Metal template promoted hydroamination of ethynylphosphines and aniline.: Asymmetric synthesis, coordination chemistry, and the imine-enamine tautomerism of P-chiral iminophosphines

In the presence of the organopalladium template containing the orthometalated (S)-(1-dimethylamino)ethyl)naphthylene auxiliary, both diphenyl(phenylethynyl)phosphine and di(phenylethynyl)phenylphosphine were reactive toward the novel hydroamination reaction with aniline to give the corresponding bidentate iminophosphines in high yields. These novel P-N ligands exhibited an interesting imine-enamine tautomerism in solution. Spectroscopic studies confirmed that the uncoordinated ligands adopted the stable imino form but could be transformed to the enamino form upon complexation with palladium. The relative stability of the imino and enamino chelates is sensitive to the substituents on the phosphorus donor atoms. When di(phenylethynyl)phenylphosphine was used as the starting material, the hydroamination reaction gave a 4:1 mixture of the chelating diastereomeric P-chiral iminophosphine on the chiral template. These diastereomeric complexes could be separated efficiently via silica gel column chromatography. A pair of novel and enantiomerically pure P-chiral iminophosphines was obtained in high yields from the separated diastereomeric template complexes by treatment with KCN.