Evidence of the reversible formation of cationic pi-allylpalladium(II) complexes in the oxidative addition of allylic acetates to palladium(0) complexes

It has been established that the oxidative addition of allylic acetate with the palladium(o) complex, generated from a mixture of Pd(dba)(2) + 2PPh(3) , affords a cationic pi-allylpalladium(Ii) complex. This reaction is revers ible and proceeds through at least two successive equilibria. The overall e quilibrium constant has been determined in THF and DME The acetate ion does not act as a simple leaving group, but plays the role of a nucleophile abl e to react with cationic pi-allylpalladium(Ii) complexes to reproduce the a llylic acetate; a process that may be responsible for the racemization of c hiral allylic acetates. In DMF, no significant ion pairing occurs and free ions are formed. On the other hand, in THF almost complete ion pairing occu rs and almost no free ions are produced.