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Relevant academic research and scientific papers

Mononuclear half-sandwich iridium and rhodium complexes through C?H activation: Synthesis, characterization and catalytic activity

A series of mononuclear half-sandwich cyclometalated group 9 (Ir and Rh) metal complexes were synthesized in good yields through metal-mediated C?H bond activation. These air-stable C, N-chelate mode complexes have similar solid state structures. Both experimental results and DFT calculations confirmed that no binuclear complexes were generated in this reaction. The iridium complex 3a exhibited good catalytic activity for the reduction of both electron-rich and electron-poor aryl imines with low catalyst loading in the presence of formic acid/triethylamine (F/T) azeotropic mixture. All complexes were fully characterized by elemental analysis and IR and NMR spectroscopies. The structures of 1a, 1b, 2a, 3a and 4b (see chemical structure formula in Scheme 1 and Scheme 2) were further confirmed by single-crystal X-ray analysis.

Iron-Catalyzed Oxidative Amination of Benzylic C(sp3)–H Bonds with Anilines

Iron-catalyzed oxidative amination of benzylic C(sp3)–H bonds with anilines bearing electron-withdrawing groups (EWGs) or electron-donating groups (EDGs) is realized based on simple variations of N-substituents on imidazolium cations in novel ionic Fe(III) complexes. The structural modification of the imidazolium cation resulted in regulation of the redox potential and the catalytic performance of the iron metal center. Using DTBP as oxidant, [HItBu][FeBr4] showed the highest catalytic activity for anilines bearing EWGs, while [HIPym][FeBr4] was more efficient for EDG-substituted anilines. This work provides alternative access to benzylamines with the advantages of both a wide substrate scope and iron catalysis.

Synthesis of NHC-Iridium(III) Complexes Based on N-Iminoimidazolium Ylides and Their Use for the Amine Alkylation by Borrowing Hydrogen Catalysis

Anionic NHC ligands recently developed in our group, derived from N-iminoimidazolium ylides, were used to synthesize NHC-iridium(III) complexes. Their catalytic activities were evaluated in the amine alkylation of anilines using borrowing hydrogen catalysis. The high-yielding synthesis of a small library of complexes allowed a rapid screening of the ideal steric bulk of the NHC unit and basicity of the anionic tether for the investigated model reaction. A bulky aromatic N group on the imidazolidene moiety is required to achieve high catalytic activity, and the latter is proportional to the basicity of the anionic group. A selected substrate scope of the reaction was performed, providing fair to excellent yields of the desired alkylated anilines.

Imine reduction with me2s-bh3

Although there exists a variety of different catalysts for hydroboration of organic substrates such as aldehydes, ketones, imines, nitriles etc., recent evidence suggests that tetra-coordinate borohydride species, formed by activation, redistribution, or decomposition of boron reagents, are the true hydride donors. We then proposed that Me2S-BH3 could also act as a hydride donor for the reduction of various imines, as similar compounds have been observed to reduce carbonyl substrates. This boron reagent was shown to be an effective and chemoselective hydroboration reagent for a wide variety of imines.

Ru(II) complexes containing (2-(pyren-1-ylmethylene)hydrazinyl)benzothiazole: Synthesis, solid-state structure, computational study and catalysis in N-alkylation reactions

Reactions of (2-(pyren-1-ylmethylene)hydrazinyl)benzothiazole (L) with ruthenium(II) prefabricated precursors [RuHCl(CO)(EPh3)3] and [RuH2(CO)(EPh3)3] (E = P or As) afforded new Ru(II) complexes [RuCl(CO)(EPh3)2(L)] and [RuH(CO)(EPh3)2(L)] (E = P or As) (1–4). All the Ru(II) complexes (1–4) were characterized by IR, NMR spectroscopies, ESI-mass spectrometry and elemental analyses. The solid-state structures of Ru(II) complexes (2 and 3) were established by single crystal X-ray analyses and revealed distorted octahedral geometries around the ruthenium(II) ion and mono anionic bidentate N^N coordination mode for hydrazine ligand. The Ru(II) complexes 2 and 3 were also analyzed using Hirshfeld surface analysis and DFT calculations. Moreover, all the complexes (1–4) were utilized in the N-alkylation reactions of amines using alcohol. Complex 3 was found to be highly active towards N-alkylation of different aromatic amines with alcohol.

Borrowing Hydrogen-Mediated N-Alkylation Reactions by a Well-Defined Homogeneous Nickel Catalyst

We report herein a well-defined and bench-stable azo-phenolate ligand-coordinated nickel catalyst which can efficiently execute N-alkylation of a variety of anilines by alcohol. We demonstrate that the redox-active azo ligand can store hydrogen generated during alcohol oxidation and redelivers the same to an in-situ-generated imine bond to result in N-alkylation of amines. The reaction has wide scope, and a large array of alcohols can directly couple to a variety of anilines. Mechanistic studies including deuterium labeling to the substrate establishes the borrowing hydrogen method from alcohols and pinpoints the crucial role of the redox-active azo moiety present on the ligand backbone. Isolation of the ketyl intermediate in its trapped form with a radical quencher and higher kH/kD for the alcohol oxidation step suggest altogether a hydrogen-atom transfer (HAT) to the reduced azo backbone to pave alcohol oxidation as opposed to the conventional metal-ligand bifunctional mechanism. This example clearly demonstrates that an inexpensive base metal catalyst can accomplish an important coupling reaction with the help of a redox-active ligand backbone.

Phosphorous(v) Lewis acids: Water/base tolerant P3-trimethylated trications

The water/base intolerance of the previously reported electrophilic phosphonium cations has been overcome by replacing the labile electron-withdrawing groups generally attached to phosphorus (e.g. -F, -OAr, -CF3) with methyl groups. Tri-phosphorus(v) tricationic species, accessible in one-pot from commercially available materials, are air and water/base tolerant, yet are sufficiently Lewis acidic for catalysis.

Synthesis of N-Substituted 3-Amino-4-halopyridines: A Sequential Boc-Removal/Reductive Amination Mediated by Br?nsted and Lewis Acids

N-Substituted 3-amino-4-halopyridines are valuable synthetic intermediates, as they readily provide access to imidazopyridines and similar heterocyclic systems. The direct synthesis of N-substituted 3-amino-4-halopyridines is problematic, as reductive aminations and base-promoted alkylations are difficult in these systems. A high yielding deprotection/alkylation protocol mediated by trifluoroacetic acid and trimethylsilyl trifluoromethanesulfonate is described, providing access to a wide scope of N-substituted 3-amino-4-halopyridines. This protocol furnishes many reaction products in high purity without chromatography. Similar reductive amination conditions were also established for deactivated anilines.

Heterogeneous cobalt catalysts for reductive amination with H2: General synthesis of secondary and tertiary amines

Heterogeneous Co@NC catalysts were prepared, characterized and applied for the reductive amination of aldehydes and ketones with H2 gas. The Co catalyst Co@NC (800-2 h) was found to be active and selective for the reductive amination of aldehydes and ketones using H2 gas. Thus, general synthesis of secondary and tertiary amines was developed by the Co-catalyzed reductive amination with H2 gas, and various secondary and tertiary amines can be obtained in high yields. Moreover, a practical synthesis of N-substituted isoindolinones was also presented by a one step process with the Co@NC (800-2 h) catalyst. The Co@NC (800-2 h) catalyst is reusable at least five times without evident loss of activity.

Metal-free hydrogenation catalyzed by an air-stable borane: Use of solvent as a frustrated Lewis base

In recent years 'frustrated Lewis pairs' (FLPs) have been shown to be effective metal-free catalysts for the hydrogenation of many unsaturated substrates. Even so, limited functional-group tolerance restricts the range of solvents in which FLP-mediated reactions can be performed, with all FLP-mediated hydrogenations reported to date carried out in non-donor hydrocarbon or chlorinated solvents. Herein we report that the bulky Lewis acids B(C6Cl5)x(C6F5)3-x (x=0-3) are capable of heterolytic H2 activation in the strong-donor solvent THF, in the absence of any additional Lewis base. This allows metal-free catalytic hydrogenations to be performed in donor solvent media under mild conditions; these systems are particularly effective for the hydrogenation of weakly basic substrates, including the first examples of metal-free catalytic hydrogenation of furan heterocycles. The air-stability of the most effective borane, B(C6Cl5)(C6F5)2, makes this a practically simple reaction method.