The imidazolium salt
[(S,S)-tBuNC3H3NCHPhCHPhNH2]PF6, (S,S)-11·HPF6 is a precursor to the enantiopure “Kaibene”
ligand, tBu-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·HPF6 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·HPF6 or (S,S)-12·HPF6 react with base
and AgI or CuI to give a total of four M(Kaibene)2I 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)](PF6) (17) and [IrHCl(cod)((S,S)-12)](PF6) (18), respectively, each as a mixture of six configurational
isomers. In contrast, the salt (S,S)-11·HPF6 reacts with [Ir(OtBu)(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·HPF6 reacts with silver oxide to yield [Ag2((S,S)-12)2](PF6)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)](PF6). 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, NH2, and a cyclometalated
phenyl group. The two iridium hydride compounds, 18 and 22, are catalysts for the hydrogenation of a range of ketones
(turnover number up to 499, turnover frequency up to 249 h–1, with er (enantiomeric ratio) up to 35:65 R:S).