Iridium and Rhodium Complexes Containing Enantiopure Primary Amine-Tethered N-Heterocyclic Carbenes: Synthesis, Characterization, Reactivity, and Catalytic Asymmetric Hydrogenation of Ketones

摘要

The imidazolium salt [( S , S )- t BuNC_(3)H_(3)NCHPhCHPhNH_(2)]PF_(6), ( S , S )-11· HPF_(6) is a precursor to the enantiopure “Kaibene” ligand, t Bu-Kaibene, ( S , S )-11 featuring a tert -butyl group on the N -heterocyclic carbene (NHC) ring-nitrogen atoms. It has been prepared in high yield and purity by refluxing a chiral cyclic sulfamidate with 1- tert -butylimidazole. Similarly ( S , S )-12· HPF_(6) with a mesityl group at the imidazolium ring-nitrogen atom has been prepared in the same fashion and serves as a source of Mes-Kaibene, ( S , S )-12 . These bidentate Kaibene ligands feature an NHC and a primary amine separated by a chiral linker. Salts ( S , S )-11· HPF_(6) or ( S , S )-12· HPF_(6) react with base and AgI or CuI to give a total of four M(Kaibene)_(2)I compounds (M = Ag or Cu). At 22 °C, the amine-functionalized imidazolium cations undergo oxidative addition to iridium(I) in [IrCl(cod)]_(2) (cod = 1,5-cyclooctadiene) to generate iridium(III) hydride R-Kaibene compounds [IrHCl(cod)( ( S , S )-11 )](PF_(6)) ( 17 ) and [IrHCl(cod)( ( S , S )-12 )](PF_(6)) ( 18 ), respectively, each as a mixture of six configurational isomers. In contrast, the salt ( S , S )-11· HPF_(6) reacts with [Ir(O t Bu)(cod)]_(2) to produce a bimetallic iridium compound with ( S , S )-11 as the bridging ligand. This compound contains interesting NH···Cl and NH···Ir noncovalent intramolecular interactions. Salt ( S , S )-12· HPF_(6) reacts with silver oxide to yield [Ag_(2)( ( S , S )-12 )_(2)](PF_(6))_(2) ( 20 ). Reagent 20 serves as an efficient transmetalation reagent to deliver to each rhodium in [RhCl(cod)]_(2) 1 equiv of ( S , S )-12 as a bidentate ligand to give [Rh(cod)( ( S , S )-12 )](PF_(6)). In the reaction between [IrCl(cod)]_(2) and 20 , ( S , S )-12 ends up coordinated in an iridium(III) hydride complex ( 22 ) as a tridentate ligand via the NHC, NH_(2), and a cyclometalated phenyl group. The two iridium hydride compounds, 18 and 22 , are catalysts f