Quinap and Congeners: Atropos PN ligands for Asymmetric Catalysis

Among the range of P,N-chelating ligands that have been employed in asymmetric catalysis, those relying on atropisomerism for the stability of individual enantiomers form a definable class. These APN (atropos P,N) ligands require a specific type of biaryl, with one component carrying a pendant phosphine unit, most commonly diaryl substituted, and the other bearing an sp2-nitrogen adjacent to the biaryl link. When substituents in the biaryl inhibit rotation about the linking bond, stable nonracemizing six-membered ring chelates can be formed. This Perspective relates the background to the initial synthesis in 1993 of Quinap, the original member of the series, and initial observations on its effectiveness in asymmetric catalysis. The current state of play in development of syntheses of this and other members of the APN ligand family is assessed, and their applications in asymmetric catalysis are presented. These include hydroboration and diboration of alkenes, 1,3-dipolar cycloadditions, alkynylation of iminium salts in a three-component (A3) condensation, and conjugate additions of Cu acetylides.