Electronic effects in the nickel-catalysed hydrocyanation of styrene applying chelating phosphorus ligands with large bite angles‡

Abstract

Chelating phosphorus ligands with a rigid backbone and a large natural bite angle were applied in the nickel-catalysed hydrocyanation of styrene. The para substituents in the diphenylphosphanyl moiety of the 4,6-bis(diphenylphosphanyl)-2,8-dimethylphenoxathiine (Thixantphos) ligands were varied and their electronic effects on the activity and selectivity of the catalytic experiments were investigated. The activity of the nickel complexes decreased when electron-donating substituents lead to a more basic phosphorus while electron-withdrawing substituents led to a higher activity. The results of variable temperature ³¹P-{¹H} NMR experiments on the in situ catalysts are discussed in relationship to the catalytic performance. 4,6-Bis(diphenylphosphanyl)-2,8-dimethylphenoxathiine (Thixantphos) L^1d and the complexes [NiCl₂L^1d] 1 and [Ni(CN)₂L^1a] 2 (p-Me₂N on phenyl) have been characterised by single-crystal X-ray diffraction. Complex 2 represents the first crystal structure of a monomeric dicyanonickel(II) complex with a P–P chelating ligand. The geometries of ligand L^1d and complex 1 were predicted by molecular mechanics calculations.