Platinum catalysts migration

The metal-catalysed hydrogenation of the higher olefins exhibit general features which are similar to those observed with the n-butenes. Thus, for example, the hydrogenation of hex-1-ene over Adams platinum catalyst [144] is accompanied by very low amounts of double-bond migration the relative rates of isomerisation and hydrogenation are in the ratio 0.03 1. Similarly, in the liquid phase hydrogenation of the n-pentenes over platinum—charcoal and iridium—charcoal [145], little or no isomerisation... 

Although it is now almost fifty years since Speier and his colleagues first announced the chloroplatinic acid-catalyzed hydrosilation of olefins, we are still far from complete control of the chemistry. A particular problem is the suppression of double bond migration. A solution of this problem will require a more detailed understanding of the factors affecting the relative rates of P-hydride elimination from an alkyl group and of the reductive elimination of Si-H from a platinum silyl hydride complex. Another factor which is poorly understood is suppression of the irreversible reduction of the platinum catalyst to Pt° metal. Both of these problems can greatly increase costs of production of certain products. 

Papers (4, 47, 48) demonstrate that, while the character of the carrier (silica gel, active carbon) of the active component has no pronounced influence on the process of hydrogenation, there are distinct differences in the effect of the active components themselves. Side reactions occurred on rhodium and palladium catalysts, while on platinum catalysts they could not be observed in most cases (migration of the double bond, cis-trans isomerization). These reactions occurred only if a sufficient amount of hydrogen was present in the reaction mixture (part of hydrogen is irreversibly consumed by hydrogenation). Neither the carrier alone nor the catalyst in an inert atmosphere provoked any side reactions, which shows that hydrogen in one of its forms participates directly in the isomerization process. 

Methyl migration has been noted in the hydrogenolysis of (515) and related compounds over palladium and platinum catalysts. Cleavage of the three-membered ring occurs on catalytic hydrogenation of prazepam (516X in contrast to electrolysis which reduces only the imine function. ... 

Usually, platinum catalysts stimulate extensive isomerization of nonfunctional-ized olefins. Nevertheless, they have been screened for asymmetric hydroformylation, where migration of the double bond to a (achiral) terminus is not desired. Frequently, this problem has been overcome by use of styrenes as model substrates, which direct the hydroformylation to the position next to the aryl ring [118]. Whether the olefin insertion or the subsequent carbonylation is the regio-chemistry determining step is still under discussion [119,120]. 

In some cases the tendency of double bond to migration rather than saturation is so large that even platinum does not prevent it. In these cases homogeneous catalysts may be useful. A case in point is coronopilin (7), which affords either dihydrocoronopilin (8) or the isomerized product isocoronopilin (9), depending on the catalysts. 

A platinum-rhenium composite catalyst supported on the granular activated carbon (Pt-Re/C, 5 wt-Pt%, mixed molar ratio of Pt/Re = 2) [10] was prepared by a "dry-migration method" [33,34] as follows (1) The Pt/C catalyst prepared earlier (5 wt-metal%) was evacuated at 180°C for 1 h (2) The mixture (molar ratio of Pt/Re = 2) of the Pt/C catalyst and a cyclopentadienylrhenium tricarbonyl complex (Re(Cp)(CO)3) were stirred under nitrogen atmosphere at room temperature for 1 h and then heated at 100° for 1 h, with the temperature kept at a constant (3) This mixture was further stirred under hydrogen atmosphere at 240°C for 3 h and finally (4) the Pt-Re/C composite catalyst was evacuated at 180°C for 1 h. A platinum-tungsten composite catalyst supported on the granular activated carbon (Pt-W/C, 5 wt-Pt%, mixed molar ratio of Pt/W = 1) [5,6] was also prepared similarly by the dry-migration method. All the catalysts were evacuated inside the reactor at 150°C for 1 h before use. 

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