Enantiomerically pure rhodium complexes bearing 1,5-diphenyl-1,5- cyclooctadiene as a chiral diene ligand. Their use as catalysts for asymmetric 1,4-addition of phenylzinc chloride

Article abstract of DOI:10.1021/ol0524914

(Chemical Equation Presented) A rhodium complex coordinated with 1,5-diphenyl-1,5-cyclooctadiene (Ph-cod), [RhCl((R)-Ph-cod)]₂, was obtained enantiomerically pure through optical resolution of diastereomeric isomers [Rh(Ph-cod)((R)-1,1′-binapht

Full text of DOI:10.1021/ol0524914

ORGANIC
LETTERS
2005
Vol. 7, No. 26
5889-5892
Enantiomerically Pure Rhodium
Complexes Bearing
1,5-Diphenyl-1,5-cyclooctadiene as a
Chiral Diene Ligand. Their Use as
Catalysts for Asymmetric 1,4-Addition of
Phenylzinc Chloride
Asato Kina, Kazuhito Ueyama, and Tamio Hayashi*
Department of Chemistry, Graduate School of Science, Kyoto UniVersity,
Sakyo, Kyoto 606-8502, Japan
thayashi@kuchem.kyoto-u.ac.jp
Received October 14, 2005
ABSTRACT
A rhodium complex coordinated with 1,5-diphenyl-1,5-cyclooctadiene (Ph-cod), [RhCl((R)-Ph-cod)]2, was obtained enantiomerically pure through
optical resolution of diastereomeric isomers [Rh(Ph-cod)((R)-1,1 -binaphthyl-2,2 -diamine)]BF4. The enantiomerically pure rhodium complexes
′
′
showed high catalytic activity and enantioselectivity (up to 98% ee) in the asymmetric 1,4-addition of phenylzinc chloride to
ketones and esters in the presence of chlorotrimethylsilane.
r,â-unsaturated
Since our first report on the preparation of a chiral diene
ligand and its successful use for rhodium-catalyzed asymmet-
ric 1,4-addition,1 the chemistry of chiral diene ligands has been
undergoing a rapid development. The chiral dienes so far
reported to be effective as chiral ligands are all those based
on bicyclic diene skeletons. They are bicyclo[2.2.1]hepta-
2,5-diene (nbd*),1,2 bicyclo[2.2.2]octa-2,5-diene (bod*),3-6
bicyclo[3.3.1]nona-2,6-diene (bnd*),7,8 and bicyclo[3.3.2]-
deca-2,6-diene (bdd*)8 (Figure 1). We have prepared C2-
symmetric chiral dienes, which are substituted with benzyl
or aryl substituents one each at the two double bonds, by
way of catalytic asymmetric hydrosilylation1,2 or optical
(5) (a) Shintani, R.; Okamoto, K.; Otomaru, Y.; Ueyama, K.; Hayashi,
T. J. Am. Chem. Soc. 2005, 127, 54. (b) Shintani, R.; Tsurusaki, A.;
Okamoto, K.; Hayashi, T. Angew. Chem., Int. Ed. 2005, 44, 3909. (c)
Shintani, R.; Okamoto, K.; Hayashi, T. Chem. Lett. 2005, 34, 1294.
(6) (a) Fischer, C.; Defieber, C.; Suzuki, T.; Carreira, E. M. J. Am. Chem.
Soc. 2004, 126, 1628. (b) Defieber, C.; Paquin, J.-F.; Serna, S.; Carreira,
E. M. Org. Lett. 2004, 6, 3873. (c) Paquin, J.-F.; Defieber, C.; Stephenson,
C. R. J.; Carreira, E. M. J. Am. Chem. Soc. 2005, 127, 10850. (d) Paquin,
J.-F.; Stephenson, C. R. J.; Defieber, C.; Carreira, E. M. Org. Lett. 2005,
7, 3821.
(7) Otomaru, Y.; Tokunaga, N.; Shintani, R.; Hayashi, T. Org. Lett. 2005,
7, 307.
(8) Otomaru, Y.; Kina, A.; Shintani, R.; Hayashi, T. Tetrahedron:
Asymmetry 2005, 16, 1673.
(1) Hayashi, T.; Ueyama, K.; Tokunaga, N.; Yoshida, K. J. Am. Chem.
Soc. 2003, 125, 11508.
(2) Shintani, R.; Ueyama, K.; Yamada, I.; Hayashi, T. Org. Lett. 2004,
6, 3425.
(3) Tokunaga, N.; Otomaru, Y.; Okamoto, K.; Ueyama, K.; Shintani,
R.; Hayashi, T. J. Am. Chem. Soc. 2004, 126, 13584.
(4) (a) Otomaru, Y.; Okamoto, K.; Shintani, R.; Hayashi, T. J. Org.
Chem. 2005, 70, 2503. (b) Shintani, R.; Kimura, T.; Hayashi, T. Chem.
Commun. 2005, 3213. (c) Hayashi, T.; Tokunaga, N.; Okamoto, K.; Shintani,
R. Chem. Lett. 2005, 1480. (d) Shintani, R.; Okamoto, K.; Hayashi, T. Org.
Lett. 2005, 7, 4757.
10.1021/ol0524914 CCC: $30.25
© 2005 American Chemical Society
Published on Web 11/30/2005

Here, we report our studies on the chiral diene-rhodium
complexes where the diene is not chiral but prochiral until
its coordination to a metal. We chose 1,5-diphenyl-1,5-
cyclooctadiene (1, Ph-cod) as a prochiral diene because 1,5-
cyclooctadiene (cod) is well-known to coordinate to late
transition metals forming stable chlelate diene-metal com-
plexes11 and the diene 1 is expected to form C2-symmetric
chiral diene moiety on coordination to a metal. The chiral
environment brought about by the enantioface-selective
coordination of the diene 1 turned out to be very powerful,
giving rise to high enantioselectivity in the rhodium-catalyzed
asymmetric 1,4-addition of a phenylzinc reagent to R,â-
unsaturated ketones and esters.
Figure 1. Chiral bicyclic diene ligands.
A diagonally substituted cyclic diene, 1,5-diphenyl-1,5-
cyclooctadiene (1, Ph-cod), was obtained in a high yield by
the palladium-catalyzed cross-coupling12 between phenyl-
magnesium bromide and 1,5-dibromo-1,5-cyclooctadiene,
which is accessible from 1,5-cyclooctadiene by bromination
with bromine followed by dehydrobromination with potas-
sium tert-butoxide.13 Treatment of diene 1 with [RhCl-
(ethylene)2]2 in benzene brought about ligand substitution
giving a quantitative yield of racemic [RhCl(Ph-cod)]2 (dl-
2). Optical resolution of the Ph-cod complex dl-2 was
conducted according to the Gru¨tzmacher’s method9 (Scheme
1). Thus, dl-2 was treated with (R)-1,1′-binaphthyl-2,2′-
resolution of their intermediates3,4 or the dienes themselves.7,8
Carreira reported C1-symmetric bod* ligands which are
readily accessible from (-)-carvone.6 These chiral diene
ligands have been demonstrated to be highly effective,
especially in rhodium-catalyzed aryl transfer reactions. High
catalytic activity and/or high enantioselectivity was observed
in asymmetric 1,4-addition of arylboron reagents to N-
sulfonylarylimines3,7 and R,â-unsaturated ketones, esters,
amides, and aldehydes.1,2,4,6,8 In the arylative cyclization of
alkynes bearing an aldehyde or enoate moiety,5 high chemose-
lectivity and high enantioselectivity was achieved by use of
a chiral diene ligand.
Recently, Gru¨tzmacher reported [Rh((R)-Ph-dbcot)(MeCN)2]-
OTf as a new type of chiral diene-rhodium complex, where
Ph-dbcot stands for 5-phenyldibenzo[a,e]cyclooctene (Figure
2).9 This chiral rhodium complex, obtained in an enantio-
Scheme 1. Preparation of Ph-cod (1) and Its Rhodium
Complexes
Figure 2. Gru¨tzmacher’s chiral diene-rhodium complex.
merically pure form through optical resolution of a mixture
of diastereomeric rhodium complexes coordinated with the
diene and (R)-1,1′-binaphthyl-2,2′-diamine, was used as a
catalyst for asymmetric reactions including asymmetric 1,4-
addition of phenylboronic acid. This chiral diene complex
is very different from those of the chiral bicyclic dienes such
as substituted bod* in that the chirality of the prochiral diene
Ph-dbcot is generated and fixed on coordination to a metal.10
Unfortunately, the Ph-dbcot/rhodium catalyst was not so
enantioselective as other chiral diene-rhodium catalysts
probably due to its C1-symmetric structure lacking a sub-
stituent on one of the two double bonds.
diamine (R)-4 and AgBF4 in dichloromethane to give [Rh-
(Ph-cod)((R)-1,1′-binaphthyl-2,2′-diamine (4))]BF4 (3), which
(9) La¨ng, F.; Breher, F.; Stein, D.; Gru¨tzmacher, H. Organometallics
2005, 24, 2997.
(10) It has been well-documented that coordination of prochiral alkenes
to a metal with either of their enantiotopic faces generates chirality on the
alkene moiety. As a pioneering work: Paiaro, G.; Panunzi, A. J. Am. Chem.
Soc. 1964, 86, 5148.
(11) For a example of the cod/rhodium complexes: Ibers, J. A.; Snyder,
R. G. J. Am. Chem. Soc. 1962, 84, 495.
(12) Hayashi, T.; Konishi, M.; Kobori, M.; Kumada, M.; Higuchi, T.;
Hirotsu, K. J. Am. Chem. Soc. 1984, 106, 158.
(13) Detert, H.; Rose, B.; Mayer, W.; Meier, H. Chem. Ber. 1994, 127,
1529.
5890
Org. Lett., Vol. 7, No. 26, 2005

is a mixture of diastereomeric isomers in a ratio of one to
one. Fractional crystallization of the diastereomeric mixture
from tetrahydrofuran and benzene gave 29% yield (58% yield
based on one diastereomer) of one of the diastereomeric
isomers 3. 1H NMR showed that its diastereomeric purity is
>99% and the complex keeps its purity in solution at room
temperature for a week, indicating that the dissociation of
the diene causing epimerization is not taking place. Removal
of the chiral diamine (R)-4 by the reaction with concd
hydrochloric acid in acetonitrile gave enantiomerically pure
[RhCl(Ph-cod)]2 (2), whose absolute configuration was
determined to be (1R,5R) by its X-ray crystal analysis (vide
infra). We will denote its configuration (R) hereafter for
simplicity. Cationic complex [Rh((R)-Ph-cod)(MeCN)2]BF4
((R)-5) was also prepared from the chloro-bridge dimer,
[RhCl((R)-Ph-cod)]2 ((R)-2), by abstraction of the chloride
with AgBF4 in acetonitrile. The retention of the >99%
enantiomeric purity during these transformations was con-
firmed by the reaction of (R)-2 with (R)-4 and AgBF4, which
gave back the diastereomerically pure complex [Rh((R)-
Ph-cod)((R)-4)]BF4 ((R,R)-3) in a quantitative yield.
Figure 4. Selected bond distances and angles for [RhCl(R)-Ph-
cod)]2 (R)-2.
bonds (CRdCâ and CR′dCâ′) of the Ph-cod 1 are not
parallel to each other but twisted by 9.4°. As a result, the
whole structure of the 1,5-cyclooctadiene moiety is not in a
higher symmetry than C2. This twisted coordination manner
of Ph-cod 1 is similar to that of Ph-bnd*, whose basic
skeleton is bicyclo[3.3.1]nona-2,6-diene.8
The enantiomerically pure rhodium complexes, (R)-2,
(R,R)-3, and (R)-5, were first examined for their catalytic
activity and enantioselectivity in the asymmetric addition of
phenylboronic acid to 2-cyclohexenone (6a)15,16 (Scheme 2).
The X-ray crystal structure of [RhCl((R)-Ph-cod)]2 ((R)-
2), which contains a CH2Cl2 solvent molecule, is shown in
Figure 3. The complex adopts the chloro-bridge dimeric
Scheme 2. Asymmetric 1,4-Addition of Phenylboronic Acid
to 2-Cyclohexenone (6a)
The chloro-bridge dimer (R)-2 (3 mol % of Rh) catalyzed
the reaction at 50 °C to give 90% yield of 3-phenylcyclo-
hexanone (7a) after the reaction time of 6 h. However, the
enantiomeric purity of 7a thus obtained was not so high as
we expected, being only 43% ee (R). Fortunately, it turned
out that the enantiomeric purity of the 1,4-addition product
7a is strongly dependent on the progress of the reaction, the
higher % ee at the lower conversion. For example, the
reaction stopped after 20 min reaction time gave (R)-7a of
91% ee, although the yield was only 3%. We reasoned that
the lower % ee at higher conversion would be caused by
racemization of the catalyst under the reaction conditions,
which takes place probably by dissociation of the diene 1
from rhodium and recoordination on the other enantioface.
To realize the high chemical yield and high enantioselec-
tivity at the same time, we looked for a reaction system where
Figure 3. ORTEP illustration of [RhCl((R)-Ph-cod)]2‚CH2Cl2 ((R)-
2) with thermal ellipsoids drawn at the 50% probability level.
Hydrogens are omitted for clarity.
structure, two rhodium atoms and two chlorine atoms
forming a folded diamond shape. Two double bonds of the
Ph-cod ligand coordinate to one rhodium atom in a chelate
coordination manner. Absolute configuration (1R,5R) of the
coordinated diene was determined by the Flack parameter.14
Figure 4 shows the structure of the diene-rhodium moiety
of [RhCl((R)-Ph-cod)]2 ((R)-2) and selected bond distances
and angles around the rhodium atom. The two phenyl
substituents on the double bond are situated at the second
and fourth quadrants in a C2 fashion, thereby constructing a
good chiral environment around the rhodium center. The
distance between rhodium and the carbon bonded to phenyl
(Rh-CR ) 2.14 Å) is longer than that between rhodium
and unsubstituted carbon (Rh-Câ ) 2.09 Å). Two double
(15) For reviews: (a) Hayashi, T.; Yamasaki, K. Chem. ReV. 2003, 103,
2829. (b) Fagnou, K.; Lautens, M. Chem. ReV. 2003, 103, 169.
(16) For leading references: (a) Sakai, M.; Hayashi, H.; Miyaura, N.
Organometallics 1997, 16, 4229. (b) Takaya, Y.; Ogasawara, M.; Hayashi,
T.; Sakai, M.; Miyaura, N. J. Am. Chem. Soc. 1998, 120, 5579. (c) Hayashi,
T.; Takahashi, M.; Takaya, Y.; Ogasawara, M. J. Am. Chem. Soc. 2002,
124, 5052.
(14) Flack, H. D. Acta Crystallogr. 1983, A39, 876.
Org. Lett., Vol. 7, No. 26, 2005
5891

the asymmetric 1,4-addition proceeds rapidly under catalysis
by the chiral diene-rhodium complexes, hopefully, rapidly
enough for the reaction to be completed before the catalyst
racemization becomes a serious problem. It was found that
the addition of phenylzinc chloride in the presence of
chlorotrimethylsilane is very rapid,17 the 1,4-addition to
2-cyclohexenone (6a) being completed within 20 min at 0
°C. Thus, to a solution of 3 mol % of the rhodium catalyst
(R)-2 in THF were added at 0 °C chlorotrimethylsilane (1.5
equiv) and phenylzinc chloride (1.4 equiv) in THF succes-
sively, and the mixture was stirred at the same temperature
for 20 min. Hydrolysis with 3 N HCl18 gave 89% yield of
the 1,4-addition product 7a, which is an R isomer of 81%
ee (entry 1 in Table 1). Higher enantioselectivity was
observed in the asymmetric addition to 2-cyclopentenone
(6b). The chloro-bridge dimer (R)-2 catalyzed the asymmetric
addition of phenylzinc chloride to 6b efficiently to give a
high yield of (R)-3-phenylcyclopentanone (7b) of 87% ee
(entry 2). Use of cationic rhodium complex, (R,R)-3, which
bears the chiral diamine ligand (R)-4 and Ph-cod ligand with
R configuration, brought about a slightly better result, (R)-
7b of 90% ee being obtained in 92% yield (entry 3). The
reaction catalyzed by (R)-5, which is also a cationic rhodium
complex but does not contain the diamine (R)-4, gave the
product (R)-7b of the same enantiomeric purity (90% ee)
(entry 4), indicating that the diamine (R)-4 on the complex
(R,R)-3 does not affect the enantioselectivity probably
because the diamine is free from rhodium during the catalytic
reaction. The very low enantioselectivity (14% ee) observed
for the reaction catalyzed by a 1:1 mixture of diastereoiso-
mers (R,R)-3 and (S,R)-3 (entry 5) may support the dissocia-
tion of diamine 4 from rhodium at a stereocontrolling step.
The present asymmetric 1,4-addition system which consists
of the Ph-cod/rhodium catalyst, phenylzinc chloride, and
chlorotrimethylsilane, is particularly effective for cyclic R,â-
unsaturated esters. The addition to five-membered ring
lactone 6c and to six-membered ring lactone 6d gave the
corresponding 1,4-phenylation products with 96% ee and
98% ee, respectively (entries 6 and 7)
Table 1. Rhodium-Catalyzed Asymmetric Addition of
Phenylzinc Chloride to Cyclic Enones and Enoates 6 in the
Presence of ClSiMe3 Catalyzed by Rh/Ph-cod Complexesa
catalyst
(3 mol % of Rh)
entry substrate
yield (%) of 7b % eec
1
2
3
4
5
6d
7
6a
6b
6b
6b
6b
6c
6d
(R)-2
(R)-2
(R,R)-3
(R)-5
(R,R)-3/(S,R)-3 (1/1)
(R,R)-3
(R,R)-3
89
89
92
80
91
86
99
81 (R)
87 (R)
90 (R)
90 (R)
14 (R)
96 (R)
98 (R)
a The reaction was carried out with substrate 6 (0.30 mmol), PhZnCl
(0.42 mmol), ClSiMe3 (0.45 mmol), and a catalyst (9.0 µmol Rh, 3.0 mol
% Rh) in 1.0 mL of THF at 0 °C for 20 min, unless otherwise noted.
b Isolated yield of 7 after acidic hydrolysis followed by silica gel
chromatography. c Determined by HPLC analysis with a chiral stationary
phase column: Chiralcel OD-H for 7a, Chiralcel OB-H for 7b, Chiralpak
AD-H for 7c, and Chiralcel OG for 7d. d Carried out with 6c (0.30 mmol),
PhZnCl (0.60 mmol), ClSiMe3 (0.63 mmol), rhodium catalyst (6.0 mol %
of Rh) in THF (1.0 mL) at 30 °C for 1 h.
of the enantiotopic faces. The chiral diene-rhodium com-
plexes were found to show high catalytic activity and high
enantioselectivity (up to 98% ee) in the asymmetric 1,4-
addition of phenylzinc chloride to R,â-unsaturated ketones
and esters in the presence of chlorotrimethylsilane. The high
enantioselectivity demonstrates that the Ph-cod ligand con-
structs efficient chiral surroundings around the rhodium and
keeps its coordination to rhodium during the catalytic
reaction.
In summary, we have succeeded in the preparation of
enantiomerically pure rhodium complexes to which 1,5-
diphenyl-1,5-cyclooctadiene (1, Ph-cod) coordinates with one
Acknowledgment. This work was partially supported by
a Grant-in-Aid for Scientific Research, the Ministry of
Education, Culture, Sports, Science and Technology, Japan.
A.K. thanks the Japan Society for the Promotion of Science
for the award of a fellowship for graduate students.
(17) Rhodium-catalyzed 1,6-addition of arylzinc reagents to dienones
has been reported to be accelerated by the addition of chlorotrimethylsi-
lane: Hayashi, T.; Yamamoto, Y.; Tokunaga, N. Angew. Chem., Int. Ed.
2005, 44, 4224.
(18) Before hydrolysis, the 1,4-addition product is formed as a silyl enol
ether, 3-phenyl-1-(trimethylsilyloxy)cyclohex-1-ene. The formation of silyl
enol ether as the 1,4-addition product has been reported in the rhodium-
catalyzed asymmetric 1,4-addition of an aryltitanate reagent in the presence
of chlorotrimethylsilane: Tokunaga, N.; Yoshida, K.; Hayashi, T. Proc.
Natl. Acad. Sci. U.S.A. 2004, 101, 5445.
Supporting Information Available: Experimental pro-
cedures and spectroscopic data. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL0524914
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