A Divergent Approach to Indoles and Oxazoles from Enamides by Directing-Group-Controlled Cu-Catalyzed Intramolecular C–H Amination and Alkoxylation

Pd(II)/LA-catalyzed acetanilide olefination with dioxygen

Transition-metal-catalyzed aromatic olefination through direct C-H activation represents an atom and step-economic route for versatile pharmaceutical syntheses, and in many cases, different stoichiometric oxidants are frequently employed for achieving a reasonable catalytic efficiency of the transition metal ions. Herein, we report a Lewis acid promoted Pd(II)-catalyzed acetanilide olefination reaction with atmospheric dioxygen as the oxidant source. The linkage of the Lewis acid to the Pd(II) species through a diacetate bridge significantly improved its catalytic efficiency, and independent kinetic studies on the olefination step revealed that adding the Lewis acid significantly accelerated the olefination rate as well as the C-H activation step. A strong basicity of the internal base in the Pd(II) salt also benefited the olefination reaction plausibly through base-assisted β-hydride elimination.

The advent of electrophilic hydroxylamine-derived reagents for the direct preparation of unprotected amines

Electrophilic aminating reagents have seen a renaissance in recent years as effective nitrogen sources for the synthesis of unprotected amino functionalities. Based on their reactivity, several noble and non-noble transition metal catalysed amination reactions have been developed. These include the aziridination and difunctionalisation of alkenes, the amination of arenes as well as the synthesis of aminated sulfur compounds. In particular, the use of hydroxylamine-derived (N-O) reagents, such as PONT (PivONH3OTf), has enabled the introduction of unprotected amino groups on various different feedstock compounds, such as alkenes, arenes and thiols. This strategy obviates undesired protecting-group manipulations and thus improves step efficiency and atom economy. Overall, this feature article gives a recent update on several reactions that have been unlocked by employing versatile hydroxylamine-derived aminating reagents, which facilitate the generation of unprotected primary, secondary and tertiary amino groups.

Directing group switch in copper-catalyzed electrophilic C–H amination/migratory annulation cascade: divergent access to benzimidazolone/benzimidazole

We present here a copper-catalyzed electrophilic ortho C–H amination of protected naphthylamines with N-(benzoyloxy)amines, cyclization with the pendant amide, and carbon to nitrogen 1,2-directing group migration cascade to access N,N-disubstituted 2-benzimidazolinones. Remarkably, this highly atom-economic tandem reaction proceeds through a C–H and C–C bond cleavage and three new C–N bond formations in a single operation. Intriguingly, the reaction cascade was altered by the subtle tuning of the directing group from picolinamide to thiopicolinamide furnishing 2-heteroaryl-imidazoles via the extrusion of hydrogen sulfide. This strategy provided a series of benzimidazolones and benzimidazoles in moderate to high yields with low catalyst loading (66 substrates with yields up to 99%). From the control experiments, it was observed that after the C–H amination an incipient tetrahedral oxyanion or thiolate intermediate is formed via an intramolecular attack of the primary amine to the amide/thioamide carbonyl. It undergoes either a 1,2-pyridyl shift with the retention of the carbonyl moiety or H₂S elimination for scaffold diversification. Remarkably, inspite of a positive influence of copper in the reaction outcome, from our preliminary investigations, the benzimidazolone product was obtained in good to moderate yields in two steps under metal-free conditions. The N-pyridyl moiety of the benzimidazolone was removed for further manipulation of the free NH group.

A novel directing group switch strategy is explored in a copper-catalyzed divergent synthesis of benzimidazolone via electrophilic C–H amination/cyclization/1,2-C → N directing group migration cascade and benzimidazole through the extrusion of H₂S.

Metal-catalyzed reactions for the C(sp2)–N bond formation: achievements of recent years

The review deals with the main catalytic methods for the C(sp2)–N bond formation, including Buchwald–Hartwig palladium-catalyzed amination of aryl and heteroaryl halides, renaissance of the Ullmann chemistry, i.e., the application of catalysis by copper complexes to form the carbon–nitrogen bond, and Chan–Lam reactions of (hetero)arylboronic acids with amines. Also, oxidative amination with C–H activation, which has been booming during the last decade, is addressed. Particular attention is paid to achievements in the application of heterogenized catalysts.

The bibliography includes 350 references.

Synthesis of Imidazoles and Oxazoles via a Palladium-Catalyzed Decarboxylative Addition/Cyclization Reaction Sequence of Aromatic Carboxylic Acids with Functionalized Aliphatic Nitriles

We herein report an efficient approach for the assembly of multiply substituted imidazoles and oxazoles in a single-step manner. These transformations are based on a decarboxylation addition and annulation of readily accessible aromatic carboxylic acids and aliphatic nitriles and exhibit good functional group compatibility and a high step economy. The reaction is scalable, and as-prepared products could be transformed into practical skeletons. Importantly, the late-stage derivatization of Momelotinib highlights the potential utility of this methodology.

Nickel-Catalyzed Regio- and Stereospecific C-H Coupling of Benzamides with Aziridines

A nickel-catalyzed C-H coupling of 8-aminoquinoline-derived benzamides with aryl- and alkyl-substituted aziridines has been disclosed. The current strategy provides direct access to benzolactams by the C-H alkylation-intramolecular amidation cascade event with the concomitant removal of the aminoquinoline auxiliary. The regioselectivity of ring opening of aziridines can be controlled by the substituents. The reaction with chiral aziridines proceeds with inversion of configuration, thus suggesting an S_N2-type nucleophilic ring-opening pathway.

Palladium-Catalyzed Sequential C-H Activation/Amination with Diaziridinone: An Approach to Indoles

Indoles are an important class of molecules. This paper describes an efficient palladium-catalyzed synthesis of indoles from 2-iodostyrenes and di-t-butyldiaziridinone with a simultaneous installation of two C-N bonds. The reaction process likely proceeds through the oxidative insertion of Pd to aryl iodide and subsequent vinyl C-H activation to from a pallada(II)cycle intermediate, which is bisaminated by di-t-butyldiaziridinone to give the indole product.

Iron-Catalyzed Cycloaddition of Amides and 2,3-Diaryl-2H-azirines To Access Oxazoles via C-N Bond Cleavage

A novel and efficient iron-catalyzed cycloaddition reaction using readily available 2,3-diaryl-2H-azirines and primary amides is reported. A wide range of trisubstituted oxazoles could be achieved in good yields with good functional group compatibility. In this transformation, two C-N bonds were cleaed and new C-N and C-O bonds were formed.

Transient directing group controlled regiodivergent C(sp3)–H and C(sp2)–H polyfluoroalkoxylation of aromatic aldehydes

A novel method for achieving regiodivergent C(sp3)–H and C(sp2)–H polyfluoroalkoxylation in the o-methyl benzaldehyde framework by altering the transient directing group is disclosed.

Bidentate Directing Groups: An Efficient Tool in C-H Bond Functionalization Chemistry for the Expedient Construction of C-C Bonds

During the past decades, synthetic organic chemistry discovered that directing group assisted C-H activation is a key tool for the expedient and siteselective construction of C-C bonds. Among the various directing group strategies, bidentate directing groups are now recognized as one of the most efficient devices for the selective functionalization of certain positions due to fact that its metal center permits fine, tunable, and reversible coordination. The family of bidentate directing groups permit various types of assistance to be achieved, such as N,N-dentate, N,O-dentate, and N,S-dentate auxiliaries, which are categorized based on the coordination site. In this review, we broadly discuss various C-H bond functionalization reactions for the formation of C-C bonds with the aid of bidentate directing groups.

Synthesis of quinazolin-4(1H)-ones via amination and annulation of amidines and benzamides

Quinazolinones have broad applications in the biological, pharmaceutical and material fields. Studies on the synthesis of these compounds are therefore widely conducted. Herein, a novel and highly efficient copper-mediated tandem C(sp2)-H amination and annulation of benzamides and amidines for the synthesis of quinazolin-4(1H)-ones is proposed. This synthetic route can be useful for the construction of quinazolin-4(1H)-one frameworks.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

Synthesis of Indoles via Electron-Catalyzed Intramolecular C-N Bond Formation

A new protocol for the preparation of N-substituted indole-3-carboxylates has been developed. The key C-N bond formation occurs under transition-metal-free conditions employing a t-BuOK/DMF system without special initiators or additives. Across a number of substrates, indoles were afforded in yields higher or comparable to those obtained under transition-metal-catalyzed conditions. While demonstrating high functional group tolerance, new conditions are particularly attractive for manufacturing halogenated indoles that cannot be made in a pure form using other metal-based catalytic methods.

Recent Progress in the Transition Metal Catalyzed Synthesis of Indoles

Indole is the most frequently found heterocyclic core structures in pharmaceuticals, natural products, agrochemicals, dyes and fragrances. For about 150 years, chemists were absorbed in finding new and easier synthetic strategies to build this nucleus. Many books and reviews have been written, but the number of new syntheses that appear in the literature, make necessary continuous updates. This reviews aims to give a comprehensive overview on indole synthesis catalyzed by transition metals appeared in the literature in the years 2016 and 2017.

A comprehensive overview of directing groups applied in metal-catalysed C-H functionalisation chemistry

The present review is devoted to summarizing the recent advances (2015-2017) in the field of metal-catalysed group-directed C-H functionalisation. In order to clearly showcase the molecular diversity that can now be accessed by means of directed C-H functionalisation, the whole is organized following the directing groups installed on a substrate. Its aim is to be a comprehensive reference work, where a specific directing group can be easily found, together with the transformations which have been carried out with it. Hence, the primary format of this review is schemes accompanied with a concise explanatory text, in which the directing groups are ordered in sections according to their chemical structure. The schemes feature typical substrates used, the products obtained as well as the required reaction conditions. Importantly, each example is commented on with respect to the most important positive features and drawbacks, on aspects such as selectivity, substrate scope, reaction conditions, directing group removal, and greenness. The targeted readership are both experts in the field of C-H functionalisation chemistry (to provide a comprehensive overview of the progress made in the last years) and, even more so, all organic chemists who want to introduce the C-H functionalisation way of thinking for a design of straightforward, efficient and step-economic synthetic routes towards molecules of interest to them. Accordingly, this review should be of particular interest also for scientists from industrial R&D sector. Hence, the overall goal of this review is to promote the application of C-H functionalisation reactions outside the research groups dedicated to method development and establishing it as a valuable reaction archetype in contemporary R&D, comparable to the role cross-coupling reactions play to date.

Palladium(II)-Catalyzed Mono- and Bis-alkenylation of N-Acetyl-2-aminobiaryls through Regioselective C-H Bond Activation

We developed palladium-catalyzed oxidative coupling of olefins with N-acyl 2-aminobiaryls through a sequence of ortho C-H bond activation/alkene insertion/reductive elimination. Furthermore, we controlled the selectivity of mono- and bis-alkenylation products with the solvent effect. The developed protocol was promising for a broad substrate scope ranging from activated olefins with a wide variety of functional groups to unactivated olefins.