Design and synthesis of wide bite angle phosphacyclic ligands

Abstract

By examining structure activity relationships for a given catalytic reaction it is possible to discover what ligand features and parameters lead to stable and highly active/selective catalyst systems. With this knowledge in hand it may be possible to rationally design next generation ligands and catalysts to affect improved substrate transformations, with higher selectivities and faster reaction times. The success of Burk’s DuPhos ligands in asymmetric hydrogenation demonstrated that chiral phosphacycles can be a potent source of chiral induction, whilst in a similar vein the work of van Leeuwen and Kamer established the wide bite angle xanthene based ligands as excellent catalysts in a range of reactions including hydroformylation. In a preliminary study with Osborne they showed that combining these wide bite angle ligand backbones with Burk’s phospholane moieties led to a new powerful ligand in asymmetric allylic substitution. To examine the potential of combining these two ligand features further we designed and synthesised nine new C2-symmetric bidentate wide bite angle bisphosphacyclic ligands, featuring phosphetane, phospholane or diazaphospholane rings, aiming at a wide diversity of steric and electronic properties. The application of these ligands as chiral auxiliaries in transition metal catalysed reactions, including; hydrogenation, hydrocyanation, hydroformylation and allylic alkylation has been investigated. Good to excellent enantioselectivities were observed in all reactions, with maximum ee’s of 92.5% observed in hydrogenation, using N-(3,4-dihydro-1-napthalenyl)-acetamide as substrate, and of 96.2% in the alkylation of 1,3-diphenyl-2-propenyl acetate.