Abstract:

Kinetics and stereochem. of Rh/Al2O3-catalyzed hydrogenations of p-(Me3C)2C6H4 (I), p-(Me3C)2C6H4 (II), Me3CPh (III), and p-MeC6H4CMe3 (IV) were studied as models for the mechanism of hydrogenation of benzene rings. At approx. atm. pressure, the rates are 0 order in arene, 1st order in H and increased in the order III, I \textless IV \textless II. In competition expts. the order of reactivity is III \textgreater IV \textgreater I \textgreater II. The formation of cyclohexene-type intermediates, dependent on increased pressure, was discussed. A dissociative mechanism due to bulky CMe3 groups was rejected; the orientation of the CMe3 groups, however, affect both the rate of hydrogenation and the proportion of products formed via subsequent alternate reaction paths. [on SciFinder(R)]