Cobalt(III)- and Rhodium(III)-Catalyzed C–H Amidation and Synthesis of 4-Quinolones: C–H Activation Assisted by Weakly Coordinating and Functionalizable Enaminone

Rh(III)-catalyzed C(sp2)-H functionalization/[4+2] annulation of oxadiazolones with iodonium ylides to access diverse fused-isoquinolines and fused-pyridines

In this study, a Rh(III)-catalyzed C-H/N-H [4+2] annulation of oxadiazolones with iodonium ylides has been developed, which afforded a series of diverse fused-isoquinolines and fused-pyridines in moderate to high yields. These divergent synthesis protocols featured mild conditions, broad substrate scope, and functional-group compatibility. In addition, scale-up synthesis, related applications and preliminary mechanistic explorations were also accomplished.

"Alkene-to-Alkene" Difunctionalization of Enaminones for the Synthesis of Polyfunctionalized Alkenes by Transition-Metal-Free C-H and C-N Bond Transformation

The three-component reactions of enaminones, disulfides, and alcohols for the synthesis of polyfunctionalized alkenes have been realized via the C-H and C-N bond transformation on enaminones. The reactions proceed in a novel "alkene-to-alkene" difunctionalization mode without using any transition metal. The application of the alkene products in the synthesis of divergent sulfenyl heteroaryls, including sulfenylated pyrazoles, pyrimidines, and isoxazoles, via simple annulation has also been verified.

Iridium-Catalyzed Regioselective B(4)-Alkenylation and B(3,5)-Dialkenylation of o-Carboranes

Iridium(I)-catalyzed regioselective B(4)-alkenylation has been developed from o-carboranyl sulfoxonium ylides and alkynes through B(4)-H activation. The sequential B(4)- and B(6)-alkenylation afforded B(3,5)-dialkenylated o-carboranyl sulfoxonium ylides in one pot. Eventually, two alkenyl groups, the same or different, were introduced at positions 3 and 5 of the carborane. Sulfoxonium ylide used as a directing group remains available for further functionalization and is converted to B-alkenylated o-carboranyl trichloromethyl ketones.

Rh-Catalyzed C-H Activation/Annulation of Enaminones and Cyclic 1,3-Dicarbonyl Compounds: An Access to Isocoumarins

A facile access to isocoumarins has been established via rhodium(III)-catalyzed C-H bond activation and intramolecular C-C cascade annulation of enaminones and cyclic 1,3-dicarbonyl compounds. The synthetic protocol features a wide range of substrates with high functional group tolerance, mild reaction conditions, and the selective cleavage of the enaminone C-C bond. Notably, the cyclic 1,3-dicarbonyl compounds can in situ-generate iodonium ylide as a carbene precursor to prepare polycyclic scaffolds by reacting with PhI(OAc)₂. The application of this method to prepare useful synthetic precursors and bioactive skeletons is also exemplified.

Three-Component Fusion to Pyrazolo[5,1-a]isoquinolines via Rh-Catalyzed Multiple Order Transformation of Enaminones

A facile and practical method for the synthesis of fused tricyclic pyrazolo[5,1-a]isoquinolines has been realized via the reactions of enaminones, hydrazine hydrochloride, and internal alkynes. By means of Rh catalysis, the extraordinary high-order bond functionalization, including the transformation of aryl C-H, ketone C═O, and alkenyl C-N bonds in the enaminones, marks the major feature of the cascade reactions. The results disclose the individual advantage of enaminones in the design of novel and efficient synthetic methods.

Recent advances in the synthesis of pharmaceutically active 4-quinolone and its analogues: a review

4-Quinolone and its analogs are heterocyclic classes of organic compounds displaying biologically active and a broad spectrum of pharmaceutical drug scaffolds. 4-Quinolone is the first-line chemotherapeutic treatment for a wide spectrum of bacterial infections. Recently, 4-quinolone and its derivatives have been shown to have the potential to cure and regulate various acute and chronic diseases, including pain, ischemia, immunomodulation, inflammation, malarial, bacterial infection, fungal infection, HIV, and cancer, based on several reports. This review highlights and provides brief information to better understand the development of experimental progress made to date in the synthetic protocol towards 4-quinolone and its analogs. Thus, classical synthesis protocol, metal-free reaction protocol, and transition metal-catalyzed reaction procedures are briefly discussed along with the pharmaceutical activities of selected 4-quinolone derivatives.

Cobalt Homeostatic Catalysis for Coupling of Enaminones and Oxadiazolones to Quinazolinones

Transition metal catalysis has revolutionized modern synthetic chemistry for its diverse modes of coordination reactivity. However, this versatility in reactivity is also the predominant cause of catalyst deactivation, a persisting issue that can significantly compromise its synthetic value. Homeostatic catalysis, a catalytic process that can sustain its productive catalytic cycle even when chemically disturbed, is proposed herein as an effective tactic to address the challenge. In particular, a cobalt homeostatic catalysis process has been developed for the water-tolerant coupling of enaminones and oxadiazolones to quinazolinones. Dynamic covalent bonding serves as a mechanistic handle for the preferred buffering of water onto enaminone and reverse exchange by a released secondary amine, thus securing reversible entry into cobalt's dormant and active states for productive catalysis. Through this homeostatic catalysis mode, a broad structural scope has been achieved for quinazolinones, enabling further elaboration into distinct pharmaceutically active agents.

Cu-Catalyzed tandem cyclization and coupling of enynones with enaminones for multisubstituted furans & furano-pyrroles

A synthetic strategy that efficiently constructs complex molecular diversity in a few steps will always be embraced by organic chemists. Here, we report a cascade reaction of enynones with enaminones via carbene insertion and aryl migration to engineer distinctive multisubstituted furans with an all-carbon quaternary center, and could extend the protocol in the same pot towards furano-pyrrole bis-heterocycles. Heterogeneity of this protocol was proved with the upshot of divergent chemical space under a relatively mild reaction environment.

Cp*CoIII-catalyzed formal [4 + 2] cycloaddition of 2-phenyl-1H-imidazoles to afford imidazo[1,2-c]quinazoline derivatives

A synthetic protocol based on Cp*Co^III-catalyzed C-H amidation/annulation of 2-aryl-1H-imidazoles with 1,4,2-dioxazol-5-ones was developed to give imidazo[1,2-c]quinazoline derivatives with broad substrate scope in moderate to good yields. The method has good prospects of application in the synthesis of imidazo[1,2-c]quinazoline drugs.

Copper-catalyzed direct sulfenoamination of saturated ketones via in situ formed enaminones

A sequential and efficient protocol for the synthesis of α-thiolated enaminones has been developed using copper-TEMPO systems. This reaction features a broad substrate scope to afford the desired product in good to excellent yields with high stereoselectivity. A preliminary mechanistic study suggests that the in situ formed enaminone acts as the key intermediate.

Rhodium-Catalyzed Regioselective Double Annulation of Enaminones with Propargyl Alcohols: Rapid Access to Arylnapthalene Lignan Derivatives

We present here a rhodium-catalyzed oxidative three-point double annulation of enaminones with propargylic alcohols via a C-H and a C-N bond activation to access arylnaphthalene-based lignan derivatives. The key step in the reaction is the regioselective insertion of propargylic alcohol into the rhoda-cycle, a result of hydroxyl rhodium coordination. Necessary control experiments and KIE studies were conducted to determine the mechanism.

Cobalt(III)-catalyzed C-H amidation of N,N-dialkyl thiobenzamides by sulfur coordination

An efficient inexpensive cobalt(III)-catalyzed intermolecular amidation of N,N-dialkyl thiobenzamides with 1,4,2-dioxazol-5-ones via C-H bond activation is described. The reaction proceeds with high functional group tolerance under external oxidant free conditions, providing a straightforward approach for the direct modification of thioamide derivatives, which are prevalent organic motifs found in vital biological and pharmaceutical molecules.

Tunable Trifunctionalization of Tertiary Enaminones for the Regioselective and Metal-Free Synthesis of Discrete and Proximal Phosphoryl Nitriles

This paper reports an unprecedented trifunctionalization of tertiary enaminones for the synthesis phosphoryl nitriles by the reactions of enaminones with diarylphosphine oxides and trimethylsilyl cyanide (TMSCN) without the use of any metal reagent. Employing tetrabutyl ammonium hydroxide (TBAH) as the catalyst (0.2 equiv) enables discrete cyanophosphonation. On the other hand, selective proximal cyanophosphonation has been realized in the presence of acetic acid only (AcOH).

Rh(III)-Catalyzed [5 + 1] Annulation of Indole-enaminones with Diazo Compounds To Form Highly Functionalized Carbazoles

A novel Rh(III)-catalyzed C-H activation/annulation cascade of indole-enaminones with diazo compounds was reported to construct diversely functionalized carbazole frameworks. The most notable characteristic is that this transformation could smoothly furnish a novel [5 + 1] cyclization product with good to excellent yields (up to 95%), accompanied by the thorough removal of acetyl and N,N-dimethyl groups of two substrates from the target products, rather than the normally expected [4 + 2] cyclization products.

O-Directed C-H functionalization via cobaltacycles: a sustainable approach for C-C and C-heteroatom bond formations

This review focuses on providing comprehensive highlights of the recent advances in the field of cobalt-catalysed C-H functionalization and related synthetic concepts, relying on these through oxygen atom coordination. In recent years, 3d transition metal (Fe, Co, Cu & Ni) catalysed C-H functionalization reactions have received immense attention on account of its higher abundance and low cost, as compared to noble metals such as Ir, Rh, Ru and Pd. Among the first-row transition metals, cobalt is one of the extensively used metals for sustainable synthesis due to its unique reactivity towards the functionalization of inert C-H bonds. The functionalization of the inert C-H bond necessitates a proximal directing group. In this context, strongly coordinating nitrogen atom directed C-H functionalizations have been well explored. Nevertheless, strongly coordinating nitrogen-containing scaffolds, such as pyridine, pyrimidine, and 8-aminoquinoline, have to be installed and removed in a separate process. In contrast, C-H functionalizations through weakly coordinating atoms, such as oxygen, are largely underdeveloped. Since the oxygen atom is a part of many readily available functional groups, such as aldehydes, ketones, carboxylic acids, and esters, it could be used as directing groups for selective C-H functionalization reactions without any modification. Thus, the use of 3d transition metals, such as cobalt, along with weakly coordinating (oxygen) directing groups for C-H functionalization reactions are more sustainable, especially for the large-scale production of pharmaceuticals in industries. During the last decade, notable progress has been made using this concept. Nonetheless, almost all the reports are restricted to the formation of C-C and C-N bond. Therefore, there is a wide scope for developing this area for the formation of other bonds, such as C-X (halogens), C-B, C-S, and C-Se.

Synthesis of Sulfimides and N-Allyl-N-(thio)amides by Ru(II)-Catalyzed Nitrene Transfer Reactions of N-Acyloxyamides

The N-acyloxyamides were employed as effective N-acyl nitrene precursors in reactions with thioethers under the catalysis of a commercially available Ru(II) complex, from which a variety of sulfimides were synthesized efficiently and mildly. If an allyl group is contained in the thioether precursor, the [2,3]-sigmatropic rearrangement of the sulfimide occurs simultaneously and the N-allyl-N-(thio)amides were obtained as the final products. Preliminary mechanistic studies indicated that the Ru-nitrenoid species should be a key intermediate in the transformation.

Rhodium-catalyzed enone carbonyl directed C–H activation for the synthesis of indanones containing all-carbon quaternary centers

Rhodium(iii)-catalyzed enone carbonyl directed C–H activation/annulation of α-aroyl ketene dithioacetals with diazo compounds has been realized for the synthesis of β-quaternary indanones.

Synthesis of Benzothiadiazine-1-oxides by Rhodium-Catalyzed C-H Amidation/Cyclization

A rhodium-catalyzed C-H amidation/cyclization sequence provides benzothiadiazine-1-oxides from sulfoximines and 1,4,2-dioxazol-5-ones in good yields. The reaction is characterized by a high functional group tolerance and, in contrast to most previous transformations of this type, is well-suited for S-alkyl-S-aryl-substituted sulfoximines.

Rhodium(III)-catalyzed C4-amidation of indole-oximes with dioxazolones via C-H activation

A novel method for the Rh(iii)-catalyzed oxime-directed C-H amidation of indoles with dioxazolones has been developed. This strategy provides an exclusive site selectivity and the directing group can be easily removed. This transformation features a wide substrate scope, good functional group tolerance and excellent yields, and may serve as a significant tool to construct structurally diverse indole derivatives for the screening of potential pharmaceuticals in the future.

A three-component reaction of phosphorus ylides with isocyanates: facile synthesis of 2-amino-3-carboxylate-4-quinolones

A three-component reaction between one molecule of phosphorus ylides (P-ylides) and two molecules of isocyanates for the rapid assembly of 2-amino-3-carboxylate-4-quinolones is described. The mechanism may involve the addition of a P-ylide to an isocyanate followed by 1,3-H shift to form a carbamoyl stabilized P-ylide. The intermediate then reacts with another aryl isocyanate via Wittig/ketenimine-ketene rearrangement/6π-electrocyclization/1,3-H shift to finally afford the 4-quinolones.

Carbamates: A Directing Group for Selective C-H Amidation and Alkylation under Cp*Co(III) Catalysis

The selective reactivity of carbamate and thiocarbamate toward alkylation and amidation is reported under stable, high-valent, cost-effective cobalt(III) catalysis. This method reveals the wide possibility of designing a different branch of synthetically challenging yet highly promising asymmetric catalysts based on BINOL and SPINOL scaffolds. Late-stage C-H functionalization of l-tyrosine and estrone was also achieved through this approach. The mechanistic study shows that a base-assisted internal electrophilic substitution mechanism is operative here.

Synthesis of Diastereoenriched α-Aminomethyl Enaminones via a Brønsted Acid-Catalyzed Asymmetric aza-Baylis-Hillman Reaction of Chiral N-Phosphonyl Imines

An effective chiral GAP methodology for preparing α-aminomethyl enaminones through a (R)-CSA-catalyzed asymmetric aza-Baylis-Hillman reaction is reported. Excellent yields and high diastereoselectivity could be obtained under mild conditions and convenient GAP techniques. The confirmations of the absolute configuration of N-phosphonyl imine and chiral enaminone by X-ray diffraction provides an explicit explanation of the chirality mechanism for GAP chemistry.

Synthesis of Lactams via Ir-Catalyzed C-H Amidation Involving Ir-Nitrene Intermediates

x-membered lactams were synthesized via either an amidation of sp³ C-H bonds or an electrophilic substitution of arenes via Ir-nitrene intermediates. With the employment of a readily available iridium catalyst in dichloromethane or hexafluoro-2-propanol, a wide range of lactams were synthesized in good to excellent yields with high selectivity.

Rhodium(iii)-catalyzed diamidation of olefins via amidorhodation and further amidation

Rh(iii)/Co(iii)-catalyzed diamidation of amide-tethered olefins has been realized using dioxazolones and arylsulfonamides as different classes of amidating reagents.

Ru(ii)-catalyzed C6-selective C–H acylmethylation of pyridones using sulfoxonium ylides as carbene precursors

In this study, we describe a method using sulfoxonium ylides as carbene precursors to achieve C6-selective acylmethylation of pyridones catalyzed by a ruthenium(ii) complex. This approach featured mild reaction conditions, moderate to excellent yields, high step economy, and had excellent functional group tolerance with good site selectivity. Besides, gram-scale preparation, synthetic utility, and mechanistic studies were conducted. It offers a direct and efficient way to synthesize pyridone derivatives.

In this study, we describe a method using sulfoxonium ylides as carbene precursors to achieve C6-selective acylmethylation of pyridones catalyzed by a ruthenium(ii) complex.

Non-C2-Symmetric Chiral-at-Ruthenium Catalyst for Highly Efficient Enantioselective Intramolecular C(sp3)-H Amidation

A new class of chiral ruthenium catalysts is introduced in which ruthenium is cyclometalated by two 7-methyl-1,7-phenanthrolinium heterocycles, resulting in chelating pyridylidene remote N-heterocyclic carbene ligands (rNHCs). The overall chirality results from a stereogenic metal center featuring either a Λ or Δ absolute configuration. This work features the importance of the relative metal-centered stereochemistry. Only the non-C₂-symmetric chiral-at-ruthenium complexes display unprecedented catalytic activity for the intramolecular C(sp³)-H amidation of 1,4,2-dioxazol-5-ones to provide chiral γ-lactams with up to 99:1 er and catalyst loadings down to 0.005 mol % (up to 11 200 TON), while the C₂-symmetric diastereomer favors an undesired Curtius-type rearrangement. DFT calculations elucidate the origins of the superior C-H amidation reactivity displayed by the non-C₂-symmetric catalysts compared to related C₂-symmetric counterparts.

Rhodium(III)-Catalyzed Regioselective C(sp2)-H Functionalization of 7-Arylpyrazolo[1,5-a]pyrimidines with Dioxazolones as Amidating Agents

Rh(III)-catalyzed C-H functionalization of 7-arylpyrazolo[1,5-a]pyrimidines was developed wherein the pyrazolo[1,5-a]pyrimidine moiety is reported for the first time to direct the C-H bond activation. Various 7-arylpyrazolo[1,5-a]pyrimidines underwent smooth C-H amidation with alkyl-, aryl-, and heteroaryl-substituted dioxazolones to afford the products in moderate to good yields. Mechanistic studies suggest that a six-membered rhodacycle intermediate involving N1 might play a key role in the regioselective catalytic cycle.