Chiral Ditopic Cyclophosphazane (CycloP) Ligands: Synthesis, Coordination Chemistry, and Application in Asymmetric Catalysis

Abstract

Abstract A series of dichlorocyclophosphazanes [{ClP(μ‐NR)} 2 ] containing chiral and achiral R groups was obtained from simple commercially available amines and PCl 3 . Their condensation reactions with axially chiral biaryl diols yielded ansa ‐bridged chiral cyclophosphazane (CycloP) ligands. This highly modular methodology allows extensive elaboration of the ligand set, in which the chirality can be introduced at the diol bridge and/or the amido R group. This provides the possibility to observe match and mismatch effects in catalysis. A series of twenty CycloP ligands was synthesized and characterized by multinuclear NMR spectroscopy, HRMS, elemental analysis, and in selected cases, single‐crystal X‐ray diffraction. These studies show that all of the ditopic CycloP ligands are C 2 symmetric, rendering their metal coordination sites symmetry equivalent. Two well‐established enantioselective reactions were explored by using late‐transition metal CycloP complexes as catalysts; the gold‐catalyzed hydroamination of γ‐allenyl sulfonamides and the asymmetric nickel‐catalyzed three‐component coupling of a diene and an aldehyde. The steric demands of the CycloP ligands have a subtle influence on the reactivity and selectivity observed in both reactions. Good enantiomeric ratios (e.r.) as high as 89:11 in the gold‐catalyzed reaction and 92:8 in the nickel‐catalyzed bis‐homoallylation reaction were observed.