Computational Studies of Reactions of 1,2,4,5-Tetrazines with Enamines in MeOH and HFIP.

The reaction between 1,2,4,5-tetrazines and alkenes in polar solvents proceeds through a Diels-Alder cycloaddition along the C-C axis (C3/C6 cycloaddition) of the tetrazine, followed by dinitrogen loss. By contrast, the reactions of 1,2,4,5-tetrazines with enamines in hexafluoroisopropanol (HFIP) give 1,2,4-triazine products stemming from a formal Diels-Alder addition across the N-N axis (N1/N4 cycloaddition). We explored the mechanism of this interesting solvent effect through DFT calculations in detail and revealed a novel reaction pathway characterized by C-N bond formation, deprotonation, and a 3,3-sigmatropic rearrangement. The participation of an HFIP molecule was found to be crucial to the N1/N4 selectivity over C3/C6 due to the more favored initial C-N bond formation than C-C bond formation.

Synthesis of α-Difluoromethylene Ethers via Photoredox-Induced Hyperconjugative Ring Opening of gem-Difluorocyclopropanes.

Fluorinated compounds have found widespread applications in pharmaceuticals, agrochemicals, and materials science. Precise construction of α-difluoromethylene ether (CF2-O) moiety in organic molecules is of high demand. Herein, a visible light-promoted reaction protocol for the synthesis of α-difluoromethylene ether from gem-difluorocyclopropane is described. The key ring-opening step is induced by hyperconjugative interaction of cyclopropane with photo-oxidized aromatic rings. This reaction is easy scale-up, and the products bearing a synthetic handle enable their further manipulation.

Photoredox-Catalyzed gem-Difluoromethylenation of Aliphatic Alcohols with 1,1-Difluoroalkenes to Access α,α-Difluoromethylene Ethers.

A switchable synthesis of alcohols and ketones bearing a CF2-OR scaffold using visible-light promotion is described. The method of PDI catalysis is characterized by its ease of operation, broad substrate scopes, and the ability to switch between desired products without the need for transition metal catalysts. The addition or absence of a base plays a key role in controlling the synthesis of the major desired products.

Photoredox Catalysis-Enabled C-H Difluoromethylation of Heteroarenes with Pentacoordinate Phosphorane as the Reagent.

Difluoromethylated heterocyclic compounds have found broad applications in numerous bioactive molecules. Herein, we report photoredox catalysis-induced direct C-H difluoromethylation of heterocycles by using bis(difluoromethyl) pentacoordinate phosphorane (PPh3(CF2H)2, 1) as the reagent. A variety of heterocycles, such as quinoxalin-2(1H)-one, thiophene, indole, and coumarin, are readily tailored with a difluoromethyl group. The method is featured as transition-metal-free by using an organic compound Erythrosin B as the catalyst and O2 as the oxidant.

Divergent Construction of Azaheterocycles via Alkoxyl Radical-Triggered C-C Bond Cleavage/Cyclization of N-Functionalized Acrylamides.

An array of redox-neutral alkylation/cyclization cascade reactions of N-functionalized acrylamides with cycloalkyl hydroperoxides were achieved via the alkoxyl radical-triggered C-C bond cleavage. Through adjusting the radical acceptors on the N atom, a variety of keto-alkylated chain-containing azaheterocycles, including indolo[2,1-a]isoquinolin-6(5H)-ones, quinoline-2,4-diones, and pyrido[4,3,2-gh]phenanthridines were constructed by a one-pot procedure with good yields and excellent functional group tolerance.

Iron-catalyzed alkoxyl radical-mediated C-C bond cleavage/phosphorothiolation: a new approach to functionalized S-alkyl phosphorothioates.

An efficient iron-catalyzed alkoxyl radical-mediated C-C bond cleavage/phosphorothiolation cascade is presented. This protocol features mild and redox neutral conditions, wide substrate scope and easy scalability, allowing straightforward access to functionalized S-alkyl organophosphorus compounds in moderate to good yields.

Synthesis of Axially Chiral Biaryls via Enantioselective Ullmann Coupling of ortho-Chlorinated Aryl Aldehydes Enabled by a Chiral 2,2'-Bipyridine Ligand.

The first nickel-catalyzed highly enantioselective reductive Ullmann coupling of ortho-chlorinated aryl aldehyde was achieved under mild reaction conditions with a chiral 2,2'-bipyridine ligand (+)-DTB-SBpy, thus providing axially chiral biphenyl or binaphthyl dials with up to 99 % yield and 99.5:0.5 er. The versatility of the products as common synthetic intermediates for diverse axially chiral ligands, catalysts, and functional molecules was demonstrated by short-step transformations. This protocol also allowed the concise and highly enantioselective formal total synthesis of biologically active natural products (+)-kotanin, (-)-isoschizandrin and (+)-gossypol.

Nucleophilicities and Nucleofugalities of Thio- and Selenoethers.

Rate constants for the reactions of dialkyl chalcogenides with laser flash photolytically generated benzhydrylium ions have been measured photometrically to integrate them into the comprehensive benzhydrylium-based nucleophilicity scale. Combining these rate constants with the previously reported equilibrium constants for the same reactions provided the corresponding Marcus intrinsic barriers and made it possible to quantify the leaving group abilities (nucleofugalities) of dialkyl sulfides and dimethyl selenide. Due to the low intrinsic barriers, dialkyl chalcogenides are fairly strong nucleophiles (comparable to pyridine and N-methylimidazole) as well as good nucleofuges; this makes them useful group-transfer reagents.

Visible Light-Driven, Copper-Catalyzed Aerobic Oxidative Cleavage of Cycloalkanones.

A visible light-driven, copper-catalyzed aerobic oxidative cleavage of cycloalkanones has been presented. A variety of cycloalkanones with varying ring sizes and various α-substituents reacted well to give the distal keto acids or dicarboxylic acids with moderate to good yields.

Iron-Catalyzed Thiolation and Selenylation of Cycloalkyl Hydroperoxides via C-C Bond Cleavage.

A cheap iron-catalyzed C-C bond cleavage/thiolation and selenylation of cycloalkyl hydroperoxides are presented. This redox-neutral protocol provides efficient access to diverse distal keto-functionalized thioethers and selenium compounds. Remarkably, only some amounts of disulfides are required for this transformation.

Metal-Free, Visible-Light-Induced Selective C-C Bond Cleavage of Cycloalkanones with Molecular Oxygen.

A metal-free, visible-light-induced oxidative C-C bond cleavage of cycloketones with molecular oxygen is described. Cooperative Brønsted-acid catalysis and photocatalysis enabled selective C-C bond cleavage of cycloketones to generate an array of γ-, δ- and ϵ-keto esters under very mild conditions. Mechanistic studies indicate that singlet molecular oxygen (1 O2 ) is responsible for this transformation.

Nickel-catalyzed Suzuki Coupling of Cycloalkyl Silyl Peroxides with Boronic Acids.

A nickel-catalyzed Suzuki alkyl-aryl coupling of cycloalkyl silyl peroxides with boronic acids is reported. The primary and secondary ketoalkyl electrophiles generated through C-C bond cleavage were amenable, providing rapid access to a variety of distal arylated alkyl ketones. A radical pathway is proposed for this reaction.

Copper-Catalyzed Cyanoalkylation of Amines via C-C Bond Cleavage: An Approach for C(sp3)-N Bond Formations.

The efficient copper-catalyzed cyanoalkylation of amines via C-C bond cleavage has been demonstrated. Distinctive features of this procedure involves mild conditions, broad range of nitrogen nucleophiles, high selectivity, and good functional group tolerance, thus providing a useful approach for the C(sp3)-N bond formations. Most importantly, this protocol is applicable to the late-stage functionalization of natural products, amino acid esters, and drugs. Mechanistic studies suggest that a radical intermediate was involved in this transformation.

Iminyl Radical-Triggered 1,5-Hydrogen-Atom Transfer/Heck-Type Coupling by Visible-Light Photoredox Catalysis.

An efficient iminyl radical-triggered 1,5-hydrogen-atom transfer/Heck-type coupling cascade has been achieved through visible-light photoredox catalysis. A variety of unactivated C(sp3)-H bonds have been alkenylated efficiently and selectively with easily available alkenes, providing an elegant route to γ-alkenylated ketone.

Redox-Neutral Cyanoalkylation/Cyclization of Olefinic 1,3-Dicarbonyls with Cycloketone Oxime Esters: Access to Cyanoalkylated Dihydrofurans.

Metal-catalyzed cyanoalkylation/cyclization of olefinic 1,3-dicarbonyls with cycloketone oxime esters has been developed under redox-neutral conditions. This protocol provided a straightforward approach to diverse cyanoalkylated 2,3-dihydrofurans via a tandem ring-opening/addition/cyclization process.

Metal-Free, Visible-Light-Promoted Decarboxylative Radical Cyclization of Vinyl Azides with N-Acyloxyphthalimides.

A visible-light-mediated decarboxylative cyclization of N-acyloxylphthalimides with vinyl azides has been developed under metal-free conditions. This protocol features mild conditions, a broad substrate scope, and an excellent functional group tolerance, thus providing a facile and efficient access to substituted phenanthridines. Control experiments revealed that the reaction proceeded via a radical process.

Visible-Light-Mediated Dual Decarboxylative Coupling of Redox-Active Esters with α,ß-Unsaturated Carboxylic Acids.

An efficient visible-light-induced decarboxylative coupling between α,ß-unsaturated carboxylic acids and alkyl N-hydroxyphthalimide esters has been developed. A wide range of redox-active esters derived from aliphatic carboxylic acids (1°, 2° and 3°) proved viable in this dual decarboxylation process, affording a broad scope of substituted alkenes in moderate to excellent yields with good E/Z selectivities. This redox-neutral procedure was highlighted by its mild conditions, operational simplicity, easy accessibility of carboxylic acids, and excellent functional-group tolerance.

Palladium-Catalyzed Alkylarylation of Acrylamides with Unactivated Alkyl Halides.

An efficient palladium-catalyzed alkylarylation of acrylamides with unactivated alkyl halides has been developed. This method is highlighted by its broad substrate scope and excellent functional group tolerance. In addition to alkyl halides, fluoroalkyl halides and benzyl bromides also participated well in this transformation. A detailed mechanistic investigation suggests that a radical pathway is probably involved in the cyclization process.

Recent advances in catalytic decarboxylative acylation reactions via a radical process.

The decarboxylative cross-coupling reactions of carboxylic acids have emerged as an efficient strategy for the C-C bond and C-X bond formations. In this field, the decarboxylative cross-coupling of α-keto acids has been developed as an attractive strategy to introduce a carbonyl group into versatile organic molecules. In particular, radical-mediated decarboxylative reactions have attracted much attention of chemists owing to their mild conditions, excellent functional group tolerance and high reactivity. In this review, we highlight the recent advances in the radical decarboxylative acylation reactions of α-keto acids.

Transition-Metal-Free Oxyfluorination of Olefinic Amides for the Synthesis of Fluorinated Heterocycles.

A series of fluorinated 4H-3,1-benzoxazines and iminoisobenzofurans have been synthesized through the electrophilic fluorocyclization of olefinic amides. This methodology is highlighted by its mild conditions, wide substrate scope, and good functional group tolerance.