Iridicycle-Catalysed Imine Reduction: An Experimental and Computational Study of the Mechanism

Accessing para-Alkylphenols via Iridium-Catalyzed Site-Specific Deoxygenation of Alcohols

An iridium-catalyzed and phenol-directed deoxygenation of benzylic alcohols comes as an alternative access to 4-alkylphenols, featuring low catalyst loading (S/C up to 20,000, TOF up to 12,400 h-1), high functionality compatibility, and excellent site-selectivity. The applications in late-stage modification of steroids and gram-scale total synthesis of a Gastrodia elata extract are highlighted. Mechanistically, the intermediacy of quinone methide controls the site-selectivity, and the formation of iridium hydride serves as the rate-limiting step.

Iridium-Catalyzed Acid-Assisted Hydrogenation of Oximes to Hydroxylamines

We found that cyclometalated cyclopentadienyl iridium(III) complexes are uniquely efficient catalysts in homogeneous hydrogenation of oximes to hydroxylamine products. A stable iridium C,N-chelation is crucial, with alkoxy-substituted aryl ketimine ligands providing the best catalytic performance. Several Ir-complexes were mapped by X-ray crystal analysis in order to collect steric parameters that might guide a rational design of even more active catalysts. A broad range of oximes and oxime ethers were activated with stoichiometric amounts of methanesulfonic acid and reduced at room temperature, remarkably without cleavage of the fragile N-O bond. The exquisite functional group compatibility of our hydrogenation system was further demonstrated by additive tests. Experimental mechanistic investigations support an ionic hydrogenation platform, and suggest a role for the Brønsted acid beyond a proton source. Our studies provide deep understanding of this novel acidic hydrogenation and may facilitate its improvement and application to other challenging substrates.

Ruthenacycles and Iridacycles as Transfer Hydrogenation Catalysts

In this review, we describe the synthesis and use in hydrogen transfer reactions of ruthenacycles and iridacycles. The review limits itself to metallacycles where a ligand is bound in bidentate fashion to either ruthenium or iridium via a carbon–metal sigma bond, as well as a dative bond from a heteroatom or an N-heterocyclic carbene. Pincer complexes fall outside the scope. Described are applications in (asymmetric) transfer hydrogenation of aldehydes, ketones, and imines, as well as reductive aminations. Oxidation reactions, i.e., classical Oppenauer oxidation, which is the reverse of transfer hydrogenation, as well as dehydrogenations and oxidations with oxygen, are described. Racemizations of alcohols and secondary amines are also catalyzed by ruthenacycles and iridacycles.

Synthesis, Structure, and Catalytic Hydrogenation Activity of [NO]-Chelate Half-Sandwich Iridium Complexes with Schiff Base Ligands

A series of N,O-coordinate iridium(III) complexes with a half-sandwich motif bearing Schiff base ligands for catalytic hydrogenation of nitro and carbonyl substrates have been synthesized. All iridium complexes showed efficient catalytic activity for the hydrogenation of ketones, aldehydes, and nitro-containing compounds using clean H₂ as reducing reagent. The iridium catalyst displayed the highest TON values of 960 and 950 in the hydrogenation of carbonyl and nitro substrates, respectively. Various types of substrates with different substituted groups afforded corresponding products in excellent yields. All N,O-coordinate iridium(III) complexes 1-4 were well characterized by IR, NMR, HRMS, and elemental analysis. The molecular structure of complex 1 was further characterized by single-crystal X-ray determination.

Probe metal binding mode of imine covalent organic frameworks: cycloiridation for (photo)catalytic hydrogen evolution from formate

Metalation of covalent organic frameworks (COFs) is a critical strategy to functionalize COFs for advanced applications yet largely relies on the pre-installed specific metal docking sites in the network, such as porphyrin, salen, 2,2'-bipyridine, etc. We show in this study that the imine linkage of simple imine-based COFs, one of the most popular COFs, readily chelate transition metal (Ir in this work) via cyclometalation, which has not been explored before. The iridacycle decorated COF exhibited more than 10-fold efficiency enhancement in (photo)catalytic hydrogen evolution from aqueous formate solution than its molecular counterpart under mild conditions. This work will inspire more functional cyclometallated COFs to be explored beyond catalysis considering the large imine COF library and the rich metallacycle chemistry.

Amide Iridium Complexes As Catalysts for Transfer Hydrogenation Reduction of N-sulfonylimine

Sulfonamide moieties widely exist in natural products, biologically active substance, and pharmaceuticals. Here, an efficient water-soluble amide iridium complexes-catalyzed transfer hydrogenation reduction of N-sulfonylimine is developed, which can be carried out under environmentally friendly conditions, affording a series of sulfonamide compounds in excellent yields (96-98%). In comparison with organic solvents, water is shown to be critical for a high catalytic transfer hydrogenation reduction in which the catalyst loading can be as low as 0.001 mol %. These amide iridium complexes are easy to synthesize, one structure of which was determined by single-crystal X-ray diffraction. This protocol gives an operationally simple, practical, and environmentally friendly strategy for synthesis of sulfonamide compounds.

pH-Dependent transfer hydrogenation or dihydrogen release catalyzed by a [(η6-arene)RuCl(κ2-N,N-dmobpy)]+ complex: a DFT mechanistic understanding

The reaction mechanism of the pH-dependent transfer hydrogenation of a ketone or the dehydrogenation of formic acid catalyzed by a [(η6-arene)RuCl(κ2-N,N-dmobpy)]+ complex in aqueous media has been investigated using the density functional theory (DFT) method. The TM-catalyzed TH of ketones with formic acid as the hydrogen source proceeds via two steps: the formation of a metal hydride and the transfer of the hydride to the substrate ketone. The calculated results show that ruthenium hydride formation is the rate-determining step. This proceeds via an ion-pair mechanism with an energy barrier of 14.1 kcal mol-1. Interestingly, the dihydrogen release process of formic acid and the hydride transfer process that produces alcohols are competitive under different pH environments. The investigation explores the feasibility of the two pathways under different pH environments. Under acidic conditions (pH = 4), the free energy barrier of the dihydrogen release pathway is 4.5 kcal mol-1 that is higher than that of the hydride transfer pathway, suggesting that the hydride transfer pathway is more favorable than the dihydrogen release pathway. However, under strongly acidic conditions, the dihydrogen release pathway is more favorable compared to the hydride transfer pathway. In addition, the ruthenium hydride formation pathway is less favorable than the ruthenium hydroxo complex formation pathway under basic conditions.

One catalyst, multiple processes: ligand effects on chemoselective control in Ru-catalyzed anti-Markovnikov reductive hydration of terminal alkynes

The ligand effect through kinetic and thermodynamic control on the chemoselectivity of one-catalyst multi-step catalysis.

Methanol as hydrogen source: transfer hydrogenation of aromatic aldehydes with a rhodacycle

A rhodacycle catalyses efficient hydrogenation of aldehydes, deriving the hydrogen from methanol.

Enhanced Catalytic Activity of Iridium(III) Complexes by Facile Modification of C,N-Bidentate Chelating Pyridylideneamide Ligands

A set of aryl-substituted pyridylideneamide (PYA) ligands with variable donor properties owing to a pronounced zwitterionic and a neutral diene-type resonance structure were used as electronically flexible ligands at a pentamethylcyclopentadienyl (Cp*) iridium center. The straightforward synthesis of this type of ligand allows for an easy incorporation of donor substituents such as methoxy groups in different positions of the phenyl ring of the C,N-bidentate chelating PYA. These modifications considerably enhance the catalytic activity of the coordinated iridium center toward the catalytic aerobic transfer hydrogenation of carbonyls and imines as well as the hydrosilylation of phenylacetylene. Moreover, these PYA iridium complexes catalyze the base-free transfer hydrogenation of aldehydes, and to a lesser extent also of ketones. Under standard transfer hydrogenation conditions including base, aldehydes are rapidly oxidized to carboxylic acids rather than reduced to the corresponding alcohol, as is observed under base-free conditions.

Small "Yaw" Angles, Large "Bite" Angles and an Electron-Rich Metal: Revealing a Stereoelectronic Synergy To Enhance Hydride-Transfer Activity

Cyclometalated complexes are an important class of (pre)catalysts in many reactions including hydride transfer. The ring size of such complexes could therefore be a relevant aspect to consider while modulating their catalytic activity. However, any correlation between the cyclometalating ring size and the catalytic activity should be drawn by careful assessment of the pertinent geometrical parameters, and overall electronic effects thereof. In this study, we investigated the vital role of key stereoelectronic functions of two classes of iridacyclic complexes-five-membered and six-membered cycles-in manupulating the catalytic efficiency in a model hydride-transfer reaction. Our investigation revealed that there exists an interesting multidimensional synergy among all the relevant stereoelectronic factors-yaw angle, bite angle, and the electronic properties of both the ligand and the metal center-that governs the hydride donor ability (hydricity) of the complexes during catalysis. Thus the six-membered chelate complexes with small yaw and large bite angles, strong donor ligand, and electron-rich metal were found to be better catalysts than their five-membered analogues. A frontier molecular orbital analysis supported the significant role of the above stereoelectronic synergistic effect associated with the chelate ring to control the hydride donor ability of the complexes.

Iridacycles for hydrogenation and dehydrogenation reactions

Iridacycles are a group of cyclometalated metal complexes, which have recently been shown to be versatile catalysts for a range of reactions. This Feature Article provides an account of the work carried out by our groups. We start with an introduction to the variety of iridacycles and how they entered into catalysis. The following sections provide an overview of the discovery and applications in catalysis of the iridacycles originated from our labs, including transfer hydrogenation with formic acid, hydrogenation with H₂, dehydrogenation and borrowing-hydrogen reactions. Where possible, mechanistic insight is also presented. A notable advantage of these iridacycles is their ease of preparation, stability to air and water and high modularity. With only one coordination site available for substrate activation, the iridacycles differ from most homogeneous catalysts structure-wise.