Ruthenium-Catalyzed Cycloisomerization of 2,2′-Diethynyl- biphenyls Involving Cleavage of a Carbon-Carbon Triple Bond

Article abstract of DOI:10.1002/chem.201504937

A ruthenium complex catalyzes a new cycloisomerization reaction of 2,2′-diethynylbiphenyls to form 9-ethynylphenanthrenes, thereby cleaving the carbon-carbon triple bond of the original ethynyl group. A metal-vinylidene complex is generated from one of th

Full text of DOI:10.1002/chem.201504937

DOI: 10.1002/chem.201504937
Communication
&
Arenes
Ruthenium-Catalyzed Cycloisomerization of 2,2’-Diethynyl-
biphenyls Involving Cleavage of a Carbon–Carbon Triple Bond
Takanori Matsuda,*[a] Kotaro Kato,[a] Tsuyoshi Goya,[b] Shingo Shimada,[a] and
Masahiro Murakami*[b]
We previously reported an intramolecular hydroarylation re-
Abstract: A ruthenium complex catalyzes a new cycloiso-
merization reaction of 2,2’-diethynylbiphenyls to form 9-
ethynylphenanthrenes, thereby cleaving the carbon–
action of 2,6-dialkynylbiphenyls (Scheme 1a).[4] The two alkynyl
groups, which were located on the opposite sides of one of
the two phenyl rings, were both activated by p acidic gold
carbon triple bond of the original ethynyl group. A metal–
complexes to undergo the intramolecular hydroarylation reac-
vinylidene complex is generated from one of the two eth-
tion. Two vinylene bridges were formed between the two
ynyl groups, and its carbon–carbon double bond under-
goes a [2+2] cycloaddition with the other ethynyl group
to form a cyclobutene. The phenanthrene skeleton is con-
structed by the subsequent electrocyclic ring opening of
the cyclobutene moiety.
A reaction of terminal alkynes with complexes of transition
metals, such as ruthenium, rhodium, and tungsten, generates
the corresponding metal–vinylidene (M=C=C) complexes.[1]
A
number of catalytic reactions involving those transition-metal-
vinylidene complexes as the key intermediate have been de-
veloped in the past two decades.[2] In most of the catalytic pro-
cesses, reactions of the intermediate metal–vinylidene com-
plexes are associated with their metal–carbon double bond.
On the other hand, the carbon–carbon double bonds of the
metal–vinylidene complexes are also reactive to participate in
various pericyclic reactions.[2d,e,h] However, there have been no
examples of catalytic reactions in which the carbon–carbon
double bond of the metal vinylidenes undergoes a [2+2] cy-
cloaddition with a carbon–carbon unsaturated bond.[3] Herein,
we report a skeletal rearrangement reaction of 2,2’-diethynylbi-
phenyls in which a carbon–carbon double bond of an inter-
mediate ruthenium–vinylidene complex generated from an
ethynyl group undergoes an intramolecular [2+2] cycloaddi-
tion with another ethynyl group. Finally, the original carbon–
carbon triple bond is cleaved to furnish 9-ethynylphenan-
threnes.
Scheme 1. Intramolecular hydroarylation reactions
phenyl groups in parallel to construct a pyrene skeleton. We
attempted an analogous twofold intramolecular hydroarylation
reaction with 2,2’-dialkynylbiphenyls using various p acidic cat-
alysts. In contrast to our expectation, a new skeletal rearrange-
ment reaction was identified with 2,2’-bis(arylethynyl)biphenyls
that was transformed to azulenophenanthrenes (Scheme 1b).[5]
It is likely that this unique reaction worked with 2,2’-dialkynyl-
biphenyls, because the two alkynyl substituents on the two
phenyl rings can get in proximity to each other through axis
rotation.[6]
[a] Prof. Dr. T. Matsuda, K. Kato, S. Shimada
Department of Applied Chemistry
Tokyo University of Science
Another method has also been reported in order to formally
carry out the intramolecular hydroarylation reaction.[7] in which
a phenanthrene skeleton is constructed from the 2-ethynylbi-
phenyl by using a ruthenium catalyst. The 6p electrocyclization
occurs with an intermediate ruthenium–vinylidene complex to
1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601 (Japan)
E-mail: mtd@rs.tus.ac.jp
[b] T. Goya, Prof. Dr. M. Murakami
Department of Synthetic Chemistry and Biological Chemistry
Kyoto University
form
a vinylene bridge between the two phenyl rings
Katsura, Kyoto 615-8510 (Japan)
E-mail: murakami@sbchem.kyoto-u.ac.jp
(Scheme 1c). This electrocyclic pathway led us to study a reac-
tion of 2,2’-diethynylbiphenyl (1a) with ruthenium catalysts.
Thus, 1a was treated with a ruthenium catalyst generated in
situ from [RuCl2(PPh3)(p-cymene)] (10 mol%) and NH4PF6
Supporting information for this article and ORCID for one of the authors
are available on the WWW under http://dx.doi.org/10.1002/
chem.201504937.
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Communication
(20 mol%) in 1,2-dichloroethane at room temperature. After
24 h, 1a was consumed to give a different hydrocarbon prod-
uct of a same molecular weight. To our surprise, neither
pyrene or 4-ethynylphenanthrene, but 9-ethynylphenanthrene
(2a) was exclusively produced and isolated in 76% yield
[Eq. (1)].[8,9]
A significant skeletal change occurred with the unexpected
product 2a that incited us to carry out labeling experiments
for a mechanistic information. Thus, two kinds of labeled bi-
phenyls [13C2]1a and [D2]1a were prepared. The two terminal
ethynyl carbons of the biphenyl [13C2]1a were labeled with 13C
nuclides, and another labeled substrate [D2]1a had the two
ethynyl termini labeled with deuterium atoms. When the bi-
phenyl [13C2]1a was subjected to the cycloisomerization reac-
tion, the two 13C nuclides were found to constitute the ethynyl
group at the 9-position of the resulting phenanthrene [13C2]2a
[Eq. (2)].
Scheme 2. Proposed mechanism.
trocyclic process, generating new vinylidene ruthenium species
C. An ethynyl group is liberated from C to produce 2a togeth-
er with the regeneration of the ruthenium(II) catalyst. Neither
4-ethynylphenanthrene nor pyrene was found in the reaction
mixture, indicating that the [2+2] cycloaddition proceeded to
6p electrocyclization with the intermediate A.
Identical reaction conditions were applied to the octa-1,7-
diyne, the structure of which was significantly more flexible
than 1a. However, an analogous cycloisomerization reaction
failed to occur, suggesting that the reaction mode observed
with 2,2’-diethynylbiphenyl (1a) could be attributed to the
structural rigidity of the biphenyl backbone.
Other substituted 2,2’-diethynylbiphenyls were also subject-
ed to the cycloisomerization reaction. Substrates 1b and 1c,
with methyl and methoxy substituents at the 4 and 4’ posi-
tions, respectively, were suitable substrates, whereas the reac-
tion of 1d bearing electron-withdrawing CF3 groups was slug-
gish (Figure 1a). Methyl and fluoro substituents were also tol-
erated at the 5 (5’) positions (Figure 1b). However, the at-
tempted cycloisomerization reaction of 3,3’,5,5’-tetramethyl de-
rivative 1g, 2,2’-diethynyl-1,1’-binaphthyl (1h), and diyne 1i
carrying an internal alkyne moiety were unsuccessful; the start-
ing materials remained unchanged (Figure 1c).[10]
On the other hand, when the biphenyl [D2]1a was used, one
of the deuterium atoms was located at the 10-position of the
resulting phenanthrene skeleton, and the other deuterium
atom was found at the terminal position of the 9-ethynyl sub-
stituent [Eq. (3)].
In summary, we have described the new ruthenium-cata-
lyzed cycloisomerization reaction of 2,2’-diethynylbiphenyls af-
fording 9-ethynylphenanthrenes. The carbon–carbon triple
bond of the ethynyl substituent was cleaved and its terminal
sp carbon was coupled with another terminal sp carbon of the
other ethynyl substituent to form a new ethynyl group at the
9-position of the phenanthrene.
On the basis of the results of the labeling experiments, we
assume the mechanism depicted in Scheme 2 for the rutheni-
um-catalyzed cycloisomerization. First, one of the ethynyl
groups of 1a reacts with the ruthenium(II) to generate vinyli-
dene ruthenium complex A. The other ethynyl group under-
goes a [2+2] cycloaddition with the cumulated carbon–
carbon double bond of A, forming cyclobutenylidene rutheni-
um B. Then, the four-membered ring opens through a 4p elec-
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A
Schlenk tube was charged with 2,2’-diethynylbiphenyl
1 (0.200 mmol), [RuCl2(PPh3)(p-cymene)] (0.020 mmol, 10 mol%),
and NH4PF6 (0.040 mmol, 20 mol%), and then the tube was evacu-
ated and backfilled with nitrogen. 1,2-Dichloroethane (4.0 mL) was
added by syringe through the septum, and the mixture was stirred
at room temperature. The disappearance of the starting material
was checked by GC analysis. The reaction mixture was concentrat-
ed under reduced pressure. The residue was subjected to prepara-
tive thin-layer chromatography on silica gel (hexane/AcOEt) to
afford 9-ethynylphenanthrenes 2.
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Acknowledgements
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[8] No reaction occurred in the absence of NH4PF6 and in toluene, THF, or
1,4-dioxane.
This work was supported by JSPS, Japan (Grant-in-Aid for Sci-
entific Research (C) No. 25410054) and the Sumitomo Founda-
tion. We thank S. Shiose for help with some experiments.
[9] Conversion of 1a to 2a under FVT conditions was reported. M. Sarobe,
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[10] 2-Ethynyl-2’-vinylbiphenyl was also unreactive under the reaction condi-
tions and 2,2’-diethynyl-3,3’-bithiophene gave a complex mixture of
products.
Keywords: alkynes · cycloisomerization · phenanthrenes ·
ruthenium · vinylidene ligands
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Received: December 8, 2015
Published online on January 8, 2016
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www.chemeurj.org
1943
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim