Enantioselective hydrogenation of methyl acetoacetate

Table 41.18 Homogeneous, biphasic and heterogeneous enantioselective hydrogenations of methyl acetoacetate with Ru-BINAP [117],...

A more specific interaction between a zeolite surface and a chiral catalyst was recently uncovered (58). It was found that the Ru-binap catalyst can be specifically withheld on the outer surface of Beta zeolites. Such a heterogeneous catalyst is relevant for the highly enantioselective hydrogenation of methyl acetoacetate to (R) or (S) 3-hydroxymethylbutyrate, with typical ee values of about 95 %. 

Keane MA, Webb G (1992) The enantioselective hydrogenation of methyl acetoacetate over supported nickel catalysts I. The modification procedure. 1 Catal 136 1 Keane MA (1997) Interaction of optically active tartaric acid with a nickel-sUica catalyst role of both the modification and reaction in determining enantioselectivity. Langmuir 13 41... 

MA Keane. The role of catalyst activation in the enantioselective hydrogenation of methyl acetoacetate over silica-supported nickel catalysts. Can. J. Chem. 72 372-381, 1994. 

Table 4.7. Rate of formation of (R)-(-)-MHB (mmol h g ) and ee values in the enantioselective hydrogenation of methyl acetoacetate on deposited nickel-kieselguhr catalysts, promoted with 1% noble metals and modified with (2R,3R)-tartaric acid (according to summarized data of Orito et al. ).

Yokozeki, M., Shimokoshi, K., and nMiyazaki, E. (1985) Enantioselective hydrogenation of methyl acetoacetate over a modified nickel surface MNDO calculation of stability for intermolecular interactions, J. Phys. Chem. 89,2397 -2400. 

Nitta, Y., Sekine, F., Sasaki, J., Imanaka, T., and Teranishi, S (1983) Conversion dependence of enantioselective hydrogenation of methyl acetoacetate with modified Ni-silica catalyst, J. Catal. 79,211- 214. 

Fu L., Kung H.H., and Sachtler, W.M.H. (1987) Particle size effect on enantioselective hydrogenation of methyl acetoacetate over silica-sup-ported nickel catalyst, J. Mol Catal 42, 29-36. 

Bennett, A., Cristie, S., Keane, M.A., Peacock, R.D., and Webb, G. (1991) Enantioselective hydrogenation of methyl acetoacetate over nickel catalysts modified with tartaric acid, Catalysis Today, 10, 363 -370. 

Table 4.9. The influence of the catalysts preparation method, the nature of the support, the reduction temperature (Td and the added palladium on enantioselectivity in the hydrogenation of methyl acetoacetate (MAA) to (i )-(-)methyl hydroxybutyrate (according to Nitta et al. ).

In contrast to earlier polymer-supported complex catalysts in which complexes were immobilized through electrostatic interaction, covalent bonds, or coordinative bonds, in this case the complex is captured in Ihe elastomer network by occlusion in a dense polymer in Ihe absence of any supplementary chemical bonding and only as result of steric restrictions. In the hydrogenation of methyl acetoacetate by this catalyst an ee of 70% was obtained in polyethyleneglycol solution at 60 °C. Afer regeneration of the catalyst and reuse, its activity and enantioselectivity were almost unchanged. 

Enantioselective hydrogenation of prochiral ketones has rarely been studied in aqueous biphasic media. In addition to the chiral bisphosphonic acid derivatives of 1,2-cyclohexanediamine [130], the protonated 4,4 -, 5,5 -, and 6,6 -amino-methyl-substituted BINAP (diamBINAP 2HBr) ligands (Scheme 38.7) served as constituents of the Ru(II)-based catalysts in the biphasic hydrogenations of ethyl acetoacetate [131, 132]. These catalysts were recovered in the aqueous phase and used in at least four cycles, with only a marginal loss of activity and enantio-selectivity. 

Izumi et al. then developed another type of catalyst, Raney nickel modified by tartaric acid [ 14]. Using this, methyl acetoacetate could be hydrogenated into methyl P-hydroxybutyrate with an ee of up to 80%. Unfortunately, only some specific substrates were reduced enantioselectively. However, some interesting developments were later realized (vide infra). 

Smith et al reported that the hydrogenation of 2-methylcinnamic acid and 2-methylpent-2-enoic acid on 1 % Pd-sihca catalysts modified with l-(iS)-e c/o-bomyloxytrimethylsilane in MeOH at 25°C results in chiral products with ee s of 22.5% and 11.6%. These results noted that modified Pt and Pd catalysts can hydrogenate enantioselectively systems with 1,3-conjugated bonds, but the transfer of these aspects for hydrogenation of 1,4-double bonds as in methyl acetoacetate (MAA) has definite difficulties because it needs to assume the enol form (En) to react (Scheme 5.32.). 

Another path of manufacture of practical catalysts is using immobilized chiral metal complexes. Thus, the complex [Rh-BESIAP] was occluded in an elastomeric t5T)e polydimethylsiloxane membrane, which gave a re-generable active membrane-catalyst with the same enantioselectivity as the homogeneous catalyst in the hydrogenation of acetoacetic acid ester into methyl (7 )-(-)-3-hydroxybutyrate, that can be pol5mierized into polyester (Scheme 7.17.). 

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