Enantioselective Hydrosilylation of Ketones Catalyzed by a Readily Accessible N‐Heterocyclic Carbene–Ir Complex at Room Temperature

Abstract

A series of functionalized azolium compounds were synthesized from chiral α‐amino acid derivatives such as β‐amino alcohols. Reaction of hydroxy‐amide‐functionalized azolium salts thus obtained with Ag2O afforded N‐heterocyclic carbene‐Ag (NHC–Ag) complexes. Subsequent treatment of the resulting silver compound with [IrCl(cod)]2 yielded monodentate IrCl(cod)(NHC), which was stable in air. The NHC–Ir complex facilitated efficient asymmetric hydrosilylation of ketones using (EtO)2MeSiH under ambient conditions. A chiral ligand containing an isobutyl stereodirecting group was found to be the best ligand for the functionalized NHC–Ir complexes that were examined. A linear relationship was found between the catalyst ee and the product ee. The hydroxy functional group on the NHC ligand side‐arm not only induced stereocontrol but also enhanced the reaction rate.

Operationally Simple Enantioselective Silane Reduction of Ketones by the [Ir(OMe)(cod)]<sub>2</sub>/Azolium Catalytic System

“…Although p-methoxypropiophenone (13), which contained an electron-donating substituent, was reduced to produce the alcohol 13a in 53% yield, higher product yield (85%) was obtained in the reduction of p-chloropropiophenone (14), which contained an electron-withdrawing substituent (Table 1, Entry 8 vs. Entry 9).…”

“…We developed a series of chiral IrCl(NHC)(cod) complexes via the transmetalation reaction of a hydroxyamide-functionalized NHC/Ag complex with [IrCl(cod)] 2 using the Ag 2 O method (Figure 1) [13]. Interestingly, the Ir-catalyzed enantioselective silane reduction (ESR) reaction of ketones was successfully carried out in the presence of a small amount of AgBF 4 at room temperature without temperature control [14]. Subsequently, we investigated the ESR reaction using [Ir(cod) 2 ]BF 4 (1) as an Ir catalyst precursor without AgBF 4 additive (Figure 1) [15].…”

A leap forward in iridium–NHC catalysis: new horizons and mechanistic insights

“…Efficient asymmetric hydrosilylation (ee´s up to 96%) was achieved with 139c under ambient conditions using (EtO)2MeSiH for a broad variety of aromatic and aliphatic ketones ( Figure 17). 99…”

Hydrosilylation of Carbonyl Compounds Catalyzed by Iridium(I) Complexes with (−)‐Menthol‐Based Phosphorus(III) Ligands

“…The iridium complexes, in particular the commercially available complex – [{Ir( μ ‐Cl)(COD)} 2 ] (COD= cis , cis ‐1,5‐cyclooctadiene), are mainly known as efficient catalysts of the hydrosilylation of the carbon‐carbon unsaturated bonds in alkenes, alkynes, and α , β ‐unsaturated ketones [12–14] . In general, the application of Ir(I) and Ir(III) catalysts in the hydrosilylation of aldehydes and ketones is quite well studied, with particular emphasis on complexes containing N ‐heterocyclic carbene ligands [15–26] . However, the high catalyst loadings and the use of expensive organosilicon reagent excess (Ph 2 SiH 2 or (EtO) 2 MeSiH), make these processes economically unattractive.…”

“…[12][13][14] In general, the application of Ir(I) and Ir(III) catalysts in the hydrosilylation of aldehydes and ketones is quite well studied, with particular emphasis on complexes containing N-heterocyclic carbene ligands. [15][16][17][18][19][20][21][22][23][24][25][26] However, the high catalyst loadings and the use of expensive organosilicon reagent excess (Ph 2 SiH 2 or (EtO) 2 MeSiH), make these processes economically unattractive. The application of iridium complexes as catalysts in the hydrosilylation of esters has been relatively neglected (only two available reports have been published in the last decade [27,28] ), leaving the hydrosilylation of carbonyl compounds with these catalysts rather unexplored.…”