Transition Metal-Catalyzed Asymmetric Addition of Organoboron Reagents to lmines

Chiral amines are important building blocks in organic synthesis, they also present in numerous natural products, biologically active compounds and pharmaceutical agents. In recent years, the use of various organoboron reagents in transition metal-catalyzed reactions has attracted considerable attentions because of their ready avilibility, low toxicity, good air and moisture stability as well as high functional group compatibility. This review summarizes the remarkable progress and advances in transition metal-catalyzed asymmetric addition of organoboron reagents to imines over the past few years, providing an overview of the recent achievements in stereoselective synthesis of a-chiral amines by using chiral auxiliary or chiral catalysis strategies. At the present time, rhodium-based asymmetric catalysis has proven to be the most efficient and reliable approach to furnish highly optically active alpha-chiral amines. Varieties of chiral ligands including monophosphines, biphosphines, amidomonophosphanes, phosphoramidites, phosphites, dienes, sulfur-olefins, phosphorus-olefins have showed the great potential in many asymmetric additions of organoboron reagents to imines, enabling the access of both secondary and tertiary a-chiral amines with good to excellent enantioselectivities. On the other hand, important progress has been made in developing effective palladium catalysts mainly based on chiral pyridine-oxazoline and phosphine-oxazoline ligands. However, future studies in this area towards the objective of developing more efficient, practical and general methods remain challenging.