Design of Chiral Ligands for Asymmetric Catalysis: From C₂‐Symmetric P,P‐ and N,N‐Ligands to Sterically and Electronically Nonsymmetrical P,N‐Ligands

Abstract

For a long time, C2-symmetric ligands have dominated in asymmetric catalysis. More recently, nonsymmetrical modular P,N-ligands have been introduced. These ligands have been applied successfully in various metal-catalyzed reactions and, in many cases, have outperformed P,P-or N,N-ligands.M ost asymmetric catalysts that have been developed so far are metal complexes with chiral organic ligands. The chiral ligand modifies the reactivity and selectivity of the metal center in such a way that one of two possible enantiomeric products is formed preferentially. Based on this concept, many metal complexes have been found that catalyze various reactions with impressive enantioselectivity. Despite impressive progress in this field, the design of suitable chiral ligands for a particular application remains a formidable task. The complexity of most catalytic processes precludes a purely rational approach based on mechanistic and structural criteria. Therefore, most new chiral catalysts are still found empirically, with chance, intuition, and systematic screening all playing important roles. Nevertheless, for certain reactions such as Rh-catalyzed hydrogenation (1, 2) or Pd-catalyzed allylic substitution (3, 4), the mechanism is known, allowing at least a semirational approach to catalyst development. Moreover, useful general concepts have been developed during the last three decades that greatly facilitate the development of new chiral ligands, even in the absence of mechanistic information. Some of these concepts are described in the following sections, mainly from the perspective of our own research.

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