Synthesis of aminophosphine phosphinite ligand bis(oxazaphospholano)-ethane with C₂-symmetric axis and its application in asymmetric hydrogenation

A new class of C₂-symmetric (R,R)-bis(oxazaphospholano)ethane (BOAPE) 1-4 was synthesized by the condensation reaction of readily available optically active amino alcohols with 1,2-bis(dichlorophosphino)ethane. These new ligands bear not only C₂-symmetry axis but also a rigid ring. They were characterized by ¹H NMR, ³¹P NMR and ¹³C NMR spectra. The enantioselective yields were obtained by Rh complex catalyzed asymmetric catalytic hydrogenation of N-benzoyldehydroamino acid derivatives and α-functionalized ketones in up to 99% ee and 98% ee, respectively. This new class of (R,R)-BOAPE gave much higher efficiency and enantionselectivity than their corresponding non C₂-asymmetric aminophosphine phosphinite (AMPP). Among these ligands, (R,R)-Ph-BOAPE (2) was the best one. The t _1/2 and turnover of frequency (TOF) of the catalyst [Rh(COD)(R,R)-Ph-BOAPE]BF₄ for the asymmetric hydrogenation of methyl N-benzoyl cinnamate were 12 min and 6.5 min^-1, respectively.