Palladium(II)‐catalysed H/D allylic exchange in alkenes: An intermediacy of palladium(IV)?

Evidence is presented that the dimeric π‐allylic species [(η3‐allyl)PdCl]2is 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 Li2Pd2Cl6were 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.