Abstract
A new class of bis-cyclometalated iridium(III) catalysts containing two inert cyclometalated 6-tert-butyl-2-phenyl-2H-indazole bidentate ligands or two inert cyclometalated 5-tert-butyl-1-methyl-2-phenylbenzimidazoles is introduced. The coordination sphere is complemented by two labile acetonitriles, and a hexafluorophosphate ion serves as a counterion for the monocationic complexes. Single enantiomers of the chiral-at-iridium complexes (>99% er) are obtained through a chiral-auxiliary-mediated approach using a monofluorinated salicyloxazoline and are investigated as catalysts in the enantioselective conjugate addition of indole to an α,β-unsaturated 2-acyl imidazole and an asymmetric Nazarov cyclization.
Highlights
Cyclometalated transition metal complexes are of tremendous importance as intermediates in C–H functionalization reactions but have gained increasing attention for their interesting catalytic and biological properties, not least due to the pioneering contributions of the Pfeffer laboratory on the formation and applications of metallacycles [1,2,3,4]
Our initial design was based on iridium(III) complexes in which two inert cyclometalated 5-tertbutyl-2-phenylbenzoxazoles (IrO [7]) or 5-tert-butyl-2-phenylbenzothiazoles (IrS [8]) are complemented by two labile acetonitrile molecules and hexafluorophosphate served as the counterion for the monocationic complexes (Figure 1a)
The smooth bis-cyclometalation to provide rac-IrInd was noteworthy and the yield was significantly higher compared to rac-IrBim and the previously reported rac-IrO and rac-IrS [7,8]
Summary
Cyclometalated transition metal complexes are of tremendous importance as intermediates in C–H functionalization reactions but have gained increasing attention for their interesting catalytic and biological properties, not least due to the pioneering contributions of the Pfeffer laboratory on the formation and applications of metallacycles [1,2,3,4]. Our group has recently introduced bis-cyclometalated iridium and rhodium complexes as chiral catalysts for application in asymmetric catalysis [5,6]. The strongly σ-donating phenyl ligands labilize the acetonitrile ligands by exerting a strong trans effect [22]. This is important for catalysis, during which one or two acetonitrile ligands are replaced by a substrate or reagent, while the helically arranged inert cyclometalated ligands provide the asymmetric induction
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