Trimethylsilyl-1,3,4-oxadiazoles—new useful synthons for the synthesis of various 2,5-disubstituted-1,3,4-oxadiazoles

Photoredox/Cobalt-Catalyzed Cascade Oxidative Synthesis of 2,5-Disubstituted 1,3,4-Oxadiazoles under Oxidant-Free Conditions

An efficient oxidant-free, photoredox-mediated cascade cyclization strategy for the synthesis of 1,3,4-oxadiazoles by using an organo acridinium photocatalyst and a cobaloxime catalyst has been developed. Various acylhydrazones have been transformed into the corresponding 1,3,4-oxadiazole products in up to 96% yield, and H2 is the only byproduct. Mechanistic experiments and density functional theory (DFT) calculation studies indicate carbon-centered radicals rather than oxygen-centered radicals as π-radicals produced by the oxidation of photoexcited Mes-Acr+* along with deprotonation, which is responsible for this transformation. The practical utility of this method is highlighted by the one-pot gram-scale synthesis starting directly from commercially available aldehydes and acylhydrazides.

Unconventional Transformations of Difluorocarbene with Amines and Ethers

ConspectusFluorine-containing compounds are extensively involved in various fields originating from intriguing and unique characteristics of fluorine atom; notably, in pharmaceuticals, the involvement of a fluorine atom or a fluorine-containing group is a chief technique for improving the pesticide effect and developing new drugs. Difluorocarbene, one of the most important and powerful fluorine-containing reagents, is widely employed and studied in many areas mainly to assemble gem-difluoromethyl molecules, including but not limited to the abundant reactions between difluorocarbene with nucleophilic substrates, Wittig reaction with ketones or aldehydes, cascade reaction with both a nucleophile and an electrophile, or [2+1] cycloaddition with alkenes or alkynes. However, its unconventional and intriguing protocols beyond as a difluoromethyl synthon have rarely been studied, and thus, it is highly desired given its abundance, inexpensiveness and peculiar properties. In this Account, we mainly discuss our discovery with unconventional transformations of difluorocarbene, instead of as a sole difluoromethyl source (different from other dihalocarbene), actually can serve as an electron acceptor to activate C-X bonds (X = N and O) and thus promote a myriad of fascinating transformations for the assembly of versatile valuable products with various aza-compounds (primary/secondary/tertiary amines as well as NH₃ and NaNH₂ and so on) and aliphatic ethers in the absence of transition metals and expensive ligands. Inspired by the electron-deficient characteristics of difluorocarbene, we first found that the isocyanides could be readily formed in situ when the unoccupied orbital of difluorocarbene meets the lone-pair of primary amines; in basic condition, a cascade defluorination and cyclizations could afford plethora of valuable N-containing heterocycles. Meanwhile, we disclosed that cyano anion could be accessible in situ as well when difluorocarbene and NaNH₂ or NH₃ were mixed up in suitable basic conditions, and thus a series of aryl nitrile compounds were obtained in the presence of Pd catalysis and ArI. Interestingly, when difluorocarbene encountered secondary amines, formamides were rendered under mild reactions. Of note, concomitant functionalizations of C and N moieties via cleavage of the unstrained C(sp³)-N bond in the absence of metal and oxidant are sparce, which indeed significantly add versatility and diversity to products. Gratifyingly, by uitilizing difluorocarbene and cyclic tertiary amines, we achieved difluorocarbene-mediated deconstructive functionalizations for the first time, showing successive C(sp³)-N bond scission of amines and simultaneous functionalization of C and N atoms which would be introduced into the products in the absence of transition metals and oxidants. This method provides a brand-new while very universal synthetic pathway to selectively cleave inert unactivated Csp³-N bonds, in which halodifluoromethyl reagents act as both C1 synthon and halo (Cl, Br, I) sources. Fascinatingly, nitrogen ylides are generated in situ from difluorocarbene and tertiary amines, and an intriguing and universal approach for deaminative arylation or alkenylation of tertiary amines was disclosed for the first time in appropriate basic conditions, which represents an intriguing reaction mode to lead to a formal transition-metal free Suzuki cross coupling. Besides, we also disclosed that difluorocarbene could proceed novel atom recombination to render meaningful 2-fluoroindoles or 3-(2,2-difluoroethyl)-2-fluoroindoles from ortho-vinylanilines, 3-fluorined oxindoles from 2-aminoarylketones, in which difluorocarbene acts as a C1 synthon and F1 source simultaneously. Last but not the least, we recently found that the lone-pair-electron of oxygen could trap difluorocarbene as well to form oxonium ylide, which eventually leads to C-O bond cleavage with the formation of difluoromethyl ethers.

Metal-Free C-H Difluoromethylation of Imidazoles with the Ruppert-Prakash Reagent

The reaction of trimethyl(trifluoromethyl)silane-tetrabutylammonium difluorotriphenylsilicate (CF₃SiMe₃-TBAT) with a series of imidazoles gives products of the formal difluorocarbene insertion into the C-H bond at the C-2 position (i.e., C-difluoromethylation). According to NMR spectra, the corresponding 2-(trimethylsilyl)difluoromethyl-substituted derivatives are likely formed as the intermediates in the reaction, and then, they slowly convert to 2-difluoromethyl-substituted imidazoles. Quantum chemical calculations of two plausible reaction mechanisms indicate that it proceeds through the intermediate imidazolide anion stabilized through the interaction with solvent molecules and counterions. In the first proposed mechanism, the anion reacts with difluorocarbene without an activation barrier, and then, the CF₂ moiety of the adduct attacks the CF₃SiMe₃ molecule. After the elimination of the CF₃ anion, 2-(trimethylsilyl)difluromethyl-substituted imidazole is formed. Another possible reaction pathway includes silylation of imidazolide anion at the N-3 atom, followed by the barrierless addition of difluorocarbene at the C-2 atom and then by 1,3-shift of the SiMe₃ group from N-3 to the carbon atom of the CF₂ moiety. Both proposed mechanisms do not include steps with high activation barriers.

Indolyl-α-keto-1,3,4-oxadiazoles: Synthesis, anti-cell proliferation activity, and inhibition of tubulin polymerization

A series of novel indolyl-α-keto-1,3,4-oxadiazole derivatives have been synthesized by employing molecular iodine-mediated oxidative cyclization of acylhydrazones. In vitro anti cell proliferation activity of these derivatives against various cancer cells lines such as human lymphoblast (U937), leukemia (Jurkat & SB) and human breast (BT474) was investigated. Among the synthesized indolyl-α-keto-1,3,4-oxadiazoles 19a-p, only one compound (19e) exhibited significant antiproliferative activity against a panel of cell lines. The compound 19e with 3,4,5-trimethoxyphenyl motif, endowed strong cytotoxicity against U937, Jurkat, BT474 and SB cancer cells with IC₅₀ values of 7.1, 3.1, 4.1, and 0.8 µM, respectively. Molecular docking studies suggested a potential binding mode for 19e in the colchicine binding site of tubulin. When tested for in vitro tubulin polymerizaton, 19e inhibited tubulin polymezations (IC₅₀ = 10.66 µM) and induced apoptosis through caspase 3/7 activation. Further, the derivative 19e did not cause necrosis when measured using lactate dehydrogenase assay.

1,3,4-Oxadiazoles by Ugi-Tetrazole and Huisgen Reaction

Easy to perform, functional group tolerant, and short syntheses of the privileged scaffold oxadiazole are highly desired. Here, a metal-free protocol for MCR-based synthesis of 2,5-disubstituted 1,3,4-oxadiazoles via a Ugi-tetrazole/Huisgen sequence was developed. Optimization and scope and limitations of this short and general sequence are described. The reaction was also successfully performed on a gram scale.

Oxidant- and hydrogen acceptor-free palladium catalyzed dehydrogenative cyclization of acylhydrazones to substituted oxadiazoles

An efficient method for the synthesis of 2,5-disubstituted 1,3,4-oxadiazoles has been developed through palladium(0) catalyzed dehydrogenative cyclization ofN-arylidenearoylhydrazides without oxidants and hydrogen acceptors.

Ligand-free Cu(ii)-mediated aerobic oxidations of aldehyde hydrazones leading to N,N'-diacylhydrazines and 1,3,4-oxadiazoles

A Cu(ii)-mediated synthesis of N,N'-diacylhydrazines and 1,3,4-oxadiazoles from aldehyde hydrazones has been developed. This is the first time that the synthesis of N,N'-diacylhydrazines and 1,3,4-oxadiazoles using N,N-dimethylamides as the acylation reagent and O₂ in air as the oxidation reagent is reported. These reactions offered several advantages including simple workups, ligand-free inexpensive metal salts as mediators, high yields, and wide scope of substrates.

Synthesis of new 2,5-disubstituted-1,3,4-thiadiazole derivatives and their in vivo anticonvulsant activity

A series of 2,5-disubstituted-1,3,4-thiadiazole derivatives were synthesized by the reaction of 3-(2-cyanopropan-2-yl)-N-(5-(piperazine-1-yl)-1,3,4-thiadiazol-2-yl)benzamide with various sulfonyl chlorides and evaluated for their anticonvulsant activity in MES test. Rotorod method was employed to determine the neurotoxicity. The purity of the compounds is confirmed on the basis of their elemental analysis. The structures of all the new compounds are established on the basis of 1H NMR and mass spectral data. Out of fifteen compounds, three were found to be potent anticolvunstants. The same compounds showed no neurotoxicity at the maximum dose administered (100 mg/kg).

Efficient Oxidative Cyclisation of Acid Hydrazides to 2,5-Disubstituted 1,3,4-Oxadiazoles Catalysed by Bu4NI with t-BuOOH as Oxidant

Acid hydrazides or araldehyde N-acylhydrazones can be converted in good yields to, respectively, symmetrical or unsymmetrical, 2,5-disubstituted 1,3,4-oxadiazoles at 60 °C by a Bu4NI-catalysed procedure which requires the presence of a base and 2.5 equiv. of t-butyl hydroperoxide.

Cu(II) catalyzed imine C-H functionalization leading to synthesis of 2,5-substituted 1,3,4-oxadiazoles

A direct access to symmetrical and unsymmetrical 2,5-disubstituted [1,3,4]-oxadiazoles has been accomplished through an imine C-H functionalization of N-arylidenearoylhydrazide using a catalytic quantity of Cu(OTf)(2). This is the first example of amidic oxygen functioning as a nucleophile in a Cu-catalyzed oxidative coupling of an imine C-H bond. These reactions can be performed in air atmosphere and moisture making it exceptionally practical for application in organic synthesis.

Copper-mediated direct arylation of 1,3,4-oxadiazoles and 1,2,4-triazoles with aryl iodides

The copper-mediated direct arylation of 1,3,4-oxadiazoles and 1,2,4-triazoles with aryl iodides proceeds efficiently in the presence of suitable ligands and bases. This method allows the installation of a variety of aryl moieties bearing a functional group such as ketone, ester, or nitrile so as to enable the facile construction of various functionalized oxadiazole and triazole core pi systems.