Ring expansion reactions of ethyl cyclopropylideneacetate and benzosilacyclobutenes: Formal σ bond cross metathesis

Article abstract of DOI:10.1016/j.tetlet.2010.09.031

The ring expansion reaction of ethyl cyclopropylideneacetate (1) with benzosilacyclobutenes proceeded smoothly in the presence of Ni(cod) ₂-TOPP (tribiphenyl-2-yl phosphite) to give benzosilacycloheptenes in good yields. This formal σ bond metathesis reaction proceeded in a selective manner. The Ni-catalyzed reaction of 1 with biphenylenes was also examined.

Full text of DOI:10.1016/j.tetlet.2010.09.031

Tetrahedron Letters 51 (2010) 6028–6030
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Tetrahedron Letters
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Ring expansion reactions of ethyl cyclopropylideneacetate
and benzosilacyclobutenes: formal
r bond cross metathesis
⇑
Shinichi Saito , Takahiro Yoshizawa, Shinya Ishigami, Ryu Yamasaki
Department of Chemistry, Faculty of Science, Tokyo University of Science, Kagurazaka, Shinjuku, Tokyo 162-8601, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The ring expansion reaction of ethyl cyclopropylideneacetate (1) with benzosilacyclobutenes proceeded
Received 25 August 2010
Revised 9 September 2010
Accepted 10 September 2010
Available online 16 September 2010
smoothly in the presence of Ni(cod)2-TOPP (tribiphenyl-2-yl phosphite) to give benzosilacycloheptenes
in good yields. This formal
r bond metathesis reaction proceeded in a selective manner. The Ni-catalyzed
reaction of 1 with biphenylenes was also examined.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Ring expansion
Benzosilacyclobutene
Biphenylene
Methylenecyclopropane
Ni catalyst
The strained C–Si and C–C bonds of four-membered cyclic
compounds are reactive bonds and many attractive reactions
Since very similar metalacycles have been postulated as the
intermediates in the transition metal-catalyzed reactions of ben-
zosilacyclobutenes as well as the nickel-catalyzed reactions of 1
and alkynes, we expected that benzosilacyclobutenes would be
suitable substrates for the Ni-catalyzed reaction of 1. In this Letter,
we report the nickel-catalyzed ring expansion reaction of 1 and
benzosilacyclobutenes. The reaction of 1 with biphenylenes, which
are structurally similar compounds, was also examined.
r
have been reported. The transition metal-catalyzed reactions of
these compounds generally involve the cleavage of the four-
membered ring, and five-membered metalacycles are considered
as the key intermediates. For example, the ring expansion reac-
tions of benzosilacyclobutenes proceeded in the presence of Pd,1
Ni,2 or Co3 catalysts, and the intermediacy of the five-membered
metalacyclic compounds has been established in some reac-
tions.3,4 The ring expansion reactions of biphenylene were also
catalyzed and/or mediated by the Ni,5 Rh,6 or other metal com-
plexes,7 and these reactions would also proceed via similar
metalacyclic intermediates.
We have been showing interest in the reactivity of ethyl cyclo-
propylideneacetate (1) in the presence of Ni(0) catalyst and have
developed new methods for the synthesis of medium-sized carbo-
cyclic compounds. The [3+2+2] cycloaddition of 1 with alkynes
gave substituted cycloheptadienes8 and the [4+3] cycloaddition
of 1 with 1,3-dienes gave cycloheptenes.9 Dienynes also turned
out to be good substrates, and the [4+3+2] cycloaddition reaction
of 1 with dienynes proceeded efficiently to yield the bicyclic
cyclononadiene derivatives.10 We assumed that nickelacycles such
as nickelacyclopentadiene were initially formed by the reaction of
the Ni(0) species with alkynes, which further reacted with 1 to give
the product.
The Ni-catalyzed ring expansion reaction of 1 with 1,1-di-
methyl-2,3-benzo-1-sila-2-cyclobutene (2a) was examined under
various reaction conditions and the results are summarized in
Table 1. Compound 1 reacted with 2a in the presence of Ni(cod)2
(10 mol %) and PPh3 (40 mol %). When a solution of 1 (1 equiv)
and 2a (1.2 equiv) in toluene was slowly added to a solution of
the nickel catalyst at 100 °C over 5 h, compounds 3a and 4a were
isolated in 44% combined yield (entry 1).11 The ratio of 3a to 4a
was close to unity. The yields of the product decreased when
PBu3 (L2) was used as the ligand for this reaction (entry 2). The
reaction did not proceed efficiently when the reaction was carried
out in the presence of tris(2-biphenyl)phosphine (L3, entry 3) or
triphenylphosphite (L4, entry 4). On the other hand, bulky phos-
phite ligands turned out to be suitable for this reaction. For
example, the combined yield of 3a and 4a was 39% when the
reaction was carried out in the presence of tris[2,4-di(tert-
butyl)phenyl]phosphite (L5, entry 5). The combined yield
increased to 61% when the reaction was catalyzed by Ni(cod)2-
TOPP12 (tribiphenyl-2-yl phosphite, L6, entry 6). The highest yield
of the product was achieved when a solution of the substrates
was slowly added to a mixture of the catalyst in toluene over
⇑
Corresponding author. Tel.: +81 3 5228 8715; fax: +81 3 000 0000.
E-mail address: ssaito@rs.kagu.tus.ac.jp (S. Saito).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.09.031

S. Saito et al. / Tetrahedron Letters 51 (2010) 6028–6030
6029
Table 1
Table 2
Nickel-catalyzed ring expansion reaction of 1 with 2a
Nickel-catalyzed ring expansion reaction of 1 with 2a–e
Ni(cod)2 (10 mol%)
Ni(cod)2 (10 mol%)
Me
Me Si
R
CO2Et
1
CO2Et
ligand (40 mol%)
TOPP (L6) (40 mol%)
R Si
+
+
toluene, 100 ºC
dropwise addition
of 1 and 2a
toluene, 100 ºC
dropwise addition
of 1 and 2 for 2 h
1
2a-e
(1.2 equiv)
2a
(1 equiv)
CO2Et
Si
EtO2C
CO2Et
Me
Me
+
Si
EtO2C
Me
R
R
Me
+
Si
Si
R
R
3a
4a
3a-e
4a-e
Entry
Ligand
Addition time (h)
Yield (%) (3a + 4a)
Ratioa (3a:4a)
1
2
3
4
5
6
7
8
L1
L2
L3
L4
L5
L6
L6
L6
5
5
5
5
5
5
2
1
44
14
0
Trace
39
61
63
53
47:53
50:50
—
Entry
Compd
Yield (%) (3 + 4)
Ratioa (3:4)
1
2
3
4
5
2a (R = Me)
2b (R = Et)
2c (R = n-Bu)
2d (R = Ph)
2e (R = i-Pr)
63
69
58
47
8
52:48
55:45
50:50
—
56:44
50:50
52:48
49:51
83:17b
>95:<5c
a
The ratio was determined by the isolated yields of 3 and 4, unless otherwise
noted.
a
The ratio was determined by the isolated yields of 3a and 4a.
b
The ratio was determined by the NMR spectrum of the mixture of 3d and 4d.
The minor isomer was not detected by NMR analysis.
c
2 h (entry 7). The yields of 3a and 4a decreased when the addition
time was 1 h (entry 8).
Table 3
Nickel-catalyzed ring expansion reaction of 1 with biphenylenes
R1
R2
R1
R2
Ni(cod)2 (10 mol%)
CO2Et
P
3
PPh3 (L1) (40 mol%)
P
P
+
3
O
L1
toluene, 100 ºC
dropwise addition
of 1 and 5 for 5 h
R3
R3
3
PBu3
L4
1
5a-c
(1.2 equiv)
(1 equiv)
L3
L2
CO2Et
t-Bu
P
EtO2C
R1
O
P
O
R1
R1
R1
t-Bu
3
3
R3
R3
R3
R3
R2
R2
R2
R2
L5
L6
6a-c
not isolated
The scope of this reaction was studied under the optimized
reaction conditions and the results are summarized in Table 2.
The ring expansion reaction proceeded smoothly when other alkyl
groups such as ethyl group (entry 2) or n-butyl group (entry 3)
were introduced to the silicon atom of the benzosilacyclobutene.
The yield of the products decreased when the phenyl group was
introduced (entry 4). In this reaction, however, the E-isomer (3d)
was isolated as the major isomer. The introduction of a more bulky
isopropyl group resulted in the significant decrease of the yield and
the predominant formation of the E-isomer (entry 5).
Entry
Compd
R1
R2
R3
Yield (%)
1
5a
5a
5b
5c
H
H
Me
H
H
H
Me
H
H
H
H
Me
70
2a
3
(16)b
74
4
56
a
TOPP (L6) was used as the ligand.
The product was not isolated in pure form.
b
Encouraged by the successful conversion of benzosilacyclobut-
enes, we examined the Ni-catalyzed reaction of 1 with biphenyl-
enes (5a–c). The results are summarized in Table 3. To a mixture
of Ni(cod)2 (10 mol %) and PPh3 (40 mol %) in toluene was slowly
added a solution of 1 and biphenylene (5a) in toluene at 100 °C,
and the ring expansion product (6a) was isolated in 70% yield
(entry 1). In contrast to the reaction of 1 with 2a, no isomeric com-
pound was isolated. The proper selection of the ligand was critical
for the progress of this reaction, and PPh3 (L1) turned out to be the
best ligand: the yield of 6a decreased when TOPP (L6) was used
(entry 2).13 This reaction proceeded smoothly with other biphenyl-
ene derivatives such as 5b and 5c (entries 3 and 4).14 In the reac-
tion of 5c, the selective cleavage of the less sterically hindered
C–C bond proceeded to give the ring expansion product 6c
exclusively.
A possible mechanism of the ring expansion reaction of 2 is pre-
sented in Scheme 1. Since the insertion of Pd and Pt complex to the
silacyclobutane ring has been reported in the literature,3,4 we
assume that the insertion of the Ni(0) species to the silacyclobu-
tane ring of 2 would proceed, leading to the formation of 7. The
more reactive Si–Ni bond would react with 1, and a seven-
membered nickelacycle would be formed. The cyclopropylmeth-
yl-butenyl rearrangement15 and reductive elimination would
proceed to give 3 and 4 as the final products. We assume that
the regiochemistry of the product is determined when the cyclo-
propylmethyl-butenyl rearrangement (i.e., the formation of 9 from

6030
S. Saito et al. / Tetrahedron Letters 51 (2010) 6028–6030
2. (a) Hirano, K.; Yorimitsu, H.; Oshima, K. J. Am. Chem. Soc. 2007, 129, 6094–6095;
R
CO2Et
(b) Hirano, K.; Yorimitsu, H.; Oshima, K. Org. Lett. 2006, 8, 483–485.
3. Agenet, N.; Mirebeau, J. H.; Petit, M.; Thouvenot, R.; Gandon, V.; Malacria, M.;
Aubert, C. Organometallics 2007, 26, 819–830.
4. For the isolation of the structurally related 1-metala-2-silacyclopentanes by
the reaction of silacyclobutanes with transition metals, see: (a) Yamashita, H.;
Tanaka, M.; Honda, K. J. Am. Chem. Soc. 1995, 117, 8873–8874; (b) Schubert, U.;
Rengstl, A. J. Organomet. Chem. 1979, 170, C37–C39; (c) Cundy, C. S.; Lappert, M.
F. J. Chem. Soc., Dalton Trans. 1978, 665–673.
R Si
R
Si
Ni(0)
2
R
3 (4)
5. (a) Schaub, T.; Backes, M.; Radius, U. Organometallics 2006, 25, 4196–4206; (b)
Muller, C.; Lachicotte, R. J.; Jones, W. D. Organometallics 2002, 21, 1975–1981;
(c) Edelbach, B. L.; Lachicotte, R. J.; Jones, W. D. Organometallics 1999, 18, 4040–
4049; (d) Edelbach, B. L.; Lachicotte, R. J.; Jones, W. D. Organometallics 1999, 18,
4660–4668; (e) Schwager, H.; Spyroudis, S.; Vollhardt, K. P. C. J. Organomet.
Chem. 1990, 382, 191–200; (f) Eisch, J. J.; Piotrowski, A. M.; Han, K. I.; Kruger, C.;
Tsay, Y. H. Organometallics 1985, 4, 224–231.
6. Iverson, C. N.; Jones, W. D. Organometallics 2001, 20, 5745–5750.
7. (a) Edelbach, B. L.; Lachicotte, R. J.; Jones, W. D. J. Am. Chem. Soc. 1998, 120,
2843–2853; (b) Shibata, T.; Nishizawa, G.; Endo, K. Synlett 2008, 765–768.
8. (a) Yamasaki, R.; Terashima, N.; Sotome, I.; Komagawa, S.; Saito, S. J. Org. Chem.
2010, 75, 480–483; (b) Yamasaki, R.; Sotome, I.; Komagawa, S.; Azumaya, I.;
Masu, H.; Saito, S. Tetrahedron Lett. 2009, 50, 1143–1145; (c) Komagawa, S.;
Takeuchi, K.; Sotome, I.; Azumaya, I.; Masu, H.; Yamasaki, R.; Saito, S. J. Org.
Chem. 2009, 74, 3323–3329; (d) Saito, S.; Komagawa, S.; Azumaya, I.; Masuda,
M. J. Org. Chem. 2007, 72, 9114–9120; (e) Maeda, K.; Saito, S. Tetrahedron Lett.
2007, 48, 3173–3176; (f) Komagawa, S.; Saito, S. Angew. Chem., Int. Ed. 2006, 45,
2446–2449; (g) Saito, S.; Masuda, M.; Komagawa, S. J. Am. Chem. Soc. 2004, 126,
10540–10541; For review, see: (h) Komagawa, S.; Yamasaki, R.; Saito, S. J.
Synth. Org. Chem. Jpn. 2008, 66, 974–982.
CO2Et
Ni
R
R
Ni
R
Si
Si
R
7
9
CO2Et
α
Ni
β
R
Si
CO2Et
R
8
1
Scheme 1. Proposed mechanism.
9. Saito, S.; Takeuchi, K. Tetrahedron Lett. 2007, 48, 595–598.
10. Saito, S.; Maeda, K.; Yamasaki, R.; Kitamura, T.; Nakagawa, M.; Kato, K.;
Azumaya, I.; Masu, H. Angew. Chem., Int. Ed. 2010, 49, 1830–1833.
11. The stereochemistry of 3a was confirmed by the following NOE experiment.
CO2Et
8) proceeds. The E-isomer (3) would be isolated preferentially
when a large steric hindrance exists between the ethoxycarbonyl
group and the substituent (R), and/or the s-trans conformation8d,10
of the C(carbonyl)–C(a)–C(b)–Si bond is preferred.
H
The Ni-catalyzed reaction of 5 would proceed via a similar path-
way. In the reaction of 5, however, the conformation of the seven-
membered metalacyclic intermediate, which corresponds to 8,
would be restricted so that the E-isomer is isolated exclusively.
In summary, we developed a new Ni-catalyzed ring expansion
NOE Me
Si
Me
12. This ligand was used for the [4+3] cycloaddition of 1 with 1,3-dienes. See, Ref.
9.
13. The reactivity of the Ni(cod)2-TOPP catalyst was low in the [3+2+2] cycloaddition
of 1 with alkynes (Ref. 8d). In the reaction of 1 with benzosilacyclobutene, the
reaction of 1 and benzosilacyclobutenes (2). The formal
r bond
cross metathesis proceeded in a selective manner, and seven-
membered silacyclic compounds were isolated. The Ni-catalyzed
reaction of 1 and biphenylenes (5) proceeded smoothly as well.
The study provided a new method for the synthesis of cyclic
seven-membered compounds in a selective manner. Further exten-
sion of this study is on going.
Ni(cod)2-TOPP complex, which would be a weaker
p donor, might insert
selectively to the reactive C–Si bond of benzosilacyclobutene. As a result,
product might be isolated in higher yield by suppressing the possible
dimerization of 1. In most reactions of 1, Ni(cod)2-PPh3 complex was the best
catalyst among the catalyst we tested. For the dimerization of 1, see, Kawasaki,
T.; Saito, S.; Yamamoto, Y. J. Org. Chem. 2002, 67, 4911–4915.
14. The stereochemistry of 6b was confirmed by the following NOE experiment.
CO2Et
Supplementary data
H
NOE
H
Supplementary data (experimental procedures, characteriza-
tion data, and copies of NMR spectra of 3, 4, and 6.) associated with
this article can be found, in the online version, at doi:10.1016/
j.tetlet.2010.09.031.
15. Binger, P.; Doyle, M. J.; Benn, R. Chem. Ber. 1983, 116, 1–10.
References and notes
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Takeyama, Y.; Nozaki, K.; Matsumoto, K.; Oshima, K.; Utimoto, K. Bull. Chem.
Soc. Jpn. 1991, 64, 1461–1466.