Time-integrated one-pot synthesis, X-ray structure, and redox properties of electrochromic 1,3-diarylisobenzofurans

Article abstract of DOI:10.1246/cl.130479

1,3-Bis[4-(dimethylamino)phenyl]isobenzofuran (1a) and a derivative were efficiently prepared through a sequence of reactions in a single batch reactor (time integration). The o-quinodimethane-type π-conjugation in 1a was confirmed by bond alternation determined by a low-temperature X-ray analysis. Brightly colored heterocycle 1a undergoes reversible two-stage one-electron oxidation, and electrolysis of 1a induces a vivid change in UV-vis-NIR absorptions exhibiting several isosbestic points (electrochromism).

Full text of DOI:10.1246/cl.130479

doi:10.1246/cl.130479
1244
Published on the web June 15, 2013
Time-integrated One-pot Synthesis, X-ray Structure, and Redox Properties
of Electrochromic 1,3-Diarylisobenzofurans
Toshiyuki Hamura,*1 Ryosuke Nakayama,1 Keisuke Hanada,2 Yuto Sakano,2 Ryo Katoono,2
Kenshu Fujiwara,2 and Takanori Suzuki*2
1School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337
2Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo, Hokkaido 060-0810
(Received May 23, 2013; CL-130479; E-mail: tak@mail.sci.hokudai.ac.jp)
1,3-Bis[4-(dimethylamino)phenyl]isobenzofuran (1a) and
mino)phenyl groups at 1,3-positions was prepared in 63% yield
by treatment of methyl 2-formylbenzoate (2) with 3.3 equiv of
[4-(dimethylamino)phenyl]magnesium bromide (¹40 ¼ 0 °C)
and subsequent acidic work-up (Scheme 3). Similarly, unsym-
metrical IBF derivative 1b, having a phenyl and a (dimethyl-
amino)phenyl group at 1,3-positions, was obtained by sequential
nucleophilic addition of two aryl Grignard reagents to the
starting material 2. IBFs 1a and 1b were isolated as brilliant
orange- and yellow-colored crystals, and their solutions are
highly fluorescent (Table 1). The absorption and emission of 1a
are shifted to longer-wavelength than those of 1b, which can be
accounted for by the higher HOMO-level of 1a than that of 1b
(vide infra) due to the presence of two electron-donating amino
groups.
To gain insight into the precise molecular structure, a low-
temperature X-ray structural analysis8 was conducted on 1a
(Figure 1). The crystal contains two crystallographically inde-
pendent molecules (P21/n, Z = 8), whose structural parameters
are nearly identical. The molecule adopts a planar geometry, and
thus the aryl groups at 1,3-positions lie on the same plane as the
IBF core [dihedral angles: 3.1(4) and 8.1(3)° in molecule-1;
6.2(3) and 16.6(4)° in molecule-2]. From the comparisons of
bond lengths of the IBF skeleton, the bond alternation expected
for the oQD-type conjugation is evident (Tables 2 and S17).
Thus, bonds a, c, and e have a much smaller ³-bond order9
a derivative were efficiently prepared through a sequence of
reactions in a single batch reactor (time integration). The
o-quinodimethane-type ³-conjugation in 1a was confirmed
by bond alternation determined by a low-temperature X-ray
analysis. Brightly colored heterocycle 1a undergoes reversible
two-stage one-electron oxidation, and electrolysis of 1a induces
a vivid change in UV-vis-NIR absorptions exhibiting several
isosbestic points (electrochromism).
o-Quinodimethane (oQD) is an attractive molecule that
contains a cross-conjugating ³-electron system. This hydro-
carbon as well as its derivatives are considered versatile building
blocks in synthetic chemistry.1 At the same time, the oQD
skeleton provides a unique scaffold in designing novel organic
redox systems since the Clar’s sextet is generated upon one-
electron transfer, thus stabilizing the resulting ionic states
(Scheme 1). Recently, we found that bis(diarylmethylene)dihy-
drophenanthrenes (DMPs), novel oQD-type electron donors,
exhibit an interesting electrochromic behavior (Scheme 2; Ar:
4-MeOC6H4).2 For further pursuing utility of the oQD skeleton
in material sciences, we now design isobenzofuran (IBF)-type
electron donors 1, whose preparation, X-ray structure, and
spectral and redox properties are reported herein along with the
electrochromic behavior.3
By considering a smaller number of aryl groups in 1 than
in DMP, stronger electron-donating groups (e.g., 4-Me2NC6H4)
would be necessary to stabilize the cationic states of 1.
According to our recent one-pot synthetic method4 for IBF,5
symmetric IBF derivative 1a6 appended with two (dimethyla-
CO2Me
1.1 equiv
CHO
MgBr
3.3 equiv
Me2N
2
MgBr
–40 → 0 °C
1.3 equiv
Me2N
–40 → 0 °C
MgBr
→
–40
0 °C
e−
e−
e−
CF3CO2H
CF3CO2H
1a (63%)
X
X
X
X
X
X
1b (58%)
e−
oQD
Scheme 3. One-pot synthesis of IBFs 1a and 1b.
Scheme 1. Redox scheme of an oQD-based electron donor.
Table 1. UV-vis and fluorescence spectral data of 1a and 1b
measured in MeCN
Ar
Ar
1a
1b
Ar
Ar
UV-vis: -max/nm (log ¾)
455 (4.00),
348sh (4.35),
324 (4.39),
276sh (4.05),
245sh (4.28)
557
441 (3.96),
338sh (3.79),
308 (4.04),
273sh (4.09),
252 (4.34)
543
O
Ar
Ar
DMP
oQD
1
Fluorescence: -max/nm
Scheme 2. Molecular design based on the oQD skeleton.
Chem. Lett. 2013, 42, 1244-1246
© 2013 The Chemical Society of Japan
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1245
NMe2
NMe2
NMe2
(Y = NMe2)
e−
e−
e−
O
O
O
e−
1a,1b+
2+
1a,1b
Y
NMe2 1a
Y
(Y = H)
follow-up reaction (e.g. dimerization)
Scheme 4. Redox scheme of IBFs 1a (Y = NMe2) and 1b
(Y = H).
Figure 1. Two views of ORTEP drawings of 1a (molecule-1)
determined by an X-ray analysis measured at 150 K along with
bond notation for Table 2. a) Top view; b) side view.
Table 2. Bond length (d)a of 1a determined by X-ray analysis
and Pauling ³-bond order (p)b calculated based on d
Bond
d/¡
p
a
b
c
d
e
f
1.424(3)
1.354(3)
1.424(3)
1.383(3)
1.440(3)
1.445(3)
1.375(3)
0.31
0.88
0.31
0.65
0.19
0.15
®
Figure 2. Cyclic voltammograms of (a) 1a and (b) 1b
measured in MeCN (0.1 M Et4NClO4, E/V vs. SCE, Pt
electrode, 500 mV s¹1).
g
aAveraged over assumed C2-symmetry for the two crystallo-
graphically independent molecules. bFrom the following linear
correlation reported in ref 9: d (C-C) = 1.463 ¹ 0.124p.
+0.59 V),10 in which the positive charges of the dication can be
delocalized over the extended ³-electron system. On the other
hand, reference compound 1b with only one dimethylamino
group shows irreversible oxidation at +0.26 V (Figure 2b),
showing that 1b is a weaker electron donor than 1a and that 1b•+
is less stable than 1a•+.11
Upon electrochemical oxidation in MeCN, clean conversion
of 1a was observed exhibiting several isosbestic points
(Figure 3). Thus, the intensity of the bands at 320 and 450 nm
decreases with concomitant appearance of new absorptions in
the NIR region (-max: 725, 810, and 970 nm). Based on the two-
stage one-electron oxidation process of 1a in the voltammogram
as well as development of the low-energy absorption bands, it is
most probable that the observed chromism results from the
conversion between 1a and 1a•+, although the cation radical is
too labile to be isolated as salts. Since the electrochromic
systems exhibiting the NIR-spectral changes are rare,13 the
electron-donating IBF can serve as a novel scaffold in devel-
oping interesting materials in the less-developed category of
electrochromism.
(p: 0.31, 0.31, 0.19) than bonds b and d (0.88, 0.65). The
slightly smaller value for bond d than for b can be accounted
for by conjugation of bond d with the aryl group and oxygen
through bonds f and g.
The pronounced bond alternation should be diminished
upon electron transfer as shown in Scheme 1, and the resulting
cationic species would be stabilized by delocalization of positive
charges over the electron-donating aryl groups at 1,3-positions
(Scheme 4). In fact, bis(dimethylamino)-derivative 1a under-
ox
goes reversible two-stage one-electron oxidation at +0.09 (E1
)
and +0.29 (E2ox) V (Figure 2a).10 The donating ability of 1a is
similar to that of N,N,N¤,N¤-tetramethyl-1,4-phenylenediamine
ox
ox
ox
(E1 +0.10 V, E2 +0.62 V),10 and the smaller ¦E (ÔE2
¹
E1ox) in 1a can be accounted for by a reduced on-site Coulombic
repulsion in dication 1a2+. In fact, the ¦E value of 1a is close to
ox
ox
that of N,N,N¤,N¤-tetramethyl-1,4-benzidine (E1 +0.41 V, E2
Chem. Lett. 2013, 42, 1244-1246
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1246
0.4
Monk, R. J. Mortimer, D. R. Rosseinsky, Electrochromism
and Electrochromic Devices, Cambridge Univ. Press, Cam-
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Experimental detail and selected spectral data are given in
Supporting Information (ref 7).
4
0.3
0.2
0.1
0
5
6
7
8
Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
Crystal data of 1a: MF C24H24N2O, FW 356.47, monoclinic
P21/n, a = 11.184(4), b = 29.905(8), c = 11.946(4) ¡, ¢ =
¹3
108.779(4)°, V = 3782.7(19) ¡3, μ(Z = 8) = 1.252 g cm
,
T = 150 K, R = 7.13%, wR2 = 20.19%, GOF = 0.995.
CCDC 933252.
W. C. Herndon, C. Párkányi, J. Chem. Educ. 1976, 53, 689.
200
400
600
800
1000
1200
9
λ/nm
10 Cyclic voltammetry was conducted in MeCN containing
0.1 M Et4NClO4 as a supporting electrolyte (E/V vs. SCE,
Pt electrode, scan rate 500 mV s¹1). For the irreversible
oxidation wave of 1b, the oxidation potential was estimated
as Epeak ¹0.03 V. Ferrocene undergoes one-electron oxida-
tion at +0.38 V under the similar conditions.
Figure 3. A continuous change in UV-vis-NIR spectrum
upon constant current electrochemical oxidation of 1a
(1.4 © 10¹5 M) in MeCN containing 0.05 M Et4NClO4 (30 ¯A,
every 30 s).
11 In the voltammogram of 1-[4-(dimethylamino)phenyl]-3-
phenyl-IBF 1b, the reduction peak in the return cycle
appeared in the far cathodic region (Figure 2b), suggesting
formation of a closed-shell dication through dimerization of
1b•+ at 3-position. Similar voltammograms were observed in
“dynamic redox systems” exhibiting reversible C-C bond
formation/cleavage upon electron transfer (ref 12).
12 T. Suzuki, E. Ohta, H. Kawai, K. Fujiwara, T. Fukushima,
Synlett 2007, 851.
13 a) M. C. Aragoni, M. Arca, F. A. Devillanova, F. Isaia, V.
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H. Ohno, Chem. Lett. 2013, 42, 1001. e) T. Suzuki, Y.
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This work was supported by Grant-in-Aid for Scientific
Research on Innovative Areas: “Organic Synthesis Based on
Reaction Integration” (No. 2105) from MEXT, Japan.
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Chem. Lett. 2013, 42, 1244-1246
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/