Author(s)

Satoshi Sakaguchi^*, Chika Nagao, Ryo Ichihara, Shogo Matsuo

Department of Chemistry and Materials Engineering, Faculty of Chemistry, Materials and Bioengineering, Kansai University, Suita, Japan.

An operationally simple protocol was designed for the enantioselective silane reduction (ESR) of ketones using air- and moisture-stable [Ir(OMe)(cod)]₂ (cod = 1,5-cyclooctadiene) (3) as a metal catalyst precursor. This reaction was driven by chiral hydroxyamide-functionalized azolium salt 2. The catalytic ESR reaction could be performed under benchtop conditions at room temperature. Treatment of 2 with 3 in THF yielded the monodentate IrCl(NHC)(cod) (NHC = N-heterocyclic carbene) complex 4 in 93% yield, herein the anionic methoxy ligand of 3 serves as an internal base that deprotonates the azolium ring of 2. The well-defined Ir complex 4 catalyzed the ESR reaction of propiophenone (6) with (EtO)₂MeSiH using the pre-mixing reaction procedure. Based on this success, the catalytic ESR reaction was designed and implemented using an in situ-generated NHC/Ir catalyst derived from 2 and 3. Thus, a wide variety of aryl ketones could be reduced to the corresponding optically active alcohols in moderate to excellent stereoselectivities at room temperature without temperature control. Since the high catalytic activity of 3 was observed, we next evaluated several other transition metal catalyst precursors for the catalytic ESR reaction under the influence of 2. This evaluation revealed that Ir(acac)(cod) (acac = acetylacetonate) (28) and [IrCl(cod)]₂ (5) can be successfully used as metal catalyst precursors in the ESR reaction.

Asymmetric Catalysis, Enantioselective Reduction, Hydrosilylation Reaction, N-Heterocyclic Carbene, Iridium

Sakaguchi, S. , Nagao, C. , Ichihara, R. and Matsuo, S. (2024) Operationally Simple Enantioselective Silane Reduction of Ketones by the [Ir(OMe)(cod)]₂/Azolium Catalytic System. International Journal of Organic Chemistry, 14, 1-19. doi: 10.4236/ijoc.2024.141001.