F. Robert, D. Sinou / Journal of Organometallic Chemistry 604 (2000) 99–102
101
3.1. Hydrogenation
Hydrogenation was performed under normal pres-
sure and at 25°C. The solvent, the substrate, the surfac-
tant, the rhodium complex [Rh(COD) ]PF6 and the
2
phosphane (3) were placed in a deaerated hydrogena-
tion flask and stirred for 15 min in an argon atmo-
sphere. Then argon was replaced by hydrogen and the
reaction was followed by a volumetric measurement at
Scheme 2.
2
5°C. When the reaction was complete, the mixture was
rhodium in a micellar system. We notice also in this
latter case the formation of metallic rhodium as in
methanol. However, in methanol as the solvent, reduc-
tion occurs probably via the homogeneous system and
also via metallic rhodium, giving lower enantioselectivi-
ties and also unreproducible results. In the presence of
SDS, it was previously assumed that the reaction occurs
only in the micelle [30,31,34]; in this case, the reduction
is only due to the soluble organometallic complex
which is in the micelle, and so the enantioselectivity will
be higher.
extracted with chloroform in the case of the methyl
ester, the conversion was determined by NMR and the
enantioselectivity by glc. In the case of the acid, the
solvent was evaporated and the residue dissolved in
ethanol was esterified with diazomethane; then the
enantioselectivity was measured by glc. The enan-
tiomeric excess (% ee90.5%) was determined by glc on
the methyl ester of phenylalanine with a 10 m capillary
column coated with XE-60-
.2. Transfer hydrogenation
Transfer hydrogenation was performed by mixing the
L-valine-tert-butylamide.
3
We also tested some of these ligands for the asym-
metric reduction of acetophenone via transfer hydro-
genation [36] with isopropanol as the hydride source
rhodium (5%) or the ruthenium complex (0.1%), the
ligand (ligand/metal=1), KOH (25%), i-PrOH, and
acetophenone under a nitrogen atmosphere at the de-
sired temperature. Conversion and e.e. were determined
by glc with a capillary column Cydex-B (25 m).
(
Scheme 2). The catalysts prepared from [Rh(COD)Cl]2
and ligands 3b or 3e are very active (94 and 99%
conversion, respectively, after 24 h at 50°C); however,
the enantioselectivities are very low: 16 and 5% ee,
respectively. The catalyst prepared by mixing
RuCl (PPh ) and one equivalent of ligand 3d is also
2
3 3
3
very active using a ratio [substrate]/[catalyst] of 10
Acknowledgements
(
100% conversion after 1 h at 80°C), but the enantiose-
lectivity is again very low (4%).
In conclusion, we have shown that 4-(dialkylamino)-
We are grateful to the MENESR (fellowship to F.B.)
and the R e´ gion Rh oˆ ne-Alpes for financial support.
1-(diphenylphosphanyl)butanes, easily obtained from
tartaric acid, are effective ligands in the reduction of
unsaturated aminoacid precursors. The problem of the
decrease in enantioselectivity, due probably to the for-
mation of metallic rhodium, can be circumvented by
performing the reduction in water in the presence of a
surfactant. The use of these ligands in hydrogen trans-
fer reactions gave very active catalysts, exhibiting how-
ever very low enantioselectivities. Work is currently
directed towards the modification of the nitrogen sub-
stituents in order to obtain more understanding of the
influence of the substituents at the nitriogen on both
the activity and the enantioselectivity of the catalyst,
and so to increase the enantioselectivity of the
reduction.
References
[
[
[
[
1] W.S. Knowles, M.J. Sabacky, J. Chem. Soc. Chem. Commun.
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5
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[
[
[
[
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3. Experimental
[
[
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The syntheses of ligands 3a–d have been previously
described [26]. Sodium dodecyl sulfate (SDS) is from a
commercial source and was used as obtained.
[12] O. Reiser, Angew. Chem. Int. Ed. Engl. 32 (1993) 547.