Asymmetric Michael reactions of α-substituted acetates with cyclic enones catalyzed by multifunctional chiral Ru amido complexes

Article abstract of DOI:10.1016/j.jorganchem.2003.12.002

Well-defined 16-electron chiral Ru amido complexes, Ru[(R,R)-diamine](η⁶-arene), efficiently catalyze asymmetric Michael additions of Michael donors to cyclic enones to give adducts in high yields and with excellent ee's. β-Ketoesters or nitroa

Full text of DOI:10.1016/j.jorganchem.2003.12.002

Journal of Organometallic Chemistry 689 (2004) 1377–1381
www.elsevier.com/locate/jorganchem
Asymmetric Michael reactions of a-substituted acetates with
cyclic enones catalyzed by multifunctional chiral
Ru amido complexes
*
Takao Ikariya , Hui Wang, Masahito Watanabe, Kunihiko Murata
Department of Applied Chemistry, Graduate School of Science and Engineering,
Tokyo Institute of Technology and Frontier Collaborative Research Center, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan
Received 9 December 2003; accepted 9 December 2003
Abstract
Well-defined 16-electron chiral Ru amido complexes, Ru[(R,R)-diamine](g6-arene), efficiently catalyze asymmetric Michael
additions of Michael donors to cyclic enones to give adducts in high yields and with excellent ees. b-Ketoesters or nitroacetate as
Michael donors react with 2-cyclopentenone in toluene or t-butyl alcohol containing the Ru amido catalyst (S/C ¼ 50) to afford the
Michael adduct in 99% yield and with up to 92% ee. The outcome of the reaction was delicately influenced by the structures of the
diamine and arene ligands as well as reaction conditions.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Multifunction; Ru amido complex; Michael reaction; Asymmetric; C–C bond formation
1. Introduction
bearing sufficient Brønsted basicity, leading to the corre-
sponding malonate complex with a metal–C bond, was
found to be a crucial step for the catalytic C–C bond
formation [10]. We have expanded the scope of the Mi-
chael reaction catalyzed by the Ru amido complex and
found that asymmetric Michael additions of a-substituted
acetates including b-ketoesters, methyl cyanoacetate, and
nitroacetates to cyclic enones proceeded smoothly to give
the Michael adducts with good to excellent ees. We now
describe the details of the reaction of acetates with 2-cy-
clopentenone. The conceptual metal–NH bifunctional
effect, ‘‘Noyori effect’’, of the chiral amido complex,
which was originally developed in asymmetric transfer
hydrogenation of ketones as shown in Scheme 1, has been
successfully extended to enantioselective C–C bond for-
mation.
Enantioselective catalytic Michael additions are one of
the most important types of C–C bond formations be-
cause of the versatility of the products as chiral building
blocks. There are many reports on enantioselective Mi-
chael type reactions catalyzed by chiral metal catalyst
systems [1]. They include copper [2], nickel [3], cobalt [4],
rhodium [5], palladium [6], and heterobimetallic com-
plexes [1a,7] as well as the chiral organic compounds as
chiral catalysts [8]. We have recently reported that well-
defined chiral Ru amido complexes, Ru[(R,R)-TsDPEN]-
(g6-arene) (1) [9] and Ru[(R,R)-MsDPEN](g6-arene)
(TsDPEN ¼ N-(p-toluenesulfonyl)-1,2-diphenylethylene-
diamine, MsDPEN ¼ N-methane-sulfonyl-1,2-diphenyl-
ethylenediamine), efficiently initiate the catalytic
enantioselective Michael addition of malonates to cyclic
enones to provide the corresponding Michael adducts in
high yields with excellent ees [10b]. The reversible de-
protonation of malonates with the Ru amido complex
2. Results and discussion
A chiral Ru catalyst, Ru[(R,R)-diamine](g6-arene)
(1a–f) has proven to efficiently effect enantioselective
Michael addition of a-substituted acetates such as
* Corresponding author. Tel.: +81357342636; fax: +81357342637.
E-mail address: tikariya@apc.titech.ac.jp (T. Ikariya).
0022-328X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2003.12.002

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T. Ikariya et al. / Journal of Organometallic Chemistry 689 (2004) 1377–1381
DH
2
D
(NuH)
Rn
Ts
N
Ts
Rn
Ar
Ar
Ar
Ar
N
Ru
Ru
N
H
N
H(Nu)
H
1
H
Rn
Ar = C H
6
5
Ts
N
δ–
N
Scheme 3.
Ar
Ar
δ+
M
R
H(Nu)
R
O
O
δ –
H
R'
X
of methyl acetoacetate 3b with 2a (acetate:enone:Ru ¼
50:50:1) in toluene containing Ru[(R,R)-TsDPEN](hmb)
(HMB ¼ hexamethylbenzene) provided the adduct 4b in
99% yield and with 91% ee with respect to carbon atom
on the cyclopentanone, although with a 1:1 mixture of
two diasteromers with a single stereogenic center at the
cyclopentanone ring (Scheme 3). The outcome of the
reaction was significantly influenced by the reaction
conditions and the structure of the arene and diamine
ligands as observed in the reaction of malonates re-
ported previously [10b]. As shown in Table 1, the durene
complex 1a, which is an excellent catalyst for the reac-
tion of malonates, worked well but with relatively low
enantioselectivity. The ee value of the product increases
in the order of the durene-complex (1a)<HMB-complex
(1b), possibly due to the steric reasons. The effect of the
solvent on the catalyst performance is particularly sig-
nificant in this reaction. Although t-butyl alcohol, tol-
uene, and THF worked equally well for the reaction of
malonates [10b], toluene was the best choice of the sol-
vent in the reaction of b-ketoesters, as shown in Table 1.
The reaction in toluene provided the adduct with up to
91% ee. The methanesulfonyl diamine (MsDPEN)
complex 1c gave the product quantitatively but with a
slightly low ee.
H
R'
H δ+
δ– O
R
C
δ+
R' X = H (Nu)
a possible transition state
Scheme 1.
Scheme 2.
acetoacetates and ethyl nitroacetate to 2-cyclopentenone
2a, giving the corresponding Michael adducts with good
to excellent ees as summarized in Scheme 2. The reaction
Similarly, t-butyl acetoacetate 3c reacted with 2a to
give the adduct 4c in a reasonably high yield although
Table 1
Asymmetric Michael reactions of 2-cyclopentenone and dimethyl malonate 3a and acetates 3b–d, and acetylacetone 3e catalyzed by chiral Ru amido
complexesa
Entry
Catalyst
Donor
Solvent
Temperature (°C) Time (h)
Yieldb (%)
eec (%)
Configurationd
1
2
1a
1b
1a
1b
1b
1b
1c
1b
1c
1b
1b
3a
3a
3b
3b
3b
3b
3b
3c
3c
3d
3e
(CH3)3COH
(CH3)3COH
(CH3)3COH
(CH3)3COH
THF
40
40
40
40
40
40
40
30
30
30
30
24
24
24
24
24
24
24
48
48
24
24
99
98
98
99
99
99
99
98
88
22
96
95
98
64
70
84
91
85
75e
72e
17
0
S [10b]
S [10b]
S
S
3
4
5
S
6
Toluene
Toluene
S [10b]
7
S
S
S
–
–
8
Toluene
Toluene
9
10
11
THF
(CH3)3COH
a Unless otherwise noted, the reaction was carried out using 1.0 mmol of Michael acceptors and donors (1:1) in 1.0 ml of solvent. The molar ratio of
acceptor:donor:Ru is 50:50:1 (S/C ¼ 50).
b Isolated yield after flash chromatography on the silica gel.
c Determined by HPLC analysis.
d Determined from the sigh of rotation of the decarboxylation products.
e Determined by 13C NMR of ketals of the decarboxylation product with (2R,3R)-butanediol (see Section 2.1).

T. Ikariya et al. / Journal of Organometallic Chemistry 689 (2004) 1377–1381
1379
t-amyl alcohol at the lower temperature of 0 °C, pro-
vided the product with up to 92% ee (entry 4). Notice-
ably, alcoholic solvents provided better catalyst
performance in this reaction. The p-cymene complexes
1e,f gave unsatisfactory results.
We have found that the 16-electron metal amido
complex 1 with a M/NH bifunctional unit efficiently
effected asymmetric catalytic Michael reactions of
a-substituted acetates with cyclic enones leading to the
corresponding adducts with good to excellent ees. The
Brønsted basicity of the amido group in complex 1 is
responsible for the excellent catalyst performance in this
C–C bond formation. It has been reported that Michael
addition promoted by metal complex catalysts proceeds
through the nucleophilic attack of the O-bound metal
enolate or the C-nucleophile attached to the metal to the
olefin carbon in a 1,4-addition mode [2,7]. More recently,
Hayashi reported highly efficient 1,4-addition of organ-
oboronic acids to a,b-unsaturated ketones catalyzed by
chiral Rh complexes, in which insertion of the olefinic
part of the enone to the M–C bond was postulated [5e].
We have shown the possible mechanism of the addition
of malonates to enones based on the single-crystal X-ray
analysis of the Ru malonato complex and NMR inves-
tigation of the reaction mixture of malonate and the Ru
amido complex [10b]. In comparison with the case of
malonates, the reaction of a-substituted acetates with
enones possibly proceeds via a similar transition state as
postulated for the transfer hydrogenation and the C–C
bond formation [10b] (Scheme 5). We are now working
on further expansion of the scope of the reaction and
studies aimed at clarifying the mechanism of the Michael
reaction catalyzed by the Ru amido complexes.
Scheme 4.
with a lower ee value due to steric reasons. The reaction
of ethyl cyanoacetate 3d and 2a gave a 1:1 adduct in 22%
yield and with low ee in addition to the 2:1 adduct as a
major product (52% yield) probably due to the strong
interaction of the CN group with the Ru metal [11]. It
should be noted that acetylacetone 3e was far less
enantioselective although highly reactive, possibly be-
cause acetylacetone has favored the chelating enolate
formation, resulting in the replacement of the chiral
diamine ligand. In fact, acetylacetone favors the enolate
form in the solution [12].
In a similar manner, ethyl nitroacetate 3f readily re-
acts with 2a to give almost quantitatively the corre-
sponding Michael adduct with an excellent ee, although
with a 1:1 diastereomer ratio (Scheme 4). The ee value of
the product was determined by 13C NMR spectroscopy
of the ketal derived from the denitration product with
(2R,3R)-butanediol. It should be noted that tuning of
the chiral diamine and arene ligands as well as the re-
action conditions significantly affected the enantiomeric
excess of the reaction products as listed in Table 2. For
example, complex 1b bearing TsDPEN and HMB li-
gands, which was highly effective for the Michael reac-
tion of dimethyl malonate 3a and cyclic enone 2a, was
found to be far less effective for the reaction of nitro-
acetate 3f, giving the product in 33% yield with 32% ee
(entry 1). However, the use of the MsDPEN ligand
(complex 1c) caused a significant improvement in the
reactivity and selectivity, leading to the Michael adduct
6a with 82% ee in a quantitative yield. The complex 1d
bearing the more electron-withdrawing TfDPEN ligand
gave the product with up to 91% ee, and the reaction in
2.1. Experimental
All experiments were performed in an atmosphere of
dry argon using standard Schlenk tube techniques. An-
hydrous toluene, acetone, THF, and CH2Cl2 were pur-
chased from Kanto Chemical Co. Inc. 1H NMR and 13
C
NMR spectra were recorded on JEOL JNM-LA300
Table 2
Asymmetric Michael reactions of 2-cyclopentenone 2a and ethyl nitroacetate 3f catalyzed by chiral Ru amido complexesa
Entry
Catalyst
Solvent
Temperature (°C)
Time (h)
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
1b
1c
1d
1d
1d
1e
1f
(CH3)3COH
(CH3)3COH
(CH3)3COH
(CH3)2CHCH2OH
Toluene
30
30
30
0
48
48
24
48
48
48
24
33
99
98
95
86
83
81
32
82
91
92
92
57
55
0
(CH3)3COH
Toluene
30
30
a Unless otherwise noted, the reaction was carried out using 1.0 mmol of Michael acceptors and donors (1:1) in 1.0 ml of solvent. The molar ratio of
acceptor:donor:Ru is 50:50:1 (S/C ¼ 50).
b Isolated yield after flash chromatography on the silica gel.
c Determined by 13C NMR of corresponding ketals derived from the denitration product and (2R; 3R)-butanediol. The absolute configuration was
not determined.

1380
T. Ikariya et al. / Journal of Organometallic Chemistry 689 (2004) 1377–1381
Rn
nm; retention time for the four stereoisomers: (major)
22.8 min (minor) 29.4 min (major) 35.8 min (minor) 41.8
Ts
N
Ts
N
Rn
25
Ar
Ar
Ar
Ar
K
eq
min. ½aꢁD – 83.5(c 1.35 CHCl3). MS (EI, 70 eV) 198
+ CH R(CO CH )
Ru
Ru
2
2
3
(Mþ). Absolute configuration of cyclopentanone ring
was determined by converting to a known diketone,
CO CH
2
3
N
H
N
H
C
H
H
R
1
29
Ar = C H
6
5
5
(R)-3-(2-oxyopropyl)cyclopentanone. ½aꢁD – 79.1 (c 0.5
benzene) (lit. ½aꢁD – 70.6 benzene), 92% ee (R) [13]).
Rn
2.4. 3-[(acetyl)(tert-butoxycarbonyl)methyl]-1-cyclo-
pentanone 4c
Ts
N
O
4
Ar
Ar
Ru
C
N
Obtained as a 1:1 mixture of two diasteromers with a
1
H
H
single stereogenic center at the cyclopentanone ring. H
NMR (300 MHz, CDCl3): d 1.47 (s, 9H), 1.49 (s, 9H),
1.63–2.02 (m, 2H), 2.24 (S, 3H), 2.26 (S, 3H), 2.16–2.34
(m, 8H), 2.45 (d, J ¼ 7:3 Hz, 1H), 2.50 (d, J ¼ 7:3 Hz,
1H), 2.81–2.87 (m, 2H), 3.32 (d, J ¼ 4:9 Hz, 1H), 3.35
(d, J ¼ 4:9 Hz, 1H). 13C NMR (75 MHz, CDCl3): d
27.13, 27.62, 27.76, 27.80, 28.95, 29.17, 35.63, 35.71,
37.95, 38.12, 42.55, 42.99, 65.76, 65.96, 82.49, 82.52,
Scheme 5.
Fourier transform spectrometer with tetramethylsilane
as an internal standard. High performance liquid chro-
matography (HPLC) analysis was performed on a
JASCO PU-980 or JASCO PU1580 equipped with a
UV-970 or UV-1510 detector. Optical rotation was
measured with a JASCO DIP-370 digital polarimeter.
25
167.44, 167.50, 201.64, 201.77, 217.30. ½aꢁD – 50.6(c 1.10
CHCl3). MS (CI, 70 eV) 241(M+1).
The enantiomeric excess was determined by the rela-
tive intensities of diastereomeric ketals obtained by
the reaction of (R)-3-(2-oxyopropyl)cyclopentanone, the
decarboxylation adduct of 3-[(acetyl)(tert-butoxycar-
bonyl)methyl]-1-cyclopentanone, with (2R,3R)-butane-
diol catalyzed by p-TsOH in toluene [14]. Ketal of (R)-3-
(2-oxyopropyl)cyclopentanone with (2R,3R)-butanediol,
13C NMR (75 MHz, CDCl3): d 16.47, 16.87, 17.04, 17.14,
26.08, 31.24, 33.02, 37.28, 45.56, 46.08, 77.82, 78.05,
78.20, 78.49, 117.03. Ketal of (S)-3-(2-oxyopropyl)
cyclopentanone with (2R,3R)-butanediol, 13C NMR
(75 MHz, CDCl3): d 16.46, 16.82, 17.06, 17.13, 25.98,
31.68, 33.47, 37.93, 45.87, 45.92, 77.80, 78.05, 78.07,
78.45, 116.94. Absolute configuration of cyclopentanone
ring was determined by converting to a known diketone,
2.2. Typical experimental procedure for Michael reactions
of cyclic enone with methyl acetoacetate catalyzed by
Ru[(1R,2R)-N-(p-toluenesulfonyl)-1,2-diphenylethylene-
diamine](hexamethylbenzene)
Methyl acetoacetate (108 ll, 1.0 mmol), 2-cyclopen-
tenone (84 ll, 1.0 mmol), and toluene (1.0 ml) were added
to Ru[(1R,2R)-N-(p-toluenesulfonyl)-1,2-diphenylethy-
lenediamine] (hexamethylbenzene) (12.6 mg, 0.02 mmol)
and the mixture was degassed by freeze–thaw cycles. The
mixture was stirred at 40 °C for 24 h, then was evaporated
with a vacuum pump and purified with flash column
chromatography (silica gel, eluent:hexane/acetone ¼ 9/1)
to give 3-[(acetyl)(methoxycarbonyl)methyl]-1-cyclopen-
tanone 4b with 91% ee with a single stereogenic center at
the cyclopentanone ring in 99% isolated yield.
29
(R)-3-(2-oxyopropyl)cyclopentanone. ½aꢁD – 77.6 (c 0.5
CHCl3) (lit. ½aꢁD – 70.6 benzene), 92% ee (R) [13]).
2.5. 3-(diacetylmethyl)-1-cyclopentanone 4e
2.3. 3-[(acetyl)(methoxycarbonyl)methyl]-1-cyclopen-
tanone 4b
1H NMR (300 MHz, CDCl3): d 1.30–1.41 (m, 1 H),
1.56–1.65 (m, 1H), 2.04 (s, 3H), 2.05 (s, 3H), 1.95–2.22
(m, 4H), 2.70–2.78 (m, 1H), 3.51 (d, J ¼ 10:5 Hz, 1H).
13C NMR (75 MHz, CDCl3): d 27.02, 29.12, 29.39,
35.79, 37.51, 42.19, 74.00, 202.27, 202.48, 216.28.
Obtained as a 1:1 mixture of two diastereomers with a
1
single stereogenic center at the cyclopentanone ring. H
NMR (300 MHz, CDCl3): d 1.52–1.65 (m, 2H), 1.84 (dd,
J ¼ 10:5 Hz, 18.0 Hz, 1H), 1.96 (dd, J ¼ 11:0 Hz, 17.9
Hz, 1H), 2.13–2.51 (m, 8H), 2.26 (s, 3H), 2.28 (s, 3H),
2.81–2.93 (m, 2H), 3.48 (d, J ¼ 5:1 Hz, 1H), 3.52 (d,
J ¼ 4:6 Hz, 1H), 3.76 (s, 3H), 3.78 (s, 3H). 13C NMR (75
MHz, CDCl3): d 27.11, 27.23, 29.07, 29.34, 35.54, 35.62,
37.73, 37.86, 42.43, 42.59, 52.34, 63.90, 64.19, 168.55,
168.65, 201.21, 201.37, 216.71, 216.72.
2.6. Typical experimental procedure for the reaction of 2-
cyclopentenone with ethyl nitroacetate catalyzed by
Ru[(1R,2R)-N-trifluoromethanesulfonyl-1,2-diphenylethyl-
enediamine](hexamethylbenzene)
HPLC separation conditions: CHIRALPAK AS (4.6
mm i.d. ꢀ 250 mm); eluent, hexane/IPA ¼ 80/20; flow
rate 1.0 ml/min; temperature 30 °C; detection UV 210
Ethyl nitroacetate (111 ll, 1.0 mmol), 2-cyclopente-
none (84 ll, 1.0 mmol), and t-butyl alcohol (1.0 ml) were
added to Ru[(1R,2R)-N-trifluoromethanesulfonyl-1,2-

T. Ikariya et al. / Journal of Organometallic Chemistry 689 (2004) 1377–1381
1381
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diphenylethylenediamine](hexamethylbenzene) (12.0 mg,
0.02 mmol) and the mixture was degassed by freeze-
thaw cycles. The mixture was stirred at 30 °C for 24 h,
then evaporated with a vacuum pump and purified with
flash column chromatography (silica gel, eluent:hexane/
ethyl acetate ¼ 5/1) to give 3-[(ethoxycarbonyl)(ni-
tro)methyl]-1-cyclopentanone 4f with 91% ee as 1:1
mixture of two diasteromers with a single stereogenic
center at the cyclopentanone ring in 98% isolated yield.
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Obtained as a 1:1 mixture of two diastereomers with a
1
single stereogenic center at the cyclopentanone ring. H
NMR (300 MHz, CDCl3): d 1.32 (t, J ¼ 7:1 Hz, 3H),
1.34 (t, J ¼ 7:1 Hz, 3H), 1.76–1.88(m, 2H), 2.07–2.61 (m,
10H), 3.15–3.18 (m, 2H), 4.31 (q, J ¼ 7:1 Hz, 2H), 4.34
(q, J ¼ 7:1 Hz, 2H), 5.13 (d, J ¼ 2:7 Hz, 1H), 5.16 (d,
J ¼ 2:2 Hz, 1H). 13C NMR (75 MHz, CDCl3): d 13.64,
13.67, 25.41, 26.17, 37.12, 37.36, 37.70, 40.87, 41.39,
63.07, 63.10, 90.68, 90.89, 163.13, 163.20, 214.66, 214.85.
[a]2D4 – 78.5(c 1.5 CHCl3). MS (CI, 70 eV) 216 (M+1).
The enantiomeric excess was determined by the rel-
ative intensities of diastereomeric ketals obtained by the
reaction of the denitration product with (2R,3R)-bu-
tanediol catalyzed by p-TsOH in toluene. Major ketal
compound: 13C NMR (75 MHz, CDCl3): d 14.16, 16.76,
17.03, 30.11, 34.08, 37.68, 40.17, 44.26, 60.06, 78.18,
116.64, 172.68. Minor ketal compound: 13C NMR (75
MHz, CDCl3): d 14.16, 16.86, 17.03, 29.71, 33.65, 37.29,
40.35, 43.93, 60.06, 78.30, 116.64, 171.92.
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Acknowledgements
This work was financially supported by a Grant-in-
aid from the Ministry of Education, Science, Sports and
Culture of Japan (No. 14078209) and partially sup-
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