An HFIP-assisted, cobalt-catalyzed three-component electrophilic C-H amination/cyclization/directing group removal cascade to naphtho[1,2-d]imidazoles.

A concise and efficient method has been developed herein for the synthesis of valuable naphtho[1,2-d]imidazole derivatives. It involves an earth-abundant cobalt-catalyzed electrophilic ortho C-H amination/cyclization/directing group removal cascade with O-benzoloxyamines using paraformaldehyde as a one carbon synthon. Picolinamide has been utilized as a traceless directing group. A boosting effect of HFIP is found in the whole process. The reaction conditions are very simple and allow easy handling, making this methodology valuable and appealing.

A 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.

The emergence of the C-H functionalization strategy in medicinal chemistry and drug discovery.

Owing to the market competitiveness and urgent societal need, an optimum speed of drug discovery is an important criterion for successful implementation. Despite the rapid ascent of artificial intelligence and computational and bioanalytical techniques to accelerate drug discovery in big pharma, organic synthesis of privileged scaffolds predicted in silico for in vitro and in vivo studies is still considered as the rate-limiting step. C-H activation is the latest technology added into an organic chemist's toolbox for the rapid construction and late-stage modification of functional molecules to achieve the desired chemical and physical properties. Particularly, elimination of prefunctionalization steps, exceptional functional group tolerance, complexity-to-diversity oriented synthesis, and late-stage functionalization of privileged medicinal scaffolds expand the chemical space. It has immense potential for the rapid synthesis of a library of molecules, structural modification to achieve the required pharmacological properties such as absorption, distribution, metabolism, excretion, toxicology (ADMET) and attachment of chemical reporters for proteome profiling, metabolite synthesis, etc. for preclinical studies. Although heterocycle synthesis, late-stage drug modification, 18F labelling, methylation, etc. via C-H functionalization have been reviewed from the synthetic standpoint, a general overview of these protocols from medicinal and drug discovery aspects has not been reviewed. In this feature article, we will discuss the recent trends of C-H activation methodologies such as synthesis of medicinal scaffolds through C-H activation/annulation cascade; C-H arylation for sp2-sp2 and sp2-sp3 cross-coupling; C-H borylation/silylation to introduce a functional linchpin for further manipulation; C-H amination for N-heterocycles and hydrogen bond acceptors; C-H fluorination/fluoroalkylation to tune polarity and lipophilicity; C-H methylation: methyl magic in drug discovery; peptide modification and macrocyclization for therapeutics and biologics; fluorescent labelling and radiolabelling for bioimaging; bioconjugation for chemical biology studies; drug-metabolite synthesis for biodistribution and excretion studies; late-stage diversification of drug-molecules to increase efficacy and safety; cutting-edge DNA encoded library synthesis and improved synthesis of drug molecules via C-H activation in medicinal chemistry and drug discovery.

Overcoming the Deallylation Problem: Palladium(II)-Catalyzed Chemo-, Regio-, and Stereoselective Allylic Oxidation of Aryl Allyl Ether, Amine, and Amino Acids.

We report herein a Pd(II)/bis-sulfoxide-catalyzed intramolecular allylic C-H acetoxylation of aryl allyl ether, amine, and amino acids with the retention of a labile allyl moiety. Mechanistically, the reaction proceeds through a distinct double-bond isomerization from the allylic to the vinylic position followed by intramolecular carboxypalladation and the ß-hydride elimination pathway. For the first time, C-H oxidation of N-allyl-protected amino acids to furnish five-membered heterocycles through 1,3-syn-addition is established with excellent diastereoselectivity.

Copper-Catalyzed Electrophilic Ortho C(sp2)-H Amination of Aryl Amines: Dramatic Reactivity of Bicyclic System.

A practical copper-catalyzed, 2-picolinamide-directed ortho C-H amination of anilines with benzoyl-protected hydroxylamines has been disclosed that proceeds smoothly without any external stoichiometric oxidant or additives. Remarkably, besides anilines, bicyclic naphthyl or heterocyclic amines furnished amination products with five- and six-membered cyclic and acyclic amines at the ortho position selectively. This electrophilic C-H amination also proceeds smoothly in water under slightly modified reaction conditions.

Exceedingly Fast, Direct Access to Dihydroisoquinolino[1,2- b]quinazolinones through a Ruthenium(II)-Catalyzed Redox-Neutral C-H Allylation/Hydroamination Cascade.

A ruthenium(II)-catalyzed redox-neutral synthesis of dihydroisoquinoline-fused quinazolinone derivatives has been accomplished through the merger of C-H activation and alkene difunctionalization using quinazolinone as an inherent directing group. This intermolecular reaction proceeds rapidly and is complete within 10 min, providing the annulation product in high yields without any stoichiometric metal oxidant. Mechanistically, this tandem reaction proceeds through directed ortho C-H allylation followed by hydroamination with the proximal -CONH group, to furnish 6-methyl-5,6-dihydro-8 H-isoquinolino[1,2- b]quinazolin-8-ones in a single operation. The carboxylic acid additive has a dual role in the formation of active catalyst and protodemetalation.