Stereoselective synthesis of chiral multidentate ligands with As2NP or As4P donor atoms*

Abstract

The asymmetric bidentate ligand (±)-(2-aminophenyl)(2-chlorophenyl)methylphosphine has been prepared via chemoselective cleavage of the phenyl group from (±)-(2-aminophenyl)methylphenylphosphine using lithium in tetrahydrofuran (thf), to give (2-aminophenyl)methylphosphine upon hydrolysis, followed by deprotonation of the secondary phosphine with sodium in thf and subsequent reaction with 1,2-dichlorobenzene. The chlorophenyl-substituted tertiary phosphine has been resolved by the method of metal complexation. The absolute configuration of the R enantiomer of the ligand has been assigned by a crystal structure determination of the diastereomeric palladium(II) complex [(S _P ),(R)]-[(2-aminophenyl)(2- chlorophenyl)methylphosphine-N,P]{1-[1- (dimethylamino)ethyl]naphthyl-C ² ,N} palladium(II) hexafluorophosphate. Reaction of (±)-(2-aminophenyl)(2-chlorophenyl)methylphosphine with an equimolar quantity of sodium (2-dimethylarsinophenyl)methylarsenide in thf at -20 ± 5 °C gave a 1:1 diastereomeric mixture of the chiral pentadentate ligands (R _As *,R _As *,S _P *)-(±)- and (R _As *,S _As *,S _P *)-(±)-{2-[(2-dimethylarsinophenyl) methylarsinophenyl}{2-[(2-dimethylarsinophenyl) methylarsinoamino]phenyl}methylphosphine. The chiral quadridentate ligand (R _As *,S _P *)-(±)-1-[(2- aminophenyl)methylphosphino]-2-[(2-dimethylarsinophenyl)methylarsino] benzene can be isolated, however, when the coupling reaction is performed at 50 ± 5 °C. The chiral multidentate ligands have been isolated by complexation to cobalt(III) and the structures of the three complexes determined by X-ray analyses. It is clear from the structural data that the quadridentate ligand has formed a single dichlorocobalt(III) complex with cis-α stereochemistry and in which the stereogenic arsenic and phosphorus atoms of the ligand have opposite relative configurations. Two other complexes have also been isolated from the coupling reaction: trans-dichlorobis[1,2-phenylenebis(dimethylarsine)]cobalt( III) chloride and bis[(2-aminophenyl)methylphenylphosphine]dichlorocobalt(III) chloride. The latter is isolated as an isomeric mixture. The formation of asymmetric bidentate (±)-(2-aminophenyl)methylphenylphosphine is believed to result from reduction of the chloro group in the tertiary phosphine precursor by the sodium arsenide reagent. Metal-assisted methylation of a (2-dimethylarsinophenyl)methylarsino moiety by methanol is postulated to account for the formation of 1,2-phenylenebis(dimethylarsine) in the reaction. Optically active analogues of the three multidentate ligands have also been synthesized by reaction of (R)-(2-aminophenyl)(2-chlorophenyl)methylphosphine with sodium (2-dimethylarsinophenyl)methylarsenide in thf and similarly isolated by complexation to cobalt(III).