1,1-Dibromo-1-alkenes as Valuable Partners in the Copper-Catalyzed Direct Alkynylation of Azoles

Recent Developments in Transition-Metal Catalyzed Direct C-H Alkenylation, Alkylation, and Alkynylation of Azoles

The transition metal-catalyzed C–H bond functionalization of azoles has emerged as one of the most important strategies to decorate these biologically important scaffolds. Despite significant progress in the C–H functionalization of various heteroarenes, the regioselective alkylation and alkenylation of azoles are still arduous transformations in many cases. This review covers recent advances in the direct C–H alkenylation, alkylation and alkynylation of azoles utilizing transition metal-catalysis. Moreover, the limitations of different strategies, chemoselectivity and regioselectivity issues will be discussed in this review.

Facile synthesis of 2-alkynyl oxazoles via a Ce(OTf)3-catalyzed cascade reaction of alkynyl carboxylic acids with tert-butyl isocyanide

We developed an efficient and novel protocol to synthesize 2-alkynyl oxazoles from tert-butyl isocyanide and alkynyl carboxylic acids. This method allowed the synthesis of diversely functionalized oxazoles under mild reaction conditions, coupled with operational simplicity and these functionalized oxazoles showed a certain degree of biological activity. Moreover, compounds 2b, 2h, 2k, 2n, 2p and 2t exhibited good anticancer activities in human gastric cancer cells (MGC803) and human bladder tumor cells (T24), with IC₅₀ below 20.0 μM.

Synthesis of alkynes from non-alkyne sources

This review contains a compilation of the literature for the synthesis of alkynes from non-alkyne sources.

Oxidant speciation and anionic ligand effects in the gold-catalyzed oxidative coupling of arenes and alkynes

The mechanism of the gold-catalyzed oxidative cross-coupling of arenes and alkynes has been studied in detail combining stoichiometric experiments with putative reaction intermediates and DFT calculations. Our data suggest that ligand exchange between the alkyne, the Au(i)-catalyst and the hypervalent iodine reagent is responsible for the formation of both an Au(i)-acetylide complex and a more reactive "non-symmetric" I(iii) oxidant responsible for the crucial Au(i)/Au(iii) turnover. Further, the reactivity of the in situ generated Au(iii)-acetylide complex is governed by the nature of the anionic ligands transferred by the I(iii) oxidant: while halogen ligands remain unreactive, acetato ligands are efficiently displaced by the arene to yield the observed Csp2-Csp cross-coupling products through an irreversible reductive elimination step. Finally, the nature of competitive processes and catalyst deactivation pathways has also been unraveled. This detailed investigation provides insights not only on the specific features of the species involved in oxidative gold-catalyzed cross couplings but also highlights the importance of both ancillary and anionic ligands in the reactivity of the key Au(iii) intermediates.

Manganese-Catalyzed Divergent Markovnikov Addition and [2+2+2] Cycloaddition of 2-Carbonyl Indanone with Terminal Alkyne

Herein, we report on manganese-catalyzed regioselective Markovnikov addition and [2+2+2] cycloaddition of 2-carbonyl indanones with terminal alkynes. This method provides an efficient approach for the construction of all-carbon quaternary centers and complex polycyclic hydrocarbon frameworks by the formation of new carbon-carbon single bonds in a regio- and stereoselective manner. Preliminary mechanistic experiments involving deuterium labeling, kinetic, catalytic, and stoichiometric reactions with plausible intermediates were performed to shed light on the reaction mechanism.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

Synthesis of 1,3-Diynes via Cadiot-Chodkiewicz Coupling of Volatile, in Situ Generated Bromoalkynes

A convenient Cadiot-Chodkiewicz protocol that facilitates the use of low molecular weight alkyne coupling partners is described. The method entails an in situ elimination from a dibromoolefin precursor and immediate subjection to copper-catalyzed conditions, circumventing the hazards of volatile brominated alkynes. The scope of this method is described, and the internal 1,3-diyne products are preliminarily evaluated in ruthenium-catalyzed azide-alkyne cycloadditions.

Nickel(ii)-catalyzed tandem C(sp2)–H bond activation and annulation of arenes with gem-dibromoalkenes

A nickel(ii)/silver(i)-catalyzed tandem C(sp²)–H activation and intramolecular annulation of arenes with dibromoalkenes has been successfully achieved, which offers an efficient approach to the 3-methyleneisoindolin-1-one scaffold. Attractive features of this system include its low cost, ease of operation, and its ability to access a wide range of isoindolinones.

A nickel(ii)/silver(i)-catalyzed tandem C(sp²)–H activation and intramolecular annulation of arenes with dibromoalkenes has been successfully achieved, which offers an efficient approach to the 3-methyleneisoindolin-1-one scaffold.

Copper-catalyzed direct C–H fluoroalkenylation of heteroarenes

Efficient copper-catalyzed direct C–H fluoroalkenylation of heterocycles using various gem-bromofluoroalkenes as electrophiles is reported for the first time.

Silver-catalysed reactions of alkynes: recent advances

Silver is a less expensive noble metal. Superior alkynophilicity due to π-coordination with the carbon-carbon triple bond makes silver salts ideal catalysts for alkyne-based organic reactions. This review highlights the progress in alkyne chemistry via silver catalysis primarily over the past five years (ca. 2010-2014). The discussion is developed in terms of the bond type formed with the acetylenic carbon (i.e., C-C, C-N, C-O, C-Halo, C-P and C-B). Compared with other coinage metals such as Au and Cu, silver catalysis is frequently observed to be unique. This critical review clearly indicates that silver catalysis provides a significant impetus to the rapid evolution of alkyne-based organic reactions, such as alkynylation, hydrofunctionalization, cycloaddition, cycloisomerization, and cascade reactions.

One-pot sequential alkynylation and cycloaddition: regioselective construction and biological evaluation of novel benzoxazole-triazole derivatives

Individually, benzoxazole and triazole moieties are of significant biological interest owing to their importance in drugs and pharmaceuticals. To assess their combined biological impact when woven into one molecule, we designed a novel, regioselective, multicomponent, one-pot (MCOP) approach for the construction of benzoxazole-linked triazoles. The synthesis has been achieved in two sequential steps involving copper-catalyzed alkynylation of benzoxazole followed by a 1,3-dipolar cycloaddition reaction. By combination of these two bioactive units into one core, a series of new benzoxazole-triazole scaffolds has been synthesized and subjected to in vitro antibacterial and anticancer evaluation. Tests against clinical isolates of Staphylococcus aureus and Escherichia coli showed potent Gram-negative activity for compounds 4{1,1,1}, 4{1,1,4}, and 4{1,2,1}. The cytotoxicity of the synthesized library was determined against three cancer cell lines: HeLa, SKBr3, and Hep G2. Compound 4{2,2,2} showed significant cytotoxicity against all the cell lines. These preliminary bioassay evaluations strongly suggest the promise and scope of these novel molecules as therapeutic agents in medical science.

Base-mediated direct fluoroalkenylation of 2-phenyl-1,3,4-oxadiazole, benzothiazole and benzoxazole with gem-difluoroalkenes

Direct α-fluorovinylation of 2-phenyl-1,3,4-oxadiazole, benzothiazole and benzoxazole with gem-difluoroalkenes under the assistance of KHMDS or NaH at room temperature was developed.

Cycloaddition-based formal C-H alkynylation of isoindoles leading to the synthesis of air-stable fluorescent 1,3-dialkynylisoindoles

Reaction of N-alkylisoindoles with (bromoethynyl)triisopropylsilane afforded 1,3-bis(triisopropylsilylethynyl) isoindoles in high yields. The formal C-H alkynylation proceeds under transition-metal-free conditions through [4 + 2] cycloaddition of the pyrrole ring of isoindole with bromoalkyne followed by ring-opening of the product.

Sequential three-component synthesis of 1,4-bis[6,9-dihydro-6-oxo-9-phenyl-1H-purin-2-yl]piperazines

A simple one-pot efficient method for the synthesis of 1,4-bis[6,9-dihydro-6-oxo-9-phenyl-1 H-purin-2-yl]piperazines by a domino three-component process involves an aza-Wittig reaction/heterocyclisation in the presence of sodium ethoxide as catalyst. Ethyl 1-phenyl-5-[(triphenylphosphoranylidene)amino]-1H-imidazole-4-carboxylate reacted with aromatic isocyanate to give carbodiimide intermediates, followed by addition of piperazines to give the corresponding guanidine intermediates. The guanidine intermediates were cyclised in the presence of a catalytic amount of sodium ethoxide to give 1,4-bis(6,9-dihydro-6-oxo-9-phenyl-1 H-purin-2-yl)piperazine derivatives in good yields.

Copper-catalyzed CuAAC/intramolecular C-H arylation sequence: Synthesis of annulated 1,2,3-triazoles

Step-economical syntheses of annulated 1,2,3-triazoles were accomplished through copper-catalyzed intramolecular direct arylations in sustainable one-pot reactions. Thus, catalyzed cascade reactions involving [3 + 2]-azide-alkyne cycloadditions (CuAAC) and C-H bond functionalizations provided direct access to fully substituted 1,2,3-triazoles with excellent chemo- and regioselectivities. Likewise, the optimized catalytic system proved applicable to the direct preparation of 1,2-diarylated azoles through a one-pot C-H/N-H arylation reaction.