Chiral cyclometalated iridium complexes for asymmetric reduction reactions

Abstract

A series of chiral iridacycles have been synthesised and shown to be moderately enantioselective in asymmetric transfer hydrogenation reactions.

A Cyclometalated NHC Iridium Complex Bearing a Cationic (η5‐Cyclopentadienyl)(η6‐phenyl)iron Backbone**

“…Cyclometalated iridium(III) compounds in particular are well known for specific applications in photochemistry and as antibacterial and anticancer agents in biology [8] . In general, they are easily accessible via a base‐assisted C−H activation step [9] . Numerous examples for the use of cyclometalated iridium(III) complexes in homogenous catalysis have been reported in the past: hydrogenation, [10] transfer hydrogenation, [11] dehydrogenation, [11c,12] reductive amination, [13] hydroamination, [14] hydrosilylation [11c,15] and racemization reactions [16] are catalyzed by iridium compounds.…”

Ruthenacycles and Iridacycles as Transfer Hydrogenation Catalysts

“…Xiao and coworkers screened 15 different chiral iridacycles based on 4,5-diaryl-substituted 2-phenyl-oxazolines and imidazolines in the enantioselective reductive amination of acetophenone with p -anisidine using the formic acid triethylamine azeotrope in isopropanol as reductant at ambient temperature [ 76 ]. From this screening, iridacycle 82 came out as the best one, inducing the highest enantioselectivity.…”

“…Xiao and coworkers tested 15 different chiral iridacycles based on 4,5-diaryl-substituted 2-phenyl-oxazolines and imidazolines in the enantioselective hydrogenation of N -benzyl 2-phenylpyridinium bromide using the azeotropic mixture of formic acid and triethylamine in isopropanol as solvent at room temperature [ 76 ]. Best results (55% ee ) were obtained with iridacycle 85 .…”

Recent Development in the Synthesis and Catalytic Application of Iridacycles

“…We have recently reported a range of chiral iridacycles, obtained by cyclometalation of [Cp*IrCl 2 ] 2 with chiral oxazoline and imidazoline ligands via CÀ H bond activation under generally mild reaction conditions. [27] These iridacycles appeared mostly as a mixture of two diastereoisomers, presumably due to the small difference between their thermodynamic stability. Some examples (21)(22)(23)(24)(25)(26) are shown in Scheme 6.…”

“…In a related work, we examined the catalytic activity of a series of oxazoline and imidazoline-based iridacycles in direct asymmetric reductive amination (DARA) of acetophenone with p-methoxy aniline and in ATH of N-benzyl pyridinium bromide salts. [27] Among them, the 3,4,5-trimethoxy imidazoline-bearing iridacycle, complex 23, was found the most effective catalyst for DARA. Applying 23 to DARA of a range of ketones with amines afforded chiral amines with high yields and moderate enantioselectivities in most cases.…”