Highly enantioselective synthesis of silahelicenes using Ir-catalyzed [2+2+2] cycloaddition

Article abstract of DOI:10.1039/c1cc16762f

Silahelicenes, which contain two silole moieties in a helically chiral structure, were synthesized by a chiral Ir-catalyzed intermolecular [2+2+2] cycloaddition of tetraynes with diynes along with a Ni-mediated intramolecular [2+2+2] cycloaddition. The photophysical properties of the obtained highly enantiomerically enriched silahelicenes (up to 93% ee) were also measured. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c1cc16762f

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Highly enantioselective synthesis of silahelicenes using Ir-catalyzed
[2+2+2] cycloadditionw
Takanori Shibata,*a Toshifumi Uchiyama,a Yusuke Yoshinami,a Satoshi Takayasu,a
Kyoji Tsuchikamaa and Kohei Endob
Received 1st November 2011, Accepted 17th November 2011
DOI: 10.1039/c1cc16762f
Silahelicenes, which contain two silole moieties in a helically chiral
structure, were synthesized by a chiral Ir-catalyzed intermolecular
[2+2+2] cycloaddition of tetraynes with diynes along with a
Ni-mediated intramolecular [2+2+2] cycloaddition. The photo-
physical properties of the obtained highly enantiomerically
enriched silahelicenes (up to 93% ee) were also measured.
silicon atom and the p* orbital of the 1,3-butadiene moiety
interact to provide a s*–p* conjugated system. The resulting
low LUMO level leads to various characteristic optoelectronic
features.12 For example, high fluorescence quantum yield was
achieved with a ladder-type dibenzosilole.13 Moreover, the
high electron transport ability of siloles has been used as a
key component in organic light-emitting diodes (OLED).
Against these backgrounds, we focused on the enantio-
selective synthesis of silahelicenes as a new class of chiral
compounds, where two unique p-conjugated systems of helicene
and silole merge, and we expected that it would show new
optoelectronic properties.14 In recent years, organic molecules,
which emit circularly polarized light (CPL), have attracted
much attention,15 because they have tremendous potential in
optical devices.16 Actually, helicity is a promising structure of
CPL emission.17 We report here the first synthesis of chiral
silahelicenes using the Ir-catalyzed enantioselective [2+2+2]
cycloaddition.18
Helicenes are ortho-fused poly-aromatic compounds with
helical chirality. In helicene-like compounds, a poly-aromatic
skeleton contains non-aromatic motif(s). Due to their structurally
unique helical chirality, helicenes and helicene-like compounds
have attracted great attention as a source of chirality in
asymmetric synthesis, a chiral material, and a chiral discriminator
of biomolecules.1 Regarding the construction of a helicene
structure, the oxidative cyclization of stilbene derivatives
under photo-irradiation was accepted as a general protocol
until around 1990. Since then, other approaches have been
reported, including the Diels–Alder reaction,2 carbenoid
coupling,3 transition metal-catalyzed [2+2+2] cycloaddition
of alkynes,4 radical cyclization,5 olefin metathesis,6 the Friedel–
Crafts reaction,7 cycloisomerization,8 and electrophilic aromatic
cyclization.9 For the preparation of chiral helicenes, resolution of
We planned a consecutive inter- and intramolecular [2+2+2]
cycloaddition for construction of the silahelicene skeleton
(Scheme 1). The intermolecular reaction of a silicon-tethered
tetrayne having a conjugated diyne moiety with an ortho-
phenylene-bridged diyne will give regioisomeric cycloadducts
A and B. The obtained triyne A will undergo an intramolecular
reaction to give a desired silahelicene. Therefore, regio- and
stereocontrol were required for the efficient synthesis of chiral
silahelicenes.
racemic compounds was generally used until 1999, when Stara
´
and Stary´ reported their pioneering work on the enantioselective
intramolecular [2+2+2] cycloaddition of triynes using a chiral
Ni catalyst (up to 48% ee).10 Recently, Tanaka et al. enhanced
this strategy using a chiral Rh catalyst for the preparation of
[7]helicene-like compounds containing two 2H-pyran moieties
(up to 85% ee).11a They further reported consecutive [2+2+2]
cycloadditions of tetraynes possessing a 1,3-diyne moiety and
diynes for the synthesis of higher-order [9]helicene-like compounds,
which also contained two 2H-pyran moieties (up to 60% ee).11b,c
In contrast, silacyclopentadiene (silole) also has a unique
structure, where the s* orbital of two exocyclic s-bonds of the
We chose silicon-tethered tetrayne 1a as a model substrate and
subjected it to an intermolecular [2+2+2] cycloaddition with an
ortho-phenylene-tethered diyne using an achiral Ir–DPPBenz
catalyst
(DPPBenz:
1,2-bis(diphenylphosphino)benzene)
(Scheme 2).19 The reaction with 2a possessing an ethoxy-
carbonyl group on its alkyne termini proceeded regioselectively
to give the desired cycloadduct 3aa (triyne A in Scheme 1), but
in moderate yield,20 and the generation of axial chirality was
ascertained by HPLC analysis using a chiral column. In
contrast, the reaction with 2b possessing a methoxymethyl
group gave almost a 1 : 1 regioisomeric mixture of cycloadducts
3ab and 4ab. These results imply that the substituent on the
alkyne termini plays a key role in the regioselectivity in the
cycloaddition.
a Department of Chemistry and Biochemistry, School of Advanced
Science and Engineering, Waseda University, Shinjuku, Tokyo,
169-8555, Japan. E-mail: tshibata@waseda.jp;
Fax: +81-3-5286-8098; Tel: +81-3-5286-8098
b Waseda Institute for Advanced Study, Shinjuku, Tokyo,
169-8050, Japan
w Electronic supplementary information (ESI) available: CCDC
reference number 844257. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c1cc16762f
We next screened several chiral phosphorus ligands in
the intermolecular reaction of 1a with 2a (Table 1).
c
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Chem. Commun., 2012, 48, 1311–1313 1311

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Table 1 Screening of chiral ligands in the Ir-catalyzed cycloaddition
Entry
Liganda
Time/h
Yield (%)
ee (%)
1
2
3
4
5
(S,S)-MeDUPHOS
(S,S)-MeBPE
(S,S)-CHIRAPHOS
(S)-BINAP
(S,S)-EtFerroTANE
1
5
5
5
4
85
35
71
ꢀ38
ꢀ35
NDb
ꢀ94
39
o5
74
a
MeDUPHOS: 1,2-bis(2,5-dimethylphospholano)benzene, MeBPE:
CHIRAPHOS:
1,2-bis(2,5-dimethylphospholano)ethane,
2,3-
bis(diphenylphosphino)butane, EtFerroTANE: 1,10-bis(2,4-diethyl-
b
phospholano)ferrocene. Not determined.
Table 2 Transformation into helically chiral silahelicene 5aa
Scheme 1 Possible reaction scheme.
Entry Ir or Ni complex
Time/h Yield (%) ee (%)
1
2
3
4
[IrCl(cod)]2 + 2DPPBenz
3
2
2
4
24
76
—
—
93
[IrCl(cod)]2 + 2rac-BINAP
[IrCl(cod)]2 + 2BIPHEPa
[Ni(cod)2] + 2PPh3
Trace
Trace
94
a
BIPHEP: 2,20-bis(diphenylphosphino)-1,10-biphenyl.
We next focused on the stereospecific transformation of
axially chiral triyne 3aa into helically chiral silahelicene 5aa by
an intramolecular [2+2+2] cycloaddition (Table 2). We examined
a few Ir–diphosphine complexes (entries 1–3) and obtained the
desired silahelicene 5aa by using DPPBenz. However, the yield
was low, and the decrease in enantiomeric excess was observed
in the cycloadduct (entry 1). In the presence of a stoichiometric
amount of [Ni(cod)2] with triphenylphosphine, the reaction
proceeded at room temperature in excellent yield, and an almost
perfect transfer from axial to helical chirality was achieved
(entry 4).22–24 The enantiomeric excess (93%) is highest, which
is derived from the [2+2+2] cycloaddition approach.25
Scheme 2 Effect of substituents on the regioselectivity.
When MeDUPHOS was used, which gave an excellent asymmetric
induction of axial chirality in several alkyne-trimerizations,18
the reaction proceeded efficiently within 1 h at 100 1C, and
cycloadduct 3aa was obtained in good yield and ee (entry 1).21
For MeBPE and CHIRAPHOS, the results were worse in
terms of both yield and ee (entries 2 and 3). BINAP was an
ineffective ligand in this reaction (entry 4). After further
screening of several ligands, we finally found that EtFerroTANE
gave the best results, and the enantiomeric excess exceeded
90% (entry 5). To convert the triyne 3aa into silahelicene by
subsequent intramolecular [2+2+2] cycloaddition in a single
pot, the reaction mixture obtained in entry 5 was further
stirred at a higher reaction temperature (150 1C), but we could
not detect the formation of silahelicene 5aa.
We examined the scope of tetrayne and diyne substrates
under the optimal reaction conditions (Table 3). Electron-
donating and -withdrawing groups could be installed to the
benzene rings of tetrayne termini, and high enantioselectivity
was achieved in the Ir-catalyzed intermolecular reaction
(entries 1 and 2). Naphthalene-tethered diyne 2c also reacted
to give axially chiral triyne 3ac. In each case, the yield was
moderate because the starting materials were not completely
consumed. The Ni-mediated transformation of triynes 3ba,
3ca, 3ac proceeded with almost no loss of enantiomeric excess,
and the corresponding silahelicenes were obtained.
We measured the photophysical properties of the obtained
silahelicenes 5aa, 5ba, 5ca, and 5ac (Table 4). In UV-vis spectra
of 5aa, 5ba, and 5ca, lmax were observed at 293–296 nm.
c
1312 Chem. Commun., 2012, 48, 1311–1313
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Table 3 Enantioselective synthesis of chiral silahelicenes
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Triyne 3
Silahelicene 5
Yield (%)
ee (%)
Yield (%)
ee (%)
Entry
1
2
3
39 (3ba)
51 (3ca)
50 (3ac)
94
92
93
84 (5ba)
97 (5ca)
75 (5ac)
92
90
92
13 S. Yamaguchi, C. Xu and K. Tamao, J. Am. Chem. Soc., 2003, 125,
13662–13663.
14 Recently, a helicene was used as emission layer in OLED:
Table 4 Photophysical properties of silahelicenes 5aa, 5ba, 5ca, 5ac
S. Sahasithiwat, T. Mophuang, L. Menbangpung, S. Kamtonwong
and T. Sooksimuang, Synth. Met., 2010, 160, 1148–1152.
15 H. Maeda, Y. Bando, K. Shimomura, I. Yamada, M. Naito,
K. Nobusawa, H. Tsumatori and T. Kawai, J. Am. Chem. Soc.,
2011, 133, 9266–9269, and references therein.
UV-vis
l (log e)
Emission
la (FF)
Entry
Helicene
1
2
3
4
5aa
5ba
5ca
5ac
296 nm (4.60)
293 nm (4.51)
295 nm (4.57)
320 nm (4.74)
464 nm (0.030)
466 nm (0.033)
465 nm (0.029)
457 nm (0.084)
16 Selected examples: (a) P. Dyreklev, M. Berggren, O. Inganas, M. R.
¨
¨
Andersson, O. Wennerstrom and T. Hjertberg, Adv. Mater., 1995, 7,
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2011, 50, 3684–3687.
a
Quantum yield was determined by using quinine sulfate as a
standard.
In contrast, a red shift was observed in the UV-vis spectra of
5ac. With regard to fluorescence spectra, silahelicene 5ca has a
slightly shorter absorption wavelength, and its fluorescence
quantum yield was the highest among the four helicenes.
In conclusion, we realized the first synthesis of silahelicenes,
which have helical chirality including silole moieties. Inter-
molecular Ir-catalyzed enantioselective [2+2+2] cycloaddition
and the subsequent Ni-mediated intramolecular stereospecific
[2+2+2] cycloaddition provided highly enantiomerically enriched
silahelicenes. The extreme stability of silahelicene 5aa26 is
noteworthy for the application in optical devices. We expect that
these structurally unique silahelicenes will exhibit electron transport
ability and will use them as emitting layer in an OLED.27
We thank Prof. Yukio Furukawa and Heisuke Sakai (Waseda
University, Japan) for their helpful discussion. This work was
supported by Grant-in-Aid for Scientific Research from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan (No. 22350046) and Rohm Foundation.
18 (a) T. Shibata, T. Fujimoto, K. Yokota and K. Takagi, J. Am.
Chem. Soc., 2004, 126, 8382–8383; (b) T. Shibata and
K. Tsuchikama, Chem. Commun., 2005, 6017–6019; (c) T. Shibata,
K. Tsuchikama and M. Otsuka, Tetrahedron: Asymmetry, 2006, 17,
614–619; (d) T. Shibata, Y. Arai, K. Takami, K. Tsuchikama,
T. Fujimoto, S. Takebayashi and K. Takagi, Adv. Synth. Catal.,
2006, 348, 2475–2483; (e) T. Shibata, T. Chiba, H. Hirashima,
Y. Ueno and K. Endo, Heteroat. Chem., 2011, 22, 363–370.
19 Ir-catalyzed [2+2+2] cycloaddition for the synthesis of non-chiral
multiaromatic siloles: T. Matsuda, S. Kadowaki, T. Goya and
M. Murakami, Org. Lett., 2007, 9, 133–136.
20 Undesired cycloadduct 4aa (triyne B in Scheme 1) could not be
detected in the reaction mixture.
21 Cycloadduct 4aa was obtained in 9% yield.
22 Stereospecific transformation from axial chirality to helical chirality
for the synthesis of chiral helicenes; (a) M. Miyasaka, A. Rajca,
M. Pink and S. Rajca, J. Am. Chem. Soc., 2005, 127, 13806–13807;
(b) K. Nakano, Y. Hidehira, K. Takahashi, T. Hiyama and
K. Nozaki, Angew. Chem., Int. Ed., 2005, 44, 7136–7138.
23 A catalytic amount of the Ni complex (20 mol%) gave only a trace
amount of silahelicene 5aa.
24 We ascertained the structural details of silahelicene 5aa by X-ray
crystallographic analysis (see ESIw).
25 [Ni(cod)2]-mediated reaction of 1a and 2a did not proceed at all.
26 Silahelicene 5aa remained intact in air for almost 2 years.
27 As a preliminary experiment, 5aa was used as an emission layer in
OLED, and the emission of blue fluorescence was observed. The
detailed results will be disclosed in the near future.
Notes and references
1 (a) C. Schmuck, Angew. Chem., Int. Ed., 2003, 42, 2448–2452;
(b) A. Urbano, Angew. Chem., Int. Ed., 2003, 42, 3986–3989.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 1311–1313 1313