Diastereo- and enantioselective asymmetric hydrogenation of α-amido-β-keto phosphonates via dynamic kinetic resolution

Article abstract of DOI:10.1021/ol303105d

Dynamic kinetic resolution of various α-amido-β-keto phosphonates via asymmetric hydrogenation proceeded efficiently to give the corresponding β-hydroxy-α-amido phosphonates in high diastereo- and enantioselectivities (up to 99:1 syn/anti, 99.8% ee). The addition of catalytic amounts of CeCl₃ 3 7H₂O is necessary to achieve both good selectivity and catalytic efficiency under mild reaction conditions.

Full text of DOI:10.1021/ol303105d

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Diastereo- and Enantioselective
Asymmetric Hydrogenation of
r‑Amido-β-keto Phosphonates
via Dynamic Kinetic Resolution
Xiaoming Tao,† Wanfang Li,† Xiaoming Li,† Xiaomin Xie,† and Zhaoguo Zhang*,†,‡
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University,
800 Dongchuan Road, Shanghai 200240, China, and State Key Laboratory of
Organometallic Chemistry, Shanghai Institute of Organic Chemistry,
345 Lingling Road, Shanghai 200032, China
zhaoguo@sjtu.edu.cn
Received November 11, 2012
ABSTRACT
Dynamic kinetic resolution of various R-amido-β-keto phosphonates via asymmetric hydrogenation proceeded efficiently to give the
corresponding β-hydroxy-R-amido phosphonates in high diastereo- and enantioselectivities (up to 99:1 syn/anti, 99.8% ee). The addition of
catalytic amounts of CeCl3 7H2O is necessary to achieve both good selectivity and catalytic efficiency under mild reaction conditions.
3
Chiral β-hydroxy-R-amino phosphonates have received
considerable attention in the past decades because of their
prevalence in bioorganic and medicinal chemistry owing
to their unique biological activities as well as their
potential uses as peptide mimics and haptens of catalytic
antibodies (Figure 1).1 Presently, a number of methods
are available for the synthesis of optically pure and
enriched β-hydroxy-R-amino phosphonates,2 via resolu-
tion, enzymatic methods, or asymmetric CÀC and CÀP
bond-forming reactions. However, these synthetic routes
sufferedfrom eitherlow stereoselectivity orpoor efficiency.
Accordingly, the search for effective, highly stereoselective,
and atom-economic processes to obtain β-hydroxy-R-amino
phosphonates is of great significance.
Ruthenium-catalyzed asymmetric hydrogenation via
dynamic kinetic resolution (DKR) has turned out to be
an elegant and powerful synthetic tool to control two
adjacent stereogenic centers with high levels of enantio-
selectivity and diastereoselectivity in one single chemical
† Shanghai Jiao Tong University.
‡ Shanghai Institute of Organic Chemistry.
(1) (a) Dhawan, B.; Redmore, D. Phosphorus Sulfur 1987, 32, 119–
144. (b) Kafarski, P.; Lejczak, B. Phosphorus, Sulfur Silicon Relat. Elem.
1991, 63, 193–215. (c) Kukhar’, V. P.; Svistunova, N. Y.; Solodenko,
V. A.; Soloshonok, V. A. Russ. Chem. Rev. 1993, 62, 261–278. (d)
Hirschmann, R.; Smith, A. B.; Taylor, C. M.; Benkovic, P. A.; Taylor,
S. D.; Yager, K. M.; Sprengeler, P. A.; Benkovic, S. J. Science 1994, 265,
234–237.
operation.3 This reaction was reported by Noyori et al.4 in
5
1989 and Genet et al. in 1991 for the synthesis of threo-
^
nine. The hydrogenation of R-acetamido-β-keto esters and
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1992, 3, 377–378. (h) Ktmz, H. Synthesis 1992, 90–93. (i) Benmmi, Y. L.;
Hanessinn, S.; Herve, Y. Synlett 1993, 35–38. (j) Shatzmiller, S.;
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r
10.1021/ol303105d
XXXX American Chemical Society

more generally of noncyclic R-amino-β-keto esters with
Ru(II) catalysts afforded the corresponding β-hydroxy
esters with excellent diastereoisomeric and enantiomeric
excesses.6 The stereochemical course of the hydrogenation
reaction is highly dependent on the nature of the protected
amino groups. In the current paradigm, hydrogenation of
the N-acyl-protected R-amido-β-keto esters yields the syn
diastereomer,7 whereas the hydrochloride salt of R-amino-
β-keto esters afforded the anti diastereomer.8 However,
up to now, the only example of an efficient DKR of an
R-acetamido-β-keto phosphonates with Ru-BINAP sys-
tem afforded the corresponding syn R-acetamido-β-hydroxy
phosphonates was reported by Noyori et al. in 1995.9 There-
fore, asymmetric hydrogenation of R-amido-β-keto phos-
phonates is still a challenging work.
The asymmetric hydrogenation was carried out with 1 mol %
of [RuCl(benzene)(S)-SunPhos]Cl under 10 bar of H2 at
50 °C in MeOH for 24 h with dimethyl (1-acetamido-
2-oxo-2-phenylethyl)phosphonate (1a) as the standard
substrate. We obtained full conversions and excellent
diastereoisomeric and enantiomeric excesses of the desired
product 2a11 (Table 1, entry 1, syn/anti 97:3, ee 98.0%).
Because subtle changes in geometric, steric, and/or elec-
tronic properties of chiral ligands can lead to dramatic
variations of reactivity and stereselectivity,12 (S)-Tol-Sun-
Phos and two commercially available chiral bidentate
ligands, (S)-SegPhos and (S)-MeO-Biphep, were also
tested under the same reaction conditions (Figure 2). They
showed lower activity, and the diastereo- and enantios-
electivities varied to some extent. As illustrated in Table 1,
(R)-Tol-SunPhos (entry 2, syn/anti 94:6, ee 99.9%),
(S)-SegPhos (entry 3, syn/anti 89:11, ee 95.7%), and
(S)-Meo-Biphep (entry 4, syn/anti 89:11, ee 96.7%) were
inferior to (S)-SunPhos. On the basis of the above results,
(S)-SunPhos was the ligand of choice.
Results of the optimization of solvents, hydrogen pres-
sure, andreactiontemperaturesare summarizedin Table1.
Solvent is important for the catalytic efficiency and dia-
stereoisomeric and enantiomeric excesses of the asym-
metric hydrogenation reaction (Table 1, entries 1, 5À8).
Protic solvents resulted in high catalytic activities and
diastereoisomeric and enantiomeric excesses, providing
Figure 1. Biologically active β-hydroxy-R-amino phosphonates.
Our group has designed some atropisomeric C2-symmetric
biaryl biphosphine-SunPhos ligands and explored their
applications in asymmetric hydrogenation of functiona-
lized ketones.10 In this paper, we disclose a general and
highly diastereo- and enantioselective hydrogenation reac-
tion of R-amido-β-keto phosphonates.
The catalyst was prepared from [RuCl2(benzene)]2 and
(S)-SunPhos (Figure 2) by refluxing them in degassed
dichloromethane/ethanol (v/v = 1:1) for 1.5 h, and
then the solvent was removed under reduced pressure.10a
Figure 2. Structures of chiral bidentate ligands.
(6) (a) Mashima, K.; Matsumura, Y.; Kusano, K.; Kumobayashi,
H.; Sayo, N.; Hori, Y.; Ishizaki, T.; Akutagawa, S.; Takaya, H. Chem
Commun. 1991, 609–610. (b) Coulon, E.; Andrade, M. C. C. d.;
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complete conversions of 1a to 2a with good to excellent
syn/anti ratios ranging from 95:5 to 97:3 and ee’s up to
98.2%. Aprotic solvents such as dichloromethane or THF
gave 2a with up to 99.0% ee but lower diastereoselectivities
(Table 1, entries 7 and 8). The results depicted in Table 1
showed that the efficiency and diastereoisomeric and enan-
tiomeric excesses of the hydrogenation were strongly depen-
dent on the temperature (Table 1, entries 1 and 9À11).13
Lower reaction temperature remarkably decreased the reac-
tion rate and diastereoisomeric and enantiomeric excesses,
while higher reaction temperature slightly decreased
6467–6470. (c) Makino, K.; Hiroki, Y.; Hamada, Y. J. Am. Chem. Soc.
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(11) Absolute configurations of 2a and 2l were assigned as (1S,2S) on
the basis of their optical rotation and 31P NMR; see ref 9.
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Zhang, Z. Org. Lett. 2011, 13, 3876–3879. (g) Fan, W.; Li, W.; Ma, X.;
Tao, X.; Li, X.; Yao, Y.; Xie, X.; Zhang, Z. Chem. Commun. 2012, 48,
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B
Org. Lett., Vol. XX, No. XX, XXXX

enantioselectivity. However, the efficiency and diastereo-
isomeric and enantiomeric excesses were not affected by
the hydrogen pressure in our optimization.
enantioselectivity remained excellent. The best results were
achieved with CeCl3 7H2O as additive; perfect enantio-
3
selectivity and diastereoselectivity were obtained (Table 2,
entries 3). There was no detectable difference between
anhydrous CeCl3 and CeCl3 7H2O as additives. The dia-
3
stereo- and enantioselectivities decreased slightly when
aqueous solutions of Brønsted acids, except for HBF4,
were tested (entries 7À10), where the diastereoselectivity
was better than those without additives (entry 7 vs 1, syn/
anti98:2vs97:3). Wheniodine was used asthe additive, the
enantioselectivity remained but the diastereoselectivity
increased slightly to 98:2 (entry 11).
Table 1. Optimization of Ligands and Reaction Conditionsa
entry
ligand
solvent
syn/antib
ee of synb (%)
1
2
3c
4d
5
L1
L2
L3
L4
L1
L1
L1
L1
L1
L1
L1
L1
L1
MeOH
MeOH
MeOH
MeOH
EtOH
97:3
94:6
89:11
89:11
96:4
95:5
87:13
85:15
95:5
97:3
98:2
97:3
97:3
98.0
99.9
95.7
96.7
97.7
98.2
98.1
99.0
94.4
98.7
96.6
97.8
98.0
Table 2. Effects of Additives on the DKR of 1aa
6
i-PrOH
CH2Cl2
THF
7
8
9e
10f
11g
12h
13i
MeOH
MeOH
MeOH
MeOH
MeOH
entry
additive
none
syn/antib
ee of synb (%)
1
2
97:3
97:3
99:1
99:1
98:2
99:1
98:2
97:3
96:4
96:4
98:2
98.0
97.4
98.8
98.6
98.1
97.2
99.9
97.2
94.5
94.7
97.9
H2O
3
CeCl3 7H2O
3
4
CeCl3
ZnCl2
a Unless otherwise stated, all reactions were carried out with a
substrate (1 mmol) concentration of 0.5 M in solvent under 10 bar of
H2 at 50 °C for 24 h, S/C = 100:1. Conversion: >99%. b Determined by
HPLC on a Chiralpak AD-H column. c Conversion: 90%. d Conversion:
94%. e At 30 °C, conversion: 80%. f At 70 °C. g At 90 °C. h Under 30 bar.
i Under 50 bar.
5
6
BF3 Et2O
3
7
HBF4 aq
HF aq
HCl aq
HBr aq
I2
8
9
10
11
It has been reported that additives played some crucial
roles in improving the reactivity and diastereo- and en-
antioselectivity in many asymmetric reactions.14 In our
preliminary work, we used Lewis acids or aqueous solu-
tions of Brønsted acids as additives in the asymmetric hydro-
genation of R-ketoesters and achieved better results.10c,e
Accordingly, we evaluated several additives for the asym-
metric hydrogenation of 1a by using 1 mol % of [RuCl-
(benzene)(S)-SunPhos]Cl as catalyst and 5 mol % of
additives in an attempt to promote the diastereo- and
enantioselectivity. Lewis acids are among the most useful
reagents in reactions with ketones as substrates. For
a Unless otherwise stated, all reactions were carried out with a
substrate (1 mmol) concentration of 0.5 M in MeOH under 10 bar of
H2 at 50 °C for 24 h, S/C/additive = 100:1:5. Conversion: >99%.
b Determined by HPLC on a Chiralpak AD-H column.
On the basis of these results, the optimized reaction
conditions were therefore set as the following: 1 mol % of
[RuCl(benzene)L1]Cl as the catalyst, CeCl3 7H2O as the
additive, MeOH as the solvent with a substrate concentra-
tion of 0.5 M, and 10 bar of H2 at 50 °C.
3
Under the optimized reaction conditions, a variety of
protected R-amido-β-keto phosphonates were hydroge-
nated, and the results are depicted in Table 3. The diastereo-
and enantioselectivities were highly dependent on the amino
protecting groups: benzoyl-protected substrate (1c) gave
low diastereoselectivity and high enantioselectivity (entry
3, syn/anti 55:45, ee 96.5%); when benzyloxycarbonyl
was employed as the amino protecting group, diastereo-
and enantioselectivity for the hydrogenation turned out
to be very high (entry 2, syn/anti 95:5, ee 99.5%); the
acetyl-protected substrate (1a) had accomplished the
highest diastereo- and enantioselectivity (entry 1, syn/anti
99:1, ee 98.8%). Therefore, acetyl-protected substrates
were chosen in the following investigation. A series of
acetyl-protected R-amido-β-keto phosphonates bearing a
different substituent at the para position on the phenyl
group were studied. The results showed that the electron
example, CeCl3 7H2O was used in the selective reduction
3
of the carbonyl group of R,β-unsaturated ketones with
NaBH4.15 When Lewis acids were used as additives in the
hydrogenation of our model substrate 1a with ruthenium
as the catalyst (Table 2, entries 3À6), improved diastereo-
selectivities (syn/anti 98:2À99:1) were obtained and the
(14) (a) Togni, A. Angew. Chem., Int. Ed. 1996, 35, 1475–1477. (b)
€
Vogl, E. M.; Groger, H.; Shibasaki, M. Angew. Chem., Int. Ed. 1999, 38,
1570–1577. (c) Xiao, D.; Zhang, X. Angew. Chem., Int. Ed. 2001, 40,
3425–3428. (d) Chi, Y.; Zhou, Y. G.; Zhang, X. J. Org. Chem. 2003, 68,
4120–4122. (e) Moessner, C.; Bolm, C. Angew. Chem., Int. Ed. 2005, 44,
7564–7567. (f) Legault, C. Y.; Charette, A. B. J. Am. Chem. Soc. 2005,
127, 8966–8967. (g) Hou, G.-H.; Xie, J.-H.; Wang, L.-X.; Zhou, Q.-L.
J. Am. Chem. Soc. 2006, 128, 11774–11775. (h) Akiyama, T. Chem. Rev.
2007, 107, 5744–5758. (i) Zhou, Y.-G. Acc. Chem. Res. 2007, 40, 1357–
1366.
(15) Gemal, A. L.; Luche, J. L. J. Am. Chem. Soc. 1981, 103, 5454–
5459.
Org. Lett., Vol. XX, No. XX, XXXX
C

selectivity (entry 8, syn/anti 76:24). Electronic influences
of the para substituents were presumed to affect the
coplanarity of the benzene rings with CdO in the transi-
tion state,16 thereby generating an asymmetric bias. Simple
ortho- and meta-substituted substrates were also hydro-
genated to give the products with excellent diastereo- and
enantioselectivities (entries 4 and 5). Heteroaryl group was
also tolerated and excellent diastereo- and enantioselec-
tivities were obtained (entry 12).
Table 3. Asymmetric Hydrogenation of R-Amido-β-keto
Phosphonatesa
Hydrogenation ofanaliphaticanalogue alsogavemode-
rate to excellent diastereo- and enantioselectivities (Table 3,
entries 13À16, syn/anti 75:25À98:2, ee 91.7À95.9%).
Asymmetric hydrogenation of dimethyl (1-acetamido-2-
oxopropyl)phosphonate (1m) afforded the corresponding
alcohol (2m) in 98:2 syn/anti and 95.2% ee (entry 13).11
Enantiopure 2m has been applied in the practical synthesis
of fosfomycin.17 Furthermore, the reaction proceeded
smoothly on multigram scale with excellent diastereo- and
enantiofacial discrimination; up to 97:3 syn/anti and 94.0%
ee under 10 bar of hydrogen pressure and 50 °C using a
catalyst loading of 0.01 mol % of [RuCl(benzene)(S)-
SunPhos]Cl (entry 14) were obtained.
In conclusion, we have developed a convenient and
general protocol for the diastereo- and enantioselective
hydrogenation of a variety of R-amido-β-keto phospho-
nates via dynamic kinetic resolution, and excellent diastereo-
and enantioselectivity were obtained. Additives play a crucial
role in improving the diastereo- and enantioselectivity in our
entry
1
R1
C6H5
2 syn/antib
2 ee of synb (%)
1c
2d
3
1a
1b
1c
1d
1e
1f
99:1
95:5
55:45
95:5
95:5
94:6
94:6
76:24
96:4
98:2
96:4
97:3
98:2
97:3
98:2
94:6
75:25
98.8
99.5
96.5
93.2
97.3
94.2
99.8
95.6
95.5
95.0
97.5
93.6
95.2
94.0
92.5
91.7
95.9
C6H5
C6H5
4
o-CH3C6H4
m-CH3C6H4
p-CH3C6H4
p-OMeC6H4
p-F-C6H4
p-ClC6H4
p-BrC6H4
p-CF3C6H4
2-furyl
5
6
7
1g
1h
1i
8
9
10
11
12
13c
14e
15
16
17
1j
1k
1l
1m
1m
1n
1o
1p
CH3
CH3
C2H5
CH(CH3)2
C6H5CH2
a Unless otherwise stated, all reactions were carried out with a
substrate (1 mmol) concentration of 0.5 M in MeOH under 10 bar of
H2 at 50 °C for 24 h. S/C/additive = 100:1:5. Conversion: >99%.
b Determined by HPLC. c Absolute configurations of 2a and 2m were
assigned as (1S, 2S) on the basis of their optical rotation and 31P NMR
(see ref 9); the configuration of other products can be assigned as (1S, 2S)
according to the well-established general trend (see ref 8). d Absolute
configuration was not known. e S/C/additive = 10000:1:10.
reaction. In the presence of CeCl3 7H2O as the additive,
3
dramatically increased diastereoisomeric and enantiomeric
excesses up to 99:1 syn/anti and 99.8% ee were achieved.
Acknowledgment. We thank the National Natural
Science Foundation of China for financial support.
density of the aromatic ring has a strong effect on the
diastereo- and enantioselectivities (entries 6À11, syn/anti
76:24À98:2, ee94.2À99.8%). Thestrongelectron-donating
groups (MeO) on the aromatic ring afforded the corre-
sponding product (2g) in 94:6 syn/anti in the presence of
Supporting Information Available. Experimental de-
tails of substrate synthesis and NMR and/or HPLC data
of 1 and 2. This material is available free of charge via the
Internet at http://pubs.acs.org.
CeCl3 7H2O (entry 7), while the electron-withdrawing
3
groups (F) on the aromatic ring decreased the diastereo-
(17) Kolodiazhnyi, O. I. Tetrahedron: Asymmetry 2005, 16, 3295–
3340.
(16) Casy, A. F.; Drake, A. F.; Ganellin, C. R.; Mercer, A. D.;
Upton, C. Chirality 1992, 4, 356–366.
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX