Rapid Access to Nanographenes and Fused Heteroaromatics by Palladium-Catalyzed Annulative π-Extension Reaction of Unfunctionalized Aromatics with Diiodobiaryls

Article abstract of DOI:10.1002/anie.201707486

Efficient and rapid access to nanographenes and π-extended fused heteroaromatics is important in materials science. Herein, we report a palladium-catalyzed efficient one-step annulative π-extension (APEX) reaction of polycyclic aromatic hydrocarbons (PAHs) and heteroaromatics, producing various π-extended aromatics. In the presence of a cationic Pd complex, triflic acid, silver pivalate, and diiodobiaryls, diverse unfunctionalized PAHs and heteroaromatics were directly transformed into larger PAHs, nanographenes, and π-extended fused heteroaromatics in a single step. In the reactions that afford [5]helicene substructures, simultaneous dehydrogenative ring closures occur at the fjord regions to form unprecedented larger nanographenes. This successive APEX reaction is notable as it stiches five aryl–aryl bonds by C?H functionalization in a single operation. Moreover, the unique molecular structures, crystal-packing structures, photophysical properties, and frontier molecular orbitals of the thus-formed nanographenes were elucidated.

Full text of DOI:10.1002/anie.201707486

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Accepted Article
Title: Rapid Access to Nanographenes and Fused Heteroaromatics
by Palladium-Catalyzed Annulative π-Extension Reaction of
Unfunctionalized Aromatics with Diiodobiaryls
Authors: Wataru Matsuoka, Hideto Ito, and Kenichiro Itami
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201707486
Angew. Chem. 10.1002/ange.201707486
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10.1002/anie.201707486
Angewandte Chemie International Edition
COMMUNICATION
Rapid Access to Nanographenes and Fused Heteroaromatics by
Palladium-Catalyzed Annulative π-Extension Reaction of
Unfunctionalized Aromatics with Diiodobiaryls
Wataru Matsuoka,[1] Hideto Ito,*[1] and Kenichiro Itami*[1,2,3]
Abstract: Efficient and rapid access to nanographenes and π-
extended fused heteroaromatics is important in materials science.
Herein, we report a palladium-catalyzed efficient one-step annulative
π-extension (APEX) reaction of polycyclic aromatic hydrocarbons
(PAHs) and heteroaromatics, producing various π-extended
aromatics. In the presence of a cationic Pd complex, triflic acid,
silver pivalate, and diiodobiaryls, diverse unfunctionalized PAHs and
heteroaromatics were directly transformed into larger PAHs,
nanographenes, and π-extended fused heteroaromatics in a single
step. In the reactions that afford [5]helicene substructures,
simultaneous dehydrogenative ring closures occur at the fjord
regions to form unprecedented larger nanographenes. This
successive APEX reaction is notable as it stiches five aryl–aryl
bonds by C–H functionalization in a single operation. Moreover, the
1b).[6,7] APEX involves the one-step π-extension of
unfunctionalized template aromatics along with the construction
of one or more new fused aromatic rings. Currently, various
types of transformations are categorized as APEX reactions.[6]
Although many APEX reactions of heteroaromatics have been
reported, the APEX reactions of inactive aromatic hydrocarbons
such as benzenes, naphthalenes, and PAHs are still limited and
thus regarded as challenging subjects.[7,8] Development of new
APEX chemistry for PAHs is highly demanded in the field of
nanocarbon and material sciences. In this paper, we report a
novel APEX reaction of various PAHs and heteroaromatics with
diiodobiaryls effectively promoted by a Pd catalyst for the
efficient and rapid access to nanographenes and π-extended
heteroaromatics.
unique
molecular
structures,
crystal-packing
structures,
photophysical properties, and frontier molecular orbitals of the thus-
formed nanographenes were elucidated.
a) General stepwise π-extensioin of unfunctionalized aromatics
cross-
coupling
X
cyclo-
dehydrogenation
(Scholl reaction)
halogenation
or
metalation
π-Extended fused aromatics have recently received much
attention in synthetic chemistry and material sciences.[1] In
particular, nanographenes and polycyclic aromatic hydrocarbons
(PAHs) show superior physicochemical properties over small
aromatics, arising from their extended π-conjugation, narrow
HOMO–LUMO gaps, long-wavelength absorptions and
emissions, lower redox potentials, strong π–π interactions, and
higher mechanical strengths.[2] The general synthesis methods
for nanographenes are based on stepwise protocols involving
double
cross-coupling
dihalogenation
H
H
X
X
unfunctionalized
aromatics
· PAHs
· nanographenes
· GNRs
cross-coupling
reactions,
Diels–Alder
reactions,
b) This work: one-step annulative π-extension (APEX)
[2+2+2]cyclotrimerization, cyclization of acetylenes, and
cyclodehydrogenation such as Scholl reaction.[3,4] In addition, the
sequence of cross-coupling reaction/cyclodehydrogenation and
double cross-coupling reaction of dihaloarenes has been
regarded as reliable synthetic routes to nanographenes (Figure
1a).[3,5]
H
Ar
Ar
H
Annulative
π-Extension
(APEX)
template aromatics
π-extended fused aromatics
Pd cat.
Ag salt
TfOH
I
I
Recently, we established a novel synthetic concept, APEX,
for the efficient and rapid synthesis of large PAHs,
nanographenes, and π-extended fused heteroaromatics (Figure
ClCH2CH2Cl
50 °C
[1]
W. Matsuoka, Dr. H. Ito, Prof. Dr. K. Itami
Graduate School of Science, Nagoya University, Chikusa, Nagoya
464-8602, Japan
E-mail: ito.hideto@g.mbox.nagoya-u.ac.jp (HI)
E-mail: itami@chem.nagoya-u.ac.jp (KI)
Prof. Dr. K. Itami
Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya
University, Chikusa, Nagoya 464-8601, Japan
Prof. Dr. K. Itami
JST-ERATO, Itami Molecular Nanocarbon Project, Nagoya
University, Chikusa, Nagoya 464-8602, Japan
Z
Z
• aromatic hydrocarbons
• heteroaromatics
• large PAHs
• nanographenes
• π-extended heteroaromatics
[2]
[3]
Figure 1. Schematic illustration of general stepwise π-extension of
unfunctionalized aromatics and annulative π-extension (APEX) of aromatic
compounds (this work).
Supporting information for this article is given via a link at the end of
the document.
1
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10.1002/anie.201707486
Angewandte Chemie International Edition
COMMUNICATION
Reaction conditions were screened for
a
novel APEX
reaction, enabling efficient and rapid access to complex π-
reaction of phenanthrene (1a) with various π-extending agents,
metal catalysts, additives, etc (see Supporting Information for
detailed discussion on the modification of reaction conditions).
The use of Pd(MeCN)4(BF4)2 (5 mol%) in the presence of silver
pivalate (AgOPiv, 2 equiv) and trifluoromethanesulfonic acid
(TfOH, 2 equiv) effectively catalyzes the APEX reaction of 1a
with 2,2-diiodobiphenyl (2a) in 1,2-dichloroethane. After heating
the mixture at 50 °C for 15 h, dibenzo[g,p]chrysene (3aa) was
obtained in 77% isolated yield (Scheme 1). Notably, the K-region
of phenanthrene was selectively π-extended, and the yield of
3aa was much higher than that observed in our previous K-
region-selective APEX reaction.[7a]
extended fused heteroaromatics.
During the investigation of substrate scope, novel
successive APEX reactions were discovered in some
combinations of substrates and π-extension units (Figure 2).
When chrysene (1h) was used as a template PAH under the
standard APEX reaction conditions, highly fused nanographene
7ha was obtained as the major product in 32% yield, while the
initially expected ‘usual’ APEX product 3ha was not obtained
(Figure 2a). Nanographene 7ha is most likely produced by two-
fold APEX reactions at both the K-regions of chrysene (1h) to
initially
form
3ha,
followed
by
the
Scholl-type
cyclodehydrogenation at the fjord region of [5]helicene
substructure in 3ha. A similar successive APEX reaction also
occurred when 3,6-di-tert-butylphenanthrene (1i) was treated
with 9-iodo-10-(2-iodophenyl)phenanthrene (2g), producing
dibenzonaphthopentaphene 7ig in 29% yield (Figure 2h). These
successive APEX reactions enable the rapid access to
structurally complex nanographenes that are otherwise difficult
to synthesize in only one step and can be regarded as an
advanced synthetic methodology for nanographenes.
5 mol% Pd(MeCN)4(BF4)2
2.0 equiv. AgOPiv
2.0 equiv. TfOH
I
+
I
ClCH2CH2Cl (2.0 mL)
50 °C, 15 h
1a
2a
3aa
(0.10 mmol)
(2.0 equiv.)
77%
isolated yield
Scheme 1. Pd-catalyzed APEX reaction of phenanthrene (1a) with 2,2ʹ-
diiodobiphenyl (2a) under optimized reaction conditions.
We were extremely excited to have unsymmetrically π-
extended nanographene 7ha that is otherwise difficult to
synthesize. Thus, the structures and fundamental properties of
7ha were investigated. X-ray diffraction analysis showed that the
novel nanographene 7ha has a characteristic conformation and
packing structure in the solid state. Compound 7ha consists of
both planar and twisted aromatic regions derived from many
fused rings and the remaining [5]helicene moiety (Figures 2b
and 2c).[9] The red-filled phenanthrene moiety is highly twisted
due to steric repulsions between the hydrogen atoms at the fjord
and cove regions. In the packing structure, the enantiomer pairs
of 7ha are longitudinally and alternately aligned, and two types
of face-to-face π–π stacking modes were observed according to
the orientation of 7ha (Figure 2d and 2e). One mode is an
orthogonal π–π stacking in which the longitudinal phenanthrene
moieties of 7ha are located next to each other (Figures 2d and
2f, red and blue molecules). The other mode is a parallel π–π
stacking in which the longitudinal phenanthrene moieties of 7ha
are alternately stacked while avoiding the overlapping of bulky
twisted phenanthrene moieties (Figures 2d and 2g, green and
yellow molecules). The π–π stacking distances in each stacking
mode were estimated to be 3.4–3.6 Å, indicating that the
moderate-to-weak π–π interactions continuously work in the
longitudinal direction. These columnar 1D-stacking ability[10] and
dense packing property are the requisite features for excellent
charge-carrier mobility in applications to organic semiconductors
such as OFET and OPV.[11]
With the optimized reaction conditions in hand (Table S1),
the substrate scope of diiodobiaryls 2 and PAHs 1 was explored
(Table 1). 4,4ʹ-Di-tert-butyl-2,2ʹ-diiodobiphenyl (2b) also afforded
the corresponding product 3ab in
Diiodobiaryls bearing electron-withdrawing
a
good yield (72%).
chloro and
trifluoromethyl groups (2c and 2d) effectively worked as π-
extending agents, affording products 3ac and 3ad in high yields
(3ac: 90%, 3ad: 84%). On the other hand, the yields of APEX
reaction products were low when diiodobiaryls bearing an
electron-donating methoxy group (2e and 2f) were used in the
reaction (3ae: 31%, 3af: 48%). 2,7-Di-tert-butylphenanthrene
(1b) was found to be a good template for the APEX reaction,
producing 3ba in 91% yield. Methylene-bridged phenanthrene
1c and benzoanthracene (1d) participated in this APEX reaction,
and the K-regions on these substrates were selectively π-
extended, affording 3ca and 3da in 53% and 40% yields,
respectively. When 2,7-di-tert-butylpyrene (1e) was used, both
the K-regions at the periphery were simultaneously π-extended,
furnishing hexabenzotetracene 3ea in 49% yield. Other PAHs
such as corannulene (1f) and fluoranthene (1g) also participated
in this APEX reaction, albeit with lower yields of products 3fa
and 3ga.
Then, the catalytic system was used in the APEX reactions
of heteroaromatics (Table 1). To our delight, benzothiophene
(5a), substituted benzothiophenes 5b and 5c, 2,3-
diphenylthiophene (5d), and N-methylindole (5e) participated in
this APEX reaction, affording various phenanthrothiophenes 6aa,
6ba, 6ca, and 6da, and dibenzocarbazole (6ea) in moderate
yields. Interestingly, the reaction of N-phenylpyrrole (5f) with 4
equiv
of
2a
afforded
a
double
APEX
product,
tetrabenzocarbazole 6fa, in an acceptable yield (47%). These
results show the great potential of the newly developed APEX
2
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Table 1. Substrate scope of PAHs (1), heteroaromatics (5) and diiodobiaryls 2 in the Pd-catalyzed APEX reaction.
5 mol% Pd(MeCN)4(BF4)2
2.0 equiv. AgOPiv
2.0 equiv. TfOH
Ar
Ar
Ar
I
I
+
or
or
ClCH2CH2Cl (2.0 mL)
50 °C, 15 h
Z
Ar
Z
heteroarenes (5)
6
PAHs (1)
2a
(2.0 equiv.)
3
isolated yield
(0.20 mmol)
isolated yield
(0.10 mmol)
tBu
tBu
R1
R3
2a: R1 = R2 = R3 = H
2b: R1 = R2 = tBu, R3 = H
2c: R1 = Cl, R2 = R3 = H
2d: R1 = CF3, R2 = R3 = H
2e: R1 = OMe, R2 = R3 = H
2f: R1 = R2 = H, R3 = OMe
I
I
tBu
tBu
R2
1b
1c
1d
1e
1f
1g
tBu
Cl
CF3
OMe
OMe
tBu
3aa
74%
3ab
72%
3ac
84%
3ad
84%
3ae
31%
3af
48%
tBu
tBu
tBu
tBu
3ba
91%
3ca
53%
3da
40%
3ea
49%[a]
3fa
3ga
46%
28%
Cl
tBu
Ph
Ph
S
N
N
S
S
S
Cl
Me
Ph
6aa
52%[b]
6ba
58%[c]
6ca
68%[c]
6da
40%
6ea
6fa
47%[d]
39%
[a] Pd(MeCN)4(BF4)2 (10 mol%), 2a (4 equiv), AgOPiv (4 equiv), and TfOH (4 equiv) were used. [b] 10 mol% Pd catalyst was used at 80 °C. [c] 10 mol% Pd
catalyst was used. [d] 10 mol% Pd catalyst, 4 equiv of 2a, 4 equiv of AgOPiv, and 4 equiv of TfOH were used.
To elucidate the electronic properties, DFT calculations were
carried out. Geometry optimizations were conducted at the
B3LYP/6-31G(d) level of theory, and single-point calculations
using the 6-311++(d,p) basis sets were used to obtain their
molecular orbital energy levels (Figure 3a). As shown in Figure
4a, the HOMO and LUMO are delocalized over the entire
molecule, but tend to be localized on the planar surface of 7ha.
On the other hand, HOMO–1/LUMO+1 and HOMO–2/LUMO+2
are localized on the half sides of molecule. TD-DFT calculations
(B3LYP/6-311G++(d,p)) showed that the energetically lowest
HOMO→LUMO transition is possible (λ = 458 nm, f (oscillator
strength) = 0.3569). The UV–visible absorption spectrum of 7ha
in CH2Cl2 indicates that 7ha has broad absorption bands
between 300 and 470 nm (Figure 3b). In particular, the observed
longest absorption maximum at 442 nm is consistent with the
calculated HOMO→LUMO transition. The molar absorptivity (ε)
of 7ha in CH2Cl2 at 442 nm was measured to be 4.38 × 104 cm–1
mol–1 L. Compound 7ha also showed
a
broad emission
spectrum in CH2Cl2 with the maximum emission wavelength at
464 nm, whose shape resembles mirror image of the
a
absorption spectrum. While 7ha emits a blue-green fluorescence
in the CH2Cl2 solution (Figure 3c), its powder shows a yellow
emission in the solid state (Figure 3d), indicating that the
columnar π–π interactions effectively work even in the
amorphous solid state of 7ha.
3
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COMMUNICATION
Figure 2. [a],[b] One-step successive APEX reactions of chrysene (1h) and 3,6-di-tert-butylphenanthrene (1i), and X-ray crystallographic structures of 7ha[12]
.
[b],[c] Extracted thermal ellipsoid structures of 7ha in the X-ray crystallographic analysis. [d] Top and [e] side views of the packing structure of 7ha in a single
crystal obtained by recrystallization from THF/Et2O. THF and Et2O are indicated as purple molecules in the top view, and these are omitted in the side view for
clarity. [f],[g] Two representative π–π stacking modes in the packing structure of 7ha.
mode in the solid state. This APEX reaction has immense
potential for applications in the precise synthesis and late-stage
functionalization of various unfunctionalized nanographenes and
π-extended fused aromatics. Further investigations on detailed
reaction mechanism and applications towards the synthesis of
larger nanographenes are currently underway.
Acknowledgements
Figure 3. Electronic and photophysical properties of 7ha. [a] Pictorial frontier
This work was supported by the ERATO program of the JST
(JPMJER1302 to K.I.) and JSPS KAKENHI Grant Numbers
JP26810057, JP16H00907 (H.I.) and the Sumitomo Foundation
(141495 to H.I.). We thank Dr. Yasutomo Segawa for the X-ray
diffraction analyses. ITbM is supported by the World Premier
International Research Center Initiative (WPI), Japan.
molecular orbitals and energy levels calculated using the B3LYP/6-
31G(d)//B3LYP/6-311++G(d,p) level of theories. [b] Normalized UV–visible
absorption and emission spectra of 7ha in CH2Cl2. [c] Blue-green emission of
7ha in CH2Cl2 excited by 365-nm light. [d] Solid-state yellow emission of 7ha
excited by 365-nm light.
In summary, novel Pd-catalyzed APEX reactions of various
PAHs with diiodobiaryls were established. The discovered Pd
catalyst/AgOPiv/TfOH reaction system enables the direct APEX
reactions of not only PAHs, but also various heteroaromatics in
one step with a high regioselectivity. Moreover, in the reactions
forming the fjord region by APEX, subsequent ring closures
occur, affording large fused nanographenes in a single operation.
The obtained nanographene consists of both a flat and helical
PAH region, forming an effective 1D-columnar π–π stacking
Keywords: polycyclic aromatic hydrocarbons • π-extension •
palladium • nanographenes
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10.1002/anie.201707486
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COMMUNICATION
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[12] CCDC 1548289 [(7ha)3·THF·0.5Et2O] contains the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[6]
[7]
For a review on APEX reactions, see: H. Ito, K. Ozaki, K. Itami, Angew.
Chem.
Int.
Ed.
2017,
56,
Accepted
Articles.
DOI:
10.1002/anie.201701058.
For our contributions to APEX chemistry for the synthesis of graphene
nanoribbons and π-extended heteroaromatics, see: a) K. Ozaki, K.
Kawasumi, M. Shibata, H. Ito, K. Itami, Nat. Commun. 2015, 6, 6251; b)
Y. Yano, H. Ito, Y. Segawa, K. Itami, Synlett 2016, 27, 2081; c) K. Kato,
5
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10.1002/anie.201707486
Angewandte Chemie International Edition
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COMMUNICATION
Wataru Matsuoka, Hideto Ito*, Kenichiro
Itami*
Page No. – Page No.
Rapid Access to Nanographenes and
Fused Heteroaromatics by Palladium-
Catalyzed Annulative π-Extension
Reaction of Unfunctionalized
A novel one-step annulative π-extension reaction of various unfunctionalized
aromatics has been demonstrated, providing facile and rapid access to π-extended
polycyclic aromatic hydrocarbons, π-extended fused heteroaromatics and
nanographenes. In the presence of palladium catalyst, silver pivalate and
trifluoromethanesulfonic acid the aromatic templates were π-extended with a
variety of diiodobiaryls as π-extending agents in good yields and regioselectivity.
Aromatics with Diiodobiaryls
6
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