Enantioselective synthesis, crystal structure, and photophysical properties of a 1,1′-bitriphenylene-based sila[7]helicene

Article abstract of DOI:10.1002/ejoc.201403565

A 1,1′-bitriphenylene-based sila[7]helicene was synthesized with a high ee value by enantioselective double [2+2+2] cycloaddition of a biaryl-linked tetrayne with a silicon-linked bis(propargylic alcohol) as a key step. This sila[7]helicene exhibited a high tolerance toward racemization, which could be explained by the X-ray crystal structure analysis. With respect to the photophysical properties, this sila[7]helicene exhibited a relatively high fluorescence quantum yield and circularly polarized luminescence activity.

Full text of DOI:10.1002/ejoc.201403565

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201403565
Enantioselective Synthesis, Crystal Structure, and Photophysical Properties of
a 1,1Ј-Bitriphenylene-Based Sila[7]helicene
Koichi Murayama,[a] Yasuaki Oike,[a] Seiichi Furumi,[b] Masayuki Takeuchi,[c]
Keiichi Noguchi,[d] and Ken Tanaka*[a,e,f]
Keywords: Luminescence / Helicenes / Triphenylenes / Cycloaddition / Rhodium
A 1,1Ј-bitriphenylene-based sila[7]helicene was synthesized
with a high ee value by enantioselective double [2+2+2]
cycloaddition of a biaryl-linked tetrayne with a silicon-linked
bis(propargylic alcohol) as a key step. This sila[7]helicene
exhibited a high tolerance toward racemization, which could
be explained by the X-ray crystal structure analysis. With
respect to the photophysical properties, this sila[7]helicene
exhibited a relatively high fluorescence quantum yield and
circularly polarized luminescence activity.
would afford an 1,1Ј-bitriphenylene-based sila[7]helicene
(Scheme 1). Recently, Nozaki, Nakano, and co-workers
reported the synthesis of an enantiopure 3,3Ј-biphen-
anthrene-based sila[7]helicene[12] by the platinum-catalyzed
intramolecular double hydroarylation followed by resolu-
tion with chiral HPLC,[4] while the enantioselective synthe-
sis of the sila[7]helicene has not been achieved. In this pa-
per, we disclose the enantioselective synthesis, crystal struc-
ture, and photophysical properties of the 1,1Ј-bitripheny-
lene-based sila[7]helicene.
Introduction
Silafluorene derivatives have attracted much attention in
materials chemistry because of their use for organic electro-
luminescent compounds.[1] There have been several reports
on the transition-metal-catalyzed synthesis of sila-
fluorenes.[2–4] For example, several research groups have re-
ported the intra- and intermolecular cross-coupling reac-
tions to produce silafluorenes.[2] Murakami and co-workers
reported a conceptually different strategy, the iridium-cata-
lyzed [2+2+2] cycloaddition of silicon-linked diynes with
monoynes, for the synthesis of silafluorenes.[3] On the other
hand, our research group reported the enantioselective syn-
thesis of 1,1Ј-bitriphenylene-based carbo- and phospha[7]-
helicenes by the rhodium-catalyzed double [2+2+2] cyclo-
addition of a biaryl-linked tetrayne with carbonyl- or
phosphorus-linked diynes (Scheme 1).[5–11] We anticipated
that the rhodium-catalyzed double [2+2+2] cycloaddition
of the biaryl-linked tetrayne with a silicon-linked diyne
[a] Department of Applied Chemistry, Faculty of Engineering,
Tokyo University of Agriculture and Technology
Koganei, Tokyo184-8588, Japan
[b] Department of Applied Chemistry, Faculty of Science,
Tokyo University of Science
Scheme 1. Enantioselective synthesis of 1,1Ј-bitriphenylene-based
[7]helicenes by rhodium-catalyzed [2+2+2] cycloadditions.
1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan
[c] Organic Materials Group, National Institute for Materials
Science
Results and Discussion
1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
[d] Instrumentation Analysis Center, Tokyo University of
Agriculture and Technology
Our research group previously reported that a silyl-sub-
stituted propargylic alcohol is a highly reactive substrate in
the rhodium-catalyzed [2+2+2] cycloaddition with a biaryl-
linked diyne (Scheme 2).[13]
Koganei, Tokyo184-8588, Japan
[e] Department of Applied Chemistry, Graduate School of Science
and Engineering,
Tokyo Institute of Technology
Thus, silicon-linked bis(propargylic alcohol)
2 was
Ookayama, Meguro-ku, Tokyo 152-8550, Japan
E-mail: ktanaka@apc.titech.ac.jp
selected as a cycloaddition partner of the biaryl-linked
tetrayne 3. Diyne 2 was readily prepared in two steps
through the silylation of protected propargylic alcohol 1
followed by deprotection (Scheme 3). Various cationic
rhodium(I)/axially chiral biaryl bisphosphine complexes
http://www.tuat.ac.jp/~tanaka-k/
[f] ACT-C, Japan Science and Technology Agency (JST),
4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201403565.
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K. Tanaka et al.
SHORT COMMUNICATION
based carbo[7]helicene,[5] but significantly smaller than that
(2980°) of the 3,3-biphenanthrene-based sila[7]helicene.[4]
For comparison, the rhodium-catalyzed double [2+2+2]
cycloaddition of biaryl-linked diyne 6 with 2 was examined
(Scheme 5). Interestingly, not the corresponding C–Si bond
cleavage product 8 but the double [2+2+2] cycloaddition
product 7 was obtained in good yields. Therefore, steric
strain of sila[7]helicene 5 might facilitate C–Si bond
cleavage.
In order to confirm the formation of (+)-4 from (+)-5,
the reaction of (+)-5 with the cationic rhodium(I)-
BArF4/(S)-segphos complex was examined. Indeed, (+)-4
was generated in good yield without racemization at room
temperature (Scheme 6).
We previously reported that the racemization of a carb-
onyl-linked 1,1Ј-bitriphenylene proceeds in (CH2Cl)2 solu-
tion at 80 °C.[5] The thermal stability of 1,1Ј-bitriphenylene-
based sila[7]helicene 5 toward racemization was examined,
which revealed that no racemization was observed in
(CH2Cl)2 solution at 80 °C for 24 hours. This observation
is consistent with the recent report by Nozaki, Nakano, and
co-workers, which discloses the extraordinarily high
tolerance of the phenanthrene-based sila[7]helicene toward
racemization.[4]
Scheme 2. Rhodium-catalyzed [2+2+2] cycloaddition of a biaryl-
linked diyne with a silyl-substituted propargylic alcohol.
were screened for the reaction of tetrayne 3[5] and diyne 2
at room temperature for 16 hours, which revealed that the
use of a cationic rhodium(I)-BArF4/(S)-segphos complex
shows the highest catalytic activity, while not the desired
sila[7]helicene but biaryl (+)-4 was obtained with a high ee
value (Scheme 3).
It is well known that transition-metal complexes react
with aryl[2-(hydroxymethyl)phenyl]dimethylsilanes to form
[1,2]oxasilole derivatives through transmetallation of in situ
generated tetraorganosilicon intermediates.[14] In order to
minimize the formation of the undesired biaryl (+)-4, the
same reaction was conducted for 1 hour to give the desired
1,1Ј-bitriphenylene-based sila[7]helicene (+)-5 in 18% yield
The X-ray crystal structure of enantiopure (+)-5 is shown
with 85% ee (Scheme 4) along with (+)-4 in ca. 10% yield. in Figure 1. The angle between two formal C=C double
–
The use of the BF4 counteranion improves the enantio- bonds of the annelated cyclopentadiene fragment is 48.1°,
selectivity, but the product yield decreases (Scheme 4). The which is larger than that for the carbonyl-linked 1,1Ј-bitri-
optical rotation value (1378°) of 5, which is calculated as phenylene (37.2°)[5] and close to that of the 3,3-biphen-
100% ee, was close to that (1087°) of the 1,1-bitriphenylene- anthrene-based sila[7]helicene (53°).[4] This feature accounts
Scheme 3. Enantioselective synthesis of 1,1Ј-bitriphenylene-based biaryl 4 by rhodium-catalyzed [2+2+2] cycloaddition.
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1,1Ј-Bitriphenylene-Based Sila[7]helicene
Scheme 4. Enantioselective synthesis of 1,1Ј-bitriphenylene-based sila[7]helicene (+)-5 by rhodium-catalyzed [2+2+2] cycloaddition.
Scheme 5. Rhodium-catalyzed [2+2+2] cycloaddition of biaryl-linked diyne 6 with silicon-linked bis(propargylic alcohol) 2.
Scheme 6. Rhodium-mediated formation of (+)-4 from (+)-5.
for the higher tolerance of 5 towards racemization. On the
other hand, the sum of the five dihedral angles [C15–C16–
C17–C18, C16–C17–C18–C19, C17–C18–C19–C36, C18–
Figure 1. X-ray crystal structure analysis of enantiopure (+)-5 with
C19–C36–C35, and C19–C36–C35–C34] is 119.1°, which is
ellipsoids at 30% probability.
significantly larger than those for the carbonyl-linked 1,1Ј-
bitriphenylene (90.7°)[5] and the 3,3-biphenanthrene-based
sila[7]helicene (99.5°).[4] Therefore, triphenylene-based
Absorption and emission spectra of 5 are shown in Fig-
sila[7]helicene 5 is more distorted than the carbonyl-linked ure 2. The absorption maximum of 5 is 269 nm, which is
1,1Ј-bitriphenylene and 3,3-biphenanthrene-based sila[7]- longer than that for the 3,3-biphenanthrene-based sila[7]-
helicene. The absolute configuration of (+)-5 was deter- helicene (ca. 260 nm)[4] presumably because of the more ex-
mined to be P by the anomalous dispersion method.[15]
tended delocalization of the π-electrons over triphenylene
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K. Tanaka et al.
SHORT COMMUNICATION
than phenanthrene. Compound 5 exhibits a strong blue alcohol) as a key step. The thus obtained 1,1Ј-bitriphen-
luminescence with λmax = 482 nm, which is also longer than ylene-based sila[7]helicene exhibits a high tolerance toward
that for the 3,3-biphenanthrene-based sila[7]helicene racemization, which could be explained by the X-ray crystal
(450 nm).[4] The fluorescence quantum yield of 5 in CHCl3 structure analysis. With respect to the photophysical prop-
is relatively high (15%) for helicene derivatives,[16] although erties, this sila[7]helicene exhibits a relatively high fluores-
those of the 3,3-biphenanthrene-based sila[7]helicene cence quantum yield and circularly polarized luminescence
(23%)[4] and the 1,1-bitriphenylene-based carbo[7]helicene activity.
(32%)[5] are higher.
Experimental Section
Typical Procedure: (Scheme 4). Under an atmosphere of argon,
[Rh(cod)2]BArF (11.8 mg, 0.010 mmol) and (S)-segphos (6.1 mg,
4
0. 010 mmol) were dissolved in CH2Cl2, and the mixture was stirred
at room temperature for 10 min. H2 was introduced to the resulting
solution in a Schlenk tube. After the mixture was stirred at room
temperature for 1 h, it was concentrated to dryness. To
a
(CH2Cl)2 (1.0 mL) solution of the residue and (8.4 mg,
2
0.0500 mmol) was added a (CH2Cl)2 (1.0 mL) solution of 3
(20.1 mg, 0.0500 mmol). The mixture was stirred at room tempera-
ture for 1 h. The resulting solution was concentrated and purified
by a preparative TLC (hexane/ethyl acetate/CH2Cl2/CH3OH =
100:60:60:3) to give (+)-5 (5.1 mg, 0.0091 mmol, 18% yield, 85%
ee).
Supporting Information (see footnote on the first page of this arti-
Figure 2. UV/Vis (blue line) and fluorescence (red line) spectra of
cle): Experimental procedures, characterization data, and copies of
5 at 7.8ϫ10–6 m in CHCl3 at 25 °C.
1
the H NMR and 13C NMR spectra are presented.
The corresponding circular dichroism (CD) spectra of
(+)-5 (91% ee) and (–)-5 (91% ee) in CHCl3 is shown in
Figure 3. It was found that these enantiomers show strong
Cotton effects, which are mirror images. Thus, we at-
tempted to measure circularly polarized luminescence
(CPL) of (+)-5, as shown in Figure 3.[17] The value is glum
= –0.016 at 482 nm, which is larger than that for the 3,3-
biphenanthrene-based sila[7]helicene (glum = –0.0035 at
470 nm)[4] but smaller than that for the 1,1-bitriphenylene-
based carbo[7]helicene (glum = –0.030 at 428 nm).[5,18]
Acknowledgments
This work was supported partly by ACT-C from Japan Science
and Technology Agency (JST, Japan). We thank Nanotechnology
Platform Project of MEXT, Japan. We also thank Mr. Yuta
Miyauchi (TUAT) for his valuable assistance, Takasago Inter-
national Corporation for the gift of Segphos, and Umicore for
generous support in supplying the rhodium complex.
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Conclusions
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Received: December 3, 2014
Published Online: January 22, 2015
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© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2015, 1409–1414