Tethering a metal complex to its phosphate counterion via a phosphine ligand enables a new strategy in asymmetric counteranion-directed catalysis (ACDC). A straightforward, scalable synthetic route gives access to the gold(I) complex of a phosphine displaying a chiral phosphoric acid function. The complex generates a catalytically active species with an unprecedented intramolecular relationship between the cationic Au(I) center and the phosphate counterion. The benefits of tethering the two functions of the catalyst are demonstrated here in a tandem cycloisomerization/nucleophilic addition reaction, by attaining high enantioselectivity levels (up to 97% ee) at an unusually low 0.2 mol % catalyst loading. Remarkably, the method is also compatible with a silver-free protocol.
The
development of catalytic enantioselective transformations,
enabling the construction of complex molecular scaffolds from simple
precursors, has been a long-standing challenge in organic synthesis.
Recent achievements in transition-metal catalyzed enantioselective
functionalizations of carbon–hydrogen (C–H) bonds represent
a promising pathway toward this goal. Over the last two decades, iridium
catalysis has evolved as a valuable tool enabling the stereocontrolled
synthesis of chiral molecules via C–H activation. The development
of iridium-based systems with various chiral ligand classes, as well
as studies of their reaction mechanisms, has resulted in dynamic progress
in this area. This review aims to present a comprehensive picture
of the enantioselective functionalizations of C–H bonds by
chiral iridium complexes with emphasis on the mechanisms of the C–H
activation step.
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