Fengphos, a Ferrocene-Based Chiral Diphosphine: Synthesis and Applications

摘要

New, effective chiral ligands are generally found by serendipity, since ab initio design is, to date, not possible. However, once a basic ligand structure is known to impart high selectivity to a catalyst, it is often possible to rationally design new variants on the basis of a structural hypothesis. Ever since the first highly selective diphosphines with a ferrocene backbone, such as bppfa and bppfOH (Figure 1) introduced by Kumada and Hayashi,proved to be effective ligands for a variety of asymmetric transformations, new ligand families with a range of structural variations and types of chiral elements have been described.Many of these have a mix of chiral elements: planar chirality due to the ferrocene moiety, central chirality due to stereogenic carbon and/or phosphorous centers and sometimes even axial chirality. Which of these chirality elements is most important for the magnitude and the sense of induction was an obvious scientific question. Moreover, the high modularity and good synthetic access to many of these ferrocene-based ligands made them very attractive from a commercial point of view.