Abstract: | Evidence is presented that the dimeric π-allylic species (η3-allyl)PdCl]2 is not intermediate in the Li2Pd2Cl6-catalysed allylic H/D exchange in alkenes. Neither H/D exchange in α-methylstyrene, nor enrichment of (η3-2-PhC3H4)PdCl]2, was observed when the latter complex was incubated at 100°C in D3CCOOD either in the presence or in the absence of PhC(CH3)?CH2, respectively. The kinetics of H/D exchange in α-methylstyrene catalysed by Li2Pd2Cl6 were studied in some detail. The exchange proceeds at highest rates when reduction of palladium(II) takes place and is much slower in the presence of 1,4-benzoquinone as a palladium reoxidant. The exchange rate is directly proportional to the alkene and catalyst concentrations and independent of the reoxidant concentration. It is suggested that the palladium(II)-catalysed exchange involves an intermediate hydrid-allyl species where palladium has a formal oxidation state of IV. |