Optical resolution, configurational stability, and coordination chemistry of the P-chiral heterocyclic diphosphine 1,1′-diphenyl-3,3′,4,4′-tetramethyl-2,2′-diphosphole-3, 3′-diene

Abstract

The optical resolution of the C2-symmetrical racemic ligand (RP*S C*)-1,1′-diphenyl-3,3′,4,4′-tetramethyl-2, 2′-diphosphole-3,3′-diene (±)-(1) has been achieved efficiently via fractional crystallization of a pair of diastereomeric palladium(II) complex cations containing the appropriate form of the chelating diphosphine ligand and orthometalated (S)-(1-(dimethyl-amino)ethyl)naphthalene. The absolute stereochemistries of both diastereomeric complexes have been confirmed by single-crystal X-ray crystallography. The liberation of the optically active diphosphine ligand from the corresponding resolved complexes involved two steps: the removal of the naphthylamine ligand with concentrated hydrochloric acid followed by the decomposition of the intermediate [Pd(1)Cl2] species with aqueous potassium cyanide. Thus, (RPSC)-(-)-(1) and (SPRC)-(+)-(1) were obtained in high yields with [α]D = ±124.3° (C6H6). These resolved ligands are configurationally stable, as their optical purities remained unchanged upon heating at 140°C for 48 h in benzene and in propanol. They are able to form the optically active digold complexes [Au2(μ-1)Cl2], despite the fact that the two rigid heterocyclic rings are linked with a carbon-carbon bond of restricted rotation.