Heterogeneous catalytic hydrogenation of olefinic substrates by poly-NAP

Article abstract of DOI:10.1016/S0040-4039(00)00377-4

With our previously described poly-NAP, various olefinic substrates were reduced with selectivities comparable to those obtained by BINAP. For substrates which contained a methyl ester, the selectivities were higher than those observed for their carboxylic acid analogues. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00377-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 3323–3326
Heterogeneous catalytic hydrogenation of olefinic substrates by
poly-NAP
a
a
b
a,∗
Rob ter Halle, Emmanuelle Schulz, Michel Spagnol and Marc Lemaire
a
Laboratoire de catalyse et synthèse organique, UCBL, CPE, 43, Bd du 11 Novembre 1918, 69622 Villeurbanne Cedex,
France
b
Rhône-Poulenc, CRIT-Carrières, 85, avenue des Frères-Perret, BP 62, 69192 St-Fons Cedex, France
Received 9 December 1999; accepted 2 March 2000
Abstract
With our previously described poly-NAP, various olefinic substrates were reduced with selectivities comparable
to those obtained by BINAP. For substrates which contained a methyl ester, the selectivities were higher than those
observed for their carboxylic acid analogues. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: asymmetric reactions; heterogeneous catalysis; hydrogenation; dehydro-amino acids; olefins.
One of the first examples of successful enantioselective catalysis was described as the hydrogenation
of prochiral acetamidoacrylic acid derivatives. By the introduction of diphosphine ligands such as
1
2
3
4
DIOP, CHIRAPHOS, BINAP and DUPHOS enantioselectivities of up to 99.8% were obtained in
the rhodium- or ruthenium-catalyzed reactions. Besides these acetamidoacrylic acid derivatives, a wide
5
6
variety of other functionalized olefins, such as itaconates or enol esters, have been successfully reduced.
To extend the utility of these (already) useful catalysts we decided to heterogenize BINAP in order
to obtain a catalyst which could combine both the advantages of homogeneous catalysis (high enan-
tioselectivities and TONs) and heterogeneous catalysis (easy work-up procedures and reusability). As
7
recently described, our philosophy of heterogenization was to polymerize a methylamine functionalized
BINAP with a diisocyanate (Scheme 1) to yield poly-NAP. This is in contrast with recently published
8
work of Bayston et al. in which a functionalized BINAP was anchored to an already existing polymer.
Simultaneous to our work, however, Chan et al. published the synthesis of chiral polyester-supported
9
BINAP ligands, also containing a chiral diol in the main chain. These chelating polymers were tested
in the Ru-catalyzed hydrogenation of 2-arylacrylic acids to the reduced products in high yields and
enantioselectivities. These activities and enantioselectivities, arising from a double-induction, proved
to be maintained during several runs.
∗
Corresponding author. Fax: 33 4 72 43 14 08; e-mail: marc.lemaire@univ-lyon1.fr (M. Lemaire)
0
040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00377-4
tetl 16657

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Scheme 1. Synthesis of polymerized BINAP
It was shown that with poly-NAP, various β-keto-esters could be hydrogenated with excellent enan-
tioselectivities and low catalyst/substrate ratios. Furthermore, the catalyst could easily be reused by
simple filtration without loss of either selectivity or activity. In this article, we wish to present the
extension of this heterogeneous catalyst, poly-NAP, towards the hydrogenation of olefinic substrates.
The catalyst was prepared by modification of the procedure described by Noyori et al., which gives the
1
0
11
RuCl ·dmf complex and the procedure described by Takaya et al., which yields the RuCl complex.
2
2
Both catalysts were brown solids and used without purification for the hydrogenation.
Since the degree of enantioselection is highly affected by the hydrogen pressure, by the substrate
concentration, the temperature and the solvent,12 we decided to perform the same reaction with BINAP,
parallel to our catalytic tests, in order to get a real comparison between the homogeneous and hetero-
geneous hydrogenation systems. The results for the reduction of α-acetamido acrylic acid 1 and methyl-
α-acetamido cinnamate 2, yielding the products 3 and 4, respectively, are summarized in Table 1.
Table 1
Hydrogenation of substrates 1 and 2
As shown in Table 1, the results for poly-NAP and BINAP are comparable for substrate 2. Substrate 1
was reduced with our polymeric catalyst with a slightly lower enantioselectivity as compared to BINAP.
The advantage of this heterogeneous catalyst is the easier work-up procedure (a filtration allows the

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obtention of the reduced products). Most importantly, the residue could be reused without loss of activity
or enantioselectivity as was observed for substrate 1.
Encouraged by these results we decided to test other substrates with unsaturated double bonds. The
hydrogenation of substrates 5a and 5b at 50°C are summarized in Table 2.
Table 2
Hydrogenation of itaconic acid and dimethyl itaconate
Comparable results were obtained for both poly-NAP and BINAP for substrate 5b. In the case of
the itaconic acid, bearing a hydroxy instead of a methoxy group, the enantioselectivity was slightly
reduced (entries 1 and 2). The same effect was observed for the dehydro-amino acids, where the acid
also conducted to a small decrease of enantioselectivity, whereas the ester gave comparable results for
both poly-NAP and BINAP.
Surprisingly, tiglic acid 7 was hydrogenated with the poly-NAP–RuCl complex to give product 8,
2
with the same enantioselectivity as obtained for the corresponding BINAP complex (see Table 3).
Table 3
Hydrogenation of tiglic acid
In conclusion, we can say that dehydro-amino acids, itaconic acid and tiglic acid were quantitatively
reduced with enantioselectivities similar to those obtained by the BINAP in the ruthenium-catalyzed
hydrogenation. The methyl esters of these substrates gave enantioselectivities which were comparable to

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326
those obtained by the BINAP reduction. After filtration, the catalyst can be reused without loss of either
activity or enantioselectivity.
This method offers a heterogeneous alternative for BINAP-catalyzed hydrogenations of olefinic
substrates.
1. Experimental
RuCl catalyst: 5 mg (0.0058 mmol) of (S)-poly-NAP was stirred with 1.3 mg (0.0027 mmol) of a 8:1
2
mixture of EtOH:benzene for 1 h at 50°C. The solvent was removed under reduced pressure to give a
brown solid.
RuCl ·dmf catalyst: 5 mg (0.0058 mmol) of (S)-poly-NAP was stirred with 1.3 mg (0.0027 mmol) of
2
[
RuCl (benzene)] in 1 mL DMF at 100°C for 1 h. The solvent was removed under reduced pressure to
2 2
yield a brown solid.
Typical procedure: To the prepared catalyst were added the substrate and 2 mL of MeOH. The resulting
suspension was stirred overnight at 50°C under 40 bars of hydrogen pressure. After centrifugation of the
suspension, the liquid phase was recovered by syringe. The free carboxylic acids were converted to their
methyl esters by addition of SOCl and further stirring for 2 h. The esters were analyzed either by GC or
2
™
HPLC to determine both conversion and enantiomeric excess (GC: Supelco β-dex 225, 30 m. HPLC:
J. T. Baker, Chiracel OD, heptane:isopropanol 9:1, 1 mL/min).
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