Catalytic Enantioselective Synthesis of Axially Chiral Polycyclic Aromatic Hydrocarbons (PAHs) via Regioselective C-C Bond Activation of Biphenylenes

Article abstract of DOI:10.1021/jacs.9b12205

Chiral polycyclic aromatic hydrocarbons (PAHs) are expected to have unusual physical properties due to their chirality and expanded π-conjugated system. Indeed, PAHs are promising compounds as chiral recognizers and organic semiconductors. Therefore, an efficient method for the synthesis of chiral PAHs is strongly desired. In contrast to helically chiral PAHs, there are only limited examples of the enantioselective synthesis of axially chiral PAHs. Herein, we report the catalytic enantioselective synthesis of benzo[b]fluoranthene-based axially chiral PAHs in excellent yields and enantioselectivities (up to >99%, >99% ee) by regioselective cleavage of the sterically hindered C-C bond of biphenylenes. The consecutive cyclizations could provide polycyclic PAHs with two chiral axes. The obtained chiral PAHs have high ? values (up to ? = 8.9 × 10⁴), quantum yields (up to φ = 0.67), and circularly polarized luminescence (CPL) property (|g_lum| = up to 3.5 × 10^-3).

Full text of DOI:10.1021/jacs.9b12205

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Article
Catalytic Enantioselective Synthesis of Axially Chiral Polycyclic Aromatic
Hydrocarbons (PAHs) via Regioselective C-C Bond Activation of Biphenylenes
Hideaki Takano, Natsumi Shiozawa, Yoshitane Imai, Kyalo Stephen Kanyiva, and Takanori Shibata
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b12205 • Publication Date (Web): 12 Feb 2020
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Catalytic Enantioselective Synthesis of Axially Chiral Polycyclic
Aromatic Hydrocarbons (PAHs) via Regioselective C-C Bond
Activation of Biphenylenes
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[
a]
[a]
[b]
[c]
Hideaki Takano, Natsumi Shiozawa, Yoshitane Imai, Kyalo Stephen Kanyiva, and Takanori
[a]
Shibata*
a
Department of Chemistry and Biochemistry, School of Advanced Science and Engineering, Waseda
University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan
b
Department of Applied Chemistry, Faculty of Science and Engineering, Kindai University, 3–4-1
Kowakae, Higashi-Osaka, Osaka 577–8502, Japan
c
Global Center of Science and Engineering, School of Advanced Science and Engineering, Waseda
University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan
E-mail: tshibata@waseda.jp
ORCID: 0000-0003-0744-9292 (H.T.), 0000-0003-4415-7432 (Y.I.), 0000-0001-9546-8367 (K.S.K.),
0
000-0003-4436-8264 (T.S.)
ABSTRACT:
Chiral polycyclic aromatic hydrocarbons (PAHs) are expected to have unusual physical properties due
to their chirality and expanded π-conjugated system. Indeed, PAHs are promising compounds as chiral
recognizers and organic semiconductors. Therefore, an efficient method for the synthesis of chiral PAHs
is strongly desired. In contrast to helically chiral PAHs, there are only limited examples of
enantioselective synthesis of axially chiral PAHs. Herein, we report the catalytic enantioselective
synthesis of benzo[b]fluoranthene-based axially chiral PAHs in excellent yields and enantioselectivities
(
up to >99%, >99% ee) by regioselective cleavage of the sterically hindered C-C bond of biphenylenes.
The consecutive cyclizations could provide polycyclic PAHs with two chiral axes. The obtained chiral
4
PAHs have high ε values (up to ε = 8.9×10 ), quantum yields (up to Φ = 0.67) and circularly polarized
-
3
luminescence (CPL) property (|g | = up to 3.5×10 ).
lum
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Introduction
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Polycyclic aromatic hydrocarbons (PAHs) have attracted much attention because of their unique
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physical properties and they have been studied for possible applications as organic electronic materials.
Thus, PAHs have been recognized as synthetic targets for the development of new methodologies and as
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templates for the bottom-up synthesis of nanocarbons. For example, Itami and co-workers established
the annulative π-extension (APEX) reaction for the synthesis of fused aromatic hydrocarbons from
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simple aromatic compounds. Recently, they further achieved living APEX polymerization to synthesize
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graphene nanoribbon from phenanthrene as a simple initiator.
Chiral PAHs have the potential for use in new types of optical devices because they likely have
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circularly polarized luminescence (CPL) properties. For instance, in 2016, Nozaki and co-workers
reported [7]helicene-like compounds possessing a fluorene skeleton and those showed large g values
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(dissymmetry factor) for CPL (up to |g | = 3.0×10 ). In contrast, axially chiral PAHs also have
lum
relatively strong CPL: Hirose, Matsuda, and co-workers synthesized a 13,13’-bibenzo[b]perylenyl
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derivative, which had a high fluorescence yield (Φ = 0.64) as well as a strong CPL (|g | = 5×10 ).
lum
However, in both cases, the enantioenriched molecules were prepared from racemic compounds by
HPLC resolution using the chiral stationary phase column.
t-Bu
t-Bu
H
H
t-Bu
t-Bu
Φ = 0.39, |glum| = 3.0x10-3
Φ = 0.64, |glum| = 5x10-3
Hirose and Matsuda (2016)
Nozaki (2016)
Figure 1. Selected examples of chiral PAHs
For further studies of chiral PAHs, the catalytic enantioselective synthesis of chiral carbohelicenes
have been developed. The most well-established method for the preparation of enantioenriched
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benzenoid helicene is transition-metal-catalyzed [2+2+2] cycloaddition. In 2013, Stará, Starý, and
co-workers synthesized chiral dibenzo[6]helicenes 2 from triyne 1 via nickel-catalyzed enantioselective
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[2+2+2] cycloaddition (Scheme 1, a). In 2018, Tanaka and co-workers reported the rhodium-catalyzed
enantioselective synthesis of dibenzo[7]helicenes 4 via [2+2+2] cycloaddition and of twisted anthracene
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e
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via [2+1+2+1] cycloaddition from tetrayne 3 (Scheme 1, b). In particular, the dibenzo[7]helicenes 4
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showed relatively strong CPL property (|g | = 2.2×10 ). Cycloisomerization was also used for the
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synthesis of helically chiral helicenes. In 2017, Alcarazo and co-workers achieved an enantioselective
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synthesis of [6]carbohelicene 7 from diyne 6 by using a chiral gold catalyst (Scheme 1, c).
Contrastingly, there are no examples of the enantioselective construction of PAHs possessing a biaryl
system without any heteroatom as far as we know. While there are several approaches for the
asymmetric synthesis of axially chiral biaryl compounds such as binaphthyl, a heteroatom such as
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a) Stará, Starý (2013)
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chiral Ni cat.
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4% yield, 64% ee
21% yield
glum| = 2.2x10-3
|glum| = 7.5x10-4
|
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R1
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chiral Au cat.
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Scheme 1. Successful examples for enantioselective synthesis of helically chiral PAHs
To synthesize enantioenriched axially chiral PAHs, we developed a strategy involving the
more-hindered site-selective cleavage of a biphenylene skeleton. While biphenylene is an antiaromatic
hydrocarbon, which exhibits enormous strain at central 4-membered ring, transition metal complexes
readily cleave the strained C-C bond to provide dibenzometallacyclopentadiene complexes as useful C-4
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units for the construction of various ring systems. We previously reported the regioselective cleavage
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of the sterically more hindered C-C bond of biphenylenes 8 and subsequent intramolecular cyclization
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with alkene in the presence of a Rh catalyst. In this reaction, the alkene moiety acted as both a
directing group and a reaction site to form dihydrobenzo[b]fluoranthenes 9. Since catalytic and
regioselective activation of a sterically hindered C-C bond is still rare, it seems that this methodology
has the potential to form complex PAHs. Moreover, we previously reported the enantioselective
intermolecular cyclization of biphenylene and ortho-substituted aryl alkynes for the synthesis of axially
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chiral phenanthrene derivatives.
Against this background, we assumed that, if an enantioselective cyclization of biphenylene 10
proceeds with regioselective cleavage of a sterically hindered C-C bond assisted by the coordination of
an alkyne moiety, an axially chiral benzo[b]fluoranthene derivative 11 could be obtained via an
enantiodetermining insertion of an alkyne moiety to a Rh-C bond, and enantioselective synthesis of
unique axially chiral PAHs can be realized. In this study, we achieved a Rh-catalyzed enantioselective
synthesis of benzo[b]fluoranthene-based chiral PAHs in excellent yields with excellent
enantioselectivities. In addition, we also obtained various poly-conjugated axially chiral PAHs by
consecutive cyclizations.
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a) Our previous work
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Rh(cod)(SIMes)]BARF (2 mol%)
xylene, 135 °C
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up to 97% yield
14 examples
b) This work
Ar2
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R
Chiral cationic Rh cat.
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Regioselective
Reductive
elimination
C-C bond cleavage
R
Ar1
Ar1
R
Ar2
Ar2
Enantio-determining insertion
[Rh]
[Rh]
R
R
R
R
•
Highly regio- and enantioselective reaction
• Enantioselective synthesis of axially chiral PAHs
Synthesis of complex axially chiral PAHs
Novel photophysical properites such as CPL
•
•
Scheme 2. Our previous work and the concept of this work
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Discussion
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We chose a biphenylene derivative 10aa as a model substrate and screened various cationic chiral Rh
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catalysts (Table 1). When chiral diphosphine ligands L1-3 were used, the desired product 11aa was
obtained in high yield, albeit with poor enantioselectivity (Entries 1-3). In the case of chiral
N-heterocyclic carbenes (NHC) ligands L4 and L5, enantioselectivities were moderate or low (Entries 4,
5). We supposed that stronger trans effect of alkyne moieties increased the distance between chiral
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moiety and substrate. As a result, enantiocontrol of the reaction became more difficult. Therefore, we
tried chiral diene ligand L6, and chiral product 11aa was obtained with the highest ee of 78% (Entry 6).
Screening of the solvent at 135 °C (bath temperature) revealed that cyclopentyl methyl ether (CPME)
was the best choice with regard to both yield and ee (Entries 6-9). The prepared [RhCl(L6)] gave the
2
same results as the catalyst prepared in situ (Entries 9 and 10). The reaction temperature was crucial for
the enantioselectivity and we achieved almost quantitative yield with excellent enantioselectivity at
80 °C (Entries 10-12). The comparable result was obtained also in chlorobenzene at 80 °C (Entry 13).
We assumed that racemization proceeded at the high temperature. We measured that the rate constant of
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-1
racemization for 11aa at 160 °C (433 K) is 8.99×10 s and calculated that the flipping energy of the
‡
axial chirality (∆G ) is 35.7 kcal/mol according to the Eyring equation. This result means that the
1
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half-life of 11aa at 80 °C (353 K) is 18 years and that it hardly racemizes at the temperature. The
reaction could be conducted by the ten times scale without loss of yield and ee (Entry 14). The amount
of catalyst could be reduced to 5 mol% of Rh with a slight decrease of yield and ee (Entry 15). We
further investigated the present reaction using the conditions in entry 12.
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a
Table 1. Screening of reaction conditions
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Solvent, 24 h
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Me
PPh2
P
Me
L2: (S,S)-MeDUPHOS L3: (S,S)-CHIRAPHOS
L1: (S)-BINAP
Ph Ph
Ph Ph
Me
Ph
N
N
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Ph
L6: (R,R)-Ph-bod
i-Pr i-Pr
i-Pr Me
L5
L4
Entry
Rh Catalyst
Solvent
Xylene
Xylene
Xylene
Xylene
Xylene
Xylene
PhCl
Bath Temperature / °C
Yield / %
81
Ee / %
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-2
b
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[Rh(cod) ]BARF + L1
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[Rh(cod) ]BARF + L2
>99
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[Rh(cod) ]BARF + L3
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[RhCl(cod)(L4)]+NaBARF
[RhCl(cod)(L5)]+NaBARF
[RhCl(coe) ] +2 L6+2 NaBARF
87
-42
-4
b
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77
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80
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96
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96
2
2
[RhCl(coe) ] +2 L6+2 NaBARF
>99
68
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2
[RhCl(coe) ] +2 L6+2 NaBARF
Dioxane
CPME
CPME
CPME
CPME
PhCl
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2
[RhCl(coe) ] +2 L6+2 NaBARF
>99
>99
99
2 2
1
0
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[RhCl(L6)] + 2 NaBARF
2
1
[RhCl(L6)] + 2 NaBARF
2
1
1
2
3
[RhCl(L6)] + 2 NaBARF
99
2
[RhCl(L6)] + 2 NaBARF
80
>99
>99
94
2
c
1
4
[RhCl(L6)] + 2 NaBARF
CPME
CPME
80
2
d
1
5
[RhCl(L6)] + 2 NaBARF
80
2
a
The reaction was conducted in a sealed Schlenk tube: 10aa (0.05 mmol), Rh catalyst (0.01 mmol of
b
c
Rh), solvent (0.5 mL). 10aa (0.10 mmol), Rh catalyst (0.01 mmol of Rh), solvent (0.5 mL). 10aa
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0.50 mmol), Rh catalyst (0.1 mmol of Rh), solvent (5.0 mL). The reaction was conducted in the
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presence of Rh catalyst (5 mol%) for 48 h. BARF = tetrakis[(3,5-trifluoromethyl)phenyl]borate
Next, we screened various substrates 10 which have different functional groups at the ortho position
of the phenyl ring (Table 2). Although the reaction proceeded smoothly to give 11ba, which has a longer
alkyl chain, in excellent yield with excellent enantioselectivity, an alkoxy group had a negative effect on
enantioselectivity (11ca) and the ee was moderate in both CPME and chlorobenzene. This was most
likely because dissociation of the chiral diene was caused by the coordination of an oxygen atom to the
metal center. 2-Biphenyl and 1-naphthyl groups could be used as substituents on the alkyne terminus
(11da, 11ea). When electron-withdrawing groups were attached to the ortho position of the phenyl ring
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9
0
(11fa-ha), the reactions also proceeded with high yields and ees. In particular, in the reaction of
CF -substituted substrate 10fa, chlorobenzene (98% ee) gave higher ee than CPME (85% ee). Based on
3
the results of X-ray single-crystal analysis of 11ha, the absolute configuration was assigned to be R
1
7,18
isomer.
For the synthesis of complex PAHs, we synthesized pyrene-substituted substrates 10ia and
1
0ja, and pyrene-benzo[b]fluoranthene biaryl compounds 11ia and 11ja were obtained in excellent
yields and ees. Overall, this reaction proceeded smoothly with various kinds of ortho-substituted aryl
groups on the alkyne terminus.
Furthermore, we checked the effects of substituents on the ortho-phenylene-tether (Table 3). A
methoxy group as an electron-donating group and a halogen atom as an electron-withdrawing group
could be installed at all positions on the ortho-phenylene-tether (11ab-ai) with high yields and ees.
Regarding the reaction of 10ae, 10af, and 10ai, chlorobenzene was used as a solvent. We next examined
naphthalene-tethered compounds. When 1-alkynyl-2-biphenylene-substituted substrate 10aj was used,
dibenzo[b,l]fluoranthene 11aj was obtained in moderate yield with high enantioselectivity, but 30% of
inseparable starting material 10aj remained. The starting material was completely consumed in
chlorobenzene along with a slight decrease of ee. In the case of the 2,3-naphthylene-tethered compound
1
0ak, the reaction proceeded smoothly and gave dibenzo[b,k]fluoranthene 11ak. Thiophene-tethered
compound 10al was also suitable for this reaction to give 11al in excellent yield with excellent
enantioselectivity.
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a
Table 2. Screening of the ortho-substituents on phenyl ring
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
R
[RhCl{(R,R)-Ph-bod}] (10 mol%)
Ar1
Ar1
2
NaBARF (20 mol%)
CPME, 80 °C, 24-48 h
R
1
0
11
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
n-C H
O(n-C H )
Ph
8
17
8 17
1
1ba
11ca
99%, 68% ee
99%, 64% eeb
11da
>99%, 98% ee
11ea
9
3%, 97% ee
91%, 93% ee
>
>
CF3
Cl
Br
17
1fab
X-ray structure of 11ha
1
11ga
91%, 94% ee
11ha
96%, 92% ee
(
Thermal ellipsoids shown
at 50% probability)
8
9%, 98% ee
t-Bu
t-Bu
t-Bu
11ia
11ja
>99%, 99% ee
>99%, 98% ee
a
b
1
0 (0.05 mmol), [RhCl{(R,R)-Ph-bod}] (0.005 mmol), NaBARF (0.01 mmol), CPME (0.5 mL).
2
The reaction was conducted in chlorobenzene.
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a
Table 3. Screening of ortho-phenylene tethers
1
2
3
4
5
6
7
8
9
Me
Ar2
Ar2
[RhCl{(R,R)-Ph-bod}] (10 mol%)
2
NaBARF (20 mol%)
CPME, 80 °C, 24-48 h
Me
1
0
11
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
R
R
R
F
Me
Me
Me
Me
11ai; 98%, 97% eec
1
1ab (R = OMe); 82%, 93% ee
11ac (R = F); >99%, 94% eeb
11ad (R = OMe); 61%, 97% ee 11ag (R = OMe); 88%, 97% ee
11ae (R = F); >99%, 98% eec
11ah (R = F); 79%, 93% ee
11af (R = Cl); 92%, 91% eec
S
Me
Me
Me
11aj; 53%, 84% ee (10aj; 30%)
11ak; 98%, 91% ee
11al; 98%, 97% ee
94%, 78% eec
a
b
1
0 (0.05 mmol), [RhCl{(R, R)-Ph-bod}] (0.005 mmol), NaBARF (0.01 mmol), CPME (0.5 mL)
2
c
The reaction was conducted at 100 °C. The reaction was conducted in chlorobenzene.
To synthesize more complex PAHs, we used various substrates with two alkyne moieties and two
biphenylene skeletons, and undertook consecutive cyclizations (Scheme 3). When 12a was used, only
para-phenylene-tethered chiral bis(benzo[b]fluoranthene) 13a as an optically active diastereomer was
obtained in high yield with excellent enantioselectivity. Next, we synthesized regioisomers 12b and 12c,
where two alkyne moieties and two biphenylene skeletons were installed to the core benzene ring. When
para-dialkynyl para-biphenylenyl compound 12b was used, chiral nonacyclic compound 13b was
isolated in 93% yield with >99% ee, along with a slight amount of separable meso product (4%).
Starting from meta-dialkynyl meta-biphenylenyl 12c, compound 13c was afforded in moderate yield
with excellent enantioselectivity. In this case, separable meso product (24%) was also obtained probably
due to a steric effect around the symmetrical axes.
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1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
n-C H
n-C H
8 17
8
17
[
RhCl{(R,R)-Ph-bod}] (20 mol%)
2
NaBARF (40 mol%)
CPME, 80 °C, 48 h
n-C H
n-C H
8 17
8
17
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
12a
13a; 89%, 98% ee
single diastereomer)
(
n-C H
8
17
n-C H
8 17
same as above
n-C H
8
17
n-C H
8 17
1
2b
n-C H
1
3b; 93%, >99% ee (meso; 4%)
n-C H
8
17
8
17
n-C H
8
17
n-C H
8 17
same as above
12c
13c; 53%, >99% ee (meso; 24%)
Scheme 3. Consecutive cyclizations for complex axially chiral PAHs
19
Finally, we measured the photophysical properties of some of the synthesized PAHs. The UV/Vis
absorption spectra measured in DCM are shown in Figure 2. The benzo[b]fluoranthene skeleton itself
has a relatively high ε value in the visible-light region. The spectra of 11ia and 11ja were red-shifted
compared to those of 2-tert-butylpyrene and 2,7-di-tert-butylpyrene. Moreover, the spectra of 11ia and
1
1ja were slightly shifted compared to that of 11ba. These observations can be explained in terms of
internal interaction between the pyrene ring and benzo[b]fluoranthene skeleton. Regarding polycyclic
products 13, the UV spectrum of 13a was almost the same as that of 11ba, but the ε value of 13a was
higher than that of 11ba. Compared to the spectrum of a benzo[b]fluoranthene unit, the spectra of
nonacyclic aromatic hydrocarbons 13b and 13c were shifted to a longer wavelength side. In addition,
13b had a higher ε value than 13c and other benzo[b]fluoranthene derivatives.
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1
2
3
4
5
6
7
8
9
a
4.0
1
1
1
2
2
1ba
3.5
3.0
2.5
2.0
1.5
1.0
1ia
1ja
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
-
tert-butylpyrene
,7-di-tert -butylpyrene
0.5
0.0
300
320
340
360
380
400
420
Wavelength λ (nm)
9
.0
.0
.0
.0
.0
.0
b
1
1ba
8
7
6
5
4
13a
13b
1
3c
3.0
2.0
1.0
0.0
300
350
400
450
500
Wavelength λ (nm)
-
5
-5
Figure 2. a, UV/Vis spectra of 11 and related compounds (11ba; 1.0×10 M, 11ia; 1.0×10 M, 11ja;
-
5
-5
-5
1
.0×10 M, 2-tert-butylpyrene; 1.0×10 M, 2,7-di-tert-butylpyrene; 1.0×10 M). b, UV/Vis spectra of
-5
-5
-5
-5
1
1ba and 13 (11ba; 1.0×10 M, 13a; 1.0×10 M, 13b; 1.0×10 M, 13c; 1.0×10 M).
The fluorescence spectra and quantum yields of selected products were also measured (Figure 3).
Interestingly, the shape of the fluorescence spectrum of 11ia was almost the same as that of 11ja.
Furthermore, these spectra were shifted to a longer wavelength compared to that of 11ba. This result
probably suggested that internal energy was transferred from excited pyrene to the benzo[b]fluoranthene
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skeleton. Compound 11ia showed the same level of quantum yield as 11ba (Φ = 0.29 and 0.27,
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
2
0
respectively). Although pyrene derivatives 11ia showed low quantum yields due to a loose bolt effect,
11ja had a relatively high quantum yield (Φ = 0.67). Compound 13a had almost the same photophysical
-1
-1
properties as 11ba. The Stokes shifts of 13b and 13c (9064 cm (1.12 eV) and 8774 cm (1.09 eV),
respectively) were significantly larger than that of 13a (88 nm), which can be recognized that they are
derived from the structural change of fused benzo[b]fluoranthene skeleton in the excited state. The
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
1
quantum yields of both 13b and 13c were relatively high (Φ = 0.43 and 0.53, respectively). Moreover,
-5
we measured CPL spectra of 11ja, 13b, and 13c in 1.0×10 M DCM solution. As a result,
-
3
pyrene-substituted 11ja had a stronger CPL property (|g | = 3.5×10 ) than p-extended compounds 13b
lum
-
4
-4
(|g | = 6.4×10 ), and 13c (|g | = 3.1×10 ).
lum
lum
a
1
.8
.6
.4
.2
0
1
1
1
2
2
1ba
0
0
0
0
1ia
1ja
-tert-butyl pyrene
,7-di-tert -butylpyrene
300
350
400
450
500
550
600
650
Wavelength λ (nm)
1
.8
.6
b
1
1ba
0
0
13a
1
3b
3c
1
0.4
0.2
0
300
350
400
450
500
550
600
650
Wavelength λ (nm)
Figure 3. a, Fluorescence spectra of 11 and related compounds (concentration (excitation
-5
-5
-5
wavelength): 11ba; 1.0×10 M (353.0 nm), 11ia; 1.0×10 M (344.5 nm), 11ja; 1.0×10 M (345.0 nm),
-
5
-5
2
-tert-butylpyrene; 1.0×10 M (322.5 nm), 2,7-di-tert-butylpyrene; 1.0×10 M (339.5 nm)) b,
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-
5
Fluorescence spectra of 11ba and 13 (concentration (excitation wavelength): 13a; 1.0×10 M (353.0
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
-
5
-5
nm), 13b; 1.0×10 M (340.5 nm), 13c; 1.0×10 M (338.5 nm))
a
Table 4. Quantum yields of selected compounds
Compound
1ba
1ia
1ja
2-tert-Butylpyrene
Φ
Compound
Φ
22
1
0.27
0.29
0.67
0.082
2,7-Di-tert-butylpyrene
0.12
0.28
0.43
0.53
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
13a
13b
1
13c
a
-5
-5
Excitation wavelength (concentration) 11ba; 353.0 nm (3.4×10 M), 11ia; 344.5 nm (2.2×10 M), 11ja; 345.0 nm
-
6
-5
-5
(8.2×10 M ), 2-tert-butylpyrene; 322.5 nm (2.8×10 M), 2,7-di-tert-butylpyrene; 339.5 nm (1.3×10 M), 13a; 353.0
-5
-6
-6
nm (1.9×10 M), 13b; 340.5 nm (5.5×10 M), 13c; 338.5 nm (7.5×10 M).
Conclusion
We have achieved a catalytic enantioselective synthesis of axially chiral PAHs via regioselective
cleavage of biphenylenes. Axially chiral benzo[b]fluoranthene derivatives were obtained in excellent
yields and ees with broad functional group tolerance. Moreover, PAHs which have two axial chiralities
could be synthesized by consecutive cyclizations with high diastereoselectivity. Some of the obtained
PAHs have photophysical properties such as strong UV absorption, fluorescence and CPL. Further
investigations on the synthesis of highly complex PAHs with chirality and the application of
fluoranthene derivatives are ongoing in our laboratory.
Acknowledgement
This work was supported by Waseda University Grant for Special Research Projects (New
Developments in Research). H.T. is grateful to the Japan Society for the Promotion of Science for
financial support (JSPS KAKENHI Grant Number 18J13634). We thank Mr. Koki Shiba, Mr. Yota
Suzuki and Prof. Koji Ishihara (Waseda University) for their support of measurement of UV-Vis and
fluorescence spectra. We are grateful to Dr. Shoichi Hosoya (Proteomics Research Support Unit,
Research Core, Tokyo Medical and Dental University (TMDU)) for the support of measurement of
MALDI-TOF MS. We also thank Mr. Daiki Kaji (Kindai University) for the measurement of CPL
spectra.
Experimental Section
General procedure for a Rh-catalyzed enantioselective reaction
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[
RhCl{(R,R)-Ph-bod}]2 (0.005 mmol, 10 mol%), NaBARF (0.01 mmol, 20 mol%) and
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
substrates 10 (0.05 mmol, 1.0 equiv) were placed in a sealed Schlenk tube, which was then evacuated
and backfilled with argon (×3). To the reaction vessel was added dehydrated CPME (0.5 mL, prepared
by argon bubbling for 30 sec). The solution was then stirred at 80 °C (bath temperature) for 24-48 h.
The reaction mixture was cooled to room temperature and the solvent was evaporated to dryness. The
obtained crude products were purified by preparative thin-layer chromatography (PTLC) to give
products 11.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Data Availability
Crystallographic data for 11ha has been deposited at the Cambridge Crystallographic Data Center
with the code CCDC 1961208. All other data including experimental detail, characterization data for
new compounds and photophysical properties are available on the Supporting Information.
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Shen, D.; Xu, Y.; Shi, S.-L. A Bulky Chiral N-Heterocyclic Carbene Palladium Catalyst Enables Highly
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13) Shibata, T.; Nishizawa, G.; Endo, K. Iridium-Catalyzed Enantioselective Formal [4+2]
Cycloaddition of Biphenylene and Alkynes for the Construction of Axial Chirality. Synlett 2008, 765–
68.
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(14) See the Supporting Information for the details of screening of conditions (S9).
(15) (a) Cheeseman, T. P.; Odell, A. L.; Raethel, H. A. trans-Effect Order for Alkene, Alkyne, Phosphine,
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Arsine, Stibine, and Sulphide Ligands from Studies of Diethylamine Exchange reactions of
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L,PtCl ,[ C]NHEt in Various Solvents. Chem. Commun. (London), 1968, 1496–1498. (b) Appleton, T.
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G.; Clark, H. C.; Manzer, L. E. The trans-influence: its measurement and significance. Coord. Chem.
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for octahedral complexes by using an NMR probe. Dalton Trans. 2019, 48, 1814–1822.
(16) See the Supporting Information for the experimental and calculated data in details (S10-11).
(17) Single crystal of 11ha for X-ray analysis was obtained as a co-crystal with hexane. For the clarity,
hexane was omitted from the figure in Table 2. CCDC 1961208 contains the supplementary
crystallographic data of 11ha.
(18) As a representative example, we simulated the CD spectra of (R)-11ja and it was in good
agreement with that of measured CD spectra of the major enantiomer of 11ja. Thus, we showed all
cycloadducts in R-form. See Supporting Information in details (S16).
(19) CD spectra of 11ba, 11ia, 11ja, and 13 were shown in the Supporting Information.
(20) Schloman, W. W.; Morrison, H. Organic Photochemistry. 35. Structural Effects on the Nonradiative
Decay of Alkylbenzenes. The Nature of the “α-Substitution Effect”. J. Am. Chem. Soc. 1977, 99, 3342–
345.
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(21) We measured UV-Vis and fluorescence spectra, and quantum yields of 11aj, 11ak, and 11al.
Compound 11ak showed the highest quantum yield of 72%. See the Supporting Information in details
(S12-13).
(22) Hu, J. Y.; Era, M.; Elsegood, M. R. J.; Yamato, T. Synthesis and Photophysical Properties of
Pyrene-Based Light-Emitting Monomers: Highly Pure-Blue-Fluorescent, Cruciform-Shaped
Architectures. Eur. J. Org. Chem. 2010, 72–79.
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R
Ar2
Ar2
Ar1
Ar1
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cat. [RhCl{(R,R)-Ph-bod}]2
R
Ph
Ph
up to >99% yield
up to >99% ee
(
R,R)-Ph-bod
R
CPL
R
R
t-Bu
R
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