A enantioselective hydrogenation

Lightfoot, A., Schnider, P. and Pfaltz, A. Enantioselective Hydrogenation of Olefins with Iridium-Phosphanodihydrooxazole Catalysts. Angew. Chem. Int. Ed. 1998, 37, 2897-2899. 

Minder, B., Mallat, T., Pickel, K.H., Steiner, K., Baiker, A. Enantioselective Hydrogenation of Pyruvate in Supercritical Fluids. Catalysis Letters. 1995, 34, 1-9. 

Genet, J.P, Pinel, C., Ratovelomanana-Vidala, V., MaUarL S., Pfister, X., Bischoff, L., Cano De Andrade, M.C., Darses, S., Galopin, C., and Laffitte, J.A., Enantioselective hydrogenation reactions with a fuU set of preformed and prepared in situ chiral diphosphine-ruthenium(II) catalysts. Tetrahedron Asymmetry, 5, 675, 1994. 

Minder B, Mallat T, Pickel KH, Steiner K, Baiker A. Enantioselective hydrogenation of ethyl pyruvate in supercritical fluids. Catal Lett 1995 34 1-9. 

Minder B, Mallat T, Baiker A. Enantioselective hydrogenation in supercritical fluids. Limitations of the use of supercritical CO2. In von Rohr PR, Trepp C, eds. High Pressure Chemical Engineering Proceedings of the 3rd International Symposium on High Pressure Chemical Engineering, Zurich, Switzerland, 7-9 October, 1996. Amsterdam Elsevier, 1996 139-144. 

Catalytic asymmetric hydrogenation was one of the first enantioselective synthetic methods used industrially (82). 2,2 -Bis(diarylphosphino)-l,l -binaphthyl (BINAP) is a chiral ligand which possesses a Cg plane of symmetry (Fig. 9). Steric interactions prevent interconversion of the (R)- and (3)-BINAP. Coordination of BINAP with a transition metal such as mthenium or rhodium produces a chiral hydrogenation catalyst capable of inducing a high degree of enantiofacial selectivity (83). Naproxen (41) is produced in 97% ee by... 

A number of enantioselective hydrogenation reactions in ionic liquids have also been described. In all cases reported so far, the role of the ionic liquid was mainly to open up a new, facile way to recycle the expensive chiral metal complex used as the hydrogenation catalyst. 

William Knowles at the Monsanto Company discovered some years ago that u-amino acids can be prepared enantioselectively by hydrogenation of a Z enam-ido acid with a chiral hydrogenation catalyst. (S)-Phenylalanine, for instance, is prepared in 98.7% purity contaminated by only 1.3% of the (H) enantiomer when a chiral rhodium catalyst is used. For this discovery, Knowles shared the 2001 Nobel Prize in chemistry. 

These examples are part of a broader design scheme to combine catalytic metal complexes with a protein as chiral scaffold to obtain a hybrid catalyst combining the catalytic potential of the metal complex with the enantioselectivity and evolvability of the protein host [11]. One of the first examples of such systems combined a biotinylated rhodium complex with avidin to obtain an enantioselective hydrogenation catalyst [28]. Most significantly, it has been shovm that mutation-based improvements of enantioselectivity are possible in these hybrid catalysts as for enzymes (Figure 3.7) [29]. 

Table 2 Enantioselective hydrogenation of a-acylamino acrylic acid derivatives...

Chitosan (Fig. 27) was deposited on sihca by precipitation. The palladium complex was shown to promote the enantioselective hydrogenation of ketones [80] with the results being highly dependent on the structure of the substrate. In the case of aromatic ketones, both yield and enantioselectiv-ity depend on the N/Pd molar ratio. Low palladium contents favored enan-tioselectivity but reduced the yield. Very high conversions were obtained with aliphatic ketones, although with modest enantioselectivities. More recently, the immobilized chitosan-Co complex was described as a catalyst for the enantioselective hydration of 1-octene [81]. Under optimal conditions, namely Co content 0.5 mmolg and 1-octene/Co molar ratio of 50, a 98% yield and 98% ee were obtained and the catalyst was reused five times without loss of activity or enantioselectivity. 

Chitin (Fig. 27) was supported on silica by grinding the two solids together. The Pt complex was tested as a catalyst in the enantioselective hydrogenation of racemic 1-phenylethanol to obtain (i )-l-cyclohexylethanol [82]. Up to 65% yield with 100% ee was obtained and the catalyst was reused five times with almost the same results. 

An iron complex-catalyzed enantioselective hydrogenation was achieved by Morris and coworkers in 2008 (Scheme 13) [49]. Reaction of acetophenone under moderate hydrogen pressure at 50°C catalyzed iron complex 12 containing a tetradentate diimi-nodiphosphine ligand in the presence of BuOK afforded 1-phenylethanol with 40% conversion and 27% ee. 

Hydrogenation of the free acids over unmodified catalyst occurred slowly, proceeded to completion in 20 h and gave racemic product as expected Enantioselective hydrogenation occurred at a slower rate over alkaloid-modified catalyst, cinchonidine modification providing an excess of S-product and cinchonine an excess ofR-product... 

Racemic and enantioselective hydrogenations of tiglic acid each exhibited an apparent activation energy of 17 kJ mol (268 to 308 K). Enantiomeric excess was constant at 20 to 23% over the range 273 to 308 K but lower, 13%, at 268 K. Enantioselective hydrogenation of trifluorotiglic acid exhibited an activation energy of 23 kJ mol" (253 to 323 K) and a temperature-independent enantiomeric excess of 13 2%. 

No rate enhancement of the enantioselective hydrogenation pathway is expected, in the manner adduced for the Pt-catalysed reaction, because the process is not one of simple H-atom addition across a carbon-oxygen double bond. 

The similarities of above experimental results inspired us to investigate the role of SE in heterogeneous catalytic enantioselective hydrogenation reactions. In heterogeneous catalytic reaction the SE means that a given template molecule interacts with the prochiral substrate in the liquid phase in such a way that one of the prochiral sites is preferentially shielded. If the substrate is shielded then its adsorption onto the metal can take place with its unshielded site resulting in ED. 

The principles of the SE were applied for two enantioselective hydrogenation reactions (i) hydrogenation of P-keto esters over Ni-tartrate and (ii) hydrogenation of a-keto esters over cinchona-Pt/Al203 catalysts. In this respect the tartaric acid - P-keto ester system gave a negative result. Neither the substrate nor the modifier have bulky substituents required for SE. 

One of the most interesting side reactions taking place during the enantioselective hydrogenation is the transesterification of the substrate or the reaction product. If the enantioselective hydrogenation of ethyl pyruvate was performed in methanol as a solvent the formation of methyl pyruvate and methyl lactate was observed. CD appeared to be an effective catalyst for the above transesterification reaction. 

CDj a The simplified reaction scheme for the enantioselective hydrogenation of a-keto esters over cinchona-Pt/Al203 catalyst can be written as follows ... 

In this scheme, due to the rate acceleration effect, the enantioselective hydrogenation is much faster than the two racemic hydrogenation reactions (k > kp, k > k ). Please note that the rate constants for the hydrogenation reactions of are pseudo fist order, which contains in a... 

By homogeneous reaction of the conjugated double bond system selectively the C=C double bond is hydrogenated [63-66] the ester function is not affected. Moreover, by action of the chiral catalyst, a chiral hydrogenated product is created with good enantioselectivity. 

Catalytic asymmetric hydrogenation is a relatively developed process compared to other asymmetric processes practised today. Efforts in this direction have already been made. The first report in this respect is the use of Pd on natural silk for hydrogenating oximes and oxazolones with optical yields of about 36%. Izumi and Sachtler have shown that a Ni catalyst modified with (i ,.R)-tartaric acid can be used for the hydrogenation of methylacetoacetate to methyl-3-hydroxybutyrate. The group of Orito in Japan (1979) and Blaser and co-workers at Ciba-Geigy (1988) have reported the use of a cinchona alkaloid modified Pt/AlaO.i catalyst for the enantioselective hydrogenation of a-keto-esters such as methylpyruvate and ethylpyruvate to optically active (/f)-methylacetate and (7 )-ethylacetate. 

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