ISOMERIZATION OF (PI-ALLYL)PALLADIUM COMPLEXES VIA NUCLEOPHILIC DISPLACEMENT BY PALLADIUM(0) - A COMMON MECHANISM IN PALLADIUM(0)-CATALYZED ALLYLIC SUBSTITUTION

Treatment of (pi-allyl)palladium complexes such as 6 and 9 with Pd(PPh3)4 leads to rapid isomerization at -15-degrees-C in tetrahydrofuran and other solvents. At 0-degrees-C and in the presence of more than 2 equiv of triphenylphosphine per palladium, the phosphine attacks the pi-allyl group to give allylic phosphonium salts 7 with concomitant formation of a palladium(0)-phosphine complex, and isomerization of 6 is observed. Attack by PPh3 on 6 was shown to be stereospecific and to proceed with inversion. Studies of the Pd(0)-catalyzed substitution of 1c (X = OAc) with several different nucleophiles support the hypothesis that Pd(0) acts as a nucleophile on (pi-allyl)palladium complexes, in a reaction that leads to loss of stereospecificity in these systems.