Synthesis of P-chiral phosphines via chiral metal template promoted asymmetric furan Diels-Alder reaction

The organoplatinum complex containing ortho-metalated (S)-(1-(dimethylamino)ethyl)-naphthalene as the chiral auxiliary has been used to promote the asymmetric [4+2] Diels-Alder reaction between phenyldivinylphosphine and 2-diphenylphosphinofuran. The reaction was complete in 6 days at room temperature, with the formation of four isomeric diphosphino-substituted oxanorbornene metal complexes in the ratio of 4:2:2:1. Only the exo-cycloaddition products were formed. The formation of stereogenic carbon centers within the oxanorbornene skeleton are highly stereoselective, with all four cycloadducts adopting the same absolute configurations. However, the stereocontrol at the external phosphorus stereogenic center is less efficient (S_p:R_p = 2:1 for the template cycloadducts). The chiral naphthylamine auxiliary could be removed chemoselectively by treatment with concentrated hydrochloric acid, and further ligand liberation of the dichloro complexes with aqueous cyanide gave the diphosphino-substituted oxanorbornene ligands. Hydrogenation of the double bonds in the cycloadduct stabilizes the phosphorus stereogenic center of the free diphosphine ligand which otherwise undergoes inversion of absolute configuration.