Axially chiral phosphinamine ligands in asymmetric catalysis

One of the current approaches to prepare enantiomerically pure materials is to use catalytic quantities of chiral transition metal complexes in a homogeneous medium. The catalytic activity originates from the metal and the asymmetry of the metal - catalysed process is induced by the chiral organic ligands attached to that metal. Commonly used donor atoms, which include phosphorus, nitrogen, oxygen and sulfur, help to electronically tune the metal. A vast number of mono-, bi- and polydentate ligands have been successfully applied in asymmetric catalysis. In this review, an attempt is made to systemise the role which bidentate, axially chiral phosphinamine ligands play in asymmetric catalysis. The ligands are classified, not by the reaction to which their metal complexes have been applied, but by the biaryl and other groups present which induce chirality. These biaryl groupings include 3,5-dihydro-4H-dinaphthazepines, 1-naphthyl-2-naphthylamines, 1,1'-biarylphosphiteoxazolines, 1,1'-binaphthyloxazolines, isoquinolines, quinazolinones and our work on pyrazine- and quinazoline-containing axially chiral ligands. Within this sub-classification the ligands are described, where feasible, in the chronological order in which they were reported so that the development of ligand architectural design can be more easily monitored. The asymmetric transformations to which metal complexes of these ligands have been applied include palladium catalysed allylic substitutions, copper catalysed 1,4-additions to enones and rhodium catalysed hydroboration of vinylarenes. Excellent enantioselectivities, regioselectivities and reactivities have been achieved in each of these processes.