Regioselective Activation of a Sterically More Hindered C?C Bond of Biphenylenes Using an Alkene as Both a Directing Group and a Reaction Moiety

Article abstract of DOI:10.1002/chem.201804044

The Rh-catalyzed intramolecular reaction of 1-(2-vinylaryl)-substituted biphenylenes was used to construct a dihydrobenzo[b]fluoranthene skeleton. This transformation was achieved by regioselective C?C bond cleavage of a sterically more hindered biphenylene site by using alkene as both a directing group and a reaction moiety. Furthermore, we measured and analyzed the photophysical properties of the new multicyclic fused compounds.

Full text of DOI:10.1002/chem.201804044

DOI: 10.1002/chem.201804044
Communication
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Hydrocarbons |Hot Paper|
Regioselective Activation of a Sterically More Hindered CÀC Bond
of Biphenylenes Using an Alkene as Both a Directing Group and a
Reaction Moiety
[a]
[a]
[b]
[a]
Hideaki Takano, Takeharu Ito, Kyalo Stephen Kanyiva, and Takanori Shibata*
cycloalkenones from squaric acids possessing alkenyl teth-
[5b]
Abstract: The Rh-catalyzed intramolecular reaction of 1-
2-vinylaryl)-substituted biphenylenes was used to con-
ers: selective CÀC bond activation was achieved through the
bidentate coordination of ketone and a hetero atom, or a dike-
tone as a directing group. Although directing groups contain-
ing hetero atoms have the potential to cleave a CÀC bond in a
diastereoselective manner, they are typically difficult to remove
(
struct a dihydrobenzo[b]fluoranthene skeleton. This trans-
formation was achieved by regioselective CÀC bond cleav-
age of a sterically more hindered biphenylene site by
using alkene as both a directing group and a reaction
moiety. Furthermore, we measured and analyzed the pho-
tophysical properties of the new multicyclic fused com-
pounds.
[6]
after the reaction. Against this background, selective cleav-
age of a sterically hindered CÀC bond without a directing
group or with a traceless directing group is still challenging.
In contrast, biphenylene is a strained hydrocarbon that was
[7]
first synthesized by Lothrop in 1941. It can react with transi-
tion metal complexes driven by the release of ring strain of
the centered 4-membered ring to give a dibenzometallacyclo-
pentadiene intermediate, which is widely used in organic syn-
Transition-metal-catalyzed carbon–carbon bond activation has
been the subject of great interest because it can be used to re-
construct the carbon framework of organic compounds with
perfect atom economy and can enable the construction of
new carbon frameworks that can be accessed by other proto-
[8]
thesis. While a sterically less hindered site is usually selective-
ly cleaved, there is only one example in which a sterically hin-
dered site is selectively cleaved by the aid of a directing
group: Koga, Matsubara and co-workers reported the stoichio-
metric reaction of 1-(2-pyridyl)biphenylene with chloro(1,5-cy-
clooctadiene)iridium(I) dimer to give an 18-electron iridium
[
1]
cols only with some difficulty, if at all. Among various mecha-
[
2]
nisms of CÀC bond cleavage, the most notable approach is
oxidative addition to a strained CÀC bond such as in cyclobu-
[9]
complex (Scheme 1a).
[
3]
tane derivatives. Murakami, Ito and co-workers achieved an
intramolecular reaction of 4-membered carbocycles involving
successive cleavage of a CÀC bond of cyclobutanone and an
[
4]
etheric CÀO bond. Since then, many intramolecular transfor-
mations including CÀC bond activation of various other cyclic
[
5]
as well as acyclic substrates have been reported.
Nevertheless, regioselective CÀC bond activation is still
[
5]
being studied. Dong and co-workers reported Rh-catalyzed
intramolecular carboacylation via CÀC bond activation of ben-
[
5f]
zocyclobutenone. They elucidated the reaction mechanism
based on the DFT calculations: selective cleavage of less hin-
dered and more reactive CÀC bond is followed by isomeriza-
tion of rhodacycle through decarbonylation and carbonyl-
Scheme 1. Regioselective transformation of biphenylene.
[
5i]
ation. Yamamoto and co-workers developed a synthesis of bi-
In the course of our investigation of CÀC bond activation,
we reported the formal [4+1] cycloaddition of biphenylene
with alkene to give 9,9-disubstituted fluorene derivatives
[
a] H. Takano, T. Ito, Prof. Dr. T. Shibata
Department of Chemistry and Biochemistry
School of Advanced Science and Engineering, Waseda University
[10]
(
Scheme 1b). When 1-substituted biphenylenes were used in
3
-4-1 Okubo, Shinjuku, Tokyo 169-8555 (Japan)
E-mail: tshibata@waseda.jp
this reaction, a sterically less hindered CÀC bond was selective-
ly cleaved, as in previous examples.
[
b] Dr. K. S. Kanyiva
Global Center for Science and Engineering
School of Advanced Science and Engineering, Waseda University
We considered that a new transformation initiated by a steri-
cally more hindered site of biphenylene could be achieved by
an intramolecular reaction that used an alkene as both a di-
recting group and a reaction site (Scheme 2). First, an alkene
moiety coordinates to the metal center, which may put the
3
-4-1 Okubo, Shinjuku, Tokyo 169-8555 (Japan)
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/chem.201804044.
Chem. Eur. J. 2018, 24, 1 – 6
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Communication
(
Entry 5). In this reaction, almost no solvent effect was ob-
served (Entries 6 and 7). When the prepared [Rh(cod)(SIMes)]-
BARF was used, the amount of catalyst could be reduced to
2
mol% and excellent yield was achieved, along with the for-
mation of a negligible amount of 3a (>20:1; Entry 9).
With the optimized reaction conditions in hand (Entry 9 in
Table 1), we examined various substituted biphenylene deriva-
tives 1 (Table 2). Both electron-donating and -withdrawing
groups were available at the 3- and 4-positions on the arene
of 1-(2-vinylphenyl)biphenylenes, and the corresponding 6-
and 7-methoxy and halogenated dihydrobenzofluoranthenes
were obtained (2b–2 f). Regarding the substituent at the 5-po-
sition, a methoxy group deactivated the substrate, and the
yield of 2g was low, however 5-halogenated substrates gave
good results, and the corresponding 5-halogenated products
2h and 2i were obtained in high yields. The steric effect at the
6-position was significant: a fluorine atom was so small that
there was little steric effect and the reaction gave 2j in excel-
lent yield. In contrast, the reaction of naphthyl-substituted 1k
did not proceed at all. Actually, naphthyl-fused 2l with an ex-
Scheme 2. Concept of selective cleavage of sterically more hindered CÀC
bond of biphenylene in the intramolecular reaction.
[11]
metal close to the sterically hindered site of biphenylene.
Oxidative addition of the metal complex to a CÀC bond gives
a dibenzometalacyclopentadiene intermediate, and the alkene
moiety is inserted intramolecularly. As a result, a formal [4+1]
or [4+2] cycloadduct can be formed as a multicyclic fused
compound. To the best of our knowledge, there is no example
of a catalytic and intramolecular transformation initiated by
the direct activation of a sterically more hindered site of biphe-
nylene.
Table 2. Screening of substrates.
We chose 1-(2-vinylphenyl)biphenylene (1a) as a model sub-
strate and examined an intramolecular reaction using various
[10]
transition metal catalysts (Table 1). When neutral iridium or
rhodium catalyst with BINAP as a diphosphine ligand was
used, no desired products were observed (Entries 1 and 2).
However, in the presence of cationic catalysis, dihydroben-
zo[b]fluoranthene derivative 2a was obtained as a formal
[4+2] cycloadduct, albeit in low yields, together with a small
amount of dehydrogenated product 3a (Entries 3 and 4).
While we screened various diphosphines for the cationic rhodi-
um catalyst, the yield of 2a could not be improved. Therefore,
we turned our attention to the N-heterocyclic carbene (NHC)
[
12]
ligand for the cationic Rh complex, which was prepared in
[
13]
situ from [RhCl(cod)(SIMes)] and NaBARF: the reaction pro-
ceeded smoothly with the complete consumption of 1a within
2
h, and cycloadduct 2a was obtained in high isolated yield
Table 1. Screening of the reaction conditions.
Entry
Catalyst (X)
[IrCl(cod)] (10)
Additive (Y)
Time [h] Yield [%]
1
2
3
4
5
2
BINAP (20)
BINAP (20)
BINAP (20)
BINAP (20)
NaBARF (10)
NaBARF (10)
NaBARF (10)
–
40
40
40
40
2
ND
ND
8
34
87
83
87
89
92
[RhCl(cod)] (10)
[Ir(cod) ]BARF (20)
[Rh(cod) ]BARF (20)
2
2
2
[RhCl(cod)(SIMes)] (10)
[RhCl(cod)(SIMes)] (10)
[RhCl(cod)(SIMes)] (10)
[Rh(cod)(SIMes)]BARF (10)
[Rh(cod)(SIMes)]BARF (2)
[
a]
6
7
8
9
2
2
2
2
[
b]
–
[
a] Chlorobenzene was used as a solvent. [b] The catalyst (5 mol%) was
used.
[a] 1,4-Dioxane was used. [b] Chlorobenzene was used.
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tended p-conjugated system was obtained in a moderate
of non-aromatic compounds 2 and aromatic compounds 3.
The UV/Vis absorption and fluorescence spectra measured in
acetonitrile are shown in Figure 1, and quantum yields both in
solution and in the solid state are shown in Table 4. An elec-
tron-donating methoxy group on a dihydrobenzofluoranthene
scaffold (2d) induced a redshift in both UV and fluorescence
spectra, whereas the redshift in the UV spectrum of oxidized
compound 3d was slight. While p-extension (2l) had no mean-
ingful effect on the shift in the UV and fluorescence spectra, a
significant redshift was observed in both spectra of 3l. While
the peaks in the fluorescence spectra of 2a, 2 f and 2l split,
those of 2d and 3 were broad, which suggests that the excit-
ed-state structures of 2d and 3 may be polarized; therefore,
yield. A substituent on the biphenylene ring was tolerated
(
2m). In place of a vinyl group, 1,1-disubstituted olefin could
also be used and dihydrobenzofluoranthene containing a qua-
ternary carbon was obtained (2n and 2o), albeit in low to
moderate yield because of low conversion.
We next oxidized selected products 2 using DDQ (2,3-di-
chloro-5,6-dicyano-1,4-benzoquinone) and obtained dehydro-
genated compounds 3 in moderate to high yields (Table 3). We
measured their photophysical properties and compared those
Table 3. Oxidation to benzo[b]fluoranthene derivatives.
[
a,b]
Table 4. Quantum yields of 2 and 3.
Comp.
F (solution)
F (solid)
Comp.
F (solution)
F (solid)
2
2
2
2
a
d
f
0.35
0.34
0.28
0.14
0.31
0.20
0.08
0.14
3a
3d
3 f
3l
0.78
0.11
0.07
0.15
0.44
0.23
0.09
0.25
l
[
a] Excitation wavelength 2a; 269 nm, 2d; 276 nm, 2 f; 273 nm, 2l;
2
1
3
65 nm, 3a; 300 nm, 3d; 306 nm, 3 f; 302 nm, 3l; 323 nm. [b] 2a; 2.2ꢁ
À4
À4
À4
À5
À5
0
m, 2d; 1.3ꢁ10 m, 2 f; 1.4ꢁ10 m, 2l; 5.9ꢁ10 m, 3a; 7.5ꢁ10 m,
À4
À4
À4
d; 1.4ꢁ10 m, 3 f; 1.9ꢁ10 m, 3l; 1.3ꢁ10 m.
À4
À4
À4
À5
Figure 1. (a) UV/Vis spectra of 2 in acetonitrile solution (2a; 2.2ꢁ10 m, 2d; 1.3ꢁ10 m, 2 f; 1.4ꢁ10 m, 2l; 5.9ꢁ10 m). (b) UV/Vis spectra of 3 in acetoni-
À5
À4
À4
À4
À5
trile solution (3a; 7.5ꢁ10 m, 3d; 1.4ꢁ10 m, 3 f; 1.9ꢁ10 m, 3l; 1.3ꢁ10 m). (c) Fluorescence spectra of 2 in acetonitrile solution (2a; 2.2ꢁ10
269 nm), 2d; 1.3ꢁ10 m (276 nm), 2 f; 1.4ꢁ10 m (273 nm), 2l; 5.9ꢁ10 m (265 nm)). (d) Fluorescence spectra of 3 in acetonitrile solution (3a; 7.5ꢁ10 m
300 nm), 3d; 1.4ꢁ10 m (306 nm), 3 f; 1.9ꢁ10 m (302 nm), 3l; 1.3ꢁ10 m (323 nm)).
m
À5
À5
À5
À6
(
(
À5
À5
À5
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Communication
they are stabilized by a solvation effect of acetonitrile, and
compounds 2a, 2 f, and 2l could not have a polarized struc-
ture in the excited state. Notably, the quantum yields of 2
(except for 2 f) in the solid state are almost the same as those
in solution. Although the quantum yields of 3 strongly de-
pended on the substituents, similar effects were not observed
in compounds 2 (Table 4). These results mean that the new
carbon skeleton of dihydrobenzo[b]fluoranthenes 2 has differ-
ent physical properties than the known carbon skeleton of
benzo[b]fluoranthenes 3.
To investigate the structure of dihydrobenzofluoranthene,
[14]
we conducted geometry optimization using Gaussian 09.
The calculated dihedral angle roughly agreed with the results
of the X-ray crystal analysis (Table 5). In a comparison of 2a
and 4, 2a has a more planar structure due to the fluorene scaf-
Scheme 3. Mechanistic experiment.
of alkene to the rhodium center is critical for the selective CÀC
bond activation as a directing group, and that CÀC bond acti-
vation and alkene insertion are consecutive.
[
a]
Table 5. Calculated dihedral angles of 2a, 3a and 4.
In summary, we have developed the first regioselective intra-
molecular reaction of biphenylene with alkene via direct cleav-
age of sterically more hindered CÀC bond. In this reaction, an
alkene moiety acts as both a directing group to promote oxi-
dative addition and a reaction site, and a multicyclic fused-ring
system is constructed, which is difficult to access by other pro-
tocols. Additionally, the obtained dihydrobenzofluoranthene
scaffold has photophysical properties different than those of
benzofluoranthene derivatives. An asymmetric variant of the
present transformation is under investigation in our laboratory.
Dihedral angle
C1-C2-C3-C4
2a [8]
2i [8]
3a [8]
4 [8]
19.8
19.8
0.0
24.5
(20.2)
[a] wB97XD/6–311+G(d,p) and the value of dihedral angle from crystal
structure in parenthesis.
[15]
Acknowledgements
fold. Additionally, we calculated the value of NICS(0) of 2a
and 3a (Table 6). Certainly, ring d was aromatized by oxidation
H.T. is grateful to the Japan Society for the Promotion of Sci-
ence for the fellowship support. This work was supported by
Waseda University Grant for Special Research Projects. We are
grateful to Umicore for generous support and a supply of
(
+3.3 to À4.5), and ring b simultaneously became more antiar-
Table 6. NICS(0) of 2a and 3a.
[
RhCl(cod)]2.
Conflict of interest
Ring
2a
3a
The authors declare no conflict of interest.
a
b
c
d
e
À7.1
+1.9
À6.1
+3.3
À6.6
À5.5
+4.8
À6.9
À4.5
À8.0
Keywords: biphenylene
hydrocarbons · regioselective · rhodium
·
CÀC
bond
activation
·
[a] GIAO B3LYP/6–311+G(d,p)//wB97XD/6–311+G(d,p).
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[
[
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Version of record online: && &&, 0000
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COMMUNICATION
&
Hydrocarbons
H. Takano, T. Ito, K. S. Kanyiva, T. Shibata*
& – &&
&
Regioselective Activation of a
Sterically More Hindered CÀC Bond of
Biphenylenes Using an Alkene as Both
a Directing Group and a Reaction
Moiety
Two in one: We achieved the first direct
regioselective CÀC bond activation of
biphenylene by using alkene as both a
directing group and a reaction site. In
this reaction, an alkene moiety guided a
cationic rhodium catalyst to the sterical-
ly hindered CÀC bond of biphenylene,
followed by oxidative addition, insertion
and reductive elimination to give dihy-
drobenzo[b]fluoranthene derivatives as
a new cyclic hydrocarbon. Furthermore,
we measured various photophysical
properties of dihydrobenzo[b]fluoran-
thene and analyzed its structure.
&
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