Optically active asymmetric bidentate ligands. Crystal and molecular structure of [SP-4-4-(R),(R)]-(–)589-{1-[1-(dimethylamino)ethyl]naphthyl-C2,N}[1-(diphenylphosphino)-2-(methylphenylphosphino)ethane-P,P′]-palladium(II) hexafluorophosphate

Abstract

Asymmetric bidentate (±)-1-(diphenylphosphino)-2-(methylphenylphosphino)ethane has been resolved by the separation by fractional crystallisation of internally diastereomeric palladium(II) complexes containing the racemic ligand and orthometallated (R)-dimethyl[1-(1-naphthyl)ethyl]amine. The absolute configuration of the S enantiomer of the ligand has been assigned by a crystal structure determination of the least-soluble diastereomeric palladium(II) complex (R,R)-(–)₅₈₉-{1-[1-(dimethylamino)ethyl]naphthyl-C²,N}[1-(diphenylphosphino)-2-(methylphenylphosphino)ethane-P,P′]palladium(II) hexafluorophosphate. The X-Ray analysis also showed the methylphenylphosphino group of the di(tertiary phosphine) to be trans to the amino group of the resolving agent. This trans arrangement is retained in dichloromethane but facile intermolecular cis–trans isomerisation occurs in more polar solvents. Chemoselective cleavage of a phenyl group from the diphenylphosphino moiety of the free diphosphine occurred in the presence of lithium in tetrahydrofuran to give a separable 1 : 1 diastereomeric mixture of (R*,R*)- and (R*,S*)-1-(ethylphenylphosphino)-2-(methylphenylphosphino)ethane upon further reaction with bromoethane.