Palladium-Complex-Promoted Asymmetric Synthesis of Stereoisomeric P-Chiral Pyridylphosphines via an Unusual Exo−Endo Stereochemically Controlled Asymmetric Diels−Alder Reaction between 2-Vinylpyridine and Coordinated 3,4-Dimethyl-1-phenylphosphole

The organopalladium complex containing ortho-metalated ( S )-(1-(dimethylamino)ethyl)naphthalene as the chiral auxiliary has been used successfully to promote the asymmetric [4 + 2] Diels−Alder reaction between 3,4-dimethyl-1-phenylphosphole and 2-vinylpyridine. The pyridyl group in the resulting phosphanorbornene cycloadducts can be located stereospecifically in the exo or endo position by controlling the electronic properties of the organopalladium promoter. In the exo-cycloaddition process, the P−N bidentate ligand (−)-2-[(1α,2α( S ),4α,7 S )-5,6-dimethyl-7-phenyl-7-phosphabicyclo[2.2.1]hept-5-ene-2-yl]pyridine was produced stereoselectively. In the endo-cycloaddition process, however, a pair of separable diastereomeric palladium template complexes containing the naphthylamine auxiliary and the enantiomeric forms of 2-[(1α,2β( R / S ),4α,7( R / S )-5,6-dimethyl-7-phenyl-7-phosphabicyclo[2.2.1]hept-5-ene-2-yl]pyridine were obtained. In these diastereomeric complexes, the endo-cycloadducts coordinated on palladiun as monodentate ligands via only their phosphorus donor atoms. The pyridyl-nitrogen atoms are not involved in metal complexation. The absolute configurations and the coordination properties of the exo- and endo-pyridylphosphines have been established by single-crystal X-ray analyses.