Optical resolution and the stereoelectronic properties of chelating (±)-[(methylsulfinyl)methyl]diphenylphosphine

Abstract

Optical resolution of the asymmetric chelating agent (±)-Ph₂PCH₂S(O)Me has been achieved via fractional crystallization of a pair of diastereomeric palladium(II) cationic complexes containing the sulfinyl-substituted ligand and ortho-metallated (S)-[1-(dimethylamino)ethyl]naphthalene. The X-ray structural analysis of the perchlorate salt of the less-soluble diastereomer confirmed that the sulfinyl-substituted phosphine co-ordinated to the palladium via phosphorus and oxygen with the non-co-ordinated sulfur having an R absolute configuration. An unusual electronic repulsion is observed between palladium and the non-bonded sulfur lone pair thus directing the methyl substituent on sulfur to a sterically unfavored axial position. Furthermore, a two-dimensional rotating frame Overhauser enhancement ¹H NMR study of the complex in CDCl₃ confirmed that this intramolecular electronic interaction outweighs the general steric considerations in solution. Optically pure (R)-(+)-Ph₂PCH₂S(O)Me was displaced from the resolving palladium complex with 1,2-bis(diphenylphosphino)ethane.