Catalytic deuterium exchange reactions with organics. XXVII. The alkylbenzenes on self-activated Group VIII transition metals

Abstract

Exchange reactions between heavy water and the alkylbenzenes have been investigated on the following self-activated Group VIII transition metal oxides: PtO₂,2H₂O; PdO; Rh₂O₃; IrO₂,2H₂O; RuO₂,H₂O; ReO₂; and Ni₂O₃. All oxides except Ni₂O₃ are self-activated by benzene up to 180°. Order of ease of self-activation is Pt > Pd > Rh, Ru, Ir > Re > Ni. Trends in deuteration rates of the alkylbenzenes on self-activated platinum oxide are generally similar to those obtained on hydrogen prereduced platinum. At 130°, some differences in reactivity are observed and these are attributed to reagent displacement effects from the presence of small percentages of dimer (1%) associated with the self-activation process. Compared with hydrogen prereduced catalysts, significant differences in isotope orientation are observed with certain alkylbenzenes on self-activated catalysts. Multiple deuteration effects which are accentuated on self-activated catalysts have been used to confirm isotope orientation and also the participation of a π-complex mechanism for the exchange process. Isomerization and exchange in cis- and trans-stilbenes on self-activated platinum have also been interpreted by the dissociative n-complex substitution mechanism. The advantages of self-activated catalysts in general deuterium and tritium labelling work have been evaluated. Possible correlations between exchange results and electron spin resonance data for charge-transfer adsorption on the above oxides are discussed. Preliminary kinetic studies with self-activation are also reported.