Triflic acid mediated cascade cyclization of aryldiynes for the synthesis of indeno[1,2-c]chromenes: Application to dye-sensitized solar cells

Article abstract of DOI:10.1002/chem.201405860

A new triflic acid (TfOH)-mediated cascade cyclization of ortho-anisole-substituted aryldiynes is described for the construction of indeno[1,2-c]chromenes. The cascade cyclization proceeds through an unusual TfOH-induced alkyne-alkyne cyclization followed by nucleophilic attack of the methoxy group on the benzylidene cation, which is completely different to the cyclization of ortho-aniline- or ortho-thioanisole-substituted aryldiynes. A new class of organic dyes with the indeno[1,2-c]chromene framework as both donor and π-linker were synthesized. These compounds exhibit high photovoltaic performances in dyesensitized solar cells (DSCs).

Full text of DOI:10.1002/chem.201405860

DOI: 10.1002/chem.201405860
Full Paper
&
Donor–Acceptor Systems
Triflic Acid Mediated Cascade Cyclization of Aryldiynes for the
Synthesis of Indeno[1,2-c]chromenes: Application to Dye-
Sensitized Solar Cells
Hua Jiang,[a, b] Giovanni Ferrara,[b] Xuan Zhang,[b] Kazuaki Oniwa,[b] Ashraful Islam,[c]
Liyuan Han,[c] Ying-Ji Sun,[a] Ming Bao,[a] Naoki Asao,[b] Yoshinori Yamamoto,[a, b] and
Tienan Jin*[b]
Dedicated to Professor Iwao Ojima on the occasion of his 70th birthday
Abstract: A new triflic acid (TfOH)-mediated cascade cycliz-
ation of ortho-anisole-substituted aryldiynes is described for
the construction of indeno[1,2-c]chromenes. The cascade
cyclization proceeds through an unusual TfOH-induced
alkyne-alkyne cyclization followed by nucleophilic attack of
the methoxy group on the benzylidene cation, which is
completely different to the cyclization of ortho-aniline- or
ortho-thioanisole-substituted aryldiynes. new class of
A
organic dyes with the indeno[1,2-c]chromene framework as
both donor and p-linker were synthesized. These
compounds exhibit high photovoltaic performances in dye-
sensitized solar cells (DSCs).
Introduction
Activation of alkynes through coordination with p-
electrophiles, such as Brønsted acids, iodine, Lewis acids, and
transition metals, followed by reaction with various nucleo-
philes is a useful way to promote a range of versatile organic
transformations.[1] In this context, electrophile-mediated cas-
cade cyclization reactions of aryldiynes bearing nucleophiles in
the proximity of the alkyne moiety have been used as a direct
synthetic method for the construction of p-conjugated
polycycles. These processes generally proceed through a
5-endo-dig nucleophilic addition followed by a 6-endo-dig
hydroarylation (Scheme 1a).[2] For example, we and Ohno et al.
have independently developed a gold-catalyzed cascade cycli-
zation of the aniline-substituted aryldiynes, which afforded the
corresponding benzo[a]carbazoles.[2a–c] We also reported an
iodine-mediated cascade cyclization of thioanisole-substituted
aryldiynes to synthesize iodine-substituted benzo[b]naph-
tho[2,1-d]thiophenes.[2f] In contrast, we found that triflic acid
(TfOH) promoted the cascade cyclization of anisole-substituted
Scheme 1. Electrophile-promoted cascade cyclization of ortho-nucleophile-
substituted aryldiynes.
aryldiynes through a completely different pathway, affording
indeno[1,2-c]chromene derivatives in good to high yields
under mild reaction conditions (Scheme 1b).
[a] H. Jiang, Dr. Y.-J. Sun, M. Bao, Dr. Y. Yamamoto
Indeno[1,2-c]chromene, having a tetracyclic skeleton, is a
precursor of Gnetuhainin S, which shows antioxidant activity;[3]
however, a very limited number of synthetic methods for the
construction of the indeno[1,2-c]chromene framework have
been reported.[4] For example, Li et al. reported an iron-
mediated intramolecular [3+2] cycloaddition of 2-(2-(ethynyl)-
phenoxy)-1-arylethanones,[4a] and Wu et al. reported a Pd-
catalyzed intermolecular cascade cyclization of 2-alkynyl-
halobenzene with 2-alkynylphenol.[4b] Herein, we describe
a novel TfOH-mediated cascade cyclization of ortho-anisole-
substituted aryldiynes for construction of the corresponding
State Key Laboratory of Fine Chemicals and School of Chemistry
Dalian University of Technology, Dalian 116023 (China)
[b] H. Jiang, Dr. G. Ferrara, X. Zhang, K. Oniwa, Dr. N. Asao, Dr. Y. Yamamoto,
Dr. T. Jin
WPI-Advanced Institute for Materials Research (WPI-AIMR)
Tohoku University, 2-1-1, Aoba-ku Katahira, Sendai 980-8577 (Japan)
E-mail: tjin@m.tohoku.ac.jp
[c] Dr. A. Islam, Dr. L. Han
Photovoltaic Materials Unit, National Institute for Materials Science
1-2-1 Sengen, Tsukuba 305-0047 (Japan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201405860.
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indeno[1,2-c]chromenes. This framework has been applied in
the synthesis of new organic JH dyes (donor–p-linker–acceptor
dyes) as a new donor moiety as well as a p-linker for use in
high-performance dye-sensitized solar cells (DSCs).
CH3CN and low addition temperature (below 08C) is a key
factor for achieving the high yield of 2a, which may be one of
the reasons why the reaction requires an excess amount of
TfOH (2 equiv). The structure of 2a was confirmed unambigu-
ously by reference to the authentic compound 4a,[4b] which
was prepared by bromination of the 11-position of 2a with N-
bromosuccinimide (NBS) followed by Suzuki–Miyaura coupling
of the resulting 3a with phenylboronic acid (Scheme 2). We
noted that the reaction with the ortho-phenol substituted aryl-
diyne 1a’ instead of 1a resulted in no reaction under the stan-
dard conditions, probably due to intermolecular hydrogen-
bond formation of 1a’ under the present conditions (entry 11).
This observation implies that the existence of the ortho-anisole
substituent is indispensable for the present transformation. As
indicated in Scheme 1a, we reported previously that the cyclo-
addition of thioanisole-substituted aryldiynes in the presence
of iodine as electrophile produced iodine-substituted benzo[b]-
naphtho[2,1-d]thiophenes.[2f] Interestingly, the reaction of 1a
with iodine at room temperature afforded indeno[1,2-c]chro-
mene dimer 4b, without forming either iodinated 3a’ or the
potential 5-iodo-6-phenylnaphtho[1,2-b]benzofuran product
(Scheme 3). We then prepared iodinated 3a’ from 2a upon
treatment with iodine. The reaction proceeded smoothly to
give the corresponding dimer 4b in 76% yield, which clearly
indicated that this dimer was generated through the formation
of 3a’ followed by dimerization in the presence of iodine.[5]
With the optimal conditions in hand, we investigated the
substrate scope, as shown in Table 2. Various aryldiynes having
aromatic groups at R were examined. A variety of functional ar-
omatic rings bearing Me, MeO, Br, F, CN, and NO2 groups were
tolerated regardless of the electronic properties, affording the
corresponding tetracycles 2b–g in good to high yields
(entries 2–7). Biphenyl and phenanthrenyl groups were also
suitable aromatic substituents, giving the corresponding tetra-
cycles 2h and 2i in 77 and 74% yields, respectively (entries 8
and 9). Substrate 1j, having a conjugated cyclic enyne moiety,
Results and Discussion
Initially, based on our previous results as shown in
Scheme 1a,[2a] we investigated a range of Lewis acidic metal
catalysts using the anisole-substituted aryldiyne 1a as a
substrate at room temperature (Table 1). Unfortunately, the
Table 1. Screening of reaction conditions for cyclization of 1a.[a]
Entry
Electrophile
Solvent
Yield [%][b]
1
2
3
4
5
6
7
8
9[e]
10[e]
11[f]
NaAuCl4 (5 mol%)
PtCl2 (5 mol%)
Cu(OTf)2 (5 mol%)
AuCl/AgOTf (5 mol%)
TfOH (5 mol%)
TfOH (2 equiv)
Tf2NH (2 equiv)
CF3CO2H (2 equiv)
TfOH (2 equiv)
EtOH
0
0
0
4
5
56[c]
47
0
98[c]
50[d]
0
toluene
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
TfOH (1 equiv)
TfOH (2 equiv)
[a] Reaction conditions: 1a (0.2 mmol), electrophile, solvent (0.01m), RT,
10 h. [b] Yield based on 1H NMR spectroscopic analysis, determined using
CH2Br2 as an internal standard. [c] Isolated yield. [d] 1a was recovered in
50% yield. [e] The reaction was performed at 08C for 3 h then at RT for
10 h. [f] 2-{[2-(Phenylethynyl)phenyl]ethynyl}phenol (1a’) was used
instead of 1a; compound 1a’ was recovered quantitatively.
reactions with NaAuCl4, PtCl2, Cu(OTf)2, and AuCl/
AgOTf catalysts were almost entirely unproductive
(entries 1–4). We were pleased to find that although
the reaction with a catalytic amount of TfOH gave
a poor yield of the indeno[1,2-c]chromene product
2a (entry 5), the reaction did proceed with an excess
Scheme 2. Determination of structure of 2a through the preparation of 4a.
amount of TfOH (2 equiv) to produce 2a in 56%
yield; decomposition of 1a was observed at room
temperature (entry 6). Further screening of other
Brønsted acids showed that the strongly acidic Tf2NH was as
active as TfOH, whereas the less acidic CF3CO2H was complete-
ly inactive (entries 7 and 8). To our delight, when TfOH
(2 equiv) was added dropwise at 08C then the mixture was
slowly warmed to room temperature, the yield of 2a increased
to 98% (entry 9). It was also found that the use of 1 equiv of
TfOH under the same conditions led to a decrease in the yield
of 2a to 50%, and 1a was recovered in 50% yield (entry 10). It
was noted that the use of other solvents such as methanol
and chloroform resulted in either no reaction or a complex
mixture. The use of a low concentration of 1a (0.01m) in
was somewhat unstable under the optimal conditions, giving
the corresponding product 2j in 40% yield (entry 10). Sub-
strate 1k, having a methoxy group on the central phenyl ring
of aryldiynes, was unstable at 08C, but the corresponding
product 2k could be obtained in 52% yield at the lower reac-
tion temperature of À208C [Eq. (1)]. The reaction with 1l (R1 =
H, R2 =OMe), having an electron-donating methoxy group at
the para-position of the alkynyl moiety of the anisole ring, af-
forded the corresponding product 2l in 98% yield, which is
higher than that with 1m (R1 =OMe, R2 =H), having a methoxy
group at the meta-position of the alkynyl moiety [Eq. (2)]. It
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Scheme 3. Cascade cyclization of 1a with iodine.
Table 2. TfOH-mediated cascade cyclization of anisole-substituted
aryldiynes.[a]
A proposed reaction mechanism is shown in Scheme 4.
Initially, the p-electrophilic TfOH activates the electron-rich
triple bond at the ortho-position of the anisole moiety,[6] which
may induce the 5-endo-dig alkyne-alkyne cyclization[7] to afford
Entry
R (1)
2
Yield [%][b]
1
2
3
4
5
6
7
8
9
Ph (1a)
2a
2b
2c
2d
2e
2 f
2g
2h
2i
98
72
84
90
81
79
73
77
74
40
4-Me-C6H4 (1b)
2-MeO-C6H4 (1c)
4-Br-C6H4 (1d)
4-F-C6H4 (1e)
4-CN-C6H4 (1 f)
4-NO2-C6H4 (1g)
4-C6H5-C6H4 (1h)
9-phenanthrenyl (1i)
1-cyclohexenyl (1j)
10
2j
[a] Reaction conditions: 1 (0.2 mmol), TfOH (0.4 mmol), CH3CN, 08C for
3 h, then at RT for 10 h. [b] Isolated yield.
Scheme 4. Proposed reaction mechanism.
was noted that the diyne substrate, having a strong electron-
withdrawing NO2 group at R1 (R2 =H), produced only a trace
amount of the corresponding product, and the starting sub-
strate was recovered in 90% yield. The 2- and 3-methoxy-sub-
stituted aryldiynes 1n and 1o also reacted smoothly under the
standard conditions to give the corresponding pentacyclic
products 2n and 2o in 99 and 79% yields, respectively
[Eqs. (3) and (4)]. It was noted that aryldiynes having an alkyl-
substituent at R are completely inactive under the present con-
ditions. Furthermore, the present reactions did not afford any
benzofuran side-products and the lower yields observed in
some cases were ascribed to the decomposition of the starting
aryldiynes.
the benzylidene cation B.[6] Intramolecular nucleophilic attack
of the methoxy group on B forms chromene oxonium ion C.
Finally, demethylation of
C produces the corresponding
product 2a.
Organic dyes composed of a donor–p-linker–acceptor (D–p–
A) structure have been widely employed in dye-sensitized solar
cells (DSCs) as panchromatic sensitizers because of their broad
absorption, high molar extinction coefficients, flexible molecu-
lar engineering, and efficient charge transfer.[8] The electron-
donating property of the conjugated dienol ether moiety in
the indeno[1,2-c]chromene structure and our interest in
developing high-performance DSCs[9] led us to design and
synthesize new D–p–A dyes by using the indeno[1,2-c]-
chromene framework as a new electron-donating moiety as
well as a p-linker.
The synthetic routes to JH dyes are shown in Scheme 5. The
11-bromo-6-phenylindeno[1,2-c]chromenes 3b and 3c were
synthesized by Pd-catalyzed CÀN couplings[10] of 2d with N,N-
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porting Information), dyes JH344 and JH375, having amines as
strong electron-donors, exhibit higher molar extinction coeffi-
cients (e) than dye JH132 in CHCl3 solution. The JH dyes on
a transparent TiO2 film show broad absorption spectra, and
the onsets of JH dyes extend to 640, 710, and 740 nm, respec-
tively, which are desirable for harvesting the solar spectrum
and generating a large photocurrent. The ionization potentials
(IP) of JH dyes in the range of À5.42 to À5.30 eV are sufficient-
ly lower than the redox potential of IÀ/I3 (À5.2 eV),[13] which is
À
desirable for the regeneration of the dye through the reaction
of the oxidized dye with iodide. The excited-state oxidation
potentials (S+/*) of JH dyes on TiO2 were calculated to be in
the range of À3.22 to À3.16 eV, which lie above the conduc-
tion band edge (À4.2 eV)[14] of TiO2. Consequently, an efficient
electron injection from the adsorbed JH dyes into the conduc-
tion band edge of TiO2 is expected.
To gain further insight into the frontier orbitals of JH dyes
and the electron-donating properties of the indeno[1,2-c]chro-
mene moiety, we calculated their molecular orbitals by using
the hybrid DFT energy functional B3LYP/6-31G+ +**. The cal-
culated HOMO and LUMO isodensities superimposed on the
molecular structures of the JH dyes are shown in Figure 1. It is
Scheme 5. Synthesis of JH dyes for dye-sensitized solar cells based on the
indeno[1,2-c]chromene framework. Reagents and conditions: a) 3b’:
Pd(OAc)2 (2.5 mol%), P(tBu)3 (5 mol%), NaOtBu, HNPh2, p-xylene, 1208C,
12 h; 3c’: [Pd2(dba)3] (dba=dibenzylideneacetone; 2 mol%), 2,2’-bis(diphe-
nylphosphino)-1,1’-binaphthyl (BINAP; 6 mol%), NaOtBu, pyrrolidine, tolu-
ene, 808C, 12 h. b) NBS (1.05 equiv), CHCl3/AcOH (10:1), 08C. c) 5a and 5b:
i. nBuLi, THF, À788C, iPrOB(pin) then RT (pin=pinacolato). ii. [Pd(PPh3)4]
(10 mol%), K2CO3, 5’-bromo-[2,2’-bithiophene]-5-carbaldehyde, DME/H2O
(5:1), reflux, 12 h. d) 5c: Pd(OAc)2 (2 mol%), PCy3·HBF4 (4 mol%), PivOH
(30 mol%), [2,2’-bithiophene]-5-carbaldehyde, K2CO3, DMAc, 1008C, 2 d.
e) JH132: NCCH2CO2H, CH3CO2NH4, AcOH, 1208C, 10 h; JH344 and JH375:
NCCH2CO2H, piperidine, CHCl3, 608C, 12 h.
diphenylamine and pyrrolidine, followed by bromination of the
11-position of 3b’ and 3c’ with NBS in high yields, respectively.
Sequential lithiation and boronation of the 11-positions of 3a
and 3b followed by the Suzuki–Miyaura cross-couplings of the
resulting boronates with 5’-bromo-[2,2’-bithiophene]-5-carbal-
dehyde afforded the corresponding aldehydes 5a and 5b in
88 and 54% yields, respectively. However, we failed to obtain
5c by following this two-step method because of unsuccessful
Suzuki–Miyaura coupling from 3c. Fortunately, 5c was synthe-
sized through Pd-catalyzed direct cross-coupling of 3c with
[2,2’-bithiophene]-5-carbaldehyde in 53% yield by using the
method reported by Fagnou et al.[11] It was mentioned that 5a
and 5b could also be synthe-
Figure 1. The calculated HOMO and LUMO isodensities of JH dyes showing
the regions of delocalization.
clear that the HOMO of JH132 is delocalized mainly on the
indeno[1,2-c]chromene moiety where the dienol ether unit re-
sides, which indicates the electron-donating nature of the
indeno[1,2-c]chromene moiety. Similarly, the HOMOs of JH344
and JH375 are delocalized over the indeno[1,2-c]chromene
moiety and the diphenylamine or pyrrolidine donor. On the
other hand, the LUMOs of JH dyes are mainly delocalized on
the p-bridge and on the electron-accepting cyanoacrylic unit,
sized by using Fagnou’s direct
cross-coupling, but the yields
Table 3. Optoelectronic properties and DSC performances of JH dyes.
were much lower than that
using the two-step method. The
resulting aldehydes 5a–c reacted
with cyanoacetic acid under
Knoevenagel condensation con-
ditions[12] to afford the desired
organic dyes, JH132, JH344, and
JH375 in good yields.
Dye
lmax [nm][a]
(eꢁ104 mÀ1 cmÀ1
lonset
[nm][b]
IP
[eV][c]
E0-0
[eV][d]
S+/
*
Jsc
Voc
[V]
FF
h
)
[eV][e]
[mAcmÀ2
]
[%]
JH132
JH344
JH375
442 (2.60)
414 (3.37)
408 (3.35)
562
572
578
À5.42
À5.32
À5.30
2.20
2.16
2.14
À3.22
À3.16
À3.16
11.22
13.38
14.34
0.64
0.69
0.65
0.71
0.71
0.72
5.07
6.56
6.71
[a] Absorption maxima were measured in chloroform (10À5 m). [b] Absorption measured in chloroform (10À5 m).
[c] Ionization potential (IP) of absorbed dyes on the nanocrystalline TiO2 film was determined with a photo-
emission yield spectrometer (Riken Keiki, AC-3E). [d] E0-0 was determined from the onset of the absorption in
As shown in Table 3 (for the
absorption spectra, see the Sup-
solution. [e] The excited-state oxidation potentials (S+/*) were calculated from the expression of S+/*=IP+E0-0
.
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and has significant contributions from the carboxylic groups.
These HOMO and LUMO distributions are appropriate for
efficient charge transfer and electron injection into the TiO2
conduction band. Good correlation between the calculated
and experimental HOMO and LUMO energy levels was found
by using the present DFT calculations. This clearly confirms
that the present D–p–A structural motif is an excellent design
for efficient charge transfer in DSCs.
Acknowledgements
This work was supported by a Scientific Research (B) award
from the Japan Society for Promotion of Science (JSPS) (No.
25288043) and a World Premier International Research Center
Initiative (WPI), MEXT, Japan.
Keywords: aryldiynes
· cyclization · domino reactions ·
The photovoltaic performances of DSCs based on JH dyes
indeno[1,2-c]chromenes · organic dyes · triflic acid
were measured under AM 1.5 G irradiation at 100 mWcmÀ2
;
the results are summarized in Table 3 (IPCE and I-V curves, are
shown in the Supporting Information). The JH132 dye, with
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the indeno[1,2-c]chromene as
a donor moiety, showed
a power conversion efficiency (h) of 5.07%. Under the same
conditions, the overall h of JH344 and JH375 were 6.56 and
6.71%, respectively. The highest h for the latter device is attrib-
uted to the highest short-circuit current (Jsc) of 14.34 mAcmÀ2
.
We ascribe the highest Jsc of JH375 to the strong electron-do-
nating substituent of pyrrolidine,[9d] which is consistent with
the widest absorption and incident photon-to-current efficien-
cy (IPCE) responses of JH375 on TiO2. On the other hand, the
highest open-circuit voltage (Voc) of 0.69 V for the JH344
device may be attributed to the decreased intermolecular p-p
interaction of the diphenylamine-based JH344 dye.[9d]
Conclusion
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We have developed an efficient method for constructing
indeno[1,2-c]chromenes through a TfOH-mediated cascade
cyclization of aryldiynes having an ortho-anisole substituent.
The reaction proceeds through an unusual TfOH-induced
alkyne-alkyne cyclization followed by cycloaddition between
the methoxy group and the benzylidene cation, which is com-
pletely different from the reported enediyne cyclization path-
ways. A new class of JH dyes incorporating the indeno[1,2-
c]chromene framework has been synthesized that exhibit high
photon-to-current efficiencies for DSCs. The HOMO and LUMO
isodensities were calculated and the results suggest that the
use of an indeno[1,2-c]chromene framework with an enol
ether donor and a conjugated diene p-linker constitutes
a promising new design for organic dyes.
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Typical procedure for the TfOH-mediated cyclization of 1a
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Received: October 28, 2014
Published online on && &&, 0000
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FULL PAPER
&
Donor–Acceptor Systems
H. Jiang, G. Ferrara, X. Zhang, K. Oniwa,
A. Islam, L. Han, Y.-J. Sun, M. Bao,
N. Asao, Y. Yamamoto, T. Jin*
Rerouting the cascade: An unprece-
dented triflic acid (TfOH)-mediated cas-
cade cyclization of ortho-anisole-substi-
tuted aryldiynes for the construction of
indeno[1,2-c]chromenes is reported (see
scheme). A new class of JH dyes
(donor–p-linker–acceptor dyes) with the
indeno[1,2-c]chromene framework as
both donor and p-linker were synthe-
sized, which exhibited high photovoltaic
performance in dye-sensitized solar cells
(DSCs).
&& – &&
Triflic Acid Mediated Cascade
Cyclization of Aryldiynes for the
Synthesis of Indeno[1,2-c]chromenes:
Application to Dye-Sensitized Solar
Cells
Chem. Eur. J. 2015, 21, 1 – 7
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ