Highly enantioselective sequential hydrogenation of ethyl 2-Oxo-4-arylbut-3-enoate to ethyl 2-hydroxy-4-arylbutyrate

Article abstract of DOI:10.1021/jo801140j

(Chemical Equation Presented) The hydrogenation of (E)-ethyl 2-oxo-4-arylbut-3-enoate with [NH₂Me₂]⁺[{RuCl [(S)-SunPhos]}₂(μ-Cl₃)] gave ethyl 2-hydroxy-4- arylbutyrate with 94-96% ee. Further investigation has proved that the hydrogenation proceeded via a sequential hydrogenation of C=O and C=C bonds, which is sensitive to the reaction temperature. Hydrolysis of ethyl 2-hydroxy-4-phenylbutyrate (ee 93%) provided the 2-hydroxy-4-phenylbutyric acid with 81% yield at 99% ee after a single recrystallization from 1, 2-dichloroethylene.

Full text of DOI:10.1021/jo801140j

Highly Enantioselective Sequential Hydrogenation of Ethyl
2-Oxo-4-arylbut-3-enoate to Ethyl 2-Hydroxy-4-arylbutyrate
Qinghua Meng,† Lufeng Zhu,† and Zhaoguo Zhang*,†,‡
School of Chemistry and Chemical Technology, Shanghai Jiaotong UniVersity, 800 Dongchuan Road,
Shanghai 200240, China, and Shanghai Institute of Organic Chemistry, 354 Fenglin Road,
Shanghai 200032, China
zhaoguo@sjtu.edu.cn
ReceiVed May 26, 2008
The hydrogenation of (E)-ethyl 2-oxo-4-arylbut-3-enoate with [NH2Me2]+[{RuCl [(S)-SunPhos]}2(µ-
Cl3)] gave ethyl 2-hydroxy-4-arylbutyrate with 94-96% ee. Further investigation has proved that the
hydrogenation proceeded via a sequential hydrogenation of CdO and CdC bonds, which is sensitive to
the reaction temperature. Hydrolysis of ethyl 2-hydroxy-4-phenylbutyrate (ee 93%) provided the 2-hydroxy-
4-phenylbutyric acid with 81% yield at 99% ee after a single recrystallization from 1, 2-dichloroethylene.
Introduction
to prepare such ACE inhabitors. A number of routes to (R)-2-
hydroxy-4-phenylbutyrate have been developed, including clas-
sical resolution of the hydroxyl racemic acid;3 catalytic enan-
tioselective reduction of a prochiral ketone by chemical,4
microbial, or enzymatic reduction;5 and chiral pool synthesis.6
Enantioselective hydrogenation of ethyl 2-oxo-4-phenylbu-
tanoate is likely one of the most economic and efficient method;
however, it suffers from instability of the R-ketoesters and
sensitivity of enantioselective hydrogenation to substrate purity.1i,4f
Therefore, the search for effective, highly enantioselective, and
Asymmetric hydrogenation catalyzed by transition metal
complexes containing optically active phosphine ligands has
attracted significant interest in industry and academia for its
synthetic utility.1 A variety of commercially important angio-
tensin-converting enzyme (ACE) inhibitors containing an (S)-
homophenylalanine moiety can be synthesized from various
chiral building blocks.2 Among them, enantiomerically pure
ethyl (R)-2-hydroxy-4-phenylbutyrate is a useful intermediate
† Shanghai Jiaotong University.
(3) Attwood, M. R.; Hassall, C. H.; Krohn, A.; Lawton, G.; Redshaw, S.
J. Chem. Soc., Perkin Trans. 1 1986, 1011.
‡ Shanghai Institute of Organic Chemistry.
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Building Blocks and Fine Chemicals; Beller, M., Bolm, C., Eds.; Wiley-VCH:
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Homogeneous Catalysis with Organometallic Compounds, 2nd ed.; Cornils, B.,
Herrmann, W. A., Eds.; Wiley-VCH: Weinheim, 2000; Chapter 3.3.1. (c) Noyori,
R.; Ohkuma, T. Angew. Chem., Int. Ed. 2001, 40, 40. (d) Noyori, R. Angew.
Chem., Int. Ed. 2002, 41, 2008. (e) Knowles, W. S. Angew. Chem., Int. Ed.
2002, 41, 1998. (f) Rossen, K. Angew. Chem., Int. Ed. 2001, 40, 4611. (g) Pfaltz,
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10.1021/jo801140j CCC: $40.75
Published on Web 08/22/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 7209–7212 7209

Meng et al.
SCHEME 1. Synthesis of Ethyl 2-Oxo-4-arylbut-3-enoatesa
SCHEME 2. Asymmetric Hydrogenation of 1a with L1-L4
a Reagents and conditions: (a) KOH, MeOH, 20-40 °C, 10-15 h; (b)
AcCl, EtOH, 70 °C, 6-8 h.
practical approaches to (R)-2-hydroxy-4-phenylbutyrate is still
of significance.
Recently we have been focusing on the design and application
of new biaryl phosphine ligands.7 Initial work in this area has
resulted in the identification of a series of bidentate ligands
highly effective in enantioselective hydrogenation of ꢀ-keto-
esters,7a,b R-ketoesters,7c,e and ꢀ-keto sulfones.7d Asymmetric
hydrogenation of monofunctionalized olefins or ketones has been
investigated extensively in recent decades, and high enantiose-
lectivities have usually been attained with substrates that have
another functional group at a neighboring position; in contrast,
bifunctionalized and even polyfunctionalized olefins or ketones
have rarely attracted attention. In this paper, we report a highly
enantioselective synthesis of ethyl 2-hydroxy-4-arylbutyrates by
sequential hydrogenation of (E)-ethyl 2-oxo-4-arylbut-3-enoates,
which can be prepared conveniently by two steps starting from
the corresponding aryl aldehydes and pyruvic acid (Scheme 1).8
TABLE 1. Optimization Studies of Catalytic Asymmetric
Hydrogenation of 1aa
entry catalyst
additive
temp (°C) product
ee (%)b
1
2
3
4
5
6
7
8
4
4
4
4
4
4
5
5
6
6
6
6
6
6
6
1 N H2SO4 aq
1 N CSA aq
1 N HBF4 aq
1 N HCl aq
1 N H3BO3 aq
CeCl3 ·7H2O
none
70
70
70
70
70
70
70
70
70
70
70
70
30
50
50
2a
2a
2a
2a
2a
80
90
84
93
88
Results and Discussion
2a/3, 95/5c 75/45d
Based on the initial success of asymmetric hydrogenation for
functionalized ketones, the catalyst [RuCl(benzene)L]Cl was
readily prepared from [Ru(benzene)Cl2]2 and a diphosphine
ligand by refluxing them in degassed ethanol/dichloromethane
for 1 h.7 The chiral bidentate ligands L1-L4 were tested for
the asymmetric hydrogenation of ethyl 2-oxo-4-phenylbut-3-
enoate (1a). The reaction was conducted with 1 mol % of
[RuCl(benzene)L]Cl as catalyst at 70 °C in EtOH for 12 h.
Under 14 bar of H2, ꢀ,γ-unsaturated R-ketoester 1a was
successfully reduced to (S)-ethyl 2-hydroxy-4-phenylbutyrate
with complete conversion and good ee. Enantioselectivity of
the reaction was greatly dependent on ligands employed: with
(S)-Binap as ligand, 67% ee was obtained; with (S)-DifluorPhos,
(S)-SegPhos, and (S)-SunPhos as ligands, the corresponding ee
values 75%, 85%, and 86% were achieved (Scheme 2).
2a
2a
2a
2a
2a
2a
78
91
90
94
82
49
1 N HCl aq
none
9
10
11e
12f
13
14
15g
1 N HCl aq
1 N HCl aq
1 N HCl aq
1 N HCl aq
1 N HCl aq
1 N HCl aq
2a/3, 70/30 96/96d
2a/3, 76/24 95/95d
2a/3, 79/21 94/94d
a Unless otherwise noted, all reactions were carried out with
a
substrate (1 mmol) concentration of 0.20 M in EtOH under 14 bar of H2
for 12 h, substrate/catalyst/additive ) 100/1/6, conversion 100%. b ee
values were determined by HPLC on
a Chiralpak OD-H column.
c Molar ratio of 2a/3 was determined by 1H NMR. d ee values of 2a/3.
e ClCH2CH2Cl as the solvent. f Toluene as the solvent. g P (H2) ) 41
bar.
It has been reported that additives play a crucial role in
improving the reactivity and enantioselectivity of many asym-
metric reactions.7,9 Accordingly, we evaluated a number of
additives for asymmetric hydrogenation of 1a by using 1 mol
% of [RuCl(benzene)(S)-SunPhos]Cl (4) as catalyst and 6 mol
% of the additive in an attempt to promote the enantioselectivity.
As shown in Table 1, among the tested aqueous solutions of
Brønsted acids, the proper counteranion was essential for
achieving better enantioselectivity (Table 1, entries 1-5), and
the addition of catalytic amounts of 1 N HCl aqueous solution
improved the enantiomeric excess up to 93% (Table 1, entry
4). In the presence of CeCl3 ·7H2O, an effective additive in Ru-
catalyzed enantioselective hydrogenation of aromatic R-keto-
esters,7c,e the hydrogenation of 1a afforded a mixture of ethyl
2-hydroxy-4-phenylbutanoate (2a) and (E)-ethyl 2-hydroxy-4-
phenylbut-3-enoate (3) with decreased ee values (Table 1, entry
6, molar ratio of 2a/3 ) 95/5). Different ruthenium complexes
of the same ligand also influenced the ee values: up to 94% ee
(7) (a) Sun, Y.; Wan, X.; Guo, M.; Wang, D.; Dong, X.; Pan, Y.; Zhang, Z.
Tetrahedron: Asymmetry 2004, 15, 2185. (b) Wan, X.; Sun, Y.; Luo, Y.; Li, D.;
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Q.; Zhang, H.; Jiang, L.; Zhang, Z. Org. Lett. 2005, 7, 5425. (d) Wan, X.; Zhang,
H.; Sun, Y.; Fan, W.; Zhang, Z. Org. Lett. 2007, 9, 5613. (e) Meng, Q.; Sun,
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Audrain, H.; Thorhauge, J.; Hazell, R. G.; Jorgensen, K. A. J. Org. Chem. 2000,
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R.; Raval, P.; Gite, A.; Goswami, A.; Sadhwani, N.; Gandhi, N.; Patel, H.;
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7210 J. Org. Chem. Vol. 73, No. 18, 2008

Sequential Hydrogenation of Ethyl 2-Oxo-4-arylbut-3-enoate
was achieved with [NH2Me2]+[{RuCl [(S)-SunPhos]}2(µ-Cl3)]
(6)10 as the catalyst in the presence of 1 N HCl (Table 1, entry
10), but with RuCl2[(S)-SunPhos](DMF)m (5)11 as the catalyst,
only 78% ee was obtained (Table 1, entry 7). On the basis of
these findings, the catalyst system [NH2Me2]+[{RuCl [(S)-
SunPhos]}2(µ-Cl3)] (6) and catalytic amounts of 1 N HCl was
tested across a range of solvents, reaction temperature, and
hydrogen pressure (Table 1, entries 11-15). As a matter of fact,
aprotic solvents (toluene or 1, 2-dichloroethane) gave lower
enantioselectivity than ethanol (Table 1, entries 11 and 12 vs
entry 10). At lower temperature, 1a was also hydrogenated with
complete conversion and gave a mixture of 2a and 3 with
slightly higher ee values. Increasing the hydrogen pressure
improved the rate of hydrogenation of the CdC bond while
decreasing the enantioselectivities (Table 1, entry 14 vs entry
15). The results indicated that the reactivity of the CdO bond
toward hydrogenation is better than that of the CdC bond;
however, all attempts to achieve selective hydrogenation of the
CdO bond proved in vain. The optimized reaction conditions
were therefore set as the following: 1 mol % of [NH2Me2]+
[{RuCl [(S)-SunPhos]}2(µ-Cl3)] (6) as the catalyst, 6 mol % of
1 N HCl as the additive, and ethanol as the solvent, with the
substrate concentration of 0.2 M, and 14 bar of H2 at 70 °C.
TABLE 2. Asymmetric Hydrogenation of 1 with
[NH2Me2]+[{RuCl[(S)-SunPhos]}2(µ-Cl3)] (6)a
Under the optimized reaction conditions, a series of (E)-ethyl
2-oxo-4-arylbut-3-enoates were hydrogenated using [NH2Me2]+
[{RuCl [(S)-SunPhos]}2(µ-Cl3)] (6) as catalyst (Table 2). High
enantioselectivities have been achieved with all tested substrates.
The results show that the electron density and steric factors of
the aromatic ring of the substrate have little effect on the
enantioselectivities (ee 94-96%, entries 1-9).
When the substrate/catalyst ratio increased from 100 to 2000,
the hydrogenation of 1a still proceeded smoothly, keeping the
enantioselectivity almost unchanged (ee 94%, entry 1 vs ee 93%,
entry 10). Hydrolysis of (S)-ethyl 2-hydroxy-4-phenylbutyrate
(ee 93%) provided the (S)-2-hydroxy-4-phenylbutyric acid (7)
with 81% yield at 99% ee after a single recrystallization from
1,2-dichloroethylene (Scheme 3). Compared with ethyl 2-oxo-
4-phenylbutanoate (8), the ꢀ,γ-unsaturated R-ketoesters 1 are
more stable, easier to make, and more suitable for asymmetric
hydrogenation. This is a promising procedure for the preparation
of enantiomerically pure 2-hydroxy-4-phenylbutyric acid at large
scale.
a All reactions were carried out under 14 bar of hydrogen with a
substrate (1 mmol) concentration of 0.20 M in EtOH under 14 bar of H2
at 70 °C for 12 h. substrate/6/1 N HCl ) 100/1/6, conversion 100%.
b ee values were determined by HPLC analysis. c Configuration was
determined to be S by comparing the specific rotation with reported data
or by analogy. d 1a (30 mmol), S/C ) 2000, 24 h, conversion was
100%. e ee value after purification.
There are three possible reaction pathways from 1a to 2a
(Figure 1): (A) the CdC bond of 1a is first hydrogenated to
generate saturated ethyl 2-oxo-4-phenylbutanoate (8), and then
the CdO bond of 8 is reduced;12 (B) the CdO bond of 1a is
first hydrogenated to give allyllic alcohol 3, whose CdC double
bond is then reduced; or (C) allyllic alcohol 3 isomerizes to
8,13 followed by hydrogenation of 8 directly in the presence of
ruthenium catalyst.
FIGURE 1. Three possible reaction pathways from 1a to2a.
SCHEME 3. Purification of (S)-Ethyl
2-Hydroxy-4-phenylbutyrate
Our results have proved that the CdO bond of 1a is easier
to be hydrogenated to generate allyllic alcohol 3 (Table 1, entries
13-15). However, we cannot determine whether the double
(10) (a) Mashima, K.; Nakanura, T.; Matsuo, Y.; Tani, K. J. Organomet.
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Lambert, R. M. J. Am. Chem. Soc. 2006, 128, 7329.
bond can be isomerized prior to the reduction of the CdO
double bond. To gain further insight into the reaction pathway,
three experiments were conducted (Scheme 4). Hydrogenation
of 8 gave 2a with 91% ee, somewhat lower than 94% (Table 1,
entry 10); hydrogenation of 1a and racemic 3 at 1:1 ratio in
one pot afforded 2a with only 36% ee; hydrogenation of racemic
J. Org. Chem. Vol. 73, No. 18, 2008 7211

Meng et al.
SCHEME 4. Asymmetric Hydrogenation of 8, 1a, and/or
Racemic 3
recrystallization. This reaction has provided a useful procedure
for the preparation of the key intermediates for ACE inhibitors.
Experimental Section
Typical Procedure for the Asymmetric Hydrogenations. The
catalyst [NH2Me2]+[{RuCl [(S)-SunPhos]}2(µ-Cl3)] (6) was dis-
solved in degassed ethanol (20 mL) containing 1 N HCl aqueous
solution (240 µL), and then the solution was put into four vials
equally. To these vials were introduced the ꢀ,γ-unsaturated R-ke-
toesters (1 mmol), and then the four vials were taken into one
autoclave. The autoclave was purged three times with H2, and the
pressure of H2 was set to 14 bar. The vials in the autoclave were
stirred under the specified reaction conditions. After cooling to
ambient temperature and release of the hydrogen, the autoclave
was opened and the solvent was evaporated. The enantiomeric
excess was determined by HPLC after passing the samples through
a short pad of silica gel with hexanes and ethyl acetate.
Procedure for the Purification of the Hydrogenation
Product. (S)-Ethyl 2-Hydroxy-4-phenylbutyrate (6.13 g, 30 mmol)
was dissolved in EtOH/H2O (40/40 mL). LiOH ·H2O (1.39 g, 33
mmol) was then added, and the mixture was stirred at 40 °C for
2 h. The solvent was evaporated under reduced pressure at 40 °C,
and the residue was acidified by 6 mol/L H2SO4 aqueous solution
and extracted with ethyl acetate (30 mL × 3). After the organic
layer was separated and dried over MgSO4, the solvent was
evaporated, and the crude product was recrystallized from 1,
2-dichloroethylene to give 7 (4.38 g, 81%). After transfer of 7 to
2a in refluxing EtOH with a drop of concentrated sulfuric acid, the
3 yielded racemic 2a. On the basis of the experimental results
observed, the isomerization of allylic alcohol 3 to saturated
ketone 8 could be ruled out. Because we have not isolated or
detected compound 8 during and after the reaction, we assume
that hydrogenation of the CdO bond occurred prior to the
hydrogenation of the CdC bond in 1a; with the accumulation
of the primary hydrogenation product, hydrogenation of the
CdC bond in 3 occurred directly to give the completely
hydrogenated product 2a.
In conclusion, we have developed a convenient protocol for
highly enantioselective preparation of ethyl 2-hydroxy-4-aryl-
butyrate by hydrogenation of (E)-ethyl 2-oxo-4-arylbut-3-enoate.
Further investigation has proved that the hydrogenation proceeds
via a sequential hydrogenation of the CdO and CdC bonds.
The enantiomeric excess of ethyl 2-hydroxy-4-phenylbutyrate
can be easily upgraded from 93% to >99% after simple
1
ee value of 7 was determined to be 99%. Data for 7: H NMR
(400 MHz, CDCl3) δ 1.97-2.06 (m, 1H), 2.14-2.23 (m, 1H), 2.80
(t, J ) 8.0 Hz, 2 H), 4.27 (dd, J ) 4.0, 8.0 Hz, 1 H), 7.18-7.31
(m, 5H).13C NMR (100 MHz, CDCl3) δ 31.0, 35.6, 69.5, 126.3,
1
128.48, 128.54, 147.0, 179.8. Data for 2a: H NMR (400 MHz,
CDCl3) δ 1.28 (t, J ) 7.2 Hz, 3 H), 1.90-1.99 (m, 1H), 2.10-2.16
(m, 1H), 2.70-2.79 (m, 2H), 2.81 (br, 1H), 4.17-4.20 (m, 1H),
4.21 (q, J ) 7.2 Hz, 2 H), 7.20-7.31 (m, 5H). 13C NMR (100
MHz, CDCl3) δ 14.0, 30.9, 35.8, 61.5, 69.6, 125.8, 128.2, 128.4,
141.0, 175.1. HPLC (Chiralcel OD-H column, hexane/iPrOH 95/
5, 0.8 mL min-1, 220 nm): tR (major) ) 10.3 min, tR (minor) )
15.6 min.
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Lightfoot, A.; Pfaltz, A.; Rosner, T.; Schnider, P.; Zimmermann, N. Chirality
2000, 12, 442. (d) van der Drift, R. C.; Bouwman, E.; Drent, E. J. Organomet.
Chem. 2002, 650, 1. (e) Uma, R.; Cre´visy, C.; Gre´e, R. Chem. ReV. 2003, 103,
27. (f) Cadierno, V.; Garc´ıa-Garrido, S. E.; Gimeno, J. Chem. Commun. 2004,
232. (g) Ito, M.; Kitahara, S.; Ikariya, T. J. Am. Chem. Soc. 2005, 127, 6172.
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Acknowledgment. We thank the National Natural Science
Foundation of China, the Science and Technology Commission
of Shanghai Municipality, and the Education Commission of
Shanghai Municipality for financial support.
Supporting Information Available: NMR and/or HPLC data
of compounds 1, 2, 3, 7 and 8. This material is available free
of charge via the Internet at http://pubs.acs.org.
´
Varela.Alvarez, A.; Sordo, J. A. J. Am. Chem. Soc. 2006, 128, 1360. (j) Crochet,
P.; Ferna´ndez-Zu´mel, M. A.; Gimeno, J.; Scheele, M. Organometallics 2006,
25, 4846.
JO801140J
7212 J. Org. Chem. Vol. 73, No. 18, 2008