Alkanes, addition deuterium exchange

In fact, in addition to the many pieces of evidence for the existence of alkane a-complexes, including the direct observations of such complexes by and discussed above, hydrogen-deuterium exchange prior... 

It will not have escaped notice that adsorbed species implicated in the exchange of alkanes with deuterium are formally the same as those invoked in the hydrogenation of alkenes indeed the reiteration of the alkyl-alkene transformation (process 6.J) held responsible for multiple exchange in linear and branched alkanes, and designated the afi exchange mechanism, is on the face it of identical with the old and well-tried Horiuti-Polanyi mechanism for alkene hydrogenation. This will be discussed further in the next chapter (sections 7.1 and 7.21), but briefly it supposes the sequential addition of two hydrogen atoms to some adsorbed form of the alkene, e.g. 

The early use of deuterium in place of hydrogen in the study of catalytic hydrogenation led to the recognition that the process was not simply the addition of H2 to the double bond. Horiuti and Polanyi proposed that both H2 and alkene (1) are bound to the catalyst surface and transformed to products by a sequence of elementary steps, which they represented as shown in Scheme 1, where an asterisk ( ) represents a vacant site on the catalyst.The last step, (d), is virtually irreversible under the usual hydrogenation conditions, but can be observed in the exchange reactions of D2 with alkanes. The mechanism accounts for the isomerization of an alkene if the reversal of step (c), which involves the formation of the alkyl intermediate (3), involves the abstraction of a hydrogen atom other than the one first added, and is coupled with the desorption of the alkene, (2) - (1). At present, the bond between the alkene and the metal often is represented as a ir-complex (4), as in equation (7). ... 

Evidence for alkane activation has also been seen by the observation of H/D exchange between two alkanes, an alkane and an arene, or an alkane and THE Using CpRe(PPh3)2H2 as the photo catalyst, thousands of turnovers have been observed. While the intermediate responsible for this catalysis was not identified, it does not appear to be [CpRe(PPh3)H2] undergoing Rem/Rev oxidative addition/reductive elimination, since no deuterium incorporation was observed in the dihydride catalyst [99]. Several other metal hydrides are known to catalyze H/D exchange between alkanes and deuterated benzene, such as Ir(PMe3)2H5 [100],CpMo(dmpe)H3 [101], and Re[P(c-hexyl)3]2H7 [102]. 

There is evidence with nickel and platinum that molecular addition is unimportant virtually the same alkane profile is given by non-equilibrated and equilibrated mixtures of hydrogen and deuterium. Unfortunately the measurements with platinum were made with propene where direct addition as shown by semi-logarithmic plots is absent (it appears with ethene but not propene, Figure 7.9). In addition to the evidence provided by the high values of q and s for ruthenium and osmium, which incidentally overcome the extensive alkene exchange to make ethane-rf2 the major product, the orders in deuterium are often somewhat... 

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