Functionalization of Metal-Protected Chiral Phosphines via Simple Organic Transformations

With metal as protection, the introduction of functionalities on coordinated chiral phosphines using
organic transformations was demonstrated. Bis(diphenylphosphino)-substituted oxa- and azanorbornene
metal complexes were subjected to a series of organic transformations including hydrogenation,
hydroboration, electrophilic addition, and dihydroxylation reactions. Hydrogenation of the oxanorbornene
double bond stabilizes the free diphosphino-substituted oxanorbornene ligand, which is otherwise prone
to the retro-cycloaddition reaction. Hydroboration of the oxanorbornenic double bond using borane,
followed by oxidation with alkaline hydrogen peroxide, generated two regioisomeric products with the
introduction of a hydroxy group at the exo position. However, regioselective hydroboration could be
achieved with the use of 9-BBN as the hydroborating agent. Stereoselective electrophilic addition of the
oxanorbornenic double bond with phenylselenenyl chloride resulted in the formation of a sole anti-addition product. Subsequent oxidative syn-elimination of the resultant selenide product was also shown
to proceed regioselectively to give the vinyl chloride complex. Dihydroxylation of the oxa- and
azanorbornenic double bond with osmium tetraoxide proceeded stereoselectively with the introduction
of two hydroxy group at the exo positions. Liberation of the functionalized chiral phosphine ligands
from the metal complexes was also illustrated.