Zirconium-Catalyzed Synthesis of 6-(Trifluoromethyl)-2H-pyran-2-ones via C-C Bond Cleavage

A strategy for the annulation reaction of alkynones with ethyl 4,4,4-trifluoro-3-oxobutanoate through C-C bond cleavage is described. The zirconium-catalyzed transformation provides access to a wide range of structurally diverse 6-(trifluoromethyl)-2H-pyran-2-ones in moderate to good yields, utilizing Na2CO3 as a base. Further transformations into trifluoromethylated arene derivatives have been demonstrated as well. Furthermore, plausible reaction pathways are proposed by conducting various control experiments and isolating a β-diketone intermediate (X-ray) containing an intramolecular hydrogen bond.

Continuous flow reactions in the preparation of active pharmaceutical ingredients and fine chemicals

Herein, we present an overview of continuous flow chemistry, including photoflow and electroflow technologies in the preparation of active pharmaceutical ingredients (APIs) and fine chemical intermediates. Examples highlighting the benefits and challenges associated with continuous flow processes, mainly involving continuous thermal, photo- and electrochemical transformations, are drawn from the relevant literature, especially our experience and collaborations in this area, with emphasis on the synthesis and prospective scale-up.

Recent advances in selective mono-/dichalcogenation and exclusive dichalcogenation of C(sp2)-H and C(sp3)-H bonds

Organochalcogen compounds are prevalent in numerous natural products, pharmaceuticals, agrochemicals, polymers, biological molecules and synthetic intermediates. Direct chalcogenation of C-H bonds has evolved as a step- and atom-economical method for the synthesis of chalcogen-bearing compounds. Nevertheless, direct C-H chalcogenation severely lags behind C-C, C-N and C-O bond formations. Moreover, compared with the C-H monochalcogenation, reports of selective mono-/dichalcogenation and exclusive dichalcogenation of C-H bonds are relatively scarce. The past decade has witnessed significant advancements in selective mono-/dichalcogenation and exclusive dichalcogenation of various C(sp2)-H and C(sp3)-H bonds via transition-metal-catalyzed/mediated, photocatalytic, electrochemical or metal-free approaches. In light of the significance of both mono- and dichalcogen-containing compounds in various fields of chemical science and the critical issue of chemoselectivity in organic synthesis, the present review systematically summarizes the advances in these research fields, with a special focus on elucidating scopes and mechanistic aspects. Moreover, the synthetic limitations, applications of some of these processes, the current challenges and our own perspectives on these highly active research fields are also discussed. Based on the substrate types and C-H bonds being chalcogenated, the present review is organized into four sections: (1) transition-metal-catalyzed/mediated chelation-assisted selective C-H mono-/dichalcogenation or exclusive dichalcogenation of (hetero)arenes; (2) directing group-free selective C-H mono-/dichalcogenation or exclusive dichalcogenation of electron-rich (hetero)arenes; (3) C(sp3)-H dichalcogenation; (4) dichalcogenation of both C(sp2)-H and C(sp3)-H bonds. We believe the present review will serve as an invaluable resource for future innovations and drug discovery.

C-C Bond Cleavage Mediated Reaction for Constructing 3-Carbonyl Imidazo[1,5-a] Pyridines from 1,3-Dicarbonyl Compounds and Pyridin-2-ylmethanamines

A [4 + 1] cyclization and C-C bond cleavage process mediated reaction for constructing 3-carbonyl imidazo[1,5-a] pyridines from 1,3-dicarbonyl compounds and pyridin-2-ylmethanamines has been developed. Various 1,3-dicarbonyl compounds are applicable, and selectivity could be achieved. Importantly, this strategy could be extended to an atom economy method by employing a cyclic 1,3-dicarbonyl compound, and it provided a new view for C-C bond cleavage reactions.

Divergent Synthesis of α-(1,3,5-Triazinylthio)-ketones and Thiazolo[3,2-a][1,3,5]triazines from 1,3-Dicarbonyl compounds or Chalcones with 1,3,5-Triazine-2-thiols

An efficient synthesis of α-(1,3,5-triazinylthio)-ketones from 1,3-dicarbonyl compounds with 1,3,5-triazine-2-thiols has been developed. The reaction proceeds through the C-C bond cleavage and C-S bond reconstruction of 1,3-dicarbonyl compounds, and β-keto esters, β-keto amides, and 1,3-diones were tolerated. In addition, the annulation of 1,3,5-triazine-2-thiols with chalcones has been achieved for the synthesis of thiazolo[3,2-a][1,3,5]triazines. The method occurred in moderate to good yields and tolerated chalcone with a broad functional group.

Copper-catalyzed three-component annulation toward pyrroles via the cleavage of two C-C bonds in 1,3-dicarbonyls

The first copper-catalyzed three-component annulation of α,β-unsaturated ketoximes, 1,3-dicarbonyls and paraformaldehyde has been documented. This novel strategy achieved the two C-C bond cleavage of 1,3-dicarbonyl compounds directly as a single-carbon synthon and provided a new and highly efficient method for the synthesis of 2,3-disubstituted pyrroles in moderate to good yields with broad functional group compatibility.

Organophotoredox-Catalyzed Sulfurization of Alkenes and Alkynes: Selective and Controlled Synthesis of Sulfoxides, β-Hydroxysulfoxides, and β-Keto Sulfides

A visible-light organophotoredox-catalyzed sulfurization of alkenes and alkynes with aromatic and heteroaromatic thiols using eco-friendly air (O₂) as the sole oxidant has been demonstrated. The established method delivers a novel, benign, and metal-free strategy for the difunctionalization of alkenes and alkynes using organophotoredox catalyst Eosin Y. The selective and controlled synthetic approach shows good substrate generality to afford sulfoxides, β-hydroxysulfoxides, and β-keto sulfides in high yield and excellent regioselectivity.

α-Sulfenylation between 4-Hydroxydithiocoumarin and 1,3-Dicarbonyl Compounds: A Key Precursor for the Synthesis of New Pyrazole Derivatives

An efficient synthetic protocol for the α-sulfenylation of 1,3-dicarbonyl compounds is reported through a cross dehydrogenative coupling reaction with 4-hydroxydithiocoumarins in the presence of 10 mol% KI and 1 equiv. TBHP in toluene under reflux conditions. Some of the products are utilized for the synthesis of substituted new pyrazole derivatives on reaction with phenylhydrazine in ethanol at room temperature. In addition, α-benzylation is also achieved on treatment with benzyl bromide using K2CO3/CH3CN under mild conditions. The salient features of the present protocol are good yields, mild reaction conditions, shorter reaction time, no byproducts were formed (sulfoxide/sulfone), and no deacylation occurs during the process. In the present protocol, 4-hydroxydithiocoumarin is converted into a suitable electrophile through a radical substitution pathway, which undergoes ultimately C–S bond formation with 1,3-dicarbonyl compounds by a nucleophilic substitution reaction.

Direct Synthesis of α-Sulfenylated Ketones under Electrochemical Conditions

We investigated the electrochemical sulfenylation reaction in both batch and continuous flow regimes, involving thiophenols/thiols and enol-acetates to yield α-sulfenylated ketones, without using additional oxidants or catalysts. Studies with different electrolytes were also performed, revealing that quaternary ammonium salts are the best mediators for this reaction. Notably, during the study of the reaction scope, a Boc-cysteine proved to be extremely tolerant to our protocol, thus increasing its relevance. The methodology also proved to be scalable in both batch and continuous flow conditions, opening up possibilities for further studies since these relevant functional groups are important moieties in organic synthesis.

The journey of C–S bond formation from metal catalysis to electrocatalysis

This perspective describes the journey of C–S bond constructions starting from transition metal catalysis through oxidant catalysis, photocatalysis and very recently employed electrocatalysis by using various sulfur surrogates.

Visible light-driven copper(ii) catalyzed aerobic oxidative cleavage of carbon–carbon bonds: a combined experimental and theoretical study

By switching on visible blue light, aerobic oxidation of various substrates, such as α-substituted, β-substituted and α-halo styrenes, was first realized with a copper(ii) catalyst.

Electrochemical α-Thiolation and Azidation of 1,3-Dicarbonyls

A highly efficient electrochemical α-thiolation and azidation of 1,3-dicarbonyl compounds is developed. This electrochemical process is conducted under mild conditions without the use of chemical oxidant, and exhibits a wide...

Production of Alkyl Aryl Sulfides from Aromatic Disulfides and Alkyl Carboxylates via a Disilathiane-Disulfide Interchange Reaction

The results of this study show that disilathiane is an effective mediator in the synthesis of alkyl aryl sulfides with disulfides and alkyl carboxylates. Mechanistic studies suggest that disilathiane promotes cleavage of the sulfur-sulfur bond of disulfides to generate thiosilane as a key intermediate. Diselenides were also applicable to this transformation to produce the corresponding selenides.

Direct alkylation of N,N-dialkyl benzamides with methyl sulfides under transition metal-free conditions

Amides are a fundamental and widespread functional group, and are usually considered as poor electrophiles owing to resonance stabilization of the amide bond. Various approaches have been developed to address challenges in amide transformations. Nonetheless, most methods use activated amides, organometallic reagents or transition metal catalysts. Here, we report the direct alkylation of N,N-dialkyl benzamides with methyl sulfides promoted by the readily available base LDA (lithium diisopropylamide). This approach successfully achieves an efficient and selective synthesis of α-sulfenylated ketones without using transition-metal catalysts or organometallic reagents. Preliminary mechanism studies reveal that the deprotonative aroylation of methyl sulfides is promoted by the directed ortho-lithiation of the tertiary benzamide with LDA.

Direct C-C Bond Cleavage of 1,3-Dicarbonyl Compounds as a Single-Carbon Synthon: Synthesis of 2-Aryl-4-quinolinecarboxylates

A novel [2 + 1 + 3] cyclization reaction for the synthesis of 2-aryl-4-quinolinecarboxylates from aryl methyl ketones, arylamines, and 1,3-dicarbonyl compounds has been established. This metal-free process achieved the C-C bond cleavage of 1,3-dicarbonyl compounds directly as a single-carbon synthon. The reaction is highly efficient and has good substrate compatibility while operating under mild conditions. This method has good practicability and successfully realized the synthesis of bioactive molecules.

Tris-(2-pyridylmethyl)amine-ligated Cu(II) 1,3-diketonate complexes: anaerobic retro-Claisen and dehalogenation reactivity of 2-chloro-1,3-diketonate derivatives

We report synthetic, structural and reactivity investigations of tris-(2-pyridylmethyl)amine (TPA)-ligated Cu(ii) 1,3-diketonate complexes. These complexes exhibit anaerobic retro-Claisen type C-C bond cleavage reactivity which exceeds that found in analogs supported by chelate ligands with fewer and/or weaker pyridyl interactions.

Assessing Methodologies to Synthesize α-Sulfenylated Carbonyl Compounds by Green Chemistry Metrics

α-Sulfenylated carbonyl compounds are important both as active pharmaceutical ingredients and as intermediates in organic synthesis. Owing to their relevance in synthetic organic chemistry, this Minireview focuses on assessing the most relevant synthetic procedures based on green chemistry metrics. The Minireview starts with the traditional routes and then focuses on more recently developed methodologies. These routes include sulfenylating reagents using organocatalysis, cross-dehydrogenative couplings using in situ halogenations to prevent reactive intermediates in high concentrations, oxidative couplings using terminal oxidants such as DDQ or TEMPO, and redox-neutral couplings using transition metal catalysis. These methodologies have been evaluated on the basis of atom economy, E factor, and the safety and toxicity of the transformations and the solvents used. Besides using green metrics to evaluate these novel methodologies, the synthetic utility is also assessed with regard to the availability of starting materials and the generality of the reactions. This Minireview aims to inspire researchers to apply green assessments to other methodologies and also for them to take measures to increase the greenness of their developed transformations.

Selective synthesis of α-organylthio esters and α-organylthio ketones from β-keto esters and sodium S-organyl sulfurothioates under basic conditions

We described herein a selective method to prepare α-organylthio esters and α-organylthio ketones by the reaction of β-keto esters with sodium S-benzyl sulfurothioate or sodium S-alkyl sulfurothioate (Bunte salts) under basic conditions in toluene as the solvent at 100 °C. When 4 equivalents of a base were used, a series of differently substituted α-thio esters were obtained with up to 90% yield. On the other hand, employing 2 equivalents of a base, α-thio ketones were achieved after 18 h under air. Furthermore, after a shorter reaction time, the isolation of keto-enol tautomers was possible, revealing them as significant intermediates for the mechanism elucidation.

Chemoselective α-Sulfidation of Amides Using Sulfoxide Reagents

The direct α-sulfidation of tertiary amides using sulfoxide reagents under electrophilic amide activation conditions is described. Employing convenient and readily available reagents, selective functionalization takes place to generate isolable sulfonium ions en route to α-sulfide amides. Mechanistic studies identified activated sulfoxides as promoters of the desired transformation and enabled the extension of the methodology from benzylic to aliphatic amide substrates.

Selective C–C bond cleavage of amides fused to 8-aminoquinoline controlled by a catalyst and an oxidant

Herein, copper-catalyzed direct C–C bond cleavage of amides fused to 8-aminoquinoline as a directing group to form urea in the presence of amines and dioxygen is reported.

Microwave assisted synthesis of β-keto thioethers and furan derivatives by thiol directed multicomponent reactions

A series of β-keto thioethers tethered with cyclic-1,3-dicarbonyls (4) and a tetra-substituted furan derivative (5) has been prepared by multicomponent reaction under microwave heating conditions.

Ruthenium(ii)-catalyzed chemoselective deacylative annulation of 1,3-diones with sulfoxonium ylides via C-C bond activation

The first successful example of deacylative annulation of 1,3-diones with sulfoxonium ylides was achieved through Ru(ii)-catalyzed C-C bond activation. The excellent chemoselectivity and broad substrate scope render this method a practical and versatile approach for the preparation of (hetero)aryl and alkenyl substituted furans, which are valuable units in many biologically active compounds and functional materials. A preliminary mechanistic study reveals that this process involves a deacylative α-ruthenation to generate key alkyl Ru(ii) intermediates with the release of a benzoic acid fragment.

Thiol-free chemoenzymatic synthesis of β-ketosulfides

A preparation of β-ketosulfides avoiding the use of thiols is described. The combination of a multicomponent reaction and a lipase-catalysed hydrolysis has been developed in order to obtain high chemical diversity employing a single sulfur donor. This methodology for the selective synthesis of a set of β-ketosulfides is performed under mild conditions and can be set up in one-pot two-step and on a gram-scale.

Synthesis of α-arylthioacetones using TEMPO as the C3 synthon via a reaction cascade of sequential oxidation, skeletal rearrangement and C-S bond formation

Here, we present an unprecedented pathway to α-sulfenylated carbonyl compounds from commercially available thiols and universally employed TEMPO and its analogues, which act as C3 synthons through skeletal rearrangement under simple and metal-free conditions. Mechanism studies suggest that this reaction involves a consecutive radical oxidation and cation coupling process. TEMPO analogues and thiols serve as oxidants and reductive reagents, respectively, along the radical process, while in the coupling process, the former ones afford C3 synthons to couple with related sulfur sources.

Substrate-switched dual functionalization of alkenes: catalyst-free synthetic route for β-hydroxy and β-keto thioethers

A substrate-controlled dual functionalization of alkenes under catalyst-free and solvent-free conditions to provide β-hydroxy thioethers and β-keto thioethers is described.

One pot preparation of α-dithioacetal/α-diselenoacetal amides via a dual-C–S/C–Se bond formation and C–C bond cleavage cascade of 3-oxo-butanamides

A new and practical approach to α-dithioacetal/α-diselenoacetal amides has been developed.

1,4-Refunctionalization of β-diketones to γ-keto nitriles via C–C single bond cleavage

The 1,4-refunctionalization of β-diketones to γ-keto nitriles was realized via an Fe-catalyzed cascade radical process.

Direct access to α-sulfenylated amides/esters via sequential oxidative sulfenylation and C–C bond cleavage of 3-oxobutyric amides/esters

An efficient, environmentally benign and unprecedented synthesis of various α-sulfenylated amides/esters has been developed under oxygen atmosphere.

Cs2CO3-promoted cross-dehydrogenative coupling of thiophenols with active methylene compounds

α-Sulfenylation of carbonyl compounds has been achieved via Cs₂CO₃-promoted aerobic cross-dehydrogenative coupling of thiophenols with active methylene compounds.

Regioselective thiolation of electron rich arenes and heterocycles in recyclable catalytic media

A convenient and novel approach has been developed for the synthesis of unsymmetrical diaryl sulfides by the reaction of sulfonyl hydrazides with phenols, aromatic amines and heterocycles using a [Bmim][Br] ionic liquid through C–S bonds formation.

Aerobic Copper-Mediated Domino Three-Component Approach to 2-Aminobenzothiazole Derivatives

An unprecedented three-component reaction involving a 2,2'-diaminodiaryl disulfide, copper cyanide, and an electrophile is described. This transformation is based on an oxidative copper-mediated S-cyanation as a key step and involves a cyanation/cyclization/acylation domino sequence enabling a rapid and efficient synthesis of diversely substituted 2-aminobenzothiazole derivatives. Notably, this reaction proceeds via an original mechanism involving an intermolecular migration of the acyl group.