Enantioselective ruthenium-mediated hydrogenation: Developments and applications

Article abstract of DOI:10.1016/S0022-328X(98)00680-9

A general preparation of chiral ruthenium(II) catalysts and the homogeneous enantioselective hydrogenation of prochiral olefins and keto groups are presented. Some applications to the synthesis of biologically active compounds are reported.

Full text of DOI:10.1016/S0022-328X(98)00680-9

Journal of Organometallic Chemistry 567 (1998) 163–171
Enantioselective ruthenium-mediated hydrogenation: developments and
applications
Virginie Ratovelomanana-Vidal, Jean-Pierre Geneˆt *
Laboratoire de Synthe`se Se´lecti6e Organique et Produits Naturels associe´ au CNRS (UMR 7573), 11 rue P. et M Curie,
75231 Paris Cedex 5, France
Received 12 December 1997; received in revised form 2 February 1998
Abstract
A general preparation of chiral ruthenium(II) catalysts and the homogeneous enantioselective hydrogenation of prochiral olefins
and keto groups are presented. Some applications to the synthesis of biologically active compounds are reported. © 1998 Elsevier
Science S.A. All rights reserved.
Keywords: Ruthenium; Catalysts; Hydrogenation; Dynamic kinetic resolution
1. Introduction
2. Transition metal catalysts
Asymmetric catalysis [1–12] is now an important
part in drug design and development. A great number
of optically active compounds are synthesized using
asymmetric catalytic routes and among them asymmet-
ric hydrogenation provides a practical access to chiral
building blocks of biological interest. One concern of
the synthetic organic chemist is to develop new asym-
metric catalytic methodologies using transition metal
complexes. For a few years, we have been interested in
homogeneous enantioselective hydrogenation using chi-
ral Ru(II) catalysts. We have developed a general syn-
thesis of chiral Ru(II) catalysts compatible with a wide
variety of chiral ligands. We report herein an overview
of the utility of this method for highly enantioselective
hydrogenation of olefins and functionalized keto
groups as well as some applications to the synthesis of
pharmaceutical intermediates and biologically active
compound.
2.1. Chiral ligands
In the 1970’s, Kagan, Knowles and others discovered
efficient ligands for Rh-mediated hydrogenation of de-
hydroaminoacids. Since this time, a great number of
diphosphines have been prepared and used as chiral
ligands [13,14] for homogeneous catalysis. The most
typical ligands are chiral bidentate diphosphines such as
1,2 and 1,4 diphosphines which can be classified related
to their structures: those bearing chirality on the carbon
skeleton (CHIRAPHOS, CBD, PROPHOS, DIOP, Me
or Et-DuPHOS, i-Pr BPE …) including atropoisomeric
type (BINAP, MeO-BIPHEP, BIPHEMP, BICHEP …)
and on the phosphorus center (DIPAMP, BIPNOR
[15]). Chiral bidentate ligands such as bisaminophos-
phinites (BPPM, CAP) and ferrocenyl phosphines
(BPPFA, BPPFOH …) have also been developed.
Among these chiral diphosphines, only few useful 1,3-
substituted diphosphines have been prepared
(SKEWPHOS). Some diphosphines used in asymmetric
catalysis are shown in Fig. 1.
* Corresponding author. Fax:
genet@ext.jussieu.fr
+33 44071062; e-mail:
0022-328X/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.
PII S0022-328X(98)00680-9

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2.2. Chiral ruthenium(II) catalysts
The BINAPRu(II) complexes were highly efficient
catalysts for the asymmetric hydrogenation of a large
variety of prochiral substrates [23] including olefins,
carbonyl groups and imines.
Heiser and coworkers [24] described the synthesis of
atropoisomeric diphosphine ruthenium dicarboxylato
complexes P*PRu(OCOR)2 (R=CF3 and CH3) using
the (COD)Ru(p3-(CH2)2CCH3)2 as starting material.
The first chiral ruthenium catalyst Ru2Diop3Cl4 pre-
pared from RuCl2(PPh3)3 and excess of DIOP was
discovered in 1975 by James [16]. Later, Ikariya re-
ported the synthesis of
a
dinuclear catalyst
Ru2Cl4Binap2, NEt3 2 [17] obtained from [RuCl2(cod)]n
with (R) or (S)-BINAP and triethylamine in toluene at
110°C. This complex was used to prepare the first
mononuclear catalyst BinapRu(OAc)2 3 [18,19] synthe-
sized by Noyori in 1986.
Some other syntheses of chiral Ru(II) catalysts con-
taining atropoisomeric diphosphines have also been
described [25].
We have focused our efforts on the development of a
new general and practical route to ruthenium com-
plexes of any kind of diphosphines [26–28]. Our inves-
tigations afforded the first general synthesis of chiral
Ru(II) catalysts, 5 and 6, prepared from the commer-
cially available (COD)Ru(p3-(CH2)2CCH3)2 4 and a
wide variety of diphosphines including P-chiral species.
This synthesis was recently improved and the BI-
NAPRu(II) dicarboxylate complex was obtained more
conveniently by sequential treatment of [RuCl2
(benzene)2]2 with BINAP in DMF at 100°C followed by
reaction of sodium acetate at room temperature [20].
Some other BinapRu(II) catalysts have also been
synthesized under milder conditions [21,22].
Fig. 1. Some representative diphosphines.

V. Rato6elomanana-Vidal, J.-P. Geneˆt / Journal of Organometallic Chemistry 567 (1998) 163–171
165
This general method was recently improved and the
Ru-catalysts 6 were synthesized in situ [29] under milder
conditions in a one step reaction from (COD)Ru(p3-
(CH2)2CCH3)2 4.
50°C under 3 bars of hydrogen pressure. The in situ
(R)-BinapRuBr2 and (R)-MeO–BiphepRuBr2 afforded
(R)-methyl succinic acid with very high enantio-
selectivity.
A large variety of chiral Ru(II) catalysts have been
prepared using this methodology, including the
DuPHOSRuBr2 [29] and SkewphosRuBr2 complexes
[30]
3.1.2. Asymmetric hydrogenation of Naproxen
precursor
An application of this methodology to the synthesis
of the antiinflammatory drug Naproxen was achieved
with good selectivity (87–90%).
3. Homogeneous asymmetric hydrogenation
3.1. Asymmetric hydrogenation of olefins [29]
The Ru-catalyzed hydrogenation of a number of
h,i-unsaturated acids was considered. The hydrogena-
tion of tiglic acid is discussed hereafter as a representa-
tive example. The reaction was best carried out in
methanol under 3–4 bars of hydrogen pressure. The
different chiral Ru-catalysts performed enantioselective
hydrogenation with degrees of selectivities which de-
pended on the ligands. The best results were obtained
with atropoisomeric ligands i.e. BINAP and MeO–
BIPHEP. The C2 symmetric bisphospholanobenzene
Me–DuPHOS and CBD gave good selectivities (80 and
70% e.e.) while DIOP and DIPAMP were less efficient
(51 and 25% e.e., respectively) (Table 1).
3.1.3. Asymmetric hydrogenation of N-acetyl
dehydroaminoacids
In a similar manner, h-aminoacids derivatives were
easily obtained from the corresponding amido substi-
tuted olefins in good yields and enantiomeric excesses.
3.1.1. Asymmetric hydrogenation of itaconic acid
Hydrogenation of itaconic acid was carried out at
Table 1
3.1.4. Asymmetric hydrogenation of allylic alcohol
The hydrogenation of an allylic alcohol, possessing a
chiral azetidinone unit was achieved diastereoselectively
and in good yield by using the in situ (R)-MeO–
BiphepRuBr2, leading to the corresponding 1i-methyl-
carbapenem intermediate.
Asymmetric hydrogenation of tiglic acid
3.2. Asymmetric hydrogenation of ketones
Halogen-containing Ru(II) complexes prepared in
situ were very efficient catalysts for the enantioselective
hydrogenation of various functionalized ketones

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Table 2
3.2.1.2. Unsaturated i-ketoesters. Furthermore, i-ke-
toesters having an unsaturated alkyl chain were
chemoselectively hydrogenated to optically pure unsat-
urated i-hydroxyesters under controlled reaction con-
ditions (4–6 bars, 40–80°C) and very short reaction
times (Table 3).
Asymmetric hydrogenation of i-ketoesters
3.2.2. Asymmetric hydrogenation of
i-ketophosphonates [32]
The in situ prepared (P*P)RuBr2 catalyzed the enan-
tioselective hydrogenation of various i-ketophospho-
nates in alcoholic media to give the corresponding
i-hydroxyphosphonic esters with excellent enantiofa-
cial discrimination either at atmospheric pressure or
100 bars of hydrogen with temperature varying from
r.t. to 50°C (Table 4).
3.2.3. Asymmetric hydrogenation of
i-ketothiophosphonates [32]
The asymmetric hydrogenation of i-ketothiophos-
phonates was performed in methanol under 10–100
bars of hydrogen pressure at room temperature to
afford i-hydroxythiophosphonates with very good se-
lectivities (e.e. 90–94%) (Table 5).
3.2.1. Asymmetric hydrogenation of i-ketoesters
3.2.1.1. Saturated i-ketoesters. The asymmetric hydro-
genation of i-ketoesters to i-hydroxyesters under at-
mospheric pressure with 2% of chiral Ru(II) catalysts
found a wide generality and was applicable to alkyl or
aryl substituted i-ketoesters [31]. At room temperature,
conversion was not complete. At higher temperature
(50–78°C), excellent conversion and high chiral induc-
tion were reached with the in situ halogen containing
Ru(II) catalysts (e.e.=87–99%) (Table 2).
In addition to these examples, a screening of ligands
[29] was performed with the standard substrate methy-
lacetoacetate: the catalysts (R,R)-DiopRuBr2, (S,S)-
CbdRuBr2 prepared in situ provided the i-hydroxy-
butyrate under 10 bars of hydrogen pressure and 80°C
with very poor e.e. (5–22% e.e.). A higher enantiofacial
discrimination was observed using (R)-PROPHOS or
(S,S)-CHIRAPHOS as ligands at 20 bars and 50°C
leading to the i-hydroxyester with selectivity approach-
ing 60%. Interestingly, the bisphospholanobenzene lig-
and Me–DuPHOS afforded i-hydroxybutyrate with
e.e. up to 87%.
Table 3
Asymmetric hydrogenation of unsaturated i-ketoesters
This reaction has been extended to other i-ketoesters
bearing additional functional groups. High optical puri-
ties have been obtained. Some examples are given
below.

V. Rato6elomanana-Vidal, J.-P. Geneˆt / Journal of Organometallic Chemistry 567 (1998) 163–171
167
Table 4
Table 6
Asymmetric hydrogenation of i-ketophosphonates
Asymmetric hydrogenation of phenylthiosulfides
3.2.4. Asymmetric hydrogenation of phenylthiosulfides
[33]
In addition to these experiments with atropoisomeric
chiral ligands, we pointed out that the SkewphosRuBr2
catalyst prepared in situ was also very efficient for
asymmetric hydrogenation of various prochiral ketones
[30](Table 7).
Because organosulfur compounds are widely used in
organic syntheses, the asymmetric hydrogenation of i
and k phenylthio ketones was investigated. All reactions
were carried out with 2% of chiral dibromide rutheniu-
m(II) complexes prepared in situ. The presently re-
ported hydrogenation procedure showed to be
compatible with a divalent sulfur. The chiral alcohols
were obtained in good yields with e.e. ranging from 70
to 97% (Table 6). These enriched phenylthio alcohols
were interesting building blocks for natural product
synthesis [34].
Table 7
Asymmetric hydrogenation with (R,R) or (S,S)-SkewphosRuBr2
Table 5
Asymmetric hydrogenation of i-ketothiophosphonates

168
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A general sense of the enantioselectivity has been
4.1. Cyclic i-ketoesters [38]
proposed by Noyori and coworkers for the BINAP
assisted hydrogenation of substituted ketones [19]b
bearing an heteroatom at the h, i or k position. The
absolute configuration of the hydroxy-bearing position
can be predicted by the absolute configuration of the
atropoisomeric ligand as shown below.
The kinetic dynamic resolution applied to racemic
cyclic i-ketoesters such as 2-substituted-3-oxo car
boxylic esters revealed that the stereochemical course of
the reaction was influenced by the structures of the
substrates: in the case of five membered ring (Table 8,
entry 1), the hydrogenation carried out with the in situ
(R)-BinapRuBr2 proceeded with high diastereo and
enantioselectivity (e.e.=85%) to give the corresponding
(2R,3R) trans products. In contrast, the diastereoselec-
tivity was decreased to some extent (d.e.=47%) by
increasing the ring size of the racemic cyclic i-ketoester
from five to six membered ring by using the
This observation is applicable to any atropoisomeric
ligands such as MeO–BIPHEP or BIPHEMP etc.
4. Dynamic kinetic resolution
Racemic substrates possessing an epimerisable center
such as h-substituted i-ketoesters can be converted to
one single h-substituted i-hydroxyester under con-
trolled hydrogenation conditions. This reaction where
the chiral ruthenium(II) catalyst can select one of the
diastereoisomeric transition states and which resulted in
a single product is the so-called dynamic kinetic
resolution.
Table 8
Dynamic kinetic resolution of cyclic i-ketoesters
The first example of dynamic kinetic resolution of
h-acetamido i-ketoesters was pointed out in 1989
simultaneously by Noyori and our group [35–37].

V. Rato6elomanana-Vidal, J.-P. Geneˆt / Journal of Organometallic Chemistry 567 (1998) 163–171
169
Table 9
temperature (80 bars and 80°C) exclusively in dichloro-
methane with a new type of in situ chiral Ru(II) catalysts
easily prepared by mixing the chiral ligand (BINAP and
MeO–BIPHEP) and (COD)Ru(p3-(CH2)2CCH3)2. The
anti h-chloro i-hydroxyesters were synthesized with very
high diastereoselection when the i-ketoesters were sub-
stituted with a linear alkyl side chain (Table 9, entries 1
and 2, d.e.=99% and 93%) although the diastereoselec-
tion was decreased (Table 9, entry 3, d.e.=70%) with a
ramified side chain. In all cases, enantiomeric excesses
were satisfactory (e.e. up to 98%).
Dynamic kinetic resolution of h-chloro i-ketoesters
The reaction was then applied to h-chloro i-ketoesters
bearing an aromatic side chain under the same reaction
conditions (Table 10), i.e. high pressure and temperature
(80 bars, 80°C) in dichloromethane to afford the corre-
sponding anti chlorhydrins (d.e.=88–97%) with excel-
lent enantio and diastereofacial discrimination (e.e. up to
99%).
5. Synthetic applications
These enantiomerically enriched h-chloro i-hydroxy
esters were precursors for the synthesis of optically
active glycidates [39] which were key intermediates in
the synthesis of the potent channel blockers diltiazem®
and of Taxotere® side chain.
same catalyst (Table 8, entry 2), while enantiomeric
excess of the major (2S,3S) product was very high
(e.e.=91%). Finally, an ideal dynamic kinetic resolution
was achieved in the case of cyclic i-ketoesters containing
the tetralone structure (Table 8, entry 3) under 10 bars
and 80°C with the in situ (R)-MeO–BiphepRuBr2 which
was an extremely effective catalyst: the racemic tetralone
was hydrogenated with an excellent level of enantio and
diastereoselectivity (d.e.=97%, e.e.=95%) to afford the
trans cyclic i-hydroxyester.
Table 10
Dynamic kinetic resolution of h-chloro i-ketoesters
4.2. h-Chloro i-ketoesters [39]
Behavior of the open-chain h-substituted i-ketoesters
was somewhat different from that of the cyclic substrates.
Concerning the h-chloro i-ketoesters, the dynamic ki-
netic resolution was attempted at high pressure and

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Acknowledgements
The sequential ruthenium catalyzed hydrogenation
[29,31] and electrophilic amination [40] was applied to
the synthesis of enantiomerically pure anti N-Boc-h-
hydrazino-i-hydroxyesters.
We would like to thank Dr R. Schmidt (Hoffman La
Following this strategy, some heterocycles were syn-
thesized such as (3S,4S)-4-hydroxy-2,3,4,5-tetrahy-
dropyridazine-3-carboxylic acid [41] which was an
unusual aminoacid constituent of luzopeptin A.
Roche) for a generous gift of (R) and (S)-MeO–
BIPHEP and Dr P. Savignac for samples of i-ke-
tophosphonates and thiophosphonates.
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