Synthesis of rhodium(I) and iridium(I) complexes of chiral N-heterocyclic carbenes and their application to asymmetric transfer hydrogenation

Abstract

Rhodium and iridium complexes of chiral NHC-phenolimine and NHC-amine ligands have been prepared and studied for asymmetric transfer hydrogenation. X-ray and NMR spectroscopy show that for NHC-phenolimine complexes abstraction of chloride results in a change in ligand coordination from NHC only to chelating NHC-imine. Complexes of NHC-amines are inactive for transfer hydrogenation, whereas complexes of NHC-phenolimines are active at room temperature for a range of aryl containing ketones. Enantioselectivity is very sensitive to the NHC N-substituent resulting in a switch in the predominant enantiomer.

Preparation and Catalytic Performances of a Molecularly Imprinted Ru‐Complex Catalyst with an NH2 Binding Site on a SiO2 Surface

“…The configurations of the products were determined according to a literature method. [102][103][104] Activation energies (E a ) were estimated in the temperature range 308-328 K and DH and DS at 318 K were calculated using the estimated E a .…”

Asymmetric Transfer Hydrogenation of Aromatic Ketones Using Rhodium Complexes of Chiral N‐Heterocyclic Carbenes Derived from (S)‐Pyroglutamic Acid

“…The highest enantioselectivity obtained was 56 % ee with 3-chloroacetophenone. [11] In the same year, Kuang et al applied a chelated ferrocene-based planar chiral NHC-rhodium(I) complex to the ATH of acetophenone and cyclohexylphenylketone to afford the corresponding secondary alcohols with 60 and 67 % ee, respectively. [12] It should be noted that these are the highest enantioselectivities that have been reported in the literature.…”

Ruthenium(II) complexes derived from C2‐symmetric ferrocene‐based chiral bis(phosphinite) ligands: synthesis and catalytic activity towards the asymmetric reduction of acetophenones

“…The reduction of unsaturated molecules can be conducted either by direct hydrogenation with gaseous hydrogen, which is more widely applied, or by hydrogen obtained in situ from a suitable donor such as propan‐2‐ol or formic acid . The latter has drawn great attention in the last decade due to its operational simplicity, use of inexpensive and benign reagents and there being no need for high‐pressure hydrogen that can be potentially dangerous . Asymmetric transfer hydrogenations of functionalized prochiral ketones and imines, as an efficient method for producing enantiopure alcohols and amines, have drawn great attention due to their potential for applications in the fine chemicals, pharmaceuticals and agrochemical industries and in new materials …”