How Does Palladium Coordination Affect the Electrophilicities of Allyl Cations? Development of a Robust Kinetic Method for Following Reactions of [(η3-Diarylallyl)Pd(PPh3)2]+ with Nucleophiles

Kinetics of the reactions of [(eta(3)-1,3-diarylallayl)-Pd(PPh3)(2)](+) complexes with carbanions, enamines, amines, and triphenylphosphine have been investigated photometrically in dichloromethane, DMSO, and acetonitrile solutions at 20 degrees C. Amines were found to react both at palladium (substitution of PPh3) and at the allyl ligands, causing complex and poorly reproducible kinetics. Excellent reproducibility of the second-order rate constants for the attack at the allyl ligand was achieved when the reactions were studied in the presence of traces of PPh3 and fumaronitrile. The second-order rate constants (k(2)) for the attack of nucleophiles at the allyl ligands were found to follow the correlation log k(2) = s(N)(N + E), where s(N) and N are solvent-dependent nucleophile-specific parameters and E is an electrophilicity parameter (J. Am. Chem. Soc. 2001, 123, 9500-9512). While the electrophilicities of the free 1,3-bis(3,5-difluorophenyl)allyl (E = 6.11) and the 1,3-bis(4-dimethylaminophenyl)allyl cations (E = -7.50) differ by more than 13 orders of magnitude, the electrophilicities of the corresponding Pd(PPh3)(2) complexes were found to be almost independent of the nature of the substituents (E approximate to -14), showing that Pd(PPh3)(2) coordination reduces the electrophilic reactivities of allyl cations by 7-20 orders of magnitude.