Sterically Demanding Diphosphonite Ligands –Synthesis and Application in Nickel‐Catalyzed Isomerization of 2‐Methyl‐3‐Butenenitrile

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Abstract
The synthesis of a novel class of sterically demanding diphosphonites 1–8, based on rigid backbones, is described. The starting materials are all commercially available and the methodology allows for a modular approach. All ligands have been fully characterized, including an X‐ray crystal structure for compound 1, 4,5‐bis{di[(2‐tert‐butyl)phenyl]phosphonito}‐9,9‐dimethylxanthene. The coordination of these diphosphonite ligands towards Ni(II) and Ni(0) precursors is investigated, both by NMR spectroscopy as well as X‐ray crystallography and compared with the behaviour of diphosphine ligands such as Xantphos. The molecular structure for complex 9, trans‐[NiBr2(1)] is described in detail. The nickel‐catalyzed isomerization of 2‐methyl‐3‐butenenitrile to 3‐pentenenitrile is studied, a relevant step in the industrially important hydrocyanation of butadiene (the DuPont adiponitrile process). Good activities and selectivities to the desired 3‐pentenenitrile are obtained in this reversible CC bond activation reaction.