Rhenium-catalyzed [2 + 2] cycloadditions of norbornenes with internal and terminal acetylenes

Article abstract of DOI:10.1246/cl.2007.1162

Treatment of norbornenes with internal and terminal acetylenes in the presence of a catalytic amount of [ReBr(CO)₃(thf)]₂ gave cyclobutene derivatives in good to excellent yields. Copyright

Full text of DOI:10.1246/cl.2007.1162

1162
Chemistry Letters Vol.36, No.9 (2007)
Rhenium-catalyzed [2 þ 2] Cycloadditions of Norbornenes
with Internal and Terminal Acetylenes
Yoichiro Kuninobu,ꢀ Peng Yu, and Kazuhiko Takaiꢀ
Division of Chemistry and Biochemistry, Graduate School of Natural Science and Technology,
Okayama University, Tsushima, Okayama 700-8530
(Received June 27, 2007; CL-070686; E-mail: kuninobu@cc.okayama-u.ac.jp; ktakai@cc.okayama-u.ac.jp)
Treatment of norbornenes with internal and terminal acety-
lenes in the presence of a catalytic amount of [ReBr(CO)3(thf)]2
gave cyclobutene derivatives in good to excellent yields.
Table 1. Investigation of various additivesa
[ReBr(CO)3(thf)]2 (2.5 mol %)
additive (5.0 mol %)
CO2Me
CO2Me
+ MeO2C
CO2Me
toluene, 115 °C, 24 h
1a
2a
3a
Yield/%b
Yield/%b
[2 þ 2] Cycloadditions of olefins with acetylenes is one of
the most powerful tools to synthesize cyclobutene derivatives.
Following Woodward–Hoffman rules, [2 þ 2] cycloaddition
reactions usually proceed under UV-irradiation conditions.
However, using transition-metal complexes sometimes enables
the reaction to be promoted without UV-irradiation. There have
been many reports on metal-mediated1 or catalyzed [2 þ 2] cy-
cloadditions of norbornenes with acetylenes; the following metal
complexes have been used: ruthenium,2 iron,3 cobalt,4 rhodium,5
nickel,6 palladium,7 and copper.8 Recently, we have been inves-
tigating the catalytic abilities of rhenium complexes.9,10 In the
process, we found that a rhenium complex also has the ability
to catalyze [2 þ 2] cycloadditions between norbornenes and
acetylenes.
Additive
none
Entry
Entry
Additive
benzonitrile
1
2
3
4
30
30
28
36
8
9
5
10
1
Sc(OTf)3
Y(OTf)3
In(OTf)3
N,N-dimethylimidazolidinone
tetrahydrothiophene
t-BuNC
10
11
43
5
6
7
PPh3
N,N-dimethylaniline
pyridine
5
5
1
12
13
14
PhCH2NC
52
58
71
2,6-Me2C6H3NC
2,6-i-Pr2C6H3NC
a2a (2.0 equiv.). b1H NMR yield.
By treatment of norbornene (1a) with dimethyl but-2-
ynedioate (2a) in the presence of a rhenium complex, [Re-
Br(CO)3(thf)]2, as a catalyst, [2 þ 2] cycloaddition reaction pro-
ceeded and cyclobutene derivative 3a was formed in 30% yield
(Table 1, Entry 1). Another rhenium complex, ReBr(CO)5, also
provided 3a in the same yield.11 To the best of our knowledge,
this is the first example of rhenium-catalyzed [2 þ 2] cycloaddi-
tion between a norbornene derivative and an acetylene.
To improve the yield of 3a, several additives were examined
(Table 1). The yield of 3a did not increase by addition of various
Lewis acids and bases (Table 1, Entries 2–10). However, when
tert-butyl isocyanide was added, the yield of 3a was increased
(Table 1, Entry 11). Benzyl isocyanide and 2,6-dimethylphenyl
isocyanide also provided 3a in moderate yields, respectively
(Table 1, Entries 12 and 13). By using a bulky isocyanide, 2,6-
diisopropylphenyl isocyanide, cyclobutene derivative 3a was
obtained in 71% yield (Table 1, Entry 14).12
By increasing the amounts of the rhenium catalyst, [Re-
Br(CO)3(thf)]2, and 2,6-diisopropylphenyl isocyanide, the yield
of cyclobutene derivative 3a was increased slightly (Table 2,
Entry 1). Norbornadiene (1b) also gave the corresponding cyclo-
butene 3b; however, the yield of 3b was low (Table 2, Entry 2).
In this reaction, a 1:2-adduct was not formed. Treatment of
benzonorbornadiene (1c) with acetylene 2a provided cyclobu-
tene 3c in 68% yield (Table 2, Entry 3). When the reaction
was conducted at higher temperature (150 ꢁC), the yield of 3c
increased to 92% yield (Table 2, Entry 4).
Table 2. Reactions between norbornene 1 and dimethyl but-2-
ynedioate (2a)a
[ReBr(CO)3(thf)]2 (5.0 mol %)
2,6-iPr2C6H3NC (10 mol %)
+
MeO2C
CO2Me
toluene, 24 h
2a
1
CO2Me
CO2Me
3
Temp/°C
Entry
Norbornene
Yield/%b
1
2
115
3a 79 (86)
3b 37 (41)
1a
1b
115
3
4
115
150
3c 68 (70)
3c 92 (95)
1c
1c
a2a (2.0 equiv.) bIsolated yield. The yield determined by
1H NMR is reported in parentheses.
duced the corresponding cyclobutene derivatives 3e and 3f in
57 and 77% yields, respectively (Table 3, Entries 2 and 3). It
is usually difficult to obtain cyclobutene derivatives from termi-
nal acetylenes.13,14 By using a rhenium catalyst, cyclobutene
derivatives 3g–3j were also obtained from terminal acetylenes
2e–2h (Table 3, Entries 4–7). In these reactions, trimerization
products of acetylenes were not detected.
Next, we investigated the reactivities of several acetylenes
(Table 3). Diethyl but-2-ynedioate (2b) also afforded 3d; howev-
er, the yield of 3d was low (Table 3, Entry 1). By using acety-
lenes having a phenyl or an alkyl group, 2c and 2d, also pro-
The proposed reaction mechanism is as follows (Scheme 1):
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.9 (2007)
1163
Table 3. Reactions between norbornadienes 1 and acetylenes 2a
Commun. 1976, 722. b) T. Mitsudo, T. Kondo, Synlett 2001, 3,
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Jordan, S. P. Kibbee, J. D. Goddard, W. Tam, J. Org. Chem.
2006, 71, 3793.
[ReBr(CO)3(thf)]2 (5.0 mol %)
2,6-iPr2C6H3NC (10 mol %)
R'
+
R
R'
R
toluene, 24 h
1
2
3
Norbornene
Acetylene
Yield/%b
Entry
Temp/°C
40 (45)
57 (60)
1
1c
1c
1c
1c
1c
EtO2C
CO2Et 2b
150
180
150
150
150
3d
3
4
A. Greco, A. Carbonaro, G. Dall’Asta, J. Org. Chem. 1970, 35,
271.
2c
3
2c
Me 2d
3e
3f
MeO2C
Ph
K. C. Chao, D. K. Rayabarapu, C.-C. Wang, C.-H. Cheng, J. Org.
Chem. 2001, 66, 8804.
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B. M. Trost, M. Yanai, K. Hoogsteen, J. Am. Chem. Soc. 1993,
115, 5294.
Y. Takenaka, H. Ito, M. Hasegawa, K. Iguchi, Tetrahedron 2006,
62, 3380.
MeO2C
MeO2C
77 (82)
5
6
4
H
2e
2f
3g 76 (77)
3h 72 (73)
5
EtO2C
O
H
7
8
9
H
22 (41)
6
1c
2g
150
3i
7
1b
Ph
H
2h
115
3j 17 (20)
a) Y. Kuninobu, A. Kawata, K. Takai, J. Am. Chem. Soc. 2005,
127, 13498. b) Y. Kuninobu, A. Kawata, K. Takai, Org. Lett.
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Takai, J. Am. Chem. Soc. 2006, 128, 202. d) Y. Kuninobu, Y.
Nishina, M. Shouho, K. Takai, Angew. Chem., Int. Ed. 2006,
45, 2766. e) Y. Kuninobu, Y. Nishina, K. Takai, Org. Lett.
2006, 8, 2891. f) Y. Kuninobu, A. Kawata, K. Takai, J. Am. Chem.
Soc. 2006, 128, 11368. g) Y. Kuninobu, Y. Nishina, C. Nakagawa,
K. Takai, J. Am. Chem. Soc. 2006, 128, 12376. h) Y. Kuninobu,
Y. Inoue, K. Takai, Chem. Lett. 2006, 35, 1376.
a2a (2.0 equiv.) bIsolated yield. The yield determined by
c
1H NMR is reported in parentheses. Benzonorbornadiene (2.0
equiv.), acetylene (1.0 equiv.), ReBr(CO)5 was used as a cata-
lyst.
R
+
10 Rhenium-catalyzed reactions have also been reported by other
groups, see: a) F. E. Kuhn, A. Scherbaum, W. A. Herrmann, J.
R
R'
R'
¨
ReI
Organomet. Chem. 2004, 689, 4149. b) M. R. Luzung, F. D.
Toste, J. Am. Chem. Soc. 2003, 125, 15760. c) K. A. Nolin,
R. W. Ahn, F. D. Toste, J. Am. Chem. Soc. 2005, 127, 12462.
d) H. Kusama, H. Yamabe, Y. Onizawa, T. Hoshino, N. Iwasawa,
R
R
R'
Angew. Chem., Int. Ed. 2005, 44, 468. e) L. L. Ouh, T. E. Muller,
¨
ReIII
ReI
Y. K. Yan, J. Organomet. Chem. 2005, 690, 3774.
11 This reaction did not proceed using Re2(CO)10, ReBr(CO)3-
(2,20-bipyridyl), Re(C5Me5)(CO)3, ReCl3, ReCl3(PMe2Ph)3,
ReCl3O(PPh3)2, and Re2O7.
R'
Scheme 1. Proposed mechanism of the formation of cyclobu-
tene derivatives.
12 The role of isocyanides is not clear. However, addition of the
isocyanide inhibited the polymerization of acetylenes.
13 Ruthenium complexes, RuCl2(PPh3)3 and RuH2(CO)(PPh3)3,
which are sometimes employed as catalysts to promote [2 þ 2]
cycloadditions of norbornenes with acetylenes, did not give cyclo-
butene derivatives 3g and 3h.
14 For ruthenium-catalyzed [2 þ 2] cycloadditions using terminal
acetylenes, see: T. Mitsudo, H. Naruse, T. Kondo, Y. Ozaki, Y.
Watanabe, Angew. Chem., Int. Ed. Engl. 1994, 33, 580. However,
the yields of cyclobutenes are low because of cyclic trimerization
of acetylenes.
15 There are some reports on transition-metal-catalyzed synthesis of
[2 þ 2] cycloadducts by the reactions between bicyclic alkenes
and acetylenes via metalacyclopentene intermediates, see: a) T.
Mitsudo, K. Kokuryo, T. Shinsugi, Y. Nakagawa, Y. Watanabe,
Y. Takegami, J. Org. Chem. 1979, 44, 4492. b) D.-J. Huang,
D. K. Rayabarapu, L.-P. Li, T. Sambaiah, C.-H. Cheng, Chem.
Eur. J. 2000, 6, 3706. c) K. C. Chao, D. K. Rayabarapu, C.-C.
Wang, C.-H. Cheng, J. Org. Chem. 2001, 66, 8804.
(1) coordination of a norbornene and an acetylene to a rhenium
center; (2) formation of a rhenacyclopentene intermediate;9f,15,16
(3) reductive elimination.
In summary, we have succeeded in the [2 þ 2] cycloaddition
of norbornenes with both internal and terminal acetylenes using
a rhenium complex, [ReBr(CO)3(thf)]2, as a catalyst and an
isocyanide, 2,6-diisopropylphenyl isocyanide, as an additive.
Recently, we have reported rhenium-catalyzed insertion of
acetylenes into a carbon–carbon single bond of non-strained
cyclic compounds under mild conditions.9f In the first step of
the ring-enlargement, we have postulated the formation of a
rhenacyclopentene intermediate by the reaction of the rhenium
catalyst, a ꢀ-keto ester and a terminal acetylene. To the best
of our knowledge, this is the first example of rhenium-catalyzed
[2 þ 2] cycloaddition, and this result supports the mechanism
for the ring-enlargement.
References and Notes
1
2
a) M. Frank-Neumann, M. Miesch, L. Gross, Tetrahedron Lett.
1990, 31, 5027. b) D. Neville, W. S. Murphy, Tetrahedron Lett.
1996, 37, 5221.
16 In this step, the orientation of the acetylene is not clear.
17 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
a) T. Mitsudo, K. Kokuryo, Y. Takegami, J. Chem. Soc., Chem.
Published on the web (Advance View) August 18, 2007; doi:10.1246/cl.2007.1162