Rhodium-catalyzed enantioselective synthesis, crystal structures, and photophysical properties of helically chiral 1,1′-bitriphenylenes

Article abstract of DOI:10.1021/ja300278e

The highly enantioselective synthesis of helically chiral 1,1′-bitriphenylenes has been achieved via rhodium-catalyzed double [2 + 2 + 2] cycloaddition of biaryl-linked tetraynes with 1,4-diynes (up to 93% ee). Crystal structures and photophysical properties of these helically chiral 1,1′-bitriphenylenes have also been studied.

Full text of DOI:10.1021/ja300278e

Communication
pubs.acs.org/JACS
Rhodium-Catalyzed Enantioselective Synthesis, Crystal Structures,
and Photophysical Properties of Helically Chiral 1,1′-Bitriphenylenes
Yayoi Sawada,† Seiichi Furumi,‡,§ Atsuro Takai,∥ Masayuki Takeuchi,∥ Keiichi Noguchi,⊥
and Ken Tanaka*,†
⊥
†Department of Applied Chemistry, Graduate School of Engineering, and Instrumentation Analysis Center, Tokyo University of
Agriculture and Technology (TUAT), Koganei, Tokyo 184-8588, Japan
‡Applied Photonic Materials Group and ∥Organic Materials Group, National Institute for Materials Science (NIMS), 1-2-1 Sengen,
Tsukuba, Ibaraki 305-0047, Japan
§PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
S
* Supporting Information
Thus, we first screened various axially chiral biaryl bisphosphine
ligands (Figure 2) for the rhodium(I)-catalyzed enantioselective
ABSTRACT: The highly enantioselective synthesis of
helically chiral 1,1′-bitriphenylenes has been achieved via
rhodium-catalyzed double [2 + 2 + 2] cycloaddition of
biaryl-linked tetraynes with 1,4-diynes (up to 93% ee).
Crystal structures and photophysical properties of these
helically chiral 1,1′-bitriphenylenes have also been studied.
riphenylene and fluorene derivatives have attracted much
T
attention in materials chemistry because of their rigid,
planar, conjugated structures.1 For example, triphenylenes bear-
ing long aliphatic side chains form discotic liquid crystals
through self-assembly.2 Oligo- or polyfluorene derivatives are
employed in organic light-emitting diodes (OLEDs) and
organic field-effect transistors (OFETs).3 On the other hand,
helicenes have also attracted much attention as potential
candidates for optical or electronic functional materials because
of their unique helical chirality as well as rigid conjugated
structures.4 By combining these three types of fascinating π-
conjugated structures, we designed new [7]helicenes, helically
chiral 1,1′-bitriphenylenes, bearing both triphenylene and
fluorene cores, as shown in Figure 1.5 We anticipated that these
Figure 2. Structures of chiral bisphosphine ligands.
double [2 + 2 + 2] cycloaddition of biaryl-linked tetrayne 1a with
dialkynyl ketone 2a, leading to helically chiral 1,1′-bitripheny-
lene 3aa (Table 1, entries 1−7). Pleasingly, the reactions of 1a
and 2a proceeded at room temperature, and the use of (S)-xyl-
Segphos as a ligand furnished 3aa in good yield with the highest
ee value (entry 5). Although the Deiters group previously
reported a synthesis of triphenylenes via [2 + 2 + 2] cyclo-
addition of a biaryl-linked diyne with alkynes using a nickel
catalyst, microwave heating was necessary.8 The use of an
axially chiral biaryl bisphosphine ligand is critical. The reaction
using a chiral nonbiaryl bisphosphine ligand, (R,R)-Me-Duphos
(Figure 2), resulted in a low yield and ee value (entry 8).
The effect of the amounts of 2a and the catalyst was then
examined. Increasing the amount of 2a improved the yield of
3aa (Table 2, entry 2 vs 1). The catalyst loading could be
reduced to 10 mol % without erosion of the product ee value,
although the product yield decreased (entry 2 vs 3). Thus, the
scope of the rhodium-catalyzed enantioselective [2 + 2 + 2]
cycloaddition of biaryl-linked tetraynes 1 with dialkynyl ketones
2 was examined. Not only dihexynyl ketone 2a but also dido-
decynyl ketone 2b and phenyl-, chloro-, and benzyloxy-substituted
dipentynyl ketones 2c−e could participate in this reaction when
the cationic rhodium(I)/(S)-xyl-Segphos catalyst was used
(entries 4−7). With respect to tetraynes, not only 1a but also
Figure 1. Our design of helically chiral 1,1′-bitriphenylenes.
new [7]helicenes could be synthesized via the double [2 + 2 + 2]
cycloaddition of a biaryl-linked tetrayne with 1,4-diynes in an
enantioselective manner.6,7 In this paper, we disclose the catalytic
enantioselective synthesis, crystal structures, and optical properties
of helically chiral 1,1′-bitriphenylenes.
Our research group has reported the enantioselective syn-
thesis of helicene-like molecules via the double [2 + 2 + 2]
cycloaddition of tetraynes with dialkynyl ketones catalyzed by a
cationic rhodium(I)/axially chiral biaryl bisphosphine complex.6f
Received: January 10, 2012
Published: February 16, 2012
© 2012 American Chemical Society
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Communication
Table 1. Screening of Ligands for the Rh-Catalyzed
Scheme 1.
Enantioselective Double [2 + 2 + 2] Cycloaddition of 1a
a
with 2a
b
entry
ligand
time (h)
yield (%)
ee (%)
1
2
3
4
5
6
7
8
(S)-H8-BINAP
(S)-BINAP
16
16
16
16
16
91
16
16
72
73
67
68
63
17
71
22
39
67
74
57
91
75
86
52
(S)-Segphos
(S)-tol-Segphos
(S)-xyl-Segphos
(S)-DTBM-Segphos
(S)-Difluorophos
(R,R)-Me-Duphos
yield. Arylation of 3aa with 2-bromobiphenyl (5) followed by
dehydration9 furnished spirofluorene 6 in good yield.
Our research group recently reported the enantioselective
synthesis of helically chiral phosphafluorenes via [2 + 2 + 2]
cycloaddition of tetraynes with dialkynyl phosphine oxides cata-
lyzed by a cationic rhodium(I)/axially chiral biaryl bisphos-
phine complex.6h,10 Thus, we next examined the enantiose-
lective [2 + 2 + 2] cycloaddition of 1a with dialkynyl phosphine
oxides 2f and 2g in the presence of the cationic rhodium(I)/
axially chiral biaryl bisphosphine complex. Pleasingly, these
reactions proceeded at room temperature using (S)-Segphos as
a ligand, giving the desired helicenes 3af and 3ag in moderate
yields with good ee values (Scheme 2).
a
[Rh(cod)2]BF4 (0.010 mmol), ligand (0.010 mmol), 1a (0.050
mmol), 2a (0.060 mmol), and (CH2Cl)2 (2.0 mL) were used.
Isolated yields.
b
Table 2. Enantioselective Synthesis of Helical 1,1′-
Bitriphenylenes 3 via Rh-Catalyzed Double [2 + 2 + 2]
Cycloaddition
a
Scheme 2.
catalyst
yield ee
b
entry
1 (R1)
1a (H)
2 (R2, equiv)
2a (Me, 2)
(mol %)
3
(%) (%)
1
2
20 (+)-3aa
20 (+)-3aa
10 (+)-3aa
20 (+)-3ab
20 (+)-3ac
63
67
59
62
60
91
91
91
92
91
93
1a (H)
1a (H)
1a (H)
1a (H)
1a (H)
2a (Me, 2)
c
3
2a (Me, 2)
4
5
6
2b [(CH2)6CH3, 2]
2c (Ph, 2)
2d (Cl, 2)
20 (+)-(P)- 59
In our previous reports, racemization of a benzopyrano-fused
helical fluorenone occurred in CH2Cl2 solution at room tem-
perature6f while that of benzopyrano-fused helical phospha-
fluorenes did not occur under the same conditions, presumably
because of the longer length of the C−P bond relative to the
C−C bond.6h Similarly, racemization of fluorenone 3aa occurred
in (CH2Cl)2 solution at 80 °C (from 91% ee to 65% ee over
24 h), while no racemization of phosphafluorene 3af was observed
under the same conditions.13 Very recently, Nakano, Nozaki, and
co-workers11 synthesized λ5-phospha[7]helicenes via the double
cross-coupling reaction. They also observed the high tolerance of
the λ5-phospha[7]helicenes toward racemization. Interestingly, the
racemization of fluorene 4 in (CH2Cl)2 solution at 80 °C was
slower (from 93% ee to 81% ee over 24 h) than that of fluorenone
3aa, and no racemization of spirofluorene 6 was observed.13
To elucidate the higher tolerance of spirofluorene 6 than
fluorenone 3aa toward racemization, the crystal structures of 6
and 3aa were compared, as shown in Figures 3 and 4. The sums
of the five dihedral angles derived from the seven C−C bonds
are 90.7° for 3aa and 97.8° for 6.12 In addition, the larger angle
3ad
7
1a (H)
2e (OBn, 1.2)
20 (+)-3ae
20 (+)-3ba
10 (+)-3ba
49
74
73
91
66
65
53
de
,
8
1b (CO2n-Bu) 2a (Me, 1.2)
1b (CO2n-Bu) 2a (Me, 1.2)
1b (CO2n-Bu) 2d (Cl, 1.2)
cd f
, ,
9
d
10
20 (+)-3bd 73
a
[Rh(cod)2]BF4 (0.010 mmol), (S)-xyl-Segphos (0.010 mmol), 1
(0.050 mmol), 2 (0.060−0.10 mmol), and (CH2Cl)2 (2.0 mL) were
used. Isolated yields. [Rh(cod)2]BF4 (0.0050 mmol) and (S)-xyl-
Segphos (0.0050 mmol) were used. Ligand: (S)-Difluorophos. For
40 h. For 72 h.
b
c
d
e
f
electron-deficient tetrayne 1b reacted with dialkynyl ketones 2a
and 2d in the presence of the cationic rhodium(I)/(S)-
Difluorophos complex (10−20 mol %) to give the correspond-
ing helicenes 3ba and 3bd in high yields, although the ee values
were moderate (entries 8−10).
Fluorenone 3aa was readily derivatized into fluorene derivatives
without erosion of its ee value (Scheme 1). Hydrogenation of 3aa
in the presence of Pd/C and HCl furnished fluorene 4 in good
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Journal of the American Chemical Society
Communication
phosphafluorene 3af, and that of fluorenone 3aa was the lowest
(2.1%) among the four helicenes.16 Optical rotation values of
these helicenes were also measured. Interestingly, the optical
rotation value, which was calculated as 100% ee, was signifi-
cantly smaller for spirofluorene 6 than for 3aa, 4, and 3af.
Finally, we conducted circularly polarized luminescence (CPL)
measurements (differential emission of right circularly polarized
light vs left circularly polarized light in chiral molecular systems17)
on 4 (93% ee) and 6 (91% ee) in chloroform. Figure 5 shows the
Figure 3. Crystal structure analysis of enantiopure (+)-3aa with ellipsoids
at 30% probability. The sum of the five dihedral angles is 90.7°.
Figure 5. (a) CD and CPL spectra of (+)-(P)-4 (blue line) and
(−)-(M)-4 (red line) at 2 × 10−5 M (10 mm path length) in CHCl3
at 25 °C. (b) UV−vis and fluorescence spectra of (+)-(P)-4 in CHCl3
at 25 °C. The excitation wavelengths for fluorescence and CPL
measurements were 350 and 375 nm, respectively.
Figure 4. Crystal structure analysis of enantiopure (+)-6 with ellipsoids
at 30% probability. The sum of the five dihedral angles is 97.8°.
between the two formal CC double bonds of the annelated
cyclopentadiene fragment (Figures 3 and 4) in 6 (39.6°)
relative to 3aa [37.2° (average)] induces a larger overlap of the
two terminal benzene rings in 6 than in 3aa, resulting in a larger
steric repulsion.13,14 Indeed, the distances between two terminal
benzene rings are 4.716 and 4.685 Å for 3aa and 4.900 and 4.958
Å for 6. These larger distortions of 6 account for the higher
tolerance toward racemization. The absolute configuration of
helicene (+)-3ad possessing two chlorine atoms was determined
to be P by the anomalous dispersion method.15
corresponding circular dichroism (CD), CPL, UV−vis absorption,
and fluorescence spectra of 4. Compounds 4 and 6 exhibited CPL
activities, as the CPL spectra of (+)-(P) and (−)-(M) helicenes are
mirror images. The degree of CPL is given by the luminescence
dissymmetry ratio, which is defined as glum = 2(IL − IR)/(IL + IR),
where IL and IR are the luminescence intensities of left and right
circularly polarized light, respectively. The values were glum
=
−0.030 at 428 nm for (−)-(M)-4 and glum = −0.032 at 449 nm for
(−)-(M)-615 in chloroform, which are comparable to those for our
recently reported phthalhydrazide-functionalized [7]helicene-like
molecule18 (glum = −0.035 at 476 nm for the assembly state and
−0.021 for the molecularly dispersed state) and significantly larger
than those for helically chiral molecules reported to date.19
In conclusion, we have achieved the highly enantioselective
synthesis of helically chiral 1,1′-bitriphenylenes via rhodium-
catalyzed double [2 + 2 + 2] cycloaddition. The present method
realized the highest level of enantioselectivity in the synthesis of
helicenes reported to date. Crystal structure analysis of the
[7]helicene containing the spirofluorene moiety revealed a large
overlap and distortion of the two terminal benzene rings, which
account for the high tolerance of this helicene toward racemi-
zation. The CPL properties of [7]helicenes containing the fluorene
or spirofluorene moiety are significantly larger than those of
helicene derivatives reported to date. Future work will focus on
further functionalization of these helicenes to enhance their
chiroptical properties20,21 and application of this methodology
Absorption and emission data for representative helically
chiral 1,1′-bitriphenylenes 3aa, 4, 6, and 3af are shown in Table 3.
Table 3. Photophysical Properties of (+)-3aa, (+)-4, (+)-6,
a
and (+)-3af
absorption
fluorescence
cd
,
b
cd
,
e
compound
λmax (nm)
λmax (nm)
ϕF
[α]D25
(+)-3aa
(+)-4
292, 361
572
428
449
0.021
0.320
0.296
1397
1087
684
261, 364, 388
262, 370, 400
273, 370, 388
(+)-6
(+)-3af
487
0.218
1230
a
b
c
d
Measured in CHCl3 at 25 °C. 2 × 10−5 M. 3 × 10−5 M. Excited at
e
350 nm. Values were calculated as 100% ee.
Fluorenone 3aa and phosphafluorene 3af showed large red shifts
of the absorption and emission maxima compared with
fluorenes 4 and 6. Fluorenes 4 and 6 showed higher quantum
yields (32.0% and 29.6%, respectively) in CHCl3 solution than
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Stary, I. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 13169. (h) Fukawa,
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to the synthesis of various heteroatom-bridged helically chiral
1,1′-bitriphenylenes.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, compound characterization data, and
X-ray crystallographic information files. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
́ ́
(c) Carreno, M. C.; Gonzalez-Lopez, M.; Urbano, A. Chem. Commun.
̃
2005, 611. (d) Rajca, A.; Miyasaka, M.; Pink, M.; Wang, H.; Rajca, S.
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AUTHOR INFORMATION
Corresponding Author
tanaka-k@cc.tuat.ac.jp
■
44, 2167. (f) Carreno, M. C.; García-Cerrada, S.; Urbano, A. Chem.
̃
Eur. J. 2003, 9, 4118.
(8) McIver, A.; Young, D. D.; Deiters, A. Chem. Commun. 2008,
Notes
4750.
̀
(9) Thirion, D.; Poriel, C.; Rault-Berthelot, J.; Barriere, F.; Jeannin,
The authors declare no competing financial interest.
O. Chem.Eur. J 2010, 16, 13646.
(10) Syntheses of heterofluorenes via transition-metal-catalyzed [2 + 2 + 2]
cycloadditions of heteroatom-bridged 1,6-diynes with monoynes have
been reported. For carbazoles, see: (a) Witulski, B.; Alayrac, C. Angew.
Chem., Int. Ed. 2002, 41, 3281. (b) Alayrac, C.; Schollmeyer, D.; Witulski,
B. Chem. Commun. 2009, 1464. For silafluorenes, see: (c) Matsuda, T.;
Kadowaki, S.; Goya, T.; Murakami, M. Org. Lett. 2007, 9, 133. For diben-
zofurans and azadibenzofurans, see: (d) Komine, Y.; Kamisawa, A.; Tanaka,
K. Org. Lett. 2009, 11, 2361.
ACKNOWLEDGMENTS
■
This work was partially supported by a Grant-in-Aid for
Scientific Research (20675002) from MEXT, Japan. We thank
the Nanotechnology Network Project of MEXT, Japan. We
also thank Dr. Takeshi Suda (TUAT) for his valuable
assistance, Takasago International Corporation for the gift of
Segphos and H8-BINAP derivatives, and Umicore for generous
support in supplying a rhodium complex.
(11) Nakano, K.; Oyama, H.; Nishimura, Y.; Nakasako, S.; Nozaki, K.
Angew. Chem., Int. Ed. 2012, 51, 695.
(12) Nakano, Nozaki, and co-workers11 reported that the sums of the
five dihedral angles of oxa-, aza-, and thia[7]helicenes were 79−88°.
On the other hand, that of λ5-phospha[7]helicene was 95−100°.
(13) For discussion regarding this relationship, See ref 11 and
Groen, M. B.; Schadenberg, H.; Wynberg, H. J. Org. Chem. 1971, 36,
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̌
́
I.;
4083
dx.doi.org/10.1021/ja300278e | J. Am. Chem. Soc. 2012, 134, 4080−4083