Rhodium-catalyzed isomerization of unactivated alkynes to 1,3-dienes

Article abstract of DOI:10.1039/b605368h

A rhodium/binap complex has been found to effectively catalyze the isomerization of unactivated internal alkynes to the corresponding 1,3-dienes in the presence of an azomethine imine as the reaction promoter. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b605368h

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COMMUNICATION
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Rhodium-catalyzed isomerization of unactivated alkynes to 1,3-dienes{
Ryo Shintani, Wei-Liang Duan, Soyoung Park and Tamio Hayashi*
Received (in Cambridge, UK) 13th April 2006, Accepted 21st July 2006
First published as an Advance Article on the web 2nd August 2006
DOI: 10.1039/b605368h
terminal 1,3-dienes selectively (entry 4). Alkynes bearing a longer
alkyl substituent can also be employed to furnish internal 1,3-diene
products, although their efficient isomerization requires a higher
reaction temperature (100 uC; entries 5–8). In addition, it is worth
noting that various functional groups, such as ethers, esters,
alkenes, ketones, and carbamates, are compatible with this process.
We then carried out some experiments to probe the catalytically
active species in the present reaction system. Thus, alkyne 1a is
fully converted to 2a by the use of only 0.3 equiv of dipole 3
(eqn. 3), indicating that dipole 3 may react with a Rh/binap
complex, rather than with substrate 1a, to generate an active
rhodium catalyst. Because transition-metal hydride complexes are
typically used as catalysts for the isomerization of electron-
deficient alkynes,2 the active catalyst in this process could also be a
rhodium hydride species. Unfortunately, however, direct observa-
tion of such a species by treatment of [RhCl(cod)]2/(¡)-binap with
dipole 3 has not been successful so far. To examine the feasibility
of a rhodium hydride catalyst, we employed a readily available
rhodium hydride complex, RhH(PPh3)4, as the catalyst (eqn. 4).
Although the catalyst activity of this complex is somewhat lower,
alkyne 1a is cleanly isomerized to 1,3-diene 2a in 81% yield.
A rhodium/binap complex has been found to effectively
catalyze the isomerization of unactivated internal alkynes to
the corresponding 1,3-dienes in the presence of an azomethine
imine as the reaction promoter.
Isomerization of organic compounds catalyzed by transition-metal
complexes can provide a mild and efficient process that may not be
easily achieved under conventional thermal conditions.1 For
isomerization of internal alkynes to 1,3-dienes, several transition-
metal complexes (e.g., Ru, Ir, and Pd complexes) are known to
catalyze such a reaction when the alkyne is directly attached to an
electron-withdrawing group (eqn. 1).2 On the other hand, very
little progress has been described for the isomerization of
unactivated alkynes to the corresponding 1,3-dienes. In fact, to
the best of our knowledge, only one recent report by Mitsudo has
addressed this issue to date: a ruthenium complex can catalyze the
isomerization of several simple internal alkynes with moderate
efficiency.3,4 In this Communication, we report that a rhodium
complex can effectively catalyze the isomerization of unactivated
internal alkynes to 1,3-dienes in high yield.5
(1)
Initially, we conducted a reaction of alkyne 1a in the presence of
dipole 36,7 by the use of 5 mol% Rh/binap catalyst in 1,2-
dichloroethane at 80 uC, and found that 1a was cleanly isomerized
to 1,3-diene 2a in 89% isolated yield (eqn. 2). In comparison, no
reaction occurred in the absence of dipole 3 or the Rh/binap
catalyst, indicating both components are necessary to promote this
isomerization reaction.
ð3Þ
ð4Þ
The use of an alkyne tethered to an electron-deficient olefin led
to a tandem isomerization/Diels–Alder reaction, furnishing bicyclic
compound 4 in 75% yield along with 3% yield of uncyclized diene
2j (eqn. 5).3
ð2Þ
Under these conditions, the scope of this isomerization reaction
is illustrated in Table 1.{ Thus, substrates with a bulky primary
alkyl group and ethyl group on the alkyne selectively provide
terminal 1,3-dienes in 76–84% yield (entries 1–3). Alkynes with a
secondary alkyl group and ethyl group are also isomerized to
ð5Þ
In summary, we have developed a rhodium-catalyzed isomer-
ization of unactivated alkynes to 1,3-dienes by using dipole 3 as the
reaction promoter. Although the exact role of this dipole is unclear
at this moment, the generation of a rhodium hydride may be the
key to this catalyst system. Future studies will focus on the
investigation of the reaction mechanism as well as further
utilization of this interesting effect of 1,3-dipoles.
Department of Chemistry, Graduate School of Science, Kyoto
University, Sakyo, Kyoto 606-8502, Japan.
E-mail: thayashi@kuchem.kyoto-u.ac.jp; Fax: +81-75-753-3988;
Tel: +81-75-753-3983
{ Electronic supplementary information (ESI) available: Experimental
procedures and compound characterization data. See DOI: 10.1039/
b605368h
3646 | Chem. Commun., 2006, 3646–3647
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Table 1 Rhodium-catalyzed isomerization of alkynes to 1,3-dienesa
Entry
1b
Substrate
Product
Yield (%)
76
(E/Z)
(73/27)
2
3
84
76
(69/31)
(83/17)
4
81c
(85/15)
5d
6d
73
84
(71/29)e
(49/35/8/8)f
7d
87
83
(84/9/7)g
(81/19)h
8d
a
b
Conditions: [RhCl(cod)]2 (5 mol% Rh), (¡)-binap (5.5 mol%), dipole 3 (1.0 equiv), 1,2-dichloroethane, 80 uC. The reaction was conducted
c
d
in the presence of 7 mol% rhodium catalyst. Contaminated with an unidentified byproduct (y5%). The reaction was conducted at 100 uC.
The ratio of (1E,3E)/(1E,3Z). The ratio of (2E,4E)/(2E,4Z)/(2Z,4E)/(2Z,4Z). The ratio of (2E,4E)/(2E,4Z)/(2Z,4E). The ratio of (2E,4E)/
(2Z,4E).
e
f
g
h
2 Ketones as an EWG: B. M. Trost and T. Schmidt, J. Am. Chem. Soc.,
1988, 110, 2301; D. Ma, Y. Lin, X. Lu and Y. Lu, Tetrahedron Lett.,
1988, 29, 1045; D. Ma, Y. Yu and X. Lu, J. Org. Chem., 1989, 54, 1105;
C. Guo and X. Lu, Tetrahedron Lett., 1991, 32, 7549. Esters or
amides as an EWG: D. Ma and X. Lu, Tetrahedron Lett., 1989, 30,
843; D. Ma and X. Lu, Tetrahedron, 1990, 46, 3189. Perfluoroalkyl
groups as an EWG: Z. Wang and X. Lu, Tetrahedron, 1995, 51,
11765.
3 M. Shiotsuki, Y. Ura, T. Ito, K. Wada, T. Kondo and T. Mitsudo,
J. Organomet. Chem., 2004, 689, 3168.
4 For isomerization of propargyl ethers to dienyl ethers, see: K. Hirai,
H. Suzuki, Y. Moro-oka and T. Ikawa, Tetrahedron Lett., 1980, 21,
3413.
5 During the preparation of this manuscript, a similar transformation using
[RhCl(cod)]2 as a catalyst in the presence of a stoichiometric amount of
allyltributylstannane was reported at the 86th Annual Meeting of the
Chemical Society of Japan: H. Yasui, H. Yorimitsu and K. Oshima,
2006, presentation No. 4H3-09; H. Yasui, H. Yorimitsu and K. Oshima,
Synlett, 2006, 1783.
Support has been provided in part by a Grant-in-Aid for
Scientific Research, the Ministry of Education, Culture, Sports,
Science and Technology, Japan (21 COE on Kyoto University
Alliance for Chemistry).
Notes and references
{ General procedure for the isomerization reaction: A solution of
[RhCl(cod)]2 (2.5 mg, 10 mmol Rh) and (¡)-binap (6.8 mg, 11 mmol) in
1,2-dichloroethane (0.3 mL) was stirred for 5 min at room temperature.
Dipole 3 (34.8 mg, 0.20 mmol) and alkyne 1 (0.20 mmol) were added to it
with additional 1,2-dichloroethane (0.3 mL), and the mixture was stirred
for 24–72 h at 80–100 uC. After being cooled to room temperature, the
reaction mixture was directly passed through a pad of silica gel with
EtOAc, and the solvent was removed under vacuum. The residue was
purified by silica gel preparative TLC with Et2O/hexane to afford 1,3-diene
2.
6 H. Dorn and A. Otto, Chem. Ber., 1968, 101, 3287; H. Dorn and A. Otto,
Angew. Chem., Int. Ed. Engl., 1968, 7, 214.
1 For reviews, see: B. M. Trost and M. J. Krische, Synlett, 1998, 1;
I. J. S. Fairlamb, Angew. Chem., Int. Ed., 2004, 43, 1048; R. Uma,
C. Cre´visy and R. Gre´e, Chem. Rev., 2003, 103, 27; S. Otsuka and
K. Tani, in Transition Metals for Organic Synthesis, M. Beller, C. Bolm,
Eds., Wiley-VCH: Weinheim, Germany, 2nd edn, 2004, p. 199;
A. Hashmi and K. Stephen, in Modern Allene Chemistry, N. Krause,
A. Hashmi, K. Stephen, Eds., Wiley-VCH: Weinheim, Germany, 2004,
p. 877.
7 Azomethine imines of this type have been used in the context of [3 + 2]
dipolar cycloaddition reactions. For recent examples, see: A. Sua´rez,
C. W. Downey and G. C. Fu, J. Am. Chem. Soc., 2005, 127, 11244;
L. Pezdirc, V. Jovanovski, B. Bevk, R. Jakse, S. Pirc, A. Meden,
B. Stanovnik and J. Svete, Tetrahedron, 2005, 61, 3977; I. Panfil,
Z. Urbanczyk-Lipkowska, K. Suwinska, J. Solecka and M. Chmielewski,
Tetrahedron, 2002, 58, 1199.
This journal is ß The Royal Society of Chemistry 2006
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