Meta-analysis in asymmetric catalysis. Influence of chelate geometry on the roles of PN chelating ligands

All X-ray structures of PN ligands forming 6-ring metal complex chelates have been retrieved from the CDS database, and those lacking chelate chirality filtered out. Many of the remainder fit naturally into four main families (PPFA, FcPhox, Phox and Quinap), which have been widely applied to asymmetric catalysis in diverse ways. It is known through experimental observation that certain of these ligand structures are more effective for specific classes of reaction but there has been little by way of explanation for their divergent behaviour. In this paper we examine the wide variation of conformations within individual families of PN complexes in the solid state, establish common features, and make cross-correlations with their effectiveness in specific catalytic asymmetric reactions. The extent of rigidity in the chelate varies widely and yet flexible complexes may be extremely effective in asymmetric catalysis. These observations emphasise the importance of induced fit between reactants and catalyst and militate against over-reliance on rigid lock-and-key models.