A [4 + 1] Cyclative Capture Access to Indolizines via Cobalt(III)-Catalyzed Csp2–H Bond Functionalization

Enantioselective synthesis of α-tetrasubstituted (1-indolizinyl) (diaryl)-methanamines via chiral phosphoric acid catalysis

An enantioselective Friedel-Crafts reaction of cyclic α-diaryl N-acyl imines with indolizines catalyzed by a chiral spirocyclic phosphoric acid has been developed. The asymmetric transformation proceeds smoothly to afford α-tetrasubstituted (1-indolizinyl) (diaryl)methanamines in good yields with up to 98% ee under mild conditions.

Chiral Phosphoric Acid-Catalyzed Asymmetric Friedel-Crafts Addition of Indolizine to Cyclic N-Sulfonyl Imine

A highly efficient chiral phosphoric acid-catalyzed enantioselective Friedel-Crafts addition of indolizine to cyclic N-sulfonyl imine has been established. The newly developed protocol, which probably proceeds via a monoactivation reaction pathway, allows the access of enantioenriched sulfonamide functionalized indolizines with excellent yields (up to 99%) and enantioselectivities (up to 99%). Moreover, the synthetic utility of this protocol has been explored with some chemical transformations.

Rh(III)-Catalyzed Synthesis of Substituted Isoindoles through a Direct C-H Activation/[4 + 1] Annulation and Acyl Migration Cascade of Oxadiazolones with Diazo Compounds

A Rh(III)-catalyzed C-H bond activation for the synthesis of fused 2H-isoindole scaffolds from oxadiazolones with diazo compounds was developed. The reaction proceeded through C-H activation of oxadiazolones/[4 + 1] annulation, intramolecular cyclization, and an unusual acyl migration cascade to afford target scaffolds with good yields. These 2H-isoindole derivatives could be further transformed into intriguing drug privileged scaffolds.

Enantioselective synthesis of α-tetrasubstituted (3-indolizinyl) (diaryl)methanamines via chiral phosphoric acid catalysis

An enantioselective Friedel-Crafts reaction of cyclic α-diaryl N-acyl imines with indolizines catalyzed by a chiral spirocyclic phosphoric acid has been developed. The asymmetric transformation proceeds smoothly to afford α-tetrasubstituted (3-indolizinyl) (diaryl)methanamines in good yields with up to 98% ee under mild conditions.

The Kröhnke synthesis of benzo[a]indolizines revisited: towards small, red light emitters

Benzo[a]indolizines with an ordered arrangement of various electron-withdrawing substituents (NO2, CF3, CN, CO2R and COPh) were prepared directly from pyridinium salts and chloronitroarenes, allowing for refined control of the photophysical...

Iodine-Mediated Domino Cyclization for One-Pot Synthesis of Indolizine-Fused Chromones via Metal-Free sp3 C-H Functionalization

An efficient method for the synthesis of new indolizine-fused chromones has been accomplished from ethyl (E)-3-(2-acetylphenoxy)acrylates and pyridines in a "one-pot" manner. Facile operation in open-air, metal-free, and mild conditions renders this protocol particularly practical and attractive. Moreover, this method can simultaneously construct two molecular fragments of chromone and indolizine. Scale-up experiment and the construction of natural products further prove the practicability of this strategy.

Chiral Phosphoric Acid Catalyzed Desymmetrization of Cyclopentendiones via Friedel-Crafts Conjugate Addition of Indolizines

An organocatalytic highly diastero- and enantioselective Friedel-Crafts conjugate addition of indolizines to prochiral cyclopentenediones has been successfully developed. This desymmetric transformation provides a direct access to the desired indolizine-substituted cyclopentanediones in yields of 62-91% and excellent stereoselectivities. The utility of the approach was demonstrated by diverse late-stage functionalizations through reduction or oxidation. Importantly, the direct sp² C-H functionalization with nitromethane in one-pot process resulted in the indolizine-linked axially chiral styrene bearing a remote chiral center.

Cobalt-Catalyzed Vinylic C-H Addition to Formaldehyde: Synthesis of Butenolides from Acrylic Acids and HCHO

A carboxyl-assisted C-H functionalization of acrylic acids with formaldehyde to give butenolides is described. It is the first time that the addition of an inert vinylic C-H bond to formaldehyde has been achieved via cobalt-catalyzed C-H activation. The unique reactivity of the cobalt species was observed when compared with related Rh or Ir catalysts. γ-Hydroxymethylated butenolides were produced by the treatment of Na₂CO₃ after the catalytic reaction in one pot.

C-H bond functionalization by high-valent cobalt catalysis: current progress, challenges and future perspectives

Over the last decade, high-valent cobalt catalysis has earned a place in the spotlight as a valuable tool for C-H activation and functionalization. Since the discovery of its unique reactivity, more and more attention has been directed towards the utilization of cobalt as an alternative to noble metal catalysts. In particular, Cp*Co(III) complexes, as well as simple Co(II) and Co(III) salts in combination with bidentate chelation assistance, have been extensively used for the development of novel transformations. In this review, we have demonstrated the existing trends in the C-H functionalization methodology using high-valent cobalt catalysis and highlighted the main challenges to overcome, as well as perspective directions, which need to be further developed in the future.

[3 + 2]-Annulation of gem-Difluoroalkenes and Pyridinium Ylides: Access to Functionalized 2-Fluoroindolizines

A [3 + 2]-annulation of gem-difluoroalkenes and pyridinium ylides was developed employing ambient air as the sole oxidant in an open-vessel manner, affording a series of multifunctionalized 2-fluoroindolizines in moderate to good yields. In this reaction, gem-difluoroalkene acts as a C2 synthon and entirely avoids the competitive addition-elimination process, which provides facile access to 2-fluoroindolizines.

Ru(II)-Catalyzed C-H Hydroxyalkylation and Mitsunobu Cyclization of N-Aryl Phthalazinones

Ruthenium(II)-catalyzed C(sp²)-H functionalization of N-aryl phthalazinones with a range of aldehydes and activated ketone is described. Initial formation of hydroxyalkylated phthalazinones and subsequent Mitsunobu cyclization provided facile access to biologically relevant indazolophthalazinones. The utility of this method is highlighted by synthetic transformations into a series of potentially bioactive scaffolds.

Synthesis of Benzofulvenes via Cp*Co(III)-Catalyzed C-H Activation and Carbocyclization of Aromatic Ketones with Internal Alkynes

A highly efficient and practical synthesis of benzofulvenes was developed via ketone-directed Cp*Co(III)-catalyzed sequential C-H activation, addition, cyclization, and dehydration cascade processes between simple aromatic ketones and alkynes. The reaction tolerates a variety of functional groups, and various benzofulvenes were efficiently synthesized in 42-92% yields.

Synthesis of 1-naphthols via Cp*Co(iii)-catalyzed C–H activation and cyclization of sulfoxonium ylides with alkynes

A highly practical synthesis of 1-naphthols was developed via Cp*Co(iii)-catalyzed C–H activation and cyclization between sulfoxonium ylides and alkynes.

Cobalt(iii)-catalyzed C–H amidation of weakly coordinating sulfoxonium ylides and α-benzoylketene dithioacetals

Cobalt(iii)-catalyzed mild C–H amidation of weakly coordinating sulfoxonium ylides and α-benzoylketene dithioacetals has been realized.

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.

C-H functionalization by high-valent Cp*Co(iii) catalysis

Significant progress has been accomplished in directed C-H functionalization through the use of earth-abundant and inexpensive first-row transition metals. Among these base metals, Co is especially attractive in view of its versatile applications in C-H functionalization, in both low- and high-valent states. In this vein, catalytic Co(iii) species can be generated from the dissociation of a Cp*Co(iii) catalyst or through the oxidation of a low-valent cobalt catalyst in the presence of an oxidant. In this feature article, we will discuss the breakthroughs in Cp*Co(iii)-promoted C-H functionalization. In this field, C(sp²)-H functionalization has been extensively studied and developed. In contrast, few C(sp³)-H functionalization reactions have been reported.

Cobalt(iii)-catalyzed 1,4-addition of C–H bonds of oximes to maleimides

An oxime directed cobalt-catalyzed sp² C–H bond addition reaction was developed with a broad substrate scope and no external additives were needed.

Rhodium(III)-Catalyzed Oxadiazole-Directed Alkenyl C-H Activation for Synthetic Access to 2-Acylamino and 2-Amino Pyridines

We report herein a Rh(III)-catalyzed alkenyl C-H activation protocol for the coupling of oxadiazoles with alkynes and synthesis of 2-acylamino and 2-amino pyridines, an important heterocyclic scaffold for various naturals products and synthetic pharmaceuticals bearing a readily reacting functional group. The selective protection/deprotection of amino groups through simple solvent switching, good functional group compatibility, superior product yield, and high regioselectivity are some of the notable synthetic features witnessed in this reaction protocol.

Atom-Economical Palladium Carbon-Catalyzed de Novo Synthesis of Trisubstituted Nicotinonitriles

A de novo palladium carbon-catalyzed synthesis of trisubstituted nicotinonitriles from easily synthesized homopropagylic or homoallylic aromatic alcohols in the presence of nitriles has been explored. The mechanism proceeds with an interesting generation of a Pd(II)-C palladacycle followed by an oxidative aromatization to generate the pyridine core. The pyridine core is generated with a noteworthy C-C bond cleavage in the case of the substituted nitriles. The moderate yields and easy separation of the products lend a unique importance to this one-pot methodology.

Direct Functionalization of C-H Bonds by Iron, Nickel, and Cobalt Catalysis

Non-precious-metal-catalyzed reactions are of increasing importance in chemistry due to the outstanding ecological and economic properties of these metals. In the subfield of metal-catalyzed direct C-H functionalization reactions, recent years have shown an increasing number of publications dedicated to this topic. Nickel, cobalt, and last but not least iron, have started to enter a field which was long dominated by precious metals such as palladium, rhodium, ruthenium, and iridium. The present review article summarizes the development of iron-, nickel-, and cobalt-catalyzed C-H functionalization reactions until the end of 2016, and discusses the scope and limitations of these transformations.

Rhodium(iii)-catalyzed indole-directed carbenoid aryl C–H insertion/cyclization: access to 1,2-benzocarbazoles

A rhodium(iii)-catalyzed indole-directed aryl C–H bond carbenoid insertion cascade of 2-arylindoles with diazo compounds has been developed.

Recent advances using [Cp*Co(CO)I2] catalysts as a powerful tool for C–H functionalisation

This perspective highlights recent applications of Cp*Co^III catalysts in C–H functionalisation protocols, exemplifying both terminal couplings and heterocycle formation.

Application of primary halogenated hydrocarbons for the synthesis of 3-aryl and 3-alkyl indolizines

A transition-metal-free synthetic route to 3-aryl and 3-alkyl indolizines from electron-deficient alkenes, pyridines and primary halogenated hydrocarbons is reported for the first time using a tandem reaction.

The one-pot synthesis of quinolines via Co(iii)-catalyzed C–H activation/carbonylation/cyclization of anilines

A new and versatile Co(iii)-catalyzed C–H activation/carbonylation/cyclization cascade reaction has been disclosed.