Chiral tetrafluorobenzobarrelene ligands for the rhodium-catalyzed asymmetric cycloisomerization of oxygen- And nitrogen-bridged 1,6-enynes

Article abstract of DOI:10.1002/anie.200906792

Chemical Equation Presented The long and winding Rhoad: The asymmetric cycloisomerization of nitrogenand oxygen-bridged 1,6-enynes proceeded in the presence of a cationic rhodium complex that was coordinated by PPh ₃ and a chiral diene ligand to afford high yields of chiral 3-aza- and 3-oxabicyclo[4.1.0]heptenes and high enantioselectivities. TS = 4-toluenesulfonyl, Tf= trifluoromethanesulfonyl.

Full text of DOI:10.1002/anie.200906792

Communications
DOI: 10.1002/anie.200906792
Asymmetric Catalysis
Chiral Tetrafluorobenzobarrelene Ligands for the Rhodium-Catalyzed
Asymmetric Cycloisomerization of Oxygen- and Nitrogen-Bridged
1,6-Enynes**
Takahiro Nishimura,* Takahiro Kawamoto, Makoto Nagaosa, Hana Kumamoto, and
Tamio Hayashi*
The transition-metal-catalyzed cycloisomerization reaction of
1,n-enynes has emerged as an atom-economical process for
the preparation of diverse cyclic compounds.[1] Nitrogen- and
oxygen-bridged enynes are useful starting materials for the
construction of heterocyclic building blocks. Since Blum et al.
reported the PtCl4-catalyzed transformation of allyl propargyl
ethers into 3-oxabicyclo[4.1.0]heptenes in 1995,[2] this type of
6-endo-dig cycloisomerization has been developed by the use
of p-acidic metal[3] catalysts such as platinum[4,5] and gold[6]
(Scheme 1). The reaction is thought to be promoted by the
oxabicyclo[4.1.0]heptene derivatives with high enantioselec-
tivity using a rhodium–chiral diene[10] complex that has an
achiral monophosphine as the second ligand.
Based on the reported use of active platinum catalysts
containing a single vacant coordination site in the cyclo-
isomerization of dienes[11] or enynes,[5] we tested the catalytic
activity of a cationic rhodium complex that was coordinated
by one bidentate phosphine ligand and one triphenylphos-
phine ligand for the generation of a cationic rhodium species I
in the cycloisomerization of a nitrogen-bridged 1,6-enyne 1a
(Scheme 2). However, the use of bis(phosphine) ligands dppe
Scheme 1. Cycloisomerization of 1,6-enynes.
coordination of the alkyne moiety on the enyne to the p-
acidic metal center. For less p-acidic metals, such as rho-
dium[7] and iridium,[8] their high catalytic activities are
induced by the use of CO as a ligand and/or by performing
the reaction under a CO atmosphere. In spite of its high
synthetic utility for the construction of chiral cyclic skeletons
as well as chiral cyclopropanes, there have been only a few
reports on the asymmetric variant of the 6-endo-dig cyclo-
isomerization reaction of 1,6-enynes to afford bicyclo-
[4.1.0]heptene derivatives.[5,8,9] Shibata and co-workers
reported the asymmetric cycloisomerization of nitrogen-
bridged 1,6-enynes, catalyzed by an iridium–bis(phosphine)
complex under CO.[8] Recently, Marinetti et al. reported high
enantioselectivities from the use of a chiral platinum–mono-
phosphine complex.[5c] Herein, we report the asymmetric
cycloisomerization of 1,6-enynes to afford 3-aza- and 3-
Scheme 2. Rhodium/phosphine catalysts in the cycloisomerization of
À
1,6-enyne 1a. P P=bis(phosphine) ligand.
and binap did not afford any bicyclo[4.1.0]heptene 2a
product. Next, we focused on the use of chiral diene ligands[10]
for the rhodium-catalyzed asymmetric cycloisomerization of
1,6-enynes, based on our catalyst design that involves three
characteristic features (Scheme 3): 1) the coordination of one
phosphorus ligand to a cationic rhodium center that is also
coordinated by a diene ligand provides a single vacant site at
the square-planar rhodium(I) complex; this inhibits the
simultaneous coordination of an alkyne and an alkene
[*] Dr. T. Nishimura, Dr. T. Kawamoto, M. Nagaosa, H. Kumamoto,
Prof. T. Hayashi
Department of Chemistry, Graduate School of Science
Sakyo, Kyoto 606-8502 (Japan)
Fax: (+81)75-753-3988
E-mail: tnishi@kuchem.kyoto-u.ac.jp
thayashi@kuchem.kyoto-u.ac.jp
[**] This work has been supported by a Grant-in-Aid for Scientific
Research (S) (19105002) from the MEXT (Japan).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200906792.
Scheme 3. Catalyst design for the rhodium-catalyzed asymmetric cyclo-
isomerization of 1,6-enynes.
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moiety of the 1,6-enyne;[5,11] 2) the less hindered and less
electron-donating character of a diene ligand than a bis(phos-
phine) ligand is expected to stabilize a rhodium complex that
is coordinated by a monophosphine ligand; and 3) the C2-
symmetric chiral diene ligand offers a good chiral environ-
ment.
no catalytic activity, even though it could provide a single
coordination site as a monomeric chlororhodium species
(Table 1, entry 6). Next, we examined several C2-symmetric
tfb ligands[15] in the reaction of 1,6-enyne 1a to find a
successful catalyst for the enantioselective cycloisomerization
reactions (Table 1, entries 7–9). Chiral tfb ligand (R,R)-L1,[15b]
which has methyl group substituents, gave 2a in 48% yield,
and modest enantioselectivity (50% ee; Table 1, entry 7).
Ligand L2, which has longer alkyl chains (pentyl), gave 2a in
64% yield and 56% ee (Table 1, entry 8). (R,R)-L3,[15c] which
contains CH2OCH2OCH3 substituents, displayed a high
catalytic activity and enantioselectivity to afford 2a in 94%
yield and 80% ee (Table 1, entry 9). The nature of the
phosphorus ligands had a significant influence on both the
catalytic activity and enantioselectivity (Table 1, entries 10–
13). Tri(para-tolyl)phosphine displayed the same catalytic
activity and enantioselectivity as PPh3 (Table 1, entry 10).
However, the use of electron-deficient tris(4-trifluoromethyl-
phenyl)phosphine afforded 2a in low yield (19%) and
57% ee (Table 1, entry 11). Both tricyclohexylphosphine
(Table 1, entry 12) and triisopropyl phosphite (Table 1,
entry 13) gave very low yields of 2a. The relative and absolute
configurations of 2a that was obtained with (R,R)-L3 were
determined to be 1S,6R,7R by X-ray crystallographic analysis
(Figure 1).[16]
To estimate the catalytic activity of our rhodium catalyst,
the reaction of 1,6-enyne 1a was carried out in the presence of
[{RhCl(diene)}2] (5 mol% of Rh), PPh3 (5 mol%), and
NaBArF (10 mol%; ArF = 3,5-bis(trifluoromethyl)phenyl)
4
in 1,2-dichloroethane at 508C for 24 hours (Table 1). The
Table 1: Rhodium-catalyzed cycloisomerization of 1a.[a]
Entry
Diene
PR’3
Yield [%][b]
1
2
cod
nbd
PPh3
PPh3
9
7
3
bod
PPh3
23
4
5
tfb
tfb
tfb
(R,R)-L1
(R,R)-L2
(R,R)-L3
(R,R)-L3
(R,R)-L3
(R,R)-L3
(R,R)-L3
PPh3
-
-
PPh3
PPh3
PPh3
P(4-MeC6H4)3
P(4-CF3C6H4)3
PCy3
58
2[c]
6[d]
7
0
48 (50% ee)[e]
64 (56% ee)[e]
94 (80% ee)[e]
94 (80% ee)[e]
19 (57% ee)[e]
2
8
9
10
11
12
13
Figure 1. ORTEP illustration of 2a; thermal ellipsoids are drawn at the
50% probability level.
P(O-iPr)3
7
Table 2 summarizes the results obtained for the cyclo-
isomerization reactions of nitrogen-bridged 1,6-enynes 1. The
reaction was carried out using [RhCl(PPh3)(R,R)-L3]
(5 mol% of Rh) as a precursor to the active catalytic species
that possesses one vacant coordination site, which was
prepared by the reaction of [{RhCl(R,R)-L3}2] with PPh3 in
dichloromethane. The asymmetric cycloisomerization of 1,6-
enynes 1a–1e, which have aryl groups on the alkene moiety,
proceeded to give the corresponding bicyclic compounds 2a–
2e in good yields,[17] and enantioselectivities in the range 76–
95% ee (Table 2, entries 1–5). Enynes containing ortho sub-
stituents on the benzene ring, 2-MeO (1c) and 2,6-Me2 (1d),
increased the enantioselectivity (90 and 95% ee, respectively;
Table 2, entries 3 and 4). 1,6-Enyne 1 f, which contains a
trifluoromethanesulfonyl substituent on the nitrogen atom,
also gave cycloisomerization product 2 f in 73% yield and
73% ee (Table 2, entry 6). The reaction of 1,6-enyne 1g,
which has a methallyl group, proceeded smoothly under mild
conditions, although the enantioselectivity of the product 2g
was moderate (68% ee; Table 2, entry 7).
[a] Reaction conditions: enyne 1a (0.10 mmol), [{RhCl(diene)}2]
(2.5 mmol, 5 mol% of Rh), PPh3 (5.0 mmol), NaBArF (0.010 mmol),
4
1,2-dichloroethane (0.2 mL) at 508C for 24 h. [b] Determined by 1H NMR
spectroscopy. [c] 82% of 1a was recovered. [d] Performed without
NaBArF . [e] The ee was determined by HPLC analysis with a chiral
4
stationary phase column (Chiralpak AD-H). cod:1,5-cyclooctadiene, nbd:
norbornadiene, bod: bicyclo[2.2.2]octa-2,5-diene, tfb: tetrafluorobenzo-
barrelene, Cy: cyclohexyl.
use of rhodium complexes coordinated by cod (1,5-cyclo-
octadiene), nbd (2,5-norbornadiene), and bod (bicyclo-
[2.2.2]octa-2,5-diene)[12] gave 9%, 7%, and 23% yield of
cycloisomerization product 2a, respectively (Table 1,
entries 1–3); tfb (tetrafluorobenzobarrelene)[13] displayed a
higher catalytic activity to give 2a in 58% yield (Table 1,
entry 4). The high catalytic activity of the tfb ligand, which has
a similar bicyclo[2.2.2]octadiene skeleton to bod, is probably
due to its high p-accepting ability.[14] Reaction in the absence
of triphenylphosphine gave a low yield of 2a, which indicates
that a single coordination site at rhodium is essential for the
selective formation of the cycloisomerization product
(Table 1, entry 5). A neutral complex [{RhCl(tfb)}2] also had
The same type of cycloisomerization was observed for
oxygen-bridged 1,6-enynes [Eqs. (1) and (2)]. Thus, the
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Table 2: Rhodium-catalyzed cycloisomerization of 1,6-enyne 1.[a]
Entry
Enyne
T [8C]
t [h]
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
1a
1b
1c
1d
1e
1 f
50
40
40
60
60
60
40
24
24
24
48
24
48
24
94 (2a)
91 (2b)
87 (2c)
71 (2d)
92 (2e)
73 (2 f)
81 (2g)
80
81
90
95
76
73
68
1g
Scheme 4. Plausible catalytic cycle.
[a] Reaction conditions: enyne 1 (0.20 mmol), [RhCl(PPh3)(R,R)-L3]
(0.010 mmol, 5 mol% of Rh), NaBArF (0.020 mmol, 10 mol%), 1,2-
4
dichloroethane (0.4 mL). [b] Yield of isolated product. [c] Determined by
HPLC analysis with a chiral stationary phase column (Chiralpak AD-H).
reaction of 3a gave 3-oxabicyclo[4.1.0]heptene 4a in 86%
yield and 90% ee [Eq. (1)].[18] Very high enantioselectivity
(99% ee) was observed for the reaction of 1,6-enyne 3b,
which afforded tricyclic compound 4b [Eq. (2)].
Scheme 5. Proposed stereochemical pathway.
rhodium species A effectively shield three quadrants, and the
substituents on the tfb ligand shape the less shielded site at the
fourth quadrant. The alkene part of the 1,6-enyne approaches
the alkyne moiety that has been activated by the p-acidic
rhodium center. The 6-endo-dig cyclization step is directed by
the steric bulk of the tosyl group, the two proximal phenyl
groups of the triphenylphosphine, and the CH2OCH2OCH3
substituent on the (R,R)-L3. Therefore, cyclization from B-1
(Scheme 5) to give rhodium–carbenoid C-1 is the most
favorable, which leads to (1S,6R,7R)-2a.
In summary, we have developed a rhodium-catalyzed
asymmetric cycloisomerization reaction of nitrogen- and
oxygen-bridged 1,6-enynes that affords 3-aza- and 3-
oxabicyclo[4.1.0]heptenes in high yields and high enantiose-
lectivities. The reaction was accomplished using a cationic
rhodium complex that was coordinated by PPh3 and a chiral
diene ligand.
It has been proposed that the mechanism for the PtII-
catalyzed cycloisomerization reaction involves a platinum
carbenoid intermediate.[1,4,19] Provided that the mechanism
for the p-acidic RhI-catalyzed formation of bicyclo-
[4.1.0]heptenes is similar to that for the PtII-catalyzed
reaction, we suggest that the catalytic cycle of the cyclo-
isomerization proceeds as shown in Scheme 4. The coordina-
tion of the alkyne moiety of 1,6-enyne 1a to coordinatively
unsaturated cationic rhodium(I) species A, which was gen-
erated from [RhCl(PPh3)(R,R)-L3] and NaBArF , affords h2-
4
alkyne–rhodium species B. Species B undergoes 6-endo-dig
cyclization with the formation of two carbon–carbon bonds to
form rhodium carbenoid C. b-Hydrogen shift affords the
product (2a) whilst also regenerating the cationic rhodium
species (A). It is most likely that the configuration of the
product (2a) is determined during the carbon–carbon bond-
formation step on h2-alkyne–rhodium species B. The enantio-
selectivity achieved in this catalytic system can be rationalized
using a quadrant diagram (Scheme 5). The ligands on cationic
Experimental Section
[RhCl(PPh3)(R,R)-L3] (7.7 mg, 0.010 mmol) and NaBArF (18.4 mg
4
calculated as dihydrate, 0.020 mmol) were added to a solution of 1,6-
enyne 1a (67.9 mg, 0.20 mmol) in 1,2-dichloroethane (0.4 mL), and
the mixture was stirred at 508C for 24 hours. The mixture was passed
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through a short column of silica gel with hexane/ethyl acetate/Et3N
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subjected to flash column chromatography on silica gel (hexane/ethyl
acetate/CHCl3/Et3N = 20:1:1:1) to give 2a (63.6 mg, 0.19 mmol,
94%).
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Received: December 2, 2009
Published online: February 1, 2010
Keywords: cyclization · diene ligands · enynes · heterocycles ·
.
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