Ruthenium-Catalyzed Cycloisomerization of 2-Alkynylstyrenes via 1,2-Carbon Migration That Leads to Substituted Naphthalenes

Article abstract of DOI:10.1002/chem.201802413

A ruthenium-catalyzed carbocyclization of 2-alkynylstyrenes that involves a very rare 1,2-carbon migration of internal alkynes is reported. Various 1,2-di -and 1,4,7-trisubstituted naphthalenes are synthesized. Mechanistic studies revealed that this reaction proceeds via a disubstituted vinylidene complex as the key intermediate by 1,2-carbon migration of the 2-alkynylstyrenes.

Full text of DOI:10.1002/chem.201802413

A Journal of
Accepted Article
Title: Ruthenium-Catalyzed Cycloisomerization of 2-Alkynylstyrenes via
1,2-Carbon Migration that Leads to Substituted Naphthalenes
Authors: Takuma Watanabe, Haruka Abe, Yuichiro Mutoh, and Shinichi
Saito
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To be cited as: Chem. Eur. J. 10.1002/chem.201802413
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Ruthenium-Catalyzed Cycloisomerization of 2-Alkynylstyrenes
via 1,2-Carbon Migration that Leads to Substituted Naphthalenes
Takuma Watanabe, Haruka Abe, Yuichiro Mutoh,* and Shinichi Saito*
(a) Cycloisomerization via π-activation (non-migration)
Abstract:
A
ruthenium-catalyzed
carbocyclization
of
2-
[M]
R1
R1
alkynylstyrenes that involves a very rare 1,2-carbon migration of
internal alkynes is reported. Various 1,2-di -and 1,4,7-trisubstituted
naphthalenes are synthesized. Mechanistic studies revealed that this
reaction proceeds via a disubstituted vinylidene complex as the key
intermediate by 1,2-carbon migration of the 2-alkynylstyrenes.
R1
[M]
R2
R2
M = Pd, Pt, Au, Zn…
(b) Cycloisomerization via 1,2-hydrogen / iodine / silicon migration
R2
Naphthalenes are attractive aromatic compounds on account of
their diverse biological activity, as well as their photophysical and
fluorescence properties.[1,2] Consequently, substantial efforts
have been devoted to the development of synthetic routes to
various naphthalene skeletons.[3,4] Among these methods, a
particularly powerful and efficient strategy is based on the
H, I, SiR1
[M]
H, I, SiR1
H, I, SiR1
3
3
3
•
[M]
R2
R2
R2
R1
M = W, Rh, Ru
(c) This work: Cycloisomerization via 1,2-carbon migration
R1 R1
transition-metal-catalyzed
carbon-carbon
bond-forming
cycloaromatization[5,6] of benzo-fused 1,3-dien-5-ynes. In the
presence of catalytic amount of p-Lewis-acidic metals, such as
palladium,[6c,g] platinum,[6a,c,f] gold,[6d,h,i] and zinc,[6c] the benzo-
fused 1,3-dien-5-ynes are converted into naphthalene derivatives
without migration of the substituents (Scheme 1a). Following a
pioneering study by Merlic and Pauly,[7] which involves a
ruthenium vinylidene complex in the electrocyclization of 1,3-dien-
5-ynes, cycloaromatization reactions of 2-alkynylstyrenes that
afford naphthalene derivatives via vinylidene intermediates have
been disclosed (Scheme 1b).[8,9] These reactions proceeded via
the 1,2-migration of hydrogen atom,[8a,c,d,f,10] iodine atom,[8c,d] or
that of a trialkylsilyl group.[8b,e,11] In contrast, catalytic processes
that involve the vinylidene rearrangement of 2-alkynylstyrenes
with a carbon substituent on their alkyne terminus by 1,2-carbon
migration has not yet been reported though experimental[12] and
theoretical[13] investigations into stoichiometric internal alkyne-to-
vinylidene rearrangements have only recently been reported.
Meanwhile, we have developed the first catalytic reaction that
involves the vinylidene rearrangement of internal alkynes by 1,2-
[Ru]+
•
[Ru]+
R2
R2
R2
Scheme 1. Substituted naphthalenes from the transition-metal-catalyzed
cycloisomerization of 2-alkynylstyrenes.
Initially, we investigated the impact of the structure of the
styrene moiety in 2-(phenylethynyl)styrene derivatives (1) on the
carbocyclization using [CpRuCl(dppe)]/NaBArF ·2H2O (ArF = 3,5-
4
(CF3)2C6H3) as the catalytic system, which was found to be highly
active in the previous studies.[14,16] When a mixture of 2-
(phenylethynyl)styrene (1a), [CpRuCl(dppe)] (10 mol%), and
NaBArF4·2H2O (12 mol%) was stirred at 130 °C for 2 h in xylene,
the desired 1,2-aryl migration product, 1-phenylnaphthalene (2a),
was formed in 74% yield (Table 1). Alkyne 1b with an α-
methylstyryl group was more reactive than 1a and converted into
the corresponding 1,4-disubstituted naphthalene 2b in 91% yield
after 1 h in the presence of a smaller amount (6 mol%) of
[CpRuCl(dppe)]. This result can be rationalized by postulating that
1b adopts an appropriate conformation for the cyclization upon
the introduction of the methyl group. The observed high reactivity
of 1b is consistent with the results obtained from a tungsten-
catalyzed cycloaromatization of 2-(ethynyl)styrene derivatives.[8a]
The reaction of a-ethylstyrene derivative 1c completed in the
carbon migration.[14] In the presence of the cationic ruthenium
−
catalyst
[CpRu(dppe)]+
(Cp
=
h5-C5H5 ;
dppe
=
Ph2PCH2CH2PPh2), various 2-alkynylanilides are converted into
the corresponding 3-substituted indoles in high yields via the 1,2-
carbon migration followed by a carbon-nitrogen bond-forming
cyclization. These results encouraged us to develop new and
useful reactions that involve 1,2-carbon migration and carbon-
carbon bond formation. Herein, we report a ruthenium-catalyzed
cycloisomerization
of 2-alkynylstyrenes that generates
presence of
8 mol% of [CpRuCl(dppe)], and 1-ethyl-4-
substituted naphthalenes via 1,2-carbon migration (Scheme
1c).[15]
phenylnaphthalene (2c) was isolated in 82% yield. When the
reaction of α-phenyl derivative 1d was performed under similar
conditions, the reaction did not proceed. After a further screening
of the reaction conditions, 1,4-diphenylnaphthalene (2d) was
prepared in 77% yield upon increasing the catalyst loading and
the temperature. At this point, we assume that the presence of the
bulky substituent (i.e., a phenyl group) bound to the olefin moiety
prevents the vinylidene intermediate from adopting a suitable
conformation for the progress of the reaction.
[*]
T. Watanabe, H. Abe, Dr. Y. Mutoh, Prof. Dr. S. Saito
Department of Chemistry, Faculty of Science
Tokyo University of Science
1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601 (Japan)
E-mail: ymutoh@rs.tus.ac.jp
ssaito@rs.kagu.tus.ac.jp
Supporting information for this article is given via a link at the end of
the document.
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Table 1. Effect of the structure of the styrene moiety in 2-(phenylethynyl)styrene
derivatives (1) in the ruthenium-catalyzed 1,2-carbon migration/cyclization.[a,b]
Ethoxycarbonyl derivative 1p was converted into the targeted 2p
(43% yield), albeit that the conversion of 1p was low (47%
recovery of 1p). The effect of the substituent at the meta-position
with respect to the alkyne was much more pronounced. Whereas
the reaction of a styrene with a methoxy group 1q gave rise to a
complex mixture, the reactivity of trifluoromethyl derivative 1r was
low, but the corresponding trisubstituted naphthalene 2r was
obtained in 54% yield. We assume that the substituent at the
benzene ring of the a-methylstyryl group should simultaneously
affect the reactivity of the alkynyl and the isopropenyl groups, and
some side reactions should thus occur during the expected 1,2-
[CpRuCl(dppe)] (x mol%)
NaBArF4·2H2O (1.2x mol%)
xylene, 130 °C
1a–d
2
R
R
carbon migration/carbocyclization.
A
similar complicated
substituent effect was observed in our previously reported indole
synthesis.[14]
H
Me
Table 2. Scope of the ruthenium-catalyzed 1,2-carbon migration/cyclization with
respect to 2-alkynyl-a-methylstyrene derivatives (1).[a,b]
2a (74%)
2b (91%)
2c (82%)
(10 mol% Ru, 2 h) (6 mol% Ru, 1 h) (8 mol% Ru, 1 h)
2d (77%)[c]
(20 mol% Ru, 24 h)
R1
R1
[CpRuCl(dppe)] (x mol%)
NaBArF4·2H2O (1.2x mol%)
[a] Reaction conditions: 1 (0.4–0.5 mmol, [1] = 0.1 M in xylene), [CpRuCl(dppe)]
(6–20 mol%), NaBArF4·2H2O (7–24 mol%). [b] Isolated yields are shown in
parentheses. [c] The reaction was conducted at 180 °C in 1,2-dichlorobenzene.
xylene
130 °C, 1 h
R3
R3
R2
R2
1e–m
2
Subsequently, we studied the scope and limitations of this
reaction using various 2-alkynyl-α-methylstyrene derivatives (1)
(Table 2). Initially, we examined the impact of the R1 group. Under
the established reaction conditions described above, the aromatic
enynes 1e and 1f with 4′-methoxy and 4′-methyl groups,
respectively, were smoothly converted into the respective 1,4-
disubstituted naphthalenes 2e (88%) and 2f (90%) as expected.
The molecular structure of 2e was determined by a single-crystal
X-ray diffraction analysis and revealed that a 1,2-aryl migration
furnished the 1,4-disubstituted naphthalene.[17] The reaction of
bromide 1g was sluggish, albeit that the desired 1-aryl-4-
methylnaphthalene 2g was isolated in 77% yield when the
reaction was performed for 24 h using 20 mol% of [CpRuCl(dppe)].
This result shows that the bromo functionality is tolerated under
the applied conditions and that 2g should be readily amenable to
further functionalization of the C(sp2)−Br bond. 1,4-Disubstituted
OMe
Me
Br
O
Me
2e (88%)[c]
(6 mol% Ru)
Me
Me
2f (90%)
2g (77%)
(6 mol% Ru) (20 mol% Ru, 24 h)
F
F
CO2Et
CF3
F
MeO
naphthalenes with an electron-withdrawing group (2h and 2i[17]
)
Me
Me
Me
2h (97%)
(10 mol% Ru)
2i (88%)
(10 mol% Ru, 6 h)
2j (84%)
(10 mol% Ru)
were synthesized in the presence of 10 mol% of [CpRuCl(dppe)].
Similarly, the cycloisomerization of 3′-methoxy derivative 1j
afforded the desired product 2j in 84% yield after 1 h.
OMe
The compatibility of the substrates with an acyl or an alkyl
groups on the alkynyl group was also evaluated.[18] The benzoyl
group was tolerated under the applied reaction conditions, and
naphthyl phenyl ketone 2k was isolated in 66% yield.
O
The reactivity of 1l with a p-methoxyphenylethynyl group was
similar to that of 1a, and 2l was obtained in 77% yield. The
introduction of the methyl group at the β-position of the styrene
had a negative impact, even though the reaction of α,β-dimethyl
derivative (1m) proceeded under harsher conditions to give the
desired 1,2-dimethyl-4-phenylnaphthalene (2m) in 9% yield
together with 1,2-dimethyl-3-phenylnaphthalene (14% yield).
We further explored the effect of the substituents at the para-
and meta-position on the benzene ring relative to the 2-
phenylethynyl group (Table 3). Unexpectedly, the reaction of the
substrates (1n and 1o) with methyl or methoxy groups did not
proceed smoothly, and the corresponding 1,4,7-trisubstituted
naphthalenes (2n and 2o) were isolated in moderate yields.
Me
Me
H
2l (77%)
Me
2m (9%)[d]
2k (66%)
(20 mol% Ru, 24 h)
(20 mol% Ru, 12 h)[e]
(10 mol% Ru, 2 h)
[a] Reaction conditions: 1 (0.4–0.5 mmol, [1] = 0.1 M in xylene), [CpRuCl(dppe)]
(6–20 mol%), NaBArF4·2H2O (7–24 mol%). [b] Isolated yields are shown in
parentheses. [c] The yield was estimated based on the 1H NMR analysis of the
mixture of 2e and the corresponding non-migration product, i.e., 3-(4-
methoxyphenyl)-1-methylnaphthalene (4% yield). [d] The reaction was
conducted at 180 °C in 1,2-dichlorobenzene. [e] The yield was estimated based
on the 1H NMR analysis of the mixture of 2m and the corresponding non-
migration product, i.e., 1,2-dimethyl-3-phenylnaphthalene.
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the corresponding diarylvinylidene intermediate.[17] The isolation
of the vinylidene complex implies that the electrocyclization of 3
proceeds slower than the vinylidene rearrangement of 1 under the
applied reaction conditions. When a solution of vinylidene
complex 3a was stirred at 130 °C for 1 h, 2a was isolated in 51%
yield. The formation of 2a from 3a indicates that the vinylidene
rearrangement of internal alkynes is the crucial step in the present
reaction.
Table 3. Effect of the substituent on the benzene ring of the a-methylstyrene
moiety.[a,b]
[CpRuCl(dppe)] (x mol%)
NaBArF4·2H2O (1.2x mol%)
R
R
xylene
130 °C, 1 h
1n–r Me
2
Me
δC 354.7
BArF
NaBArF4·2H2O
(1.2 equiv)
4
1a
(4.0 equiv)
•
EtO2C
Me
MeO
+
Ru
PPh2
xylene
75 °C, 5 h
Me
2p (43%)
Me
2n (41%)
Me
Cl
2o (47%)
(6 mol% Ru)
Ru
Ph2P
(1.0 equiv)
3a (65%)
(10 mol% Ru, 24 h)
(10 mol% Ru)
PPh2
Ph2P
H
BArF
H
C
4
H
MeO
F3C
C
C
Ru
Me
Me
P
1q (complex mixture)
(6 mol% Ru, 2 h)
2r (54%)
(20 mol% Ru, 24 h)
xylene
130 °C, 1 h
P
2a (51%)
[a] Reaction conditions: 1 (0.4–0.5 mmol, [1] = 0.1 M in xylene), [CpRuCl(dppe)]
(6–20 mol%), NaBArF4·2H2O (7–24 mol%). [b] Isolated yields are shown in
parentheses.
Scheme 3. Synthesis and reaction of vinylidene complex 3a. An ORTEP
drawing for the cationic part of 3a is shown with thermal ellipsoids set to 50%
probability; only selected atoms are labeled, and all hydrogen atoms except
those of the styrene moiety are omitted for clarity.
The electronic effect on the reaction rate was studied by a
competition experiment (Scheme 2). For that purpose, a 1:1
mixture of 1e and 1h was stirred at 130 °C for 1 h in the presence
of [CpRuCl(dppe)] and NaBArF4·2H2O, and the obtained product
mixture was analyzed by 1H NMR spectroscopy. The estimated of
2e to 2h was 1.0:0.35. This result indicates that alkyne 1e with an
electron-donating group is more reactive than 1h under the
applied reaction conditions, which is consistent with the rates of
the formation of the vinylidene complex from 4′-substituted
A catalytic cycle for this reaction is proposed in Scheme 4.
The cationic ruthenium complex [CpRu(dppe)]+ should initially
react with 1 to yield the h2-alkyne complex A. The 1,2-migration
of an aryl group in A would then give rise to the vinylidene
diarylacetylenes on [CpRu(dppe)][BArF ].[12d,f]
4
[CpRuCl(dppe)] + NaBArF
4
R
Ar
R
Ar
R = OMe
1e (1 equiv)
[CpRuCl(dppe)] (10 mol%)
NaBArF4·2H2O (12 mol%)
NaCl
[Ru]+
2e + 2h
+
xylene
130 °C, 1 h
R = CO2Et
1h (1 equiv)
R
2
1
Me
1e / 1h / 2e / 2h = 0 : 0.70 : 1.0 : 0.35
(based on 1H NMR analysis)
[Ru]+
Ar
Ar
[Ru]+
Scheme 2. Competition experiment between 1e and 1h.
B
A
R
R
To gain insight into the underlying mechanistic aspects of the
present reaction, we examined the stoichiometric reaction
between 1a, [CpRuCl(dppe)], and NaBArF4·2H2O at 75 °C in
xylene (Scheme 3). After stirring for 5 h, vinylidene complex 3a
was isolated in 65% yield. In the13C{1H} NMR spectrum, 3a
exhibits a signal at d 347.7, which is characteristic for the α-carbon
atom of a vinylidene ligand. The molecular structure of 3a was
unambiguously determined by a single-crystal X-ray diffraction
analysis that confirmed the anticipated 1,2-aryl migration to form
Electrocyclization
1,2-Aryl migration
Ar
•
[Ru]+
3
R
Scheme 4. Proposed catalytic cycle.
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10.1002/chem.201802413
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[4]
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W.-X. Zhang, J. Pei, Z. Xi, Nat. Commun. 2014, 5, 4508; b) S. Ponra, M.
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complex 3, which could subsequently undergo an
electrocyclization to form B. Finally, a 1,2-hydrogen shift[8a,20]
would afford 1-arylnaphthalenes (2) with concomitant
regeneration of the cationic ruthenium catalyst.
In summary, we have demonstrated a ruthenium-catalyzed
cycloisomerization of 2-alkynylstyrenes, which proceed via a 1,2-
carbon migration. Stoichiometric reactions revealed that the
disubstituted vinylidene complex that is directly derived from the
internal alkyne is the key intermediate in this naphthalene
synthesis. This study shows that the vinylidene rearrangement of
internal alkynes by 1,2-carbon migration can be integrated into
useful catalytic reactions that involve carbon-carbon bond
formation reactions. Further studies directed toward the
development of catalytic reactions that involve 1,2-carbon
migration is ongoing.
[5]
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Acknowledgements
This research was supported in part by MEXT KAKENHI Grant
Numbers JP23105543 and JP25105747, as well as the JGC-S
Scholarship Foundation, and the Promotion Expenses for Mid-
term Research Strategic Plan by Tokyo University of Science. We
would also like to thank Mr. Koichi Takano, Dr. Shintaro Kodama,
and Prof. Youichi Ishii for elemental analysis measurements at
Chuo University (Japan).
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Conflict of interest
a) K. Maeyama, N. Iwasawa, J. Org. Chem. 1999, 64, 1344; b) J. W.
Dankwardt, Tetrahedron Lett. 2001, 42, 5809; c) T. Miura, N. Iwasawa,
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Kiyota, H. Kusama, N. Iwasawa, J. Mol. Catal. A Chem. 2004, 213, 59;
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The authors declare no conflict of interest.
Keywords: alkynes • cycloisomerization • rearrangement •
ruthenium • vinylidene ligands
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10.1002/chem.201802413
Chemistry - A European Journal
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Entry for the Table of Contents
COMMUNICATION
R1
cat. [Ru]+
R1
A ruthenium-catalyzed
cycloisomerization of 2-
Takuma Watanabe, Haruka, Abe,
Yuichiro Mutoh,* Shinichi Saito*
alkynylstyrenes that involves a very
rare 1,2-carbon migration of internal
alkynes and affords various 1,2-di -
and 1,4,7-trisubstituted naphthalenes
is reported. Mechanistic studies
revealed that this reaction proceeds
via a disubstituted vinylidene complex
as the key intermediate by a 1,2-
carbon migration of the 2-
R1 = aryl
acyl
Page No. – Page No.
R2
R2
Ruthenium-Catalyzed
1,2-Carbon Migration
Carbocyclization
Cycloisomerization of 2-
Alkynylstyrenes via 1,2-Carbon
Migration that Leads to Substituted
Naphthalenes
R1
•
[Ru]+
alkynylstyrenes.
R2
[X-ray]
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