Rh(II)-catalyzed skeletal reorganization of enynes involving selective cleavage of C-C triple bonds

Article abstract of DOI:10.1039/b805100c

Treatment of enynes with a catalytic amount of Rh(II) complex results in skeletal reorganization to give cis-configured 1-vinylcycloalkenes, the formation of which occurs via double cleavage of both C-C double and C-C triple bonds. The Royal Society of Chemistry.

Full text of DOI:10.1039/b805100c

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Rh(II)-catalyzed skeletal reorganization of enynes involving selective
cleavage of C–C triple bondsw
Kazusa Ota and Naoto Chatani*
Received (in College Park, MD, USA) 26th March 2008, Accepted 21st April 2008
First published as an Advance Article on the web 29th May 2008
DOI: 10.1039/b805100c
Treatment of enynes with a catalytic amount of Rh(II) complex
results in skeletal reorganization to give cis-configured 1-vinyl-
cycloalkenes, the formation of which occurs via double cleavage
of both C–C double and C–C triple bonds.
tion of enynes.7
Recently, skeletal reorganization of enynes, leading to
1-vinylcycloalkenes, and related cycloisomerization reactions
catalyzed by electrophilic metal complexes have been exten-
sively studied.1 These reactions are initiated by interactions
between alkynes and electrophilic metals.2 Interestingly, two
possible isomers, type I and type II, form in the skeletal
reorganization of enynes (Scheme 1).3 When enynes having a
substituent either at a terminal alkyne carbon or a terminal
alkene carbon are used, one can determine which type of
products form.
ð2Þ
It is noteworthy that substituting the malonate moiety in the
tether with a ketal moiety, as in 3, led to the selective
formation of type II isomers. This result demonstrates that
the structure of the tether had a significant effect on the
product distribution.
The formation of type II isomers involves double cleavage
of both C–C triple and C–C double bonds, a phenomenon that
is mechanistically interesting.4,5 However, the number of re-
ports describing the formation of type II products is limited. A
mixture of type I and II products was usually obtained even
when type II was formed3 and the selective formation of type
II products is rare.3f During the examination of catalysts that
can catalyze the skeletal reorganization of enynes, we found
that Rh2(O2CCF3)4 is active in the skeletal reorganization of
enynes, especially enynes with an aryl group at the alkyne
carbon. We wish to focus on selective formation of cis-con-
figured type II products, which involves double cleavage of
both C–C triple and C–C double bonds (eqn (1)).
ð3Þ
Based on the results in eqn (3), as expected, enynes 5, 7, 9, and
11 also led to the selective formation of type II products. It
was also found that enynes with an electron-withdrawing
group, such as a CF3 group on the phenyl ring, as in 13,
selectively gave the type II, as with product 14, even if they
have a malonate tether (compare with 1b in eqn (2)). In
addition, use of a sterically hindered aryl group, as in 17, also
selectively gave the type II product 18 in high yield. The
electronic and steric natures of a substituent on the phenyl
ring also affect the product distribution. In the case of 1,7-
enyne 19, the type II product was exclusively formed, even
ð1Þ
First, we examined the reaction of enyne 1a, which has a
phenyl group at the alkyne carbon and a malonate moiety in
the tether, and that selectively gave the type I product 2a-I
(eqn (2)). Interestingly, substitution of a methyl group at the
internal alkene carbon, as in 1b, increased the product yield
and gave a mixture of type I and II products in a nearly 1 : 1
ratio. Surprisingly, the type II product had a cis configura-
tion.6 This is the first example of exclusive formation of cis-
configured type II products as a result of skeletal reorganiza-
Department of Applied Chemistry, Faculty of Engineering, Osaka
University, Suita, Osaka 565-0871, Japan.
E-mail: chatani@chem.eng.osaka-u.ac.jp; Fax: +81 6 6879 7396;
Tel: +81 6 6879 7397
w Electronic supplementary information (ESI) available: Experimen-
tal, analysis and crystallographic data in CIF format (CCDC 678098).
See DOI: 10.1039/b805100c
Scheme 1 Type I vs. type II.
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Table 1 Skeletal reorganization of enynes involving selective cleavage
of C–C triple bondsa
Enyne
Type II product
Yield (%)b
73 (498%) 5 h
6
5
76 (498%)c
Scheme 2 Proposed reaction mechanism of the formation of type II
products.
Ar = 1-Naphthyl (7)
Ar = 4-CF3C6H4 (9)
8
10
72 (95%) 3 hd
67 (498%) 4 h
ity, especially the role of the tether is not fully understood.
Elucidation of the reaction mechanism is the subject of current
investigations.
12
11
Notes and references
1. Reviews: (a) G. C. Lloyd-Jones, Org. Biomol. Chem., 2003, 1, 215;
(b) A. M. Echavarren and C. Nevado, Chem. Soc. Rev., 2004, 33,
453; (c) S. T. Diver and A. J. Giessert, Chem. Rev., 2004, 104,
Ar = 4-CF3C6H4 (13) 14
Ar = 4-MeOOCC6H4 (15) 16
76 (91%)
93 (87%)
94 (498%)
76 (498%)e
1317; (d) L. Anorbe, G. Domınguez and J. Perez-Castells,
´ ´
Chem.–Eur. J., 2004, 10, 4938; (e) L. Zhang, J. Sun and
S. A. Kozmin, Adv. Synth. Catal., 2006, 348, 2271.
Ar = 2-MeC6H4 (17)
18
2. (a) A. Furstner and P. W. Davies, Angew. Chem., Int. Ed., 2007,
¨
46, 3410; (b) D. J. Gorin and F. D. Toste, Nature, 2007, 446, 395;
(c) A. S. K. Hashmi, Chem. Rev., 2007, 107, 3180.
3. Papers on the formation of type II products: (a) N. Chatani,
T. Morimoto, T. Muto and S. Murai, J. Am. Chem. Soc., 1994,
116, 6049; (b) N. Chatani, N. Furukawa, H. Sakurai and S. Murai,
Organometallics, 1996, 15, 901; (c) S. Oi, I. Tsukamoto, S. Miyano
and Y. Inoue, Organometallics, 2001, 20, 3704; (d) C. H. Oh,
S. Y. Bang and C. Y. Rhim, Bull. Korean Chem. Soc., 2003, 24,
20
19
a
Reaction conditions: enyne (0.5 mmol), Rh2(O2CCF3)4 (0.01 mmol),
b
toluene (2.5 mL) at 80 1C for 20 h. Type II selectivity is shown in
parentheses. Rh2(O2CCF3)4 (0.02 mmol) was used. The reaction
c
d
´
887; (e) C. Nieto-Oberhuber, S. Lopez, M. P. Munoz,
D. J. Cardenas, E. Bunuel, C. Nevado and A. M. Echavarren,
´
e
was carried out at 60 1C. The reaction was carried out at 100 1C
using Rh2(OOCCF3)4 (0.04 mmol).
Angew. Chem., Int. Ed., 2005, 44, 6146; (f) Y. Miyanohana and
N. Chatani, Org. Lett., 2006, 8, 2155.
4. H. Nakai and N. Chatani, Chem. Lett., 2007, 36, 1494.
5. A review: M. Tobisu and N. Chatani, Chem. Soc. Rev., 2008, 37,
300.
6. Cis configuration was confirmed by X-ray analysis of 8 and the
1H NMR coupling constant. See ESIw.
though 19 has a malonate tether, showing that chain length is
also important, see Table 1 for results.
The proposed mechanism for the formation of type II is
depicted in Scheme 2. Generation of metal–carbene complex
21 via the 5-exo pathway has already been proposed based on
trapping of the metal–carbene intermediates8 as well as DFT
calculations.9 Ring opening of 21 gives a spiro intermediate 22.
The presence of a methyl group (R = Me) on the alkene
carbon facilitates formation of type II isomers because the
methyl group stabilizes the tertiary cation in 22.10 Ring-open-
ing of 22 gives the carbene complex 23, which undergoes 1,2-H
shift to give cis-configured type II isomers.11
7. A mixture of cis and trans isomers of type II was obtained in most
cases reported thus far. See ref. 3.
8. Selected examples: (a) B. M. Trost and A. S. K. Hashmi, Angew.
Chem., Int. Ed. Engl., 1993, 32, 1085; (b) N. Chatani, K. Kataoka,
S. Murai, N. Furukawa and Y. Seki, J. Am. Chem. Soc., 1998, 120,
9104; (c) C. Nieto-Oberhuber, S. Lopez and A. M. Echavarren,
´
J. Am. Chem. Soc., 2005, 127, 6178; (d) J. Marco-Contelles,
N. Arroyo, S. Anjum, E. Mainetti, N. Marion, K. Cariou,
G. Lemiere, V. Mouries, L. Fensterbank and M. Malacria, Eur.
´
J. Org. Chem., 2006, 4618; (e) C. A. Witham, P. Mauleon,
N. D. Shapiro, B. D. Sherry and F. D. Toste, J. Am. Chem.
Soc., 2007, 129, 5838.
9. C. Nieto-Oberhuber, P. Pe
T. Lauterbach, C. Rodrıguez, S. Lo
D. J. Cardenas and A. M. Echavarren, J. Am. Chem. Soc., 2008,
´
rez-Gala
´
n, E. Herrero-Go
´
mez,
In summary, we have demonstrated that a Rh(II) complex
shows high catalytic activity for skeletal reorganization of 1,6-
and 1,7-enynes and for selective formation of cis-configured
type II products. Although the substrate scope is limited, this
is the first example of the selective formation of cis-configured
type II products. The reaction involves double cleavage of
both C–C double and triple bonds. The origin of this selectiv-
´
´
pez, C. Bour, A. Rosello
´
n,
´
130, 269.
10. Direct formation of 23 from 21 is proposed based on DFT
theoretical studies of the Au-catalyzed reaction. Even in this case,
the methyl group may stabilize the developing partial positive
charge in the transition state: see ref. 3e.
11. D. F. Taber and P. V. Joshi, J. Org. Chem., 2004, 69, 4276.
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