Chiral NHC-Iridium Complexes and Their Performance in Enantioselective Intramolecular Hydroamination and Ring-Opening Amination Reactions

摘要

A series of chiral N-heterocyclic carbene (NHC)-iridium complexes of general formula [(NHC*)Ir(diene)Cl] bearing TFB (tetrafluorobenzobarrelene), TCB (tetrachlorobenzobarrelene), BB (benzobarrelene), and COD (cyclooctadiene) as diene ligands were synthesized and fully characterized. Chiral NHC ligands used were of the type previously reported from our group, with backbone stereocenters as well as axial chirality present. The cationic NHC-iridium complexes [(NHC*)Ir(diene)][PF6] were obtained via chloride abstraction from [(NHC*)Ir(diene)Cl] with AgPF6. X-ray crystallographic data for some of the neutral and cationic complexes were obtained and analyzed. The topographic steric bulk of the chiral NHC ligands was calculated using the SambVca 2.0 program. While attempting to get a single crystal of the unstable (R-a,R-a,S,S)-[(DiPh-2-SICyoctNap)Ir(BB)][PF6] complex, a decomposition product with an unexpected pincer-type NHC*-diene ligand was obtained. The series of chiral cationic NHC-iridium complexes were either isolated or freshly made and used in the representative enantioselective intramolecular hydroamination of N-benzyl-2,2-diphenylpent-4-en-1 amine to produce methylated pyrrolidine product with varying yields and enantioselectivities. More importantly, the catalyst performance of these chiral cationic NHC-iridium complexes was expanded to the enantioselective ring-opening aminations of oxabicycles, where a highly enantioselective catalyst system was identified. Furthermore, we discovered that using the opposite axial stereochemistry on the NHC ligand completely switched the absolute configuration of the product, again showing high optical purity for the enantiomer of the product.