Pt-alumina-Cnd

Table 5.3. shows dependence of the ee values on the polarities of solvents in the hydrogenation of EtPy on Pt-zeolite catalysts in comparison with Pt-alumina-Cnd catalysts. 

The statement of Baiker that good enantioselectivity can be achieved with Pt particle sizes above 3-4 nm is based on obtaining large ensembles where modifier and reactant can adsorb best on the large crystallites. Indeed, in the hydrogenation of EtPy on Pt-alumina-Cnd (room temperature and 70 bar hydrogen) Blaser et al. found a maximal ee of 80% at 0.2 dispersion which diminished to 30% with dispersion increasing to 0.8 simultaneously with the rate of reaction. 

Von Arx et al. studied the hydrogenation of 4-oxoisophorone (2,6,6-trimethylcyclohex-2-en-l,4-dione) over Pt-alumina and Pd-alumina catalysts modified with Cnd and discovered (over Pt-alumina-Cnd) the first example of an unprecedented selectivity in hydrogenation of a sterically bin-... 

Solvent effects were studied in the enantioselective hydrogenation of l-phenylpropane-l,2-dione over Pt-alumina-Cnd catalyst. The ee values and reaction rates are dependent on the hydrogen solubilities and dielectric constants in different solvents. The highest ee, 65%, of (i )-1-hydroxy-1-phenyl-propanone was obtained in toluene. It decreased non-linearly with increasing solvent dielectric constant becoming close to zero in MeOH (Toukonitty et al. )... 

In the second hydrogenation step during the asymmetric hydrogenation of cyclohexane-1,2-dione over Pt-alumina-Cnd an enantiomeric excess of over 80% of (li ,2/ )-tra 5-cyclohexane-l,2-diol was obtained due to kinetic resolution (Sonderegger et al. ). 

Enantioseleetivity inereased with higher temperatures and higher alkaloid to oxime ratios. The presence of the alkaloid modifier resulted in a decrease of the reaction rate by a factor of as much as 140, compared to the "raeemic" hydrogenation without modifier. On 5% Pt-alumina-Cnd eatalyst, at 60 bar, 20°C, in AeOH, they showed that pyruvamides with different amido groups gave modest ec s with the best ee of 60% being observed only for trifluoroethyl pyruvamide. 

Kimzle, Baiker et al. s mthesized a cyclic imidoketone, 1-ethy 1-4,4-dimelhylp5TTolidine-2,3,5-trione (a strueturd analog of ketopantolaetone), and hydrogenated it over 5% Pt-alumina-Cnd at 15 C in toluene solution into (i )-l-ethyl-3-hydroxy-4,4-dimethylp nTolidine-2,5-dione (Scheme 5.18.) with an ee of 91%. 

The effect of acids and bases as solvents (or as additives) in the hydrogenation of EtPy on Pt-alumina-Cnd was studied by Blaser et al. The addition of AcOH to common applied solvents (toluene, ethanol) or using AcOH as the solvent exhibited unusually strong positive effects on ee"s, up to 95%, especially with catalysts modified with MeO-DHCnd (Blaser et al. ). 

Analogous to the pyruvates, the same decrease in ee with increasing solvent polarity was found in the enantioselective hydrogenation of ketopan-tolactone into pantolactone over Pt-alumina-Cnd catalyst (Schuerch et al. ). The most suitable solvent proved to be toluene ee 78%) but in acetic acid solution the strong increase of ee, which was observed in the case of pyruvates, was not seen and the optical yield of pantolactone reached only up to 35% (see results in Part 5.4.). 

Table 5.14. Effect of solvents and additives on the enantioselective hydrogenation of MePy into (5)-(+)-MeLa on 5% Pt-C-Cnd and 5% Pt-alumina-Cnd catalysts (0.5 g catalysts, 70 bar, RT) (according to Orito ).

Figure 5.10. Comparative study of enantioselective hydrogenation of EtPy into ( )-(-)-EtLa on Pt-alumina-Cnd catalyst as a function of the catalyst to substrate weight ratio in ethanol solution ( , at 60 bar, 22 C as subcritical solvent) and in the supercritical solvent ethane ( , at 60...

Figure 5.10. shows an example of the application of SC-ethane as solvent in the hydrogenation of EtPy over Pt-alumina-Cnd catalyst. For all practical purposes the ee values of the resulting EtLa did not change with increasing amounts of catalyst if the reaction was carried out in SC-ethane, whereas these values fall sharply in ethanol. 

Enantioselective hydrogenation of EtPy on Pt-alumina-Cnd catalyst at 60 bar, 40°C, in supercritical solvent ethane (subcritical condition for ethane are 48.2 bar and 32.2°C) can be compared with hydrogenation in ethanol at 60 bar, 22°C as subcritical solvent (SCF are 61 bar and 241 C) (according to Baiker... 

In the case of the Pt-alumina-Cnd-EtPy system, increasing conversion did not change the ee even at high conversions. This is in contrast to the Ni catalysts modified with tartaric acid and suggests a different mechanism of enantioselectivity. Also, in both cases the possibility of nonlinear effects must be pointed that is, increasing enantio-purity of a product of low opticalpurity upon its exposure to a chiral catalyst (Blackmond Avalos, Kagan ). 

If the reactivity of the substrate in the shielded [substrate-modifier] complexes is higher than that of the free keto ester, enantioselectivity can occur. According to Margitfalvi the quinuclidine N, which aims towards the keto carbonyl group in the keto ester, provides the increased reactivity of the keto carbonyl group. This shielding model, can e q)lain enantioselectivity and rate acceleration effects for almost all substrates used in many examples, for example, hydrogenation of MePy, MBf, ketopanto-lactone, and trifluoroacetophenone over Pt-alumina-Cnd catalysts. 

Additional crucial evidence against a mechanism based on such associations formed in solution is supported by the fact that under these conditions on Pt-alumina-Cnd catalysts C=C prochiral bonds, which are unable to form associations like Cnd-MePy, can be enantioselectively hydrogenated with good ee s. 

Baiker et al. also hydrogenated 2-melhylpent-2-enoic acid on Pt-alumina-Cnd at 20°C and 50 bar with an ee of 52%. 

It was found that reactions with unmodified catalyst proved to be much faster than those with Cnd modified catalyst, which also is quite different from the results reported for Pt-alumina-Cnd catalysts in the hydrogenation of alpha-keio esters. Reuse of catalysts resulted in almost complete loss of ee and indicates elution and absence of Cnd fi om the surface of the catalyst. 

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