Palladium‐Catalyzed Primary Amine‐Directed Decarboxylative Annulation of α ‐Oxocarboxylic Acids: Access to Indolo[1,2‐ a ]quinazolines

Copper-Catalyzed Sulfonylation Reaction of NH-Sulfoximines with Aryldiazonium Tetrafluoroborates and Sulfur Dioxide: Formation of N-Sulfonyl Sulfoximines

An efficient and practical SO₂ insertion protocol of NH-sulfoximines with aryldiazonium tetrafluoroborates and DABSO toward N-sulfonyl sulfoximines has been developed under mildly basic conditions. This transformation features easy operation, readily available substrates, and mild conditions. A tentative mechanism is proposed, which indicates that the aryldiazonium tetrafluoroborates would be radical donors under standard reaction conditions. The aryl radical produced in situ from diazonium salts would be trapped by SO₂ to generate an arylsulfonyl radical and then undergo further transformation to generate the final N-sulfonyl sulfoximines.

Divergent construction of 3-(indol-2-yl)succinimide/maleimide and fused benzodiazepine skeletons from 2-(1H-indol-1-yl)anilines and maleimides

Divergent construction of 3-(indol-2-yl)succinimide/maleimide and indoyl/pyrrolyl fused benzodiazepine skeletons from 2-(1H-indol-1-yl)anilines and maleimides is presented.

N,N-Dimethylformamide as Carbon Synthons for the Synthesis of N-Heterocycles: Pyrrolo/Indolo[1,2-a]quinoxalines and Quinazolin-4-ones

N,N-dimethylformamide (DMF) as synthetic precursors contributing especially the methyl, acyl, and amino groups has played a significant role in heterocycle syntheses and functionalization. In this protocol, a wide range of pyrrolo/indolo[1,2-a]quinoxalines and quinazolin-4-ones were obtained in moderate to good yields by using elemental iodine without any metal or peroxides. We considered that N-methyl and N-acyl of DMF participate and complete the reaction separately through different mechanisms, which displayed potential still to be explored of DMF.

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.

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).

Elemental Sulfur Mediated Synthesis of Pyrrolo[1,2-a]quinoxalines from 1-(2-Nitroaryl)pyrroles

Methods to afford pyrrolo[1,2-a]quinoxalines often require the use of prefunctionalized aniline precursors, transition metals, and/or harsh conditions. Herein we describe a simple coupling of 1-(2-nitroaryl)pyrroles and arylacetic acids, in the presence of elemental sulfur, to furnish the fused heterocycles in good yields. The conditions are compatible with many functionalities including ester, nitrile, halogen, and nitro groups. Use of benzyl alcohols and picoline coupling reagents was also attempted.

Synthesis of 4-Aryl Pyrrolo[1,2-α]quinoxalines via Iron-Catalyzed Oxidative Coupling from an Unactivated Methyl Arene

Herein, we describe the direct synthesis of pyrrolo[1,2-α]quinoxaline via oxidative coupling between methyl arene and 1-(2-aminophenyl) pyrroles. Oxidation of the benzylic carbon of the methyl arene was achieved by di-t-butyl peroxide in the presence of an iron catalyst, followed by conversion to an activated aldehyde in situ. Oxygen played a crucial role in the oxidation process to accelerate benzaldehyde formation. Subsequent Pictet-Spengler-type annulation completed the quinoxaline structure. The protocol tolerated various kinds of functional groups and provided 22 4-aryl pyrrolo[1,2-α]quinoxalines when various methyl arene derivatives were used. The developed method proceeded in air, and all catalysts, reagents, and solvents were easily accessible.

Substrate-Controlled Cu(OAc)2-Catalyzed Stereoselective Semi-Reduction of Alkynes with MeOH as the Hydrogen Source

A substrate-controlled stereoselective semi-reduction of alkynes with MeOH as the hydrogen source has been developed, and readily available Cu(OAc)₂ (copper acetate) is utilized as an optimal catalyst. The detailed investigation of the mechanism revealed distinct catalytic processes for the (Z)- and (E)-alkenes, respectively. As a result, a diversity of alkynes (including terminal, internal alkynes etc.) were compatible under the mild reaction conditions. Furthermore, the high proportion of deuterium in Z-alkenes (up to 96%) was obtained using d ₄-methanol as a solvent.

Bifunctional acidic ionic liquid-catalyzed decarboxylative cascade synthesis of quinoxalines in water under ambient conditions

An acid-functionalized ionic liquid (IL)-catalyzed cascade decarboxylative cyclization of 2-arylanilines with α-oxocarboxylic acids was developed.

Palladium-catalyzed sequential acylation/annulation of indoles with acyl chlorides using primary amine as the directing group

An efficient, palladium(ii)-catalyzed, C(sp²)–H acylation/annulation of indoles with acyl chlorides for the synthesis of substituted indolo[1,2-a]quinazolines is reported.

Pd(II)-Catalyzed C-H Acylation of (Hetero)arenes-Recent Advances

Di(hetero)aryl ketones are important motifs present in natural products, pharmaceuticals or agrochemicals. In recent years, Pd(II)-catalyzed acylation of (hetero)arenes in the presence of an oxidant has emerged as a catalytic alternative to classical acylation methods, reducing the production of toxic metal waste. Different directing groups and acyl sources are being studied for this purpose, although further development is required to face mainly selectivity problems in order to be applied in the synthesis of more complex molecules. Selected recent developments and applications are covered in this review.

Copper-catalyzed [2+3]-annulation of N-H imines with vinyl azides: access to polyaryl 2H-imidazoles

A practical method for the synthesis of 2H-imidazoles via a [2+3] annulation of N-H imines with vinyl azides using a copper catalyst is developed. In this conversion, environmentally friendly oxygen is used as the sole oxidant and N2 and H2O are the only by-products. The catalytic transformation, operating under mild conditions, is operationally simple and is considered as a readily available catalytic system having good substrate and functional compatibility with high atom-efficiency without the need for additional ligands or additives.

Palladium-Catalyzed Multistep Tandem Carbonylation/N-Dealkylation/Carbonylation Reaction: Access to Isatoic Anhydrides

A novel and efficient synthesis of isatoic anhydride derivatives was developed via palladium-catalyzed multistep tandem carbonylation/N-dealkylation/carbonylation reaction with alkyl as the leaving group and tertiary anilines as nitrogen nucleophiles. This approach features good functional group compatibility and readily available starting materials. Furthermore, it provided a convenient approach for the synthesis of biologically and medicinally useful evodiamine.

Copper- and DMF-mediated switchable oxidative C–H cyanation and formylation of imidazo[1,2-a]pyridines using ammonium iodide

The cyanation and formylation of imidazo[1,2-a]pyridines were developed under copper-mediated oxidative conditions using ammonium iodide and DMF as a nontoxic combined cyano-group source and DMF as a formylation reagent.

Palladium-catalyzed C3-selective C–H oxidative carbonylation of imidazo[1,2-a]pyridines with CO and alcohols: a way to access esters

A palladium-catalyzed C3-selective C–H oxidative carbonylation for the synthesis of various esters from imidazo[1,2-a]pyridines and CO with high efficiency and high atom economy has been developed.

A ruthenium-catalyzed free amine directed (5+1) annulation of anilines with olefins: diverse synthesis of phenanthridine derivatives

A ruthenium(ii)-catalyzed cross-ring (5+1) annulation between 2-aminobiphenyls and activated olefins is disclosed for succinct synthesis of valuable phenanthridine scaffolds.