Highly electron-donating 3,3′-diaryl-1,1′-bi(isobenzofuran)s synthesized by photochemical exocyclic [2 + 2 + 2] cycloaddition

Article abstract of DOI:10.1021/ol801358c

(Chemical Equation Presented) Facile photochemical exocyclic [2 + 2 + 2] cycloaddition produced a series of electron-donating 1,1′- bi(isobenzofuran)s having various aryl groups at the 3,3′ positions. The 1,1′-bi(isobenzofuran) skeleton with a highly coplanar structure is a useful building unit to construct the narrow bandgap π-conjugated systems with low oxidation potentials.

Full text of DOI:10.1021/ol801358c

ORGANIC
LETTERS
2008
Vol. 10, No. 16
3591-3594
Highly Electron-Donating
3,3′-Diaryl-1,1′-bi(isobenzofuran)s
Synthesized by Photochemical Exocyclic
[2 + 2 + 2] Cycloaddition
Hongyu Zhang, Atsushi Wakamiya, and Shigehiro Yamaguchi*
Department of Chemistry, Graduate School of Science, Nagoya UniVersity, Furo,
Chikusa, Nagoya 464-8602, Japan
yamaguchi@chem.nagoya-u.ac.jp
Received June 16, 2008
ABSTRACT
Facile photochemical exocyclic [2 + 2 + 2] cycloaddition produced a series of electron-donating 1,1′-bi(isobenzofuran)s having various aryl
groups at the 3,3′ positions. The 1,1′-bi(isobenzofuran) skeleton with a highly coplanar structure is a useful building unit to construct the
narrow bandgap π-conjugated systems with low oxidation potentials.
Isobenzoheteroles (benzo[c]heteroles), such as isoben-
zothiophene, -pyrrole, and -furan, are 10π electron systems
with a quinoid nature, which makes them attractive as a
unique building unit for oligomeric and polymeric π-con-
jugated compounds.1 For instance, poly(isobenzothiophene)2
is the most representative example of narrow bandgap
polymers, which have been the subject of extensive research
in the area of conducting polymers for the past two decades.3
The chemistry of isobenzofuran, however, is still in its
infancy in view of a building unit for π systems, mainly
due to the difficult synthesis of the π-extended derivatives
as well as their instability.4 Whereas isobenzofurans are long
recognized as useful dienes in the Diels-Alder reaction5 and
can provide access to a variety of acenes,6 no example of
π-conjugated molecule consisting of more than two isoben-
zofuran units has been synthesized.7 We now disclose the
synthesis of a series of 1,1′-bi(isobenzofuran)-containing
π-conjugated systems based on a simple photochemical
reaction. These compounds have highly coplanar structures
with excellent π-π stacking in the crystals and exhibit unique
electronic properties with low oxidation potentials and
(4) (a) Wittig, G.; Pohmer, L. Chem. Ber. 1956, 89, 1334. (b) Cava,
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Ratner, M. A.; Marks, T. J. J. Phys. Chem. B 2005, 109, 3126.
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(7) Calculations of oligo(isobenzofuran)s, see refs 2e and 2f.
10.1021/ol801358c CCC: $40.75
Published on Web 07/19/2008
2008 American Chemical Society

narrow bandgaps. Their facile and general synthesis, struc-
tures, and fundamental properties are presented.
lography (2c, 2f, and 2i).11 Whereas compounds 2a-i are
readily oxidized in solution by atmospheric oxygen to
generate the corresponding ring-opened tetraketones 3,12 they
are quite stable in the solid state.13
Among these compounds, the crystal structure of bis-
(bithienyl) derivative 2i is shown in Figure 1. This compound
A series of π-extended 1,1′-bi(isobenzofuran)s 2 having
various aryl groups at the 3,3′ positions were synthesized in
one pot from bis[o-(arylcarbonyl)phenyl]acetylenes 1, as
shown in Scheme 1. The precursors 1a-i were synthesized
Scheme 1
.
Synthesis of 2 by Photochemical Exocyclic [2 + 2
+ 2] Cycloaddition
Figure 1. Crystal structures of 2i: (a) ORTEP drawing with 50%
probability for thermal ellipsoids and (b) herringbone packing
structure (red, oxygen; yellow, sulfur).
via the dilithiation of bis(o-bromophenyl)acetylene followed
by the reaction with aryl aldehydes and then oxidation with
MnO2. Upon the irradiation of light (365 nm) on the dilute
THF solutions for 4 h, the dicarbonylalkynes 1a-i undergo
an exocyclic [2 + 2 + 2] cycloaddition to produce the doubly
cyclized 2a-i in moderate to good yields (40-90%).8
A similar reaction was reported in the literature9 and also
used for the preparation of 3,3′-dimethyl-1,1′-bi(isobenzo-
furan),10 which, however, was unstable and only isolated by
trapping with an electron-deficient alkyne. Our successful
isolation of the 3,3′-diaryl derivatives 2 in this form is mainly
due to their high crystallinity. The irradiation of light quickly
turned the color of the reaction mixture from colorless to
red with a reddish orange emission at room temperature. The
cyclized products then spontaneously precipitated as bright
deep red crystals within 1 h. Notably, the products 2a-i were
isolated in an analytically pure form simply by filtration
without further purification. While the isolated compounds
exhibit poor solubility in common organic solvents and some
of them could not be characterized by NMR spectroscopy,
their structures were verified by means of high-resolution
mass spectrometry, elemental analysis, and/or X-ray crystal-
has a center of symmetry between the two isobenzofuran
units with a completely coplanar conformation. This com-
pound also maintains a highly coplanar π-conjugated frame-
work with dihedral angles of 4.3° between the isobenzofuran
unit and the neighboring thiophene ring and of 2.0° between
the two thiophene rings. This fact suggests that the π-con-
jugation is effectively extended over the entire molecule.
Similar planar structures were also observed for 2c and 2f,
indicating that this is the general structural feature for the
3,3′-diaryl-1,1′-bi(isobenzofuran)s. In addition, we noted that
compound 2i forms an interesting packing structure, as shown
in Figure 1b. Each isobenzofuran moiety π-stacks with an
adjacent molecule with the interfacial distance of ca. 3.37
Å. The resulting one-dimensional columns are further packed
into a herringbone structure, where a half-part of molecules
has close proximity with the molecules in the neighboring
column. This structure might be of interest with regard to
the carrier transport.
(11) Crystal data for 2c, 2f, and 2i: see the Supporting Information.
CCDC-683557 (2c), CCDC-683558 (2f), and CCDC-683559 (2i) contain
the supplementary crystallographic data for this paper. These data can be
obtained free of charge from the Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
(8) The moderate yields for some derivatives were mainly due to the
incomplete conversion of the reaction.
(12) For example, 2a was oxidized by atmospheric oxygen to produce
the corresponding tetraketone 3a in 87% yield. See the Supporting
Information.
(9) Nakatani, K.; Adachi, K.; Tanabe, K.; Saito, I. J. Am. Chem. Soc.
1999, 121, 8221.
(13) Compounds 2a-i are also thermally stable. Upon heating the
compounds in a sealed capillary in vacuum, no decomposition was observed
until melting or sublimation. See the Supporting Information.
(10) Casey, C. P.; Strotman, N. A.; Guzei, I. A.; Beil, J. Org. Chem.
2005, I:18.
3592
Org. Lett., Vol. 10, No. 16, 2008

The absorption and fluorescence spectra of 2a-i in CH2Cl2
are shown in Figure 2. Their data are summarized in Table
were studied by cyclic voltammetry, and their data are
summarized in Table 1. The cyclic voltammogram of 2a
a
Table 1. Electrochemical Properties of 2a-i in CH2Cl2
b
b
b
b
p1
1/2
p2
p1
p2
compd Eox (V) Eox (V) Eox (V) Ered (V) Ered (V)
2a
2b
2c
2d
2e
2f
2g
2h
2i
-0.11
-0.18
+0.09
+0.31
-0.09
-0.15
+0.20
-0.18
-0.17
+0.41
-0.20
-0.23
+0.67
+0.56
+0.66
c
+0.72
+0.40
+0.85
+0.46
+0.21
c
-2.19
-2.24
-2.11
-1.85
-2.16
-2.23
-1.86
-2.15
-1.98
c
-2.41
c
-2.25
-2.03
-2.36
-2.42
-2.03
-2.35
-2.17
c
-0.19
-0.21
-0.24
-0.18
+0.35
IBF
a Potentials are given against ferrocene/ferrocenium ion couple (Fc/Fc+).
b Peak potentials. c Not observed.
1/2
shows a reversible first oxidation at Eox ) -0.20 V and
p2
an irreversible second oxidation peak at Eox ) +0.67 V
(vs Fc/Fc+) in CH2Cl2, while it shows two irreversible
p1
p2
Figure 2. Photophysical properties of 3,3′-diaryl-1,1′-bi(isobenzo-
reduction peaks at Ered ) -2.17 V and Ered ) -2.39 V
furan)s 2a-i: (a) UV/vis absorption and (b) fluorescence spectra
1/2
(see the Supporting Information). Notably, the Eox of 2a
in CH2Cl2.
1/2
is negatively shifted by 0.55 V compared to IBF (Eox
)
+0.35 V) and even more negative than that of ferrocene,
demonstrating its highly electron-donating character as well
as the significant effect of the bi(isobenzofuran) skeleton.
S1 (Supporting Information), together with those of 1,3-
diphenylisobenzofuran (IBF) for comparison.14,15 3,3′-Di-
phenyl-1,1′-bi(isobenzofuran) 2a has its absorption maximum
at 543 nm and emission maximum at 587 nm, which are
128 and 101 nm red-shifted compared to IBF (λabs ) 415
nm, λem ) 486 nm), respectively. This comparison clearly
shows the significant effect of the insertion of one isoben-
zofuran unit.
All of the substituted phenyl derivatives 2b-f show their
absorption maxima around 540-550 nm and reddish orange
emission around 590-610 nm with moderate fluorescence
quantum yields of 0.20-0.25. These data indicate that the
electron-withdrawing (2c and 2d) or electron-donating (2e
and 2f) substituents only subtly affect their photophysical
properties. In contrast, the introduction of the electron-rich
thiophene substituents significantly affects the photophysical
properties. The absorption and emission of the thiophene
derivative 2h peak at 574 and 637 nm, respectively, which
are red-shifted about 31 and 50 nm in comparison with 2a.
Moreover, the bis(bithienyl) derivative 2i exhibits an absorp-
tion at 631 nm and broad emission in the near-infrared (NIR)
region with the maximum at 723 nm, which are red-shifted
about 88 and 136 nm when compared to 2a, demonstrating
its quite narrow bandgap electronic structure.
The substituent effect on the redox properties is investi-
gated. While the introduction of the electron-withdrawing
aryl groups (2c and 2d) or electron-poor pyridyl group (2g)
to the bi(isobenzofuran) skeleton results in the increase in
p1
the oxidation potential (Eox ) +0.09 V, +0.31 V, and
+0.20 V for 2c, 2d, and 2g, respectively), the introduction
of the electron-donating groups, including even thienyl or
bithienyl groups, does not further lower the oxidation
potential compared to 2a. All of the compounds 2b, 2e, 2f,
1/2
2 h, and 2i have their first oxidation potentials around Eox
) -0.18 to -0.24 V, suggesting that their low oxidation
potentials mainly reflect the character of the bi(isobenzofu-
ran) skeleton. However, it is worth noting that all of these
compounds show reversible first oxidation peaks. This is in
contrast to the cases of compounds 2c, 2d, and 2g having
electron-withdrawing (hetero)aryl groups, which only show
irreversible oxidation processes. The stability toward the one-
electron oxidation is dependent on the aryl groups at the 3,3′
positions.
To obtain a deeper insight into the electronic structures
of these π systems, we carried out the density functional
theory (DFT) calculations of 2a-i at the B3LYP/6-31G(d)
level. All of the compounds have the high-lying HOMO
(-5.01 to -4.20 eV) and narrow HOMO-LUMO gaps (1.79
to 2.32 eV), as shown in Figure 3a. Both of their HOMOs
and LUMOs are delocalized over the entire skeleton (Figure
3b). Notable features are summarized as follows. First, the
electron-withdrawing substitutes have significant effects on
the orbital energies. Compared with 2a, compounds 2c, 2d,
Another notable feature of the present π systems 2a-i is
their low oxidation potentials. The electrochemical properties
(14) Adams, R.; Gold, M. H. J. Am. Chem. Soc. 1940, 62, 2038
.
(15) We also measured the absorption spectra of 2a-i in the solid state
using the KBr pellet containing their crystals. Compared to those in solution,
the absorption bands significantly broaden and new bands appear at the
longer wavelengths presumably due to the formation of aggregates. See
the Supporting Information
.
Org. Lett., Vol. 10, No. 16, 2008
3593

transformed into the corresponding adducts. When a reaction
of 2b with DMAD was conducted at room temperature in
an argon atmosphere, 4 was obtained in 91% yield. Similar
cycloadducts 5 and 6 were also obtained when 2a or 2e,
respectively, were treated with in situ generated benzyne or
p-benzoquinone (Scheme 2). We believe this reactivity paves
Scheme 2. Diels-Alder Reactions of
3,3′-Diaryl-1,1′-bi(isobenzofuran)s
Figure 3. DFT calculations of 2a-i (B3LYP/6-31G(d)): (a) a plot
of the Kohn-Sham HOMO and LUMO energy levels and (b)
pictorial presentation of the HOMO and LUMO of 2a.
and 2g having electron-withdrawing (hetero)aryl groups
significantly decrease both the HOMO and LUMO levels.
In contrast, second, compounds 2b, 2e, 2f, 2h, and 2i, having
electron-donating (hetero)aryl groups, only slightly increase
the HOMO levels compared to 2a. This is consistent with
the observations in the cyclic voltammetry. Third, the
introduction of the thienyl or bithienyl groups primarily
affects not the HOMO levels but the LUMO levels. The
LUMO levels of 2h and 2i are lower by 0.16 and 0.38 eV,
respectively, compared to 2a. This is the origin of the narrow
bandgap electronic structures observed in the absorption
spectra for 2h and 2i.
We also carried out the NICS(0) calculations to evaluate
the aromaticity of the isobenzofuran skeleton. The calculated
NICS(0) value of the six-membered ring of the isobenzofuran
moiety in 2a is -4.2 ppm, which is much higher than that
of benzene (-9.7 ppm),16 suggesting that the six-membered
ring indeed lacks the aromaticity and has a significant quinoid
character. The increase in the quinoid character of the
π-conjugated backbone with the detriment of the aromaticity
has been the main strategy for synthesizing narrow bandgap
π-conjugated systems.17
the way to the synthesis of bi(acene)s, such as bi(naphtha-
lene) and bi(anthrathene).
In summary, we have demonstrated that the 1,1′-bi(isoben-
zofuran) skeleton is a powerful building unit to construct
narrow bandgap π systems with low oxidation potentials. It
is particularly worth noting that a series of 3,3′-diaryl
derivatives are synthesized by a very facile photochemical
exocyclic [2 + 2 + 2] cycloaddition reaction. The simple
filtration of the spontaneously produced crystals allows us
to obtain these inherently unstable π-conjugated systems.
Further studies on the applications of these materials to
electronic devices, as well as the synthesis of substituted
bi(acene)s, are currently in progress in our laboratory.
Acknowledgment. This work was partly supported by
Grants-in-Aid (Nos. 19675001 and 17069011) from the
Ministry of Education, Culture, Sports, Science, and Tech-
nology, Japan.
Notably, the produced bi(isobenzofuran)s are useful pre-
cursors for further transformations. Preliminary experiments
revealed that the bi(isobenzofuran)s could smoothly undergo
the Diels-Alder reactions with certain dienophiles and be
Supporting Information Available: Experimental pro-
cedures, spectral data for all new compounds, photophysical
and electrochemical data, theoretical calculation, and crystal-
lographic data in CIF format and ORTEP drawings of 2c
and 2f. This material is available free of charge via the
Internet at http://pubs.acs.org.
(16) Schleyer, P. v. R.; Maerker, C.; Dransfeld, A.; Jiao, H.; Hommes,
N. J. R. E. J. Am. Chem. Soc. 1996, 118, 6317.
(17) Karikomi, M.; Kitamura, C.; Tanaka, S.; Yamashita, Y. J. Am.
Chem. Soc. 1995, 117, 6791.
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