Skita catalyst

Colloid catalysts with chiral stabilizing agents can be used as asymmetric catalysts in the reactions of prochiral compounds. One such catalyst is the well-known "Skita-catalyst"(colloidal Pt or Pd with gum-arabicum as an optical active polysaccharide as a protective colloid). These catalysts have been used very often in a number of hydrogenations of unsaturated compounds, including prochiral molecules, but never were their asymmetrizing action noted. Nevertheless, including chiral components as protective colloids in such catalysts allowed for the discovery of asymmetric effects in their action. Indeed, Balandin, Klabimovskii et al. found small... 

More general statements about catalytic hydrogenation are difficult to make since the results are affected by many factors such as the catalyst, its supports, its activators or inhibitors, solvents [27], pH of the medium [27] (Auwers-Skita rule acidic medium favors cis products neutral or alkaline medium favors Irons products [22]) and to a certain extent temperature and pressure [5]. 

In the reductive alkylation of ammonia with cyclohexanone, Skita and Keil found that, although cyclohexylamine was obtained in 50% yield over a nickel catalyst, over colloidal platinum dicyclohexylamine was produced as the predominant product even in the presence of an excess molar equivalent of ammonia. Steele and Rylander compared the selectivity to primary amine, secondary amine, and alcohol in the reductive alkylation of ammonia with 2- and 4-methylcyclohexanones over 5% Pd-, 5% Rh-, and 5% Ru-on-carbon as catalysts.18 As seen from the results shown in Table 6.2, the formation of secondary amine is greatly depressed by the methyl group at the 2 position. Thus over Pd-C the secondary amine was formed predominantly with cyclohexanone and 4-methylcyclohexanone while the primary amine was produced in 96% selectivity with 2-methylcyclohexanone. Over Ru-C the alcohol was formed quantitatively with 4-methylcyclohexanone without the formation of any amines, whereas with 2-methylcyclohexanone the alcohol was formed only to an extent of 57%, accompanied by the formation of 4 and 39% of the secondary and primary amines, respectively. These results indicate that secondary amine formation is affected by the steric hindrance of the methyl group to a much greater extents than is the formation of the primary amine or the alcohol. The results with Ru-C and Rh-C also indicate... 

Colloidal metals of the eighth group in the periodic table were among the first catalysts employed in the hydrogenation of organic compounds. Comprehensive summaries of the earlier work are available (Berkman, Morrell, and Egloff, 1 Skita, 2 Campbell and Campbell, 3 etc.). 

Platinum Catalyst for Seductions (Coli. Vol. 1, 452) Directions are given for the preparation of a colloidal platinum (or palladium) catalyst in an anhydrous, alkaline medium. This catalyst is said to be particularly valuable for the reduction of nitriles, oximes, and nitrostyrenes to pure primary amines. Skita and Keil, Ber. 65, 424 (1932). 

The work of Skita with colloidal platinum (9) led the way toward hydrogenation of the pyridine ring under low pressure conditions. The use of Adams catalyst (10), of far greater value, gave further impetus to the investigation of low pressure conversions of the ring. The latter... 

It must be pointed out, however, that physical shielding takes place only when the nitrogen atom is not in an ionized form. Hydrogenations of pyridine and 2-substituted pyridines when carried out in acid solution or as salts show that pyridine is reduced first in comparison with other pyridines. For example, under the conditions used by Skita (9) pyridine was reduced more readily than 2-picoline. The same order of reducibility was observed in hydrogenations in acetic acid in the presence of Adams catalyst (55). In another comparison of the reduction of mixed hydrochloride salts (39) it was found that pyridine reduced first. This is further amplified in the reduction of equimolecular binary mixtures of hydrochloride salts in the presence of platinum oxide (40). 

Whether traces of oxygen are required for all cases of catalytic hydrogenation is uncertain. Willstatter and Waldschmidt-Leitz (48) found that the activity of platinum or palladium for the hydrogenation of benzene was suppressed if the catalyst was carefully freed from oxygen, and that the activity was restored by treatment with this gas but this work has been criticized by Skita (49) moreover, negative results had also previously been reported by Hess (50). The question of this effect of... 

Carl Ludwig Paal and others used colloidal platinum and palladium catalysts in liquid phase hydrogenation reactions at low temperatures. Aromatic and unsaturated aliphatic compounds such as aldehydes or ketones were easily hydrogenated by either platinum or palladium. Aladir Skita collaborated with Paal in publishing a patent that described some of this work. The colloidal metals could be stabilized by the use of albumen from egg whites, but they were not really practicable and it was difficult to separate them from products. An outline of an early colloidal catalyst preparation is given in Table 3.4. 

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