Studies on the structure and equilibration of (π-allyl)palladium complexes of phosphino(oxazolinyl)ferrocene ligands

The equilibration and catalytic efficiency of (π-allyl)Pd complexes of N,P-chelates (L), [Pd(L)(η³-PhCHCHCHPh)]X, depending on their counteranions X have been studied by NMR spectroscopy and X-ray crystallography [L: 1-[4(S)-tert-butyl-2-oxazolin-2-yl]-2(_PS)-(diphenylphosphino)ferrocene 1; 1-[4(S)-tert-butyl-2-oxazolin-2-yl]-2(_PR)-(diphenylphosphino)ferrocene 2. X: Cl^- and PF₆^-]. Among the possible isomeric (π-allyl)Pd complexes, only endo-syn-syn 1n and 2n and exo-syn-syn isomers 1x and 2x were observed and the endo-isomer was found to be the major one in both cases. The endo/exo ratio determined in CDCl₃ at room temperature was dependent both on the counterions and more so on the N,P-chelates (Cl^-: 1n/1x = 9.8/1; 2n/2x = 5.3/1 vs PF₆^-: 1n/1x = 8.7/1; 2n/2x = 4.6/1). The counteranions significantly affected the rate as well as the enantioselectivity in the palladium catalyzed allylic substitution reaction. In the case of Cl^- counterion, the catalytic reaction proceeded much faster and also provided a higher enantioselectivity compared to the case of the PF₆^- counterion. We have also evaluated the relative thermodynamic stability of the palladium complexes depending on ligands 1 and 2 by an equilibration study and by X-ray crystal structure analysis for the corresponding (π-allyl)palladium complexes. The higher reactivity of the less stable palladium complex of 1 over the more stable palladium complex of 2 is explained by a steric strain-reactivity argument.