Deuterium, catalytic exchange with alkanes

Much of the work on the catalytic activation of C—H bonds in alkanes follows from the important observation of Garnett and Hodges (21) that, in the presence of the PtCl42- ion, aromatic compounds exchange hydrogen with deuterium in the D20—CH3C02D solvent. Temperatures in the range 80°-100°C are used and the solvent also contains a mineral acid to stabilize the platinum(II) from disproportionation ... 

In view of the proven multistep catalytic process, transport of the reacting species from one type of site to another before desorption as an alkane, cycloalkane, or benzene seems necessary. Since desorbed olefin plays a significant role in exchange of cycloalkanes with deuterium on Pd films even at ambient temperatures, olefins and even dienes could be responsible in the transport steps. It is useful to recall that this is, in fact, the basis of the classic theory of dualfunctional catalysis. 

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). ... 

E2S.15 Two observations must be explained in this catalytic deuteration of C-H bonds. The first is that ethyl groups are deuterated before methyl groups. The second observation is that a given ethyl group is completely deuterated before another one incorporates any deuterium. Let s consider the first observation first. The mechanism of deuterium exchange is probably related to the reverse of the last two reactions in Figure 26.20, which shows a schematic mechanism for the hydrogenation of an olefin by D2. The steps necessary for deuterium substitution into an alkane are shown below and include the dissociative chemisorption of an R-H bond, the dissociative chemisorption of D2, and the dissociation of R-D. This can occur many times with the same alkane molecule to effect complete deuteration. 

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