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

Abstract

An enantioselective hydrosilylation of prochiral ketones was achieved by using a catalytic amount of the readily accessible and air-and moisture-stable iridium complex [IrClA

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

“…For a reaction with a tertiary silane such as R 3 SiH, the reaction appears to proceed by the Ojima mechanism. In the previous report, we showed the hydrosilylation of ketone 5 with Ph 2 SiH 2 catalyzed by IrCl(cod)(NHC) complex 2c furnished a racemic mixture of the reduced product with low yield (36 %) 12b. Therefore, it is likely that the present hydrosilylation with (EtO) 2 MeSiH proceeds through a reaction pathway similar to that proposed by Ojima.…”

“…Other ketones 6 – 9 were reduced to their corresponding optically active alcohols with good enantioselectivies at room temperature (Table 2, Entries 2–5). In our previous communication, we had investigated NHC–Ir‐catalyzed enantioselective hydrosilylation of acetophenone derivatives 5 – 9 with (EtO) 2 MeSiH in 2‐methyltetrahydrofuran 12b. A comparison with previously reported results revealed that these substrates, except for 5 and 7 , could be reduced efficiently in the reaction using THF as solvent.…”

“…The desired [IrCl(cod)(NHC)] complex was purified by column chromatography on silica gel (CH 2 Cl 2 /CH 3 OH = 9:1). Complexes 2c and 2f have been previously reported 12b…”

“…Other new NHC–Ir and NHC–Ru complexes have been synthesized to further explore the applications of this kind of chiral hydroxy–amide‐functionalized NHC ligand 12. We briefly described the enantioselective hydrosilane reduction of ketones under mild conditions, catalyzed by the monodentate IrCl(cod)(NHC) complex, in a previous communication 12b. Here, we present the entire set of results from our study of the NHC–Ir‐catalyzed asymmetric hydrosilylation of ketones.…”

Dealing at Arm's Length: Catalysis with N‐Heterocyclic Carbene Ligands Bearing Anionic Tethers

“…Chiral iridium complexes with hydroxamide‐based tethered NHC ligands, such as 209 , which were originally studied for transfer hydrogenation of ketones, also enable the related asymmetric hydrosilylation of alkyl aryl ketones such as thiophene 212 (Scheme ) . Also, the hydrosilylation of propiophenone ( 214 ) with diethoxy(methyl)silane in the presence of 209 delivers the corresponding alcohol 215 with an e.r .…”