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
Organometallics, 2012, 31, 2416–2424 published on 02.03.2012
Kinetics of the reactions of [(η3-1,3-diarylallyl)Pd(PPh3)2]+ complexes with carbanions, enamines, amines, and triphenylphosphine have been investigated photometrically in dichloromethane, DMSO, and acetonitrile solutions at 20 °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 (k2) for the attack of nucleophiles at the allyl ligands were found to follow the correlation log k2 = sN(N + E), where sN 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 ≈ −14), showing that Pd(PPh3)2 coordination reduces the electrophilic reactivities of allyl cations by 7–20 orders of magnitude.