FeCl3-Mediated Oxidative Spirocyclization of Difluorenylidene Diarylethanes Leading to Dispiro[fluorene-9,5′-indeno[2,1-a]indene-10′,9″-fluorene]s

Article abstract of DOI:10.1002/anie.201507794

A novel FeCl₃-mediated oxidative spirocyclization for construction of a new class of di-spirolinked π-conjugated molecules, dispiro[fluorene-9,5′-indeno[2,1-a]indene-10′,9″-fluorene]s (DSFIIFs), has been reported. The combination of FeCl_3<

Full text of DOI:10.1002/anie.201507794

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201507794
German Edition: DOI: 10.1002/ange.201507794
Conjugation
FeCl -Mediated Oxidative Spirocyclization of Difluorenylidene
3
Diarylethanes Leading to Dispiro[fluorene-9,5’-indeno[2,1-a]indene-
0’,9’’-fluorene]s
1
Jian Zhao, Zhanqiang Xu, Kazuaki Oniwa, Naoki Asao, Yoshinori Yamamoto, and Tienan Jin*
Abstract: A novel FeCl -mediated oxidative spirocyclization
3
for construction of a new class of di-spirolinked p-conjugated
molecules, dispiro[fluorene-9,5’-indeno[2,1-a]indene-10’,9’’-
fluorene]s (DSFIIFs), has been reported. The combination of
FeCl with FeO(OH) triggered an unprecedented double one-
3
electron oxidation of difluorenylidene diarylethanes to afford
the corresponding dispirocycles in high yields. The highest
fluorescence quantum yield was up to 0.94 in solution. This
protocol is also applicable to the synthesis of the non-
spirolinked dihydroindenoindenes.
F
luorene-based p-conjugated spirocycles have attracted
attention as important optoelectronic materials in various
fields of organic electronics because of their unique structural
[
1–14]
features.
As common features of p-conjugated spiro-
cycles, the high rigidity of spiromolecules efficiently mini-
mizes the electronic interactions between the p-systems and
offers high solubility, high thermal and morphological stabil-
ity, and intense fluorescence properties, without significantly
changing their absorption and fluorescence spectra compared
with the corresponding non-spirolinked parent compounds. In
this context, many structural modifications have been made
based on the spirobifluorene (SBF) unit in attempts to study
the hole-transporting and light-emitting performances in
Scheme 1. Reported fluorene-based spirocycles and our novel method
for constructing a new class of dispirocycle DSFIIFs.
[1–10]
various electronics (Scheme 1a).
positional isomers of di-spirofluorene indenofluorenes
DSFIFs), combining both di-hydroindenofluorene and fluo-
rene units, have also been reported for high-performance
In addition, several
challenging from the perspective of the structural diversity
and the future of organic electronics.
(
Very recently, we developed an interesting synthetic
approach for constructing 9,9-bifluorenylidene (9,9’BF) and
1,2-di(9H-fluoren-9-ylidene)-1,2-diphenylethane (DFDPE;
Scheme 1b) units from the corresponding bis(biaryl) alkynes
or biaryl alkynes through a novel palladium-catalyzed dual
[11–16]
organic light-emitting diodes (OLEDs).
For example, the
[
1,2-a], [1,2-b], and [2,1-c] isomers were used as hosts in blue
and green phosphorescent OLEDs, the [2,1-b] and [1,2-a]
isomers were used as efficient blue fluorophores, and the [2,1-
a] isomer was used as an emitting layer for an excimer-based
OLED (Scheme 1a). Taking into consideration the promi-
nent role of the spirolinked p-systems in organic electronics,
the discovery of a novel class of spirocycles is highly
2
[17]
C(sp )ÀH bond activation. The facile synthetic method and
2
the structurally intriguing DFDPE, having all sp -hybridized
carbon atoms, led us to explore a new CÀH oxidation reaction
for constructing new p-conjugated fused systems. Recently,
the 5,10-dihydro-[2,1,a]-indenoindene (DII) core, having
aromatic 14p electrons, has been reported as a new class of
chromophores in OLEDs to show high hole mobility and high
[18–21]
[
*] Dr. J. Zhao, Z. Xu, Dr. K. Oniwa, Prof. N. Asao, Prof. Y. Yamamoto,
sky-blue fluorescence.
knowledge, the di-spirolinked indenoindenes, such as dispiro-
fluorene-9,5’-indeno[2,1-a]indene-10’,9’’-fluorene] (DSFIIF;
However, to the best of our
Prof. T. Jin
WPI-Advanced Institute for Materials Research (WPI-AIMR)
Tohoku University
[
Scheme 1b), have not yet been reported, presumably owing
to synthetic difficulties. Herein, we report a novel and highly
2
-1-1, Aoba-ku Katahira, Sendai 980-8577 (Japan)
E-mail: tjin@m.tohoku.ac.jp
efficient FeCl -mediated oxidative spirocyclization of
3
Prof. Y. Yamamoto
DFDPEs towards the synthesis of a new class of di-
spirolinked p-systems, that is, DSFIIFs (Scheme 1b).
As stated in Scheme 1b, DFDPEs were synthesized
efficiently through our previously reported palladium-cata-
State Key Laboratory of Fine Chemicals and School of Chemistry,
Dalian University of Technology
Dalian 116023 (China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201507794.
[17]
lyzed CÀH activation of biaryl alkynes. In the presence of
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PdCl (10 mol%), PivOH (10 mol%), and MnO (3 equiv),
two parallel two fluorene moieties having a dihedral angle of
2
2
various biaryl alkynes afforded the corresponding DFDPEs
having a variety of functional groups on either the diphenyl or
difluorenylidene moiety in good to high yields (see the
Supporting Information). With these structurally intriguing
molecules in hand, we examined many different oxidants and
acids using the DFDPE 1a as a substrate in dichloromethane
at room temperature (Table 1). Interestingly, when the Lewis-
89.988. FeCl ·6H O proved to be more effective for achieving
3
2
a higher yield of 2a (entry 2). The non-acidic K [Fe(CN) ]
3
6
oxidant was ineffective (entry 3). The strong acidic oxidant of
MoCl resulted in complete decomposition of 1a (entry 4).
5
The reaction in the presence of other oxidants, such as CuCl2
and I , afforded 2a in very poor yields (entries 5 and 6). The
2
classical oxidant systems of 2,3-dichloro-5,6-dicyano-p-ben-
zoquinone (DDQ) and CF CO H (TFA) produced 2a in
acidic oxidant of anhydrous FeCl (3 equiv) was used, a novel
3
3
2
a reasonable yield of 75% with partial decomposition of 1a
entry 7), and the use of either DDQ or TFA alone did not
(
Table 1: Investigation of reaction conditions of spirocyclization of
DFDPE 1a.
promote the present spirocyclization. It was worth noting that
[
a]
the reaction with the non-oxidizing Lewis acid AlCl led to
3
decomposition of 1a (entry 8). The distinct reactivity differ-
ence between FeCl3 and AlCl3 implies that the present
spirocyclization seems to undergo a well-known one-electron
[22]
oxidation.
After screening various additives, 2a was
achieved in an excellent 99% yield when FeCl ·6H O
3
2
(
1.5 equiv) was combined with either Fe O (0.5 equiv) or
2 3
FeO(OH) (1 equiv); the combination with FeO(OH) was
more reactive (entries 9 and 10). In contrast, the use of bases,
[
b]
Entry
Oxidant or acid
Additive
t [h]
Yield [%]
a
2
1a
such as K CO3 suppressed the effective spirocyclization
2
1
2
3
4
5
6
FeCl3
–
–
–
–
–
–
1
2
58
78
0
0
5
(entry 11). We assumed that the effect of ferric oxide should
FeCl ·6H O
3
2
be to regenerate the active FeCl oxidant by the reaction with
3
K [Fe(CN) ]
12
12
20
12
12
12
24
12
24
99
0
60
36
0
3
6
HCl generated in situ. It was noted that although FeCl ·6H O
3
2
MoCl5
CuCl2
I2
DDQ
AlCl3
0
could serve as the catalyst in this reaction, the catalytic
reproducibility was not satisfactory. Consequently, the most
effective oxidant systems of FeCl ·6H O (1.5 equiv) and
14
12
75
0
[
c]
7
8
9
1
1
CF CO H
3
2
3
2
–
0
0
0
FeO(OH) (1 equiv) were chosen as optimal reaction con-
ditions for investigating the substrate scope of various
DFDPEs.
FeCl
3
·6H
2
O
Fe O
2 3
99
99
26
[
[
d]
d]
[e]
0
1
FeCl ·6H O
FeO(OH)
3
2
[f]
FeCl ·6H O
K2CO3
73
3
2
A variety of DFDPEs (1) having versatile functional
groups were investigated under the optimized reaction
conditions (Table 2). It was found that the electronic property
of the phenyl ring significantly affected the reactivity of 1. The
reactions with 1a and 1b, having an electron-donating n-butyl
and tert-butyl group, respectively, on the phenyl ring pro-
ceeded smoothly at room temperature to give the corre-
sponding dispirocycles 2a and 2b in nearly quantitative yields.
However, the reaction with 1c, having a strong electron-
donating methoxy group, required a higher temperature of
808C, thus affording the corresponding product 2c in 95%
yield. Interestingly, 1d with an electron-donating functional
group, also underwent the present spirocyclization at room
temperature to give the highly p-extended dispirocycle 2d in
95% yield. Both the unsubstituted 1e and 1 f were active at
[
a] Reaction conditions: 1a (0.1 mmol), acid or oxidant (3 equiv),
additive, CH Cl (0.05m), under dark conditions at room temperature.
[
as an internal standard. [c] DDQ (2 equiv) and CF CO H (5 equiv) were
used. [d] FeCl ·6H O (1.5 equiv) and Fe O (0.5 equiv) or FeO(OH)
2
2
1
b] Both yields were determined by H NMR spectroscopy using CH Br
2 2
3
2
3
2
2
3
(
1 equiv) were used. [e] Yield of isolated product is shown after silica gel
chromatography. [f] K CO (4 equiv) was used.
2
3
dispirocycle (2a) was formed in 58% yield along with
decomposition of 1a (entry 1). The structure of 2a was
confirmed unambiguously by X-ray crystallography analysis
(
Figure 1). In the crystal structure, the central indeno[2,1-a]-
indene core is perfectly flat, and is nearly perpendicular to the
4
2
08C, thus affording the corresponding dispirocycles 2e and
f in high yields within 12 hours, without being influenced by
the protonated or deuterated phenyl group. The presence of
the electron-donating groups on the fluorenyl moiety did not
affect the reactivity of the DFDPEs. For example, 1g and 1h,
having four methyl and four tert-butyl groups, respectively, on
the fluorenyl moieties underwent the present spirocyclization
at 408C, thus leading to the formation of the dispirocycles 2g
and 2h, respectively, in high yields. We found that the
introduction of the electron-withdrawing groups, such as an
ester (1i), CF (1j), or Br (1k and 1l), to the phenyl moiety of
3
DFDPEs lowered the reaction efficiency at low temperatures,
while the reaction at the elevated temperature of 808C gave
Figure 1. ORTEP drawing of the dispirocycle 2a. Hydrogen atoms are
omitted for clarity. Thermal ellipsoids are shown at 50% probability.
[25]
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[a]
Table 2: FeCl -mediated spirocyclization of various DFDPEs (1).
3
Entry
1
2
Scheme 2. Synthesis of 5,10-dihydroindeno[2,1-a]indene.
Recently it has become clear that in the most oxidative
aromatic couplings (Scholl type reaction) mediated by one-
electron oxidants, such as FeCl or DDQ, the oxidation could
3
preferentially occur at the most electron-rich position to form
1
2
3
1a, R=nBu
1b, R=tBu
1c, R=OMe
2a, 99% (RT)
2b, 98% (RT)
2c, 95% (808C)
[22]
radical cations. Interestingly, the density functional theory
(DFT) computational studies of the frontier molecular
orbitals revealed that the highest occupied molecular orbitals
(HOMOs) of the substrates 1a and 3a are more localized on
the C=C bonds of the conjugated diene moiety rather than on
the phenyl and fluorenyl moieties, thus indicating the highest
electron density is on the diene moiety (see Figure S1 in the
Supporting Information). These studies suggested that in the
present spirocyclization, the C=C bond of the diene in
4
1d
2d, 95% (RT)
DFDPEs should be oxidized preferably to form the radical
cation species. In addition, the intermolecular competing
reactions of the protonated 1e and the deuterated 1 f under
the standard reaction conditions for 2 hours, revealed that the
kinetic isotope effect (KIE) was 1.14 (see Scheme S1), thus
indicating that the CÀH bond cleavage of the phenyl group
5
6
1e, R=H
1 f, R=D
2e, 95% (408C)
2 f, 90% (408C)
was not the rate-determining step.
On the basis of the distinct activity of FeCl , electronic
3
effects of substrates, and electron density distribution, we
proposed a mechanism involving a one-electron oxidation
7
8
1g, R=Me
1h, R=tBu
2g, 97% (408C)
2h, 99% (408C)
reaction as shown in Scheme 3. FeCl induces double one-
3
electron oxidations of the two more-electron-rich C=C bonds
of the diene moiety in 1e to give the radical cation species A,
which can exist as the dication intermediate B. It should be
À
noted that the analogous dication having an I3 counteranion
1
2
9
1i, R =CO Et, R =H
2i, 99% (808C)
2j, 82% (808C)
2k, 99% (808C)
2l, 97% (808C)
has been isolated and characterized by X-ray crystallogra-
2
1
2
1
1
1
0
1
2
1j, R =CF , R =H
[23]
3
phy. Subsequently, the double Friedel–Crafts reactions take
1
2
1k, R =Br, R =H
place to form an arenium dication (C), which undergoes the
rapid deprotonation by chloride anions to give the dispiro-
1
2
1l, R =Br, R =Cl
[
a] Standard reaction conditions: 1 (0.1 mmol), FeCl ·6H O (1.5 equiv),
3 2
FeO(OH) (1.0 equiv), under dark conditions in CH Cl at either RT or
2
2
4
08C, or in ClCH CH Cl at 808C. Yields are those of the isolated
2 2
products.
rise to the corresponding dispirocycles 2h–l in good to
excellent yields.
The present FeCl -mediated conditions have also proven
3
effective for the non-fluorenyl-incorporated buta-1,3-dienes
(
Scheme 2). For example, the reaction of the multisubstituted
diene 3a led to the formation of the tetraphenyl-substituted
5
8
,10-dihydroindeno[2,1-a]indene (DII) 4a in 88% yield at
08C, and was synthesized previously by multistep methods in
[
18,19]
low yields.
Similarly, the reaction of the multisubstituted
diene 3b, having electron-donating p-tolyl groups, reacted at
room temperature to give the corresponding product 4b in
9
9% yield.
Scheme 3. Proposed reaction mechanism.
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cycle 2e and HCl. The DFT calculation shows that the
HOMO of B is mainly localized in the central stilbene moiety,
thus indicating the electron-donating nature of the phenyl
rings, which are favorable for reacting with the cation
reduced structural rigidity of the former. DSFIIFs exhibit the
absolute fluorescence quantum yields of 0.45 for 2b, 0.56 for
2e, and 0.45 for 2h, and are lower than the reported results of
[1,2-b]DSFIFs (Scheme 1a, 0.62–0.70). It is worth noting that
the DSFIIF 2I, having two electron-withdrawing ester groups
at the central indeno[2,1-a]indene moiety, exhibits a red-
shifted absorption and emission maxima compared to other
DSFIIF derivatives. Interestingly, 2i is highly fluorescent with
electrophiles (see Figure S1). Either the FeO(OH) or Fe O3
2
additive may react with HCl to supply the fresh FeCl oxidant,
3
thus suppressing the partial decomposition of 1e by HCl, and
it is crucial for achieving high yields of the dispirocycles.
The optical and electrochemical properties of the selected
dispirocycles 2 are summarized in Table 3 and Figure 2 (see
also Figures S2 and S3). The UV-vis absorption spectra of the
the absolute quantum yield of 0.94 in CHCl , thus implying
3
the distinct substituent effect of the ester group on the p-
conjugated indeno[2,1-a]indene moiety. The HOMO energy
levels of DSFIIFs were calculated from the oxidation
potentials by cyclic voltammetry (CV) in CH Cl , and the
2
2
lowest unoccupied molecular orbital (LUMO) energy levels
were estimated from the optical bandgap and HOMOs. The
HOMO energy levels of 2b, having two electron-donating
tert-butyl groups at the indeno[2,1-a]indene moiety is
À5.49 eV, similar to the reported [2,1-a]DSFIF(tBu)
[
24]
(
À5.48 eV, Scheme 1a). The nonsubstituted 2e (À5.68 eV)
and 2h (5.62 eV), having four tert-butyl substituents at the
fluorene moieties, exhibited slightly lower HOMOs than that
of 2b, and are similar to the reported values (À5.76 eV,
À5.66 eV, 5.61 eV) of the three [1,2-b]DSFIF isomers (Sche-
[15,24]
me 1a).
The LUMO energy levels of 2b, 2e, and 2h,
estimated to be in the range of À2.10 to À2.23 eV, are very
near to the reported DSFIF series. 2i shows both low HOMO
and LUMO energy levels of À5.95 eV and À2.80 eV with
a narrow energy bandgap of 3.15 eV resulting from the two
electron-withdrawing ester groups at the indeno[2,1-a]indene
moiety.
Figure 2. UV-vis spectra (left; solid line) and fluorescence spectra
(right; solid line) of 2b, 2e, 2h, and 2i in chloroform solution.
Normalized UV-vis absorption of spin-coated thin films on the quartz
In conclusion, we have developed a novel and highly
plates (dashed line) at room temperature. F =quantum yield.
f
efficient FeCl -mediated oxidative spirocyclization of 1,2-
3
di(9H-fluoren-9-ylidene)-1,2-diphenylethanes under mild
reaction conditions. A new class of di-spirolinked indeno-
[2,1-a]indene p-conjugated systems having a wide range of
functional groups have been synthesized in very high yields
with the highest fluorescence quantum yield of 0.94 for the
ester-substituted dispirocycle. The results led to the conclu-
sion that the present mechanism involved two one-electron
oxidations and a dication-induced Friedel–Crafts spirocycli-
zation pathway. This protocol also can be applicable for the
construction of non-spirolinked dihydroindenoindenes. The
present method provides not only a novel and valuable
spirocycle synthetic tool, but also a new type of spirolinked p-
conjugated system which is expected to be highly applicable
in the field of optoelectronics.
nonsubstituted (2e) and tBu-substituted (2b and 2h) DSFIIFs
exhibit bands of the central indeno[2,1-a]indene moiety in the
range of l = 300–360 nm in chloroform, and they are very
similar to the [1,2-b]DSFIFs having similar substituents at the
central dihydroindeno[1,2-b]fluorene moiety as reported by
[15,24]
Poriel, Rault-Berthelot et al. (Scheme 1a).
Thin-film
absorption maxima of 2b, 2e, and 2h, recorded on spin-
coated films on the quartz plates, are slightly red-shifted
compared to the corresponding solution absorption maxima,
thus implying very weak intermolecular interactions in solid
state. The photoluminescence spectra of the present DSFIIFs
show Stock shifts of 15–17 nm, which are larger than that of
[15,24]
[
1,2-b]DSFIF analogues (2–3 nm),
thus indicating the
Table 3: Selected optical and electrochemical properties of DSFIIFs
[
a]
[b]
[c]
[d]
ox
[e]
opt
[f]
[g]
[h]
DSFIIF
labs [nm]
labs [nm]
lem (lexc) [nm]
Ff
E
[V]
DE [eV]
HOMO [eV]
LUMO [eV]
2
2
2
2
b
e
h
i
358, 341, 326, 263
352, 336, 315, 261
353, 336, 318, 305, 268, 254
386, 366, 350, 325, 310, 299
359
356
353
390
390 (340)
382 (320)
382 (340)
420 (340)
0.45
0.56
0.45
0.94
0.91
1.11
1.04
1.29
À3.39
À3.45
À3.44
À3.15
À5.49
À5.68
À5.62
À5.95
À2.10
À2.23
À2.18
À2.80
[
[
a] UV-vis absorptions were measured in chloroform. [b] UV-vis absorptions were measured using as-spun thin films on the quartz plates.
c] Fluorescence maxima were measured in chloroform. [d] Absolute fluorescence quantum yield (F ) was measured by a photon-counting method
f
opt
using an integration sphere. [e] Oxidation potential (vs Ag/AgCl) was measured by cyclic voltammetry in CH Cl . [f] Optical bandgap (DE ) was
2
2
estimated from the contact between the UV-vis absorption and the fluorescence spectra. [g] HOMO was calculated from the oxidation potential.
h] LUMO was calculated from the HOMO energy and the optical bandgap.
[
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General procedure of FeCl -mediated oxidative spirocyclization of
3
1
a. 1a (0.1 mmol, 62 mg), FeCl ·6H O (0.15 mmol, 40 mg), and
3 2
FeO(OH) (0.1 mmol, 9 mg) were added to an anhydrous CH Cl2
2
(2 mL, 0.05m) solution. The reaction vial was capped. The reaction
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2 2
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Received: August 20, 2015
Revised: September 21, 2015
Published online: October 16, 2015
Angew. Chem. Int. Ed. 2016, 55, 259 –263
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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