NMR study on the coordination of dibenzylideneacetone to chiral palladium(0) units. Fluxional behaviour including an intramolecular double bond exchange

Abstract

New derivatives with the general formula Pd(L–L)(η²-dba) (dba = dibenzylideneacetone), L–L = 1-diphenylphosphino-2,1′-(1-dimethylaminopropanediyl)ferrocene, PAPF, 1; 2-[1-(dimethylamino)ethyl]-1-(diphenylphosphino)ferrocene, PPFA, 2; N,N-dimethyl-1-[1′,2-bis(diphenylphosphino)ferrocenyl]ethylamine, BPPFA, 3; 1-diphenylphosphino-2,1′-(1-dicyclohexylphosphinopropanediyl)ferrocene, PP^CyPF, 4 were synthesized from Pd₂(dba)₃·CHCl₃ and the appropriate ferrocenyl ligand. When an excess of PPFA was used, the complex Pd(PPFA)₃ with P-coordinated PPFA ligands was formed. The dba unit preferentially adopts an s-cis,trans conformation with the s-trans alkene coordinated to the Pd(L–L) fragment. Several intramolecular dynamic processes were identified: at room temperature a fast rotation of dba about the alkene–Pd bond, a Pd–N bond rupture process for complex 2 and, at elevated temperatures, an intramolecular diastereomer interconversion involving an alkene face exchange accompanied by an interchange between the coordinated and non-coordinated dba double bonds. The crystal structure of complex 2 was determined by X-ray diffraction.