MAdPHOS, a P-Stereogenic Aminodiphosphane Ligand with Adamantyl Groups: Synthesis, NH/PH Tautomerism, and Rhodium and Nickel Complexes

A novel chiral ligand, named MAdPHOS, bearing a P-stereogenic phosphane and a diadamantyl phosphane linked by a NH bridge has been synthesized. This bulky, C₁-symmetric, PNP ligand has been prepared from enantiopure tert-butylmethyl aminophosphane and was obtained as a crystalline solid. The NH/PH tautomerism, air-stability, and σ-donor capacity of MAdPHOS have been assessed herein. The free ligand has been prepared, showing much higher stability, in the solid form, than its tert-butyl analogue MaxPHOS. Both rhodium and nickel COD complexes have been prepared. The Rh-MAdPHOS complex has shown outstanding enantioselectivities in the asymmetric hydrogenation of enamides.