Abstract
A new class of bis‐cyclometalated rhodium(III) catalysts containing two inert cyclometalated 6‐tert‐butyl‐2‐phenyl‐2H‐indazole ligands and two labile acetonitriles is introduced. Single enantiomers (>99 % ee) were obtained through a chiral‐auxiliary‐mediated approach using a monofluorinated salicyloxazoline. The new chiral‐at‐metal complex is capable of catalyzing the visible‐light‐induced enantioselective α‐cyanoalkylation of 2‐acyl imidazoles in which it serves a dual function as the chiral Lewis acid catalyst for the asymmetric radical chemistry and at the same time as the photoredox catalyst for the visible‐light‐induced redox chemistry (up to 80 % yield, 4:1 d.r., and 95 % ee, 12 examples).
Highlights
Chiral transition-metal complexes are a prominent and powerful class of asymmetric catalysts, traditionally assembled from chiral organic ligands and metal salts or organometallic precursor complexes.[1]
We found that the nature of the cyclometalating ligand has a profound influence on the reactivity and stereoselectivity of the bis-cyclometalated iridium(III) and rhodium(III) complexes[2] and we were seeking to investigate ligands that differ from our previous benzoxazole and benzothiazole systems
We demonstrate that this RhInd catalyst is superior for the visible-light-induced enantioselective a-cyanoalkylation of 2-acyl imidazoles in which RhInd serves a dual function as the chiral catalyst but is involved in the photochemical induction
Summary
Chiral transition-metal complexes are a prominent and powerful class of asymmetric catalysts, traditionally assembled from chiral organic ligands and metal salts or organometallic precursor complexes.[1]. Our initial design was based on bis-cyclometalated iridium(III) and rhodium(III) complexes, in which two 5-tert-butyl-2phenylbenzoxazoles (IrO[7] and RhO[8]) or 5-tert-butyl-2-phenylbenzothiazoles (IrS[9] and RhS[10]) implement a stereogenic metal center with either a left-handed (L-configuration) or right-handed (D-configuration) overall helical topology (Figure 1). These two cyclometalated ligands are configurationally inert so that the overall stereochemical information is re-. We demonstrate that this RhInd catalyst is superior for the visible-light-induced enantioselective a-cyanoalkylation of 2-acyl imidazoles in which RhInd serves a dual function as the chiral catalyst but is involved in the photochemical induction
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