Synthesis of substituted helicenes by Ir-catalyzed annulative coupling of biarylcarboxylic acid chlorides with alkynes

Article abstract of DOI:10.1246/bcsj.20180081

A series of substituted [4] and [5]helicenes were synthesized in moderate to good yields by an Ir-catalyzed annulative coupling of biarylcarboxylic acid chlorides with internal alkynes, which involves facile C-H bond cleavage and decarbonylation. The double annulative coupling of 1, 1′:5′, 1″-ternaphthalene- 2, 2″-dicarboxylic acid dichloride with 4-octyne was also accomplished to give rise to an S-shaped double helicene. Unexpectedly, a π-extended benzofluoranthene-merged [5]- helicene was constructed through the annulative coupling and the successive C(aryl)-C(aryl) bond forming reaction when 1, 1′:4′, 1″-ternaphthalene-2, 2″-dicarboxylic acid dichloride was employed as the substrate. The crystal structure and the optical properties of the latter unique product were also investigated.

Full text of DOI:10.1246/bcsj.20180081

Advance Publication Cover Page
Synthesis of Substituted Helicenes by Ir-Catalyzed Annulative Coupling of
Biarylcarboxylic Acid Chlorides with Alkynes
Ken Kamikawa,* Hiroakira Den, Akihiro Tsurusaki, Tomoya Nagata, and Masahiro Miura*
Advance Publication on the web April 14, 2018
doi:10.1246/bcsj.20180081
© 2018 The Chemical Society of Japan
Advance Publication is a service for online publication of manuscripts prior to releasing fully edited, printed versions. Entire
manuscripts and a portion of the graphical abstract can be released on the web as soon as the submission is accepted. Note that the
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Publication manuscripts.

Synthesis of Substituted Helicenes by Ir-Catalyzed Annulative Coupling of
Biarylcarboxylic Acid Chlorides with Alkynes
Ken Kamikawa,*1 Hiroakira Den,1 Akihiro Tsurusaki,1 Tomoya Nagata,2 Masahiro Miura*2
1
Department of Chemistry, Graduate School of Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan
2
Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
kamikawa@c.s.osakafu-u.ac.jp, miura@chem.eng.osaka-u.ac.jp
Ken Kamikawa
Ken Kamikawa was born in Japan in 1969. He received his Ph.D. (1998) from Osaka City University supervised by
Prof. M. Uemura. After working as a post-doctoral fellow (1998-1999) at Massachusetts Institute of Technology with
Professor S. L. Buchwald, he joined the Department of Chemistry, Faculty of Integrated Arts and Sciences, Osaka
Prefecture University as an assistant professor (1999). He was promoted to an associate professor in 2004 and to full
professor in 2015. He was awarded the Incentive Award in Synthetic Organic Chemistry of Japan (2007).
Masahiro Miura
Masahiro Miura is a professor of the Graduate School of Engineering, Osaka University. He received his Ph.D.
degree from Osaka University in 1983 under the guidance of Prof. S. Kusabayashi and Prof. M. Nojima. He started his
academic career as assistant professor at Osaka University in 1984. He was promoted to associate professor in 1994 and
to full professor in 2005. He also worked as a Humboldt fellow with Prof. K. Griesbaum at Karlsruhe University of
Germany in 1990-1991. His current research interests include transition-metal catalysis and the synthesis of functional
molecules, especially new π-conjugated aromatic substances. He received the CSJ award for Creative work for 2012.
Abstract
synthesis, we previously reported the synthesis of azahelicene
N-oxides by a Pd-catalyzed direct C-H annulation of the
pendant Z-bromovinyl side chain (Scheme 1 (1)).3d However,
some drawbacks remain, including the need to introduce a
Z-bromovinyl side chain and to activate a C-H bond by an
electron-withdrawing N-oxide group. In order to synthesize
multi-functionalized helicenes, it is crucial for the development
of versatile methods to overcome such drawbacks.
A
series of substituted [4] and [5]helicenes were
synthesized in moderate to good yields by an Ir-catalyzed
annulative coupling of biarylcarboxylic acid chlorides with
internal alkynes, which involves facile C-H bond cleavage and
decarbonylation.
1,1’:5’,1’’-ternaphthalene-2,2’’-dicarboxylic acid dichloride
with 4-octyne was also accomplished to give rise to an
The double annulative coupling of
S-shaped double helicene.
Unexpectedly, a π-extended
benzofluoranthene-merged [5]helicene was constructed through
the annulative coupling and the successive C(aryl)-C(aryl)
Previous Work
R1
R1
Pd-Catalyzed
Intramolecular
C-H Coupling
Br
bond
forming
reaction
when
1,1’:4’,1’’-ternaphthalene-2,2’’-dicarboxylic acid dichloride
was employed as the substrate. The crystal structure and the
optical properties of the latter unique product were also
investigated.
(1)
N
N
O
O
R2
R2
Keywords: Helicene, C-H Bond Functionalization,
Ir-Catalyzed Coupling
This Work
R1
R1
Ir-Catalyzed
Annulative
Coupling
R3
R3
1. Introduction
R3
R3
+
(2)
Helicenes, which are screw-sense π-conjugated
polyaromatic compounds, are receiving immense attention due
to their potential utility as functional organic molecules.1 The
characteristic features of helicenes offer promising prospects
for applications in asymmetric catalysis, supramolecular
chemistry, liquid crystals, and organic devices.1a–1d Various
synthetic approaches for the efficient construction of twisted
conjugated aromatic systems have been actively investigated.
As one such approach, the aromatic C-H bond activation
reactions by transition-metal catalysts are promising due to
their efficient and straightforward nature that allows access to a
wide range of condensed aromatic compounds.2 Meanwhile,
there have only been few studies of helicene synthesis so far
although they could render a number of approaches to carbo-
and hetero-helicenes with asymmetrical blades.3 As part of
our continuous efforts to uncover new approaches to helicene
COCl
R2
R2
Scheme 1. Helicene synthesis involving C-H bond cleavage.
In light of these results, we focused on the development
of an Ir-catalyzed annulative coupling of biarylcarboxylic acid
chlorides with alkynes in our quest for the efficient helicene
synthesis (Scheme 1 (2)). This protocol features a unique
process that involves two successive carbon-carbon bond
formation steps accompanied by the elimination of carbon
monoxide and hydrogen chloride via facile C-H bond cleavage,
which would confer a high degree of flexibility in the design of
helicene molecules.4 Consequently, we took on the task of

synthesizing a series of [4] and [5]helicenes by using the above
methodology. The results are reported herein.
the further improvement of yield, other phosphine ligands were
also examined. Among them, JohnPhos was found to give a
comparable yield to that using PtBu3·HBF4, forming product 2
in 50% yield (entry 2). PCy3·HBF4 was also inferior to
PtBu3·HBF4 (entry 3). Therefore, PtBu3·HBF4 was confirmed
to be the ligand of choice for the substituted helicene synthesis.
After the optimization studies, we adopted the conditions to the
synthesis of other substituted [4] and [5]helicenes (Table 2).
2. Results and Discussion
In order to optimize the reaction conditions, we initially
evaluated the performance of several phosphine ligands in the
transformation of 1-phenyl-2-naphthoyl chloride (1) into
corresponding benzophenanthrene derivative 2 using 4-octyne
as the coupling partner (Table 1).
The
substrates
we
prepared
were
chloride
chloride
chloride
chloride
as
follows:
(3a),
(3b),
(3c),
1-(o-tolyl)-2-naphthoyl
1-(o-methoxyphenyl)-2-naphthoyl
1-naphthyl-2-naphthoyl
Table 1. Ligand screening for Ir-catalyzed annulative coupling
of 1-phenyl-2-naphthoyl chloride (1) with 4-octyne.
1-(phenanthren-9-yl)-2-naphthoyl
(3d)
and
1-(pyren-4-yl)-2-naphthoyl chloride (3e). These substrates were
coupled with internal alkynes 4a–c for 18 h in refluxing xylene
in the presence of [IrCl(cod)]2/PtBu3·HBF4 and a series of
substituted [4] or [5]helicene derivatives 5 with symmetric and
asymmetric blades were obtained in preparatively useful yields.
In particular, π-extended helicenes 5ea–5ec, which have a
pyrene-helicene hybrid structure, were obtained in reasonable
yields. The hybrid core derivatives are known to exhibit
unique physical properties.5 Meanwhile, multiple helicenes
with plural helicities have attracted considerable attention and
been developed actively in recent years as these compounds
may have unique optoelectronic properties.6–11 Thus, we next
examined double annulative coupling reactions for substrates
with two carboxylic acid chloride functions, anticipating the
generation of S- and W-shaped double helicenes (Figure 1).12
4-octyne (5 eq.)
[IrCl(cod)]2 (5 mol%)
ligand (10 mol%)
nPr
COCl
xylene, reflux, 18 h
nPr
1
2
Entry
Ligand
Yield/%
57
1
2
3
PtBu3·HBF4
JohnPhos
50
35
PCy3·HBF4
Table 2. Substrate scope for Ir-catalyzed annulative coupling
of 1-aryl-2-naphthoyl chloride 3 with alkynes 4.
R
R
[IrCl(cod)]2 (5 mol%)
PtBu3·HBF4 (10 mol%)
R
+
xylene, reflux
18 h
COCl
R
3a-e
R = nPr
R = nBu
4a:
4b:
5
W-Shaped Double Helicene
S-Shaped Double Helicene
C6H4(p-nBu)
4c: R =
Figure 1. Disconnection approach for the synthesis of S- and
W-shaped double helicenes.
R
R
R
R
R
MeO
Me
nPr
R
nPr
ClOC
4-octyne (10 eq)
5ca: 51%
5cb: 43%
5cc:
5aa: 49%
5ab: 54%
5ac:
5ba: 48%
5bb: 41%
5bc: 26%
[IrCl(cod)]2 (10 mol%)
PtBu3·HBF4 (20 mol%)
24%
45%
xylene, reflux, 18 h
6 %
nPr
COCl
nPr
6
7
R
R
Scheme 2. Synthesis of S-shaped double helicene 7 by double
Ir-catalyzed coupling.
R
R
Initially,
we
prepared
5da: 34%
5db: 52%
5dc: 38%
5ea: 39%
5eb: 44%
5ec:
47%
1,1’:5’,1’’-ternaphthalene-2,2’’-dicarboxylic acid dichloride (6)
by cross-coupling and subsequent conventional transformations
(see Supporting Information for details) and conducted the
reaction with 4-octyne in the presence of 10 mol% of Ir catalyst
In the first instance, 1 was reacted with 4-octyne for 18 h in
refluxing xylene in the presence of [IrCl(cod)]2 together with
PtBu3·HBF4, which was an optimal ligand in our previous
study.4 Under these conditions, desired product 2 was
furnished in 57% yield (entry 1). The major byproduct for
this reaction was 1-phenyl-2-naphthoic acid (27%), which was
formed by the hydrolysis of unreacted starting material 1. For
under standard conditions.
To our delight, the double
coupling reaction proceeded to afford desired S-shaped double
helicene 7 albeit in a low yield. Other by-products were also
formed but could not be identified.

1,1’:4’,1’’-ternaphthalene-2,2’’-dicarboxylic acid dichloride (8)
was prepared and subjected to the double annulative coupling
as above. However, the desired W-shaped product could not
4-octyne (10 eq)
[IrCl(cod)]2 (10 mol%)
PtBu3·HBF4 (20 mol%)
be obtained.
In this case, benzo[j]fluoranthene-merged
helicene 9 was isolated in 8% yield instead (Scheme 3) and its
bonding nature was confirmed by X-ray diffraction analysis
COCl
xylene, reflux, 18 h
8 %
nPr
nPr
COCl
(vide infra).
A plausible reaction mechanism for the
formation of 9 is illustrated in Scheme 4. The formation of 9
may be rationalized by two successive coupling processes
involving a single Ir-catalyzed annulative coupling with alkyne
(First Ir Cycle) and an Ir-catalyzed direct C(aryl)-C(aryl)
coupling (Second Ir Cycle). Initially, acyliridium species A is
formed and subsequent decarbonylation and C-H bond
cleavage take place to give iridacycle intermediate B.4
Insertion of the alkyne gives seven-membered iridacycle
intermediate C and subsequent reductive elimination gives
singly cyclized product 10. Then, thus-formed 10 enters the
second Ir-catalyzed C-C bond formation process (Second Ir
Cycle). Another acyl chloride unit reacts with the Ir catalyst
8
9
nPr
nPr
nPr
nPr
not detected
Scheme 3. Synthesis of 9 via successive C-H bond cleavage.
One of the possible reasons for the low yield may be the
increased molecular distortion by the plural helicities. Next,
expecting the formation of a W-shaped double helicene,
nPr
9
Ir
8
Ir
nPr
COCl
10
First Ir Cycle
Second Ir Cycle
Cl
Ir
Ir Cl
Cl2Ir
nPr
O
nPr
COCl
O
Cl2Ir
COCl
nPr
nPr
nPr
C
nPr
E
A
nPr
D
CO, HCl
nPr
Ir
Cl
CO, HCl
COCl
B
Scheme 4. Plausible reaction mechanism for the formation of 9.
to form acyl iridium D and to avoid steric repulsion between
one of the n-propyl groups and the acyl iridium group, a
rotation around the axis occurs to produce iridacycle
intermediate E. Finally, reductive elimination gives product 9.
Single crystals of 9 suitable for X-ray diffraction analysis were
obtained as yellow prisms by room-temperature vapor diffusion
of hexane into chloroform solution. The dihedral angle
defined by the two least squares planes in the [5]helicene
moiety was measured as 50.9°, which is within the typical
range of those of [5]helicene derivatives,12 whereas the
benzo[j]fluoranthene moiety in 9 was found to be almost flat
(Figure 2 (b)). The crystal packing of 9 showed that the P-
and M-isomers of the [5]helicene moiety formed a pair of
molecules via the intermolecular π-π stacking between the
benzo[j]fluoranthene moieties, whose interplanar distance was
calculated as 3.525 Å. In addition, CH-π interaction was
observed between both edged aromatic rings of the
benzo[j]fluoranthene and [5]helicene moieties with a distance
of 2.776 Å (Figure 2 (c)).

Figure 2. X-ray crystallography of 9 (a: Front View, b: Side
View, c: Crystal Packing).
explained by the paring nature of two molecules as found in the
X-ray diffraction analysis. The quantum yield (Φf) of 9 in
CHCl3 solution is 0.16, which is higher than that of [5]helicene
(Φf = 0.04)18 and benzo[j]fluoranthene (Φf = 0.07).19 The
observed lifetime (τs) is 7.6 ns, which is shorter than that of
[5]helicene (τs = 25.5 ns)18 and comparable to that of
benzo[j]fluoranthene (τs = 8.0 ns).19 The quantum yield and
the observed lifetime of 9 in powder form are 0.073 and 12–13
ns, respectively.20
Although π embedded in a [5]helicene moiety, such as pyrene,5
corranulene,13 and others,14 were well investigated, to the best
our knowledge, the reported fluoranthene−[5]helicene hybrid
derivative is only benzo[j]indeno[1,2,3-hi]chrysene (11)15,16
(see ref 15 for the structure of 11). As 9 showed a green
emission in CHCl3 solution (Figure 3, (a)) and a yellow
emission in powder form (Figure 3, (b)), we investigated its
photophysical properties (Figure 4).
3. Conclusion
We have succeeded in constructing a series of substituted
[4] and [5]helicenes by the Ir-catalyzed annulative coupling of
biarylcarboxylic acid chlorides with internal alkynes in
moderate to good yields. This protocol enabled us to integrate
(a)
(b)
three different components into
a framework, thereby
delivering multi-functionalized helicenes. We also succeeded
in synthesizing S-shaped double helicene 7. During the
course of this work, we unexpectedly synthesized
benzofluoranthene-merged helicene 9, which showed unique
optical properties.
4. Experimental
Typical Procedure for Reactions of 3 with 4 in Table
2. The carboxylic acid chlorides 3 were prepared by the
procedure described in Supporting Information. To the crude
mixture of acid chloride 3 was added [IrCl(cod)]2, tBu3P·HBF4,
and p-xylene (0.2 M). After the addition of alkyne (5 equiv.),
the reaction mixture was stirred at 160 ℃ (bath temperature)
for 18 h. The volatiles were removed under reduced pressure at
room temperature. The residue was chromatographed on silica
gel with eluent (hexane/EtOAc or hexane/CHCl3) to give 5.
Figure 3. Photographs of (a) green emission in CHCl3 solution
and (b) yellow emission in powder of 9 excited at 365 nm.
Supporting Information
Detailed experimental procedures, spectroscopic data for
the products, theoretical studies, and crystallographic data for 9
are
described.
This
material
is
available
on
http://dx.doi.org/xxxxxx/bcsj.20180081.
Acknowledgement
This work was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas "Precisely Designed Catalysts
with Customized Scaffolding" (#JP16H01039 to K.K.) from
JSPS; a Grant-in-Aid for Scientific Research (B) (#15H03780);
Figure 4. UV-vis absorption in CHCl3 solution (blue line),
fluorescence in CHCl3 solution (green line), and fluorescence
in powder form (orange line) of 9.
a
Grant-in-Aid for Challenging Exploratory Research
(#16K13956 to K.K.); a Grant-in-Aid for Research Activity
start-up (#16H07126 to A.T.); and a Grant-in-Aid for Specially
Promoted Research (#17H06092 to M.M). K.K. also thanks
the Naito Foundation and the Yamada Science Foundation. A.
T. thanks Takasago International Corporation for a research
fellowship. The authors thank Professor Ikuko Miyahara
(Osaka City University). Professor Hideki Fujiwara (Osaka
Prefecture University) is gratefully acknowledged for providing
The UV-vis spectrum of 9 in CHCl3 showed complicated
absorptions with λmax of 318, 356, and 402 nm and a shoulder
at 432 nm. The longest absorption (432 nm) was significantly
red-shifted compared to those of [5]helicene (350 nm)5d and
benzo[j]fluoranthene (365 nm),17 which should be ascribed to
the extended π-conjugation. Time–dependent (TD) DFT
calculations at B3LYP/6-311+G(2d,p)/B3LYP/6-31G(d) level
of theory revealed that this absorption is assignable to the
HOMO–LUMO transition (λcalc = 460.8 nm, f = 0.180). The
HOMO (–5.26 eV) is delocalized over the whole molecule,
whereas the LUMO (–1.97 eV) is mainly delocalized over the
benzo[j]fluoranthene moiety together with some contribution of
access
to
UV-vis
and
fluorescence
spectroscopy
instrumentation.
The authors would also like to thank
Professor Satoshi Minakata and Professor Youhei Takeda
(Osaka University) for the measurement of absolute quantum
yield and photoluminescence lifetime.
All theoretical
the [5]helicene moiety (Figure S1).
The fluorescence
calculations were carried out at the Research Center for
Computational Science (Japan).
spectrum of 9 in CHCl3 solution is a mirror image of the longer
wavelength region of the UV-vis spectrum (370–460 nm)
(Figure 4), the maxima of which (λmax = 484 and 510 nm) are
also red-shifted compared to those of [5]helicene (375 nm)5d
The fluorescence
spectrum of 9 in powder form (λmax = 560 nm) is broad and
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