Ruthenium-Catalyzed Direct C–H Amidation of Arenes: A Mechanistic Study

Iridium-catalyzed regioselective C–H sulfonamidation of 1,2,4-thiadiazoles with sulfonyl azides in water

We have developed a regioselective C–N cross-coupling of 1,2,4-thiadiazoles with sulfonyl azides through iridium catalysis in water. This method tactically linked the 1,2,4-thiadiazoles and sulfonamides together, and the novel molecules increased the diversity of 1,2,4-thiadiazoles which may have potential applications.

We have developed a regioselective C–N cross-coupling of 1,2,4-thiadiazoles with sulfonyl azides through iridium catalysis in water.

Computational Study on the Fate of Oxidative Directing Groups in Ru(II), Rh(III), and Pd(II) Catalyzed C-H Functionalization

Activation of C-H bonds assisted by a directing group is indispensable in organic synthesis. Among them, utilizing oxidative directing groups that can serve as an internal oxidant to drive the Mⁿ/Mⁿ⁺² catalytic cycle has recently become a promising strategy. A survey of published reactions involving N-alkoxyamides or N-acyloxyamides reveals that not all N-O bonds act as an internal oxidant. We have therefore systematically investigated the effect of the oxidative groups on a model reaction catalyzed by Ru(II), Rh(III), and Pd(II) complexes. DFT calculations show that N-methoxy and N-acyloxy groups oxidize Ru(II) to Ru(IV) and Rh(III) to Rh(V), but cannot oxidize a cyclo-Pd(II) intermediate to Pd(IV). The stability of the metal imido intermediate 7-M (M = Ru, Rh, and Pd) controls whether the oxidation occurs or not. N-Acyloxy groups show a more pronounced selectivity than N-methoxy to oxidize Ru(II) and Rh(III) species, while no distinctive effect is observed for Pd(II).

Crystal structure of (η6-1-methyl-4-isopropylbenzene)-[5-bromo-2-(2-pyridyl)phenyl-κ2 C,N]-chloro-ruthenium(II), C21H21BrClNRu

C21H21BrClNRu, triclinic, P1̄ (no. 2), a = 8.2185(4) Å, b = 10.1626(5) Å, c = 11.9356(6) Å, α = 100.399(4)°, β = 90.005(4)°, γ = 93.434(4)°, V = 978.68(9) Å3, Z = 2, R gt(F) = 0.0353, wR ref(F 2) = 0.0676, T = 291(2) K.

Investigations on the Anticancer Potential of Benzothiazole-Based Metallacycles

A series of 2-phenylbenzothiazole derivatives and their corresponding organometallic ruthenium(II) and osmium(II) complexes were synthesized, designed to exploit both, the attributes of the half-sandwich transition metal scaffold and the bioactivity spectrum of the applied 2-phenylbenzothiazoles. All synthesized compounds were characterized via standard analytical methods. The obtained organometallics showed antiproliferative activity in the low μM range and are thus at least an order of magnitude more potent than the free ligands. ESI-MS measurements showed that the examined compounds were stable in aqueous solution over 48 h. Additionally, their binding preferences to small biomolecules, their cellular accumulation and capacity of inducing apoptosis/necrosis were investigated. Based on the fluorescence properties of the selected ligand and the corresponding ruthenium complex, their subcellular distribution was studied by fluorescence microscopy, revealing a high degree of colocalization with acidic organelles of cancer cells.

Theoretical studies on the N–X (X = Cl, O) bond activation mechanism in catalytic C–H amination

A favorable S_N2-type N–Cl bond cleavage mechanism are proposed for Rh-catalysed C–H amination, which also works for N–O bond cleavage in Rh, Ru, and Pd analogous systems. These results could provide new understanding of C–H amination.

Ruthenium(ii)-catalyzed [5 + 1] annulation reaction: a facile and efficient approach to construct 6-ethenyl phenanthridines utilizing a primary amine as a directing group

A ruthenium(ii)-catalyzed [5 + 1] annulation reaction between 2-arylanilines and cyclopropenones employing a free amine as a directing group has been developed.

Cobalt-catalysed unactivated C(sp3)–H amination: two-state reactivity and multi-reference electronic character

A remarkable two-state reactivity scenario and an unusual multi-reference character have been computationally found in Co-catalysed C(sp³)–H amination. In addition, the investigation on the additive, aminating reagent, metal center, and auxiliary ligand provides implications for development of new catalytic C–H functionalization systems.

Ruthenium-catalyzed ortho-selective CAr–H amination of heteroaryl arenes with di-tert-butyldiaziridinone

Application of an oxidative amination reagent (di-tert-butyldiaziridinone) to the Ru₃(CO)₁₂-catalyzed ortho-selective C_Ar–H amination reaction is described.

[Cp*RhIII ]/Ionic Liquid as a Highly Efficient and Recyclable Catalytic Medium for C-H Amidation

A [Cp*Rh^III ]-catalyzed direct C-H amidation is carried out in ionic liquid. Both C(sp² )-H bonds of (hetero)arenes and alkenes and unactivated C(sp³ )-H bonds can be easily amidated with high functional-group tolerance and excellent yields under these conditions. Notably, using [Cp*Rh^III ]/[BMIM]BF₄ (BMIM=1-butyl-3-methylimidazolium) as the green and recyclable medium is environmentally benign, in light of characteristics such as the reusability of the expensive rhodium catalyst, avoidance of highly toxic organic solvents, and mild reaction conditions, as well as a short reaction time.

Ruthenium-catalyzed C–H allylation of arenes with allylic amines

The Ru-catalyzed pyridyl-directed C–H allylation of arenes with allylic amines was carried out in the presence of 5 mol% of [Ru(p-cymene)Cl₂]₂ and 0.5 equiv. of AgOAc in CF₃CH₂OH.

Emerging Developments Using Nitrogen-Heteroatom Bonds as Amination Reagents in the Synthesis of Aminoarenes

Aminoarenes constitute valuable building blocks in organic synthesis and an essential skeleton ubiquitously found in ligands, agrochemicals, and pharmaceuticals. This Synopsis presents recent amination methods using nitrogen-heteroatom bonds as a powerful and versatile platform to rapidly synthesize diverse aminoarenes, with a focus on aryne amino functionalization and transition-metal-catalyzed arene C-H amination.

Transition Metal-Catalyzed C-H Amination: Scope, Mechanism, and Applications

Catalytic transformation of ubiquitous C-H bonds into valuable C-N bonds offers an efficient synthetic approach to construct N-functionalized molecules. Over the last few decades, transition metal catalysis has been repeatedly proven to be a powerful tool for the direct conversion of cheap hydrocarbons to synthetically versatile amino-containing compounds. This Review comprehensively highlights recent advances in intra- and intermolecular C-H amination reactions utilizing late transition metal-based catalysts. Initial discovery, mechanistic study, and additional applications were categorized on the basis of the mechanistic scaffolds and types of reactions. Reactivity and selectivity of novel systems are discussed in three sections, with each being defined by a proposed working mode.

Half-sandwich rhodium and iridium metallamacrocycles constructed via C-H activation

Half-sandwich rhodium and iridium complexes with carboxylic acid ligands were combined with pyrazine, 4,4'-bipyridine (bpy) or trans-1,2-bis(4-pyridyl)-ethylene (bpe) to give a series of tetranuclear macrocycles. The metallamacrocycles [(Cp*Rh)4()2(pyrazine)2][OTf]2 (), [(Cp*Rh)4()2(bpy)2][OTf]2 (), [(Cp*Rh)4()2(bpe)2][OTf]4 () and [(Cp*Ir)4()2 (pyrazine)2] () ( = 3-(2-pyridyl)acrylic acid, = 1,4-di(4-carboxyphenyl)benzene) were characterized by elemental analysis, NMR, IR and single-crystal X-ray analyses. Due to the different structures of the carboxylate ligands, the complexes , and were synthesized through double-site C-H activation, and complexes were obtained by one-site C-H activation.

A remote coordination booster enhances the catalytic efficiency by accelerating the generation of an active catalyst

A remote Ru(II)(terpy)2 unit incorporated in conjugation with the [NHC-Ru(II)(para-cymene)] catalytic site, acts as a "coordination booster" for enhancing the catalytic efficiency to achieve excellent performance in selective oxidative scission of various carbon-carbon multiple bonds to the corresponding aldehydes, ketones and diketones. Generation of an active catalyst via oxidative loss of para-cymene from the precatalyst was found to be accelerated by the "coordination booster" through the electronic effect.

A potential role of a substrate as a base for the deprotonation pathway in Rh-catalysed C–H amination of heteroarenes: DFT insights

DFT studies suggest that basic substrates assist the C–H activation step in Rh-catalysed reactions and transport protons towards the protodemetallation step.

Iridium-Catalyzed Direct C-H Sulfamidation of Aryl Nitrones with Sulfonyl Azides at Room Temperature

Ir(III)-catalyzed direct C-H sulfamidation of aryl nitrones has been developed to synthesize various sulfamidated nitrones in moderate to excellent yields with excellent regioselectivity and broad functional group tolerance. This transformation could proceed smoothly at room temperature with low catalyst loading in the absence of external oxidants, acids, or bases. Molecular nitrogen was released as the sole byproduct, thus providing an environmentally benign sulfamidation process. And this protocol could efficiently apply to synthesize the substituted benzisoxazoline via one-step transformation from the product.

Transition-metal-catalyzed C-N bond forming reactions using organic azides as the nitrogen source: a journey for the mild and versatile C-H amination

Owing to the prevalence of nitrogen-containing compounds in functional materials, natural products and important pharmaceutical agents, chemists have actively searched for the development of efficient and selective methodologies allowing for the facile construction of carbon-nitrogen bonds. While metal-catalyzed C-N cross-coupling reactions have been established as one of the most general protocols for C-N bond formation, these methods require starting materials equipped with functional groups such as (hetero)aryl halides or their equivalents, thus generating stoichiometric amounts of halide salts as byproducts. To address this aspect, a transition-metal-catalyzed direct C-H amination approach has emerged as a step- and atom-economical alternative to the conventional C-N cross-coupling reactions. However, despite the significant recent advances in metal-mediated direct C-H amination reactions, most available procedures need harsh conditions requiring stoichiometric external oxidants. In this context, we were curious to see whether a transition-metal-catalyzed mild C-H amination protocol could be achieved using organic azides as the amino source. We envisaged that a dual role of organic azides as an environmentally benign amino source and also as an internal oxidant via N-N2 bond cleavage would be key to develop efficient C-H amination reactions employing azides. An additional advantage of this approach was anticipated: that a sole byproduct is molecular nitrogen (N2) under the perspective catalytic conditions. This Account mainly describes our research efforts on the development of rhodium- and iridium-catalyzed direct C-H amination reactions with organic azides. Under our initially optimized Rh(III)-catalyzed amination conditions, not only sulfonyl azides but also aryl- and alkyl azides could be utilized as facile amino sources in reaction with various types of C(sp(2))-H bonds bearing such directing groups as pyridine, amide, or ketoxime. More recently, a new catalyst system using Ir(III) species was developed for the direct C-H amidation of arenes and alkenes with acyl azides under exceptionally mild conditions. As a natural extension, amidation of primary C(sp(3))-H bonds could also be realized on the basis of the superior activity of the Cp*Ir(III) catalyst. Mechanistic investigations revealed that a catalytic cycle is operated mainly in three stages: (i) chelation-assisted metallacycle formation via C-H bond cleavage; (ii) C-N bond formation through the in situ generation of a metal-nitrenoid intermediate followed by the insertion of an imido moiety to the metal carbon bond; (iii) product release via protodemetalation with the concomitant catalyst regeneration. In addition, this Account also summarizes the recent advances in the ruthenium- and cobalt-catalyzed amination reactions using organic azides, developed by our own and other groups. Comparative studies on the relative performance of those catalytic systems are briefly described.

Mechanistic studies on the Rh(III)-mediated amido transfer process leading to robust C-H amination with a new type of amidating reagent

Mechanistic investigations on the Cp*Rh(III)-catalyzed direct C-H amination reaction led us to reveal the new utility of 1,4,2-dioxazol-5-one and its derivatives as highly efficient amino sources. Stepwise analysis on the C-N bond-forming process showed that competitive binding of rhodium metal center to amidating reagent or substrate is closely related to the reaction efficiency. In this line, 1,4,2-dioxazol-5-ones were observed to have a strong affinity to the cationic Rh(III) giving rise to dramatically improved amidation efficiency when compared to azides. Kinetics and computational studies suggested that the high amidating reactivity of 1,4,2-dioxazol-5-one can also be attributed to the low activation energy of an imido-insertion process in addition to the high coordination ability. While the characterization of a cationic Cp*Rh(III) complex bearing an amidating reagent was achieved, its facile conversion to an amido-inserted rhodacycle allowed for a clear picture on the C-H amidation process. The newly developed amidating reagent of 1,4,2-dioxazol-5-ones was applicable to a broad range of substrates with high functional group tolerance, releasing carbon dioxide as a single byproduct. Additional attractive features of this amino source, such as they are more convenient to prepare, store, and use when compared to the corresponding azides, take a step closer toward an ideal C-H amination protocol.

Ru(ii)-catalyzed amidation reactions of 8-methylquinolines with azides via C(sp3)–H activation

Ru(ii)-catalyzed amidation reactions of 8-methylquinolines with azides have been developed through C(sp³)–H bond activation to give quinolin-8-ylmethanamines in good yields.