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-[(1 alpha,2 alpha(S),4 alpha,7S)-5,6-dimethyl-7-phenyl-7-phosphabicyclp[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-[(1 alpha,2 beta(R/S),4 alpha,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.