Facile Synthesis of Azahelicenes and Diaza[8]circulenes through the Intramolecular Scholl Reaction

Article abstract of DOI:10.1002/chem.202102269

Carbazole-based aza[7]helicenes and hetero[9]helicenes were successfully obtained via the intramolecular Scholl reaction of 3,6-bis(biphenyl-2-yl)carbazole congeners, while the reaction of 3,6-bis(naphthylphenyl)-appended carbazole gave a triple helicene via an unexpected simultaneous double aryl rearrangement. DFT calculations suggested that the rearrangement proceeded via an arenium cation intermediate. In addition, the reaction of methoxy-appended substrate gave an azahepta[8]circulene via the concurrent C?C bond formation. These helical dyes showed circularly polarized luminescence. The azahepta[8]circulene was further transformed into deeply saddle-distorted dibenzodiaza[8]circulenes as the first example of its solution-based synthesis and unambiguous structural determination.

Full text of DOI:10.1002/chem.202102269

Accepted Article
Accepted Article
Title: Facile Synthesis of Azahelicenes and Diaza[8]circulenes through
the Intramolecular Scholl Reaction
Authors: Chihiro Maeda, Shuichi Nomoto, Koki Akiyama, Takayuki
Tanaka, and Tadashi Ema
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.202102269
Link to VoR: https://doi.org/10.1002/chem.202102269
01/2020

10.1002/chem.202102269
Chemistry - A European Journal
RESEARCH ARTICLE
Facile Synthesis of Azahelicenes and Diaza[8]circulenes through
the Intramolecular Scholl Reaction
Chihiro Maeda,*[a] Shuichi Nomoto,[a] Koki Akiyama,[a] Takayuki Tanaka,[b] and Tadashi Ema*[a]
[a]
Dr. C. Maeda, S. Nomoto, K. Akiyama, Prof. T. Ema
Division of Applied Chemistry, Graduate School of Natural Science and Technology, Okayama University,
Tsushima, Okayama 700-8530, Japan
E-mail: cmaeda@okayama-u.ac.jp
ema@cc.okayama-u.ac.jp
[b]
Dr. T. Tanaka
Department of Chemistry, Graduate School of Science, Kyoto University,
Sakyo-ku, Kyoto 606-8502, Japan
Supporting information for this article is given via a link at the end of the document.
Abstract: Carbazole-based aza[7]helicenes and hetero[9]helicenes
were successfully obtained via the intramolecular Scholl reaction of
3,6-bis(biphenyl-2-yl)carbazole congeners, while the reaction of 3,6-
bis(naphthylphenyl)-appended carbazole gave a triple helicene via an
unexpected simultaneous double aryl rearrangement. DFT
calculations suggested that the rearrangement proceeded via an
arenium cation intermediate. In addition, the reaction of methoxy-
appended substrate gave an azahepta[8]circulene via the concurrent
C–C bond formation. These helical dyes showed circularly polarized
luminescence. The azahepta[8]circulene was further transformed into
deeply saddle-distorted dibenzodiaza[8]circulenes as the first
example of its solution-based synthesis and unambiguous structural
determination.
(a)
[8]Circulene
Hetero[8]circulenes
X
N
N
X
X
X
X
X
1 report
no X-ray structures
no solution-state properties
many reports
with X-ray structures
on-surface synthesis
Ito (2020)
(b) This work
Ar
Ar
R
R
N
Bn
2a–h
rearrangement
NBn
NBn
Introduction
Scholl reaction
3
Triple helicene
1a–c
Aza[7]helicenes
X
Circulene derivatives and congeners have been recognized as
unique polycyclic aromatic hydrocarbons (PAHs), and their
structural and electronic properties have attracted considerable
attention.[1–6] [6]Circulene consisting of six ortho-fused benzene
rings adopts a planar structure, while larger circulenes often adopt
nonplanar saddle-shaped structures to reduce the strain. Among
the circulenes, hetero[8]circulenes, which have one or more fused
heterole rings around the cyclooctatetraene (COT) skeleton
instead of fused benzene rings, have been actively investigated
owing to their optelectronic properties and possible antiaromatic
contribution from the central COT rings (Figure 1a). Pristine
[8]circulene adopts a saddle-shaped structure,[3] while four or
eight heterole-containing [8]circulenes adopt nearly planar
structures.[4,5] Diheterole-based [8]circulenes occupies an
important position to exploit deeper understanding in their
properties and to create a new class of highly strained PAHs.
However, their intrinsic high strain energies have hampered their
synthesis in the solution state and only on-surface synthesis of
the p-extended diaza[8]circulene has recently been achieved by
Ito and co-workers.[6]
R
N
MeO
OMe
X
NBn
N
N
Bn
1f–h (X = NMe, O, S)
Aza[9]helicenes
Bn
Azahepta[8]circulene
4
Diaza[8]circulenes
Figure 1. (a) Structures of [8]circulenes. (b) Synthesis of azahelicenes and
diaza[8]circulenes in this work.
As another strained motif, helicenes have also received
considerable attention during the last decade, and their structural
and chiroptical properties have been actively studied.[7–10] We
have explored the synthesis and chiroptical properties of chiral
carbazole-based BODIPY analogues, and recently reported the
synthesis of azahelicene-fused boron complex, where the novel
carbazole-based azahelicene was developed via the short step
synthesis.[11] In this paper, we synthesized a series of carbazole-
based azahelicenes via the intramolecular Scholl reaction of 3,6-
bis(biphenyl-2-yl)carbazole congeners 2a–h to investigate their
chiroptical properties (Figure 1b). Aza[7]helicenes 1a–c and
aza[9]helicenes 1f–h were successfully obtained, while triple
helicene 3 was produced from naphthyl-appended 2e via a double
aryl rearrangement. In addition, azahepta[8]circulene 4 was
formed from 2d.[12] We envisioned that 4 would serve as a useful
1
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10.1002/chem.202102269
Chemistry - A European Journal
RESEARCH ARTICLE
intermediate for the synthesis of diaza[8]circulenes. Fortunately,
(a)
(b)
we
succeeded
in
the
transformation
of
4
into
dibenzodiaza[8]circulenes and their unambiguous structural
determination by X-ray diffraction analysis for the first time. Here,
we report the synthesis, structures, and photophysical properties
of the azahelicenes and aza[8]circulenes.
C4
C3
C4
C3
17.1°
19.1°
C1
C2
C1
C2
Results and Discussion
(c)
(d)
The FeCl3-mediated intramolecular Scholl reaction of N-benzyl-
3,6-bis(biphenyl-2-yl)carbazole (2a) gave azahelicene 1a
(Scheme 1). Similarly, azahelicenes 1b and 1c bearing methyl
and tert-butyl groups were obtained in 95% and 42% yields,
respectively.[13] Interestingly, the reaction of methoxy-substituted
2d gave azahepta[8]circulene 4 via the concurrent aromatic
coupling,[14,15] and the reaction of naphthyl-appended 2e gave
triple helicene 3 via an unexpected double aryl rearrangement.[16]
4 was characterized by NMR and mass spectroscopy, while 3 was
elucidated by the X-ray diffraction analysis (Figure 2c). The X-ray
crystal structure of 3 revealed the aza[7]helicene moiety and two
[4]helicene moieties all of which adopted the same helicities.
Furthermore, we conducted the reaction of 2f–h with
benzoheterole frameworks, which gave hetero[9]helicenes 1f–h.
1h was successively oxidized by mCPBA to 1i with sulfonyl
groups. The structures of 1b, 1c, 3, and 1h were confirmed by the
X-ray diffraction analysis, which revealed the helical structures
(Figure 2). The torsion angles of the carbazole moieties (C1-C2-
C3-C4) increase as the size of the substituents increase, while
that in 1h was relatively small probably because of the stacking of
the benzothiophene moieties. The enantiomers of helicenes were
successfully separated by passing through a chiral column, and
their absolute configurations were deduced by TD-DFT
calculations (Supporting Information).[17]
C4
C3
20.9°
C2
C4
C3
13.1°
C2
C1
C1
Figure 2. X-ray crystal structures of (a) 1b, (b) 1c, (c) 3, and (d) 1h. Hydrogen
atoms and solvent molecules are omitted for clarity. The torsion angles of the
carbazole moieties (C1-C2-C3-C4) are shown.
DFT calculations were performed to reveal the reaction
mechanism of the rearrangement from 2e to 3 at the (U)B3LYP/6-
31G(d) level using SMD solvation effect of dichloromethane
(Scheme 2 and S10 in Supporting Information). 3-[2-(Naphthalen-
2-yl)phenyl]carbazole (R) was used as a model compound. We
calculated two pathways from R to the helicenes (A5 and A8) via
the arenium cation and radical cation intermediates according to
the literatures.[10j,16] First, the arenium cation pathway is proposed
as follows (Scheme 2). Protonation of R at the 2-position of the
naphthalene moiety gives A1.[18] C–C bond formation between the
3-position of the carbazole moiety and the 1-position of the
naphthalene moiety provides A2 with the bond length of 1.63 Å.
Oxidation and deprotonation of A2 affords A3, followed by the 1,2-
phenyl shift to A4. The C–C bond lengths linked between the
carbazole moiety and the naphthalene moiety in A3 and A4 are
1.55 Å and 1.46 Å, respectively, indicating relatively tight bonds
at these stages. Finally, deprotonation of A4 affords [5]helicene
A5 as a rearrangement product. On the other hand, C–C bond
formation of A1 at the 4-position of the carbazole moiety gives
rise to A6. The oxidation and deprotonation of A6 gives A7 which
is transformed into [6]helicene A8. The transition state energy
from A1 to A2 and A6 is 4.1 and 4.7 kcal/mol, respectively.[19] The
kinetically favored formation of A2 over A6 as well as the large
stabilization from A2 into A3 might promote the rearrangement to
A5 via the arenium cation pathway. Next, another possible
pathway via the radical cation intermediate is proposed as follows
(Scheme S10 in Supporting Information). The oxidation of R gives
rise to a radical cation species R1. C–C bond formation of R1 at
the 3-position of the carbazole moiety forms spiro-fused adduct
R2. The subsequent release of hydrogen radical gives R3,
equivalent to A3. The 1,2-aryl shift of the phenyl group and the
deprotonation provide [5]helicene A5. On the other hand, C–C
bond formation of R1 at the 4-position of the carbazole moiety
forms 3,4-fused adduct R4. The subsequent release of hydrogen
radical and proton gives [6]helicene A8. The transition state
energy from R1 to R2 and R4 is 27.2 and 24.9 kcal/mol,
respectively, and the rearrangement via the radical cation
pathway is considered to be disfavored. In the present study,
MeO
OMe
R
R
R
R
FeCl3
MeNO2
or
CH2Cl2
NBn
NBn
N
Bn
1a: R = H 86%
1b: R = Me 95%
1c: R = tBu 42%
2a–d
4 93%
chloranil
BF3•OEt2
CH2Cl2
NBn
3 31%
NBn
2e
X
X
conditions
1f: X = NMe 40% (B)
1g: X = O 53% (C)
1h: X = S 78% (A)
A: FeCl3, MeNO2
B: DDQ, MSA
C: DDQ, BF3•OEt2
X
X
mCPBA
CH2Cl2
CH2Cl2
NBn
NBn
1i: X = SO2 71%
2f–h
Scheme 1. Synthesis of azahelicenes.
2
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10.1002/chem.202102269
Chemistry - A European Journal
RESEARCH ARTICLE
therefore, rearrangement via the arenium cation intermediate is
considered to be dominant from the result of the calculations.
H
– 2H+
– 2e–
H
– H+
H
rearrangement
(favored)
H
N
H
N
H
N
H
N
H
TS (4.1)
A2 (0.3)
A3 (–32.3)
A4 (–30.5)
A5 (–77.1)
+ H+
H
H
(disfavored)
– 2H+
– 2e–
N
H
N
H
– H+
R
H
A1 (0.0)
H
TS (4.7)
N
H
N
H
N
H
A6 (–1.4)
A7 (–24.0)
A8 (–77.7)
TSA1–A6
TSA1–A2
A2
A6
A1
A7
A4
A1
A4
A3
TSA1–A2
A2
A5
A5
A8
A3
Scheme 2. Proposed reaction mechanism of the rearrangement from 3-[2-(naphthalen-2-yl)phenyl]carbazole (R) to helicene A5 via arenium cation intermediates.
The values in parenthesis are relative energies in kcal/mol. Potential energy profiles and selected optimized structures are shown. An energy gap between chloranil
and tetrachlorohydroquinone is included in A3 and A7, while that between [chloranil×H]- and tetrachlorohydroquinone is included in A5 and A8.
Chiroptical properties of the optically pure helicenes were
investigated (Figure 3 and Table 1). All the helical dyes showed
(a)
200
100
0
4
2
absorption at 250–450 nm and fluorescence at 400–550 nm with
the fluorescence quantum yields of 0.08–0.31, and the Cotton
effects and CPL signals were observed at the regions. The CD
and CPL spectra of 1a–c are similar, while the gabs and glum values
slightly increase with an increase of sizes of the substituents, and
1c recorded the maximum |glum| of 0.0035. On the other hand, the
Cotton effects and CPL intensities of azahepta[8]circulene 4 and
triple helicene 3 were much weaker. The chiroptical properties of
hetero[9]helicenes 1f–i changed upon the replacement of the
hetero atoms. (P)-Enantiomers of 1f–i similarly showed the
positive Cotton effects at 300 nm. For the longest wavelength
regions, however, (P)-1g–i showed the negative Cotton effects,
while (P)-1f showed the positive Cotton effect. The sign of CPL is
consistent with the sign of the Cotton effects, suggesting that
excited-state structures are similar to the ground-state structures.
Electrochemical properties were investigated by cyclic
voltammetry. The oxidation potentials of 1b and 1c with alkyl
substituents were 0.65 V and 0.66 V, respectively, which were
slightly lower than that of 1a (0.68 V). For hetero[9]helicenes, 1g
and 1h showed one oxidation wave at 0.59 V and 0.61 V,
respectively, while 1f showed two oxidation waves at 0.22 and
0.60 V. The low first oxidation potential of 1f might be ascribed to
the fused carbazole trimer structure.
0
-20
-40
0
I/
Δ
Δε
-100
-200
-2
-4
8
6
4
2
0
1a
1b
1c
4
3
ε
250
300
350 400
wavelength / nm
450
500
400
450
wavelength / nm
500
550
600
(b)
2
1
20
0
200
100
0
-20
0
I/
Δε
Δ
-100
-200
-1
-2
8
6
4
2
0
1f
1g
1h
1l
ε
250
300
350 400
wavelength / nm
450
500
400
450
500
wavelength / nm
550
600
Figure 3. Chiroptical spectra in CH2Cl2. (a) 1a–c, 3, and 4, and (b) 1f–i (For CD
and CPL, solid line: (P)-enantiomer, dotted line: (M)-enantiomer). Insets show
the enlarged CD spectra of (P)-enantiomers.
3
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10.1002/chem.202102269
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RESEARCH ARTICLE
lengths of the COT moiety are relatively long (1.44 and 1.47 Å)
and are arranged alternately. The longer C–C bond lengths of the
COT moiety indicate negligible 8p antiaromatic contribution, being
Table 1. Chiroptical results in CH2Cl2.
[a]
[b,c]
compd
(P)-1a
(P)-1b
(P)-1c
(P)-4
l
A [nm]
gabs [10-3
-4.9
]
l
F [nm][b]
FF
glum [10-3
]
analogous
to
previously
reported
hetero[8]circulenes.
418
419
419
418[d]
422
412
401
414[d]
440
432, 454
432, 455
432, 456
435
0.28
0.27
0.31
0.17
0.10
0.24
0.30
0.08
0.10
-3.2
Consistently, the nucleus-independent chemical shift (NICS)
value at the central position of the COT moiety of 7a is calculated
to be +9.10 ppm, that is ascribable to the induced paratropicity
arising from the surrounding polycyclic aromatics.
-5.9
-3.4
-5.4
-3.5
+0.23
-3.2
+0.25
-0.39
+0.29
-0.72
-1.1
(P)-3
458, 480
456
(a)
Bond lengths [Å]
C1–C2: 1.499(4)
C2–C3: 1.405(5)
(P)-1f
(P)-1g
(P)-1h
(P)-1i
+0.45
-0.92
-1.6
420, 441
432, 457
493
F
C3–C4: 1.465(4)
C4–C5: 1.441(5)
C1 C2
C8
E
C3
D
A
C7
C4
C5–C6: 1.473(5)
C6–C7: 1.444(5)
C7–C8: 1.468(5)
C8–C1: 1.406(5)
-0.73
-0.26
C6 C5
C
B
[a] gabs = De/e at the maximum wavelength of the longest Cotton effects. [b]
Excited at l = 320 nm. [c] glum = 2(IL-IR)/(IL+IR). Average values at lF ± 10 nm.
[d] Shoulder.
NICS(0) values [ppm]
Ring A : +5.21
Ring B : –9.70
Ring C : –9.26
Ring D : –1.35
Ring E : –7.85
Ring F : –8.37
HO
TfO
OTf
OH
PhNTf2
DMAP
Et3N
BBr3
4
CH2Cl2
40 °C
59%
CH2Cl2
rt
60%
N
Bn
N
Bn
5
6
H2O, K3PO4
RNH2, K3PO4
Pd2dba3, Xantphos
dry m-xylene
(b)
Pd2dba3, Xantphos
m-xylene
140 °C
Bond lengths [Å]
C1–C2: 1.467(2)
C2–C3: 1.441(2)
C3–C4: 1.470(2)
C4–C5: 1.447(2)
C5–C6: 1.466(2)
C6–C7: 1.442(3)
C7–C8: 1.472(3)
C8–C1: 1.444(2)
120–140 °C
R
N
O
C1 C2
C8
C7
C3
A
D
E
C4
C6 C5
7a: R = Bn 60%
7b: R = Ph 52%
N
Bn
N
Bn
C
8 39%
B
Scheme 3. Synthesis of dibenzohetero[8]circulenes.
NICS(0) values [ppm]
Ring A : +9.10
Ring B : –7.11
Ring C : –10.10
Ring D : –3.60
Ring E : –8.38
We expected that azahepta[8]circulene
4
might be
transformed into diaza[8]circulenes (Scheme 3).[20] Demethylation
of 4 with BBr3 gave dihydroxy-substituted 5, which was converted
into 5 with triflate groups. X-ray diffraction analysis of 6 revealed
the structure of highly deformed azahepta[8]circulene framework
(Figure 4a). To our delight, double amination reaction of 6 with
benzylamine under palladium catalysis successfully provided
dibenzodiaza[8]circulene 7a. The reaction with aniline similarly
provided the corresponding diaza[8]circulene 7b. During the
investigation of the amination reaction, azaoxa[8]circulene 8 was
found to be obtained under the wet conditions.[8b] 8 was obtained
in 39% yield in the presence of water under the amine-free
conditions. The structure of 7a was confirmed by the X-ray
Figure 4. X-ray crystal structures of (a) 6 and (b) 7a. Hydrogen atoms, solvent
molecules, and peripheral substituents in the side views are omitted for clarity.
The UV/vis absorption and fluorescence spectra of 7a, 7b,
and 8 are shown in Figure 5. The hetero[8]circulenes showed two
main absorption bands at 250–350 nm and 350–500 nm, and the
latter band of 7a was assigned as the transition from the HOMO
or HOMO-3 to the LUMO (Supporting Information). These
molecular orbitals are based on a carbazole framework, and the
allowed transitions are peculiar to aza[8]circulenes (Figure 6).
The spectra of 7a and 7b are quite similar, while that of 8 is blue-
diffraction
analysis,
revealing
the
saddle-shaped
diaza[8]circulene framework as the first example of diheterole-
based [8]circulenes (Figure 4b). The mean plane deviation of the
dibenzoazahepta[8]circulene
moiety
of
6
and
the
dibenzodiaza[8]circulene moiety of 7a are 0.78 and 0.90 Å,
respectively, indicating the distorted p-framework. The C–C bond
4
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10.1002/chem.202102269
Chemistry - A European Journal
RESEARCH ARTICLE
shifted. As compared to the delocalized HOMO of 7a (state 172), Conclusion
8 showed the localized HOMO at the carbazole moiety (state 148),
and the stabilized HOMO seems to contribute to the blue-shifted
absorption. They showed fluorescence at 500–650 nm with the
fluorescence quantum yields of less than 0.02. The broad
fluorescence is different from tetraaza[8]circulenes probably due
to the flexible framework in this system, and the benzo-fused
moieties might accelerate vibration energy relaxation.
Consequently, the diaza[8]circulenes have electronic states and
flexible structure similar to tetraaza[8]circulenes and pristine
[8]circulenes, respectively.
In conclusion, double intramolecular oxidative aromatic
coupling enables the transformation of substituted carbazoles into
aza[7]helicenes 1a–c, hetero[9]helicenes 1f–i, triple helicene 3,
and azahepta[8]circulene 4. DFT calculations suggested that the
unexpected double rearrangement from 2e to 3 proceeded via the
arenium cation intermediate. These helical dyes showed CPL with
the |glum| in the range of 2.5 ´ 10-4–3.5 ´ 10-3. Furthermore,
azahepta[8]circulene 4 was converted into diaza[8]circulenes 7a
and 7b and azaoxa[8]circulene 8 via the double amination
reaction. X-ray diffraction analysis of 7a revealed the saddle-
shaped diaza[8]circulene framework. In the present study, the X-
ray crystal structure and solution-state properties of diheterole-
based [8]circulenes have been reported for the first time. The
short-step synthesis of azahelicenes and aza[8]circulenes in this
work will be applicable to the synthesis of related helicenes and
PAHs. Further investigations on the development of novel
helicenes and circulenes are ongoing in our laboratory.
8
absorption fluorescence (lex = 440 nm)
7a
7b
8
FF = 0.01
FF = 0.01
FF = 0.02
6
4
2
0
ε
Deposition Numbers 2061453 (for 1c), 2061454 (for 1h),
2061455 (for 7a), 2061456 (for 1b), 2061457 (for 6), 2061458 (for
3) contain the supplementary crystallographic data for this paper.
These data are provided free of charge by the joint Cambridge
Crystallographic Data Centre and Fachinformationszentrum
300
400 500
wavelength / nm
600
700
Figure 5. UV/vis absorption and fluorescence spectra in CH2Cl2.
Karlsruhe
Access
Structures
service
www.ccdc.cam.ac.uk/structures.
8
7a
E / eV
–1.0
Acknowledgements
This work was supported by JSPS KAKENHI Grant Number
18K05083. We thank Prof. Suga and Prof. Mitsudo (Okayama
University) for CV measurements and Dr. K. Takaishi (Okayama
University) for TD-DFT calculations.
173
149
–2.0
–3.0
Keywords: carbazoles • circularly polarized luminescence •
circulenes • dyes • helicenes
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Figure 6. Molecular orbital diagrams of 7a and 8 calculated at the B3LYP/6-
31G* level.
5
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10.1002/chem.202102269
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RESEARCH ARTICLE
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6
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RESEARCH ARTICLE
Entry for the Table of Contents
Intramolecular Scholl reaction of carbazole derivatives afforded highly strained azahelicenes and azahepta[8]circulene which was
transformed into dibenzodiaza[8]circulenes. The diaza[8]circulene structure was characterized for the first time by X-ray diffraction
analysis which revealed the saddle-shaped p-conjugated polyaromatics.
7
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