Unusual NHC-Iridium(I) Complexes and Their Use in the Intramolecular Hydroamination of Unactivated Aminoalkenes

Abstract

N-heterocyclic carbene (NHC) ligands with naphthyl side chains were employed for the synthesis of unsaturated, yet isolable [(NHC)Ir(cod)]+ (cod=1,5-cyclooctadiene) complexes. These compounds are stabilised by an interaction of the aromatic wingtip that leads to a sideways tilt of the NHC-Ir bond. Detailed studies show how the tilting of such N-heterocyclic carbenes affects the electronic shielding properties of the carbene carbon atom and how this is reflected by significant upfield shifts in the 13C NMR signals. When employed in the intramolecular hydroamination, these [(NHC)Ir(cod)]+ species show very high catalytic activity under mild reaction conditions. An enantiopure version of the catalyst system produces pyrrolidines with excellent enantioselectivities. Tilt and turn(over): An additional binding interaction that tilts the N-heterocycle of a particular class of monodentate NHCs is key to the synthesis of cationic IrI species. Such tilting leads to an upfield shift of the carbene signal, which was shown to be a general phenomenon for NHCs. When employed as catalysts in the intramolecular hydroamination, these compounds show excellent activity and enantioselectivity (see figure, TOF=turnover frequency).