Metal-templated chiral Brønsted base organocatalysis.

Abstract

Inert octahedral chiral-at-metal complexes are an emerging class of asymmetric catalysts that exploit the globular, rigid nature and stereochemical options of octahedral compounds. While the central transition metal serves as a structural anchorpoint and provides metal centrochirality, catalysis is mediated through the organic ligand sphere, thereby merging the branches of transition metal catalysis and organocatalysis. Here we report the development of inert octahedral 3-aminopyrazolato iridium(III) complexes as novel chiral Brønsted base catalysts and demonstrate their merit with applications to highly effective asymmetric sulfa-Michael and aza-Henry reactions, permitting catalyst loadings down to 0.02 and 0.25 mol%, respectively. The observed high stereocontrol can be rationalized by a bifunctional mode of action in which the iridium catalyst, after the initial proton transfer, controls a ternary complex through defined hydrogen bonding interactions. This work reveals the potential of octahedral metal complexes as chiral scaffolds for the design of high-performance asymmetric catalysts.