Intermolecular oxidative Friedel-Crafts reaction triggered ring expansion affording 9,10-diarylphenanthrenes

Article abstract of DOI:10.1021/acs.orglett.0c03283

A novel intermolecular tandem oxidative aromatic coupling between arylidene fluorenes and unfunctionalized aromatics mediated by a DDQ/TFA oxidation system has been developed for the construction of 9,10-diary-lphenanthrenes (DAPs). The formation of a benzylic carbocation species possessing a quaternary sp³-carbon center on the fluorene moiety by an intermolecular oxidative Friedel-Crafts reaction of two different arenes successfully triggered the subsequent ring expansion to afford DAPs.

Full text of DOI:10.1021/acs.orglett.0c03283

pubs.acs.org/OrgLett
Letter
Intermolecular Oxidative Friedel−Crafts Reaction Triggered Ring
Expansion Affording 9,10-Diarylphenanthrenes
Lu Yang, Ilya D. Gridnev, Masahiro Terada, and Tienan Jin*
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ABSTRACT: A novel intermolecular tandem oxidative aromatic coupling between
arylidene fluorenes and unfunctionalized aromatics mediated by a DDQ/TFA
oxidation system has been developed for the construction of 9,10-diary-
lphenanthrenes (DAPs). The formation of a benzylic carbocation species possessing
a quaternary sp3-carbon center on the fluorene moiety by an intermolecular
oxidative Friedel−Crafts reaction of two different arenes successfully triggered the
subsequent ring expansion to afford DAPs.
Scheme 1. Development of New Intermolecular Oxidative
Coupling of Arenes for Synthesis of DAPs
mong a variety of synthetic methods toward polycyclic
aromatic hydrocarbons (PAH),1,2 the intramolecular
A
oxidative aromatic coupling of the precisely preconstructed
multiaryl-substituted arenes using various single electron
oxidants has been considered as one of the most straightforward
and atom-economical synthetic strategies.2 On the other hand,
the intermolecular oxidative aromatic cross-coupling between
two different arenes having inequivalent reactive sites typically
produced unsymmetrical biaryls sometimes accompanied by
homocoupling side reactions.2b Consequently, the development
of a novel version of the intermolecular oxidation reaction
between different arenes toward PAHs by suppressing the
potential formation of the undesired cross-coupling and
homocoupling products is highly desirable and challenging.
The Friedel−Crafts alkylation reaction is one of the most
important methods for C−C bond formation (Scheme 1a).3
Meanwhile, functionalization of alkene by intermolecular and
intramolecular trapping of radical cations was also developed
(Scheme 1b).4 In this work, we have developed an
intermolecular oxidative Friedel−Crafts reaction triggered ring
expansion tandem process through the formation of an all-
carbon quaternary center carbocation for constructing PAH
products (Scheme 1c). This successive loss of two electrons,
followed by rearrangement to achieve aromatic compounds, has
been rarely reported.
Recently, we have been interested in developing new
intramolecular single electron oxidative tandem annulations
toward the synthesis of structurally diverse π-extended poly-
cycles.5 We have found that the formation of a radical cation
species at the alkenyl moiety of arylidene fluorenes was
considered to be a key feature to induce the subsequent
spirocyclization and ring expansion processes. In light of our
previous studies,5 we assumed that if a single electron oxidation
occurs at the alkenyl moiety of arylidene fluorenes (1, AF), the
resulting radical cation species A may induce subsequent
intermolecular oxidative Friedel−Crafts reaction with aromatic
nucleophiles, followed by a ring expansion reaction through 1,2-
Received: October 2, 2020
https://dx.doi.org/10.1021/acs.orglett.0c03283
© XXXX American Chemical Society
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A

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aryl shift in the intermediate C, leading to 1,2-diarylphenan-
threnes (3, DAPs) (Scheme 1c). However, under the single
electron oxidation conditions, some potential compete side-
reactions were considered to occur in our reaction hypothesis.
For example, (a) typical homocouplings and cross-couplings of
two arenes,2b (b) intramolecular cyclodehydrogenative coupling
of the intermediate A (R = Ar1),6 (c) 1,2-aryl shift of the
nucleophilic aromatic group (Nu) in the intermediate C (vide
infra; see Scheme 5b, presented later in this work), and (d)
formal intermolecular Friedel−Crafts reaction, may erode the
desired reaction efficiency.
As one of the simplest PAHs, DAPs have attracted significant
synthetic attention, in terms of their intriguing optoelectronic
properties and important graphene segments.7 Among many
efforts, the transformations through the formation of various
metallacycle species E or F have been considered as most
effective synthetic methods (Scheme 1d).8 In comparison, our
reaction was designed to proceed through the cleavage of three
chemical bonds to form two new C−C bonds in one shot and
the starting AFs 1 can be readily accessible by the condensation
of aryl aldehydes with 9H-fluorene (Scheme 1c). In this context,
the desired metal-free tandem oxidation is considered to be a
highly atom-economic and straightforward method to construct
DAPs.
upon treating 1a with the aforementioned oxidation condition in
the absence of 2a, a mixture of the undesired oxidative
dimerization and trimerization products of 1a were detected
by high-resolution mass spectrometry. Fortunately, the best
result was obtained with DDQ, using TFA as the solvent at room
temperature; thus, the corresponding product 3aa was formed in
a high yield of 95% (Table 1, entry 4). Among the other acids
that were examined as a solvent with DDQ, the relatively weak
acid (acetic acid, AcOH) was totally inactive and the strong acid
(methanesulfonic acid, MsOH), afforded the formal intermo-
lecular Friedel−Crafts reaction product 3aa′ as a major product
without forming the desired 3aa (Table 1, entries 5 and 6).
Other oxidants such as p-chloranil and o-chloranil, combined
with TFA, led to no reaction or a low yield of 3aa (Table 1,
entries 7 and 8). The present tandem oxidation did not occur if
TFA or DDQ was used solely (Table 1, entries 9 and 10). Note
that other byproducts arising from the aforementioned side
reactions were not observed in the reaction mixture, indicating
the indispensable role of the DDQ/TFA system for the selective
implementation of the present tandem oxidation.
Under the DDQ/TFA oxidation conditions, we investigated
the influence of various arylidene fluorenes, and aromatic
nucleophiles on the selectivity and efficiency of the present
tandem oxidation (Scheme 2). The reactions of 1a with
relatively weak nucleophiles, such as toluene and biphenyl,
afforded the corresponding products 3ab and 3ac in goods yields
with an exclusive para-selectivity at elevated temperature (80
°C). The electron-rich anisole and benzo[d][1,3]dioxole were
tested to be reactive nucleophiles, thus transforming 1a into the
corresponding DAPs 3ad and 3ae in high yields. Polycyclic
arenes such as naphthalene and corannulene also can be used as
nucleophiles, giving rise to the tandem oxidation with 1a to
produce the corresponding DAPs 3af and 3ag, albeit a low yield
of the latter, because of the low nucleophilicity of corannulene.
Negligible electronic effect of various electron-withdrawing and
electron-donating substituents on the arylidene moiety of 1b−
1g was observed for the reaction with anisole and o-xylene,
affording the corresponding DAPs 3bd−3ga in good to high
yields. Similarly, the electron-donating t-butyl and the electron-
withdrawing Br or Cl groups on the fluorene moiety did not
show a significant electronic effect on the reaction efficiency,
giving the corresponding DAPs 3hd−3jd in good yields.
Thiophene is one of the most widely employed arene
components in both small molecules and polymeric organic
electronic materials.9 The present tandem oxidation showed
high compatibility with thiophene nucleophiles. The reaction of
1a or 1k with the parent thiophene afforded the corresponding
thienyl-substituted DAPs 3ah and 3kh in 93% and 50% yields,
respectively, under the DDQ/TFA/DCE oxidation conditions.
Interestingly, the monobromo- and dibromo-substituted
thiophenes also can be used as reactive nucleophiles, trans-
forming 1a, 1k, and 1h into the corresponding DAPs 3ai−3hm
in good to high yields with an exclusive site selectivity of
thiophenes. These bromo-substituted DAPs could be further
functionalized by various coupling reactions, whereas they are
not easy to achieve by the conventional transition-metal-
mediated synthetic methods.8 Similarly, benzofuran and
benzothiophene were also competent nucleophiles, affording
the corresponding DAPs 3an−3ap in satisfactory yields. The
reaction can be applied to the gram-scale and thus the use of 5
mmol of 1a with 3-bromobenzothiophene produced 3ap in 73%
yield (1.71 g), emphasizing the practical synthetic application of
this method.
In order to investigate the feasibility of our assumptions, we
began our studies by optimizing various single electron
oxidation systems for the intermolecular reaction of a readily
accessible benzylidene fluorene 1a with o-xylene 2a as an
aromatic nucleophile (see Table 1). The previously employed
Table 1. Optimization of Oxidation Conditions for the
Formation of 3aa
a
Yield (%)
entry
oxidant
DDQ
TBPB/CuCl
DDQ
DDQ
DDQ
acid (equiv)
solvent (0.17 M)
3aa
1a
b
1
Cu(OTf)2 (0.3)
TFA (5)
TFA (10)
DCM
o-xylene
DCE
0
<5
27
(95)
0
0
0
30
0
0
86
0
22
0
99
0
96
21
75
99
c
2
d
3
4
5
6
−
−
−
−
−
−
TFA
AcOH
MsOH
TFA
TFA
TFA
e
DDQ
7
8
9
p-chloranil
o-chloranil
−
10
DDQ
−
DCE
a
1
Yields were determined by H NMR spectroscopy, using CH2Br2 as
b
an internal standard. Isolated yield was given in parentheses. 2.5
equiv of DDQ was used at 40 °C. 2 equiv of TBPB and 20 mol % of
CuCl were used in o-xylene solvent at 80 °C. The reaction
temperature was 80 °C. 3aa′ was obtained in 50% yield.
c
d
e
oxidation conditions, such as the combination of DDQ with
Cu(OTf)2 and TBPB/CuCl with trifluoroacetic acid (TFA, 5
equiv),5b,c did not afford the desired product 3aa (Table 1,
entries 1 and 2). In addition, the DDQ and TFA (10 equiv)
combination system in dichloroethane solvent at 80 °C afforded
the desired product 3aa in low yield with significant
decomposition of 1a (Table 1, entry 3). It was noted that,
B
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(DDQ/TFA/DCE solvent) were required for the reaction with
the heteroaromatic nucleophile, such as thiophene, because of
its relatively strong nucleophilicity (N = −1.01). In comparison,
the very weak benzene nucleophile (N = −6.3) and the strong N-
methylindole nucleophile (N = 5.75) showed a detrimental
effect on the reaction outcome,12 yielding no desired products.
The synthetic applications for constructing π-extended PAHs
have been investigated. For example, the Pd-catalyzed intra-
molecular C−H/C-Br coupling of the resulting product 3ap
afforded the corresponding DBC derivative 4ap, benzo[b]-
benzo[11,12]chryseno[6,5-d]thiophene, in 84% yield (Scheme
3a). We also demonstrated that the one-pot oxidation process
Scheme 2. Substrate Scope of Arylidene Fluorenes and
Aromatic Nucleophiles
Scheme 3. Synthetic Applications toward
Dibenzo[g,p]chrysene Derivatives
could directly affect the DBC framework. For example, after the
oxidation of 1a or 1f with anisole under the present reaction
conditions, the resulting reaction mixture was directly subjected
with the DDQ/MsOH oxidation system (Scheme 3b). As a
result, the intermolecular tandem oxidation and subsequent
intramolecular oxidative aromatic coupling reaction6a occurred
in one pot to give the corresponding DBCs 4ad and 4fd in good
to high yields.
Furthermore, the phenanthrene end-capped co-oligomers
could be synthesized through this 2-fold tandem oxidation. For
example, the reaction of 3 equiv of 1a with the parent thiophene
2h under the modified tandem oxidation conditions afforded the
phenanthrene end-capped thiophene co-oligomers 5ah in 33%
yield (Scheme 4a). Similarly, the 2-fold tandem oxidation of the
BF-dimer 1l with 3 equiv of 2-bromothiophene 2j led to the
corresponding co-oligomer 5lj in 42% yield (Scheme 4b). It is
interesting to stress that this 2-fold process proceeds through the
cleavage of six C−H and C−C bonds to form four new C−C
a
b
DDQ (1.5 equiv), TFA (0.17 M), rt, 4 h. Corannulene was
c
recovered in 75% yield. DDQ (1.5 equiv), TFA (10 equiv), DCE
(0.25 M), 80 °C, 4 h. Yield of a 5 mmol (1a) scale reaction is shown
in parentheses.
d
Scheme 4. Two-Fold Tandem Oxidation for Phenanthrene
End-Capped Co-oligomers
We briefly summarized the compatibility and limitation of the
employed nucleophilic aromatics based on Mayr’s database of
reactivity parameters10 in order to better understand and predict
suitable nucleophiles, with respect to optimal reaction
conditions. A rough nucleophilicity cutoff of aromatic
nucleophiles for this reaction is in the range of −4.36 < N <
−1.01 (where N represents the nucleophilicity parameter),11
indicating that a broad scope of aromatic nucleophiles with weak
to moderate nucleophilicity can be applicable in this reaction.
For example, the reaction of 1a with a weak nucleophilic toluene
(N = −4.36) required a high temperature of 80 °C (DDQ/TFA
solvent), whereas the reaction proceeded at room temperature
(DDQ/TFA solvent) with anisole, having a moderate
nucleophilicity (N = −1.18). Again, relatively mild conditions
C
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a
Scheme 5. Control Experiments and Proposed Reaction Mechanism
a
Relative Gibbs free energies of the intermediates and transition states by DFT calculation at the level of ωB97XD/6-31G(d,p) are shown in
parentheses. Trifluoroacetate was used as an anion in DFT calculation. 2,2,2-Trifluoroethanol was used as solvent in DFT calculation, because of its
same dielectric constant with TFA.
bonds in one shot, which should be highly attractive for the
construction of π-extended complex molecules.
b. In addition, we did not observe any byproducts arising from
the intermediate D′ and, thus, the 1,2-aryl shift via path b can be
excluded. Finally, the rapid deprotonation of the carbocation D
yields the aromatized product 3an.
We then performed control experiments to elucidate the
mechanism (Scheme 5a). The reaction of 1a with benzofuran 2n
under the modified oxidation conditions, DDQ (1.5 equiv) and
TFA (5.0 equiv) in DCE at room temperature, the
trifluoroacetyl and benzofuran disubstituted product 3an′ was
isolated in 68% yield, which was determined unambiguously by
X-ray crystallography (see Figure S1 in the Supporting
Information). Interestingly, upon subjecting 3an′ with TFA at
80 °C, the desired ring expansion product 3an was formed in
90% yield. These results strongly suggested that benzofuran
initially attacked preferentially at the cationic C atom on the five-
membered ring and the 1,2-aryl migration-induced ring
expansion proceeded under the acidic conditions through the
formation of a benzylic cation C. The proposed reaction
mechanism is outlined in Scheme 5b (see Figure S2 in the
Supporting Information). A single electron oxidation occurs
preferentially at the more-electron-rich alkenyl moiety of 1a by
the DDQ/TFA oxidation system to form a radical cation species
A.5b Subsequently, the benzofuran nucleophile and the radical
cation A undergoes an intermolecular Friedel−Crafts reaction,
leading to the formation of a benzylic radical species B, which
should be further oxidized to a benzylic cation C. Two 1,2-aryl
migration paths through the migration of an aryl group of the
fluorene moiety by ring expansion (path a) and the benzofuran
group (path b) in the intermediate C can be considered. The
density functional theory (DFT) calculations showed that the
activation energy barriers for the transition state TS1 through
path a and the transition state TS2 through path b are 0.8 and 6.9
kcal/mol, respectively, indicating that the 1,2-aryl migration
through a ring expansion is more favorable than that through a
benzofuran migration. In addition, the carbocation intermediate
D resulting from path a is energetically 13.3 kcal/mol more
stable than the carbocation intermediate D′ resulting from path
In conclusion, we have developed a new metal-free and highly
efficient intermolecular tandem oxidation between AFs and
various unfunctionalized aromatics for constructing structurally
important and useful DAPs under the DDQ/TFA oxidation
conditions. The formation of a radical cation species at the
alkenyl moiety of the AFs is a key feature to induce subsequent
intermolecular oxidative Friedel−Crafts reaction and ring
expansion tandem process, transforming readily accessible AFs
and unfunctionalized arenes into the desired DAPs with
excellent functional group tolerance and exclusive chemo-
selectivity. Moreover, we have demonstrated that the present
tandem oxidation can be applied to one-pot and 2-fold processes
to access π-extended PAHs and co-oligomers. Further extension
of the present synthetic protocol to construct medium-sized
ring-fused PAHs are in progress, which may open a new and
alternative avenue to access various planar and curved PAHs.
ASSOCIATED CONTENT
* Supporting Information
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03283.
Experimental procedures and characterization of related
compounds (PDF)
Accession Codes
CCDC 2027514 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 Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
D
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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
Authors
Lu Yang − Department of Chemistry, Graduate School of Science,
Tohoku University, Sendai 980-8578, Japan
Ilya D. Gridnev − Department of Chemistry, Graduate School of
Science, Tohoku University, Sendai 980-8578, Japan
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:
https://pubs.acs.org/10.1021/acs.orglett.0c03283
(10) Mayr’s Database of Reactivity Parameters. Available via the
Internet at: http://www.cup.lmu.de/oc/mayr/reaktionsdatenbank/.
(11) (a) Mayr, H.; Kempf, B.; Ofial, A. R. Acc. Chem. Res. 2003, 36,
66−77. (b) Mayr, H.; Bug, T.; Gotta, M. F.; Hering, N.; Irrgang, B.;
Janker, B.; Kempf, B.; Loos, R.; Ofial, A. R.; Remennikov, G.; Schimmel,
H. J. Am. Chem. Soc. 2001, 123, 9500−9512.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by JSPS KAKENHI Grant No.
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” (No. JP17H06447) from MEXT
(Japan).
(12) Yao, S.-J.; Ren, Z.-H.; Wang, Y.-Y.; Guan, Z.-H. J. Org. Chem.
2016, 81, 4226−4234.
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