Diastereoinduction in the Synthesis of Pallada(II)pyrrolidinones: Palladacycles with Two Pd-bonded Stereogenic Carbons

Stable palladapyrrolidinones (L-L)Pd-CH(p-MeC6H4)NBnC(O)CHPh representing endocyclic carbon-bonded palladium(II) amide enolates with two Pd-bonded sp3-hybridized stereogenic carbons and bidentate auxiliary ligands (tetramethylethylenediamine (TMEDA), bis(diphenylphosphino)ethane (dppe), and (S,S)-CHIRAPHOS) were synthesized. The syntheses proceeded with a significant diastereoinduction from the Pd-bonded stereocenter in racemic cationic palladium amide complexes [(L-L)Pd-CH(p-MeC6H4)NBnC(O)CH2Ph]+OTf − favoring the formation of cis diastereomers (denoting the relative stereochemistry of the two sp3-hybridized carbons in the adjacent positions of the square planar Pd(II) complexes), whereas epimerization of the Pd-bonded stereocenters favoring the formation of the more stable trans diastereomers was achieved under basic conditions. Studies of the stereochemical relationships between Pd-bonded stereocenters in palladapyrrolidinones bearing chiral nonracemic auxiliary ligands ((S,S)-CHIRAPHOS) indicated a lack of double diastereodifferentiation induced by the auxiliary ligands. Molecular structures of the cis diastereomers of pallada(II)pyrrolidinones bearing TMEDA and dppe ligands were established by X-ray crystallography.