Asymmetric hydrogenation reactions using a practical in situ generation of chiral ruthenium-diphosphine catalysts from anhydrous RuCl3

Article abstract of DOI:10.1016/S0040-4020(01)00067-9

A very simple in situ preparation of chiral ruthenium-diphosphine catalyst from anhydrous RuCl₃ is reported. Prochiral C = O and C = C bonds have been reduced with high enantioselectivities via ruthenium-catalyzed hydrogenation.

Full text of DOI:10.1016/S0040-4020(01)00067-9

TETRAHEDRON
Pergamon
Tetrahedron 57 (2001) 2563±2568
Asymmetric hydrogenation reactions using a practical in situ
generation of chiral ruthenium±diphosphine catalysts
from anhydrous RuCl3
p
Ã
J. Madec, X. P®ster, P. Phansavath, V. Ratovelomanana-Vidal and J.-P. Genet
Á Â Â
Laboratoire de Synthese Selective Organique et Produits Naturels, UMR 7573, Ecole Nationale Superieure de Chimie de Paris,
11 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France
Received 1 August 2000; revised 29 September 2000; accepted 2 October 2000
AbstractÐA very simple in situ preparation of chiral ruthenium±diphosphine catalysts from anhydrous RuCl3 is reported. Prochiral CvO
and CvC bonds have been reduced with high enantioselectivities via ruthenium-catalyzed hydrogenation. q 2001 Elsevier Science Ltd. All
rights reserved.
1. Introduction
complexes 5 Ru(RCp)(Binap)Cl (RˆH, CH3)20 from
Ru(RCp)(PPh3)2Cl have been reported. The Ru-based
precursors to the complexes 2, 3, 4 and 5, respectively,
Ru(COD)(h3-methylallyl)2, [RuX2(arene)]2, Ru(acac)3 and
Ru(RCp)(PPh3)2Cl are all prepared from RuCl3´nH2O. Our
contribution to this ®eld has been the development of
general synthetic methods for the preparation of chiral
ruthenium(II)-catalysts bearing various chiral diphosphines.
These preparations include the direct use of the polymeric
Transition metal-catalyzed enantioselective hydrogena-
tion1±9 of prochiral substrates represents one of the most
ef®cient method for the preparation of chiral building blocks
of biological interest. In this ®eld, several groups have
focused their research on the development of Ru-based
hydrogenation catalysts. The utility of RuCl3´nH2O as a
starting material for multistep preparations of ef®cient
ruthenium-chiral diphosphine10 catalysts is shown in
Scheme 1.26 Ikariya et al.11 have prepared a dinuclear
complex formulated as Ru2Cl4(BINAP)2(NEt3) (later eluci-
dated as [NH2Et2][{RuCl(PpP)}2(m-Cl)3] 112 with PpPˆ(R)-
p-MeO-BINAP) in a two-step sequence from RuCl3´nH2O
via the polymeric complex [RuCl2(COD)]n (CODˆcyclo-
octa-1,5-diene) in 85% yield. The ®rst mononuclear hexa-
coordinate ruthenium complex Ru(O2CR)2(PpP) 2 bearing
BINAP ligand has been reported by Noyori et al.13,14 and
prepared from Ikariya's complex 111 by treatment with
sodium acetate. An improved procedure15 for the prepa-
ration of 2 has been described starting with [RuCl2(ben-
zene)]2 (obtained from RuCl3´nH2O).
21
complex [RuCl2(COD)]n as well as the chiral catalysts
Ru(PpP)(h3-methylallyl)2 622,23 used for the preparation of
ruthenium complexes 7 de®ned by the empirical formula
Ru(PpP)X2.23 These catalysts have been also prepared
from ruthenium dicarboxylato complexes 2 by treatment
with HX or (CH3)3SiI.24 In situ-generated catalysts 7 have
been more conveniently synthesized from a 1:1 mixture of
Ru(COD)(h3-methylallyl)2 and the appropriate chiral
diphosphine by treatment with 1.5±2.2 equiv. of HX
(XˆCl, Br).23 More recently, a new cationic monohydride
8
[Ru((R,R)-Me-DuPHOS)(H)(h6-
ruthenium complex
1,3,5-cyclooctatriene)](BF4) has been discovered and
ef®ciently used for the industrial hydrogenation of tetra-
substituted ole®ns.25
Ruthenium dicarboxylato complexes 2 Ru(O2CR)2(PpP)
(RˆMe,CF3,Ar) have also been prepared from Ru-
(COD)(h3-methylallyl)2, in 2±4 steps.16,17 Treatment of
[RuX2(arene)]2 with 1 equiv. of (S)-BINAP in a 8:1 mixture
of ethanol and benzene afforded the cationic complexes 3.18
In situ preparation of Ru(acac)2[(S)-BINAP)] 419 from
Ru(acac)3 and synthesis of air stable BINAP-ruthenium
As demonstrated above, RuCl3´nH2O is the readily available
source for most of the multisteps preparations of chiral
diphosphine ruthenium complexes. Our continuing interest
in the practical preparations of chiral Ru(II)-based catalysts
prompted us to look for a more convenient way to obtain
these complexes directly from anhydrous RuCl3 avoiding,
therefore, the synthesis and handling of 1±7. To our
knowledge, only one example of the use of RuCl3´nH2O
has been described in the asymmetric hydrogenation of
the amine salt of 2-(4-isobutylphenyl)propenoic acid with
fair enantioselectivity.19 We report herein, the direct use of
Keywords: ruthenium-diphosphine; enantioselective hydrogenation.
Corresponding author. Tel.: 133-1-44-27-67-43; fax: 133-1-44-07-10-
62; e-mail: genet@ext.jussieu.fr
p
0040±4020/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(01)00067-9

2564
J. Madec et al. / Tetrahedron 57 (2001) 2563±2568
Scheme 1.
anhydrous RuCl3 as a source of in situ-generated chiral
ruthenium complexes for the asymmetric hydrogenation of
prochiral ketones and ole®ns.
conversion could be achieved in only 4 h and at 4 bar to
yield (S)-10 in 99% e.e. (entry 3) (Scheme 2).
In contrast to our method, which involves directly Ru(III)-
precatalyst, most of the enantioselective ruthenium-
mediated hydrogenation reactions proceed from Ru(II)-
precatalysts. It is reasonable to expect that Ru(III) is
reduced to Ru(II) species under the reaction conditions by
either hydrogen or the chiral phosphine. Tentative eluci-
dations for the identi®cation of the catalytic species by 31P
NMR were unsuccessful since many signals were detected
whose assigment proved to be dif®cult. Hydrogenation reac-
tions of a variety of b-keto esters were then conducted under
4 bar and 508C in methanol or ethanol using 1 mol% of the
chiral ruthenium catalyst and on a 1 mmol scale. As can be
seen, very high enantioselectivities and complete conver-
sions have been achieved under these reaction conditions
2. Results and discussion
A preliminary evaluation of the best experimental con-
ditions has been studied in the hydrogenation of methyl
acetoacetate 9 as standard substrate (Table 1). Hydro-
genation of 9 has been ®rst conducted at 50 bar and 508C
for 17 h using 1 mol% of anhydrous RuCl3 and (S)-MeO-
BIPHEP as ligand. Under these conditions (S)-methyl
3-hydroxybutyrate 10 was obtained in quantitative yield
and excellent enantiomeric excess (entry 1). Further studies
involving this catalytic system revealed that complete
Table 1. Ru-catalyzed hydrogenation of methyl acetoacetate 9 using RuCl31(S)-MeO-BIPHEP
Entry
Catalyst
Conditions
Conv. (%)
e.e.a (conf.)
P (bar)
T (8C)
t (h)
1
2
3
RuCl31(S)-MeO-BIPHEP
RuCl31(S)-MeO-BIPHEP
RuCl31(S)-MeO-BIPHEP
50
4
4
50
50
50
17
17
4
100
100
100
99 (S)
99 (S)
99 (S)
a
Enantiomeric excesses were determined by chiral GC analysis (Lipodex A column).
Scheme 2.

J. Madec et al. / Tetrahedron 57 (2001) 2563±2568
2565
Table 2. Ruthenium-catalyzed hydrogenation of b-keto esters using RuCl3 and chiral diphosphine
Entry
Substrate
Ligand PpP
Conditionsa,b
e.e.c (%)
Conf.
P (bar)
T (8C)
T (h)
1
2
(S)-Tol-BINAP
(R,R)-Me-DuPHOS
4
50
50
80
24
72
99
85
(S)-10
(R)-10
3
4
5
(S)-MeO-BIPHEP
(S)-MeO-BIPHEP
(R)-MeO-BIPHEP
4
4
4
50
50
50
24
24
24
99
99d
98e
(S)-12
(S)-14
(R)-16
6
7
(S)-MeO-BIPHEP
(R,R)-Me-DuPHOS
4
50
50
80
24
72
99
89
(R)-18
(S)-18
8
(S)-MeO-BIPHEP
10
30
40
76
(R)-20
(R)-22
9
10
(S)-MeO-BIPHEP
(S)-MeO-BIPHEP
4
4
50
80
24
24
88
95
11
12
13
(S)-MeO-BIPHEP
(S)-MeO-BIPHEP
(S)-MeO-BIPHEP
4
4
4
50
80
120
24
24
6
57
76
92
(R)-24
(R)-26
14
(S)-MeO-BIPHEP
10
120
6
50
a
1 mol% of chiral Ru-catalyst was used.
Reaction times are not optimized.
Enantiomeric excesses were determined by chiral GC analysis (Lipodex A column) except for 14 and 16.
Enantiomeric excess was determined on the benzoyl derivative by chiral HPLC (Daicel Chiralcel OJ column).
Enantiomeric excess was determined on the (S)-2-acetoxypropionyl derivative by GC analysis (DB 1701 column).
b
c
d
e
(Table 2). The test substrate methyl acetoacetate was
reduced in 99% e.e. using (S)-Tol-BINAP (entry 1) while
a lower enantiomeric excess was obtained with (R,R)-Me-
DuPHOS and the reaction required higher pressure,
temperature and reaction time for complete conversion
(85% e.e., entry 2) (Scheme 3).
508C, 24 h) was applied to b-keto esters bearing linear alkyl
chains such as methyl 3-oxo-pentanoate 11, ethyl 3-oxo-
hexanoate 13 and methyl 3-oxo-octadecanoate 15 to afford,
respectively, the corresponding b-hydroxy esters (S)-12,
(S)-14 and (R)-16 with high levels of enantioselectivity
(entries 3±5). Hydrogenation of a branched alkyl-substi-
tuted b-keto ester such as ethyl isobutyrylacetate 17 yielded
(R)-ethyl 3-hydroxy-4-methylpentanoate 18 in 99% e.e.
using (S)-MeO-BIPHEP (entry 6) and in lower enantiomeric
excess with (R,R)-Me-DuPHOS (89% e.e., entry 7). Methyl
3-oxo 3-(2-thiophenyl) propanoate 19 has been reduced to
the corresponding b-keto ester (R)-20 at 10 bar and 308C in
76% e.e. (entry 8). Hydrogenation of ethyl benzoylacetate
21 catalyzed by (S)-MeO-BIPHEP/RuCl3 under the stan-
dard conditions yielded (R)-22 in 88% e.e. (entry 9), while
the reaction at 808C afforded the desired alcohol with higher
enantioselectivity (95% e.e., entry 10). This temperature
effect was also observed in the reduction of ethyl 4-chloro-
acetoacetate 23 to (R)-24 since increase of the temperature
from 50 to 1208C allowed a much better level of enantio-
selectivity (57±92% e.e., entries 11±13). Finally, reduction
The standard set of reaction conditions (4 bar of hydrogen,
Scheme 3.

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J. Madec et al. / Tetrahedron 57 (2001) 2563±2568
Table 3. Ru-catalyzed hydrogenation of prochiral CvO and CvC bonds using RuCl31(S)-MeOBIPHEP
Entry
Substrate
Ligand PpP
Conditions
e.e. (%)
Conf.
P (bar)
T (8C)
t (h)
1
2
3
(S)-MeO-BIPHEP
(S)-MeO-BIPHEP
(S)-MeO-BIPHEP
4
4
4
50
80
99
17
17
17
69
80
88a
(S)-28
4
5
6
(S)-MeO-BIPHEP b
(S)-MeO-BIPHEP
(S)-MeO-BIPHEP
(S)-MeO-BIPHEP
4
4
5
4
50
r.t.
50
50
48
17
65
17
94c
99d
98e
79f
(S)-30
(S)-32
(R)-34
(S)-36
7
a
Enantiomeric excess was determined by chiral GC analysis (Lipodex A column).
2 mol% of chiral Ru-catalyst
b
c
d
e
f
Enantiomeric excess was determined by 1H NMR (400 MHz) with Eu(tfc)3
Enantiomeric excess was determined by chiral GC analysis (Chiralsil Val-2 column).
Enantiomeric excess was determined by chiral GC analysis (Megadex 5 column).
Enantiomeric excess was determined by chiral HPLC analysis (Chiralcel OD-H column).
of ethyl 4,4,4-tri¯uoro-3-oxo butanoate 25 at 1208C
also achieved for the ruthenium-mediated hydrogenation
of b-keto phosphonates such as dimethyl 2-oxoheptyl-
phosphonate 31 and diethyl 2-oxo 2-phenylphosphonate
33 leading, respectively, to (S)-32 (99% e.e. at room
temperature, entry 5) and (R)-34 (98% e.e., entry 6).
Comparable yields and enantioselectivities were observed
using the in situ generated Ru(II)-complexes in the hydro-
genation of compounds 2931 and 31.32 Finally, (Z)-methyl
a-acetamidocinnamate 35 was hydrogenated to the corre-
sponding (S)-N-acetylphenyl alanine methyl ester 36 in 79%
e.e. (entry 7).
afforded (R)-26 in 50% e.e., a value which compares favor-
ably with those previously reported.27,28 The above results
are comparable with those obtained using the chiral Ru(II)-
catalysts prepared in situ.23,29,30
Expansion of these studies to other substrates revealed that
the (S)-MeO-BIPHEP/RuCl3 catalyst provides a general,
enantioselective method for the reduction of functionalized
ketones and alkenes (Table 3). Hydrogenation of methyl
phenylglyoxylate 27 was carried out under the usual con-
ditions and afforded (S)-mandelate 28 in a moderate 69%
e.e. (entry 1). However, when conducted at 998C, the hydro-
genation reaction yielded quantitatively (S)-28 in 88% e.e.
(entry 3). This (S)-MeO-BIPHEP/RuCl3 system was also
found to serve as an excellent catalyst for the hydrogenation
of b-keto sulfones. Thus, (S)-1-(phenylsulfonyl)tridecan-2-
ol 30 has been prepared with high enantioselectivity (94%
e.e., entry 4). Excellent enantiofacial discrimination was
We have previously reported that in situ ruthenium-
complexes prepared from Ru(COD)(h3-methylallyl)2 were
ef®cient catalysts at 1 atm pressure of hydrogen29 and it was
therefore of interest to study the ef®ciency of the RuCl3/(S)-
MeO-BIPHEP system under these very mild conditions. As
shown in Table 4, we found that the above catalytic system
appears less reactive at atmospheric pressure and 508C
Table 4. Comparative study between RuCl31(S)-MeO-BIPHEP and in situ RuBr2[(S)-MeO-BIPHEP]
Entry
Catalyst
Conditions
Conv. (%)
e.e. (conf.)
P (bar)
T (8C)
t (h)
1
2
3
4
5
6
7
2 mol% RuBr2[(S)-MeO-BIPHEP]
2 mol% [RuCl31(S)-MeO-BIPHEP]
2 mol% [RuCl31(S)-MeO-BIPHEP]
0.1 mol% [RuCl31(S)-MeO-BIPHEP]
0.1 mol% RuBr2[(S)-MeO-BIPHEP]
0.03 mol% RuBr2[(S)-MeO-BIPHEP]
0.1 mol% [RuCl31(S)-MeO-BIPHEP]
1
1
4
4
4
50
50
Rt
50
50
50
50
48
48
48
48
48
24
48
100
0
100
85
100
100
100
99(S)
±
98(S)
99(S)
99(S)
99(S)
99(S)
4
50

J. Madec et al. / Tetrahedron 57 (2001) 2563±2568
2567
Scheme 4.
compared to the in situ generated Ru(II)-catalysts. (S)-methyl
3-hydroxybutyrate 10 was quantitatively obtained in
enantiomerically pure form using RuBr2[(S)-MeO-BIPHEP]
(entry 1) while no conversion was observed under these
conditions with the present catalytic system (entry 2).
However, at higher pressure of hydrogen (4 bar), RuCl3/
(S)-MeO-BIPHEP furnished quantitatively (S)-10 at room
temperature in 98% e.e. (entry 3). These asymmetric hydro-
genations are of potential industrial importance for the
production of chiral compounds as demonstrated by Noyori9
and us.25,33±35 Thus it was of interest to carry out these
reactions with low catalyst loading. The RuCl3/(S)-MeO-
BIPHEP system was found to display lower activity than
the corresponding homogeneous species prepared in situ.
The reaction using (S)-MeO-BIPHEP/RuCl3 with a cata-
lyst/substrate ratio of 1:1000 at 4 bar and 508C furnished
(S)-methyl 3-hydroxybutyrate 10 in only 85% yield (entry
4) while complete conversion and 99% e.e. were obtained
with the in situ RuBr2[(S)-MeO-BIPHEP] complex under
the same conditions (entry 5) or with a lower catalyst/
substrate ratio of 1:3000 (entry 6).36 However, when the
reaction was performed at higher pressure of hydrogen
(50 bar) the former system furnished quantitatively (S)-10
in 99% e.e. (entry 7) (Scheme 4).
chemicals) were placed in a Schlenk tube and degassed
by three cycles of vacuum/argon at room temperature.
Degassed methanol (2 mL) and the prochiral ketone
(1 mmol) were added. The resulting mixture was
placed under the desired hydrogen pressure and
temperature until complete conversion. All reactions were
run on a 1 mmol scale and led to the corresponding
functionalized alcohols in quantitative yields except
when speci®ed.
Acknowledgements
We are grateful to Dr R. Schmid (Hoffmann La Roche)
for the generous gift of (R) and (S)-MeO-BIPHEPˆ(R)-
(1)-6,60-dimethoxy-2,20-bis(diphenylphosphino)-1,10-bi-
phenyl and to Dr P. Savignac for samples of b-keto
phosphonates.
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Á