Chiral Phosphorus-Olefin Ligands for Asymmetric Catalysis

Abstract

Transition-metal-catalyzed asymmetric transformations are among the most powerful and straightforward strategies to access various enantioenriched compounds. Hence, considerable efforts have been focused on the development of novel chiral ligands capable of highly efficient and enantioselective catalysis. The importance of olefin as ligand in transition-metal-catalyzed reactions wasn't realized until the report of the Zeise'salt in 1827.Nevertheless, application of chiral olefins as ligands for asymmetric catalysis has been overlooked for quite a long time owing to their relatively weak binding affinity toward the central metal. Since the groundbreaking work of Hayashi and Carreira in 2003~2004, chiral dienes as steering ligands in asymmetric catalysis have emerged as a fascinating new field. Given the weak coordination ability of olefins to transition-metals, functional groups with high coordination ability were considered to incorporate into the olefin framework to create a new type of hybrid olefin ligands for asymmetric catalysis. Over the past few years, a diverse range of hybrid olefin ligands were developed for various enantioselective transformations. Among these, phosphorus-based olefins represent a particularly interesting class of ligands since the first concept demonstration by Grützmacher in 2004, combining the strong coordinating phosphorus atom and the weak coordinating olefin into one ligand molecule. Typically, three structurally different types of phosphorus-based olefins are known in the literature, including phosphine-olefins, phosphoramidite/phosphinamidite-olefins, and phosphite/phosphinite-olefins. They have been successfully utilized in a series of transition-metal-catalyzed asymmetric reactions, such as iridium-catalyzed asymmetric hydrogenation of imines, allylic substitution; rhodium-catalyzed conjugate addition of organoboron reagents to α, β-unsaturated compounds, 1, 2-addition of organoboron reagents to imines/carbonyl compounds, intramolecular hydroacylation; and palladium-catalyzed asymmetric allylic alkylation/amination/etherification of allylic esters, as well as Suzuki-coupling reactions.In many cases, the reactions occur with high enantioselectivities, allows for access to a broad range of valuable chiral products. This paper reviews the literatures in this field and summarizes the remarkable progress and advances in the use of various P-olefins as powerful ligands for diverse transition metal-catalyzed asymmetric transformations since 2004.The aim is to offer an overview of the recent achievements in the rational design and development of new hybrid chiral olefin ligands for effective enantioselective catalysis. We hope that the current success of chiral phosphorus-olefin catalysis would provide an exciting opportunity for future exploration of chiral olefin ligands in a wide variety of asymmetric reactions.