Enhanced Catalytic Activity of Oxygen‐Tethered Ir^III NHC Complexes in Aqueous Transfer Hydrogenative Reductive Amination Reactions: Experimental Kinetic and Mechanistic Study

Abstract

The synthesis and characterization of seven new Ir III complexes containing o-phenoxide or o-naphthoxide chelated N-heterocyclic carbene ligands is reported herein. The crystal structures of six of the complexes have been determined. These complexes efficiently catalyze the transfer hydrogenative reductive amination (RA) of carbonyls and amines in water. Amongst the complexes tested, the introduction of o-naphthoxide on a nitrogen atom of imidazole based NHC ligand greatly increased the catalytic activity. The catalytic system has a broad substrate scope, which allows the synthesis of a variety of amines in excellent yields and with high turnover numbers up to 490 (for ketones) and 14800 (for aldehydes). The mechanism of aqueous RA reaction with an o-aryloxide chelated NHC-Ir III catalyst has been investigated by NMR spectroscopy and kinetic measurements. These studies suggest that the transfer hydrogenation (TH) is turnover-limited by the hydride formation step. As a result of the 1 H NMR studies, the higher catalytic activity of onaphthoxide chelated catalyst (3 g) over o-phenoxide chelated one (3 b) can be attributed partly due to the faster formation of an iridium hydride, the key intermediate in the RA reactions.

A Proton-Responsive Annulated Mesoionic Carbene (MIC) Scaffold on Ir Complex for Proton/Hydride Shuttle: An Experimental and Computational Investigation on Reductive Amination of Aldehyde

“…Formation of the metal-hydride intermediate via decarboxylation of the formic acid can follow three pathways: (1) inner-sphere β-hydride elimination of the metal-formate, (2) outer-sphere hydride transfer through an ion-pair, and (3) ligand-assisted hydride transfer of the formate . The presence of a vacant site at the metal of a putative metal-formate complex is the prerequisite for a β-hydride elimination pathway.…”

Efficiency enhancement with chloride to iodide ion exchange of benzimidazolium salt as a redox mediator

“…The benzimidazoles [19,20]. Also, the -CH 3 and R-CH 2 -R' carbons were assigned at 33.7 and 34.5 ppm, and 55.9 and 57.3 ppm, respectively.…”

Recent Development in the Synthesis and Catalytic Application of Iridacycles

“…Based on these studies, the Gulcemal group synthesized a set of new O-aryloxide chelated NHCÀ Ir complexes, which gave high yields for reductive amination of various carbonyls in water. [73] Scheme 32 shows the previously reported IrÀ NHC complex 71 and modified variants 72-78. The complex 73 with an imidazole moiety gave better conversions than the benzimidazole complex 72.…”

“…As was shown before, [72] the catalytic activity was found highly affected by the structure of iridium complexes. Based on these studies, the Gulcemal group synthesized a set of new O ‐aryloxide chelated NHC−Ir complexes, which gave high yields for reductive amination of various carbonyls in water [73] . Scheme 32 shows the previously reported Ir−NHC complex 71 and modified variants 72 – 78 .…”