Ruthenium-Catalyzed Site-Selective C–H Bond Activation/Annulation Cascade toward Dibenzoazepinone Skeletons

Ruthenium(II)-Catalyzed Sequential C-H/N-H Alkene Annulation Cascade of Phenanthroimidazoles: Synthesis and Photophysical Studies

We report ruthenium(II)-catalyzed sequential C-H/N-H alkenylation cascade of phenanthroimidazole and alkenes to form novel phenanthroimidazoisoindol acrylates via dual C-H activation and aza-Michael reaction. The two nitrogen atoms of the imidazole ring act as directing groups for regioselective dual sequential ortho C-H activation. These polycyclic N-heterocycles were evaluated for their photophysical properties.

Free Amine-Directed Redox Neutral Ruthenium(II) Catalysis toward Regioselective Synthesis of Heterobiaryls

A regioselective coupling of ortho-heteroaryl anilines and 7-oxabenzonorbornadienes has been developed by leveraging free amine-directed redox-neutral Ru(II) catalysis. This protocol facilitates formal C-2 arylation of the indole moiety under mild conditions to offer valuable heterobiaryls in high yields. The reaction displays a broad substrate generality and scalability and retains efficacy in the presence of diverse pharmacophore scaffolds. Moreover, products bearing a free amine group were successfully employed in Mg(NTf2)2-catalyzed double Michael addition cascade, which led to the synthesis of intricate indole- and pyrrole-fused azaheterocycles.

Enaminone-directed ruthenium(II)-catalyzed C-H activation and annulation of arenes with diazonaphthoquinones for polycyclic benzocoumarins

The weakly coordinating enaminone functionality has been leveraged for a C-H bond activation strategy under ruthenium catalysis and employed in the regioselective annulative coupling of arenes with diazonaphthoquinones, offering polycyclic benzocoumarins in very high yields. The enaminone motif plays a dual role and the protocol operates through a Ru(II)/Ru(IV) catalytic pathway which is amenable to the diversification of various pharmacophore-coupled substrates.

Ruthenium-catalyzed hydroarylation reactions as the strategy towards the synthesis of alkylated arenes and substituted alkenes

Metal-catalyzed hydroarylation reactions are always powerful tools in organic synthesis since they can form C-C or C-heteroatom bonds in an atom and step economic manner. Medicinally and biologically relevant scaffolds can be easily and efficiently synthesized using this strategy. By tuning the directing groups that are present on the arenes, regioselectivity can be induced to the C-H activation. Metals like cobalt, rhodium and ruthenium are well known as catalysts in this type of reaction. But due to their easy availability and efficiency, Ru catalysts are found to be more preferable for hydroarylation purposes. In this review, the Ru-catalyzed hydroarylation of alkenes and alkynes, intramolecular Ru-catalyzed hydroarylation of olefin tethered arenes, modifications in the catalytic system to improve the catalytic efficiency, and carboxylate-assisted Ru-catalyzed hydroarylation reactions are discussed in detail, covering literature from 2016 to 2022.

Palladium-catalyzed interannular C-H amination of biaryl amines

A palladium-catalyzed interannular C-H amination of biaryl amines with O-benzoylhydroxylamines is reported. This reaction undergoes smoothly with operational practicality and good tolerance of functional groups, thereby providing a concise synthesis of 2,2'-diaminobiaryls. Moreover, the readily accessible scale-up synthesis and the ability to transform the products into structurally diverse N-containing heterocycles demonstrate the synthetic potential of this catalytic protocol.

Heteroarylation of Congested α-Bromoamides with Imidazo-Heteroarenes and Indolizines via Aza-Oxyallyl Cations: Enroute to Dibenzoazepinone and Zolpidem Analogues

Herein, we report a highly efficient and unprecedented approach for heteroarylation of congested α-bromoamides via electrophilic aromatic substitution of imidazo-heteroarenes and indolizines under mild reaction conditions (room temperature, metal, and oxidant free). The participation of an in situ generated aza-oxyallyl cation as an alkylating agent is the hallmark of this transformation. The method was readily adapted to synthesize novel imidazo-heteroarene-fused dibenzoazepinone architectures of potential medicinal value.

Synthesis of 7-Arylthiomethyl Dibenzo[b,d]azepines through Imidoylative Heck Cyclization and CPA-Catalyzed Thio-Michael Addition/Enantioselective Protonation

A chiral phosphoric acid-catalyzed thio-Michael addition/enantioselective protonation has been developed for the first time. The reaction applies 7-methylene-6-aryl-7H-dibenzo[b,d]azepines, products of Pd-catalyzed imidoylative Heck cyclization, as Michael acceptors in reactions with a wide range of aryl thiols. Diversified 7-[(arylthio)methyl]-7H-dibenzo[b,d]azepines bearing a benzylic stereocenter and a thermodynamically regulated biaryl axis were produced with good to excellent enantioselectivity and 14-25:1 diastereoisomeric ratios.

Ruthenium(II)-catalyzed C-H activation and (4+2) annulation of aromatic hydroxamic acid esters with allylic amides

A (4+2) annulation under Ru(II)-catalysis is reported using aromatic hydroxamic acid esters as the oxidizing directing group and allylic amides as unactivated olefin coupling partners, delivering a wide variety of aminomethyl isoquinolinones in good to excellent yields. This annulation is distinctive as allylic congeners typically result in allylation and not the annulation. Late-stage derivatization of a bioactive synthetic bile acid has been showcased.

Free Amine, Hydroxyl and Sulfhydryl Directed C-H Functionalization and Annulation: Application to Heterocycle Synthesis

Transition metal-catalyzed direct C-H bond functionalization is recognized as an efficient strategy to assemble heterocyclic frameworks. For this purpose, directing groups (DGs) installation on an organic molecule has remained a widely exploited strategy for the years. The installation of directing groups, especially for the amine, alcohol and thiol containing reactants and their removal after the reaction need additional steps. In this regard, the use of free amine, hydroxyl and sulfhydryl as directing groups in native form is advantageous and in recent times, these transformations have stirred undisputable advancements for applications to heterocycle synthesis. In this review, the aromatic sp² -C attached free amine, hydroxyl and sulfhydryl as native functionalities are shown to be useful for the construction of five to seven-membered N-, O- and S-heterocycles.

Rhodium(iii)-catalyzed [5+1] annulation of 2-alkenylphenols with maleimides: access to highly functionalized spirocyclic skeletons

A new edition of [5+1] annulation reaction of maleimides with 2-alkenylphenols has been discovered under a Rh(iii)-catalytic system. The process leads to an efficient synthesis of valued spirocyclic scaffolds bearing an oxygen-containing spiro carbon in a single step and shows a broad substrate scope with good functional group tolerance. The synthetic utility has been exemplified by synthesizing highly functionalized 2,2-disubstituted-2H-chromene skeletons and a gram-scale synthesis with a low catalyst loading.

Chemoselective Installation of Diverse Succinimides on Fused Benzimidazoles via Rhodium-Catalyzed C-H Activation/Annulation: Chemosensor for Heavy Metals

A novel Rh-catalyzed cascade C-H activation/annulation of 2-arylbenzimidazoles with maleimides is reported. Rapid chemoselective access to two structurally distinct succinimide-bearing benzoimidazoisoquinolinones is achieved, depending on the acidic and basic conditions. This atom- and step-economic strategy features a wide substrate scope, excellent functional group tolerance, and site-specific functionalization. Application of the methodology yields a novel benzimidazole-based probe as a fluorescent chemosensor for the nanomolar detection of Hg²⁺, Cu²⁺, and Fe³⁺ ions.

Transition Metal Catalyzed Free-Amine (-NH2 ) Directed C-H Bond Activation and Functionalization for Biaryl Frameworks

Transition-metal-catalyzed direct transformation of inert C-H bond has revolutionized the arsenal of main-stream organic synthesis, providing a new upfront to forge structurally enriched and biologically relevant scaffolds in a step- and atom-economical way. Past decades have accounted for the major developments in this realm, proclaiming excellent site-selectivity by exploiting a variety of coordinating directing groups (DGs). Consideration of versatile, abundant, sp³ -hybridized free-amine (-NH₂ ) functionality for the same purpose has always been a formidable task owing to its innate reactivity. In recent years, free-amine functionality has emerged as a potent DG for a wide range of C-C and C-heteroatom bonds formations and annulation cascades. In this review article, we have discussed the advancements of free-amine directed C-H activation/functionalization reactions towards biaryl frameworks made within a decade (2012 to 2021).

Palladium-Catalyzed Enantioselective 7-exo-Trig Carbopalladation/Carbonylation: Cascade Reactions To Achieve Atropisomeric Dibenzo[b,d]azepin-6-ones

Enantioselective 7-exo-trig cyclocarbopalladation-initiated carbonylation cascade reactions, leading to seven-membered dibenzo[b,d]azepin-6-ones containing a thermodynamically controlled stereogenic axis, have been realized for the first time. A series of 7-acetate- or 7-acetamide-substituted dibenzo[b,d]azepin-6-ones are obtained under atmospheric pressure of CO in good yields with excellent diastereo- and enantioselectivities. The calculated energy difference between the diastereoisomers generated from the stereogenic biaryl axis and the stereogenic center is approximately 2.8 kcal/mol, which agrees with the excellent diastereoselectivity observed.

Selective Synthesis of Dihydrophenanthridine and Phenanthridine Derivatives from the Cascade Reactions of o-Arylanilines with Alkynoates through C-H/N-H/C-C Bond Cleavage

In this paper, an unprecedented selective synthesis of dihydrophenanthridine and phenanthridine derivatives through the cascade reactions of 2-arylanilines with alkynoates is presented. Mechanistic studies showed that the formation of the dihydrophenanthridine scaffold involves an initial C(sp²)-H alkenylation of 2-arylaniline with alkynoate followed by an intramolecular aza-Michael addition. When this reaction is carried out at elevated temperature, the in situ formed substituted dihydrophenanthridine readily undergoes a retro-Mannich-type reaction to give the corresponding phenanthridine through C-C bond cleavage. Compared with literature methods, this novel protocol has advantages such as easily obtainable substrates with a free amino group, pharmaceutically privileged products, cheap catalysts, and conveniently controllable selectivity.

Benzylidene succinimides as 3C synthons for the asymmetric tandem Mannich reaction/transamidation of cyclic trifluoromethyl ketimines to obtain F3C-containing polycyclic dihydroquinazolinones

By taking advantage of benzylidene succinimides as a new class of 3C synthons, a highly diastereo- and enantioselective tandem Mannich reaction/transamidation has been established by reacting them with cyclic trifluoromethyl N-acyl ketimines. Using a Cinchona alkaloid-derived squaramide as the catalyst, the tandem reaction proceeded smoothly under mild conditions and afforded a range of F3C-containing chiral polycyclic dihydroquinazolinones with excellent results (up to 99% yield, all cases >20 : 1 dr, up to 99% ee).

RhIII-Catalyzed formal [5 + 1] cyclization of 2-pyrrolyl/indolylanilines using vinylene carbonate as a C1 synthon

A rhodium(iii)-catalyzed formal C–H [5 + 1] cyclization of 2-pyrrolyl/indolylanilines with vinylene carbonate has been explored towards the potent assembly of diverse 4-methylpyrrolo[1,2-a]quinoxalines.

Synthesis of maleimide fused benzocarbazoles and imidazo[1,2-a]pyridines via rhodium(iii)-catalyzed [4 + 2] oxidative cycloaddition

In this paper, an efficient and sustainable synthesis of maleimide-fused benzocarbazoles/imidazo[1,2-a]pyridines from the reaction of 2-arylindoles/2-arylimidazo[1,2-a]pyridines with maleimides through oxidative [4 + 2] annulation is presented.