A cationic rhodium-chiral diene complex as a high-performance catalyst for the intramolecular asymmetric [4+2] cycloaddition of alkyne-1,3-dienes

Article abstract of DOI:10.1002/anie.200702586

(Chemical Equation Presented) Two rings in one step: A rhodium-diene catalyst is more active than its rhodium-bisphosphine counterpart for the intramolecular [4+2] cycloaddition of alkyne-tethered 1,3-dienes (see scheme). A related complex with a C_2<

Full text of DOI:10.1002/anie.200702586

Angewandte
Chemie
DOI: 10.1002/anie.200702586
Catalytic Cycloaddition
A Cationic Rhodium–Chiral Diene Complex as a High-Performance
Catalyst for the Intramolecular Asymmetric [4+2]Cycloaddition of
Alkyne-1,3-Dienes**
Ryo Shintani,* Yuta Sannohe, Takaoki Tsuji, and Tamio Hayashi*
Dedicated to Süd-Chemie on the occasion of its 150th anniversary
The successful development of a transition-metal-catalyzed
asymmetric transformation requires the achievement of both
high catalytic activity and high enantioselectivity. It is there-
fore desirable to properly evaluate the relationship between
the catalyst activity and the nature of a ligand on the
transition metal in a given catalytic reaction, and develop its
asymmetric variant by employing a chiral ligand with the
required properties for high activity. In this context, we
demonstrate herein that a rhodium–diene complex is much
more active than its rhodium–bisphosphine counterpart as a
catalyst for intramolecular [4+2] cycloadditions of alkyne-
tethered 1,3-dienes, and that the use of a chiral diene ligand
leads to the development of a highly active and enantiose-
lective asymmetric variant of this reaction.
tethered 1,3-diene 1a as a model substrate [Eq. (1)]. These
reactions were carried out in the presence of 2 mol% of
rhodium catalyst in dichloromethane at 258C in a reaction
calorimeter (Omnical SuperCRC), and the data were ana-
lyzed by the reaction progress kinetic analysis method
developed by Blackmond.[8] The reaction catalyzed by the
Rh–cod complex proceeded very fast, with 97% conversion
being achieved in only 10 min (Figure 1). In contrast, the use
of rhodium–bisphosphine catalysts gave much slower reac-
Since the first report by Livinghouse in 1990,[1] many
rhodium(I) complexes, along with complexes of several other
transition metals,[2] have been shown to catalyze intramolec-
ular [4+2] cycloaddition reactions of alkyne-tethered 1,3-
dienes. Cationic rhodium(I) complexes bearing a bisphos-
phine ligand such as 1,2-bis(diphenylphosphanyl)ethane
(dppe)[3] or 1,4-bis(diphenylphosphanyl)butane (dppb)[4]
have been utilized as highly efficient catalysts for these
reactions, and some asymmetric variants using chiral bisphos-
phine ligands have also been developed.[5] Chung et al., on the
other hand, described the use of [Rh(naphthalene)(cod)]BF4
(cod = 1,5-cyclooctadiene) as an effective catalyst,[6] thereby
demonstrating that a phosphine-free rhodium–diene complex
can also show high activity for these cycloadditions.[7]
On the basis of these precedents, we initially focused on
the head-to-head comparison of several ligands to quantita-
tively evaluate their efficiency in the rhodium-catalyzed
intramolecular [4+2] cycloaddition reaction with alkyne-
Figure 1. A plot of conversion versus time for the reaction of 1a (initial
concentration: 0.10m)in CH 2Cl2 (3.0 mL)in the presence of a
rhodium catalyst (2.0 mm Rh)and AgSbF 6 (3.9 mm)at 25 8C.
[*] Dr. R. Shintani, Y. Sannohe, T. Tsuji, Prof. Dr. T. Hayashi
Department of Chemistry
Graduate School of Science
Kyoto University
Sakyo, Kyoto 606-8502 (Japan)
Fax: (+81)75-753-3988
E-mail: shintani@kuchem.kyoto-u.ac.jp
thayashi@kuchem.kyoto-u.ac.jp
tions (3–4% conversion after 10 min), thereby establishing
that the Rh–cod complex is at least 20 times more active than
its Rh–dppe and Rh–dppb counterparts under these con-
ditions (Figure 1).
The results of these kinetic studies suggested that the use
of a chiral diene ligand[9–12] would be desirable for the
development of a highly efficient asymmetric variant of this
process.[13] As shown in Equation (2), the reaction of 1a
proceeded smoothly with (S,S)-Ph-bod*[10] as the ligand to
[**] Financial support of this work was provided, in part, by a Grant-in-
Aid for Scientific Research from the Ministry of Education, Culture,
Sports, Science, and Technology, Japan (21 COE on Kyoto University
Alliance for Chemistry).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Ph-bod* as the ligand, and various substitution patterns on
the alkyne and the 1,3-diene are also tolerated, with the
corresponding cycloadducts being isolated in high yield and
with high ee values (87–95% yield, 83–99% ee; Table 1,
entries 4–8). The absolute configuration of cycloadduct 2d
(Table 1, entry 4) was established as (3R,6S) by X-ray
crystallographic analysis.[15]
A proposed catalytic cycle for this process with substrate
1a is shown in Figure 2.[1] Initial coordination of 1a to a
give cycloadduct 2a in 87% yield with a high enantioselec-
tivity of 94% ee.[14] A high yield of 2a was also obtained with
(S,S)-Bn-bod* as the ligand,[10,11] although the enantioselec-
tivity was significantly lower (5% ee). The use of a structur-
ally different chiral diene ligand, namely (S,S)-Ph-bnd*,[9c,d]
resulted in only a moderate yield and ee (58% yield, 35% ee).
Substrates with an oxygen or nitrogen atom in the tether
can be employed with high efficiency (90–96% yield, 91–
97% ee; Table 1, entries 2 and 3) in this reaction, with (S,S)-
Table 1: The rhodium-catalyzed asymmetric [4+2] cycloaddition of
alkyne-tethered 1,3-dienes 1.
Figure 2. Proposed catalytic cycle for the asymmetric [4+2] cycloaddi-
tion of 1a catalyzed by Rh–(S,S)-Ph-bod*.
cationic RhI–(S,S)-Ph-bod* complex leads to oxidative cycli-
zation to form rhodacyclopentene intermediate A, which
undergoes a suprafacial 1,3-allylic migration of rhodium to
give rhodacycloheptadiene species B. Reductive elimination
of the [4+2] cycloadduct 2a from intermediate B then
regenerates the cationic rhodium(I) complex. On the basis
of this mechanism, the stereo-determining step is the for-
mation of rhodacyclopentene A and the observed stereo-
chemical outcome can therefore be rationalized as shown in
Figure 3. The rhodacyclopentene has a (3S,4R) configuration
rather than a (3R,4S) configuration (complex A’) to avoid
steric repulsion between the propenyl group at the 4-position
and the phenyl group on the olefin of the (S,S)-Ph-bod*
Entry
1
Substrate
Product
Yield [%] ee [%]
87
90
96
94
97
91
2[a]
3
4
87
95
5
89
95
89
92
97
>99
87
6[a]
7
8[a]
83
Figure 3. Rationale for the stereochemical outcome of the asymmetric
[4+2] cycloaddition of 1a catalyzed by Rh–(S,S)-Ph-bod*.
[a] The reaction was conducted at 08C.
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ligand. This intermediate gives (3R,6S)-2a by a stereospecific
1,3-allylic migration of rhodium and subsequent reductive
elimination.
The high efficiency of the present catalysis with (S,S)-Ph-
bod* as the ligand is highlighted by the reaction of 1b at a
lower catalyst loading [Eq. (3)]. This reaction proceeds
Keywords: asymmetric catalysis · cycloaddition · diene ligands ·
enantioselectivity · rhodium
.
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acetylenedicarboxylate proceeds smoothly in the presence
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[Eq. (4)].[16] Similarly, trans-1,3-decadiene gives the corre-
sponding cycloadduct 3b with 87% ee.
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In summary, we have established that a rhodium–diene
catalyst is much more active than its rhodium–bisphosphine
counterpart for the intramolecular [4+2] cycloaddition of
alkyne-tethered 1,3-dienes, and we have developed a highly
active and enantioselective asymmetric variant by employing
a chiral diene ligand. This catalyst system can also be applied
to the intermolecular cycloaddition of 1,3-dienes and alkynes
with high efficiency.
Experimental Section
A solution of alkyne-tethered 1,3-diene 1 (0.20 mmol) in CH2Cl2
(1.7 mL) was added to a mixture of [{RhCl((S,S)-Ph-bod*)}2] (4.0 mg,
10 mmol Rh) and AgSbF6 (6.9 mg, 20 mmol) in CH2Cl2 (0.3 mL). The
resulting mixture was stirred for 1 h at 258C and was then passed
through a pad of silica gel with Et2O as eluent. After removal of the
solvent under vacuum, the residue was chromatographed on silica gel
with EtOAc/hexane as eluent to afford compound 2.
[13] A single example of intramolecular [4+2] cycloaddition cata-
lyzed by a rhodium–chiral diene catalyst with low enantioselec-
tivity has been reported (26% ee; reference [5c]). High enantio-
Received: June 13, 2007
Published online: August 23, 2007
Angew. Chem. Int. Ed. 2007, 46, 7277 –7280
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Communications
selectivities were obtained in the presence of a chiral bisphos-
phine ligand.
The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
[14] The ee value of 2a stays the same at low and complete
conversion of 1a, thereby indicating that the (S,S)-Ph-bod*
ligand remains bound to the rhodium center throughout the
reaction.
[16] Similar examples using (S)-binap as a ligand (up to 82% ee)
were reported at the 87th Annual Meeting of the Chemical
Society of Japan: D. Fujiwara, A. Kawachi, T. Shibata, 2006,
presentation no. 3D6-16.
[15] CCDC-648192 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from
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