Tandem Oxidative Ring Expansion for Synthesis of Dibenzocyclooctaphenanthrenes

Article abstract of DOI:10.1021/acs.orglett.0c01725

A novel tandem single-electron oxidative ring expansion reaction has been developed for the construction of the saddle-shaped polycyclic arenes fused with cyclooctatetraene, that is, dibenzo[3,4:7,8]cycloocta[1,2-l]phenanthrenes (dbCOTPs). The combination

Full text of DOI:10.1021/acs.orglett.0c01725

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Letter
Tandem Oxidative Ring Expansion for Synthesis of
Dibenzocyclooctaphenanthrenes
Lu Yang, Hidenori Matsuyama, Sheng Zhang, Masahiro Terada, and Tienan Jin*
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ABSTRACT: A novel tandem single-electron oxidative ring expansion reaction
has been developed for the construction of the saddle-shaped polycyclic arenes
fused with cyclooctatetraene, that is, dibenzo[3,4:7,8]cycloocta[1,2-l]-
phenanthrenes (dbCOTPs). The combination of Cu(OTf)2 catalyst with DDQ
triggered the selective oxidation of o-biphenyl-tethered methylenecirculenes fused
with a seven-membered ring, giving rise to the formation of the corresponding
eight-membered ring fused dbCOTPs. The present tandem ring expansion of a
seven- to an eight-membered ring takes place via the selective single-electron oxidation of the benzylidene moiety, intramolecular
spirocyclization, and 1,2-aryl migration sequence.
Scheme 1. Representative COT-Embedded Polycyclic Arene
Units and Synthetic Methods
olycyclic arene units with negative curvature are prevalent
in various curved nanographenes and hypothetical
P
toroidal carbon nanotubes, which have attracted increasing
attention in the field of computation and synthetic chemistry
as well as materials science in terms of their negatively curved
structural features and electronic properties.1,2 Embedding
seven- or eight-membered carbon rings into the polycyclic
aromatic hydrocarbon (PAH) frameworks has been regarded
as an intriguing strategy for the construction of the negatively
curved polycyclic arenes.3 Particularly, polycyclic arenes fused
with an eight-membered cyclooctatetraene (COT) and
benzenoid rings, such as tetraphenylene,4 dibenzo[a,e][8]-
annulene (dbCOT),5 tribenzo[a,c,e][8]annulene (tribCOT),6
and dibenzo[3,4:7,8]cycloocta[1,2-l]phenanthrene
(dbCOTP),7 displayed negatively bent structures owing to
the tub-shaped geometry of COT (Scheme 1a). They not only
can serve as important key segments of negatively curved
nanographenes1−3 and Mackay crystals8 but also can act as
COT ligands for transition metal complexes5e and host
molecules for dynamic molecular recognition.9 In this context,
the development of efficient synthetic methods for COT-
embedded polycyclic arenes is of great significance and highly
demanded. However, in contrast to the well-studied
tetraphenylene and dbCOT,4,5 synthetic methods to tribCOT
and dbCOTP are significantly rare. Nonetheless, tribCOT can
be synthesized by Diels−Alder cycloaddition of a strained
acetylene of 5,6-didehydrodibenzo[a,e]cyclooctene with furan
and subsequent deoxygenation,6 but only a single example was
reported for dimethyl-substituted dbCOTP, through cyclo-
addition of dienediyne followed by an intramolecular
homocoupling reaction (Scheme 1b).7
conditions, a single-electron oxidation took place at the alkene
moiety of the o-biphenyl-tethered methylene fluorene, giving
rise to the ring expansion of a five- to a six-membered ring to
Received: May 21, 2020
Recently, we are interested in developing new synthetic
methodologies for π-extended polycycles using single-electron
oxidation of functionalized methylene fluorenes.10 Particularly,
we have discovered that under the single-electron oxidation
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construct various π-extended PAHs.10b In light of this fact, we
envisioned that if the oxidative ring expansion takes place for
the o-biphenyl-tethered methylene dibenzo[7]annulene 1b, a
new class of polycyclic arene 2b fused with an eight-membered
ring and benzenoid rings could be formed (Scheme 1c). This
method may provide a promising and general synthetic
approach to the eight-membered ring embedded polycyclic
arenes, although it is predicted to suffer from problems with
regard to varied redox potentials and two alkene moieties of
the 1b structure for the selective single-electron oxidation.
Herein, we describe a novel single-electron oxidative ring
expansion of a seven- to an eight-membered ring for the
construction of a new series of saddle-shaped dbCOTPs fused
with an eight-membered ring and benzenoid rings.
acids may improve the single-electron oxidation ability,
enabling the formation of radical cation species.11 After an
extensive survey of the reaction conditions, we found that the
combination of DDQ (1.5 equiv) with a catalytic amount of
the strong acid, namely, trifluoromethanesulfonic acid
(TfOH), was able to induce oxidative tandem ring expansion
transforming compound 1a into 2a in 36% yield along with the
decomposition of 1a (entry 5). To our delight, the use of
Lewis acidic Cu(OTf)2 catalyst instead of TfOH could further
increase the yield of 2a to 52% (entry 6). It was also noted that
the use of Cu(OTf)2 catalyst without DDQ led to no reaction.
Finally, we were able to obtain 2a in a good yield of 71% by
using an excess of DDQ (2.5 equiv) combined with Cu(OTf)2
(30 mol %) (entry 7). The use of other oxidants such as p-
t
First, the readily available substrate 5-((4′-methyl-biphenyl-
2-yl)methylene)-5H-dibenzo[a,d][7]annulene (BMDBA) (1a)
was subjected to various oxidation conditions to develop a new
ring expansion reaction to access desired product 2a (Table 1).
chloranil and PhCO3 Bu in conjunction with the Cu(OTf)2
catalyst resulted in no reaction (entries 8 and 9). Other
nontriflate copper salts such as Cu(NTf2)2, Cu(BF4)2, and
CuCl2 combined with DDQ were examined to be totally
ineffective for the formation of 2a, resulting in decomposition
or recovery of 1a (entries 10−12). Further screening of other
metal triflates revealed that Fe(OTf)3 and In(OTf)3 were also
effective in achieving moderate yields of 2a (entries 13 and
14), indicating the important role of metal triflates for the
successful implementation of the present tandem ring
expansion reaction.
Table 1. Screening of Oxidation Conditions for Ring
a
Expansion Reaction of 1a
With the optimized DDQ/Cu(OTf)2 oxidation system in
hand, the substrate scope has been investigated to understand
the influence of substituent effect on the oxidation reactivity
(Scheme 2). The reaction of 1b without any substituents on
the aromatic rings afforded the corresponding dbCOTP 2b in
a good yield of 65%. The reactions of 1c and 1d with electron-
rich substituents such as n-heptyl and tert-butyl at the 4′-
position of the o-biphenyl moiety (R1) favor the present
oxidation, giving the corresponding dbCOTPs 2c and 2d in
68% and 80% yields, respectively. The BMDBA 1e bearing an
electron-withdrawing ester group at R1 showed low efficiency,
producing the corresponding product 2e in 38% yield. The
electronic effect on the reaction efficiency implied the
involvement of a Friedel−Crafts-type cyclization process.
The reaction of 1f having a phenyl substituent at R1 proceeded
uneventfully to afford the desired product 2f in 72% yield. The
substrate 1g bearing a methyl substituent (R1) at the 3′-
position of the o-biphenyl moiety showed steric effect on the
reaction outcome, affording the corresponding product 2g in
37% yield. The introduction of a strong electron-withdrawing
nitro group (R2) to the 5-position of o-biphenyl lowered the
reactivity of 1h and 1i, but the corresponding products 2h and
2i could be obtained in good yields at the elevated temperature
of 60 °C using excess amounts of DDQ (4 equiv) and
Cu(OTf) (60 mol %). Likewise, the substrates 1j and 1k
bearing an electron-withdrawing F or Cl at the 4-position and
an electron-donating tert-butyl at the 4′-positions of o-biphenyl
individually worked well at 60 °C with excess amounts of
DDQ and Cu(OTf)2, yielding the corresponding product 2j
and 2k in good to high yields. The substrates 1l and 1m
containing electron-withdrawing monobromo- or dibromo-
substituents on the dibenzo[a,d][7]annulene moiety also
underwent the present tandem oxidative ring expansion
reaction, affording the corresponding products 2l and 2m in
moderate yields, which can be further functionalized by various
coupling reactions. The introduction of an electron-donating
methoxy group into the 5H-dibenzo[a,d][7]annulene moiety
(1n, R3 = OMe) showed higher reactivity compared to 1l and
b
b
entry oxidant (equiv)
acid (equiv)
2a (%) 1a (%)
c
t
1
2
3
4
5
6
7
8
9
PhCO3 Bu (1.5)
CuCl (0.2) with TFA (5)
TFA (5)
0
0
0
30
44
0
99
10
10
0
99
80
0
c
DDQ (1.5)
FeCl3 (3.0)
DDQ (1.5)
DDQ (1.5)
DDQ (1.5)
DDQ (2.5)
p-chloranil (2.5)
d
e
f
none
none
0
TfOH (0.2)
36
52
71
0
0
0
Cu(OTf)2 (0.3)
Cu(OTf)2 (0.3)
Cu(OTf)2 (0.3)
Cu(OTf)2 (0.3)
Cu(NTf2)2 (0.3)
Cu(BF4)2 (0.3)
CuCl2 (0.3)
t
PhCO3 Bu (2.5)
10
11
12
13
14
DDQ (2.5)
DDQ (2.5)
DDQ (2.5)
DDQ (2.5)
DDQ (2.5)
0
0
60
40
96
99
0
Fe(OTf)3 (0.3)
In(OTf)3 (0.3)
17
a
Reaction conditions: 1a (0.1 mmol), oxidant (1.5 equiv or 2.5
equiv), acid, CH2Cl2 (0.2 M), at 40 °C for 12 h. Yields were
determined by H NMR spectroscopy using CH2Br2 as an internal
standard. The reaction was performed in o-xylene (0.2 M) at 80 °C.
The reaction was performed in 1,2-dichloroethane at 80 °C. An
unknown product was obtained in 20% yield. Yield of isolated
product is shown after silica gel chromatography.
b
1
c
d
e
f
Unfortunately, the reactions with our previously developed two
oxidation systems,10b CuCl/PhCO3 Bu and DDQ (2,3-
t
dichloro-5,6-dicyanobenzoquinone)/CF3CO2H, did not pro-
duce the desired product 2a and 1a was recovered in 30% and
44% yield, respectively (entries 1 and 2). The use of a common
10a
single-electron oxidant of FeCl3 did not give the desired
product 2a, but instead, an unknown structural isomer of 2a
(not Scholl reaction product) was obtained in 20% yield (entry
3). Moreover, the use of DDQ oxidant without any additives
could not trigger the desired tandem oxidation reaction (entry
4). It was reported that the combination of DDQ with strong
B
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respectively. The unidentical bond angles in COT are
measured to be 125.1° for C10−C18−C16, 122.2° for C4−
C10−C18, 120.9° for C3−C4−C10, and 121.1° for C4−C3−
C5. The torsion angles of the COT in 2b are 59.2° for C4−
C10−C18−C16 and 73.7° for C5−C3−C4−C10, respectively,
which are larger than that of the parent COT (56°).12 The
aromaticity of 2b was investigated with the nucleus-
independent chemical shift (NICS) by the calculation at the
B3LYP/6-31G(d,p) level (Figure 1b). The NICS(0) values of
−8.7 ppm and −9.9 ppm corresponding to the peripheral
benzene A and the phenanthrene benzene D, respectively,
indicate their remarkably high aromaticity compared to the
moderate aromatic character of the peripheral benzene C
(−5.7 ppm). The NICS(0) value of the COT ring B was
calculated to be 2.4 ppm, indicating its nonaromatic character.
The UV/vis absorption and fluorescence spectra of 2b were
measured in diluted chloroform solution (Figure S1 in the
Supporting Information). A strong absorption maximum (λabs)
value at 261 nm and two weak λabs values at 297 and 308 nm
were observed, which are relatively short compared to planar
PAHs probably due to the highly nonplanar structure of 2b.10b
The fluorescence spectra of 2b showed emission maxima (λem)
values of 361, 375, and 395 nm at the excited wavelength of
300 nm with a low fluorescence quantum yield of 5.2%.
In light of the structural analysis and the nonaromaticity, we
thought that the bared double bond of COT in 2b may exhibit
typical olefin reactivity. Thus, to demonstrate the synthetic
utility of the COT-embedded products, we carried out the
additional functionalization of dbCOTPs. Oxidation of the
bared alkene moiety of COTs in 2b and 2d with
benzeneseleninic anhydride produced the corresponding
diketone 3b and 3d in good yields. The condensation of 3b
and 3d with 1,2-diaminobenzene, respectively, in the presence
of p-toluenesulfonic acid (p-TSA) catalyst afforded a new class
of quinoxaline-fused dbCOTPs 4b and 4d in quantitative
yields (Scheme 3a).13 In addition, the selective bromination of
2b and the subsequent elimination of the resulting 9,10-
dibrominated product 5b with t-BuOK produced a strained
cyclic alkyne 5b′ (Scheme 3b). 5b′ was subjected directly with
1,3-diphenylisobenzofuran to the Diels−Alder cycloaddition,
affording the oxygen-bridged cycloadduct 6b. Subsequently,
Scheme 2. Substrate Scope of Oxidative Ring Expansion of
o-Biphenyl-tethered Methylene Dibenzo[a,d][7]annulenes
a
a
Reaction conditions: 1 (0.2 mmol), DDQ (0.5 mmol, 2.5 equiv),
Cu(OTf)2 (0.06 mmol, 30 mol %), CH2Cl2 (0.2 M), at 40 °C for 12
b
h. DDQ (4 equiv) and Cu(OTf)2 (60 mol %) were used at 60 °C for
c
12 h. DDQ (4 equiv) and Cu(OTf)2 (30 mol %) were used at 60 °C
d
for 12 h. The reaction time was 6 h.
1m having electron-withdrawing groups, affording the
corresponding product 2n in 57% yield under the standard
conditions within 6 h.
The structure of 2b was determined unambiguously by X-ray
crystallography (Figure 1a). The X-ray structure indicated that
Scheme 3. Synthetic Applications of dbCOTPs for π-
Extended Polycyclic Arenes
Figure 1. (a) ORTEP drawing of the dbCOTP 2b. Thermal ellipsoids
are shown at 50% probability. (b) The NICS(0) values were
calculated using Gaussian09 at the B3LYP/6-31G(d,p) level.
2b is a saddle-shaped molecule consisting of peripheral
benzenoid rings, a planar phenanthrene moiety, and a tub-
shaped COT ring. The C16−C18 bond length (1.336 Å)
corresponding to the nonsubstituted double bond of COT is
shortest compared to C2−C9 (1.401 Å) and C3−C5 (1.359
Å) adjacent to the benzene and phenanthrene moieties,
C
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Scheme 4. Proposed Oxidative Ring Expansion Reaction Mechanism
the deoxygenation and aromatization of 6b using the Wong’s
low-valent titanium gave the corresponding π-extended
polycyclic arene 7b in 85% yield.14
arenes embedded with a COT ring by the tandem oxidative
ring expansion. The o-biphenyl-tethered methylene
dibenzo[7]annulenes fused with a seven-membered ring
could be oxidized by the DDQ/Cu(OTf)2 oxidation system
efficiently to the corresponding saddle-shaped dibenzo-
[3,4:7,8]cycloocta[1,2-l]phenanthrenes fused with an eight-
membered ring through a single-electron oxidation, spirocyc-
lization, and 1,2-aryl migration tandem process. The present
protocol provides a new synthetic approach to polycyclic
arenes fused with an eight-membered ring, which is expected
to be highly applicable for the synthesis of diverse negatively
curved nanocarbons.
It was found that two alkene moieties in 1b were calculated
to possess high electron density by frontier molecular orbital
calculation (Figure S2 in the Supporting Information). Among
them, the benzylidene alkene was predicted to undergo single-
electron oxidation preferentially compared to the alkene in the
seven-membered ring, owing to the formation of a rather stable
radical cation species E of the former (Scheme 4).1d In light of
the electronic substituent effect, indispensable role of oxidant,
and electron density distribution, the proposed reaction
mechanism of the present tandem oxidative ring expansion
reaction is outlined in Scheme 4. First, the single-electron
oxidation occurs by the DDQ/Cu(OTf)2 oxidation system at
the electron-rich alkene moiety of 1b to form a radical cation
species E. Subsequent intramolecular Friedel−Crafts reaction
affords a spirocyclic radical species F, which will be further
oxidized to give a spirocyclic cation G. Next, the intermediate
G undergoes the 1,2-aryl migration through the formation of
an arenium cation H (path I) or the direct migration (path II),
affording the cation I. The rapid deprotonation takes place in
the cation I to give the desired product 2b. To support the
reaction mechanism, the spirocyclic compound 1a′ was
subjected to our standard conditions (eq 1). As predicted,
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01725.
Experimental procedures and characterization of related
compounds (PDF)
X-ray structure report for 2b (CCDC 1994572) (PDF)
Accession Codes
CCDC 1994572 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Tienan Jin − Research and Analytical Center for Giant
Molecules, Graduate School of Science, Tohoku University,
Sendai 980-8578, Japan; orcid.org/0000-0003-1150-
7299; Email: tetsuo.kin.a6@tohoku.ac.jp
the reaction of 1a′ did produce the desired 1,2-aryl migration
product 2a, even though the yield was low, suggesting the
involvement of the intermediate F and G in the present
tandem oxidation. In addition, the reactions of 1a under the
standard conditions were completely hampered by adding
radical scavengers of 2,2,6,6-tetramethylpiperidine 1-oxyl
(TEMPO) or 2,6-di-tert-butyl-4-methylphenol (BHT), imply-
ing that the single-electron oxidation takes place in the initial
step (Scheme S5 in the Supporting Information).
Authors
Lu Yang − Department of Chemistry, Graduate School of
Science, Tohoku University, Sendai 980-8578, Japan
Hidenori Matsuyama − Department of Chemistry, Graduate
School of Science, Tohoku University, Sendai 980-8578, Japan
Sheng Zhang − Department of Chemistry, Graduate School of
Science, Tohoku University, Sendai 980-8578, Japan; State Key
In conclusion, we have developed a novel and general
synthetic method for constructing a new class of polycyclic
D
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pubs.acs.org/OrgLett
Letter
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Laboratory of Fine Chemicals and School of Chemistry, Dalian
University of Technology, Dalian 116023, China; orcid.org/
0000-0002-9671-7668
Masahiro Terada − Department of Chemistry, Graduate School
of Science, Tohoku University, Sendai 980-8578, Japan;
orcid.org/0000-0002-0554-8652
Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by JSPS KAKENHI Grant Number
JP16H00996 in Precisely Designed Catalysts with Customized
Scaffolding and a Grant-in-Aid for Scientific Research on
Innovative Areas “Hybrid Catalysis for Enabling Molecular
Synthesis on Demand” (JP17H06447) from MEXT (Japan).
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