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

The optical resolution of the C-2-symmetrical racemic ligand (R-P*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-(dimethylamino)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)Cl-2] species with aqueous potassium cyanide. Thus, (RPSC)-(-)-(1) and (SPRC)-(+)-(1) were obtained in high yields with [alpha](D) = +/-124.3 degrees (C6H6). These resolved ligands are configurationally stable, as their optical purities remained unchanged upon heating at 140 degrees C for 48 h in benzene and in propanol. They are able to form the optically active digold complexes [Au-2(mu-1)Cl-2], despite the fact that the two rigid heterocyclic rings are linked with a carbon-carbon bond of restricted rotation.