Isotope studies of the isomerization of olefins homogeneously catalysed by palladium, platinum, and rhodium complexes

Abstract

No entry of deuterium into olefin could be detected as a result of isomerization of allylbenzene in the presence of palladium(II) catalysts in CH₃CO₂D or CH₃OD. The deuterium isotope effect on the rate of isomerization was found to be high in the case of CH₃CO₂D, but slight in the case of CH₃OD. In the case of catalysis by rhodium complexes, entry of deuterium into olefin occurs in CH₃OD but not in CH₃CO₂D. Isotope effects parallel those observed with palladium(II). The isomerization of PhCD₂CH₂CH=CH₂ catalysed by palladium(II) results in a near-to-random distribution of deuterium throughout the side chain. The results are consistent with a hydrido-π-olefin complex mechanism of isomerization. Differences between catalysis by palladium(II) and rhodium(III) are discussed in terms of possible differences in the mechanism of formation of the active catalyst. In the case of palladium it is concluded that a relatively rapid rate of σ-π interchange and hence of double bond shift occurs within the equilibrium system of catalyst and olefin, and that exchange of olefin with the substrate constitutes the slower, rate-determining step. The behaviour of platinum(II) is generally similar to that of palladium(II) but is only effective at higher temperatures.