Inherently Chiral 6,7-Diphenyldibenzo [e,g][1,4]diazocine: Enantioselective Synthesis and Application as a Ligand Platform

Inherently chiral 6,7-diphenyldibenzo[e,g][1,4]diazocine (DDD) has been synthesized enantioselectively for the first time via chiral phosphoric acid (CPA)-catalyzed cyclocondensation of readily available [1,1'-biphenyl]-2,2'-diamine (1a) and benzil (2a) in 82% yield, with 98% ee under mild reaction conditions. The strategy could also be applied to racemic biaryl diamines through kinetic resolution. The unexpectedly high interconversion energy barriers between the enantiomers (Delta G = 39.5 kcal/mol) and the chemical stability rendered DDD an ideal platform for developing new chiral ligands and catalysts. Unique inherently chiral DDD-based phosphoramidites, phosphoric acid, mono- and diphosphine ligands were prepared from optically pure diphenol derivative DDDOL as a common precursor. Preliminary asymmetric reactions catalyzed by Pd or Rh in the presence of newly developed ligands exhibited comparable or even better enantioselectivities than the corresponding BINOL- or SPINOL-derived ligands. Density functional theory calculation revealed the origin of the enantioselectivity during the process. [GRAPHICS]