S. Vijay Kumar

Cyclocondensation of arylhydrazines with 1,3-bis(het)arylmonothio-1,3-diketones and 1,3-bis(het)aryl-3-(methylthio)-2-propenones: synthesis of 1-aryl-3,5-bis(het)arylpyrazoles with complementary regioselectivity.

Two efficient highly regioselective routes for the synthesis of unsymmetrically substituted 1-aryl-3,5-bis(het)arylpyrazoles with complementary regioselectivity starting from active methylene ketones have been reported. In the first protocol, the newly synthesized 1,3-bis(het)aryl-monothio-1,3-diketone precursors (prepared by condensation of active methylene ketones with het(aryl) dithioesters in the presence of sodium hydride) were reacted with arylhydrazines in refluxing ethanol under neutral conditions, furnishing 1-aryl-3,5-bis(het)arylpyrazoles 7, in which the het(aryl) moiety attached to the thiocarbonyl group of monothio-1,3-diketones is installed at the 3-position. In the second method, the corresponding 3-(methylthio)-1,3-bis(het)aryl-2-propenones (prepared in situ by base-induced alkylation of 1,3-monothiodiketones) were condensed with arylhydrazines in the presence of potassium tert-butoxide in refluxing tert-butyl alcohol, yielding 1-aryl-3,5-bis(het)arylpyrazoles 9 with complementary regioselectivity (method A). The efficiency of this protocol was further improved by developing a one-pot, three-component procedure for the synthesis of pyrazoles 9, directly from active methylene ketones, by reacting in situ generated 3-(methylthio)-1,3-bis(het)aryl-2-propenones with arylhydrazines in the presence of sodium hydride (instead of potassium tert-butoxide as base). The structures and regiochemistry of newly synthesized pyrazoles were confirmed from their spectral and analytical data along with X-ray crystallographic data of three pairs of regioisomers.

Selected Copper-Based Reactions for C-N, C-O, C-S and C-C Bond Formation

Metal-catalyzed cross-coupling reactions belong to the most important transformations in organic synthesis. Copper catalysis has received great attention owing to the low toxicity and low cost of copper. However, traditional Ullmann-type couplings suffer from limited substrate scopes and harsh reaction conditions. The introduction of several bidentate ligands, such as amino acids, diamines, 1,3-diketones, and oxalic diamides, over the past two decades has totally changed this situation as these ligands enable the copper-catalyzed coupling of aryl halides and nucleophiles at both low reaction temperatures and catalyst loadings. The reaction scope has also been greatly expanded, rendering this copper-based cross-coupling attractive for both academia and industry. In this Review, we have summarized the latest progress in the development of useful reaction conditions for the coupling of (hetero)aryl halides with different nucleophiles. Additionally, recent advances in copper-catalyzed coupling reactions with aryl boronates and the copper-based trifluoromethylation of aromatic electrophiles will be discussed.

One-pot synthesis of functionalized benzo[b]thiophenes and their hetero-fused analogues via intramolecular copper-catalyzed S-arylation of in situ generated enethiolates.

An efficient one-pot synthesis of highly functionalized multisubstituted benzo[b]thiophenes and their hetero-fused analogues, such as thieno[2,3-b]thiophenes, indolo[2,3-b]thiophenes, and pyrazolo[3,2-c]thiophenes, has been reported. The overall strategy involves sequential base-mediated condensation of 2-bromohet(aryl)acetonitrile precursors with (het)aryl/alkyl dithioesters or other thiocarbonyl species such as dimethyl trithiocarbonate, S-methyl xanthates, methyl N-imidazolyl dithioate, N-alkyl dithiocarbamate, and phenyl isothiocyanate, followed by intramolecular copper-catalyzed arylthiolation of in situ generated enethiolates, furnishing a broad range of 2-functionalized 3-cyanobenzo[b]- and/hetero-fused thiophenes in high yields.

Diversity‐Oriented Synthesis of Substituted Benzo[b]thiophenes and Their Hetero‐Fused Analogues through Palladium‐Catalyzed Oxidative CH Functionalization/Intramolecular Arylthiolation

An efficient, high yielding route to multisubstituted benzo[b]thiophenes has been developed through palladium-catalyzed intramolecular oxidative C-H functionalization-arylthiolation of enethiolate salts of α-aryl-β-(het)aryl/alkyl-β-mercaptoacrylonitriles/acrylates or acrylophenones. The overall strategy involves a one-pot, two-step process in which enethiolate salts [generated in situ through base-mediated condensation of substituted arylacetonitriles, deoxybenzoins, or arylacetates with (het)aryl (or alkyl) dithioates] are subjected to intramolecular C-H functionalization-arylthiolation under the influence of a palladium acetate (or palladium chloride)/cupric acetate catalytic system and tetrabutylammonium bromide as additive in N,N-dimethylformamide (DMF) as solvent. In a few cases, the yields of benzo[b]thiophenes were better in a two-step process by employing the corresponding enethiols as substrates. In a few examples, Pd(OAc)2 (or PdCl2) catalyst in the presence of oxygen was found to be more efficient than cupric acetate as reoxidant, furnishing benzothiophenes in improved yields by avoiding formation of side products. The method is compatible with a diverse range of substituents on the aryl ring as well as on the 2- and 3-positions of the benzothiophene scaffold. The protocol could also be extended to the synthesis of a raloxifene precursor and a tubulin polymerization inhibitor in good yields. The versatility of this newly developed method was further demonstrated by elaborating it for the synthesis of substituted thieno-fused heterocycles such as thieno[2,3-b]thiophenes, thieno[2,3-b]indoles, thieno[3,2-c]pyrazole, and thieno[2,3-b]pyridines in high yields. A probable mechanism involving intramolecular electrophilic arylthiolation via either a Pd-S adduct or palladacycle intermediate has been proposed on the basis of experimental studies.

Synthesis of N-Functionalized/NH-Multisubstituted Indoles, Thienopyrroles, Pyrroloindoles, and Pyrazolopyrroles via Sequential One-Pot Base-Mediated and Copper-Catalyzed Inter- and Intramolecular Amination of 2-[2-Bromo(het)aryl]-3-(het)aryl-3-(methylthio)acrylonitriles

A novel, efficient route to substituted 1-N-(het)aryl/NH-2-(het)aryl-3-cyanoindoles and related pyrrolo-fused heterocycles such as thienopyrroles, pyrroloindoles, and pyrazolopyrroles has been reported. The overall protocol involves sequential cycloamination of readily available 2-[2-bromo(het)aryl]-3-(het)aryl-3-(methylthio)acrylonitrile precursors with primary amines or amides via two key C-N bond-forming processes, one base-mediated intermolecular and the other Cu-catalyzed intramolecular arylamination leading to N(1)-C(2) and N(1)-C(7a) bond formation, respectively, in a two-step one-pot procedure.

Synthesis of 2,4,5-trisubstituted thiazoles via Lawesson's reagent-mediated chemoselective thionation-cyclization of functionalized enamides.

An efficient route to 2-phenyl/(2-thienyl)-5-(het)aryl/(methylthio)-4-functionalized thiazoles via one-step chemoselective thionation-cyclization of highly functionalized enamides mediated by Lawessons reagent is reported. These enamide precursors are obtained by nucleophilic ring-opening of 2-phenyl/(2-thienyl)-4-[bis(methylthio)/(methylthio)(het)arylmethylene]-5-oxazolones with alkoxides and a variety of primary aromatic/aliphatic amines or amino acid esters, leading to the introduction of an ester, an N-substituted carboxamide, or a peptide functionality in the 4-position of the product thiazoles.

Synthesis of N-(Hetero)Aryl Carbamates via CuI/MNAO Catalyzed Cross-Coupling of (Hetero)Aryl Halides with Potassium Cyanate in Alcohols

An efficient route to N-(hetero)aryl carbamates was developed through CuI/MNAO [2-((2-methylnaphthalen-1-yl)amino)-2-oxoacetic acid] catalyzed cross-coupling of (hetero)aryl chlorides with potassium cyanate in alcohols at 120-130 °C. This method utilizes broadly available substrates to afford various N-(hetero)aryl carbamates in good to excellent yields. Moreover, (hetero)aryl bromides and (hetero)aryl iodides were also reacted at low catalyst loadings and relatively low temperatures to provide N-(hetero)aryl carbamates.