Atypical and Asymmetric 1,3-P,N Ligands

Novel seven‐membered cyclic imine‐based 1,3‐P,N ligands were obtained by capturing a Beckmann nitrilium ion intermediate generated in situ from cyclohexanone with benzotriazole, and then displacing it by a secondary phosphane under triflic acid promotion. These “cycloiminophosphanes” possess flexible non‐isomerizable tetrahydroazepine rings with a high basicity; this sets them apart from previously reported iminophophanes. The donor strength of the ligands was investigated by using their P‐κ1‐ and P,N‐κ2‐tungsten(0) carbonyl complexes, by determining the IR frequency of the trans‐CO ligands. Complexes with [RhCp*Cl2]2 demonstrated the hemilability of the ligands, giving a dynamic equilibrium of κ1 and κ2 species; treatment with AgOTf gives full conversion to the κ2 complex. The potential for catalysis was shown in the RuII‐catalyzed, solvent‐free hydration of benzonitrile and the RuII‐ and IrI‐catalyzed transfer hydrogenation of cyclohexanone in isopropanol. Finally, to enable access to asymmetric catalysts, chiral cycloiminophosphanes were prepared from l‐menthone, as well as their P,N‐κ2‐RhIII and a P‐κ1‐RuII complexes.
Complexes, chirality and catalysts: 1,3‐P,N hybrid ligands are broadly applied in cooperative catalysis, but show limited electronic variation. We report novel analogues with a high N‐basicity, based on 7‐membered cyclic imines. Their transition metal complexes demonstrated their donor strength (W), hemilabile coordination (Rh), and catalytic activity for nitrile hydration (Ru) and transfer hydrogenation (Ru, Ir). Chiral derivatives were also obtained, synthesized from l‐menthone.