Iridium-Catalyzed Asymmetric Allenylic Substitution via Kinetic Resolution Enabled by New Monodentate Ligands

Abstract

Iridium-catalyzed asymmetric allenylic substitution represents a useful method for the construction of allenes bearing an allenylic central chirality, but current success has uniformly relied on only one specific chiral bidentate ligand. Herein we address the limitation by the design of a new type of monodentate ligands leading to not only excellent enantiocontrol in allenylic substitution, but also efficient kinetic resolution of α-allenylic alcohols, a new phenomenon never observed before in iridium-catalyzed allenylic substitution. This is also a rare demonstration of non-enzymatic kinetic resolution of α-allenylic alcohols. A range of highly enantioenriched allenylic diarylmethanes and α-allenylic alcohols could be accessed under mild conditions. Control experiments and DFT studies indicated that this process proceeds by an SN1 pathway featuring a rate-determining ionization step followed by ligand-controlled enantiodetermining nucleophilic addition. The newly designed rigid and bulky ligands modified from SPHENOL was believed to assemble the key iridium-bound allenylic carbocation intermediate in a different complexation mode, thus serving as the origin of enantiocontrol and the unprecedented kinetic resolution.