Recent Advances in Aerobic Oxidative of C-H Bond by Molecular Oxygen Focus on Heterocycles

Aerobic oxidative cross-coupling represents one of the most straightforward and atom-economic methods for construction of C-C and C-X (X=N, O, S, or P) bonds using air as a sustainable external oxidant. The oxidative coupling of C-H bonds in heterocyclic compounds can effectively increase their molecular complexity by introducing new functional groups through C-H bond activation, or by formation of new heterocyclic structures through cascade construction of two or more sequential chemical bonds. This is very useful as it can increase the potential applications of these structures in natural products, pharmaceuticals, agricultural chemicals, and functional materials. This is a representative overview of recent progress since 2010 on green oxidative coupling reactions of C-H bond using O2 or air as internal oxidant focus on Heterocycles. It aims to provide a platform for expanding the scope and utility of air as green oxidant, together with a brief discussion on research into the mechanisms behind it.

N-Iodosuccinimide-Mediated Synthesis of Benzo-Fused Bisimidazoles Enabled by a One-Pot Tandem Reaction of Fluorinated Propargyl Amidines

A N-iodosuccinimide (NIS)-mediated divergent and efficient tandem reaction between fluorinated propargyl amidines and aromatic o-diamines without any metal catalyst and additive under mild reaction conditions was developed for the synthesis of benzo-fused bisimidazoles in moderate to excellent yields. Preliminary mechanistic studies suggested that this reaction proceeded by an intermediate of secondary amine derived from 5-iodomethyl imidazole, and NIS played another role of oxidation reagent to promote the formation of a benzimidazole motif.

One-pot synthesis of dimerized arenes and heteroarenes under mild conditions using Co(i) as an active catalyst

A cheap and robust methodology for dimerization of arenes and heteroarenes with new Co(i) as an active catalyst at room temperature in a shorter time.

Oxidative C-H Homocoupling of Push-Pull Benzothiazoles: An Atom-Economical Route to Highly Emissive Quadrupolar Arylamine-Functionalized 2,2'-Bibenzothiazoles with Enhanced Two-Photon Absorption

Copper(II)-catalyzed C-H/C-H coupling of dipolar 2-H-benzothiazoles end-capped with triphenylamine moieties affords highly fluorescent 2,2'-bibenzothiazoles with quadrupolar (D-π-A-π-D) architecture displaying large two-photon absorption (TPA) cross sections (543-1252 GM) in the near-infrared region. The notably higher TPA performance as compared to quadrupolar π-systems with a widely used 2,2'-bipyridine core, along with the ease of the synthesis and chelating N^N ability, makes the title biheteroaryl platform an attractive building block for a large scope of functional dyes exploiting nonlinear optical phenomena.

N-Oxide S-O chalcogen bonding in conjugated materials

Non-covalent bonding interactions, such as chalcogen bonding, can have a substantial effect on the electronic and physical properties of conjugated polymers and is largely dependent on the strength of interaction. Functional groups that are traditionally used to instill chalcogen bonding such as alkoxy or fluorine substituents can demand challenging synthetic effort, as well as have drastic effects on the electronics of a π-system. The incorporation of a N-oxide functionality into bithiazole-containing materials, a synthetically simple transformation, has been entirely overlooked until now. A systematic analysis of the effects of N-oxidation on the electronic and physical properties of bithiazole-containing materials has been undertaken. N-Oxidation has been found to affect the electronic band gap through increase of the HOMO and lowering of the LUMO. Furthermore, exceptionally strong intramolecular S-O chalcogen bonding interactions in the bithiazole core contribute to rigidification of the conjugated system. Computational analysis of this system has shown this N-oxide chalcogen bonding interaction to be significantly stronger than other chalcogen bonding interactions commonly exploited in conjugated materials.

Homocoupling Reactions of Azoles and Their Applications in Coordination Chemistry

Pyrazole, a member of the structural class of azoles, exhibits molecular properties of interest in pharmaceuticals and materials chemistry, owing to the two adjacent nitrogen atoms in the five-membered ring system. The weakly basic nitrogen atoms of deprotonated pyrazoles have been applied in coordination chemistry, particularly to access coordination polymers and metal-organic frameworks, and homocoupling reactions can in principle provide facile access to bipyrazole ligands. In this context, we summarize recent advances in homocoupling reactions of pyrazoles and other types of azoles (imidazoles, triazoles and tetrazoles) to highlight the utility of homocoupling reactions in synthesizing symmetric bi-heteroaryl systems compared with traditional synthesis. Metal-free reactions and transition-metal catalyzed homocoupling reactions are discussed with reaction mechanisms in detail.

Step-Economical Synthesis of Conjugated Polymer Materials Composed of Three Components: Donor, Acceptor, and π Units

Conjugated polymers have immense potential for their use as semiconducting materials in organic optoelectronic devices. The improvement of synthetic methods for conjugated polymers is important for the practical application of conjugated polymers. For mass production, synthetic methods must be developed by considering the concerns regarding cost and environment. Reduction in the number of synthetic steps is an efficient approach to address these concerns. The utilization of direct CH functionalization is a reasonable strategy in monomer and polymer syntheses, because the prefunctionalization steps for CC bond formation can be eliminated. This review summarizes the recent developments in the efficient syntheses of conjugated polymers as well as their monomers via direct arylation (CH/CX coupling) and cross-dehydrogenative coupling (CH/CH coupling) reactions.

FeCl3 mediated dimerization of dihydropyrrolo[2,1-a]isoquinolines and chlorination of tetrasubstituted pyrroles

We have developed a mild and direct dimerization of dihydropyrrolo[2,1-a]isoquinolines through FeCl₃-mediated oxidative homocoupling. A series of dimeric dihydropyrroloisoquinoline derivatives were obtained in acceptable to excellent yields (up to >99%). Attempts to expand this dimerization system to the synthesis of dimeric pyrroles failed and chlorination products were obtained in most cases instead of dimeric pyrroles.

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.

Oxidative C-H/C-H Coupling Reactions between Two (Hetero)arenes

Transition metal-mediated C-H bond activation and functionalization represent one of the most straightforward and powerful tools in modern organic synthetic chemistry. Bi(hetero)aryls are privileged π-conjugated structural cores in biologically active molecules, organic functional materials, ligands, and organic synthetic intermediates. The oxidative C-H/C-H coupling reactions between two (hetero)arenes through 2-fold C-H activation offer a valuable opportunity for rapid assembly of diverse bi(hetero)aryls and further exploitation of their applications in pharmaceutical and material sciences. This review provides a comprehensive overview of the fundamentals and applications of transition metal-mediated/catalyzed oxidative C-H/C-H coupling reactions between two (hetero)arenes. The substrate scope, limitation, reaction mechanism, regioselectivity, and chemoselectivity, as well as related control strategies of these reactions are discussed. Additionally, the applications of these established methods in the synthesis of natural products and exploitation of new organic functional materials are exemplified. In the last section, a short introduction on oxidant- or Lewis acid-mediated oxidative Ar-H/Ar-H coupling reactions is presented, considering that it is a very powerful method for the construction of biaryl units and polycylic arenes.

Copper and silver benzoate and aryl complexes and their implications for oxidative decarboxylative coupling reactions

A copper(ii) benzoate complex is a viable intermediate for copper-catalyzed oxidative decarboxylative coupling reactions.

CuBr-ZnI2 Combo-Catalysis for Mild CuI-CuIII Switching and sp2 C-H Activated Rapid Cyclization to Quinolines and Their Sugar-Based Chiral Analogues: A UV-Vis and XPS Study

An unprecedented CuBr-ZnI₂ combo-catalyzed mild Cu¹-Cu^III switching activation of sp² C-H of highly electron-rich arenes is reported. Anilines, aldehydes, and terminal alkynes were rapidly coupled together at ambient temperature to construct a ubiquitous quinoline framework through cyclization of the C≡C bond. This smart solvent-free strategy was exploited for the direct synthesis of valuable 4-substituted, 2,4-disubstituted, and thermally labile sugar-based chiral quinolines in good yields. In contrast to the frequently used imine-alkyne cyclization reaction, this uncommonly mild Cu^I-Cu^III combo-catalysis for a rapid three-component cyclization is expected to proceed through the formation of a flexible propargyl amine intermediate, which provides a Cu^I-procatalyst for rapid sp² C-H activation with cyclization involving transient Cu^III species. The in situ generation of transient Cu^III species was confirmed through online ultraviolet-visible spectroscopy (UV-vis), electrospray ionization mass spectrometry (ESI-MS), and X-ray photoelectron spectroscopy (XPS) analyses.

The synthesis of hydrophobic 1-alkyl-1H,1′H-2,2′-bibenzo[d]imidazoles

New synthesis of 1-alkyl-1H,1′H-2,2′- bibenzo[d]imidazoles 3 by alkylation of a disodium derivative of 1H,1′H-2,2′-bibenzo[d]imidazole 2a with alkyl halides is proposed. It is noteworthy that 1,1′-dialkylated byproducts 4 are formed in yields of <5%. The structure of all obtained compounds has been confirmed by 1H NMR, 13C NMR, and HRMS or elemental analysis.

Aromatic C–H bond cleavage by using a Cu(i) ate-complex

Ate-complex [Cu(O^tBu)₂]Na was found to be able to cleave C–H bond of benzothiazole (ArH) producing [ArCuI(O^tBu)]Na with a rate constant of 3.2 × 10⁻² mol⁻¹ L s⁻¹ at −50 °C and with an activation enthalpy of 0.73 kcal mol⁻¹ at room temperature.

Regioselective copper-diamine-catalyzed C–H arylation of 1,2,4-triazole ring with aryl bromides

A bench-top protocol using simple Cu-diamine catalysis for the regioselective C5–H arylation of 1,2,4-triazole ring with (hetero)aryl bromides.

Ligand promoted Pd-catalyzed dehydrogenative alkenylation of hetereoarenes

An efficient Pd-catalyzed cross-dehydrogenative coupling of heteroarenes with styrenes and other olefinic substrates has been developed. This alkenylation paradigm encompasses a wide range of substrates and provides a straightforward approach toward C2-E-alkenylated azole motifs.

A room-temperature synthesis of 2,2′-bisoxazoles through palladium-catalyzed oxidative coupling of α-isocyanoacetamides

A novel one-step synthesis of both symmetric and unsymmetric 2,2′-bisoxazoles starting from simple acyclic α-isocyanoacetamides was achieved.

Copper-Catalyzed Homo-Dimerization of Nitronates and Enolates under Oxygen Atmosphere

A method for copper-catalyzed oxidative dimerization of nitronates and enolates using oxygen as terminal oxidant has been developed. Cyclization through oxidative intramolecular coupling is also feasible for both nitronates and enolates. The mild reaction conditions lead to good functional group tolerance.