A novel synthetic strategy for phenazine formation is reported following self-coupling of anilines by Pd–Ag binary nanocluster-catalysed synchronous double C–N bond formation via non-radical mode of ortho-aryl C–H activation.

A novel contrast in palladium and copper catalysis is revealed to form products of different chemotypes resulting in a phenazine to azoarene twist through an altered mechanistic pathway (from non-radical C–H activation mode of C–N coupling to radical N–N coupling) during the oxidative self-coupling of anilines catalysed by Pd–Ag and Cu–Ag nanoclusters.

•36 compounds were synthesized using green synthetic protocol.•Twenty-one compounds displayed good in vitro anti-mycobacterial activity.•The most potent 3 compounds exhibited MIC of 0.78 μg/mL with therapeutic index >60.•The 3D-QSAR for anti-TB compounds has been established with significant CoMFA model.The benzo[d]thiazol-2-yl(piperazin-1-yl)methanones scaffold has been identified as new anti-mycobacterial chemotypes. Thirty-six structurally diverse benzo[d]thiazole-2-carboxamides have been prepared and subjected to assessment of their potential anti-tubercular activity through in vitro testing against Mycobacterium tuberculosis H37Rv strain and evaluation of cytotoxicity against RAW 264.7 cell lines. Seventeen compounds showed anti-mycobacterial potential having MICs in the low (1–10) μM range. The 5-trifluoromethyl benzo[d  ]thiazol-2-yl(piperazin-1-yl)methanones emerged to be the most promising resulting in six positive hits (2.35–7.94 μM) and showed low-cytotoxicity (<50% inhibition at 50 μg/mL). The therapeutic index of these hits is 8–64. The quantitative structure activity relationship has been established adopting a statistically reliable CoMFA model showing high prediction (rpred2=0.718,rncv2=0.995).The benzo[d  ]thiazol-2-yl(piperazin-1-yl)methanones scaffold has been identified as new anti-mycobacterial chemotypes. The quantitative structure activity relationship has been established adopting a statistically reliable CoMFA model showing high prediction (rpred2=0.718,rncv2=0.995).

A novel strategy for direct aryl hydroxylation via Pd-catalysed Csp2–H activation through an unprecedented hydroxyl radical transfer from 1,4-dioxane, used as a solvent, is reported with bio relevant and sterically hindered heterocycles and various acyclic functionalities as versatile directing groups.

A new synthetic strategy of tandem N-aroylmethylation-nitro reduction–cyclocondensation has been developed for the first and generalized regioselective synthesis of 2-aryl quinoxalines adopting “all water chemistry.” Water plays the critical role through hydrogen bond driven ‘synergistic electrophile–nucleophile dual activation’ for chemoselective N-aroylmethylation of o-nitroanilines, that underlines the origin of the regioselectivity, as the use of organic solvents proved to be ineffective. Water also provides beneficial effects during the nitro reduction and the penultimate cyclocondensation steps.

The catalytic potential of different metal Lewis acids has been assessed for the one-pot tandem Friedländer annulation and Knoevenagel condensation involving 2-aminobenzophenone, ethyl acetoacetate, and benzaldehyde to form 2-styryl quinoline under solvent free conditions. While various metal Lewis acids were effective in promoting the Friedländer annulation step, In(OTf)3 was the only effective catalyst for the subsequent Knoevenagel condensation reaction suggesting In(OTf)3 as the stand-alone catalyst for the tandem Friedländer–Knoevenagel reaction to form 2-styryl quinolines. The protocol is compatible with different variations of aromatic/hetero-aromatic aldehydes and α,β unsaturated aromatic aldehydes giving highly functionalized 2-aryl/heteroaryl vinyl quinolines. The catalyst can be recovered and reused to afford the desired product in very good to excellent yields.

The scope and limitations of metal salt Lewis acid catalysts were examined for the selectivity control for the formation of Friedländer and non-Friedländer products during the reaction involving 2-aminobenzophenone and ethyl acetoacetate. Among a pool of metal halides, tetrafluoroborates, perchlorates, and triflates used as catalysts, In(OTf)3 emerged as the most effective catalyst for the selective/exclusive formation of the Friedländer product. The generality of the In(OTf)3-catalysed Friedländer reaction was demonstrated by the reaction of differently substituted 2-aminoarylketones with various carbonyl compounds containing an active methylene group (e.g., β-ketoesters, cyclic/acyclic β-diketones, cyclic/acylic ketones, and aryl/heteroaryl methyl ketones) under solvent-free conditions affording the desired quinolines in 75–92% yields.

The palladium–nickel binary nanocluster is reported as a new catalyst system for Suzuki–Miyaura cross-coupling of ortho-heterocycle-tethered sterically hindered aryl bromides. The inferior results obtained with the reported Pd/Ni salts/complexes or individual Pd/Ni nanoparticles as catalyst reveal the cooperative catalytic effect of the Pd and Ni nanoparticles in the Pd–Ni nanocluster. The broad substrate scope with respect to variation of the 2-arylbenzoxazole moiety and boronic acids, which offers a means for diversity generation and catalyst recyclability, marks a distinct advantage.

Benzothiazole-2-carboxyarylalkylamides are reported as a new class of potent anti-mycobacterial agents. Forty-one target compounds have been synthesized following a green synthetic strategy using water as the reaction medium to construct the benzothiazole scaffold followed by (i) microwave-assisted catalyst-free and (ii) ammonium chloride-catalyzed solvent-free amide coupling. The anti-mycobacterial potency of the compounds was determined against H37Rv strain. Twelve compounds exhibited promising anti-TB activity in the range of 0.78–6.25 μg mL−1 and were found to be non-toxic (<50% inhibition at 50 μg mL−1) to HEK 293T cell lines with therapeutic index (TI) of 8–64. The most promising anti-TB compound 5bf showed MIC of 0.78 μg mL−1 (TI > 64). The molecular docking studies of 5bf predict it to be a ligand for the M. tuberculosis HisG, the putative drug target for tuberculosis and could serve as a guiding principle for lead optimization.

A series of twenty six structurally diverse α-aminophosphonates have been synthesized and evaluated for in vitro anti-leishmanial activity and cytotoxicity using the MTT assay. Among them, seven compounds (1–7) exhibited anti-leishmanial potency against the L. donovani promastigote with IC50 values in the low micromolar range. The structure–activity relationships were quantitatively evaluated by a statistically reliable CoMFA model with high predictive abilities (r2pred = 0.87, r2ncv = 0.985).

•Epoxide phenolysis.•Neutral condition.•Silver nanoparticles.•Recyclable catalyst.•Drug synthesis.Chemo- and regio-selective epoxide phenolysis is reported for the first time under neutral condition catalysed by silver nanoparticles. Other metal nanoparticles (e.g., Au, Pd, Cu, In, and Ru) are less effective. The choice of solvent is critical with 2-propanol being the best followed by DEF. Amongst various stabilisers used (surfactants, PEGs, tetra-alkylammonium halides) the tetra-alkylammonium halides are found to be the most effective (TBAF > TBAB > TBACl > TBAI). The role of the silver nanoparticles is envisaged as synchronous mode epoxide-phenol dual activation via a cooperative network of coordination, anion–π interaction, and hydrogen bond. The silver nanoparticles are recovered and reused for five consecutive times. The reaction has been used for the synthesis of propranolol and naftopidil as a few representative cardiovascular drugs.Synergistic activation of epoxide-phenol dual activation catalysed by in situ generated silver nanoparticles is reported for the first time for epoxide phenolysis under base-free conditions that finds application towards the synthesis of various drug molecules.

A novel strategy of ‘all water chemistry’ is reported for a concise total synthesis of the novel class anti-anginal drug ranolazine in its racemic (RS) and enantiopure [(R) and (S)] forms. The reactions at the crucial stages of the synthesis are promoted by water and led to the development of new water-assisted chemistries for (i) catalyst/base-free N-acylation of amine with acyl anhydride, (ii) base-free N-acylation of amine with acyl chloride, (iii) catalyst/base-free one-pot tandem N-alkylation and N-Boc deprotection, and (iv) base-free selective mono-alkylation of diamine (e.g., piperazine). The distinct advantages in performing the reactions in water have been demonstrated by performing the respective reactions in organic solvents that led to inferior results and the beneficial effect of water is attributed to the synergistic electrophile and nucleophile dual activation role of water. The new ‘all water’ strategy offers two green processes for the total synthesis of ranolazine in two and three steps with 77 and 69% overall yields, respectively, and which are devoid of the formation of the impurities that are generally associated with the preparation of ranolazine following the reported processes.

The catalytic potential of various protic acids has been assessed for the one pot tandem condensation–cyclisation reaction involving an aldehyde, an amine, and thioglycolic acid to form 2,3-disubstituted thiazolidin-4-ones. The catalytic potential of the various protic acids that follows the order TfOH > HClO4 > H2SO4 ∼ p-TsOH > MsOH ∼ HBF4 > TFA ∼ AcOH is improved significantly by adsorption on solid supports, in particular using silica gel (230–400 mesh size), with the resulting relative catalytic potential following the order HClO4–SiO2 > TfOH–SiO2 ≫ H2SO4–SiO2 > p-TsOH–SiO2 > MsOH–SiO2 ∼ HBF4–SiO2 > TFA–SiO2 ∼ HOAc–SiO2. The better catalytic potential of HClO4–SiO2 as compared to that of Tf–SiO2, although TfOH is a stronger protic acid than HClO4, can be rationalised through a transition state model depicting the interaction of the individual protic acid with SiO2. The catalytic efficiency of HClO4 adsorbed on various solid supports was in the order HClO4–SiO2 ≫ HClO4–K10 > HClO4–KSF > HClO4–TiO2 ∼ HClO4–Al2O3. The catalytic system HClO4–SiO2 is compatible with different variations of aldehydes (aryl/heteroaryl/alkyl/cycloalkyl) and the amines (aryl/heteroaryl/arylalkyl/alkyl/cycloalkyl) affording the desired 2,3-disubstituted thiazolidin-4-ones in 70–87% yields (43 examples). The electronic and the steric factors associated with the aldehydes and the amines provide a handle for selective thiazolidinone formation and were found to be dependent on the extent of imine formation. No significant amount of thiazolidinone formation took place during the reaction of the preformed amide (synthesised from the amine and thioglycolic acid) with benzaldehyde suggesting that the reaction proceeds through the initial reversible imine formation followed by cyclocondensation of the preformed imine with thioglycolic acid, the reversible imine formation being the determining step to control selectivity of thiazolidinone formation in competitive environments. The feasibility of a large scale reaction and catalyst recycling/reuse is demonstrated.

Three new, concise, and protecting group-free synthetic routes for (RS)- and (S)-lubeluzole are reported in higher (46–62%) overall yields compared to the reported procedures (6–35%). The key steps involve C–N bond formation via epoxide aminolysis and nucleophilic substitution of 2-chlorobenzothiazole with suitably designed precursor amines and are performed in aqueous medium. Water offers an advantage in promoting the reactions compared to organic solvents and its role is envisaged as hydrogen-bond mediated electrophile–nucleophile dual activation.

Synergistic dual activation catalysis has been devised for epoxide phenolysis wherein palladium nanoparticles induce electrophilic activation via coordination with the epoxide oxygen followed by nucleophilic activation through anion–π interaction with the aromatic ring of the phenol, and water (reaction medium) also renders assistance through ‘epoxide–phenol’ dual activation.

The scope and limitations of surfactants as catalysts for the synthesis of quinoxalines using microreactors made of the surfactants in water has been assessed. The catalytic potential followed the order: non-ionic surfactants > anionic surfactants > Brønsted acid surfactants > cationic surfactants. The non-ionic surfactant, Tween 40, is the most effective catalyst affording excellent yields within a short reaction time at room temperature and is compatible with different variations of the 1,2-diketones and 1,2-diamines. The reaction medium (spent water) containing the catalyst, as well as the catalyst itself (recovered Tween 40) can be reused for five consecutive reactions. The better catalytic efficiency of the surfactant (Tween 40) compared to the various Lewis/Brønsted acids, as well as the surfactant combined Lewis acid, suggests that surfactants, which generate microreactor assemblies at the interface, are better suited as catalytic aids to promote organic reactions in water. The inferior results obtained in organic solvents, which provide a homogeneous reaction mixture compared to those obtained in water, indicate the specific role of water. This has been depicted as a synergistic dual activation through the hydrogen bond mediated formation of supramolecular assemblies involving a water dimer and the reactants. The catalytic assistance of the surfactant could be ascribed to the ability of the surfactant molecule to undergo hydrophobic and hydrogen bond forming interactions with water and the reactants in orienting the reactants at the water interface and encapsulating inside the microreactors to facilitate the cyclocondensation.

The catalytic potential of different fluoroboric acid-derived catalyst systems viz. aq HBF4, solid supported HBF4, metal tetrafluoroborates (inorganic salts), solid supported metal tetrafluoroborates, and tetrafluoroborate based ionic liquids (organic salts) were investigated for the three component reaction (3-MCR) of 1,2-diketone, aldehyde, and ammonium salts to form 2,4,5-trisubstituted imidazoles and the four component reaction (4-MCR) involving 1,2-diketone, aldehyde, amine and ammonium acetate to form 1,2,4,5-tetrasubstituted imidazoles. The HBF4–SiO2 was found to be the stand out catalyst for both the 3-MCR and 4-MCR processes. The next most effective catalysts are LiBF4 and Zn(BF4)2 to form 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles via the 3-MCR and 4-MCR, respectively. This is the first report on the unaddressed issue of competitive formation of 2,4,5-trisubstituted imidazole during the 4-MCR involving 1,2-diketone, aldehyde, amine and ammonium acetate and highlights the influence of the catalyst systems in controlling the selective formation of tetra substituted imidazole. The metal salt of weak protic acids drive selectivity towards tetra substituted imidazole in the order tetrafluoroborates > perchlorates > triflates. The catalytic potency of tetrafluoroborates was in the order Zn(BF4)2 > Co(BF4)2 > AgBF4 ≈ Fe(BF4)2 > NaBF4 ≈ LiBF4 ≈ Cu(BF4)2. The developed protocols worked well for different diketones, various aryl, heteroaryl, and alkyl aldehydes and in the case of the preparation of 1,2,4,5-tetrasubstituted imidazoles different amines can be used. The effectiveness of different ammonium salts as nitrogen source has been investigated and ammonium acetate is proved to be the best. The HBF4–SiO2 is recyclable for five consecutive uses without significant loss of catalytic activity.

Hydrogen-bond-driven electrophilic activation for selectivity control during competitive formation of 1,2-disubstituted and 2-substituted benzimidazoles from o-phenylenediamine and aldehydes is reported. The fluorous alcohols trifluoroethanol and hexafluoro-2-propanol efficiently promote the cyclocondensation of o-phenylenediamine with aldehydes to afford selectively the 1,2-disubstituted benzimidazoles at rt in short times. A mechanistic insight is invoked by NMR, mass spectrometry, and chemical studies to rationalize the selectivity. The ability of the fluorous alcohols in promoting the reaction and controlling the selectivity can be envisaged from their better hydrogen bond donor (HBD) abilities compared to that of the other organic solvents as well as of water. Due to the better HBD values, the fluorous alcohols efficiently promote the initial bisimine formation by electrophilic activation of the aldehyde carbonyl. Subsequently the hydrogen-bond-mediated activation of the in situ-formed bisimine triggers the rearrangement via 1,3-hydride shift to form the 1,2-disubstituted benzimidazoles.

Supramolecular assemblies formed by a relay of cooperative hydrogen bonds and charge−charge interactions have been identified/characterized by (+ve) ESI and MALDI-TOF-TOF MS and MS−MS studies during the aza-Michael reaction of amines with α,β-unsaturated carbonyl compounds in the presence of ionic liquids (ILs) digging out the role of catalysis by ILs, forming the basis of rational design/selection as organocatalysts, and offering a diagnostic model to predict/rationalize the selectivity of the aza-Michael reaction in a competitive environment.

A novel synthetic strategy for phenazine formation is reported following self-coupling of anilines by Pd–Ag binary nanocluster-catalysed synchronous double C–N bond formation via non-radical mode of ortho-aryl C–H activation.

Synergistic dual activation catalysis has been devised for epoxide phenolysis wherein palladium nanoparticles induce electrophilic activation via coordination with the epoxide oxygen followed by nucleophilic activation through anion–π interaction with the aromatic ring of the phenol, and water (reaction medium) also renders assistance through ‘epoxide–phenol’ dual activation.

A novel strategy for direct aryl hydroxylation via Pd-catalysed Csp2–H activation through an unprecedented hydroxyl radical transfer from 1,4-dioxane, used as a solvent, is reported with bio relevant and sterically hindered heterocycles and various acyclic functionalities as versatile directing groups.

A novel contrast in palladium and copper catalysis is revealed to form products of different chemotypes resulting in a phenazine to azoarene twist through an altered mechanistic pathway (from non-radical C–H activation mode of C–N coupling to radical N–N coupling) during the oxidative self-coupling of anilines catalysed by Pd–Ag and Cu–Ag nanoclusters.