Ru-SYNPHOS and Ru-DIFLUORPHOS: Highly efficient catalysts for practical preparation of β-hydroxy amides

Article abstract of DOI:10.1055/s-2005-917089

Ru-SYNPHOS and Ru-DIFLUORPHOS catalysts were efficiently used for the synthesis of a wide variety of chiral β-hydroxy amides via asymmetric hydrogenation of the corresponding β-keto amides. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2005-917089

2478
LETTER
Ru-SYNPHOS® and Ru-DIFLUORPHOS®: Highly Efficient Catalysts for
Practical Preparation of b-Hydroxy Amides
Efficient
C
atalyst
i
s
for
P
r
actical Prepa
h
rationof
b
-H
a
ydroxy
A
mide
s
Touati,a Thouraya Gmiza,a Séverine Jeulin,b Coralie Deport,b Virginie Ratovelomanana-Vidal,*b
Béchir Ben Hassine,*a Jean-Pierre Genet*b
a
Laboratoire de Synthèse Organique Asymétrique et Catalyse Homogène, Faculté des Sciences de Monastir, avenue de l’Environnement,
5019 Monastir, Tunisia
Fax +216(73)500278; E-mail: bechirbenhassine@fsm.rnu.tn
b
Laboratoire de Synthèse Sélective Organique et Produits Naturels, UMR 7573 C.N.R.S., Ecole Nationale Supérieure de Chimie de Paris,
11, rue P. et M. Curie, 75231 Paris Cedex 05, France
Fax +33(1)44071062; E-mail: jean-pierre-genet@enscp.fr; E-mail: virginie-vidal@enscp.fr
Received 22 July 2005
amides 1. Enzymatic reduction of b-keto amides with
Abstract: Ru-SYNPHOS® and Ru-DIFLUORPHOS® catalysts
baker’s yeast6,7 led to 56–98% ee values. Because the
scope of baker’s yeast was limited, the biocatalytic reduc-
were efficiently used for the synthesis of a wide variety of chiral b-
hydroxy amides via asymmetric hydrogenation of the correspond-
tion of 3-oxo-3-phenylpropanamide and 3-oxobutan-
amide derivatives was reported in good yields and
enantioselectivities ranging from 43% to 98%.8 Asym-
metric reduction of acetoacetanilide with a NaBH4-L-tar-
taric acid system was reported.9 An alternative approach
to chiral b-hydroxy amides was the homogeneous catalyt-
ic asymmetric hydrogenation of b-keto amides. To the
best of our knowledge, most of these examples involved
Ru-BINAP as chiral catalysts with a limited number of
substrates. Noyori et al. reported one example of quantita-
tive Ru-BINAP-catalyzed hydrogenation reaction of N,N-
dimethyl 2-oxo-butanamide at 63 atm for 86 hours with
96% ee.10 The Ru-BINAP-promoted hydrogenation of
alkyl-substituted b-ketoamides was achieved with good
level of enantio- and diastereoselectivites.11,12 The synthe-
sis of the chiral fluoxetine intermediate 1 (R = Ph,
R¢ = Me) was described in a moderate 50% yield and
>99.9% ee after repeated recrystallization by using
{RuCl2[(S)-BINAP]} at 200 psi and 100 °C for 18
hours.13 Thus, the development of an efficient and practi-
cal route towards various chiral b-hydroxy amides could
be useful since they are precursors of optically pure 1,3-
aminoalcohols which have found a widespread use in
organic chemistry as chiral units of synthetic utility.14
ing b-keto amides.
Key words: catalysts, hydrogenation, enantioselectivity, ruthenium
Chiral b-hydroxy amides 1 are useful building blocks for
the synthesis of biologically active compounds.1 Exam-
ples include (R)-fluoxetine (2)2 used as antidepressant, N¢-
(3-hydroxy-12-methyltridecanoyl)nornicotine (3)3 as se-
lective toxic agent against the larvae of Manduca sexta
and BO-2727 (4)4 having antimicrobial activity
(Figure 1). Although the synthetic utility of this family of
compound is well established, there are only few efficient
methods for the enantioselective synthesis of such inter-
mediates. Efficient synthesis were reported based on
regioselective epoxide ring-opening reactions.2a,5
F3C
OH
O
O
R
NHR'
N
⋅HCl
H
1
2
OH
In previous communications, we have described the
synthesis of new atropisomeric diphosphines (Figure 2)
named SYNPHOS15 and DIFLUORPHOS.16
H
H
N
S
O
N
O
(CH2)8
N
COOH
OH
HO
O
O
H
H
F
4
3
F
O
O
O
PPh2
PPh2
PPh2
PPh2
O
O
MeHN·HCl
F
F
Figure 1
O
(S)-SYNPHOS
(S)-DIFLUORPHOS
The direct asymmetric reduction of b-keto amides is a
convenient route to synthesize optically active b-hydroxy
Figure 2
SYNLETT 2005, No. 16, pp 2478–2482
0
4
.1
0
.2
0
0
5
Advanced online publication: 21.09.2005
DOI: 10.1055/s-2005-917089; Art ID: G23405ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Efficient Catalysts for Practical Preparation of b-Hydroxy Amides
2479
Our studies have demonstrated both their relevant steric
and electronic properties and their catalytic performance
in ruthenium-promoted hydrogenation reactions.15,16,17c–e
As part as our continuing interest in the homogeneous
ruthenium-promoted hydrogenation reactions,17 we wish
to report in this paper a new application of chiral Ru-
SYNPHOS and Ru-DIFLUORPHOS catalysts for the
synthesis of a wide range of b-hydroxy amides with high
optical purity.
We first examined the Ru-SYNPHOS-promoted hydroge-
nation reaction of benzoylacetamide 5 as representative
substrate (Scheme 1). The catalytic tests were performed
in methanol at 50 °C under 5–10 bar of hydrogen pressure
with a substrate/catalyst ratio (S/C) of 100 (Table 1). Pre-
liminary study was carried out under 5 bar of hydrogen
pressure for 10 hours by using the in situ generated
{RuBr2[(S)-SYNPHOS]} prepared from a mixture of
(COD)Ru(2-methylallyl)2 and the diphosphine by addi-
tion of 2.2 equivalents of HBr according to our convenient
procedure.18 The b-hydroxy amide 6 was obtained in 76%
yield and 96% ee (entry 1). By increasing the pressure to
10 bar, the reaction proceeded faster and with excellent
91% yield and 96% ee (entry 2). When BINAP was used
as ligand, a comparable yield and ee (90% yield, 94% ee)
was obtained but the reaction time increased to 15 hours
(entry 3). Fully comparable results were achieved in 8
hours by using {RuBr2[(S)-MeO-BIPHEP]} and
{RuBr2[(S)-DIFLUORPHOS]} under analogous condi-
tions (entries 4 and 5, respectively: 90% and 94% yield,
96% and 93% ee).
Figure 3 Hydrogen uptake vs. time for the Ru-SYNPHOS asymme-
tric hydrogenation of 5. (1) {(RuCl[(R)-SYNPHOS)]2 (m-
Cl)3}[NH2Me2], ee = 99% (2) {RuBr2[(R)-SYNPHOS]}, ee = 96%
(3) {Ru(p-cymene)[(S)-SYNPHOS]Cl}Cl, ee = 87%
Next, a comparative study was carried out between some
Ru-SYNPHOS catalysts such as {(RuCl[(R)-SYN-
PHOS)]2(m-Cl)3}[NH2Me2] and {Ru(p-cymene)[(S)-
SYNPHOS]Cl}Cl. Results in Figure 3 are issued from a
parallel screening using the TOP INDUSTRIE parallel
hydrogenation system (TOP 1 590 000).19 This commer-
cially available equipment aims at screening several
hydrogenation reactions on small to large scale with inde-
pendent control of temperature and pressure. Individual
monitoring of hydrogen uptake led to the kinetic profiles
of each reaction. The parallel catalytic tests have been run
under the same reaction conditions (entries 2, 6, 7, 10 bar,
50 °C, S/C = 100). This graph allowed determining the
best reaction time for the Ru-SYNPHOS catalysts. When
comparing the catalytic activities of the different Ru-
SYNPHOS catalysts (Figure 3), we observed that
{(RuCl[(R)-SYNPHOS)]2 (m-Cl)3}[NH2Me2] catalyst en-
hanced exceedingly the hydrogenation reaction rates of 5
(curve 1) compared to both {RuBr2[(R)-SYNPHOS]}
(curve 2) and {Ru(p-cymene)[(S)-SYNPHOS]Cl}Cl
(curve 3).
O
O
OH
O
(S)-[Ru]-cat.
Ph
NHCH3
Ph
NHCH3
H2, P(bar)
5
6
50 °C, MeOH
Scheme 1
Table 1 Optimization of the Asymmetric Hydrogenation of 5 in Methanol
Entry
Ru catalysta
P (bar)
Time (h)
Yield (%)b
ee (%)c
1
2
3
4
5
6
{RuBr2[(S)-SYNPHOS]}
{RuBr2[(S)-SYNPHOS]}
{RuBr2[(R)-BINAP)}
5
10
10
10
10
10
10
3
76
91
90
90
94
95
96 (R)
96 (R)
94 (S)
96 (R)
93 (R)
99 (S)
15
8
{RuBr2[(S)-MeO-BIPHEP]}
{RuBr2[(S)-DIFLUORPHOS]}
8
{(RuCl[(R)-SYNPHOS)]2
(m-Cl)3}[NH2Me2]
2
7
8
{Ru(p-cymene)[(S)-SYNPHOS]Cl}Cl
10
10
10
2
75
87 (R)
98 (S)
{(RuCl[(R)-SYNPHOS)]2
(m-Cl)3}[NH2Me2]
92d
a Reaction carried out with S/C = 100.
b Isolated yield after flash chromatography.
c The ee were determined by HPLC analysis.
d Reaction was conducted on 1.5 g scale.
Synlett 2005, No. 16, 2478–2482 © Thieme Stuttgart · New York

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R. Touati et al.
LETTER
Table 2 Asymmetric Hydrogenation of b-Keto Amides 7–11 with Ru-SYNPHOS and Ru-DIFLUORPHOS Catalysts
Entry
1
Substrate Ru catalysta
Time (h)
19b
Yield (%)c Product
93
ee (conf.) (%)d
7
{(RuCl[(R)-SYNPHOS)]2
99 (S)
(m-Cl)3}[NH2Me2]
2
3
4
7
7
8
{RuBr2[(S)-SYNPHOS]}
19b
19b
1
92
91
92
15
15
99 (R)
94 (R)
>99 (S)
{RuBr2[(S)-DIFLUORPHOS]}
{(RuCl[(R)-SYNPHOS)]2
(m-Cl)3}[NH2Me2]
5
6
7
8
8
9
{RuBr2[(S)-SYNPHOS]}
1
91
92
93
16
16
>99 (R)
>99 (S)
>99 (S)
{RuBr2[(R)-DIFLUORPHOS]}
1.5
2
{(RuCl[(R)-SYNPHOS)]2
(m-Cl)3}[NH2Me2]
8
9
9
{RuBr2[(R)-SYNPHOS]}
1
5
5
2
91
70
80
91
17
17
17
>99 (S)
>99 (S)
>99 (R)
93 (S)
9
{RuBr2[(R)-DIFLUORPHOS]}
{RuBr2[(S)-DIFLUORPHOS]}
10
11
9
10
{(RuCl[(R)-SYNPHOS)]2
(m-Cl)3}[NH2Me2]
12
13
10
11
{RuBr2[(S)-SYNPHOS]}
5
92
90
18
89 (R)
95 (R)
{(RuCl[(R)-SYNPHOS)]2
(m-Cl)3}[NH2Me2]
19a
14
15
11
11
{RuBr2[(R)-SYNPHOS]}
5
5
89
91
19
19
97 (R)
97 (R)
{RuBr2[(R)-DIFLUORPHOS]}
a Reaction carried out with S/C = 100.
b Reaction time not optimized.
c Isolated yields after flash chromatography.
d The ee were determined by HPLC analysis using Chiralcel OD-H or Chiralcel OJ column.
Thus, this study was extended to a series of b-keto amides dered substrates such as 9 and 10 were hydrogenated with
prepared according to the literature.20 The range of b-keto ee in a range from 89% to 99% (entries 7, 8, 11, 12). We
amides 7–11 is illustrated in Scheme 2. The screening were pleased to find that homogeneous system based on
tests were carried out on a 1 mmol scale in methanol under Ru-DIFLUORPHOS gave good results with yields up to
10 bar hydrogen pressure at 50 °C by using 1 mol% of 90% and ee in a range from 94% to >99% (entries 3, 6, 9,
the Ru-SYNPHOS and Ru-DIFLUORPHOS catalysts 10, 15).
(Table 2). In all cases, complete conversions were
achieved. As illustrated in Table 2, all hydrogenations
exhibited both excellent level of enantioselectivities,
yields and high substrate generality. In all cases, both
O
O
OH
O
OH
O
[Ru]-cat.
or
R
NHR'
H2, (10 bar)
50 °C, MeOH
R
NHR'
R
NHR'
{(RuCl[(R)-SYNPHOS)]2
(m-Cl)3}[NH2Me2]
and
15–19
{RuBr2[(R)-SYNPHOS]} catalysts have been engaged in
the ruthenium-promoted hydrogenation of aromatic b-
keto amides 7–11 (Table 2) leading to excellent yields up
to 93% for the para-substituted compounds 7 and 8 with
no influence of the para-substituents (p-Me–Ph, entries 1,
2 or p-F–Ph, entries 4 and 5). Interestingly, the more hin-
7
8
9
R = p-Me-C6H4, R' = Me
R = p-F-C6H4, R' = Me
R = 1-Naphth, R' = Me
10 R = 2-Naphth, R' = Me
11 R = CH2Ph, R' = CH2Ph
Scheme 2
Synlett 2005, No. 16, 2478–2482 © Thieme Stuttgart · New York

LETTER
Efficient Catalysts for Practical Preparation of b-Hydroxy Amides
2481
To extend the scope of this versatile methodology with enantiomer was detected by HPLC analysis. When these
conditions established for the catalytic hydrogenation of transformations were repeated by using DIFLUORPHOS
b-aryl-substituted keto amides, this procedure was ap- ligand, slightly lower ee were obtained together with very
plied to the hydrogenation of b-alkyl-substituted keto good yields (entries 3, 6, 9: 96–99% ee, 90–93% yields).
amides 12–14 (Scheme 3, Table 3). Once again, essential-
ly quantitative conversions to enantiomerically enriched
b-hydroxy amides 20–22 (entries 1–9) were observed un-
der the same catalytic conditions. When SYNPHOS was
used as chiral ligand, the hydrogenation reactions were
performed again with excellent enantiofacial discrimina-
tion affording the corresponding enantiopure alcohols
21–22 (Table 3, entries 4, 5, 7, 8: ee >99%). Only one
O
O
OH
O
OH
O
[Ru]-cat.
or
NHR'
R
NHR'
H2, (10 bar)
R
R
NHR'
50 °C, MeOH
20–22
12 R = Me, R' = Ph
13 R = n-Pr, R' = Me
14 R = C15H31, R' = Me
Scheme 3
Table 3 Asymmetric Hydrogenation of b-Keto Amides 12–14 with Ru-SYNPHOS and Ru-DIFLUORPHOS Catalysts
Entry
1
Substrate
Ru-catalysta
Time (h)
1
Yield (%)b Product
92
ee (conf.) (%)d
12
{(RuCl[(R)-SYNPHOS)]2 (m-
99 (R)c
Cl)3}[NH2Me2]
2
12
{(RuCl[(S)-SYNPHOS)]2 (m-
Cl)3}[NH2Me2]
1
94
20
20
99 (S)c
3
4
12
13
{RuBr2[(R)-DIFLUORPHOS]}
4
90
91
96 (R)c
>99 (R)
{(RuCl[(R)-SYNPHOS)]2 (m-
0.5
Cl)3}[NH2Me2]
5
13
{(RuCl[(S)-SYNPHOS)]2 (m-
Cl)3}[NH2Me2]
0.5
92
21
21
>99 (S)
6
7
13
14
{RuBr2[(R)-DIFLUORPHOS]}
5
1
90
92
99 (R)
{(RuCl[(R)-SYNPHOS)]2 (m-
>99 (R)
Cl)3}[NH2Me2]
8
9
14
14
{(RuCl[(S)-SYNPHOS)]2 (m-
Cl)3}[NH2Me2]
1
4
94
93
22
22
>99 (S)
96 (R)
{RuBr2[(R)-DIFLUORPHOS]}
a Reaction carried with S/C = 100.
b Isolated yields after flash chromatography.
c The ee were determined by 1H NMR (400 MHz) of (+)-MTPA ester derivatives.
d The ee were determined by HPLC analysis using Chiralcel OD-H or Chiralcel OJ column.
The absolute configurations of the chiral b-hydroxy the convenient preparation of both enantiomers with high
amides 6 and 21 were assigned from [a]D value by com- level of enantioselectivites enabling the synthesis of
parison with known compounds.8,12a,13 Concerning the b- natural products and analogues of biological interest.
keto amides 7–11, we assumed that their hydrogenation
follows the same stereochemical outcome as above
according to the stereochemical model proposed for the
Acknowledgment
We would like to thank the CMCU (Comité Mixte Franco-Tunisien
pour la Coopération Universitaire) for financial support.
hydrogenation reaction of carbonyl derivatives with
ruthenium-arylphosphine catalysts.17,21
In conclusion, we outlined the development of a general
route that provides access to a wide variety of chiral b-hy-
droxy amides by using the ruthenium-promoted hydroge-
nation of corresponding b-keto amides. In this work, we
have demonstrated that the SYNPHOS and DIFLUOR-
PHOS ligands can be efficiently used in these transforma-
tions. This procedure is simple to perform22,23 and allows
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Synlett 2005, No. 16, 2478–2482 © Thieme Stuttgart · New York

2482
R. Touati et al.
LETTER
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(S)-SYNPHOS (7.5 mg, 1.2 ꢀ 10–2 mmol) and (COD)Ru(2-
methylallyl)2 (3.2 mg, 1.0 ꢀ 10–2 mmol) were placed in a 5
mL flask and 2 mL of anhyd acetone were added. A
methanolic solution of HBr (0.122 mL, 0.18 M) was added to
the resulting suspension and the reaction mixture was stirred
at r.t. for about 30 min. The solvent was removed under
vacuum. The yellow solid residue was used as catalyst for
the hydrogenation reaction. Then, MeOH (4 mL),
acetoacetanilide (5, 177 mg, 1 mmol) and the catalyst were
placed under argon in the TOP INDUSTRIE parallel
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initial pressure of 10 bar of hydrogen and the reaction was
allowed to proceed at 50 °C for 2 h. The crude reaction
mixture was purified by silica gel chromatography
(cyclohexane–EtOAc, 50:50) to afford 163 mg of (R)-6 as a
white solid (91%). Mp 125–126 °C; [a]D +43 (c 1, CHCl3);
[a]D +29 (c 1, CH3OH). IR (KBr disk): 3331 (nOH–NH), 3111
(nCH3), 1645 (nCO) cm–1. 1H NMR (300 MHz, CDCl3):
d = 2.50–2.59 (m, 2 H), 2.84 (d, J = 4.8 Hz, 3 H), 4.18 (br, 1
H), 5.09–5.16 (dd, J = 5.0, 7.6 Hz, 1 H), 5.83 (br, 1 H), 7.31–
7.39 (m, 5 H). 13C NMR (CDCl3): d = 26.0, 44.4, 70.7,
125.4, 142.9, 152.4, 172.4. MS (70 eV): m/z (%) = 179 (21)
[M+], 73 (100), 105 (26), 58 (24), 43 (25).
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Synlett 2005, No. 16, 2478–2482 © Thieme Stuttgart · New York