Homogeneous catalysis for bulk and fine chemicals. Adventures with phosphorus ligands
The article summarises the encounters the author had with phosphorus-based ligands over the past 30 years. The travel started with water-soluble bidentate phosphine ligands for asymmetric imine hydrogenation. An interlude spent on the development of a new process for the production caprolactam based on butadiene focused on the use of bulky bidentate phosphite ligands for rhodium-catalysed isomerising hydroformylation. A renewed focus on fine chemicals and asymmetric hydrogenation led to the development of monodentate phosphoramidites as revolutionary monodentate ligands for rhodium and iridium-catalysed asymmetric hydrogenations. Best of all, it was possible to produce libraries of 96 phosphoramidites ligands by parallel synthesis in a robotic system. As a result of these developments, a production process was developed for the asymmetric hydrogenation of an alpha-alkyl cinnamic acid in which a mixture of a phosphoramidite and a monodentate triarylphosphine was used. Many other substrates could also be hydrogenated with catalysts based on these ligands with excellent enantioselectivities. Another class of ligands that was previously underdeveloped is the secondary phosphineoxides that were used for hydrolytic amidation of nitriles to the alkylamides. A chiral version was used for iridium-catalysed hydrogenation of imines and the rhodium-catalysed hydrogenation of methyl 3,3-dialkyl 2-acetamido-acrylates.