Modular Synthesis of a New Type of Chiral Bis(carbene) Ligand from L‐Valinol and Iridium(I) and Rhodium(I) Complexes Thereof

Abstract

New, chiral bis(imidazol‐2‐ylidene) ligands, and their rhodium(I) and iridium(I) complexes, have been prepared by a simple six‐step synthesis starting from commercially available enantiomerically pure L‐valinol. The stepwise introduction of the imidazoles makes it possible to create chiral bis(NHC) complexes with different substituents at the terminal nitrogen atoms. Formation of IrI and RhI complexes by a one‐pot transmetallation reaction gives the endo and exo stereoisomers in different ratios, depending on the metal and the reaction conditions. Diastereomerically pure material was obtained by crystallisation.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Asymmetric Hydrosilane Reduction of Ketones Catalyzed by an Iridium Complex Bearing a Hydroxyamide‐Functionalized NHC Ligand

“…[14] The appearance of the two signals (a') corresponding to the tert-butyl group of the minor isomer might be explained by agostic interactions between the C À H bond and the metal center, although we do not have any data to corroborate this hypothesis. A similar consideration was also reported by Nagel et al [15] In the 13 C NMR spectra, we observed a characteristic doublet at d = 196.7 ppm with a typical C-Rh coupling constant of 49.7 Hz for the carbene carbon atom.…”

Cyclopentadienyl‐, Indenyl‐ and Fluorenyl‐Functionalized N‐Heterocyclic Carbene Metal Complexes: Synthesis and Catalytic Applications

“…An enantiomerically pure cyclopentadienyl-NHC carbene ligand (9) was recently prepared by reaction of a chiral imidazole tosylate derivative [21] with LiCp, followed by treatment with methyl iodide (Scheme 6). [22] This method is limited to unsubstituted cyclopentadienyl ligands since tetraalkylcyclopentadienyls are subject to lack of regioselectivity in their direct alkylation.…”

“…Cui and co-workers obtained the Sc (16, 20, 24) and Y (17,21) complexes by alkanolysis of [M(CH 2 SiMe 3 ) 3 (THF) 2 ] (M = Sc, Y) with the neutral Ind-and Flu-NHCs [generated in situ by treatment of the Ind-and Flu-imidazolium salts with Li(CH 2 SiMe 3 )]. Cui and co-workers obtained the Sc (16, 20, 24) and Y (17,21) complexes by alkanolysis of [M(CH 2 SiMe 3 ) 3 (THF) 2 ] (M = Sc, Y) with the neutral Ind-and Flu-NHCs [generated in situ by treatment of the Ind-and Flu-imidazolium salts with Li(CH 2 SiMe 3 )].…”

“…[26] A similar procedure was extended to the synthesis of other lanthanide metals ( conditions (by using ether instead of toluene as reaction solvent and by allowing longer reaction times) afforded the dimeric yttrium complex 26. Synthesis of indenyl-and fluorenyl-functionalized NHC complexes of rare earth metals (16)(17)(18)(19)(20)(21)(22)(23)(24)(25). Synthesis of indenyl-and fluorenyl-functionalized NHC complexes of rare earth metals (16)(17)(18)(19)(20)(21)(22)(23)(24)(25).…”

“…The preparation of an enantiomerically pure Cp-and indenyl-NHC ligands remained elusive until a new synthetic protocol based on the use of the recently reported chiral imidazole tosylate [21] was described. The preparation of an enantiomerically pure Cp-and indenyl-NHC ligands remained elusive until a new synthetic protocol based on the use of the recently reported chiral imidazole tosylate [21] was described.…”