Self-supported heterogeneous catalysts for enantioselective hydrogenation

Article abstract of DOI:10.1021/ja047372i

"Self-supported" catalysts were prepared by the reaction of bis-MonoPhos ligand with rhodium(I) metallic ion on the basis of molecular assembling through coordination. These polymeric metal-organic assemblies are insoluble in common organic solvents and,

Full text of DOI:10.1021/ja047372i

Published on Web 08/10/2004
Self-Supported Heterogeneous Catalysts for Enantioselective Hydrogenation
Xingwang Wang and Kuiling Ding*
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry,
Chinese Academy of Sciences, 354 Fenglin Road, Shanghai 200032, P. R. China
Received May 5, 2004; E-mail: kding@mail.sioc.ac.cn
The immobilization of homogeneous chiral catalysts using
various solid insoluble supports, including inorganic materials,
organic polymers, and membranes as supports, could solve some
problems of homogeneous catalysts such as difficult recovery of
expensive chiral catalyst and metal contaminants leached from the
catalysts in the products, to some extent.1 However, in the classical
approaches, the chiral ligands or the catalytically active units are
randomly anchored onto irregular polymers and other supports, and
the resulted immobilized catalysts often displayed reduced enan-
tioselectivity and less efficiency in the catalysis in comparison with
,2
their homogeneous counterparts. The functional metal-organic
Figure 1. (a) Self-supported chiral Rh catalyst 2a (yellow solids at the
assemblies might provide an alternative approach to heterogeneous
catalysis to overcome the problems mentioned above since these
self-assembled materials have shown permanent porosity and
bottom of the reactor) in toluene. (b) SEM image of the self-supported Rh
catalyst 2a. The scale bar indicates 5 µm.
Scheme 1. Heterogenization of Ferringa’s Catalyst by
Self-Supported Strategy
3
absorption capacity for organic guest molecules. Aoyama and co-
workers have demonstrated the catalytic properties of nonchiral
4
metal-organic solids for Diels-Alder reaction. Accordingly, the
design and synthesis of chiral metal-organic frameworks or
polymers might provide a facile strategy for asymmetric hetero-
geneous catalysis, because the bridged chiral ligand can spontane-
ously form a chiral environment on the surface of the solids or
inside the cavities of the materials for enantioselective control of
the reaction, and the metal ion acts as the catalytically active center
5
without using any supports. Therefore, the use of chiral metal-
organic assemblies can be considered as a “self-supported strategy”5
for heterogenization of homogeneous catalyst in enantioselective
reactions.6 In the present work, we report our results on the
,7
8
heterogenization of Ferringa’s MonoPhos/Rh catalyst using self-
supported strategy for enantioselective hydrogenations of R-de-
9
hydroamino acid and enamide derivatives, affording a variety of
enantioenriched biologically important compounds with high yields
and enantioselectivities.
tive substrates, including â-aryl- or alkyl-substituted dehydro-R-
amino acid and enamide derivatives 3a-d (Table 1). The reactions
The linker-bridged bis-MonoPhos ligands (1a-c) were prepared
by the reaction of hexamethylphosphorus triamide (HMPT) with
corresponding bis-BINOL derivatives6 in good yields. The ligands
were carried out at room temperature under 40 atm pressure of H
2
c
with a substrate concentration of 0.2 M. As shown in Table 1, for
all three catalysts examined, the complete conversion of the
substrates could be achieved within 10 h and the enantioselectivities
of hydrogenation were excellent (94.3-97.3% ee), which are
comparable to the cases of homogeneous catalysis at the same level
1
a-c and the catalyst precursor [Rh(cod)
toluene and dichloromethane, respectively. The solution of [Rh-
cod) ]BF in dichloromethane was added to the solution of ligands
a-c, and the orange solids precipitated immediately. After removal
2 4
]BF were dissolved in
(
1
2
4
of the solvents, the resulting powders (2a-c) were washed with
toluene to remove the trace amount of soluble low-molecular weight
materials. Elemental analysis showed that the composition of the
resulting solids were consistent with the structures of 2a-c
expected. As shown in Figure 1a, these yellow polymeric solids
were completely insoluble in toluene and, accordingly, fulfill one
of the basic prerequisites of heterogeneous catalysis, and toluene
was selected as the reaction mediate for the heterogeneous
hydrogenation of olefin derivatives. SEM images showed that these
solids were composed of micrometer particles (Figure 1b), while
powder X-ray diffraction (PXD) indicated that they were non-
crystalline solids.The self-supported catalysts 2a-c were then
submitted to the catalysis of the hydrogenation of some representa-
of catalyst loading. Particularly, the self-supported catalysts
8
demonstrated improved enantioselectivity (entries 4, 8, and 12) in
the hydrogenation of 3d in comparison with the cases using
MonoPhos/Rh homogeneous catalyst (87.8-88.8% ee) under
otherwise identical conditions.10
In an effort to probe the heterogeneous or homogeneous nature
of the above catalyst systems, the supernatants of 2c in toluene
were employed for the catalysis of hydrogenation of 3a under the
same experimental conditions; no product was observed at all. This
experiment unambiguously demonstrated the heterogeneous nature
of the present catalytic systems. The inductively coupled plasma
(ICP) spectroscopy analyses of the reaction mixtures containing
substrate 3a or product 4a after filtration of the insoluble 2c
10524
9
J. AM. CHEM. SOC. 2004, 126, 10524-10525
10.1021/ja047372i CCC: $27.50 © 2004 American Chemical Society

C O MMU N I C A T I O N S
Table 1. Enantioselective Hydrogenation of Olefin Derivatives
and high density of the catalytically active units, as well as
comparable performance to that of the free catalysts in terms of
both activity and enantioselectivity.
a
3a-d under the catalysis of 2a-c
In conclusion, we have demonstrated a self-supported strategy
in the generation of heterogeneous enantioselective catalyst through
assembly of bridged monophosphoramidite chiral ligand with Rh-
(I) ion for asymmetric hydrogenation of olefin derivatives, affording
a variety of enantioenriched amino acid and amine derivatives with
high yields and enantioselectivities. This strategy might provide a
new direction in asymmetric catalysis, particularly for the develop-
ment of practical heterogeneous asymmetric synthesis of optically
active compounds.
entry
catalyst
substrate
ee (%)b
1
2
3
4
5
6
7
8
9
2a
2a
2a
2a
2b
2b
2b
2b
2c
2c
2c
2c
3a
3b
3c
3d
3a
3b
3c
3d
3a
3b
3c
3d
95.8
95.7
96.6
97.3
94.3
94.9
94.7
96.8
95.0
95.9
96.2
95.9
Acknowledgment. Financial support from the NNSFC, CAS,
and the Major Basic Research Development Program of China
Grant G2000077506) and the Ministry of Science and Technology
of Commission of Shanghai Municipality is gratefully acknowl-
edged. This paper is dedicated to Professor Li-Xin Dai on the
occasion of his 80th birthday.
(
1
1
1
0
1
2
Supporting Information Available: Experimental procedures and
figures of SEM and PXD images of catalysts and chiral HPLC or CG
analysis of the products (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
a
All of the reactions were carried out at 25 °C under 40 atm pressure of
H2 at a substrate concentration of 0.2 M for 10 h (substrate/catalyst ) 100:
1
1
). The conversion of the substrates determined by H NMR was >99%.
References
b
Determined by HPLC on a Chiralcel AD column and GC on a Supelco
BETA-DEX120 or GAMMA-DEX 225 column.
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a
in Enantioselective Hydrogenation of 3a
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8
4
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(
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(
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(
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(
(
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