Olefins Horiuti-Polanyi mechanism

Metal-catalyzed hydrogenation of olefins proceeds via the Horiuti-Polanyi mechanism (24), generally accepted since the 1930s (Scheme 1). 

Scheme 1. Classic Horiuti-Polanyi mechanism for olefin hydrogenation (2).

As already mentioned, the stereochemistry of simple olefin hydrogenation can usually be understood by utilizing the classic Horiuti-Polanyi mechanism (1,2). A number of different mechanistic rationales have been put forth, however, to account for the stereochemical data obtained on hydrogenation of a, /3-unsaturated ketones in different media. Actually, no single explanation can be used to account for all of the stereochemical observations, but it is possible to blend the various proposals to give a mechanistic framework from which it is possible by extrapolation to obtain the desired stereochemical information. 

A valuable indirect method of probing the Horiuti-Polanyi mechanism is the study and comparison of competitive rates of hydrogenation of olefins using both homogeneous and heterogeneous catalysts. Comparisons of individual rates... 

Table 1 provides data on the hydrogenation of a range of cycloalkenes. Curiously, there is no obvious trend in the hydrogenation yields of the olefins, cyclopentene and cyclooctene being reduced faster than cyclohexene and cycloheptene. These variations are, therefore, not on account of any size selectivity of the catalyst. The heats of hydrogenation (AH) of the cycloalkenes (Cs-Cg) are different from each other [12]. However, the difference in the AH supports the variation we have observed in the hydrogenation yields of cycloalkenes. The orientation of the substrate towards the catalyst could be interpreted on the basis of the Horiuti-Polanyi mechanism [13]. 

As a result of rather extensive work on the hydrogenation of olefins 139,151 mechanism originally proposed by Horiuti and Polanyi is currently accepted ... 

A (either a transition state or a free energy minimum) reacts to form B. In hydrogenation, R in C is an alkyl group formed by reaction of H with adsorbed olefin and the mechanism is not Horiuti-Polanyi-like. Rather reaction via both A and C are among the few relatively clear examples of Rideal-Eley processes. 

Accordingly, in order to understand the roles of active sites in catalysis, we should clarify not only the intermediates but also the functions of active sites in relation to elementary processes. For this reason, it may be an interesting question whether the two reactions taking place via the same kind of intermediates occur on the same or different active sites. One good example is the isomerization of olefins via alkyl intermediates and their subsequent hydrogenation. In the Horiuti and Polanyi mechanism, complete overlapping of the intermediates, as well as the reaction routes, was tacitly assumed as described in Eq. (1). In this reaction scheme, step (1) and step ( ) are the... 

If two such types of site having different catalytic abilities coexist on a catalyst surface, the isomerization and hydrogenation reactions of olefins could proceed simultaneously but on different sites. As will be discussed later, it is rather reasonable that real catalysts involve active sites with different abilities. For this reason, the Horiuti and Polanyi mechanism which assumes a priori common alkyl intermediates for the isomerization and hydrogenation reactions, appears less logical. 

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