Regioselective Synthesis of Phenanthridines via Pd(II)-Catalyzed Annulative C(sp2)-H Activation

A robust synthesis of phenanthridines has been described via Pd(II)-catalyzed domino C(sp2)-H activation/N-arylation using oxime esters with aryl acyl peroxides in a highly regioselective manner. This protocol is compatible with acetophenone as well as benzophenone-derived oxime esters and allows modular construction of functionalized phenanthridines with wide tolerance of electronic functionality. Further transformations were conducted to synthesize key building blocks, and control experiments were performed to understand the plausible reaction mechanism.

Organic-acid catalysed Minisci-type arylation of heterocycles with aryl acyl peroxides

A metal-free method for the Minisci-type arylation of heterocycles with aryl acyl peroxides has been reported. This strategy enables the rapid and simple synthesis of a series of Minisci-type adducts...

Synthetic Strategies in the Preparation of Phenanthridinones

Phenanthridinones are important heterocyclic frameworks present in a variety of complex natural products, pharmaceuticals and displaying wide range of pharmacological actions. Its structural importance has evoked a great deal of interest in the domains of organic synthesis and medicinal chemistry to develop new synthetic methodologies, as well as novel compounds of pharmaceutical interest. This review focuses on the synthesis of phenanthridinone scaffolds by employing aryl-aryl, N-aryl, and biaryl coupling reactions, decarboxylative amidations, and photocatalyzed reactions.

Decoding Directing Groups and Their Pivotal Role in C-H Activation

Synthetic organic chemistry has witnessed a plethora of functionalization and defunctionalization strategies. In this regard, C-H functionalization has been at the forefront due to the multifarious applications in the development of simple to complex molecular architectures and holds a brilliant prospect in drug development and discovery. Despite been explored tremendously by chemists, this functionalization strategy still enjoys the employment of novel metal catalysts as well metal-free organic ligands. Moreover, the switch to photo- and electrochemistry has widened our understanding of the alternative pathways via which a reaction can proceed and these strategies have garnered prominence when applied to C-H activation. Synthetic chemists have been foraging for new directing groups and templates for the selective activation of C-H bonds from a myriad of carbon-hydrogen bonds in aromatic as well as aliphatic systems. As a matter of fact, by varying the templates and directing groups, scientists found the answer to the challenge of distal C-H bond activation which remained an obstacle for a very long time. These templates have been frequently harnessed for selectively activating C-H bonds of natural products, drugs, and macromolecules decorated with multiple C-H bonds. This itself was a challenge before the commencement of this field as functionalization of a site other than the targeted site could modify and hamper the biological activity of the pharmacophore. Total synthesis and pharmacophore development often faces the difficulty of superfluous reaction steps towards selective functionalization. This obstacle has been solved by late-stage functionalization simply by harnessing C-H bond activation. Moreover, green chemistry and metal-free reaction conditions have seen light in the past few decades due to the rising concern about environmental issues. Therefore, metal-free catalysts or the usage of non-toxic metals have been recently showcased in a number of elegant works. Also, research groups across the world are developing rational strategies for directing group free or non-directed protocols that are just guided by ligands. This review encapsulates the research works pertinent to C-H bond activation and discusses the science devoted to it at the fundamental level. This review gives the readers a broad understanding of how these strategies work, the execution of various metal catalysts, and directing groups. This not only helps a budding scientist towards the commencement of his/her research but also helps a matured mind searching out for selective functionalization. A detailed picture of this field and its progress with time has been portrayed in lucid scientific language with a motive to inculcate and educate scientific minds about this beautiful strategy with an overview of the most relevant and significant works of this era. The unique trait of this review is the detailed description and classification of various directing groups and their utility over a wide substrate scope. This allows an experimental chemist to understand the applicability of this domain and employ it over any targeted substrate.

Metal-catalysed C-Het (F, O, S, N) and C-C bond arylation

The formation of C-aryl bonds has been the focus of intensive research over the last decades for the construction of complex molecules from simple, readily available feedstocks. Traditionally, these strategies involve the coupling of organohalides (I, Br, Cl) with organometallic reagents (Mg, Zn, B, Si, Sn,…) such as Kumada-Corriu, Negishi, Suzuki-Miyaura, Hiyama and Sonogashira cross-couplings. More recently, alternative methods have provided access to these products by reactions with less reactive C-Het (F, O, S, N) and C-C bonds. Compared to traditional methods, the direct cleavage and arylation of these chemical bonds, the essential link in accessible feedstocks, has become increasingly important from the viewpoint of step-economy and functional-group compatibility. This comprehensive review aims to outline the development and advances of this topic, which was organized into (1) C-F bond arylation, (2) C-O bond arylation, (3) C-S bond arylation, (4) C-N bond arylation, and (5) C-C bond arylation. Substantial attention has been paid to the strategies and mechanistic investigations. We hope that this review can trigger chemists to discover more efficient methodologies to access arylation products by cleavage of these C-Het and C-C bonds.

Diacyl peroxides: practical reagents as aryl and alkyl radical sources

Diacyl peroxides, which can be easily synthesized from corresponding carboxylic acids, are commonly utilized as radical initiators and one electron oxidants. Under thermal, transition-metal catalysis or irradiation conditions the cleavage of relatively weak O-O bonds would occur followed by CO₂ extrusion to generate the corresponding aryl or alkyl radicals. Thus, diacyl peroxides can be employed as ideal arylating and alkylating reagents in organic synthesis, including C-H/N-H arylation/alkylation, aryl/alkyl radical addition to unsaturated bonds, hetero arylation/alkylation, eliminative arylation/alkylation, perfluoroalkylation etc. Moreover, these arylation/alkylation protocols have been successfully utilized in the synthesis and late-stage functionalization of natural products as well as bioactive molecules. In this review, recent advances on arylation and alkylation using diacyl peroxides as aryl and alkyl radical sources are summarized and discussed.

Recent advances in reactions using diacyl peroxides as sources of O- and C-functional groups

This review summarizes recent advances in reactions utilizing diacyl peroxides as O- and C-sources, with examples illustrating how the reactivity of diacyl peroxides in organic reactions can be controlled.

Mechanistic investigations on C–H activated dealkylating cyclo-amination reactions of substituted triazenes, formamidines and amidines

Catalytic dealkylating cycloamination reactions of N 1-methylated-N 1,N 3-diarylated triazenes proceed via two subsequent oxidative addition reactions, regioselectivity producing benzotriazoles by C–H and C–Br activation steps. Whereas palladium-based catalysis in the presence of dealkylating reagents and directing phosphane ligands leads to high yields, the homologous metals nickel and platinum as well as other 3d transition metals show only poor catalytic activity in similar procedures. Starting compounds have been widely varied to introduce potentially competing reaction sites and to investigate the reaction mechanism of the catalytic cyclization reactions. Yields of the benzotriazole synthesis strongly depend on the electronic and steric properties of the directing phosphane ligands, the nature of the dealkylating bases and the substitution pattern in 2- and 4-position of the aryl groups of the starting triazenes. In order to clarify the role of the catalyst, palladium-based intermediates were identified. Finally, formamidines and bulky amidines were tested in related C–H activated dealkylating cycloamination 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.

Photocatalyst-, metal- and additive-free, direct C–H arylation of quinoxalin-2(1H)-ones with aryl acyl peroxides induced by visible light

A visible light-mediated direct C–H arylation of quinoxalin-2(1H)-ones with aryl acyl peroxides has been developed.

Relative Strength of Common Directing Groups in Palladium-Catalyzed Aromatic C-H Activation

Efficient functionalization of C-H bonds can be achieved using transition metal catalysts, such as Pd(OAc)2. To better control the regioselectivity in these reactions, some functional groups on the substrate may be used as directing groups, guiding the reactivity to an ortho position. Herein, we describe a methodology to score the relative strength of such directing groups in palladium-catalyzed aromatic C-H activation. The results have been collected into a scale that serves to predict the regioselectivity on molecules with multiple competing directing groups. We demonstrate that this scale yields accurate predictions on over a hundred examples, taken from the literature. In addition to the regioselectivity prediction on complex molecules, the knowledge of the relative strengths of directing groups can also be used to work with new combinations of functionalities, exploring uncharted chemical space.

Ruthenium-catalyzed synthesis of indole derivatives from N-aryl-2-aminopyridines and alpha-carbonyl sulfoxonium ylides

Indole is a ubiquitous structural motif with important applications in many areas of chemistry. Given this, a simple and efficient Ru(ii)-catalyzed synthesis of indole via intermolecular annulation of N-aryl-2-aminopyridines and sulfoxonium ylides was proposed and accomplished. Excellent selectivity and good functional group tolerance of this transformation were observed. This protocol provides easy access to a wide variety of useful indoles in the presence of a commercially available [Ru(p-cymene)Cl2]2 catalyst. A possible mechanism for the reaction pathway was also proposed. More importantly, this reaction will offer a useful method for the construction of enantioenriched indole frameworks.

Site-selective Ru-catalyzed C–H bond alkenylation with biologically relevant isoindolinones: a case of catalyst performance controlled by subtle stereo-electronic effects of the weak directing group

A general regio- and site-selective ruthenium-catalyzed C–H bond alkenylation with the biologically relevant isoindolinone fragment serving as a weak directing group is presented.

Nickel-Catalyzed Oxidative Decarboxylative Annulation for the Synthesis of Heterocycle-Containing Phenanthridinones

A nickel-catalyzed oxidative decarboxylative annulation reaction of simple benzamides and (hetero)aromatic carboxylates has been developed. This reaction provides access to a large array of phenanthridinones and their heterocyclic analogues, highlighting the utility and versatility of oxidative decarboxylative coupling strategies for C-C bond formation.

Synthesis of Unsymmetrical 2,6-Diarylanilines by Palladium-Catalyzed C-H Bond Functionalization Methodology

3,5-Dimethylpyrazole was employed as a monodentate directing group for palladium-catalyzed ortho-sp² C-H arylation with aryl iodides. The reaction shows good functional group tolerance and outstanding selectivity for mono- ortho-arylation. Ozonolysis of ortho-arylated arylpyrazoles gave acylated biphenylamines that were further arylated to afford unsymmetrically substituted 2,6-diarylacetanilides.

A Cross-Dehydrogenative Annulation Strategy towards Synthesis of Polyfluorinated Phenanthridinones with Copper

The first cross-dehydrogenative annulation of (hetero)aromatic amides with polyfluoro(hetero)arenes is presented. This operationally simple oxidative annulation process is mediated by inexpensive copper salt, accommodates a wide range of substrates with exquisite chemo- and regioselectivity profile, and produces demanding polyfluorinated phenanthridinones in high yields (up to 92 %). Using alkenyl amides under identical conditions, the synthesis of polyfluorinated 2-quinolones has also been accomplished. Given the importance of fluorinated heterocycles in the pharmaceutical industry and drug discovery, this work is highly significant.

Synthesis of 2-methylbenzoxazoles directly from N-phenylacetamides catalyzed by palladium acetate

A method to synthesize 2-methylbenzoxazoles directly from N-phenylacetamides catalyzed by Pd(OAc)₂ in moderate to excellent yields has been achieved.

Visible-Light-Promoted C-H Arylation by Merging Palladium Catalysis with Organic Photoredox Catalysis

The use of a dual palladium/organic photoredox catalytic system enables the directed arylation of arenes with aryldiazonium salts with a broad substrate scope at room temperature under mild reaction conditions. This study thus serves as not only an alternative route for the biaryl motifs but also a new example for the application of an organic photoredox catalyst.

Metal-free denitrative arylation of β-nitrostyrenes using benzoyl peroxide: an easy access to trans-stilbenes

A novel, efficient and stereoselective synthesis of trans-stilbenes has been achieved from β-nitrostyrenes and benzoyl peroxide without using any catalyst.

Direct arylation of inactivated benzene with aryl acyl peroxides toward biaryls

A direct arylation of inactivated benzene with acyl peroxides toward biaryls under metal-free, base-free and additive-free conditions was developed.

One-pot sequential reaction to 2-substituted-phenanthridinones from N-methoxybenzamides

A one-pot synthesis of 2-bromo/chloro-phenanthridinones via an amidation of arenes followed by a regioselective halogenation reaction has been developed using a hypervalent iodine reagent.

A Domino Oxidation/Arylation/Protodecarboxylation Reaction of Salicylaldehydes: Expanded Access to meta-Arylphenols

A method that allows salicylaldehydes to be efficiently transformed into meta-arylated phenol derivatives through a cascade oxidation/arylation/protodecarboxylation sequence is presented. We demonstrate that the aldehyde functional group can be used as a convenient removable directing group to control site selectivity in C-H activation. Aldehydes are easily introduced into the starting materials and the group is readily cleaved after the C-H functionalization event.