Cobalt-Catalyzed Annulation Reactions of Alkylidenecyclopropanes: Access to Spirocyclopropanes at Room Temperature

Diastereoselective Synthesis of Cyclobutanes via Rh-Catalyzed Unprecedented C-C Bond Cleavage of Alkylidenecyclopropanes

The stereoselective synthesis of highly substituted cyclobutanes is essential for the development of lead candidates in drug discovery. Herein, we present a novel Rh(III)-catalyzed reaction pathway for synthesizing substituted cyclobutanes, which involves a concerted N-C bond formation and C-C bond cleavage between 2-aryl quinazolinones and alkylidenecyclopropanes. Notably, the combination of Rh(III) catalyst and HFIP solvent plays a critical role in facilitating the formation of cyclobutane rings.

Ru(II)-Catalyzed Decarboxylative (4 + 2)-Annulation of Benzoic Acids and Benzamides with Propargyl Cyclic Carbonates

Propargyl cyclic carbonates have emerged as versatile precursors in synthetic chemistry. However, their reactivity has so far been limited to transition metal-catalyzed substitution and cyclization reactions. Herein, we illustrate the successful employment of propargyl cyclic carbonates as coupling partners in Ru(II)-catalyzed C-H annulation of benzoic acids and benzamides. This approach allowed us to access a broad range of biologically relevant isocoumarin and isoquinolinone derivatives in good to excellent yields, utilizing bench-stable and easily accessible precursors. Preliminary mechanistic studies indicated that the C-H metalation step is both reversible and rate-determining in the reaction pathway. Furthermore, the utility of the developed methodology has been illustrated by scale-up and postfunctionalization experiments.

Ru(II)- or Rh(III)-Catalyzed Annulation of Aromatic/Vinylic Acids with Alkylidenecyclopropanes via C-H Activation

An efficient and new route for the synthesis of (E)-4-benzylideneisochroman-1-ones through tandem cascade annulation of benzoic acids with alkylidenecyclopropanes using Ru(II) as a catalyst is demonstrated. It is important to note that the reaction delivers selectively E-stereoselective 4-benzylideneisochroman-1-one derivatives in moderate to good yields, which has completely diverse selectivity as compared with previous methods. Further, the annulation was explored with less-reactive β C-H activation of vinylic acids with alkylidenecyclopropanes, leading to the highly useful α-pyrone derivatives in the presence of an Rh(III) catalyst.

Cobalt-catalyzed highly diastereoselective [3 + 2] carboannulation reactions: facile access to substituted indane derivatives

Efficient oxidative [3 + 2] annulation reaction involving aryl hydrazones and heterobicyclic alkenes has been realized with inexpensive and earth-abundant cobalt salts under aerobic conditions. The reaction proceeds via directing-group-assisted C-H activation and exo-selective migratory insertion, followed by the intramolecular nucleophilic attack of the alkylcobalt(III) species onto the imine with high anti-diastereoselectivity to provide complex indane derivatives. The generation of three contiguous stereogenic centers within the indanyl unit and the avoidance of the use of stoichiometric amounts of metal oxidants make this transformation more valuable and appealing.

Synthesis of Spirocyclopropylpyrazole Derivatives via the Cascade Reaction of Alkylidenecyclopropanes with Pyrazolidinones and Trifluoroethanol

A novel synthesis of spirocyclopropylpyrazole derivatives tethered with a trifluoromethyl unit through an unprecedented cascade reaction of alkylidenecyclopropanes with pyrazolidinones and trifluoroethanol (TFE) has been developed. Mechanistically, the reaction involves...

Recent advances in transition metal-catalyzed heteroannulative difunctionalization of alkenes via C-H activation for the synthesis of heterocycles

Heterocyclic compounds are the fundamental structural motifs distributed in natural products, pharmaceuticals and biologically active compounds. Thus, there is increasing interest in the development of novel synthetic strategies for the...

Rh(iii)-Catalyzed dienylation and cyclopropylation of indoles at the C4 position with alkylidenecyclopropanes

Herein, we report a Rh(iii)-catalyzed C–H functionalization of indoles at the C4 position with alkylidenecyclopropanes (ACPs).

Recent advances in directed sp2 C-H functionalization towards the synthesis of N-heterocycles and O-heterocycles

Heterocyclic compounds are widely present in the core structures of several natural products, pharmaceuticals and agrochemicals, and thus great efforts have been devoted to their synthesis in a mild and simpler way. In the past decade, remarkable progress has been made in the field of heterocycle synthesis by employing C-H functionalization as an emerging synthetic strategy. As a complement to previous protocols, transition metal catalyzed C-H functionalization of arenes using various directing groups has recently emerged as a powerful tool to create different classes of heterocycles. This review is mainly focussed on the recent key progress made in the field of the synthesis of N,O-heterocycles from olefins and allenes by using nitrogen based and oxidizing directing groups.

Three-Component Couplings among Heteroarenes, Difluorocyclopropenes, and Water via C-H Activation

Three-component couplings have been realized for efficiently constructing various nitrogen-containing skeletons via C-H activation, where difluorocyclopropenes have been first identified as coupling partners. Many substrates including sp² and sp³ C-H substrates were well tolerated, furnishing the corresponding products in good yields. Furthermore, a catalyst-dependent reaction was also developed, enabling divergent construction of two different frameworks. The application value of these reactions was demonstrated in gram-scale experiments with as little as 1 mol % catalyst.

Rh(III)-Catalyzed Stereoselective C-C Bond Cleavage of ACPs with N-Phenoxyacetamides: The Critical Role of the Nucleophilic Directing Group

Rh(III)-catalyzed redox-neutral chemodivergent coupling of N-phenoxyacetamides and alkylidenecyclopropanes (ACPs) has been documented. The reaction proceeds via C-H activation, regioselective migratory insertion and stereoselective β-carbon elimination followed by β-hydride elimination, resulting in o-dienylation of phenols in nonpolar solvents, whereas [3 + 2]-annulation leading to dihydrobenzofurans was realized in polar fluorinated solvents. It was observed that the nucleophilic directing group controls the elimination of β-carbon and so plays a vital role for achieving high stereoselectivities. The synthetic utility of the dienylation and annulation was demonstrated by carrying out gram scale reactions and further derivatization.

Cobalt-Catalyzed C-H Activation and [3 + 2] Annulation with Allenes: Diastereoselective Synthesis of Indane Derivatives

An unprecedented bidentate directing-group-assisted cobalt-catalyzed oxidative C-H activation of aryl hydrazones followed by a syn-diastereoselective [3 + 2] annulation reaction has been achieved, employing allenes as the annulation partners. The selective 2,3-migratory insertion of allenes with arylcobalt(III) species and the subsequent intramolecular diastereoselective nucleophilic addition of η1-allylcobalt onto the imine resulted in [3 + 2] annulation over the alternative [4 + 2] annulation. Furthermore, the oxidative annulation obviates the need for stoichiometric metal oxidants and proceeds under aerobic conditions.

Chemodivergent assembly of ortho-functionalized phenols with tunable selectivity via rhodium(III)-catalyzed and solvent-controlled C-H activation

Ortho-functionalized phenols and their derivatives represent prominent structural motifs and building blocks in medicinal and synthetic chemistry. While numerous synthetic approaches exist, the development of atom-/step-economic and practical methods for the chemodivergent assembly of diverse ortho-functionalized phenols based on fixed catalyst/substrates remains challenging. Here, by selectively controlling the reactivities of different sites in methylenecyclopropane core, Rh(III)-catalyzed redox-neutral and tunable C-H functionalizations of N-phenoxyacetamides are realized, providing access to both ortho-functionalized phenols bearing linear dienyl, cyclopropyl or allyl ether groups, and cyclic 3-ethylidene 2,3-dihydrobenzofuran frameworks under mild cross-coupling conditions. These divergent transformations feature broad substrate compatibility, synthetic applications and excellent site-/regio-/chemoselectivity. Experimental and computational mechanistic studies reveal that distinct catalytic modes involving selective β-C/β-H elimination, π-allylation, inter-/intramolecular nucleophilic substitution cascade and β-H' elimination processes enabled by different solvent-mediated and coupling partner-controlled reaction conditions are crucial for achieving chemodivergence, among which a structurally distinct Rh(V) species derived from a five-membered rhodacycle is proposed as the corresponding active intermediates.

Cobalt(iii)-catalyzed redox-neutral [4+2]-annulation of N-chlorobenzamides/acrylamides with alkylidenecyclopropanes at room temperature

An efficient synthesis of substituted 3,4-dihydroisoquinolinones through [4+2]-annulation of N-chlorobenzamides/acrylamides having a monodentate directing group with alkylidenecyclopropanes in the presence of a less expensive, highly abundant and air stable Co(iii) catalyst via a C-H activation is demonstrated. In this reaction, the N-Cl bond of N-chlorobenzamide serves as an internal oxidant and thus an external metal oxidant is avoided. The 3,4-dihydroisoquinolinone derivatives are converted successfully into the highly useful imidoyl chloride derivatives. The deuterium labeling and kinetic isolabelling studies reveal that the C-H activation is a rate-determining step in this cyclization reaction.

Rh(III)-Catalyzed Chemodivergent Coupling of N-Phenoxyacetamides and Alkylidenecyclopropanes via C-H Activation

Rh(III)-catalyzed C-H activation of N-phenoxyacetamides and chemodivergent coupling to alkylidenecyclopropanes (ACPs) have been accomplished. With the assistance of the ring strain of ACPs, the coupling can be transannulative or nonannulative, delivering 3-ethylidenedihydrobenzofurans or dienes, respectively, under different reaction conditions, and the selectivity is mainly solvent-controlled. All of the reactions proceeded under mild conditions with a good substrate scope and excellent chemo- and diastereoselectivity.

Iron-Catalyzed Ortho C-H Homoallylation of Aromatic Ketones with Methylenecyclopropanes

We report here a C-H homoallylation reaction of aromatic ketones with methylenecyclopropanes (MCPs) only using a catalytic amount of Fe(PMe₃)₄. A variety of aromatic ketones and MCPs are applicable to the reaction to form ortho-homoallylated aromatic ketones selectively via regioselective scission of the three-membered rings. The homoallylated products are amenable to further elaborations, providing functionalized 1,2-dihydronaphthalenes.

Deoxygenative C2-heteroarylation of quinoline N-oxides: facile access to α-triazolylquinolines

A metal- and additive-free, highly efficient, step-economical deoxygenative C2-heteroarylation of quinolines and isoquinolines was achieved from readily available N-oxides and N-sulfonyl-1,2,3-triazoles. A variety of α-triazolylquinoline derivatives were synthesized with good regioselectivity and in excellent yields under mild reaction conditions. Further, a gram-scale and one-pot synthesis illustrated the efficacy and simplicity of the developed protocol. The current transformation was also found to be compatible for the late-stage modification of natural products.

Highly Enantioselective Cobalt-Catalyzed (3+2) Cycloadditions of Alkynylidenecyclopropanes

Low-valent cobalt complexes equipped with chiral ligands can efficiently promote highly enantioselective (3+2) cycloadditions of alkyne-tethered alkylidenecyclopropanes. The annulation allows to assemble bicyclic systems containing five-membered rings in good yields and with excellent enantiomeric ratios. We also present a mechanistic discussion based on experimental and computational data, which support the involvement of Co^I /Co^III catalytic cycles.

Visible-light-promoted divergent functionalizations of methylenecyclopropanes

Visible-light-induced monofunctionalization and difunctionalization of MCPs via ring-opening and cyclization processes have been developed for the syntheses of difluoromethyl compounds, alkyl compounds, halides, and sulfonyl compounds.

Cobalt-Catalyzed Oxidative C-H Activation: Strategies and Concepts

Inexpensive cobalt-catalyzed oxidative C-H functionalization has emerged as a powerful tool for the construction of C-C and C-Het bonds, which offers unique potential for transformative applications to modern organic synthesis. In the early stage, these transformations typically required stoichiometric and toxic transition metals as sacrificial oxidants; thus, the formation of metal-containing waste was inevitable. In contrast, naturally abundant molecular O₂ has more recently been successfully employed as a green oxidant in cobalt catalysis, thus considerably improving the sustainability of such transformations. Recently, a significant momentum was gained by the use of electricity as a sustainable and environmentally benign redox reagent in cobalt-catalyzed C-H functionalization, thereby preventing the consumption of cost-intensive chemicals while at the same time addressing the considerable safety hazards related to the use of molecular oxygen in combination with flammable organic solvents. Considering the unparalleled potential of the aforementioned approaches for sustainable green synthesis, this Review summarizes the recent progress in cobalt-catalyzed oxidative C-H activation until early 2020.

Copper mediated C(sp2)-H amination and hydroxylation of phosphinamides

Copper mediated C(sp2)-H amination and hydroxylation of arylphosphinic acid are accomplished by adopting phosphinamide as the directing group. This method is distinguished by its wide substrate scope and excellent functional group tolerance, thus allowing for the rapid preparation of organophosphorus compounds in organic synthesis.

The regioselective annulation of alkylidenecyclopropanes by Rh(iii)-catalyzed C–H/C–C activation to access spirocyclic benzosultams

The synthesis of spirocyclic benzosultams fromN-sulfonyl ketimine and alkylidenecyclopropanes under Rh(iii) catalysis has been developed. This transformation enables the formation of two C–C bonds and a double bond with highE-selectivity.

Cascade cyclization reactions of alkylidenecyclopropanes for the construction of polycyclic lactams and lactones by visible light photoredox catalysis

A facile method for the synthesis of seven- and eight-membered ring-containing polycyclic lactams and lactones by visible light photocatalysis has been developed.

Recent advances in cobalt-catalysed C–H functionalizations

Ready availability, low cost and low toxicity of cobalt salts have redirected the attention of researchers away from noble metals, such as Pd, Rh, and Ir, towards Co in the field of C–H functionalisation.

A facile method for the synthesis of dihydroquinoline-azide from the Lewis acid-catalyzed reaction of alkylidenecyclopropanes with TMSN3

A facile method for the cascade azidation of alkylidenecyclopropanes catalyzed by In(OTf)₃is developed, affording azido compounds in moderate to good yields.

Visible-light-promoted hydroxysulfonylation of alkylidenecyclopropanes: synthesis of cyclopropane-containing β-hydroxysulfones

Cyclopropane-containing β-hydroxysulfones and cyclopropyl styrenes have been synthesised from readily available ACPs and sulfuryl chloride via visible-light photoredox catalysis.