Divergent synthesis of 2,5-Diarylphospholes based on cross-coupling reactions: Substituent effects on the optical and redox properties of benzenephospholebenzene π-systems

Article abstract of DOI:10.1246/cl.2011.975

Divergent synthesis of 2,5-diarylphosphole derivatives was achieved by using two kinds of Pd-catalyzed cross-coupling reactions. In addition, para- and P-substituent effects on the optical and redox properties of the benzenephospholebenzene π-systems were

Full text of DOI:10.1246/cl.2011.975

doi:10.1246/cl.2011.975
Published on the web September 5, 2011
975
1,4-Diaryl-7,10-dimethoxyquinoxalino[2,3-b]quinoxalines and Their Dihydro Derivatives:
Redox Switching of NIR Absorption and Fluorescence
Youhei Miura,1 Hidetoshi Kawai,1,2 Kenshu Fujiwara,1 and Takanori Suzuki*1
1Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo, Hokkaido 060-0810
2Department of Chemistry, Faculty of Science, Tokyo University of Science, Shinjuku-ku, Tokyo 162-8601
(Received June 1, 2011; CL-110467; E-mail: tak@sci.hokudai.ac.jp)
Due to the high-lying HOMO of the substituents and the
low-lying LUMO inherited by the heterocyclic skeleton, the title
quinoxalinoquinoxalines 1 exhibit absorption bands which
extend to the NIR region, whereas the dihydro derivatives 2
are less colored but emit strong green fluorescence. Thus, the
present pairs can serve as novel redox dyes for drastic spectral
changes.
I
I
I
I
I
H
N
NH2
N
N
Br
Br
a
O
O
b
NH2
3b
N
H
I
5b
4b
I
I
OMe
Ar
Ar
OMe
H
N
H
c
N
N
N
N
N
d
N
H
N
H
OMe
OMe
2c-2f
2b
Linear azapolyacenes are an interesting class of compounds,
for which not only the aromatic form with [4n + 2]³ electrons
but also the antiaromatic ([4n]³) form can exist as stable
species.1 Some of them are attracting considerable recent
attention as luminescent materials2 or metal ligands3 to gain
advanced physical properties. During the course of our studies
on redox active dyes exhibiting fluorescent switching,4 we have
found that the title heterocyclic pairs (1/2) are interesting Weitz-
type redox systems,5 whose properties can be modified by intro-
duction of various aryl groups via the Suzuki-Miyaura cou-
pling.6 Noteworthy is that the electronic spectrum of aromatic
form 1 extends to the NIR region whereas the antiaromatic
dihydro derivatives 2 exhibit strong fluorescence, showing
drastic spectral changes upon redox reactions. Here we report the
preparation, X-ray structures, and spectral properties of 1 and 2.
The parent quinoxalino[2,3-b]quinoxaline (QQ) is notorious
for its low solubility7 as are many linear polyacenes. This
problem has been often overcome by introduction of substituents
to prevent the skeleton from dense packing in the crystal.
Electron-donating substituents such as methoxy groups are
attractive in both aspects for increasing solubility and for raising
the HOMO of the electron-deficient QQ derivatives. The latter
effects would induce red shift of the absorption band toward
the NIR region. With these in mind, we have designed here
1,4-diaryl-7,10-dimethoxy-QQs 1c-1f, which would be inter-
convertible with the dihydro derivatives (H2QQs) (2c-2f).
1,4-Diiodo-7,10-dimethoxy-H2QQ (2b) was chosen as the
key synthon, which was prepared as shown in Scheme 1, by
following the preparation of 1,4,7,10-tetramethoxy-H2QQ (2a).8
Thus, 3,6-diiodo-1,2-phenylenediamine (3b)9 and diethyl oxa-
late were condensed in the presence of sodium ethoxide. The
resulting 2,3-dihydroxy-5,8-diiodoquinoxaline prefers to adopt
the keto form 4b, which was converted into 2,3-dibromo-5,8-
diiodoquinoxaline (5b) by using PBr3 (yield 66% over 2 steps).
Upon condensation of 5b and 3,6-dimethoxy-1,2-phenylenedi-
amine (3a)8 in refluxing ethanol, 2b was obtained in 94% yield.
By adopting the standard conditions for the Suzuki-Miyaura
coupling reaction between 2b and PhB(OH)2, phenyl groups can
be attached at 1,4-positions of the H2QQ skeleton to give 2c
in 90% yield. Diiodide 2b was also converted into H2QQs
2d-2f smoothly having 4-methoxyphenyl, 3,5-bis(trifluorometh-
R
R
OMe
R
R
OMe
H
N
N
N
N
N
N
N
e
f
N
H
OMe
OMe
1a-1f
2a-2f
(R = a: OMe, b: I, c: C6H5, d: 4-MeOC6H4,
e: R = 3,5-(CF3)2C6H3, f: R = 2-thienyl)
Scheme 1. Reagents and conditions: (a) (COOEt)2, NaOEt,
benzene, reflux; (b) PBr3, reflux; (c) 3,6-dimethoxy-1,2-phenyl-
enediamine (3a), EtOH, reflux; (d) arylboronic acid,
[Pd(PPh3)4], K2CO3, H2O-THF, reflux; (e) DDQ, CHCl3: (f)
Na2S2O4, THF-H2O.
R
R
OMe
OMe
R
R
OMe
OMe
R
R
OMe
OMe
H
N
H
N
H
N
N
N
N
N
N
N
H
N
H
N
N
H
Form A
Form B
Form C
(R = I, C6H5, 4-MeOC6H4, 3,5-(CF3)2C6H3, 2-thienyl)
Scheme 2. Possible isomers of H2QQs 2b-2f.
yl)phenyl, and 2-thienyl groups (yield 80, 95, and 76%,
respectively) by using the corresponding arylboronic acids.10,11
H2QQs 2b-2f thus obtained are stable yellow powders and
easily oxidized upon treatment with DDQ to fully aromatized
quinoxalinoquinoxalines QQs 1b-1f (yield 96, 98, 90, 86, and
99%, respectively) exhibiting blue-black/green-black color.
DDQ also works nicely to convert 2a to 1a (yield 93%); the
latter was first prepared here by this method.
There has been controversy concerning the structure of
H2QQs in terms of the positions of NH protons.12 Thus, dihydro
compounds 2b-2f may adopt one of the three forms shown in
Scheme 2. In the NMR spectrum of diphenyl derivative 2c in
DMSO-d6, two sharp singlets are observed at 7.14 and 6.36 ppm
for aromatic protons, thus ruling out the Form C, which had been
Chem. Lett. 2011, 40, 975-977
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

976
Table 1. Redox potentials of QQs 1a-1f and H2QQs 2a-2f
measured by cyclic voltammetry in CH2Cl2
b)
a)
Compound
R
E1ox a/V E1red/V
E
sum/V
1a
1b
1c
1d
1e
1f
2a
2b
2c
2d
2e
2f
MeO
I
C6H5
4-MeOC6H4
3,5-(CF3)2C6H3
2-thienyl
MeO
+1.47
+1.57
+1.50
+1.44
+1.57
+1.27
+0.68
+0.87
+0.75
+0.72
+0.93
+0.80
¹0.39
¹0.15
¹0.36
¹0.38
¹0.24
¹0.27
1.86
1.72
1.86
1.82
1.81
1.54
Figure 1. a) Molecular overlap in H2QQ 2c and b) that in QQ
1c. In 2c, there is another overlap in crystal, which is very close
to that shown in a).
I
C6H5
4-MeOC6H4
3,5-(CF3)2C6H3
2-thienyl
considered as the main contributor before. The higher-field
resonance, which is close to that of 3a (6.15 ppm), was assigned
to those on the dimethoxybenzene nucleus of Form A.
Preference of Form A over Form B could be rationalized by
the intramolecular hydrogen bonding of N-H£O at the peri-
positions. The same could be true for other H2QQs 2b and 2d-2f
as suggested by the similar NMR data shown in Table S1.13
In this way, it has been suggested that N-H protons localize
on the 5,12-positions of 2c in solution. According to X-ray
analysis, this is also the case in the crystal (Figure 1a). The
diphenyl-H2QQ 2c crystallizes in P212121 space group (Z = 8),
and there are two crystallographically independent molecules
having quite similar structural parameters. The phenyl groups
are rotated against the H2QQ core with the torsion angles of
30-45°, and the three rings are arranged in a clockwise or an
anticlockwise manner, respectively, in molecule 1 and molecule
2. Although the location of N-H protons cannot be determined
by differential fourier synthesis, the C-N bond lengths in the
two diazahexagons are different enough [dihydropyrazine
moiety: 1.358(9), 1.376(9), 1.383(9), and 1.394(9) ¡ in molecule
1, 1.357(9), 1.367(8), 1.376(9), and 1.401(9) ¡ in molecule 2;
pyrazine moiety: 1.293(9), 1.305(9), 1.374(9), and 1.398(9) ¡ in
molecule 1, 1.292(9), 1.293(8), 1.349(9), and 1.385(8) ¡ in
molecule 2] to assure the structural assignment of the di-
hydropyrazine and pyrazine rings to confirm adoption of Form
A (Table S213). All methoxy groups in molecules 1 and 2 are
located nearly on the same plane as the H2QQ core, with the
oxygen lone pair directing toward the N-H protons to make
short O£H contacts (ca. 2.3 ¡).
The intramolecular hydrogen bond is not the only factor that
favors the observed geometry in terms of the methoxy groups to
the core. As shown by the X-ray analysis on the corresponding
oxidized form 1c, the methoxy groups occupy quite similar
positions despite the absence of hydrogen bonding (Figure 1b),
which can be rationalized by considering the expected repulsion
between the methyl groups and nitrogen lone pairs in the
inward-directing conformer. In contrast to the case of H2QQ 2c,
the C-N bond lengths in QQ 1c fall in a narrow range of
1.334(5)-1.358(5) ¡ to indicate that both diazahexagons are the
pyrazine rings. The molecules are lying on the crystallographic
mirror plane that bisects the short axis of 1c, so that the two
phenyl rings are respectively attached in a clockwise and an
anticlockwise manner to the core with the torsion angle of 54.0°.
In both crystals of 1c and 2c, the core skeleton is overlapped
in a face-to-face manner to form one-dimensional columnar
aIrreversible wave.
Figure 2. UV-vis-NIR spectra of QQs 1c-1f in CH2Cl2 (1c:
blue; 1d: red; 1e: pink; 1f: green).
stacks, in which the electron-donating dimethoxybenzene moie-
ty is overlapped with the electron-deficient quinoxaline skeleton
(Figure 1). The columns are further packed in a herringbone
motif exhibiting several short C-H£³ contacts (Figure S113).
Not only by the packing arrangement in crystal but also by the
redox potentials (E/V vs. SCE in CH2Cl2) in Table 1, the
amphoteric character of 1 is clearly shown.
Thus, the present QQs undergo irreversible one-electron
oxidation (E1ox) as well as reversible one-electron reduction
(E1red). The second reduction process is irreversible, and the
values of E2red are omitted in Table 1. The Esum value defined as
ox
red
E1 ¹ E1 is often used for evaluating the electrochemical
amphotericity, and the small values (<2 V) indicate the narrow
ox
HOMO-LUMO gap for QQs 1. The E1 of 1f (+1.27 V) is
lower than phenyl derivative 1c (+1.50 V) or tetramethoxy
derivative 1a (+1.47 V) due to the electron-rich thienyl rings.
The highest electron-affinity is found in 1e due to the electron-
withdrawing 3,5-bis(trifluoromethyl)phenyl groups (E1
red
:
¹0.24 V). On the other hand, the dihydro derivatives 2 are not
red
amphoteric. They are much weaker electron acceptors (E1
¹2 V), but exhibit stronger electron-donating ability (E1
+0.68-+0.93 V) due to the dimethoxyphenylenediamine unit.
Such difference in electronic structure results in drastic
changes in UV-vis-NIR spectra of QQs 1 and H2QQs 2. Due to
<
ox
:
Chem. Lett. 2011, 40, 975-977
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

977
coupling reaction. They are electrochemically amphoteric and
deeply colored dyes due to electron-donating methoxy groups,
and their properties can be tuned by the electronic nature of aryl
substituents. Noteworthy is the reversible interconversion with
the dihydro 2 derivatives that emit strong fluorescence. Further
modification are now in progress to attain unique properties of
QQ and H2QQ.
This work was partly supported by the Global COE Program
(Project No. B01: Catalysis as the Basis for Innovation in
Materials Science). We are very grateful to Grant-in-Aid for
Scientific Research on Innovative Areas: Organic Synthesis
Based on Reaction Integration from MEXT, Japan.
Figure 3. UV-vis (solid lines) and fluorescence spectra
(dashed line) of H2QQs 2c-2f in CH2Cl2 (2c: blue; 2d: red;
2e: pink; 2f: green).
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
Table 2. Fluorescence spectral data of H2QQs 2c-2f in CH2Cl2
1
2
3
J. I. Wu, C. S. Wannere, Y. Mo, P. v. R. Schleyer, U. H. F.
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Chem., Int. Ed. 2006, 45, 5144.
a
Compound
Ar
Φfl
-em max/nm
2c
2d
2e
2f
C6H5
0.52
0.49
0.38
0.37
459, 488, 521
458, 487, 520
480, 503
4-MeOC6H4
3,5-(CF3)2C6H3
2-thienyl
488, 521
aDetermined by using N-methylacridinium salt (Φ = 0.84) in
MeCN as an external standard.
4
T. Suzuki, K. Wada, Y. Ishigaki, Y. Yoshimoto, E. Ohta,
H. Kawai, K. Fujiwara, Chem. Commun. 2010, 46, 4100;
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Fujiwara, T. Tsuji, Tetrahedron Lett. 2003, 44, 6837.
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the high amphoteric nature, the electronic absorption bands of
1c-1f extend to the NIR region (- >700 nm) with the absorption
maxima at 590, 585, 609, and 651 nm, respectively (Figure 2).
Such a band is also present in tetramethoxy derivative 1a (616
nm) but absent in the parent QQ12 without electron-donating
substituents. The absorption is red-shifted in 3,5-bis(trifluoro-
methyl)phenyl (1e) and 2-thienyl derivatives (1f) due to the
lower LUMO and higher HOMO, respectively, than phenyl
derivative 1c. These bands show intramolecular charge-transfer
(CT), and the nonfluorescence of 1c-1f can be also rationalized
by considering facile nonradiative decay through CT interaction.
In contrast, strong green emission (450-600 nm) is evident
for H2QQs 2c-2f. The fluorescence spectra exhibit fine
structures as their absorption bands (350-450 nm) (Figure 3).
Again, 3,5-bis(trifluoromethyl)phenyl (2e) and 2-thienyl deriv-
atives (2f) show red-shifted absorption and emission, and the
fluorescence quantum yields (Φfl) are marginally smaller in 2e
and 2f than in 2c and 2d (Table 2). Due to the drastic spectral
differences of the redox pairs of 1c-1f/2c-2f, they can serve as
novel redox dyes for drastic spectral changes. The reversible
interconversion between 1c-1f/2c-2f was demonstrated by
high-yield reduction of 1c-1f to 2c-2f (yield 100, 95, 99, and
100%, respectively) using Na2S2O4.
5
6
7
8
9
S. Noguchi, Nippon Kagaku Zasshi 1959, 80, 569.
Y. Tsubata, T. Suzuki, T. Miyashi, Y. Yamashita, J. Org.
Chem. 1992, 57, 6749.
10 Not only diiodo-H2QQ 2b but also the corresponding
dibromide can be used for Suzuki-Miyaura coupling to
give the same products.
11 a) Y. Akimoto, Bull. Chem. Soc. Jpn. 1956, 29, 460. b) A.
Guirado, J. I. López-Sánchez, A. Cerezo, D. Bautista, J.
Gálvez, Tetrahedron 2009, 65, 2254.
12 G. M. Badger, I. S. Walker, J. Chem. Soc. 1956, 122.
13 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
In summary, we have designed and efficiently prepared aryl-
substituted QQ derivatives 1 by way of the Suzuki-Miyaura
Chem. Lett. 2011, 40, 975-977
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/