Hydrogen promoted isomerization, mechanism

The complex, (3P)3RhHCO, (1) which promotes double bond hydrogenation and isomerization by the mechanism shown in Scheme 3.1,5 is... 

Dual Function Catalytic Processes. Dual-function catalytic processes use an acidic oxide support, such as alumina, loaded with a metal such as Pt to isomerize the xylenes as weH as convert EB to xylenes. These catalysts promote carbonium ion-type reactions as weH as hydrogenation—dehydrogenation. In the mechanism for the conversion of EB to xylenes shown, EB is converted to xylenes... 

Banks and Bailey 77> noted that the occurence of disproportionation, polymerization, and isomerization over similar catalysts, or simultaneously over the same catalyst, suggests a similarity of mechanism. They noted that this is not to say that one can predict a given catalyst will promote one or more of these reactions or that a given catalyst known to promote one of these reactions also will promote another. Banks and Bailey 77> proposed that the ability of the catalyst to shift hydrogen atoms is a key factor in determining the reaction course. When they contacted ethylene with a series of catalysts prepared by supporting Group VI hexacarbonyls on alumina, they obtained different products with the different hexacarbonyls (Table 10). 

Moreover, MPVO reactions are traditionally performed with stoichiometric amounts of Al(III) alkoxides. Some improvements came from the use of dinuclear AI(III) complexes that can be used in catalytic amount [6, 7]. This is why there has been an ever-increasing interest in catalytic MPVO reactions promoted by lanthanides and transition-metal systems [8]. In these cases, it is believed that reaction proceeds via formation of a metal hydride, in contrast with the mechanism accepted for traditional aluminum alkoxide systems, which involves direct hydrogen transfer by means of a cyclic intermediate [9]. As well as La, Sm, Rh and Ir complexes, Ru complexes have been found to be excellent hydrogen transfer catalysts. The high flexibility of these systems makes them very useful not only for MPVO-type reactions, but also for isomerization processes [10]. 

Free radical promoted hydrosilation of olefins typically gives poorer conversions than are produed by hydrosilation mediated by metal catalysts. However, they are not subject to isomerization of the olefin, a side reaction that can be a nuisance in metal-catalyzed hydrosilations. This is due to the extremely efficient hydrogen atom transfer reaction from Si-H to carbon radicals and the resulting very short lifetimes of the latter. The generally accepted mechanism for these processes is illustrated in Scheme 1. 

Another attempt to account for the differences between the behavior of platinum and palladium in bond shift isomerization has been presented in a recent review by Clarke and Rooney (2). This new mechanism, which is also based on the ability of platinum and not of palladium to promote the formation of metallocyclobutanes, derives from Rooney s earlier mechanism but replaces the cr-alky 1 precursor by a metallocyclobutane, and, in the transition state, the n-olefinic bonding by a zr-allylic bonding. As in the previous mechanism, it is assumed that on platinum the metallocyclobutane is formed directly, while in the case of palladium, it would result from a 12 hydrogen shift via a transient species of zi-allylic character (Scheme 28). 

All possible isomers of 1,2-butadiene were observed in the gas phase deuteration of this molecule. Such isomerization is an unusual feature of diene and alkyne hydrogenation because the desorption rate of di-unsaturated hydrocarbons is usually very slow. A thermodynamic factor can hardly have been promoting this desorption because, from evidence discussed in Section II, F, 1, 1,2-butadiene appears to be most weakly adsorbed of the C4He isomers. One mechanism of reactant isomerization involves simply the loss of a hydrogen atom from adsorbed C4H, for which three relevant structures can be written. The 2-butyne, which was the major isomer produced, was largely unexchanged, how-... 

It has been proposed that the reaction mechanism for n-hexane and n-heptane isomerization over platinum promoted WZ (PtAVZ) at temperatures about 200 °C is a bifunctional non-classic one [9,10]. Active sites are formed by the interaction of hydrogen dissociated on platinum, migrating by spillover, with WOx surface species. Room temperature... 

---