Enantioselective Synthesis of Polycyclic Aromatic Hydrocarbon (PAH)-Based Planar Chiral Bent Cyclophanes by Rhodium-Catalyzed [2+2+2] Cycloaddition

Article abstract of DOI:10.1002/chem.202001450

The enantioselective synthesis of polycyclic aromatic hydrocarbon (PAH)-based planar chiral cyclophanes was achieved for the first time by the rhodium-catalyzed intramolecular regio- and enantioselective [2+2+2] cycloaddition of tethered diyne-benzofulven

Full text of DOI:10.1002/chem.202001450

Accepted Article
Accepted Article
Title: Enantioselective Synthesis of Polycyclic Aromatic Hydrocarbon
(PAH)-Based Planar Chiral Bent Cyclophanes by Rhodium-
Catalyzed [2+2+2] Cycloaddition
Authors: Yukimasa Aida, Juntaro Nogami, Haruki Sugiyama, Hidehiro
Uekusa, and Ken Tanaka
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To be cited as: Chem. Eur. J. 10.1002/chem.202001450
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01/2020

10.1002/chem.202001450
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Enantioselective Synthesis of Polycyclic Aromatic Hydrocarbon
(PAH)-Based Planar Chiral Bent Cyclophanes by Rhodium-
Catalyzed [2+2+2] Cycloaddition
Yukimasa Aida,[a] Juntaro Nogami,[a] Haruki Sugiyama,[b] Hidehiro Uekusa,[c] and Ken Tanaka*,[a]
Abstract: The enantioselective synthesis of polycyclic aromatic
hydrocarbon (PAH)-based planar chiral cyclophanes was achieved
for the first time by the rhodium-catalyzed intramolecular regio- and
enantioselective [2+2+2] cycloaddition of tethered diyne-
benzofulvenes followed by stepwise oxidative transformations. The
thus synthesized planar chiral bent cyclophanes, that possess bent
p-terphenyl- and 9-fluorenone-cores, were converted to 9-fluorenol-
based ones with excellent ee values of >99% by diastereoselective
1,2-reduction. These 9-fluorenol-based cyclophanes exhibited high
fluorescence quantum yields, which were significantly higher than
that of an acyclic reference molecule (78–82% vs. 48%). The
bending effect on the chiroptical property was also examined, which
revealed that the anisotropy factors (gabs values) for electronic
circular dichroism (ECD) of these 9-fluorenol-based planar chiral
bent cyclophanes increase as the tether length becomes shorter.
examples of the synthesis of PAH-based chiral bent
13]
cyclophanes are quite limited.[11
–
There are two main strategies for the synthesis of the PAH-
based distorted cyclophanes. The first strategy is the
construction of an ansa chain (Type-1: Scheme 1a, left), and the
second strategy is the aromatization of a bent PAH precursor
(Type-2: Scheme 1a, right).[10] The Type-1 strategy enables the
synthesis of PAH-based chiral twisted cyclophanes. For
example, Würthner reported the racemic synthesis of PBI
(perylenebisimide)-based chiral twisted cyclophanes by the
construction of two oligo(ethylene glycol) ansa chains.[14a] These
PBI-based cyclophanes were resolved by a chiral HPLC column,
and their photophysical and chiroptical properties were
measured by UV-vis absorption, emission, and ECD (electronic
circular dichroism) spectroscopy.[14b] Subsequently, their unique
chiral self-recognition and self-discrimination properties were
also elucidated.[14c,14d] Recently, Gidron succeeded in the
racemic synthesis of chiral twisted anthracenophanes, showing
the large anisotropy factors (gabs values) for ECD, by the
Introduction
construction of the ansa-chain.[14e g]
A cyclophane is a macrocyclic molecule consisting of a
mancude-ring and an ansa-chain, some of which possess ring
strain and/or chirality.[1] Due to such unique structural features,
cyclophane derivatives have been investigated in the broad area
of chemistry, such as structural organic chemistry,[2] host-guest
chemistry,[3] and asymmetric catalysis.[4] In 1899, Pellegrin
achieved the first synthesis of a cyclophane consisting of two
benzene rings and two ethylene tethers.[5] After this report,
numerous examples of the synthesis of benzene-based
cyclophanes have been reported to date.[6] Not only benzene-
based cyclophanes but also PAH (polycyclic aromatic
hydrocarbon)-based ones, including those possessing four[7] or
more[8] annelated rings,[9] have been synthesized to date.[10]
Among them, PAH-based chiral bent cyclophanes, that possess
bent aromatic systems, are of particular interest as a subunit
model of chiral carbon nanotubes. The bending effect on the
structural, optoelectronic, and chiroptical properties is interesting
in chemistry and materials science. However, the successful
−
The Type-2 strategy enables the synthesis of PAH-based
chiral
macrocyclization to form a bent PAH precursor with small strain
and the subsequent aromatization to form PAH-based
bent
cyclophanes.[11]
A
combination
of
the
a
cyclophane with large strain is a powerful strategy, which
enables the construction of a strained bent structure as well as
π-extension (Scheme 1b, top).[11] For example, in 2012, Bodwell
reported the racemic synthesis of pyrene-based chiral bent
cyclophanes via the McMurry coupling followed by the VID
(valence isomerization dehydrogenation) reaction, although the
product yield was low (12%) (Scheme 1b, middle).[11b] In 2018,
Esser reported the racemic synthesis of dibenzopentalene-
based chiral bent cyclophanes via the Suzuki-Miyaura cross-
coupling followed by arylation and dehydration reactions with
improved product yields of 14–36% (Scheme 1b, bottom).[11c]
However, in these examples, the optical resolution of racemates
by chiral HPLC columns was required to obtain enantiopure
cyclophanes.[11a,11b,14a–e] Thus, the method that enables the
enantioselective synthesis, as well as the construction of the π-
[a]
Y. Aida, J. Nogami, Prof. Dr. K. Tanaka
extended bent structure, has been awaited.[15 18]
Department of Chemical Science and Engineering, Tokyo Institute
of Technology, O-okayama, Meguro-ku, Tokyo 152-8550, Japan
E-mail: ktanaka@apc.titech.ac.jp
−
On the other hand, the enantioselective synthesis of chiral
benzene-based cyclophanes has been achieved by the rhodium-
catalyzed [2+2+2] cycloaddition[19,20] of alkynes (Scheme
1c).[21,22] In 2007, our research group reported the first example
in the enantioselective synthesis of metacyclophanes with up to
>98% ee by the cationic rhodium(I)/(R)-H8-binap complex-
catalyzed intramolecular [2+2+2] cycloaddition of unsymmetric
triynes, although the regioselectivity was low.[22a] After this report,
in 2009, Shibata reported the enantioselective synthesis of cage
http://www.apc.titech.ac.jp/~ktanaka/
[b]
[c]
Dr. H. Sugiyama
Research and Education Center for Natural Sciences, Keio
University, Hiyoshi 4-1-1, Kohoku, Yokohama, Japan
Prof. Dr. H. Uekusa
Department of Chemistry, Tokyo Institute of Technology, O-
okayama, Meguro-ku, Tokyo 152-8550, Japan
Supporting information for this article is given via a link at the end of
the document.
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a) Strategies for synthesis of PAH-based cyclophanes
PAH
precursor
Type-1
Type-2
PAH
PAH
PAH
b) Previous works: racemic synthesis of PAH-based chiral bent cyclophanes (Type-2)
Bodwell's
route
This strategy enables...
aromatization
PAH
precursor
1) !-extension
macrocyclization
PAH
2) construction of strained bent structure
Esser's
small strain
route
large strain
Bodwell (2012)
O
OHC
CHO
McMurry
coupling
VID reaction
12% yield (one-pot)
racemate
O
O
O
OHC
O
O
CHO
H
CF3
Ar
Esser (2018)
O
H
Ar =
arylation
&
dehydration
,
O
Suzuki-Miyaura
coupling
H
O
CF3
Br
Ar
Br
14–36% yields (3 steps)
racemate
O
H
O
O
O
O
c) Previous works: catalytic enantioselective synthesis of chiral cyclophanes
Shibata (2009)
Tanaka (2012)
(CH2)n
(CH2)n
O
OMe
R
R1
RhI+/(S,S)-BDPP
catalyst
RhI+/(S,S)-Me-Duphos
catalyst
N
R2
OMe
R2
N
R1N
(CH2)n
O (CH2)n
N
R
CH2Cl2, RT
(CH2Cl)2, 80 °C
R
R
n = 10–12
up to 91% yield
up to 93% ee
n = 4–6, 10
up to 95% yield
up to 98% ee
enantioselective
[2+2+2] cycloaddition
enantioselective
[2+2+2] cycloaddition
d) This work: catalytic enantioselective synthesis of PAH-based chiral bent cyclophanes
This strategy enables...
chiral RhI+
catalyst
oxidative
aromatization
R1
1) !-extension (p-terphenyl)
R1
X
X
R1
X
2) construction of strained bent structure
3) catalytic enantioselective synthesis
29–60% yields (3 steps), >99% ee
regio- and
enantioselective
[2+2+2]
R2
Z
Z
Z
R2
H
H
R2
C (large strain)
A
cycloaddition
B (small strain)
Scheme 1. Research background.
compounds with up to 98% ee by the cationic rhodium(I)/(S,S)-
Me-duphos [2,2’,5,5’-tetramethyl-1,1’-(o-
synthesis of benzene-based planar chiral cyclophanes but could
not be applied to the synthesis of PAH-based ones.
phenylene)diphospholane] complex-catalyzed intramolecular
[2+2+2] cycloaddition of symmetric triynes (Scheme 1c, left).[22c]
In 2013, our research group reported the enantioselective
synthesis of paracyclophanes with up to 93% ee by the cationic
Inspired by these previous works (Schemes 1b and 1c), we
anticipated that the intramolecular regio- and enantioselective
[2+2+2] cycloaddition of endiyne A, consisting of an
unsymmetric benzene-linked diyne unit and a benzo-fused
unsymmetric cyclic alkene unit linked by an alkyl chain, would
afford centrochiral and planar chiral dibenzo-fused bent
cyclohexadiene B with small strain. The subsequent oxidative
rhodium(I)/(S,S)-bdpp
[2,4-bis(diphenylphosphino)pentane]
complex-catalyzed intermolecular [2+2+2] cycloaddition of
symmetric cyclic diynes with terminal monoynes (Scheme 1c,
right).[22h] These reactions enabled the highly enantioselective
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aromatization would afford planar chiral cyclophane C with a π-
extended bent mancude-ring (annelated p-terphenyl core) with
large strain (Scheme 1d).
based planar chiral bent cyclophanes in good overall yields of
29–60% with complete regio- and enantioselectivity (>99% ee).
The crystal structures and photophysical/chiroptical properties of
the thus synthesized cyclophanes are also disclosed.
a) Enantioselective [2+2+2] cycloaddition of biphenyl-linked diyne with indene
RhI+/(R)-difluorphos
catalyst
CO2Me
H
CO2Me
CO2Me
+
Results and Discussion
CH2Cl2, 40 °C
MeO2C
H
We first examined the intermolecular [2+2+2] cycloaddition
of unsymmetric biphenyl-linked diyne 1b bearing the methyl and
methoxycarbonyl groups at the alkyne termini with benzofulvene
2d under the previously reported conditions[23] used for 1a and
2a shown in Scheme 2a. Pleasingly, the desired cyclohexadiene
3bd was obtained in moderate yield with an excellent ee value
of >99% (Table 1, entry 1).[30] Screening of axially chiral biaryl
diphosphine ligands (entries 2−5) revealed that the use of (R)-
segphos afforded 3bd in almost the same yield as when using
(R)-difluorphos, that has a small dihedral angle like (R)-segphos
(entry 2). Tuning the steric bulk of the aryl groups on the
phosphorus atom in segphos (entries 6–8) revealed that the use
of (R)-xyl-segphos improved the yield of 3bd to 67% while
maintaining the ee value (entry 7). Thus, (R)-xyl-Segphos was
selected as the best ligand. Further improvement of the product
1a
2a
3aa / 96%, 92% ee
b) Unsuitable indene derivatives
RhI+ catalyst
R
R
O
2b
2b'
2c (unreactive)
c) Substrate design
(CH2)n
X = C6H4, Z = C=CHR3
in Scheme 1d
O
R1
R2
4
R1 or R2 = CO2Me
R1 or R2 ≠ CO2Me
O
regioselectivity ?
enantioselectivity ?
R3
Table 1. Regio- and enantioselective [2+2+2] cycloaddition of unsymmetrical
diyne 1b with benzofulvenes 2[a]
Scheme 2. Synthesis design.
20 mol %
[Rh(cod)2]BF4/
Ligand
Me
Me
H
CH2Cl2, 40 °C , 6 h
CO2Me
To realize this concept, it is necessary to develop the highly
regio- and enantioselective [2+2+2] cycloaddition reaction of an
unsymmetric benzene-linked diyne with an unsymmetric benzo-
fused cyclic alkene. However, although several examples of the
transition-metal-catalyzed enantioselective [2+2+2] cycloaddition
of symmetric diynes with unsymmetric cyclic alkenes[23] or
unsymmetric diynes with symmetric cyclic alkenes[24] have been
MeO2C
R
H
3
1b
2
R
(5 equiv)
O
(R)-difluorphos (Z = CF2, Ar = Ph)
(R)-segphos (Z = CH2, Ar = Ph)
Z
Z
O
O
PAr2
PAr2
(R)-tol-segphos (Z = CH2, Ar = 4-MeC6H4)
(R)-xyl-segphos (Z = CH2, Ar = 3,5-Me2C6H3)
(R)-dtbm-segphos (Z = CH2, Ar = 4-tBu-3,5-Me2C6H2)
reported,
the
transition-metal-catalyzed
regio-
and
O
enantioselective [2+2+2] cycloaddition of unsymmetric diynes
with unsymmetric cyclic alkenes has been reported in only one
example with moderate yield and ee value.[25] As promising
substrates, we have recently reported the cationic
rhodium(I)/(R)-difluorphos complex-catalyzed enantioselective
[2+2+2] cycloaddition of symmetric biphenyl-linked diyne 1a with
indene (2a), which affords the corresponding dibenzo-fused bent
cyclohexadiene 3aa with high yield and ee value (Scheme
2a).[23] We anticipated that an unsymmetric biphenyl-linked diyne
and indene might be suitable diyne and alkene units for endiyne
A shown in Scheme 1d. However, there is concern about the
cationic rhodium(I) complex-catalyzed alkene isomerization in
substituted indene 2b giving regioisomer 2b’ (Scheme 2b).[26] As
a non-isomerizable alkene, indenone (2c) was attractive, but 2c
failed to react with diynes in the presence of cationic
rhodium(I)/diphosphine complexes.[27] Thus, we came up with
tethered diyne-benzofulvene 4, consisting of an unsymmetric
biphenyl-linked diyne unit, in which one alkyne terminus is the
methoxycarbonyl group, and a benzofulvene[28,29] unit linked by
an alkyl chain, as a suitable substrate for this synthetic strategy
(Scheme 2c). Herein, we have established that this strategy
realizes the first catalytic enantioselective synthesis of PAH-
MeO
MeO
PPh2
PPh2
PPh2
PPh2
PPh2
PPh2
(R)-MeO-biphep
(R)-binap
(R)-H8-binap
Entry
Ligand
2 (R)
3
% yield[b] (% ee)
48 (>99)
49 (>99)
46 (>99)
28 (97)
1
(R)-difluorphos
(R)-segphos
2d (H)
2d (H)
2d (H)
2d (H)
2d (H)
2d (H)
2d (H)
2d (H)
2e (OMe)
2e (OMe)
(+)-3bd
(+)-3bd
(+)-3bd
(+)-3bd
(+)-3bd
(+)-3bd
(+)-3bd
(+)-3bd
(+)-3be
(+)-3be
2
3
(R)-MeO-biphep
(R)-binap
4
5
(R)-H8- binap
(R)-tol- segphos
(R)-xyl- segphos
(R)-dtbm- segphos
(R)-xyl- segphos
(R)-xyl- segphos
38 (95)
6
40 (98)
7
67 (>99)
18 (52)
8
9
10c
75 (>99)
75 (>99)
[a] [Rh(cod)2]BF4 (0.040 mmol), ligand (0.040 mmol), 1b (0.20 mmol), 2 (1.00
mmol), and CH2Cl2 (2.0 mL) were used. [b] Isolated yield. [c] Reaction was
conducted using [Rh(cod)2]BF4 (0.010 mmol), ligand (0.010 mmol), 1b (0.20
mmol), 2e (1.00 mmol), and CH2Cl2 (2.0 mL) at RT for 24 h.
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a) Synthesis of diyne 10
yield was realized by using electron-rich 4-methoxyphenyl-
substituted benzofulvene 2e (entry 9), presumably due to its
enhanced reactivity towards electron-poor diyne 1b. Finally, the
catalyst loading could be reduced to 5 mol % and the reaction
temperature could be lowered to room temperature by prolonged
reaction time (24 h) without erosion of the product yield and ee
value (entry 10).
Br
Me
a,b
I
Si(i-Pr)3
Cl
5
Cl
B(OH)2 Me
6
7 / 62%
c,d
O
O
O
B
O
8
With the optimized reaction conditions and cycloaddition
partners in hand, we next challenged the enantioselective
intramolecular [2+2+2] cycloaddition of tethered diyne-
benzofulvenes 4 giving the desired centrochiral and planar chiral
macrocycles (dibenzo-fused cyclohexadienes). The synthesis of
tethered diyne-benzofulvenes 4 is shown in Scheme 3. First,
diyne part 10 was synthesized as shown in Scheme 3a. The
Sonogashira cross-coupling of 5 with triisopropylsilylacetylene
followed by the Suzuki-Miyaura cross-coupling with aryl boronic
acid 6 furnished diyne 7. The Suzuki-Miyaura cross-coupling of
7 with aryl boronate 8 followed by desilylation furnished crude
terminal alkyne 9. The methoxycarboxylation of this crude 9
followed by removal of the tetrahydropyranyl group furnished
diyne 10. Next, benzofulvene part (E)-17 was synthesized as
shown in Scheme 3b. The propargylation of aldehyde 11 with
propargylzinc followed by silylation furnished alkyne 12. The
palladium-catalyzed diarylation of the alkyne moiety of 12 with
the aryl bromide moiety of 12 and aryl boronic acid 13 furnished
trisubstituted alkene 14. The Suzuki-Miyaura cross-coupling of
14 with aryl boronate 15 followed by desilylation furnished diol
16. Dehydration of 16 followed by E/Z isomerization with CSA
furnished (E)-17. Finally, diyne part 10 was coupled stepwisely
with dibromides 18a–h and (E)-17 furnished tethered diyne-
benzofulvenes 4a–h (Scheme 3c).
Me
CO2Me
Me
e,f
OH
O
O
9
10 / 63% from 7
a: triisopropylsilylacetylene, Pd(PPh3)2Cl2, CuI, THF/Et3N, RT
b: 6, PdCl2(PPh3)2, K2CO3, toluene/EtOH/H2O, 80 °C
c: 8, Pd(OAc)2/Sphos, K3PO4, toluene/H2O, 100 °C d: TBAF, THF, 0 °C to RT
e: nBuLi, ClCO2Me, THF, !78 °C to RT f: PPTS, MeOH/THF, 0 °C to RT
b) Synthesis of benzofulvene (E)-17
tBuPh2SiO
tBuPh2SiO
Cl
Cl
O
Cl
c
a,b
H
Br
12
B(OH)2
Br
OMe
11
13
14 / 72% from 11
OMe
tBuPh2SiO
O
d,e
B
O
HO
15
HO
HO
f
OMe
16 / 97% from 14
17
MeO
E/Z = 14:86 / 69%
E/Z = >99:1 / 51%
The results of the rhodium-catalyzed enantioselective
intramolecular [2+2+2] cycloaddition of 4a–h are shown in Table
2. We first examined the reaction of nonylene [(CH2)9]-tethered
substrate 4a under the above-optimized reaction conditions.
Gratifyingly, the desired reaction proceeded to give the
corresponding chiral macrocycle 20a in high yield with perfect
regio- and enantioselectivity (entry 1). Not only nonylene (4a)
but also other shorter alkylene [(CH2)n, n = 8–3]-tethered
substrates 4b−g were successfully converted to the
corresponding chiral macrocycles 20b−g as single regioisomers
in moderate to high yields with excellent ee values of 97–>99%
(entries 2–8). Besides, triethylene glycoxy-tethered substrate 4h
could also participate in this transformation to give chiral
macrocycle 20h in high yield with an excellent ee value (entry 9).
As shown in entries 3 and 4, both enantiomers could be
synthesized by switching the ligand chirality. Additionally,
racemates could also be synthesized by using commercially
available (±)-xyl-segphos as a ligand.
g
a: propargyl bromide, zinc powder, THF, 0 °C to RT
b: tert-butyldiphenylchlorosilane, imidazole, DMF, 80 °C
c: 13, Pd(PPh3)4, Cs2CO3, EtOH, 80 °C d: 15, Pd(OAc)2/Sphos, K3PO4, toluene/H2O, 100 °C
e: TBAF, THF, 0 °C to RT f: CSA, toluene, 110 °C g: CSA, toluene, 110 °C
c) Synthesis of tethered diyne-benzofulvenes 4
19a / 61%
19b / 70%
X
18a–h
Br
K2CO3
Me
Br
19c / 74%
19d / 77%
X
10
O
CO2Me
Br
19e / >99%
19f / 82%
DMF, 90 °C
18a [X = (CH2)9]
18b [X = (CH2)8]
18c [X = (CH2)7]
18d [X = (CH2)6]
18e [X = (CH2)5]
18f [X = (CH2)4]
18g [X = (CH2)3]
19g / >99%
19h / not isolated
19a–h
(E)-17, K2CO3
DMF, 90 °C
18h [X = CH2CH2OCH2CH2OCH2CH2]
X
4a / 81%
4b / 76%
O
4c / 74%
4d / 85%
Me
The oxidative aromatization of the thus obtained nonylene-
tethered chiral cyclohexadiene 20a with DDQ followed by
oxidative cleavage of the exo-alkene moiety with RuCl3/NaIO4
afforded the corresponding fluorenone-based planar chiral
cyclophane 21a in good yield (entry 1). Not only nonylene (20a)
but also other shorter alkylene-tethered chiral cyclohexadienes
20b−d could be aromatized by the above stepwise oxidation
reactions to give the corresponding fluorenone-based planar
4e / 63%
4f / 77%
O
CO2Me
4g / 69%
4h / 41% from 10
4a–h
MeO
Scheme 3. Synthesis of tethered diyne-benzofulvene 4. PPTS = pyridinium p-
toluenesulfonate. TBAF = Tetra-n-butylammonium fluoride. CSA = camphor
sulfonic acid.
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Table 2. Enantioselective synthesis of PAH-based planar chiral bent cyclophanes 21 and 22.
X
X
O
X
O
O
O
O
20 mol %
[Rh(cod)2]BF4/
(R)-xyl-segphos
Me
Me
H
Me
1) DDQ, CH2Cl2, RT
H
2) 20 mol % RuCl3·H2O
NaIO4, CCl4/MeCN/H2O
0 °C
CH2Cl2, RT , 24 h
O
CO2Me
MeO
MeO
O
O
O
20 / single regioisomer
4
21
NaBH4
MeO
X
THF/MeOH
0 °C to RT
MeO
O
O
Me
MeO
OH
O
22 / single diastereomer
entry
4 (X)
20 / % yield[a] (% ee)
21 / % yield[a] (% ee)
22 / % yield[a] (% ee)
(+)-22a / >99 (>99)[b]
(+)-22b / >99 (>99)
(+)-22c / 84 (>99)
(–)-22c / 80 (>99)
(+)-22d / 86 (>99)
–
1
2
3
4
5
6
7
8
9
4a [(CH2)9]
4b [(CH2)8]
4c [(CH2)7]
4c [(CH2)7]
4d [(CH2)6]
4e [(CH2)5]
4f [(CH2)4]
4g [(CH2)3]
(–)-20a / 74 (>99)
(–)-20b / 61 (>99)
(–)-20c / 45 (>99)
(+)-20c / 52 (>99)[c]
(–)-20d / 78 (>99)
(–)-20e / 86 (>99)
(–)-20f / 54 (97)
(+)-21a / 67 (>99)
(+)-21b / 65 (>99)
(+)-21c / 64 (>99)
(–)-21c / 59 (>99)
(+)-21d / 79 (>99)
21e / 0
–
–
(+)-20g / 61 (97)[c]
–
–
4h (CH2CH2OCH2CH2OCH2CH2)
(–)-20h / 85 (98)
(+)-21h / 71 (>99)
(+)-22h / 86 (>99)
[a] Isolated yield. [b] At -20 °C. [c] (S)-xyl-segphos was used. DDQ = 2,3-dichloro-5,6-dicyano-p-benzoquinone.
O
based cyclophanes 22a−d and 22h, which would show intense
emission, as single diastereomers by 1,2-reduction with NaBH4
(entries 1–4 and 8). Importantly, no racemization was observed
in these oxidative and reductive transformations. Furthermore,
these cyclophanes are configurationally stable, and no
epimerization and racemization was observed even for (+)-22a
possessing the longest ansa chain in o-C6D4Cl2 under Ar at
170 °C.
O
O
O
O
Me
N
H
O
Me
O
H
H
PhCl, 130 °C
MeO
O
H
O
N
OMe
O
(+)-20g (97% ee)
(–)-23 / 42%, 97% ee
single diastereomer
MeO
MeO
Ar'
Me
Ar'
Ar' =
P
P
O
Scheme 4. Aromatization by Diels-Alder reaction.
+
Rh
Ar'
Me
Ar'
Me
OMe
Ar'
chiral cyclophanes 21b−d in good yields (entries 2−5).
Triethylene glycoxy-tethered chiral macrocycle 20h could also
be aromatized under the same conditions to give the
corresponding fluorenone-based planar chiral cyclophane 21h in
good yield (entry 9). Unfortunately, treatment of pentylene-
tethered chiral macrocycle 20e with DDQ failed to afford the
corresponding fluorenone-based cyclophane 21e and led to an
unidentified mixture of products (entry 6). The undesired strain-
relieving carbocation rearrangement might proceed in the
oxidation of 20e with DDQ.[31] Thus, Esser’s dehydrative
aromatization method using the Burgess reagent [methyl N-
(triethylammoniumsulfonyl)carbamate], in which no carbocations
are formed, is advantageous for the aromatization of highly bent
structures.[11c] As an alternative aromatization method for the
highly bent dibenzo-fused cyclohexadiene, the Diels-Alder
reaction of 20g with N-phenylmaleimide afforded the
corresponding phenanthrene-based cyclophane 23 as a single
diastereomer (Scheme 4). Fluorenone-based cyclophanes 21a−
d and 21h could be converted to the corresponding fluorenol-
Me
Ar'
CO2Me
P
P
+
Rh
Ar'
Me
Ar'
Ar
RhI+
D
MeO2C
Ar
Ar'
E
4c
Ar'
Me
P
P
+
Rh
Ar
Ar'
–RhI+
Ar'
Ar =
OMe
MeO2C
O
Ar
F
O
Me
H
H
MeO
(29aS,214aS)-(–)-20c
O
Ar
Scheme 5. Plausible reaction mechanism.
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10.1002/chem.202001450
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Reductive elimination of Rh affords (29aS,214aS)-(–)-20c.
Alternatively, the regio- and enantioselective reaction of the
electron-deficient alkyne moiety and the cycloalkene moiety of
4c with the cationic rhodium(I)/(R)-xyl-segphos complex would
generate five-membered rhodacycle F due to avoidance of steric
repulsion similar to the formation of E from D. Alkyne insertion
gives the same seven-membered rhodacycle E. Although it is
not clear which is the main pathway, either mechanism can
explain the observed high regio- and enantioselectivities.
(+)-22a
(±)-22b
(Sp,R)-(+)-22c
(±)-22d
Single crystals of (+)-22a,[32] (±)-22b,[32] (+)-22c,[32] and (±)-
22d[32] were obtained by recrystallization from n-hexane/CHCl3
or THF, and their structures were determined by X-ray
crystallographic analyses (Figure 1). The absolute and relative
configurations of (+)-22c could also be determined to be Sp,R. In
these cyclophanes, two phenoxy moieties were perpendicular to
the p-terphenyl cores and the bend of the p-terphenyl cores
became larger as the tether length became shorter.
Figure 1. ORTEP drawings of 22a–d with ellipsoids at 50% propability.
(a)
(b)
Table 3. Bend and torsion angles of 22 and 24.[a]
torsion angles
bend angles
R1
R2
O
O
"
"'
Me
b1
B
b2
!
!'
a1
C
A
a2
MeO
OH
O
(Sp,R)-22 [R1,R2 = (CH2)n]: 22a (n = 9), 22b (n = 8), 22c (n = 7), 22d (n = 6)
(R)-24 (R1 = R2 = Me)
A[b]
B[b]
C[b]
Average
(c)
(d)
Compd
α,α’ / β,β’
α,α’ / β,β’
α,α’ / β,β’
α,α’ / β,β’
(R)-24
2.3 / 3.4
4.9 / 3.4
3.9 / 8.5
4.3 / 8.3
5.9 / 8.1
6.9 / 8.9
3.4 / 11.3
11.2 / 12.0
10.9 / 12.5
12.4 / 14.7
1.6 / 7.6
5.6 / 8.1
4.6 / 8.6
5.5 / 9.1
8.2 / 11.1
3.6 / 6.6
4.6 / 7.6
6.6 / 9.7
6.9 / 10.0
8.8 / 11.3
(Sp,R)-22a
(Sp,R)-22b
(Sp,R)-22c
(Sp,R)-22d
Compd
(R)-24
a1 / a2
b1 / b2
Average
8.5
16.3 / 15.4
10.8 / 12.9
12.7 / 12.8
10.5 / 9.8
7.9 / 7.2
1.1 / 1.0
4.1 / 0.3
3.8 / 0.4
1.6 / 1.0
2.5 / 3.4
(Sp,R)-22a
(Sp,R)-22b
(Sp,R)-22c
(Sp,R)-22d
7.0
Figure 2. X-ray structurses of (Sp,R)-(+)-22c obtained from CHCl3/n-hexane.
Crystal structures: (a) Three (Sp,R)-(+)-22c in single crystal; (b) overlapping
three (Sp,R)-(+)-22c (red, blue, and green) molecules for comparison of their
conformations. Packing structures: (c) top view; (d) side view. The solvent
molecules are omitted for clarity. The yellow and grey spaces are occupied by
solvent molecules.
7.4
5.7
5.3
[a] Determined by DFT calculations at the B3LYP/6-31G(d) level. [b]
Averages of α,α’ and β,β’ are shown.
The absolute and relative configurations of (+)-22c were
determined to be Sp,R by X-ray crystallographic analysis as
shown in the latter section (Figure 1) and thus the absolute and
relative configurations of (–)-20c could be assigned to be
As good quality single crystals of (Sp,R)-(+)-22c were
obtained, its crystal and packing structures were investigated in
detail. There were three independent molecules (Figure 2a) and
the conformations of these three molecules were very similar
(Figure 2b), suggesting that they are rigid molecules with limited
flexibility. These three independent molecules were packed with
solvent molecules, and two inclusion spaces (blue and green
circles) were observed (Figures 2c and 2d). The solvent
molecules in the blue circle could be identified as chloroform and
n-hexane, while the solvent molecules in the green circle could
not be identified. In the blue circled inclusion space, the
methoxycarbonyl group and the solvent molecule interact weakly,
forming an intricate isolated space, and thus the solvent
29aS,214aS.
A
possible mechanism for the formation of
(29aS,214aS)-(–)-20c is shown in Scheme 5. The reaction of the
diyne moiety of 4c with the cationic rhodium(I)/(R)-xyl-segphos
complex would generate five-membered rhodacycle D. Regio-
and enantioselective insertion of the cycloalkene moiety of 4c
into the electron-deficient Rh-C bond to generate seven-
membered rhodacycle E, in which the bulky benzylidene moiety
(green) is located in the opposite direction to Rh and the red-
colored benzene ring is positioned to avoid steric repulsion with
the equatorial phenyl group (blue) of the (R)-xyl-xegphos ligand.
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10.1002/chem.202001450
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a) UV-vis absorption spectra of 22a–d and 24
the solvent molecules are omitted for clarity. Spaces, which
were occupied by the solvent molecules, are shown as a yellow
surface with gray lining.
To evaluate the distortion of the p-terphenyl cores from
planarity, bend angles of benzene rings A–C and torsion angles
a,b of (Sp,R)-22a–d and (R)-24 were determined from the DFT
calculations, as shown in Table 3. As expected, average bend
angles increased as the tether length shortened {averages of α
and α’ in A–C: 3.6° [(R)-24] < 4.6° [(Sp,R)-22a] < 6.6° [(Sp,R)-
22b] < 6.9° [(Sp,R)-22c] < 8.8° [(Sp,R)-22d]; averages of β and β’
in A–C: 6.6° [(R)-9] < 7.6° [(Sp,R)-22a] < 9.7° [(Sp,R)-22b] <
10.0° [(Sp,R)-22c] < 11.3° [(Sp,R)-22d]}. A similar tendency was
observed for the values in the crystalline state (Table S7)
{averages of α and α’ in A–C: 5.6° [(+)-22a] < 6.0° [(±)-22b] <
7.3° [(Sp,R)-(+)-22c] < 9.0° [(±)-22d]; averages of β and β’ in A–
C: 8.7° [(+)-22a] < 9.2° [(±)-22b] < 10.9° [(Sp,R)-(+)-22c] < 11.0°
[(±)-22d]}. Concerning torsion angles, an average of a,b tended
to be smaller as the tether length shortened {averages of a1, a2,
b1, and b2: 8.5° [(R)-24] > 7.0° [(Sp,R)-22a] < 7.4° [(Sp,R)-22b]
> 5.7° [(Sp,R)-22c] > 5.3° [(Sp,R)-22d]}. A similar tendency was
observed for the values in the crystalline state (Table S7)
{averages of b1, b2, c1, and c2: 8.7° [(+)-22a] > 7.4° [(±)-22b] >
7.0° [(Sp,R)-(+)-22c] ≈ 7.1° [(±)-22d]}.
b) Emission spectra and color (irradiation at 365 nm) of 22a–d and 24
8 (+)-7a (+)-7b (+)-7
c
UV-vis absorption and emission spectra of PAH-based
planar chiral bent cyclophanes (+)-22a−d are shown in Figures
3a and 3b, and the data are summarized in Table 4. For
comparison, data of acyclic reference compound 24 were also
collected. Red shifts of the longest-wavelength absorption
maxima were observed by shortening the ansa chain (Figure 3a),
but these shifts were very small (395–404 nm). Indeed, the DFT
calculations of (Sp,R)-22a–d and (R)-24 revealed a small
difference in HOMO-LUMO energy gaps (0.126–0.140 ev, Table
S8). Cyclophanes (+)-22a−d and acyclic reference molecule 24
exhibited the emission color of blue to green in CHCl3 (Figure
3b). The significant red shifts of the emission maxima were
observed by shortening the ansa chain. Interestingly,
fluorescence quantum yields of (+)-22a−d were significantly
higher than that of 24 (78–82% vs. 48%), which is contrary to
previously reported PBI-,[14d] anthracene-,[14e] and pyrene-
based[11b] bent cyclophanes, fluorescence quantum yields of
which are significantly lower than those of the corresponding
acyclic molecules (Figure S9). The introduction of the ansa
chain reduces the flexibility of the molecule (Figure 2b), and thus
radiationless deactivation might be suppressed. Additionally, the
amount of conjugation in the annelated p-terphenyl core is
decreased by the bend but increased by the bend-induced
decrease of torsion angles (Table 3). These factors could
account for the observed higher fluorescence quantum yields of
cyclophanes 22a–d than acyclic reference molecule 24. As PBI,
anthracene, and pyrene are planar molecules, the introduction of
the ansa chain to these molecules induces distortion, which
decreases the fluorescence quantum yields.
c) ECD spectra of 22c
Figure 3. UV-vis absorption and emission spectra of (+)-22 and 24 in CHCl3.
ECD spectra of 22c in CHCl3.
molecule showed relatively small disorder. However, the green
circled inclusion space could have little interaction with solvent
molecules by being surrounded by the hydrophobic tether, and
thus the solvent molecules were severely disordered in the
continuous solvent channel structure.[33] In Figures 2c and 2d,
Finally, we examined the chiroptical properties of PAH-
based planar chiral bent cyclophanes 22. The optical rotation
values of 22 were 691−879° (Table 5), which are larger than
those of pyrene-based chiral bent cyclophanes (130–270°)[11a,11b]
but smaller than those of the PBI-based chiral twisted
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10.1002/chem.202001450
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cyclophane (6600°).[14c] The ECD spectra of (+)-22c and (−)-22c
in CHCl3 exhibited a mirror-image relationship (Figure 3c). Thus,
the bending effect on the anisotropy factor for ECD was
examined, which revealed that the anisotropy factor (gabs value)
for ECD of 22 increases [2.06–2.82 × 10–3 (1st large peak
wavelength) and 1.83–2.91 × 10–3 (2nd large peak wavelength)]
as the tether length becomes shorter, which corresponds to
increasing the bend and reducing the twist (Table 5).
amount of conjugation in the annelated p-terphenyl core is
decreased by the bent but increased by the bent-induced
decrease of torsion angles. These factors could account for the
observed higher fluorescence quantum yields of the
cyclophanes than the acyclic reference molecule. Furthermore,
the anisotropy factors (gabs values) for ECD of these 9-fluorenol-
based planar chiral bent cyclophanes increased as the tether
length became shorter, which corresponds to increasing the
bend and reducing the twist. The present synthesis
demonstrates the high utility of the rhodium-catalyzed regio- and
enantioselective [2+2+2] cycloaddition of diynes with cyclic
alkenes for the regio- and enantioselective construction of PAH-
based planar chiral bent cyclophane structures.
Table 4. Photophysical properties.[a]
emission
λmax / nm
ΦF
UV-vis
absorption
λmax / nm
(excitation
wavelength
/ nm)
Compd
(excitation
wavelength / nm)
24
294, 358, 395
287, 367, 396
285, 367, 395
285, 373, 398
282, 375, 404
464 (294)
483 (287)
487 (285)
500 (285)
510 (282)
0.48 (290)
0.78 (290)
0.81 (290)
0.81 (290)
0.82 (280)
Acknowledgements
(+)-22a
(+)-22b
(+)-22c
(+)-22d
This research was supported partly by Grants-in-Aid for
Scientific Research (Nos. JP19H00893 for K.T., and
JP18H04504 and 20H04661 for H.U.) and research fellowship for
young scientists (No. 17J08763 for Y.A.) from JSPS (Japan).
We thank Takasago International Corporation for the gift of H8-
binap, segphos, tol-segphos, xyl-segphos, and dtbm-segphos,
and Umicore for generous support in supplying the palladium
and rhodium complexes.
[a] Measured in CHCl3 (1.0 x 10-5 M) at 25 °C.
Table 5. Chiroptical properties.[a]
gabs / 10–3
Compd
[α]25
D
(wavelength / nm)
(+)-22a
+865
+691
+879
+835
–2.06 (313)
+1.83 (361)
Keywords: planar chiral bent cyclophanes • enantioselective
catalysis • polycyclic aromatic hydrocarbons (PAHs) • rhodium •
[2+2+2] cycloaddition
(+)-22b
(+)-22c
(+)-22d
–2.06 (312)
+2.07 (353)
–2.32 (312)
+2.44 (360)
–2.82 (313)
+2.91 (360)
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Conclusion
In conclusion, we have achieved the highly enantioselective
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of the ansa chain reduces the flexibility of the molecule, which
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Chemistry - A European Journal
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see: a) Y. Shibata, K. Tanaka, Rhodium(I)-Catalyzed [2+2+2] and [4+2]
Cycloadditions, in Rhodium Catalysis in Organic Synthesis: Methods
and Reactions (Ed.: K. Tanaka), Wiley-VCH, Weinheim, 2019, pp. 183–
228; b) K. Tanaka, TCI Mail 2018, 179, 3; c) K. Tanaka, Y. Kimura, K.
Murayama, Bull. Chem. Soc. Jpn. 2015, 88, 375; d) K. Tanaka, Y.
Shibata, Synthesis 2012, 44, 323; e) K. Tanaka, Heterocycles 2012, 85,
1017; f) K. Tanaka, Chem. Asian. J. 2009, 4, 508; g) K. Tanaka, Synlett
2007, 2007, 1977.
[9]
The synthesis of achiral carbon nanobelts has been reported. See: a) G.
Povie, Y. Segawa, T. Nishihara, Y. Miyauchi, K. Itami, Science 2017,
356, 172; b) G. Povie, Y. Segawa, T. Nishihara, Y. Miyauchi, K. Itami, J.
Am. Chem. Soc. 2018, 140, 10054.
[10] For a review on PAH-based cyclophanes, see: P. G. Ghasemabadi, T.
Yao, G. J. Bodwell, Chem. Soc. Rev. 2015, 44, 6494.
[21] For a review on the enantioselective synthesis of chiral cyclophanes by
the rhodium-catalyzed [2+2+2] cycloaddition, see: K. Tanaka, Bull.
Chem. Soc. Jpn. 2018, 91, 187.
[11] a) Y. Yang, M. R. Mannion, L. N. Dawe, C. M. Kraml, R. A. Pascal, Jr.,
G. J. Bodwell, J. Org. Chem. 2012, 77, 57; b) P. R. Nandaluru, P.
Dongare, C. M. Kraml, R. A. Pascal, Jr., L. N. Dawe, D. W. Thompson,
G. J. Bodwell, Chem. Commun. 2012, 48, 7747; c) M. Hermann, D.
Wassy, D. Kratzert, B. Esser, Chem. Eur. J. 2018, 24, 7374.
[22] For examples on the enantioselective synthesis of chiral cyclophanes
by the rhodium-catalyzed [2+2+2] cycloaddition, see: a) K. Tanaka, H.
Sagae, K. Toyoda, K. Noguchi, M. Hirano, J. Am. Chem. Soc. 2007,
129, 1522; b) K. Tanaka, H. Sagae, K. Toyoda, M. Hirano, Tetrahedron
2008, 64, 831; c) T. Shibata, T. Uchiyama, K. Endo, Org. Lett. 2009, 11,
3906; d) T. Shibata, T. Chiba, H. Hirashima, Y. Ueno, K. Endo,
Heteroat. Chem 2011, 22, 363; e) T. Shibata, T. Uchiyama, H.
Hirashima, K. Endo, Pure Appl. Chem. 2011, 83, 597; f) K. Tanaka, T.
Araki, D. Hojo, K. Noguchi, Synlett 2011, 2011, 539; g) T. Shibata, M.
Miyoshi, T. Uchiyama, K. Endo, N. Miura, K. Monde, Tetrahedron 2012,
68, 2679; h) T. Araki, K. Noguchi, K. Tanaka, Angew. Chem. Int. Ed.
2013, 52, 5617; Angew. Chem. 2013, 125, 5727; i) Y. Tahara, S.
Obinata, K. S. Kanyiva, T. Shibata, A. Mándi, T. Taniguchi, K. Monde,
Eur. J. Org. Chem. 2016, 2016, 1405; j) T. Shibata, S. Obinata, K. S.
Kanyiva, Heterocycles 2017, 95, 1121.
[12] The synthesis of PAH-based chiral carbon nanohoops has been
reported. See: a) S. Hitosugi, W. Nakanishi, T. Yamasaki, H. Isobe, Nat.
Commun. 2011, 2, 1; b) S. Hitosugi, T. Yamasaki, H. Isobe, J. Am.
Chem. Soc. 2012, 134, 12442; c) T. Matsuno, S. Kamata, S. Hitosugi,
H. Isobe, Chem. Sci. 2013, 4, 3179; d) Y. Li, A. Yagi, K. Itami, J. Am.
Chem. Soc. 2020, 142, 3246.
[13] The synthesis of a chiral carbon nanobelt has been reported. See: K. Y.
Cheung, S. Gui, C. Deng, H. Liang, Z. Xia, Z. Liu, L. Chi, Q. Miao,
Chem 2019, 5, 838.
[14] a) P. Osswald, D. Leusser, D. Stalke, F. Würthner, Angew. Chem. Int.
Ed. 2004, 44, 250; Angew. Chem. 2004, 117, 254; b) P. Osswald, M.
Reichert, G. Bringmann, F. Würthner, J. Org. Chem. 2007, 72, 3403; c)
M. M. Safont-Sempere, P. Osswald, K. Radacki, F. Würthner, Chem.
Eur. J. 2010, 16, 7380; d) M. M. Safont-Sempere, P. Osswald, M. Stolte,
M. Grüne, M. Renz, M. Kaupp, K. Radacki, H. Braunschweig, F.
Würthner, J. Am. Chem. Soc. 2011, 133, 9580; e) A. Bedi, L. J. W.
Shimon, O. Gidron, J. Am. Chem. Soc. 2018, 140, 8086; f) A. Bedi, R.
Carmieli, O. Gidron, Chem. Commun. 2019, 55, 6022; g) A. Bedi, O.
Gidron, Chem. Eur. J. 2019, 25, 3279.
[23] Y. Aida, H. Sugiyama, H. Uekusa, Y. Shibata, K. Tanaka, Org. Lett.
2016, 18, 2672.
[24] a) T. Shibata, A. Kawachi, M. Ogawa, Y. Kuwata, K. Tsuchikama, K.
Endo, Tetrahedron 2007, 63, 12853; b) Y. Aida, Y. Shibata, K. Tanaka,
J. Org. Chem. 2018, 83, 2617.
[25] Only one product (63% yield, 70% ee) was reported in ref 23. For the
nickel-catalyzed enantioselective [2+2+2] cycloaddition of two
unsymmetric monoynes with unsymmetric cyclic alkenes, see: S. Ikeda,
H. Kondo, T. Arii, K. Odashima, Chem. Commun. 2002, 2422.
[15] Recently, our research group reported the enantioselective synthesis of
chiral twisted Möbius-cycloparaphenylenes by the rhodium-catalyzed
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10.1002/chem.202001450
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[26] Indeed, we observed the alkene isomerization of the substituted indene
in the presence of the cationic rhodium(I)/biaryl diphosphine complex.
See the Supporting Information for details.
[27] Although indenone 2c failed to react with diynes, the cationic
rhodium(I)/rac-BINAP complex-catalyzed [2+2+2]
cycloaddition-
aromatization of diyne with a 3-acetoxy or 3-alkoxy-1H-inden-1-one
was recently reported. See: A. D. Manick, B. Salgues, J. L. Parrain, E.
Zaborova, F. Fages, M. Amatore, L. Commeiras, Org. Lett. 2020, DOI:
10.1021/acs.orglett.0c00235.
[28] Although benzofulvenes have not been used for the transition-metal-
catalyzed [2+2+2] cycloaddition, benzofulvenes have been used for the
[6+4] cycloaddition. See: R. N. Warrener, M. L. A. Hammond, D. N.
Butler, Synth. Commun. 2001, 31, 1167.
[29] For reviews on fulvene chemistry, see: a) P. Preethalayam, K. S.
Krishnan, S. Thulasi, S. S. Chand, J. Joseph, V. Nair, F. Jaroschik, K. V.
Radhakrishnan, Chem. Rev. 2017, 117, 3930; b) H. Hopf, in Cross
Conjugation: Modern Dendralene, Radialene and Fulvene Chemistry
(Eds.: H. Hopf, M. S. Sherburn), Wiley, New York, 2016, pp. 1–440.
[30] The rhodium(I)/(R)-Difluorophos complex-catalyzed
intermolecular
[2+2+2] cycloaddition of 1b with indene (2b) proceeded to give the
desired cycloadduct 3ba in high yield, although this cycloadduct could
not be isolated in a pure form. However, the ee value of 3ba was
moderate (ca. 45% ee). Thus, the use of benzofulvenes is crucial to
realize high enantioselectivity as well as the prohibition of the alkene
isomerization.
5 mol %
[Rh(cod)2]BF4/
(R)-difluorphos
Me
Me
CH2Cl2, 40 °C , 16 h
CO2Me
2a
(5 equiv)
MeO2C
1b
3ba / ca. 90% yield, ca. 45% ee
[31] Y. Aida, Y. Shibata, K. Tanaka, Chem. Eur. J. 2020, 26, 3004.
[32] As (+)-22b and (+)-22d were amorphous, X-ray crystallographic
analyses of (±)-22b and (±)-22d were conducted. CCDC 1992059 [(+)-
22a], 1992060 [(±)-22b], 1992061 [(+)-22c], and 1992062 [(±)-22d]
contain the supplementary crystallographic data for this paper. This
data
can
be
obtained
free
of
charge
via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or deposit@ccdc.cam.ac.uk).
[33] The solvent molecules were removed using the SQUEEZE routine of
PLATON. See: A. L. Spek, Acta Cryst. 2009, D65, 148.
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10.1002/chem.202001450
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FULL PAPER
Entry for the Table of Contents (Please choose one layout)
Layout 2:
FULL PAPER
Yukimasa Aida, Juntaro Nogami, Haruki
Sugiyama, Hidehiro Uekusa, and Ken
Tanaka*
O
O
1) RhI+-catalyzed
[2+2+2]
cycloaddition
2) oxidation
Me
O
Me
MeO
OH
O
3) reduction
O
CO2Me
Page No. – Page No.
PAH-based chiral bent cyclophanes
!Catalytic enantioselective synthesis with excellent ee values (>99% ee)
!Extended !-conjugation (annelated p-terphenyl core)
!Strained bent structures
!High fluorescence quantum yields (78–82%)
!Bent induced increase of gabs values
Enantioselective Synthesis of
Polycyclic Aromatic Hydrocarbon
(PAH)-Based Planar Chiral Bent
Cyclophanes by Rhodium-Catalyzed
[2+2+2] Cycloaddition
MeO
The highly enantioselective synthesis of polycyclic aromatic hydrocarbon (PAH)-
based planar chiral bent cyclophanes, containing a annelated bent p-terphenyl
core, was accomplished by the rhodium-catalyzed intramolecular regio- and
enantioselective [2+2+2] cycloaddition of tethered diyne-benzofulvenes followed by
stepwise oxidative transformations.
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