen atmosphere with 0.1 M Bu4NPF6 as supporting
electrolyte, AgCl/Ag as reference electrode, Au disk as working
electrode, Pt wire as counter electrode, and scan rate at 50 mV/s.
levels of -3.18, -3.04, and -2.93 eV were estimated for
2, 3, and 4, respectively, based on the onset potential of the
first reduction waves.15 This also indicates that the electron-
donating aryl substitution has an obvious effect on the
electrochemical property of the fused bispentacenequinone.
In conclusion, a fused bispentacenequinone 2 was prepared
in good yield by a photocyclization reaction. An unusual
Michael addition reaction of 2 was observed, and diaryl- and
tetraaryl-substituted fused bispentacenequinones 3 and 4 were
obtained. Crystallographic analysis disclosed that there are
R,ꢀ-unsaturated ketone structures in the fused bispentacene-
quinone, which may explain the unusual additions.
Figure 3. (a) UV-vis absorption spectra of 2-4 in chloroform
(10-5 M). (b) Normalized absorption spectra of 2,3, and bisan-
thenequinone in concentrated H2SO4.
and 850 nm with the maximum peak at 698 and 709 nm,
respectively. Compared with the absorption spectrum of
bisanthenequinone in concentrated sulfuric acid, there is a
red-shift of about 126 and 137 nm for 2 and 3, respectively.
The red-shift indicates that π-conjugation of the bisanthene-
quinone core is further extended by incorporation of ad-
ditional four benzo rings and aryl moieties.
The electrochemical properties of compounds 2-4 were
studied by cyclic voltammetry in chlorobenzene at 70 °C.
As shown in Figure 4, for compound 2, two reversible
reduction waves with half-wave potentials at -1.83 and
-1.64 V (vs Fc+/Fc) were observed, indicating that the
bispentacenequinone 2 unit can be reversibly reduced into
the radical anion and dianion. For compounds 3 and 4, only
one irreversible reduction wave was observed. LUMO energy
Acknowledgment. This work was financially supported
by Singapore NRF Competitive Research Program (R-143-
000-360-281) and NUS Young Investigator Award (R-143-
000-356-101).
Supporting Information Available: Experimental pro-
cedures, characterization data of all new compounds, nor-
malized UV-vis absorption spectra, cyclic voltammetry data,
FT-IR spectra, and X-ray crystallographic analysis data. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL101720E
(15) Chi, C.; Wegner, G. Macromol. Rapid Commun. 2005, 26, 1532.
Org. Lett., Vol. 12, No. 17, 2010
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