Asymmetric hydrogenation of ethyl pyruvate using layered double hydroxides-supported nano noble metal catalysts

Article abstract of DOI:10.1080/00397910601163893

Layered double hydroxide (LDH)-supported nano noble metal heterogeneous catalysts are synthesized by ion exchange of K2PtCl6, Na2PdCl4 and impregnation of RhCl3.3H2O followed by reduction with H2. The LDH-Rh, Pt, and Pd catalysts are tested in the enantioselective hydrogenation of ethyl pyruvate to ethyl lactate with very good yields and enantiomeric excess's (e.e.'s) of up to 72% were obtained with Pt. The catalyst was recovered and reused for several cycles with consistent activity. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910601163893

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Asymmetric Hydrogenation of Ethyl Pyruvate
using Layered Double Hydroxides–Supported
Nano Noble Metal Catalysts
Mannepalli Lakshmi Kantam a , Karasala Vijaya Kumar a & Bojja Sreedhar a
a Inorganic and Physical Chemistry Division , Indian Institute of Chemical
Technology , Hyderabad, India
Published online: 24 Feb 2007.
To cite this article: Mannepalli Lakshmi Kantam , Karasala Vijaya Kumar & Bojja Sreedhar (2007) Asymmetric
Hydrogenation of Ethyl Pyruvate using Layered Double Hydroxides–Supported Nano Noble Metal Catalysts,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
37:6, 959-964, DOI: 10.1080/00397910601163893
To link to this article: http://dx.doi.org/10.1080/00397910601163893
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Synthetic Communicationsw, 37: 959–964, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910601163893
Asymmetric Hydrogenation of Ethyl
Pyruvate using Layered Double
Hydroxides–Supported Nano Noble Metal
Catalysts
Mannepalli Lakshmi Kantam, Karasala Vijaya Kumar, and
Bojja Sreedhar
Inorganic and Physical Chemistry Division, Indian Institute of Chemical
Technology, Hyderabad, India
Abstract: Layered double hydroxide (LDH)–supported nano noble metal hetero-
geneous catalysts are synthesized by ion exchange of K2PtCl6, Na2PdCl4 and impreg-
nation of RhCl.33H2O followed by reduction with H2. The LDH–Rh, Pt, and Pd
catalysts are tested in the enantioselective hydrogenation of ethyl pyruvate to
ethyl lactate with very good yields and enantiomeric excess’s (e.e.’s) of up to 72%
were obtained with Pt. The catalyst was recovered and reused for several cycles
with consistent activity.
Keywords: Cinchonidine, enantioselective hydrogenation, ethyl pyruvate, ethyl lactate
LDH–Pt, Rh, Pd nano particles
INTRODUCTION
Asymmetric synthesis is one of the most elegant strategies for the synthesis
of drugs, agrochemicals, and fragrances/flavors. Homogeneous transition-
metal complexes with chiral ligands are the most versatile and efficient
catalysts.[1a,b] However, for industrial processes, heterogeneous catalysts are
preferred because they can be easily separated and reused. In recent years,
Received in India August 11, 2006
Address correspondence to Mannepalli Lakshmi Kantam, Inorganic and Physical
Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500 007,
India. E-mail: mlakshmi@iict.res.in or lkmannepalli@yahoo.com
959

960
M. L. Kantam et al.
heterogeneous asymmetric catalysts are playing an important and crucial role
in the development of new chemical processes.[2] Asymmetric hydrogenation
of a-ketoesters was reported two decades ago by Orito et al.,[3] in which
cinchona alkaloid–modified platinum was used for enantioselective hydro-
genation of ethyl pyruvate to ethyl lactate. Later, Blaser et al.,[4a–d] Margit-
faivi et al.,[5] Sutherland et al.,[6] Augustine et al.,[7] and Torok et al.[8] have
investigated this reaction. The highest enantiomeric excess (e.e. 95–97%)
was obtained under optimum reaction conditions using the supported Pt/
cinchonidine systems.[4,8] As far as the properties of the supported platinum
catalysts are concerned, proper platinum dispersion, the type of support
material, and pore size distribution are very important. Suitable supports
such as alumina, silica, and zeolites have been studied. Collier et al.[9] have
investigated the enantioselective hydrogenation of ethyl pyruvate by
solvent-stabilized nanoparticles of Pt and Pd prepared by metal vapor
synthesis. Recently enantioselective hydrogenation of ethyl pyruvate was
reported by supported Rh nanoclusters stabilized by polyvinylpyrrolidene
(PVP) with cinchonidine as chiral modifier.[10] The utilization of other
transition metals, such as Ru, Rh, Pd, and Ir, to replace Pt resulted in lower
enantioselectivity.[9–11]
Layered double hydroxides (LDHs), a new class of basic mixed hydrox-
ides, have recently received much attention in view of their potential useful-
ness as materials, anion exchangers, and more importantly as catalysts.[12]
The LDHs consist of alternating cationic M(II)(12x)M(III)x(OH)2xþ and
anionic An2 ZH2O layers. The positively charged layers contain edge-
.
shared metal M(II) and M(III) hydroxide octahedral, with charges neutralized
by An2 anions located in the interlayer spacing or at the edges of the
lamellae.
Recently we have reported LDH-supported nanopalladium catalyst for
Heck, Suzuki, Sonigashira, and Stille-type coupling reactions.[13] In continu-
ation of our work on LDHs, herein we report the catalytic performance of
supported LDH–Rh, Pt, and Pd nanoparticles in the enantioselective hydro-
genation of ethyl pyruvate using cinchona alkaloid as chiral modifier
(Scheme 1). The effect of some reaction parameters on the enantioselectivities
and catalytic activities have been studied in detail.
Scheme 1. Asymmetric hydrogenation of ethyl pyruvate.

LDH-Supported Nano Noble Metal Catalysts
RESULTS AND DISCUSSION
961
In asymmetric synthesis, the enantioselectivity is very sensitive to the solvent.
Asymmetric hydrogenation of ethyl pyruvate is catalyzed by LDH–M
(M ¼ Rh, Pt or Pd)/cinchonidine system. The enantioselectivity data in
Table 1 clearly indicates that among the solvents investigated, THF is the
best solvent with e.e. values 72, 70, and 68 for LDH–Pt, LDH–Rh, and
LDH–Pd, respectively. A similar effect was observed in the hydrogenation
of ethyl pyruvate using finely dispersed oxide-supported rhodium nanoclusters
with cinchonidine.[10] With LDH–Rh catalyst and cinchonidine modifier in
acetic acid as solvent, complete conversion of ethyl pyruvate to ethyl
lactate with 80% e.e. is observed, but the main drawback with this system
is leaching of the metal from the support. All the catalysts are recyclable,
and consistent activity is observed for three cycles.
During the enantioselective hydrogenation of ethyl pyruvate to ethyl
lactate catalyzed by LDH–Rh, LDH–Pt, and LDH–Pd, cinchonidine not
only functions as a chiral modifier that modifies ethyl pyruvate into (R)-
ethyl lactate but also accelerates the reaction rate. Initially the e.e.’s
increase with an increase in cinchonidine, but on further increase there is a
decrease of enantioselectivity. The decrease in e.e. when the concentration
of cinchonidine is high is due to the side reaction of cinchonidine during
the conversion of ethyl pyruvate to ethyl lactate. Also some cinchonidine
was getting hydrogenated, resulting in the decrease in enantioselectivity.[14]
First the C-C double bond in the quinuclidine part of cinchonidine is hydro-
genated followed by the hydrogenation of the aromatic ring, which causes a
drop in enantioselectivity by decreasing the efficiency of the chiral modifier
and also because of the blocking of some active sites on the surface of the
metal at higher concentration of modifier.[4a]
Table 1. Effect of solvent on the enantioselective hydrogenation of ethyl pyruvate to
ethyl lactate
Catalyst
LDH–Pt
Solvent
Time (h)
Conversion (%)
e.e. (%)
THF
6
6
6
6
6
6
6
6
6
100
100
100
100
100
100
100
100
100
72
65
57
70
60
55
68
50
45
Methanol
Toluene
THF
LDH–Rh
LDH–Pd
Methanol
Toluene
THF
Methanol
Toluene
Notes. Reaction conditions: Substrate (8.6 mmol), catalyst (50 mg), solvent (15 mL),
room temp., H2 pressure (70 bar). e.e.’s based on HPLC yields.

962
M. L. Kantam et al.
In summary, the LDH–Pt, LDH–Rh, and LDH–Pd catalysts developed
by the simple ion exchange or impregnation technique followed by
reduction are used for the asymmetric hydrogenation of ethyl pyruvate to
ethyl lactate. The best result is obtained with LDH–Pt with e.e.’s up to
72%. The catalysts were reused for three cycles with consistent activity.
EXPERIMENTAL
.
Ethyl pyruvate was used as received from Fluka. AlCl3 6H2O and
.
.
MgCl2 6H2O were purchased from Aldrich. RhCl3 3H2O was purchased
from Degussa Corporation, K2PtCl6 and Na2PdCl4 were purchased from
Loba chemicals, and all the other chemicals were procured from local com-
mercial sources and used as such without further purification. All the
solvents procured from commercial sources were distilled before use.
General Procedure for the Preparation of LDH (Mg-Al-Cl) Support
The preparation of LDH (Mg–Al–Cl) was based on the literature procedure.[15]
.
.
A mixture of MgCl2 6H2O (30.49 g, 0.15 mol) and AlCl3 6H2O (12.07 g,
0.05 mol) was dissolved in 200 ml of decarbonated and deionized water. To
this aqueous solution 100 ml of sodium hydroxide (2 M) solution at 258C
was slowly added, and a further amount of 2 M NaOH solution was added to
maintain a pH of 10 under nitrogen flow. The resultant suspension was
stirred overnight at 708C. The solid product was isolated by filtration, washed
thoroughly with deionized water, and dried overnight at 808C.
General Procedure for the Preparation of LDH–Pt and LDH–Pd
Catalysts
LDH–Pt and LDH–Pd catalysts were prepared by the ion exchange method.
K2PtCl6 (0.200 g, 0.4115 mmol) and Na2PdCl4 (0.200 g, 0.8223 mmol) were
dissolved separately in 50 ml of deionized water. To these aqueous solutions,
1 g of LDH (Mg–Al–Cl) was added and stirred overnight at room tempera-
ture under a nitrogen atmosphere. The solid catalyst was filtered and
washed thoroughly with 500 ml of deionized water, dried at 808C overnight,
and finally reduced with hydrogen. The platinum content in the LDH–Pt
catalyst was 0.6 wt.%, and the Pd content in the LDH–Pd catalyst was
0.5 wt.% as determined by atomic absorption spectroscopy.
General Procedure for the Preparation of LDH–Rh Catalyst
.
RhCl3 3H2O (0.200 g, 0.7601 mmol) was dissolved in 50 ml of deionized
water. To this aqueous solution, 1 g of LDH was added, and stirring

LDH-Supported Nano Noble Metal Catalysts
963
continued at room temperature overnight under a nitrogen atmosphere. The
solid catalyst was filtered and washed with 500 ml of deionized water until
the catalyst was free from chloride. The catalyst was dried at 808C
overnight and finally reduced with hydrogen. The Rh content in the LDH–
Rh catalyst was 0.8 wt.% as determined by atomic absorption spectroscopy.
General Procedure for the Asymmetric Hydrogenation of Ethyl
Pyruvate
The hydrogenation reactions were performed in a 100 ml stainless steel
autoclave at room temperature. To 1 g (8.6 mmol) of redistilled ethyl
pyruvate in 15 ml of solvent, 6 mg (0.02 mmol) of the chiral modifier, cinch-
onidine, and 50 mg of the catalyst were added, and the reaction mixture was
flushed with hydrogen gas several times before the autoclave was pressurized
with hydrogen gas up to 70 bar. The stirring was continued at 1000 rpm and
the progress of the reaction was monitored every hour by high-performance
liquid chromatography (HPLC).
ACKNOWLEDGMENTS
We thank the CSIR for financial support under the Task Force Project CMM-
0005.
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