Access to Biaryl Sulfonamides by Palladium-Catalyzed Intramolecular Oxidative Coupling and Subsequent Nucleophilic Ring Opening of Heterobiaryl Sultams with Amines

Construction of Biaryl Sulfonamides via Pd(II)-Catalyzed Cross-Coupling of C(sp2)-H Bonds with Iodobenzenesulfonamides

This study describes the utility of Pd(II)-catalyzed C-H arylation of benzamides for constructing biaryl sulfonamides. Sulfonamides are known for their promising applications in pharmaceuticals and agrochemicals. A literature review revealed that biaryl sulfonamides were generally constructed via the traditional cross-coupling reactions. We report a progressive method for obtaining biaryl sulfonamides via the Pd(II)-catalyzed bidentate directing group (8-aminoquinoline or picolinamide)-assisted cross-coupling of sp2 C-H bonds of aromatic carboxamides with iodobenzenesulfonamides. After the C-H arylation reactions, we attempted the removal of the 8-aminoquinoline from the synthesized biaryl scaffolds possessing the carboxamide and sulfonamide moieties using triflic acid. In some cases, we observed the occurrence of decarboxylation and Friedel-Crafts acylation, affording interesting aromatic scaffolds possessing the sulfonamide moiety. The current work contributes toward developing alternative ways for assembling various biaryl sulfonamides.

Carbonylative synthesis and functionalization of indoles

Carbonylation processes have become widely recognized as a versatile, convenient, and low-cost method for the synthesis of high-value compounds. Given the great importance of heterocyclic compounds, the carbonylative approach has become increasingly important for their synthesis. In this mini-review, as a class of benzo-fused nitrogen-containing heterocyclic compounds, we summarized and discussed the recent achievements on the synthesis and functionalization of indole derivatives via carbonylative approaches.

Synthesis of 5,6-Dihydrophenanthridines via Palladium-Catalyzed Intramolecular Dehydrogenative Coupling of Two Aryl C−H Bonds

5,6-Dihydrophenanthridines are common aza heterocycle frameworks of natural products and pharmaceuticals. Herein, we reported the first palladium-catalyzed intramolecular C−H/C−H dehydrogenative coupling reaction of two simple arenes to generate 5,6-dihydrophenanthridines. The approach features a broad substrate scope and good tolerance of functional groups, offering an efficient alternative synthesis route for important 5,6-dihydrophenanthridine compounds.

Sulfoxylate Anion Radical-Induced Aryl Radical Generation and Intramolecular Arylation for the Synthesis of Biarylsultams

Aryl radical generation from the corresponding aryl halides using an electron donor and subsequent intramolecular cyclization with arenes could be an important advancement in contemporary biaryl synthesis. A green and practically useful synthetic protocol to access diverse six- and seven-membered biarylsultams especially with a free NH group including demonstration of a gram-scale synthesis is reported herein. The sulfoxylate anion radical (SO₂^-•), generated in situ from the reagents rongalite or sodium dithionite (Na₂S₂O₄), was found to be the key single electron transfer agent forming aryl radicals from aryl halides, which upon intramolecular arylation gives biarylsultams with good to excellent yields. The approach features generation of aryl radicals that remained underexplored, use of a cheap and readily available industrial reagents, and transition metal-free, mild, and green reaction conditions.

Pd-catalysed intramolecular transformations of indolylbenzenesulfonamides: ortho-sulfonamido-bi(hetero)aryls via C2-arylation and polycyclic sultams via C3 arylation

Indolyl substituted iodo-sulfonamides deliver 2-aryl(sulfonamido)indoles in the presence of Pd(OAc)2/Ph3P/Et3N; the same reactants, using Pd(OAc)2/Ph3P/K2CO3, afford indole-fused sultams.

Heck Reaction Boosted Heterocycle Ring-Closing and Ring-Opening Rearrangement: A Strategy for the Synthesis of Indolyl-Type Ligands

A novel method for P-involved heterocycle ring-closing-ring-opening rearrangement (HRR) via the Heck reaction is disclosed. The approach enables direct installation of a phosphorus-containing aryl group onto the C2 position of indole. This new rearrangement directly transforms easily prepared indole derivatives into indolyl-derived phosphonates and phosphinic acids with high yields, and many of the products are difficult to obtain by using established methods. This new HRR reaction provides an extremely simple and step-economic method to induce C-C bond formation and P-N bond cleavage for the synthesis of a variety of indolyl-type ligands.

Synthesis of Aporphine Analogues via Palladium-Catalyzed Intramolecular Aryl-Aryl Dehydrogenative Coupling

Reported herein is an intramolecular dehydrogenative coupling of two inert aryl C-H bonds for the synthesis of aporphine analogues. The process represents a novel tool for the preparation of aporphines via palladiun-catalyzed C-H bond activation. The present reaction is compatible with various functional groups, and the coupling products have been further applied for the synthesis of natural products aporphine and zenkerine.

Recent advances in the synthesis of pyrrolo[1,2-a]indoles and their derivatives

The pyrrolo[1,2-a]indole unit is a privileged heterocycle found in numerous natural products and has been shown to exhibit diverse pharmacological properties. Thus, recent years have witnessed immense interest from the synthesis community on the synthesis of this scaffold. In light of the ever-increasing demand for pyrrolo[1,2-a]indoles in drug discovery, this review provides an overview of recent synthesis methods for the preparation of pyrrolo[1,2-a]indoles and their derivatives. The mechanistic pathway and stereo-electronic factors affecting the yield and selectivity of the product are briefly explained. Furthermore, we have attempted to demonstrate the utility of the developed methods in the synthesis of bioactive molecules and natural products, wherever offered.

Palladium-catalysed cyclisation of ynamides and propargyl tethered iodosulfonamides with boronic acids leading to benzosultams

An efficient and straightforward Pd-catalysed synthesis of diversely substituted sultams utilising ynamides and boronic acids is disclosed; toluene was found to be the most suitable solvent for this transformation. This strategy has been successfully applied to generate dihydrobenzo[d]isothiazole 1,1-dioxides and dihydro-2H-benzo[e][1,2]thiazine 1,1-dioxides. The advantages of this protocol are good functional group tolerance, broad substrate scope, high-yielding reaction and low catalyst loading to access benzofused sultams with five-/six-membered rings. The synthetic utility has been demonstrated by a gram-scale synthesis. A plausible catalytic cycle involving carbopalladation has been proposed for this transformation.

Rapid Access to Benzimidazo[1,2-a]quinoline-Fused Isoxazoles via Pd(II)-Catalyzed Intramolecular Cross Dehydrogenative Coupling: Synthetic Versatility and Photophysical Studies

An efficient and atom-economical palladium-catalyzed intramolecular cross dehydrogenative coupling (CDC) reaction has been developed for the construction of highly π-conjugated benzimidazo[1,2-a]quinoline-fused isoxazole scaffolds using molecular oxygen as sole oxidant. The approach portrayed wide substrate scope with good functional group tolerance and depicted a useful tool for the generation of fluorescence active compounds with high quantum yield. Synthetic versatility of the method via Fe-catalyzed reductive isoxazole ring cleavage toward pyridine, pyrimidine, pyrazole fused heteropolycyclic compounds has been showcase.

Recent Advances in Functionalization of Pyrroles and their Translational Potential

Among the known aromatic nitrogen heterocycles, pyrrole represents a privileged aromatic heterocycle ranging its occurrence in the key component of "pigments of life" to biologically active natural products to active pharmaceuticals. Pyrrole being an electron-rich heteroaromatic compound, its predominant functionalization is legendary to aromatic electrophilic substitution reactions. Although a few excellent reviews on the functionalization of pyrroles including the reports by Baltazzi in 1963, Casiraghi and Rassu in 1995, and Banwell in 2006 are available, they are fragmentary and over fifteen years old, and do not cover the modern aspects of catalysis. A review covering a comprehensive package of direct functionalization on pyrroles via catalytic and non-catalytic methods including their translational potential is described. Subsequent to statutory yet concise introduction, the classical functionalization on pyrroles using Lewis acids largely following an ionic mechanism is discussed. The subsequent discussion follows the various metal-catalyzed C-H functionalization on pyrroles, which are otherwise difficult to implement by Lewis acids. A major emphasize is given on the radical based pyrrole functionalization under metal-free oxidative conditions, which is otherwise poorly highlighted in the literature. Towards the end, the current development of pyrrole functionalization under photocatalyzed and electrochemical conditions is appended. Only a selected examples of substrates and important mechanisms are discussed for different methods highlighting their scopes and limitations. The aromatic nucleophillic substitution on pyrroles (being an electron-rich heterocycle) happened to be the subject of recent investigations, which has also been covered accentuating their underlying conceptual development. Despite great achievements over the past several years in these areas, many challenges and problems are yet to be solved, which are all discussed in summary and outlook.

Pd(0)-Catalyzed Intramolecular Heck reaction of 2/3-Aryl(amino)methyl-3/2-bromoindoles: Syntheses of 3,4-Benzo[c]-β-carbolines, Benzo[4,5]isothiazolo[2,3-a]indole 5,5-Dioxides, and 1,2-Benzo[a]-γ-carbolines

One-pot synthesis of 3,4-benzo[c]-β-carbolines was achieved from 2-aryl(tosylamino)methyl-3-bromoindoles via 10 mol % Pd(OAc)₂/PPh₃-mediated intramolecular Heck coupling using K₂CO₃ as a base in DMF at 110 °C with concomitant aromatization through an elimination of tosylsulfinic acid. Under identical conditions, the isomeric 3-aryl(tosylamino)methyl-2-bromoindoles upon intramolecular Heck reaction furnished benzo[4,5]isothiazolo[2,3-a]indole 5,5-dioxides instead of the expected γ-carbolines. However, synthesis of the expected γ-carboline framework, 3-tosyl-6,9-dihydro-1,2-benzo[a]-γ-carbolines, could be achieved from 3-aryl(tosylamino)methyl-2-bromoindoles containing a mesitylene sulfonyl unit as a protecting group on the indole nitrogen.

Synthesis of ortho-arylated and alkenylated benzamides by palladium-catalyzed denitrogenative cross-coupling reactions of 1,2,3-benzotriazin-4(3H)-ones with organoboronic acids

An efficient palladium-catalyzed denitrogenative cross-coupling reaction of 1,2,3-benzotriazin-4(3H)-ones with organoboronic acids is described. The reaction affords various ortho-aryl and alkenylated benzamides in good to high yields.

Recent Advances in Catalytic Synthesis of Benzosultams

Benzosultams represent one category of multi-heteroatom heterocyclic scaffolds, which have been frequently found in pharmaceuticals, agricultural agents, and chiral catalysts. Given the diversely significant functions of these compounds in organic and medicinal chemistry, great efforts have been made to develop novel catalytic systems for the efficient construction of benzosultam motifs over the past decades. Herein, in this review, we mainly summarize the recent advances in the field of catalytic synthesis of benzosultams from 2017 to August of 2020, with an emphasis on the scopes and mechanisms of representative reactions.

Steering Site-Selectivity in Transition Metal-Catalyzed C-H Bond Functionalization: the Challenge of Benzanilides

Selective C-H bond functionalization catalyzed by metal complexes have completely revolutionized the way in which chemical synthesis is conceived nowadays. Typically, the reactivity of a transition metal catalyst is the key to control the site-, regio- and/or stereo-selectivity of a C-H bond functionalization. Of particular interests are molecules that contain multiple C-H bonds prone to undergo C-H bond activations with very similar bond dissociation energies at different positions. This is the case of benzanilides, relevant chemical motifs that are found in many useful fine chemicals, in which two C-H sites are present in chemically different aromatic fragments. In the last years, it has been found that depending on the metal catalyst and the reaction conditions, the amide motif might behave as a directing group towards the metal-catalyzed C-H bond activation in the benzamide site or in the anilide site. The impact and the consequences of such subtle control of site-selectivity are herein reviewed with important applications in carbon-carbon and carbon-heteroatom bond forming processes. The mechanisms unraveling these unique transformations are discussed in order to provide a better understanding for future developments in the field of site-selective C-H bond functionalization with transition metal catalysts.

Palladium-Catalyzed Intramolecular Oxidative Arylations for the Synthesis of Fused Biaryl Sulfones

This study unveils palladium-catalyzed intramolecular oxidative cyclizations in biaryl and heterobiaryl sulfones providing direct access to fused biaryl sulfones (dibenzothiophene-5,5-dioxides). Variously substituted dibenzothiophene-5,5-dioxides could be readily prepared in good to excellent yields under optimized conditions. In addition, bromination afforded dibromo derivative of dibenzothiophene-5,5-dioxides, providing platform for late-stage diversification. The translational applications of this current protocol have successfully been demonstrated in the synthesis of 2,8-diamino derivative of dibenzothiophene-5,5-dioxides, a α₇-nicotinic acetylcholine receptor agonist analogue, and novel single fluorene-tethered dibenzothiophene-5,5-dioxide, an organic emitter.

Preparation of pyrrolizinone derivatives via sequential transformations of cyclic allyl imides: synthesis of quinolactacide and marinamide

A facile synthetic route toward the preparation of pyrrolizinone derivatives has been developed and applied for the synthesis of quinolactacide/marinamide.

Design, Sustainable Synthesis, and Programmed Reactions of Templated N-Heteroaryl-Fused Vinyl Sultams

A de novo design and synthesis of N-heteroaryl-fused vinyl sultams as templates for programming chemical reactions on vinyl sultam periphery or (hetero)aryl ring is described. The key features include rational designing and sustainable synthesis of the template, customized reactions of vinyl sultams at C═C bond or involving N-S bond cleavage, and reactions on the periphery of the heteroaryl ring for late-stage diversification. The simple, easy access to the template coupled with opportunities for the synthesis of diversely functionalized heterocyles from a single template constitutes a rare study in contemporary organic synthesis.

Oxidative C-H/C-H Coupling Reactions between Two (Hetero)arenes

Transition metal-mediated C-H bond activation and functionalization represent one of the most straightforward and powerful tools in modern organic synthetic chemistry. Bi(hetero)aryls are privileged π-conjugated structural cores in biologically active molecules, organic functional materials, ligands, and organic synthetic intermediates. The oxidative C-H/C-H coupling reactions between two (hetero)arenes through 2-fold C-H activation offer a valuable opportunity for rapid assembly of diverse bi(hetero)aryls and further exploitation of their applications in pharmaceutical and material sciences. This review provides a comprehensive overview of the fundamentals and applications of transition metal-mediated/catalyzed oxidative C-H/C-H coupling reactions between two (hetero)arenes. The substrate scope, limitation, reaction mechanism, regioselectivity, and chemoselectivity, as well as related control strategies of these reactions are discussed. Additionally, the applications of these established methods in the synthesis of natural products and exploitation of new organic functional materials are exemplified. In the last section, a short introduction on oxidant- or Lewis acid-mediated oxidative Ar-H/Ar-H coupling reactions is presented, considering that it is a very powerful method for the construction of biaryl units and polycylic arenes.

Integration of oxidative arylation with sulfonyl migration: one-pot tandem synthesis of densely functionalized (NH)-pyrroles

A one-pot synthesis of 2-aryl-3-alkyl/aryl-sulfonyl-(NH)-pyrroles from N-sulfonylpyrroles, developed for the first time, via palladium-catalyzed oxidative C-2 arylation followed by sulfonyl migration is described.

Post-synthetic diversification of pyrrole-fused benzosultams via trans-sulfonylations and reactions on the periphery of pyrrole

The synthesis of pyrrole fused benzosultams and their trans-sulfonylation reactions providing workable access to (NH)-2-arylpyrroles containing ortho-sulfonamide functionality and fluorene tethered pyrrole fused benzosultam has been described.

Intramolecular Minisci acylation under silver-free neutral conditions for the synthesis of azafluorenones and fluorenones

An intramolecular Minisci acylation under silver-free neutral conditions providing access to azafluorenones and fluorenones has been developed.

Rhodium-Catalyzed NH-Indole-Directed C-H Carbonylation with Carbon Monoxide: Synthesis of 6H-Isoindolo[2,1-a]indol-6-ones

An efficient synthesis of 6H-isoindolo[2,1-a]indol-6-ones through rhodium-catalyzed NH-indole-directed C-H carbonylation of 2-arylindoles with carbon monoxide has been developed. Preliminary mechanistic studies revealed that this reaction proceeds via N-H bond cleavage and subsequent C-H bond cleavage. Reaction monitoring via ESI-MS was used to support the formation of five-membered rhodacycle species in the catalytic cycle.

Reactivity of alkynylindole-2-carboxamides in [Pd]-catalysed C–H activation and phase transfer catalysis: formation of pyrrolo-diindolones vs. β-carbolinones

[Pd]-Catalysed reaction of 3-alkynylindole-2-carboxamidesviaC–H activation leads to pyrrolodiindolones; in contrast, under phase-transfer catalysis, the same precursors afford β-carbolinones.

Implications of dynamic imine chemistry for the sustainable synthesis of nitrogen heterocycles via transimination followed by intramolecular cyclisation

An application of dynamic imine chemistry to the sustainable synthesis of N-heterocycles from N-aryl benzylamines or imines and ortho-substituted anilines is described.

Formation of amides, their intramolecular reactions for the synthesis of N-heterocycles, and preparation of a marketed drug, sildenafil: a comprehensive coverage

A comprehensive package including new amide formation, their intramolecular reactions for the synthesis of diverse nitrogen heterocycles, and preparation of a marketed drug is described.