Skeletal reorganization of enynes catalyzed by a Ru(2)Ru(3) mixed-valence complex under an atmosphere of O2 or CO

Full text of DOI:10.1246/cl.130785

doi:10.1246/cl.130785
Published on the web September 21, 2013
1565
Skeletal Reorganization of Enynes Catalyzed by a Ru(II)-Ru(III) Mixed-valence
Complex under an Atmosphere of O2 or CO
³
Takahiro Nakae, Tomohiko Yasunaga, Motonobu Kamiya, Yoshiya Fukumoto, and Naoto Chatani*
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871
(Received August 23, 2013; CL-130785; E-mail: chatani@chem.eng.osaka-u.ac.jp)
A mixed-valence Ru(II)-Ru(III) complex was used to
catalyze the skeletal reorganization of 1,6-enynes, leading to
+
the production of 1-vinylcyclopentene derivatives. The reaction
type I
type II
proceeded optimally in an atmosphere of O2 or CO. The
presence of an alkyl group at the internal olefinic carbon in the
starting enyne resulted in an increase in product yield compared
to that bearing no substituent on the olefin moiety.
Scheme 1. Skeletal reorganization of enynes.
+
CF3
Me
O
O
O
O
The catalytic cycloisomerization of enynes has recently
attracted considerable attention because of the great diversity of
products that can be produced from them.1 In 1988, Trost
reported the Pd(II)-catalyzed skeletal reorganization of enynes,
in which two isomers of 1-vinylcyclopentenes, type I and II
were formed.2 While type I is produced via the cleavage of a
C-C double bond, type II involves a double cleavage of a C-C
double bond and a triple bond (Scheme 1).
F3C
O
Me
O
Rh
O
Ru
O
O
Rh
O
O
O
Ru
O
O
Me
CF3
O
O
Me
[Ru2(OAc)4]+
CF3
Rh2(O2CCF3)4
Scheme 2. [Rh2(OCOCF3)4] vs. [Ru2(OAc)4]X.
In 1994, we reported that [RuCl2(CO)3]2 also shows a high
catalytic activity in the skeletal reorganization of enynes which
led to the selective formation of type I products.3 In this reaction,
a CO atmosphere was crucial for the reaction to proceed. In fact,
[RuCl2(p-cymene)]2 and RuCl3¢xH2O exhibited high catalytic
activities only when the reaction was run under an atmosphere
of CO. However, no catalytic activity was observed when the
reaction was run under a N2 or Ar atmosphere. In sharp contrast,
[RuCl2(CO)3]2 showed catalytic activity even under N2. At this
stage, we speculated that the role of CO is to increase the
electrophilicity of the catalyst by coordination to a ruthenium
center, which would facilitate an interaction between the alkyne
and the catalyst because of its π-acidity.4 Since then, a wide
variety of electrophilic transition-metal complexes has been
found to catalyze the cycloisomerization of enynes.1,5 Some of
these complexes exhibit a characteristic substrate specificity. In
subsequent studies, we found that [Rh2(O2CCF3)4] also showed
a high catalytic activity for the skeletal reorganization of enynes
leading to the selective formation of type II products.5f,5g
[Rh2(O2CCF3)4] was more reactive than [Rh2(OAc)4], which
again suggests that the electrophilicity of the catalyst is
important for the reaction to proceed efficiently. The structure
of [Rh2(O2CCF3)4] features a pair of rhodium atoms, each with
an octahedral molecular geometry, defined by four acetate
oxygen atoms (Scheme 2).
R2
[Ru2(O2CPh)4(THF)2]BF4
R1
R1
1,2-dichloroethane, 60 °C
R2
Scheme 3. Skeletal reorganization of enynes.
for the skeletal reorganization of 1,6-enynes when the reaction is
run under an atmosphere of O2 or CO (Scheme 3).
The treatment of enyne 1 (0.2 mmol) with a catalytic amount
of [Ru2(OAc)4(THF)2]BF4 (0.01 mmol) in toluene (1 mL) at
80 °C under N2 for 20 h gave the expected 1-vinylcyclopentene 2
in 56% yield. Similar to the Ru(II)-catalyzed skeletal reorgan-
ization of enynes, the yield was dramatically improved to 95%
yield when the reaction was carried out under an atmosphere of
CO, even for shorter reaction time (12 h). The yield was also
improved to 65% when the reaction was run under O2 for 20 h.
Kobayashi and co-workers observed that O2 has a pronounced
effect in [Mo2(OAc)4]-catalyzed Mukaiyama aldol reactions.11
They proposed the intermediacy of an electrophilic catalytic
species which is generated by the oxidation of [Mo2(OAc)4]
under the reaction conditions. Other mixed-valence ruthenium
complexes were examined under an atmosphere of O2. The use
of [Ru2(O2CPh)4(THF)2]BF4 gave 2 in 77% yield. However,
a neutral complex, [Ru2(O2CPh)4]Cl was devoid of catalytic
activity. The use of a Ru(III)-Ru(III)-Ru(III) complex, such as
[Ru3(OAc)6(¯3-O)]OAc resulted in no yield of 2 at all. Next,
various solvents were screened under O2 in the presence of
[Ru2(O2CPh)4(THF)2]BF4. Almost no reaction occurred when
cyclohexane, 1,4-dioxane, ethanol, and acetonitrile were used as
the solvents. However, the use of 1,2-dichloroethane improved
the yield of 2 (85% at 60 °C for 15 h under O2) (Scheme 4). The
We were interested in the catalytic activity of [Ru2(OAc)4]X
complex because it is structurally similar to [Rh2(O2CCF3)4]
(Scheme 2). [Ru2(OAc)4]X was expected to have a unique
catalytic activity because it is a Ru(II)-Ru(III) mixed-valence
complex.6 However, it has rarely been used in organic synthesis.
The [Ru2(OAc)4]Cl complex was used only for hydrogenation,7
the carbonylation of amines,8 and the oxidation of alcohols9
and amines.10 We now report that [Ru2(O2CPh)4(THF)2]BF4, a
Ru(II)-Ru(III) mixed-valence complex, can be used as a catalyst
Chem. Lett. 2013, 42, 1565-1567
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1566
[Ru2(O2CPh)4(THF)2]BF4
[Ru2(O2CPh)4(THF)2]BF4
E
E
E
E
E
no reaction
Me
1,2-dichloroethane
60 °C, under CO
E = COOEt
1,2-dichloroethane
60 °C
E = COOEt
E
Me
Ph
Ph
2
15
15
1
O2, 15 h
CO, 10 h
85%
95%
Ph
[RuCl2(CO)3]2
E
E
E
E
toluene
80 °C, 1 h, under CO
Scheme 4. [Ru2(O2CPh)4(THF)2]BF4-catalyzed skeletal reor-
ganization of 1.
16
97%
Table 1. Skeletal reorganization of 1,6-enynesa
Scheme 5. [RuCl2(CO)3]2 vs. [Ru2(O2CPh)4(THF)2]BF4.
Enyne
Conditions
Product
Yield/%
[Ru2(O2CPh)4(THF)2]BF4
TsN
Me
1,2-dichloroethane
60 °C
O2, 45 hb
CO, 24 hb
E
E
27
29
E
E
17
TsN
TsN
4
3
Me
Me
18
19
O2, 15 h
CO, 60 h
10%
11%
55%
56%
E
E
O2, 15 h
CO, 9 h
E
E
90
98
Et
Et
6
Scheme 6. [Ru2(O2CPh)4(THF)2]BF4-catalyzed skeletal reor-
ganization of 17.
5
The results obtained for some enynes in the presence of
[Ru2(O2CPh)4(THF)2]BF4 as the catalyst are summarized in
Table 1. A simple enyne 3 gave the 1-vinylcyclopentene 4 only
in low yields irrespective of the atmosphere used. However, the
presence of an alkyl group at the internal olefinic carbon, as in 5
and 7, resulted in a dramatic increase in product yields.12 In the
case of enynes containing a trisubstituted olefin, as in 11 and 13,
the reaction proceeded and high yields of type I products were
formed exclusively.
We have already reported on the [RuCl2(CO)3]2-catalyzed
skeletal reorganization of enynes.4 However, the catalytic
activity of [RuCl2(CO)3]2 and [Ru2(O2CPh)4(THF)2]BF4 appear
to be different. For example, 15 is one of the most reactive
substrates in [RuCl2(CO)3]2-catalyzed reactions, but no reaction
occurred when [Ru2(O2CPh)4(THF)2]BF4 was used as the
catalyst (Scheme 5).
An enyne having a nitrogen atom in the tether 17 gave
a mixture of the 1-vinylcyclopentene derivative 18 and the
bicyclic[4.1.0]heptene derivative 19, the latter being the major
isomer (Scheme 6).
In summary, we report on the skeletal reorganization of 1,6-
enynes leading to 1-vinylcyclopentene derivatives catalyzed by
a mixed-valence Ru(II)-Ru(III) complex. The reaction gives
optimal yields when run under an atmosphere of O2 or CO.13
E
E
O2, 15 h
CO, 7 h
E
E
89
94
But
But
8
7
E
E
O2, 15 h
CO, 9 h
E
E
38
85
Ph
Ph
10
9
Me
Ph
E
E
O2, 12 h
CO, 5 h
E
E
99
99
Me
Me
Me
Ph
12
11
E
E
O2, 30 h
CO, 24 h
E
E
99
97
Me
Me
14
13
aReaction conditions: enyne (0.2 mmol), [Ru2(O2CPh)4-
(THF)2]BF4 (0.01 mmol) at 60 °C in 1,2-dichloroethane
(1 mL). The reaction was run at 80 °C. E = COOEt.
b
This work was supported, in part, by a Grant-in-Aid for
Scientific Research on Innovative Areas “Molecular Activation
Directed toward Straightforward Synthesis” from Monbusho
(The Ministry of Education, Culture, Sports, Science and
Technology).
use of 1,2-dichloroethane also improved the yield of 2 when the
reaction was run under CO.
To obtain additional information regarding the active
catalytic species, NMR and ESI-MS studies were performed.
No change was observed when [Ru2(O2CPh)4(THF)2]BF4 was
exposed to a N2 atmosphere at 60 °C. In sharp contrast, ESI-MS
showed a significant change under CO and O2. However, the
structure of the active species could not be confirmed because of
production of complex mixtures.
References and Notes
³
Present address: Department of Chemistry and Biology,
Graduate School of Science and Engineering, Ehime Uni-
versity, Matsuyama, Ehime 790-8577
Chem. Lett. 2013, 42, 1565-1567
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1567
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5
Chem. Lett. 2013, 42, 1565-1567
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/