Cycloisomerization promoted by the combination of a ruthenium-carbene catalyst and trimethylsilyl vinyl ether, and its application in the synthesis of heterocyclic compounds: 3-Methylene-2,3-dihydroindoles and 3-methylene-2,3- dihydrobenzofurans

Article abstract of DOI:10.1002/anie.200454157

Substituted N and O heterocycles have been synthesized by the cycloisomerization of dienes using a ruthenium-carbene catalyst. The products obtained with and without trimethylsilyl vinyl ether differ (see scheme, Cy = cyclohexyl, Mes = 2,4,6-trimethylphenyl, Ts = p-toluenesulfonyl).

Full text of DOI:10.1002/anie.200454157

Angewandte
Chemie
Heterocycle Synthesis
Cycloisomerization Promoted by the
Combination of a Ruthenium–Carbene Catalyst
and Trimethylsilyl Vinyl Ether, and its Application
in The Synthesis of Heterocyclic Compounds: 3-
Methylene-2,3-dihydroindoles and 3-Methylene-
2,3-dihydrobenzofurans**
Yukiyoshi Terada, Mitsuhiro Arisawa,* and
Atsushi Nishida*
Transition-metal-catalyzed cyclization of a,w-dienes is an
efficient method for the construction of carbo- and hetero-
cycles. One particularly interesting cyclization is olefin meta-
thesis,[1] in which significant progress was made after the
development of well-defined ruthenium–carbene catalysts,
such as A and B (Scheme 1). In particular, the commercially
available ruthenium–carbene catalyst B has shown enhanced
activity in ring-closing metathesis (RCM), ring-opening meta-
thesis polymerization (ROMP), and cross metathesis
(CM).[1c,2] On the other hand, the nonmetathetic behaviors
of ruthenium–carbene catalysts have been demonstrated in
recent studies of olefin metatheses.[3] When a substrate cannot
give a product by metathesis, nonmetathetic reactions, such as
the Kharasch addition,[4] olefin isomerization,[5] and hydro-
[*] Y. Terada, Dr. M. Arisawa, Prof. Dr. A. Nishida
Graduate School of Pharmaceutical Sciences
Chiba University, 1-33 Yayoi-cho, Inage-ku
Chiba 263-8522 (Japan)
Fax: (+81)43-290-2909
E-mail: arisawa@p.chiba-u.ac.jp
nishida@p.chiba-u.ac.jp
[**] This research was supported by a Grant-in-Aid for Scientific
Research on Priority Areas (A) “Exploitation of Multi-Element Cyclic
Molecules” and a Grant-in-Aid for Exploratory Research from the
Ministry of Education, Culture, Sports, Science, and Technology,
Japan.
Supporting Information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200454157
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have recently demonstrated the facile and selective
isomerization of the terminal olefin by a combina-
tion of B and trimethylsilyl vinyl ether (2a). The
aromatic enamides 3 that were obtained were
subjected to standard RCM to give the correspond-
ing indole 4 in excellent yield. The combination of
these transformations established a novel synthesis
for substituted indoles (Scheme 2).[11] Although the
active ruthenium species for our isomerization was
not characterized, we postulated that a low-valent ruthenium
species, such as a ruthenium hydride, which might be
generated by the reaction of B and 2a, catalyzed the
isomerization of an olefin via intermediate F. Based on our
hypothesis, it is possible that F, which has another olefin at an
appropriate position in the substrate, may undergo intra-
Scheme 1. Ruthenium catalysts. Cy=cyclohexyl, Mes=2,4,6-trimethylphenyl.
genation, either proceed efficiently or complete recovery of
the starting materials is observed. These discoveries have
expanded the synthetic utility of these catalysts beyond olefin
metathesis. For example, olefin isomerization can be used in
the deprotection of allyl ethers and amines,[5a–e] the synthesis
of cyclic enol ethers,[5f,g] and the formation of dienamide from
allenamide.[5h]
Another method that can be used to prepare cyclic
compounds from a,w-dienes is cycloisomerization.[6] In com-
parison to RCM, in which the carbon–carbon double bond is
formed with the release of a stoichiometric amount of ethene,
cycloisomerization proceeds without the loss of carbon units.
Therefore, selective and catalytic cycloisomerization can be
considered a highly atom-economic reaction. Recently, it has
been shown that these reactions can be catalyzed by various
transition metals, such as palladium,[7a–d,f] nickel,[7a] tita-
nium,[7e] rhodium,[7f] and ruthenium.[8,9] Although ruthenium
species that induce cycloisomerization, such as E[8a] and
[{RuCl2(p-cymene)}2], (p-cymene = p-isopropyltoluene)[8b,c]
have been identified, there are few successful reports. A
highly selective cycloisomerization using a ruthenium catalyst
was established by Itoh and co-workers, who used
[{Ru(cod)Cl2}n] (cod = 1,5-cyclooctadiene).[9]
À
molecular olefin insertion into the Ru C bond, followed by a
syn b-H elimination to produce the cycloisomerized product.
Herein, the cycloisomerization of a,w-dienes using ruthe-
nium–carbene catalyst B and 2a is reported together with its
application to the synthesis of heterocycles.
First, we examined the reaction of N,N-diallyl-p-toluene-
sulfonamide (5), which is widely used for RCM and cyclo-
isomerization. When diene 5 was stirred with catalyst B
(5 mol%) in dichloromethane at room temperature for two
hours, the RCM product 8 was obtained quantitatively. In
marked contrast, when 5 was stirred with B (5 mol%) and one
equivalent of 2a under the same conditions, the expected
cycloisomerized product 6 was isolated in 65% yield, along
with 21% of 8 (Table 1, entry 1). The same reaction at an
elevated temperature (408C) led to higher yields of 6 and 7
(86 and 14%, respectively; entry 2). The amount of 7 formed
increased at a higher temperature in toluene (entry 3). In fact,
the isomerization of 6 to 7 proceeded under the same
conditions. The use of A or C (entries 4 and 5) or ethyl
We have been studying the syntheses of nitrogen-contain-
ing heterocycles using ruthenium–carbene catalysts,[10] and
Scheme 2. Selective isomerization of a terminal olefin: the synthesis of an indole through RCM. Ts=p-toluenesulfonyl, TMS=trimethylsilyl.
[a]
Table 1: Reaction of N,N-diallyl-p-toluenesulfonamide (5).
Entry
Ru
R
Solvent
T [8C]
Yield [%][b]
5
6
7
8
1
2
3
4
5
6
7
B
B
B
A
C
D
B
2a
2a
2a
2a
2a
2a
2b
TMS
TMS
TMS
TMS
TMS
TMS
Et
CH2Cl2
CH2Cl2
toluene
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
RT
40
110
40
40
40
40
0
0
0
28
0
65
86
(22)
10
29
0
21
0
0
59
64
0
(14)
78
0
0
24
0
0
58
71
37
5
[a] RT=Room temperature, TMS=trimethylsilyl. [b] Yields in parenthesis were estimated by 1H NMR spectroscopy.
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vinyl ether (2b) (entry 7) was less effective for the reaction,
while D showed a high activity that was comparable to that of
B (entry 6). Thus, the N-heterocyclic carbene (NHC) ligand
of a ruthenium catalyst has been shown from these results to
be important for highly efficient cycloisomerizations, in terms
of regio- and chemoselectivity. The ruthenium complex
[RuCl(H)(CO)(PPh3)3],[12] which is known to catalyze cycli-
zations of enynes[12a] and isomerization of terminal ole-
fins,[12b,c] did not promote the cycloisomerization of 5, and
isomerized compounds 5’ were obtained quantitatively
instead of 6 or 7. To the best of our knowledge, this is the
first example of cycloisomerization with a a,w-diene using a
ruthenium–carbene catalyst as a precursor to the active
species. By employing our conditions, a cycloisomerized
compound can be selectively prepared even when the
substrate can readily yield a RCM product.
Next, we investigated the application of this cycloisome-
rization to the synthesis of N- and O-benzo-fused hetero-
cycles, which are important structures in many pharmacolog-
ically active compounds. Although the reaction of 1 with 2a
and B in dichloromethane heated to reflux gave enamides 3 in
quantitative yields (Table 3, entry 1), the reaction of 1 to
Table 3: Isomerization and cycloisomerization of a,w-diene 1.
Entry
B [mol%]
Conditions
Yield [%][a]
3
22
1
2
3
4
5
5
5
5
5
CH2Cl2, reflux, 1.5 h
toluene, 408C, 2 h
toluene, reflux, 2 h
xylene, reflux, 2 h
xylene, reflux, 2 h
quant.
97
65
30
12
0
3
35
68
81
The scope and limitations of this reaction are further
shown in Table 2. Reactions of 9 and 10 gave the correspond-
Table 2: Cycloisomerization of dienes.[a]
10
1
Entry
Substrate
Product
Yield [%][b]
83
[a] Yields were estimated by H NMR spectroscopy.
1
prepare the corresponding cycloisomerized product was
investigated further. As a result, when the reaction of 1 was
performed in toluene at 408C, the cycloisomerized indolidene
22 was obtained, albeit in low yield (entry 2). When the same
reaction was carried out in either toluene or xylene heated to
reflux, 22 was obtained in respective yields of 35 and 68%
(entries 3 and 4). Finally, it was found that a higher temper-
ature (in xylene at reflux) and greater amount of catalyst
(10 mol%) were necessary to obtain 22 in excellent yield
(entry 5). Initial isomerization of 1 converted it into 3, which
after cyclization led to the formation of 22. This reaction
pathway is indicated as the reaction of 3 under the optimized
conditions (in xylene at reflux) yielded 22 in moderate yield
with recovered 3 (62 and 38%, respectively). It is noteworthy
that the reaction of 3 with [{Ru(cod)Cl2}n] in isopropanol for
24 hours, which is the typical procedure reported by Itoh and
co-workers,[9] did not produce 22 at all.
2
3
87
75
4
5
6
47
18
13
[a] Conditions: B (5 mol%), 2a (1 equiv), CH2Cl2 (12.5 mm), heated to
reflux, 2 h. [b] Yield of isolated product.Bn=benzyl.
Several substituted N- and O-benzo-fused heterocycles
were synthesized under our cycloisomerization conditions
(Table 4). Compounds 23–25, which have a chloro-substituted
benzene ring, gave the corresponding indolidenes 31–33 in
respective yields of 84, 78, and 78%. On the other hand, the
reaction of 26, which has a methoxy-substituted benzene ring,
gave 34 in only 24% yield. The reaction of 27, which has a
butenyl group at the nitrogen center, also gave the cyclized
product 35 in good yield. In contrast to the reactions of the
nitrogen-containing substrates (1, 23–27), the oxygen-con-
taining substrates 28–30 readily cyclized to give the corre-
sponding heterocycles 36–38 in good yields. The 3-methylene-
2,3-dihydrobenzofurans 36–38 readily isomerized to the
corresponding 3-methylbenzofurans 39–41 under acidic con-
ditions, such as in HCl (1m) and trifluoroacetic acid.
ing products 17 and 18 in respective yields of 83 and 87%. N-
Benzoyldiallylamine (11) also gave 19 in good yield, although
N-tert-butoxycarbonyldiallylamine (12) gave 20 in only 47%
yield. The reaction of 12 heated in toluene to reflux did not
increase the yield of 20. In the case of 13, neither cyclo-
isomerization nor olefin migration occurred, perhaps because
of an electron-donating effect. Both 14 and 15 gave 21 in low
yields, along with 14’ (59 and 81%, respectively), while the
reaction of 16, which has a methallyl group, only gave
isomerized products 16’. Although the substituents at the
nitrogen center and the terminal olefin functionality are very
important, we thought that this reaction could have potential
for the construction of nitrogen-containing heterocycles as
well as carbocycles.
To obtain more detailed information on the cycloisome-
rization the reaction of 5 with B and 2 in [D8]toluene was
monitored by spectroscopic analysis. The relative amounts of
5 and the cyclized products 6 and 7 were analyzed by 1H NMR
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Table 4: Synthesis of 3-methylene-2,3-dihydroindoles and 3-methylene-2,3-dihydrobenzofurans.[a]
(0.17m) at 508C. The reaction was complete
within 10 minutes and both 5 and B were
consumed. Since the reaction rate under the
above conditions was too fast to monitor, 2b
was used instead of 2a to decrease the rate
of the reaction, and the results are summar-
ized in Figure 1. After 15 minutes of heat-
ing, 31P NMR spectroscopy showed the dis-
appearance of a signal at d = 29.8 ppm in
the spectrum of B, accompanied by the
emergence of a new signal at d = 30.6 ppm.
Similarly, 1H NMR spectroscopic analysis
showed the disappearance of a signal at d =
19.7 ppm from the spectrum of B, while a
new signal appeared at d = 14.2 ppm. These
observations show the rapid formation of
Fischer-type carbene G from the reaction
Entry
1
Substrate
Solvent
Product
Yield [%][b]
xylene
84
2
3
xylene
xylene
78
78
4
5
xylene
xylene
24[c]
58
between
B and 2b. After 29 minutes,
31P NMR spectroscopic analysis showed
that G had partially decomposed to other
unidentified ruthenium compounds (d =
46.7, 32.5 ppm). In contrast to the reaction
of B, the conversion of 5 to 6 proceeded
gradually under these conditions and was
complete after 45 minutes. The formation of
7 was not observed.
6
7
8
toluene
toluene
toluene
78
76
73
As demonstrated in the 1H and
31P NMR spectroscopic analyses, catalyst
G appears to play an important role in the
cycloisomerization process. According to
Louie and Grubbs, G shows high reactivity
for metathesis processes, such as ROMP
and RCM.[13] Therefore, we synthesized G
and confirmed that it efficiently catalyzes
the RCM reaction of 5 in toluene at 508C to
give 8 in 79% yield along with recovered 5
[a] Conditions: B (10 mol% for entries 1–5; 5 mol% for entries 6–8), 2a (1 equiv), solvent (12.5 mm),
1
heated to reflux, 2 h. [b] Yield of isolated product unless otherwise noted. [c] Determined by H NMR
spectroscopy. Enamides 26’ were also obtained in 76% yield.
spectroscopy, and the relative amounts of ruthenium com-
plexes, including B and G,[13] were analyzed by 1H and
31P NMR spectroscopy. First, we periodically monitored the
reaction of 5 with B in the presence of 2a in [D8]toluene
(21%). However, the reaction of 5 in the presence of both G
(5 mol%) and one equivalent of 2b gave 6 in 98% yield.
Although it is unclear which ruthenium species participate
in this cycloisomerization, the ruthenium hydride species is a
Figure 1. Cycloisomerization of N,N-diallyl-p-toluenesulfonamide (5) in [D8]toluene (0.17m) at 508C in the presence of B (5 mol%) and 2b
(1 equiv). The relative amounts of B, G, and decomposition products were estimated by 31P NMR spectroscopic analysis using H3PO4 (85%) as
the external standard and 1H NMR spectroscopic analysis using 1,2;5,6-dibenzanthracene (5 mol%) as the internal standard. The relative amounts
of 5 and 6 were estimated by 1H NMR spectroscopy.
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experimental
results
show
that
[RuCl(H)(CO)(PPh3)3][12] or reaction conditions using ruth-
enium–carbene catalysts without NHC ligands A and C were
less effective for cycloisomerization. Also ruthenium catalysts
B and D gave cycloisomerized products in good to excellent
yields, which indicate that the presence of NHC ligands in the
ruthenium–carbene catalyst is a key component for cyclo-
isomerization. 2) Our cycloisomerization conditions can be
used to prepare indolidenes, N-benzo-fused heterocycles. The
preparation of which have not been reported using the most
successful ruthenium catalyst [{Ru(cod)Cl2}n]. Further studies
on the ruthenium species involved in the cycloisomerization
process and an investigation of the reaction mechanism are
currently in progress.
In conclusion, a new methodology for the cycloisomeriza-
tion of dienes using a Grubbs carbene complex and trime-
thylsilyl vinyl ether has been established. The utility of this
reaction was demonstrated in the synthesis of exo-methylene
heterocyclic compounds, which could act as key intermediates
for pharmacologically active compounds. We believe that
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Keywords: cyclization · heterocycles · isomerization ·
metathesis · ruthenium
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