Oxidant-Switchable Selective Synthesis of 2-Aminobenzimidazoles via C–H Amination/Acetoxylation of Guanidines

Enroute sustainability: metal free C-H bond functionalisation

The term "C-H functionalisation" incorporates C-H activation followed by its transformation. In a single line, this can be defined as the conversion of carbon-hydrogen bonds into carbon-carbon or carbon-heteroatom bonds. The catalytic functionalisation of C-H bonds using transition metals has emerged as an atom-economical technique to engender new bonds without activated precursors which can be considered as a major drawback while attempting large-scale synthesis. Replacing the transition-metal-catalysed approach with a metal-free strategy significantly offers an alternative route that is not only inexpensive but also environmentally benign to functionalize C-H bonds. Recently metal free synthetic approaches have been flourishing to functionalize C-H bonds, motivated by the search for greener, cost-effective, and non-toxic catalysts. In this review, we will highlight the comprehensive and up-to-date discussion on recent examples of ground-breaking research on green and sustainable metal-free C-H bond functionalisation.

IBX-Mediated Organic Transformations in Heterocyclic Chemistry-A Decade Update

O-Iodoxybenzoic acid (IBX) is a very mild and efficient hypervalent iodine synthetic reagent useful to carry out several selective oxidations. The present review highlights research reports on IBX-assisted transformations in heterocyclic derivatives, particularly from 2010 onward.

Hypervalent iodine reagent-mediated reactions involving rearrangement processes

Hypervalent iodine reagents have been extensively employed in various types of oxidative organic reactions including oxidative coupling/cyclization, bifunctionalization of olefins and cyclopropane, C-H functionalization, and oxidative rearrangement reactions. In this review, the developments of the exclusive hypervalent iodine-mediated reactions involving oxidative rearrangement processes, including [1,2]-migration, Hofmann rearrangement, Beckmann rearrangement, ring contraction, ring expansion, [3,3]-sigmatropic/iodonium-Claisen rearrangement and some miscellaneous rearrangements, have been summarized.

Catalyst and solvent switched divergent C-H functionalization: oxidative annulation of N-aryl substituted quinazolin-4-amine with alkynes

The development of site-selective C-H functionalizations/annulations is one of the most challenging practices in synthetic organic chemistry particularly for substrates bearing several similarly reactive C-H bonds. Herein, we describe catalyst and solvent controlled ortho/peri site-selective oxidative annulation of C-H bonds of N-aryl substituted quinazolin-4-amines with internal alkynes. The ortho C-H selective annulation was observed using Pd-catalyst in DMF to give indole-quinazoline derivatives, while, Ru-catalyst in PEG-400 favoured the peri C-H bond annulation exclusively to furnish pyrido-quinazoline derivatives.

Oxidative Cyclization Approach to Benzimidazole Libraries

An efficient approach to the parallel synthesis of benzimidazoles from anilines is described. Library approaches to vary the N1 and C2 vectors of benzimidazoles are well established; however, C4-C7 variation has traditionally relied on 1,2-dianiline building blocks, providing limited chemical space coverage. We have developed an amidine formation/oxidative cyclization sequence that enables anilines as a diversity set for benzimidazole C4-C7 SAR generation in parallel format. The amidine annulation was achieved using PIDA or Cu-mediated oxidation to access both N-H and N-alkyl benzimidazoles. This library protocol has now been utilized for analog production in four medicinal chemistry projects. Additionally, the synthesis of aza-benzimidazoles from aminopyridines was achieved via an analogous sequence.

Methods to Access 2-aminobenzimidazoles of Medicinal Importance

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Benzimidazole (BI) and derivatives are interesting because several of these compounds have been found to have a diversity of biological activities with clinical applications. In view of their importance, the synthesis of BI and its derivatives is still considered as a challenge for synthetic chemists. Examples of compounds used in medicinal chemistry containing BI, as important nucleus, are Astemizole (antihistaminic), Omeprazole (antiulcerative) and Rabendazole (fungicide), some of these compounds have the 2- aminobenzimidazole (2ABI) as base nucleus. The structure of 2ABI derivatives contains a cyclic guanidine moiety, which is interesting because of its free lone pairs, labile hydrogen atoms and planar delocalized structure. The delocalized 10-π electron system and the extension of the electron conjugation with the exocyclic amino group, in 2ABI, making these heterocycles to have amphoteric character. The 2ABI has been used as building blocks for the synthesis of several BI derivatives as medicinally important molecules. On these bases, herein, we present a bibliographic review concerning the recent methodologies used in the synthesis of 2ABIs, including the substituted ones.

Iodine(iii) promoted ring-rearrangement reaction of 1-arylamino-2-oxocyclopentane-1-carbonitriles to synthesize N-aryl-δ-valerolactams

Environmentally benign hypervalent iodine(iii) has been utilised selectively for carbocyclic to heterocyclic ring transformation under various electronic conditions with detailed structural and mechanistic investigation.

Copper-catalysed C-H functionalisation gives access to 2-aminobenzimidazoles

This paper describes the development, optimisation and exemplification of a copper-catalysed C-H functionalisation to form pharmaceutically relevant 2-aminobenzimidazoles from aryl-guanidines. High throughput screening was used as a tool to identify a catalytically active copper source, DoE was used for reaction optimisation and a range of aryl-guanidines were prepared and exposed to the optimum conditions to afford a range of 2-aminobenzimidazoles in moderate to good yields. The methodology has been applied to the synthesis of Emedastine, a marketed anti-histamine pharmaceutical compound, with the key cyclisation step performed on a gram-scale.

Structural investigation and molecular docking studies of 2-amino-1H-benzimidazolium 2-hydroxybenzoate and 2-amino-1H-benzimidazolium pyridine 2-carboxylate single crystal

The molecular complexes of 2-amino-1H-benzimidazolium 2-hydroxybenzoate (2ABHB) and 2-amino-1H-benzimidazolium pyridine 2-carboxylate (2ABP2C) were synthesised from a donor 2-aminobenzimidazole (2AB), and acceptors, 2-hydroxybenzoate (2HB) and pyridine 2-carboxylic acid (P2C) respectively. The brown colour single crystals were successfully grown by slow evaporation solution growth technique. The densities of grown crystals 2ABHB and 2ABP2C were measured by floatation method using carbon tetrachloride and xylene as solvents. The single crystal X-ray diffraction reveals the molecular structures of compounds 2ABHB and 2ABP2C. Both the structures contain positively charged 2-aminobenzimidazolium cation, 2ABHB contains negatively charged 2-hydroxybenzoate anion and 2ABP2C contains pyridine 2 carboxylic acid anions. ¹H and ¹³C NMR spectral analyses were used to predict the presence of proton and carbon in the title compounds. The FTIR and Raman spectra have been recorded to detect the vibrational modes of functional groups present in the 2ABHB and 2ABP2C compounds. The proton transfer between the cation and anion was confirmed by UV-Vis spectra of both compounds. For both the compounds, thermal behaviour and stability of the crystals were studied using thermogravimetric and differential thermal analyses technique respectively. Further, the anti-bacterial activity of 2ABHB and 2ABP2C complex against Staphylococcus aureus, Klebsiella pneumonia, Pseudomonas eruginos and E. coli pathogens were investigated.

I2-Mediated Intramolecular C-H Amidation for the Synthesis of N-Substituted Benzimidazoles

A practical intramolecular C-H amidation methodology has been developed using molecular iodine under basic conditions. The required imine substrates were readily obtained by condensation of simple o-phenylenediamine derivatives and aldehydes. The transition-metal-free cyclization reaction described here works well with crude imines and allows for the sequential synthesis of N-protected benzimidazoles without purification of the less stable condensation intermediates. This operationally simple synthetic approach is broadly applicable to a variety of aromatic, aliphatic, and cinnamic aldehydes to produce diverse 1,2-disubstituted benzimidazole derivatives in an efficient and scalable fashion.

Iodine-mediated C–N and C–S bond formation: regioselective synthesis of benzo[4,5]imidazo[2,1-b]thiazoles

The regioselective synthesis of benzo[4,5]imidazo[2,1-b]thiazole derivatives via amination (C–N) and intramolecular cyclization (C–S) reactions in the presence of molecular iodine has been reported. This method is base and metal free and features inexpensive catalysts, with a simple procedure and a short reaction time.

Organoiodine reagent-promoted intermolecular oxidative amination: synthesis of cyclopropyl spirooxindoles

An organoiodine-promoted intramolecular direct oxidative C(sp²)–N cross-coupling reaction was developed for the preparation of cyclopropyl spirooxindoles from readily available secondary cyclopropyl carboxamides.

PhI(OAc)2-mediated 1,2-aminohalogenation of alkynes: a general access to (E)-4-(halomethylene)oxazolidin-2-ones

A new PhI(OAc)₂-mediated 1,2-aminohalogenation of prop-2-yn-1-yl carbamates and halogen sources is presented with excellent selectivity. The reaction is general for the construction of (E)-4-(halomethylene)oxazolidin-2-ones through the three-membered ring or N-radical.

Hypervalent iodine-triggered transformation of homopropargyl sulfonamides into dihalo-2,3-dihydropyrroles

A novel hypervalent iodine-triggered hydroamination of homopropargyl sulfonamides with copper halides to obtain dihalo-2,3-dihydropyrroles was efficiently developed.

Towards Uniform Iodine Catalysis: Intramolecular C-H Amination of Arenes under Visible Light

A photochemical catalytic amination of arenes is presented. The reaction proceeds under benign iodine catalysis in the presence of visible light as the initiator and provides access to a range of differently substituted arylamines. A total of 29 examples demonstrate the broad applicability of this mild oxidation method. The scope of the reaction could further be expanded to silyl-tethered derivatives, which undergo intramolecular amination upon formation of seven-membered heterocycles. Cleavage of the silicon tether provides access to the corresponding 3-substituted anilines.

Aminations with Hypervalent Iodine

Recent progress in the area of hypervalent iodine-mediated and catalyzed amination reaction of hydrocarbons is reviewed. These reactions comprise processes under both intra and intermolecular control and include the functionalization of aromatic C-H bonds as well as conversion of sp-, sp(2)-, and sp(3)-hybridized carbon atoms. These developments demonstrate that hypervalent iodine(III) methodology has reached a high level in amination chemistry. The individual reactions are discussed with a focus on mechanistic details and emphasis is made to the underlying hypervalent iodine reagents, for which structural information is available.

Metal-free regioselective C-3 acetoxylation of N-substituted indoles: crucial impact of nitrogen-substituent

Regioselective C-3 acetoxylation of the N-substituted indoles with PhI(OAc)₂ is described under metal-free conditions. The π-electron-deficient aryl-substituents on the N-atom of indoles remarkably favor the C-3 acetoxylation.