Azulenophenanthrenes from 2,2′-Di(arylethynyl)biphenyls through C-C bond cleavage of a benzene ring

Article abstract of DOI:10.1002/anie.201300570

From six to seven: 2,2′-Di(arylethynyl)biphenyls undergo a skeletal rearrangement in the presence of a platinum(II) catalyst to afford polycyclic aromatic compounds containing an azulene unit. The reaction involves C-C bond cleavage of a benzene ring, which expands into a seven-membered ring. Copyright

Full text of DOI:10.1002/anie.201300570

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DOI: 10.1002/anie.201300570
Azulene Synthesis
Azulenophenanthrenes from 2,2’-Di(arylethynyl)biphenyls through
À
C C Bond Cleavage of a Benzene Ring**
Takanori Matsuda,* Tsuyoshi Goya, Lantao Liu, Yusuke Sakurai, Shoichi Watanuki,
Naoki Ishida, and Masahiro Murakami*
Dedicated to Professor Teruaki Mukaiyama
A diverse range of skeletal-rearrangement reactions cata-
lyzed by p-acidic platinum and gold complexes have been
developed over the last decade.[1] The p-acidic metals induce
unique reorganization of organic frameworks, particularly
those with alkyne moieties, to offer direct access to molecular
structures that are difficult to construct by conventional
synthetic methods.
This idea led us to examine a reaction of 2,2’-dialkynylbi-
phenyl, and a totally unexpected result was obtained. Herein,
we report a platinum(II)-catalyzed skeletal rearrangement of
2,2’-di(arylethynyl)biphenyls that results in the construction
of an azulenophenanthrene framework. The seven-membered
ring of the azulene unit in the product is constructed through
dearomatization of the six-membered benzene ring and
À
Fꢀrstner and co-workers reported the synthesis of phen-
anthrenes by a platinum-catalyzed intramolecular hydroar-
ylation reaction of 2-alkynylbiphenyls.[2] We applied their
hydroarylation reaction to the synthesis of pyrenes
(Scheme 1); 4,10-disubstituted pyrenes were successfully
subsequent electrocyclic C C bond cleavage.
When 2,2’-bis[(4-chlorophenyl)ethynyl]biphenyl (1a) was
heated at 1208C in p-xylene for 12 h in the presence of
a platinum(II) catalyst generated in situ from [PtCl(C6F5)-
(cod)] (10 mol%) and P(OCH2CF3)3 (10 mol%), two poly-
cyclic aromatic compounds were formed in 75% combined
yield in a ratio of 73:27 [Eq. (1); cod = cycloocta-1,5-diene].
1
They were separated by chromatography and their H NMR
spectra were unlike those expected for a symmetrical pyrene,
instead they were suggestive of unsymmetrical structures. The
minor product was benzo[f]tetraphene 3a, which could be
regarded as an intramolecular dehydro-Diels–Alder product
(see below).[4,5] On the other hand, the structural elucidation
of the major product necessitated a single-crystal X-ray
analysis, which proved it to be azuleno[1,2-l]phenanthrene 2a
(Figure 1).[6] Thus, one phenyl ring of 1a had been expanded
into a seven-membered ring.[7]
Scheme 1. Pyrene synthesis through twofold hydroarylation of dialkyn-
ylbiphenyls.
synthesized by the gold-catalyzed twofold hydroarylation
reaction of 2,6-dialkynylbiphenyls.[3] A phenyl ring having two
alkynyl substituents at the 2 and 6 positions forms two
vinylene bridges with a pendant phenyl ring, in parallel, to
construct a pyrene skeleton. We envisaged that 2,2’-dialkyn-
ylbiphenyl would be an alternative starting material for
another related intramolecular twofold hydroarylation. A
pyrene skeleton would result if each phenyl ring forms one
vinylene bridge to the other phenyl ring.
[*] T. Goya, Dr. L. Liu,[+] Dr. N. Ishida, Prof. Dr. M. Murakami
Department of Synthetic Chemistry and Biological Chemistry
Kyoto University
Katsura, Kyoto 615-8510 (Japan)
E-mail: murakami@sbchem.kyoto-u.ac.jp
Homepage: http://www.sbchem.kyoto-u.ac.jp/murakami-lab/
Dr. T. Matsuda, Y. Sakurai, S. Watanuki
Department of Applied Chemistry, Tokyo University of Science
1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601 (Japan)
E-mail: mtd@rs.tus.ac.jp
We propose the pathways depicted in Schemes 2 and 3 as
plausible mechanistic scenarios for the formation of 2a and
3a from 1a. Initially, an alkyne moiety acts as a p-base,
coordinating to the p-acidic PtII center to induce 6-exo-dig
cyclization involving the other alkyne moiety. The resulting
vinylic cation A is then attacked intramolecularly by the 4-
chlorophenyl ring at two sites. Attack at the ipso-carbon atom
forms the spirocyclic intermediate B, leading to the major
[+] Present address: College of Chemistry and Chemical Engineering,
Shangqiu Normal University (China)
[**] We thank Prof. S. Kitagawa and Dr. S. Horike (both Kyoto University)
for their generosity and assistance in the X-ray crystal structural
analysis.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201300570.
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react independently with the backbone, the two alkynyl
substituents of 2,2’-dialkynylbiphenyl are located close
enough to react with each other without participation of the
backbone.
Other 2,2’-di(arylethynyl)biphenyls 1b–g were subjected
to the reorganization reaction (Table 1). The phenylethynyl
derivative 1b and (4-methoxyphenyl)ethynyl derivative 1c
gave azulenophenanthrenes 2 and benzotetraphenes 3 in 75%
and 84% combined yields, respectively, with the azuleno-
phenanthrenes predominating (entries 1 and 2). It was
possible to lower the catalyst loading to 1 mol% without
any decrease in yield in the reaction of 1c (entry 3).
Analogous results were obtained when P[OCH(CF3)2]3 and
tricyclohexylphosphane were used instead of P(OCH2CF3)3 as
the ligand of platinum (entries 4 and 5) and when [(tBuX-
Phos)AuNTf2] was used as the catalyst (entry 6). However,
PtCl2 alone gave an inferior result to the [PtCl(C6F5)(cod)]/
P(OCH2CF3)3 catalyst (entry 7). The reactions of 2- and 3-
methoxyphenylethynyl derivatives (1d and 1e, respectively)
were also examined. Whereas in the reaction of 1d the
azulenophenanthrene 2d was favored in preference to
benzotetraphene 3d (entry 8), the reaction of 1e gave no
azulenophenanthrene but an almost 1:1 mixture of isomeric
benzotetraphenes (entry 9). In the cases of diynes 1 f, having
six methoxy groups, and 1g, having sterically demanding 1-
naphthyl groups, the corresponding azulenophenanthrenes 2 f
and 2g were exclusively produced without formation of
benzotetraphenes 3 (entries 10 and 11). We prepared the
diyne 1h, which was unsymmetrically disubstituted with
methoxy and methyl groups and diyne 1i substituted with
two nitro groups, and they were subjected to the standard
reaction conditions. The p-basicities of the two arylethynyl
groups of 1h were similar, therefore a complex mixture
consisting of two isomeric azulenophenanthrenes and two
isomeric benzotetraphenes resulted. The diyne 1i failed to
react and remained intact, thus supporting the involvement of
cationic intermediates in the mechanism.
Figure 1. X-ray crystal structure of 2a (thermal ellipsoids at 50%
probability, hydrogen atoms are omitted for clarity).
Scheme 2. Mechanism for the formation of azuleno-
[1,2 L]phenanthrene 2a from 1a (M=PtII).
product 2a (Scheme 2), and attack at the ortho-carbon atom
gives intermediate E, leading to the minor product 3a
(Scheme 3). The spirocyclic structure of B consists of
a platinum-substituted cyclopentadinenyl moiety and a pen-
Scheme 3. Mechanism for the formation of benzo[f]tetraphene 3a.
dant cyclohexadienyl cation moiety. The former subsequently
attacks the latter to generate the cationic norcaradiene
intermediate C, which is relatively stable because its cationic
center is a tertiary one next to phenyl, vinyl, and cyclopropyl
groups. The six-membered ring of the norcaradiene opens
through the Cope rearrangement to afford cycloheptatriene
D.[8,9] Finally, demetalation establishes aromaticity to give the
azulene skeleton. On the other hand, the intermediate E
simply rearomatizes with demetalation to give the dehydro-
Diels–Alder adduct 3a as the minor product.
In summary, a new skeletal rearrangement reaction of
2,2’-di(arylethynyl)biphenyls is reported. An azulene skele-
ton was formed by structural reorganization between the
ethyne and arylethyne moieties. Its formal depiction is
presented in Equation (2). The s bond between the ipso-
carbon and the ortho-carbon atoms of the phenyl ring is
cleaved and s-bond between the ortho-carbon atom and the
adjacent sp-hybridized carbon atom is reformed. As a result,
Of note is the contrast observed in the reaction patterns of
2,6-dialkynylbiphenyl and 2,2’-dialkynylbiphenyl. Whereas
the two alkynyl substituents of 2,6-dialkynylbiphenyl are
located on the opposite sides of the biphenyl backbone and
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Table 1: Platinum-catalyzed reaction of 2,2’-di(arylethynyl)biphenyls 1.[a]
Entry
1
Conditions
2 (Yield [%])[b]
3 (Yield [%])[b]
1
1b
10 mol% [PtCl(C6F5)(cod)]
10 mol% P(OCH2CF3)3
2b (45)
3b (30)
2
3
4
5
1c
1c
1c
1c
10 mol% [PtCl(C6F5)(cod)]
10 mol% P(OCH2CF3)3
1 mol% [PtCl(C6F5)(cod)]
1 mol% P(OCH2CF3)3
10 mol% [PtCl(C6F5)(cod)]
10 mol% P(OCH(CF3)2)3
10 mol% [PtCl(C6F5)(cod)]
10 mol% P(c-Hex)3
2c (71)
2c (80)
2c (67)
2c (70)
3c (13)
3c (10)
3c (trace)
3c (10)
6
7
1c
1c
20 mol% [(tBuXPhos)AuNTf2]
10 mol% PtCl2
2c (61)
2c (34)
3c (trace)
–
8
9
1d
10 mol% [PtCl(C6F5)(cod)]
10 mol% P(OCH(CF3)2)3
2d (62)
3d (14)
1e
10 mol% [PtCl(C6F5)(cod)]
10 mol% P(OCH(CF3)2)3
3e + 3’e (76; ca. 1:1)
10
1 f
10 mol% [PtCl(C6F5)(cod)]
10 mol% P(OCH2CF3)3
2 f (62)
11
1g
10 mol% [PtCl(C6F5)(cod)]
10 mol% P(OCH2CF3)3
2g (78)
[a] The reaction was performed in p-xylene at 120 8C for 12 h. [b] Yield of the isolated product. tBuXPhos=2-di-tert-butylphosphino-2’,4’,6’-
triisopropylbiphenyl.
the six-membered ring is expanded into a seven-membered
ring.
into a seven-membered ring.[9] Transition-metal-catalyzed
À
reactions involving cleavage of C C bonds have attracted
During the present process, a highly stable C C bond of
considerable attention during the last decade.[10] Generally,
a particular driving force, such as release of small-ring strain
À
the six-membered aromatic ring is cleaved with expansion
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Chemie
Chem. Soc. 1966, 88, 3027 – 3034; b) E. H. White, A. A. F. Sieber,
or aromatization, is required because the thermodynamically
stable s bond is to be finally transformed into a more stable
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adds an exotic example to the repertoire of metal-catalyzed
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À
cleavage of C C bonds.
[6] CCDC 920500 (2a) 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.
[7] Lewis acid mediated dimerization of diphenylethyne to give
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Kharasch, J. Am. Chem. Soc. 1958, 80, 5978 – 5982; b) H. J.
de Liefde Meijer, U. Pauzenga, F. Jellinek, Recl. Trav. Chim.
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transformation: c) G. Bꢀchi, C. W. Perry, E. W. Robb, J. Org.
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Received: January 22, 2013
Revised: March 6, 2013
Published online: May 6, 2013
Keywords: alkynes · azulenes · platinum · polycycles ·
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rearrangements
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