Abstract
As a candidate for bifunctional asymmetric catalysts containing a half-sandwich C–N chelating Ir(III) framework (azairidacycle), a dinuclear Ir complex with an axially chiral linkage is newly designed. An expedient synthesis of chiral 2,2′-bis(aminomethyl)-1,1′-binaphthyl (1) from 1,1-bi-2-naphthol (BINOL) was accomplished by a three-step process involving nickel-catalyzed cyanation and subsequent reduction with Raney-Ni and KBH4. The reaction of (S)-1 with an equimolar amount of [IrCl2Cp*]2 (Cp* = η5–C5(CH3)5) in the presence of sodium acetate in acetonitrile at 80 °C gave a diastereomeric mixture of new dinuclear dichloridodiiridium complexes (5) through the double C–H bond cleavage, as confirmed by 1H NMR spectroscopy. A loss of the central chirality on the Ir centers of 5 was demonstrated by treatment with KOC(CH3)3 to generate the corresponding 16e amidoiridium complex 6. The following hydrogen transfer from 2-propanol to 6 provided diastereomers of hydrido(amine)iridium retaining the bis(azairidacycle) architecture. The dinuclear chlorido(amine)iridium 5 can serve as a catalyst precursor for the asymmetric transfer hydrogenation of acetophenone with a substrate to a catalyst ratio of 200 in the presence of KOC(CH3)3 in 2-propanol, leading to (S)-1-phenylethanol with up to an enantiomeric excess (ee) of 67%.
Highlights
The metal/NH bifunctional catalysis has become a pivotal concept for redox transformations based on hydrogen transfer between secondary alcohols and ketones [1,2,3,4,5].From in-depth studies on original (η6 -arene)Ru catalysts bearing chiral N-sulfonyldiamines (N–N chelate complexes) developed by Noyori and Ikariya [6,7], a fundamental hydrogen delivery process associated with alternating back and forth between 16e amido and 18e hydrido(amine) complexes has been thoroughly realized
1,1′-binaphthyl scaffold and amine functionalization, we examined a three-step sequence involving the catalytic dicyanation of accessible three-step sequence involving the catalytic dicyanation of accessible ditriflate ined a three-step sequence involving the catalytic dicyanation of accessible BINOL
Based on the smooth formation of amidoiridium and hydrido(amine)iridium in the bis(azairidacycle) system, we further explored its catalytic potential for the asymmetric transfer hydrogenation
Summary
The metal/NH bifunctional catalysis has become a pivotal concept for redox transformations based on hydrogen transfer between secondary alcohols and ketones [1,2,3,4,5]. The rapid hydrogen transfer from alcohols to the amidoiridium and from the hydrido(amine)iridium to ketones accommodates the dynamic kinetic resolution of raby combination enzymatic transesterification using Candida antarctica lipase [21]. Thanks to structure is extended by bridging ligands, exhibit characteristic properties and favorable the cooperative effect, in which thetoreaction sitesmononuclear on multiple metals play different roles, and catalytic performance compared the parent complexes [27,28,29,30,31,32,33,34]. It isbyexpected that we inherent intramolecular steric effects arising fromwith the Encouraged these works, focused our interest on a dimetallic compound discrete symmetrical bis(metallacycle) will contribute to enhance the catalytic performance a chiral diamido linkage formed by the double orthometalation of benzylic diamine compared with the.
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