Dihalo-substituted dibenzopentalenes: Their practical synthesis and transformation to dibenzopentalene derivatives

Article abstract of DOI:10.1021/ol3017353

Diiodo- and bromo, iodo-substituted dibenzopentalenes were obtained by treatment of 5,6,11,12-tetradehydrodibenzo[a,e]cyclooctene with I₂ and IBr, respectively. These dihalo-substituted pentalenes reacted with terminal ethynes in Sonogashira co

Full text of DOI:10.1021/ol3017353

ORGANIC
LETTERS
2012
Vol. 14, No. 15
3970–3973
Dihalo-Substituted Dibenzopentalenes:
Their Practical Synthesis and
Transformation to Dibenzopentalene
Derivatives
Feng Xu, Lifen Peng, Akihiro Orita,* and Junzo Otera*
Department of Applied Chemistry, Okayama University of Science, Kita-ku,
Okayama 700-0005, Japan
orita@high.ous.ac.jp; otera@high.ous.ac.jp
Received June 25, 2012
ABSTRACT
Diiodo- and bromo, iodo-substituted dibenzopentalenes were obtained by treatment of 5,6,11,12-tetradehydrodibenzo[a,e]cyclooctene with I2 and
IBr, respectively. These dihalo-substituted pentalenes reacted with terminal ethynes in Sonogashira coupling and with arylboronic acid in
SuzukiꢀMiyaura coupling to give a series of phenylethynyl- and/or aryl-substituted pentalenes. SuzukiꢀMiyaura coupling of the halopentalenes
with in situ prepared pentaleneboronic esters provided bis-, tri-, and tetra(dibenzopentalene)s. It was found that these dibenzopentalene
oligomers underwent facile electrochemical reduction and exhibited a bathochromic shift in UVꢀvis absorption spectra because of their
expanded π-systems.
Great attention has been paid to polycyclic hydrocar-
bons which possess expanded π-conjugation systems such
as pentacene and its derivatives1 because they are promis-
ing as field-effect transistor (FET) and electrolumines-
cence (EL) materials.2 More recently, besides these
aromatic compounds, considerable notice has been taken
of antiaromatic π-expanded pentalenes for their potential
applications to the organic materials.3,4 Saito4a,b and
Yamaguchi4c succeeded in synthesis of dibenzopentalenes
by reduction of silylethyne with lithium and ketoary-
lethyne with lithium naphthalenide (LiNaph), respectively.
(4) (a) Saito, M.; Hashimoto, Y.; Tajima, T.; Ishimura, K.; Nagase,
S.; Minoura, M. Chem.;Asian J. 2012, 7, 480. (b) Saito, M.; Nakamura,
M.; Tajima, T. Chem.;Eur. J. 2008, 14, 6062. (c) Zhang, H.; Karasawa,
T.; Yamada, H.; Wakamiya, A.; Yamaguchi, S. Org. Lett. 2009, 11,
3076. (d) Zerubba, U. L.; Tilley, T. D. J. Am. Chem. Soc. 2010, 132,
11012. (e) Zerubba, U. L.; Tilley, T. D. J. Am. Chem. Soc. 2009, 131,
2796. (f) Kawase, T.; Fujiwara, T.; Kitamura, C.; Konishi, A.; Hirao, Y.;
Matsumoto, K.; Kurata, H.; Kubo, T.; Shinamura, S.; Mori, H.;
Miyazaki, E.; Takimiya, K. Angew. Chem., Int. Ed. 2010, 49, 7728.
(g) Kawase, T.; Konishi, A.; Hirao, Y.; Matsumoto, K.; Kurata, H.;
Kubo, T. Chem.;Eur. J. 2009, 15, 2653. (h) Babu, G.; Orita, A.; Otera,
J. Chem. Lett. 2008, 37, 1296. (i) Willner, I.; Becker, J. Y.; Rabinovitz,
M. J. Am. Chem. Soc. 1979, 101, 395. (j) Hashmi, A. S. K.; Wieteck, M.;
(1) For reviews of pentacene, see:(a) Anthony, J. E. Angew. Chem.,
Int. Ed. 2008, 47, 452. (b) Anthony, J. E. Chem. Rev. 2006, 106, 5028.
(2) (a) Functional Organic Materials; M€uller, T. J. J., Bunz, U. H. F.,
Eds.; Wiley-VCH: Weinheim, 2007. (b) Organic Light Emitting Devices:
Synthesis Properties and Applications; Mullen, K., Scherf, U., Eds.;
Wiley-VCH: Weinheim, 2006. (c) Carbon-Rich Compounds; Haley,
M. M., Tykwinski, R. R., Eds.; Wiley-VCH: Weinheim, 2006.
(3) For a recent review of dibenzopentalenes: (a) Saito, M. Symmetry
2010, 2, 950. For recent examples of 20π antiaromatic systems:
(b) Chase, D. T.; Fix, A. G.; Rose, B. D.; Weber, C. D.; Nobusue, S.;
Stockwell, C. E.; Zakharov, L. N.; Lonergan, M. C.; Haley, M. M.
Angew. Chem., Int. Ed. 2011, 50, 11103. (c) Chase, D. T.; Rose, B. D.;
McClintock, S. P.; Zakharov, L. N.; Haley, M. M. Angew. Chem., Int.
Ed. 2011, 50, 1127. (d) Shimizu, A.; Tobe, Y. Angew. Chem., Int. Ed.
2011, 50, 6906. For an amino-substituted pentalene: (e) Yin, X.; Li, Y.;
Zhu, Y.; Kan, Y.; Li, Y.; Zhu, D. Org. Lett. 2011, 13, 1520.
€
Braun, I.; Nosel, P.; Jongbloed, L.; Rudolph, M.; Rominger, F. Adv.
Synth. Catal. 2012, 354, 555. (k) Katsumoto, K.; Kitamura, C.; Kawase,
T. Eur. J. Org. Chem. 2011, 4885. (l) Yang, J.; Lakshmikantham, M. V.;
Cava, M. P.; Lorcy, D.; Bethelot, J. R. J. Org. Chem. 2000, 65, 6739.
r
10.1021/ol3017353
Published on Web 07/16/2012
2012 American Chemical Society

Tilley4d,e and Kawase4f,g reported new methodologies for
synthesis of pentalenes taking advantage of transition-
metal-catalyzed dimerization of haloarylethynes, and
Kawase evaluated their charge mobilities in OFET
devices.4f We already established the synthesis of dibenzo-
pentalenes by use of a highly strained acetylene 5,6,11,12-
tetradehydrodibenzo[a,e]cyclooctene (1)5 which could be
obtained in a practical manner: nucleophilic addition of
alkyllithium to 1 followed by transannulation and nucleo-
philic addition of the resulting pentalenyl anion to electro-
philes provided the desired dibenzopentalenes.4h
bond formation such as in Sonogashira and Suzukiꢀ
Miyaura coupling, a new, practical avenue for a variety
of π-expanded dibenzopentalenes 3 and dibenzopentalene
oligomers 4 would be developed. We report these synthetic
results and several physical properties of the newly pre-
pared dibenzopentalenes such as UVꢀvis absorption spec-
tra and cyclic voltammetry (CV).
When 1 was treated with I2 (2.0 equiv) in propionitrile at
ꢀ78 °C to rt and with IBr (1.1 equiv) in CH2Cl2 at ꢀ78 °C,
2a and 2b were obtained in 53% and 69% yield, respec-
tively (Scheme 1). Diiodopentalene 2a and bromo, iodo-
pentalene 2b were air-stable brown powders which could
be purified by column chromatography on silica gel. The
halopentalenes 2a and 2b served as a coupling component
in Sonogashira and SuzukiꢀMiyaura coupling to provide
the corresponding substituted dibenzopentalenes 3. When
2a reacted with2.2 equiv of phenylethyne in the presence of
5.0 mol % of palladium and copper catalysts, CꢀC bond
formation proceeded, and 3a was obtained in 87% yield as
an air-stable dark green powder. By using Sonogashira
coupling, dibenzopentalenes bearing an electron-donating
and -withdrawing group, 3bꢀd, were obtained in moderate
to good yields aswell: 94% for 3b, 92% for 3c, and 56% for
3d, respectively (Scheme 1). It was found that 2b was a
convenient building block for the synthesis of unsymme-
trically substituted diethynylpentalene 3e because Sonogashira
coupling of 2b with aminophenylethyne proceeded selec-
tively at the pentalenyl iodide moiety to afford 5a in a
good yield (Scheme 1). Because the cyano-substituted
pentalenes 3d and 3e showed poor solubility, it was
required to attach ethyl groups on benzene(s) in order
to improve the solubility. Subjection of 2b to Suzukiꢀ
Miyaura coupling with 4-(diphenylamino)phenylboronic
acid provided 5b, and subsequent coupling of 5b with
4-cyanophenylboronic acid and with 4-cyanophenylethyne
afforded amino- and cyano-substituted dibenzopentalenes
Herein a similar transannulation which is effected by
electrophilic dihalogenation (I2 or IBr) to give dihalodi-
benzopentalenes (Figure 1) is described.6 We postulated
that if 2 could be used in transition-metal-catalyzed CꢀC
Scheme 1. Syntheses of Dihalodibenzopentalenes 2a,b and
Sonogashira Coupling of 2a,b with Terminal Ethynes
Figure 1. Synthetic process for dibenzopentalenes from 2.
(5) Orita, A.; Hasegawa, D.; Nakano, T.; Otera, J. Chem.;Eur. J.
2002, 8, 2000.
(6) Synthesis of dibromodibenzopentalene was achieved by bromi-
nation of tetrahydrodibenzopentalene followed by dehydrobromina-
tion: Cava, M. P.; Pohlke, R.; Mitchell, M. J. J. Org. Chem. 1963, 28,
1861. Treatment of 5,10-disilyldibenzopentalene with Br2 and I2 af-
forded dibromo- and diiododibenzopentalenes (2a), respectively; see
ref 4b.
Org. Lett., Vol. 14, No. 15, 2012
3971

3f and 3g, respectively (Scheme S1). Coupling of the
acetylenic bromide 5a with 4-cyanophenylboronic acid
proceeded smoothly to give 3h which was a regioisomer
of 3g.
investigate the antiaromaticity of 3a, the nucleus-indepen-
dent chemical shift (NICS) calculation was carried out.8
Calculations at the B3LYP/6-31G(d) level showed that
NICS (0) values of the six- and five-membered rings of 3a
were ꢀ4.5 and þ8.5, respectively. Because negative and
positive NICS values indicate the presence of diatropic and
paratropic ring currents, respectively, it is demonstrated
that the peripheral benzenes are aromatic and five-mem-
bered rings are antiaromatic.9
UVꢀvis absorption spectra of 3aꢀh and 4aꢀc were
recorded in CH2Cl2 (Figures 2a and b), and in Table S1,
λmax and εmax were summarized. Dibenzopentalene 3a
exhibited an absorption band at 495 nm (ε 3.3 ꢁ 104 L
molꢀ1 cmꢀ1), and the wavelength of λmax appeared at a
longer wavelength by 31 nm in comparison with that of
9,10-di(phenylethynyl)anthracene (8) while the εmax of 3a
was 1.6 L molꢀ1 cmꢀ1 smaller than that of 8.7 The larger
wavelength of 3a indicated that the dibenzopentalene
motif had a more largely expanded π-system than anthra-
cene. TD-DFT calculations (LC-BLYP/6-31G(d)) sug-
gested that the 495-nm absorption band of 3a was
attributable to HOMO f LUMO transition (S0ꢀS2 band).7
When bromopentalene 5c, which was prepared by
selective coupling of 2b at the iodide moiety with 3,7-
dimethyloctyloxy(DMOO)-substituted phenylethyne, was
subjected to one-pot homocoupling, bis(dibenzopentalene)
4a was successfully obtained in 50% yield (Scheme 2). In
this process, boronic ester 6 was prepared and applied to
the subsequent SuzukiꢀMiyaura coupling without purifi-
cation because of the instability of 6. Since Suzukiꢀ
Miyaura coupling of bromopentalene 5c afforded bis-
(dibenzopentalene) 4a effectively, we applied this protocol
to the synthesis of tri- and tetra(dibenzopentalene)s 4b and
4c (Scheme 2). In situ preparation of 6 and the subsequent
coupling of 6 with2agave4bin15% yield. Forpreparation
of 4c, the two-step procedure was carried out: Suzukiꢀ
Miyaura coupling between 6 and 2b proceeded selectively
at the iodopentalene moiety to give 7 in 50% yield, and the
following homocoupling of 7 by treatment with the dibor-
ane/PdCl2(dppf) system afforded 4c in 36% yield. Tri- and
tetra(dibenzopentalene)s 4b and 4c are black crystalline
powders and can be purified by column chromatography
followed by reprecipitation.
Scheme 2. Syntheses of 4aꢀc by SuzukiꢀMiyaura Coupling
Figure 2. (a) UVꢀvis absorption spectra of 3aꢀh in CH2Cl2; (b)
UVꢀvis absorption spectra of 3b and 4aꢀc in CH2Cl2.
With dibenzopentalenes 3aꢀh and 4aꢀc in hand, we
evaluated their physical properties such as NMR, UVꢀvis
absorption, and cyclic voltammetry. When the 13C NMR
of 3a was recorded in CDCl3, two sp2 carbons were
observed at a particularly low magnetic field, 148.3 and
149.1 ppm. This finding is consistent with simulation
results: DFT calculations (B3LYP/6-31G(d)) suggested
that two quartenary carbons at the ring junction would
be observed at 143.3 and 147.1 ppm.7 In order to
When electron-donating and/or -withdrawing substituents
were attached to ethynylbenzenes of dibenzopentalenes,
(7) See Supporting Information for details.
(8) Chen, Z.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer,
P.; von, R. Chem. Rev. 2005, 105, 3842.
(9) Similar NICS (1) values for six- and five-membered rings were
reported in refs 4a and 4g: ꢀ5.3 and 6.6 ppm (B3LYP/6-31G(d)** for
5,10-bis(H3Si)dibenzopentalene), ꢀ6.2 and 5.9 ppm (B3LYP/6-31G**
for dibenzopentalene).
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Org. Lett., Vol. 14, No. 15, 2012

both bathochromic and hyperchromic shifts of their
largest absorption band were observed in 3bꢀe, and
Ph2N derivative 3c demonstrated a larger shift: 60 nm red
shift in wavelength of λmax and 1.7 ꢁ 104 L molꢀ1 cmꢀ1
hyperchromic shift in εmax in comparison with those of 3a.
Aminophenyl- and/or cyanophenyl-substituted pentalenes
3fꢀh showed a shorter wavelength of λmax than amino- and
cyanophenylethynylpentalene 3e indicating that substitu-
tion of the pentalene motif with a phenyl group at 5 and 10
positions would be less efficient in the expansion of its
π-system. In DMOO-substituted dibenzopentalenes 3b
and 4aꢀc, bis(dibenzopentalene) 4a had an absorption
band at 532 nm without a vibronic structure, and a 21 nm
(773 cmꢀ1) red shift from that of 3b was observed
(Figure 2b). Although 4b and 4c showed a similar batho-
chromic shift upon homologation of the dibenzopentalene
subunit, a significant decrease of the shifts was observed:
13 nm (448 cmꢀ1) for 4b relative to 4a, 9 nm (298 cmꢀ1) for
4c relative to 4b. DFT calculations which were performed
on a simplified model compound, bis(dibenzopentalene)
(C16H9ꢀC16H9) 9, suggested that 9 adopted a 52.5° or
129.7° dihedral angle between dibenzopentalene subunits in
the optimized conformers. The simulation result indicated
that the HOMOs and LUMOs in 4aꢀc were expanded
through the staggered π-system conjugation between diben-
zopentalene subunits (Figure S1).10
conjugative interaction of the dibenzopentalene motif with
phenyl groups. The electron-withdrawing cyano group
enabled a facile reduction in 3d and 3e. In a series of
DMOO-substituted dibenzopentalenes 3b and 4aꢀc, the
longer oligomer underwent the more facile reduction
(Figure S3). This finding indicated that π-system expan-
sion between dibenzopentalene subunits played a pivotal
role to decrease LUMO levels.
In conclusion, we prepared diiodo- and bromo, iodo-
dibenzopentalenes by treatment of highly strained cyc-
lic acetylene with I2 and IBr, respectively. These halo-
substituted dibenzopentalenes served as building blocks for
the synthesis of a series of phenylethynyl- and/or phenyl-
substituted dibenzopentalenes and dibenzopentalene oli-
gomers. Investigation of their UVꢀvis absorption and
cyclic voltammetry revealed that their wavelength of λmax
and reduction potential could be tuned by changing
substituents on the 5,10-positions of the dibenzopenta-
lene motif. In dibenzopentalene oligomers, their π-system
was expanded by a conjugative interaction between di-
benzopentalene subunits resulting in a bathochromic
shift in λmax and facile electrochemical reduction. Fur-
ther development of these halo-substituted dibenzo-
pentalenes to synthesize substituted pentalenes and their
application to organic materials such as OFET are under
investigation.
Cyclic voltammetry was recorded for 3aꢀh, 4aꢀc, and 8
in CH2Cl2 (Table S1, Figure S2). All of the dibenzopenta-
lenes underwent a smooth reduction and exhibited their
half-wave potentials in a rangebetweenꢀ0.96andꢀ1.33V
while reversible oxidation was observed only in Ph2N-
derivatives 3c and 3eꢀh (Figures S2a,b). Dibenzopenta-
lene 3a demonstrated a more positive reduction potential
in comparison with that of 8 indicating that a dibenzopen-
talene motif had a deeper LUMO level than anthracene. It
was shown that phenylethynyl-substituted dibenzopental-
ene 3a underwent facile reduction in comparison with5,10-
bis(triisopropylsilyl)- and dimethyl-dibenzopentalene
(Ered = ꢀ1.48 V4a and ꢀ1.9 V,4i respectively) exhibiting
that substitution with pheynylethynyl groups expanded
the π-conjugated system efficiently. The reduction poten-
tial of 5,10-diaryldibenzopentalene 3f was more negative
than those of 3aꢀh, which is diagnostic of the inefficient
Acknowledgment. This work was supported by a
Grant-in-Aid for Scientific Research on Innovative
Areas “Organic Synthesis based on Reaction Integra-
tion. Development of New Methods and Creation of
New Substances” (No. 2105), matching the fund sub-
sidy for private universities from the Ministry of Edu-
cation, Culture, Sports, Science and Technology, Japan, the
Japan Society for the Promotion of Science (JSPS) through
its “Funding Program for World-Leading Innovative
R&D on Science and Technology (FIRST Program)”
and Okayama Prefecture Industrial Promotion Foundation.
Supporting Information Available. Synthetic proce-
dures and characterization data of all new compounds,
Scheme S1, Table S1, Figures S1ꢀ3, simulation result for
NMR and TD-DFT calculations of 3a, and UVꢀvis
absorption spectrum of 8. This material is available free
of charge via the Internet at http://pubs.acs.org.
(10) Although the similar bathochromic shift was observed in a series
of oligo(9,10-anthrylenes), the red shift was ascribed to the different
substitution patterns because anthrylene subunits were orthogonal to
€
€
each other: Muller, U.; Adam, M.; Mullen, K. Chem. Ber. 1994, 127, 437.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 15, 2012
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