Mechanisms of Mn(OAc)(3)-based oxidative free-radical additions and cyclizations

Strategies and Mechanisms of First-Row Transition Metal-Regulated Radical C-H Functionalization

Radical C-H functionalization represents a useful means of streamlining synthetic routes by avoiding substrate preactivation and allowing access to target molecules in fewer steps. The first-row transition metals (Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) are Earth-abundant and can be employed to regulate radical C-H functionalization. The use of such metals is desirable because of the diverse interaction modes between first-row transition metal complexes and radical species including radical addition to the metal center, radical addition to the ligand of metal complexes, radical substitution of the metal complexes, single-electron transfer between radicals and metal complexes, hydrogen atom transfer between radicals and metal complexes, and noncovalent interaction between the radicals and metal complexes. Such interactions could improve the reactivity, diversity, and selectivity of radical transformations to allow for more challenging radical C-H functionalization reactions. This review examines the achievements in this promising area over the past decade, with a focus on the state-of-the-art while also discussing existing limitations and the enormous potential of high-value radical C-H functionalization regulated by these metals. The aim is to provide the reader with a detailed account of the strategies and mechanisms associated with such functionalization.

Recent Advances in Manganese(III)-Assisted Radical Cyclization for the Synthesis of Natural Products: A Comprehensive Review

Natural products play an important part in synthetic chemistry since they have many pharmacological properties and are used as active drug compounds in pharmaceutical chemistry. However, synthesis of these complex molecules is difficult due to the requirement of various synthetic steps, which include highly regio- and stereoselectivity. Therefore, oxidative radical cyclization assisted by manganese(III) acetate serves as an important step in obtaining spiro-, tricyclic, tetracyclic, and polycyclic derivatives of these compounds. Manganese(III)-based reactions offer a single-step regio- and stereoselective cyclizations and α-acetoxidations, reducing the number of synthetic steps. Also, the manganese(III)-mediated oxidative free radical cyclization method has been successfully applied for the synthesis of cyclic structures found in many natural products. This article presents a broad overview of manganese(III)-based radical reactions of natural products as a key step in overall synthesis. The authors have classified natural product synthesis processes assisted by manganese(III) acetate as intermolecular, intramolecular, oxidation, acetoxidation, aromatization, and polymerization reactions, respectively.

Multigram Synthesis of Dimethyl Stellane-1,5-Dicarboxylate as a Key Precursor for ortho-Benzene Mimics

Herein, we present previously unavailable C(sp3 )-rich polycyclic hydrocarbon scaffolds that have the potential to become valuable tools in medicinal chemistry and crop science as saturated bioisosteres of benzenoids. We have developed a scalable protocol (up to 50 g from a single synthetic run) for the synthesis of tricyclo[3.3.0.03,7 ]octane (bisnoradamantane or stellane) 1,5-dicarboxylic acid derivatives. X-ray crystallographic analysis of the stellane 1,5-dicarboxylic acid dimethyl ester has revealed that this scaffold is an optimal saturated isostere for ortho-disubstituted benzene where substituents exhibit in-plane topology. The synthetic protocol is based on the oxidative cyclization of dimethyl octahydropentalene-2,5-dicarboxylate (DMOD) through lithiation followed by I2 oxidation. The reaction outcome is determined by the stereochemistry of the substrate. While the endo,endo cis-DMOD, exclusively gives the "unwanted" Claisen cyclization product, the exo,endo cis- and exo,exo cis- stereoisomers afford the desired stellane 1,5-dicarboxylic acid dimethyl ester quantitatively. DFT computations have revealed that the reaction proceeds via the dianion of dimethyl octahydropentalene-2,5-dicarboxylate, which undergoes SET oxidation by I2 to form a radical anion. The subsequent cyclization followed by a second SET oxidation gives the desired stellane derivative.

An Ionic-Radical Approach to Vicinally Functionalized Cyclopentanones and Cyclohexanones

A versatile strategy to access vicinally functionalized cyclopentanones and cyclohexanones is described. It takes advantage of new reagents, α-xanthyl enones, which can be prepared from the reaction of xanthate salts with the corresponding epoxyketones under mild conditions. β-Functionalization of these compounds can be performed by conjugate additions without affecting the xanthate moiety. This significantly expands the pool of xanthate substrates, allowing the synthesis of open chain and fused bicyclic building blocks useful in the synthesis of natural products.

Photoredox-Catalyzed Oxidation of Anions for the Atom-Economical Hydro-, Amido-, and Dialkylation of Alkenes

Photoredox catalysis has become a powerful method to generate free radical intermediates in organic synthesis. This report describes the use of photoredox catalysis to directly oxidize common nucleophilic anions to access electrophilic 1,3-dicarbonyl and amidyl radical intermediates. First, conjugate bases of 1,3-dicarbonyls were oxidized to neutral radical species for intramolecular hydro- and dialkylation of alkenes. This overall redox-neutral process provided cyclopentanone products in excellent yields (up to 96%). The scope included a variety of styrene radical acceptors and products with newly formed vicinal quaternary carbons. This process was then extended to the synthesis of pyrrolidinones by alkene amidoalkylation that proceeded via N-aryl amidyl radical intermediates in good yield (up to 85%). These reactions were characterized by their mild conditions, high atom economy, and the absence of stoichiometric byproducts. Mechanistic and computational studies supported a stepwise proton-coupled electron transfer mechanism, where an "electron borrowing" photocatalyst oxidizes an anion and reduces a benzylic radical after bond formation.

Copper-Catalyzed Intermolecular Enantioselective Radical Oxidative C(sp3 )-H/C(sp)-H Cross-Coupling with Rationally Designed Oxazoline-Derived N,N,P(O)-Ligands

The intermolecular asymmetric radical oxidative C(sp³ )-C(sp) cross-coupling of C(sp³ )-H bonds with readily available terminal alkynes is a promising method to forge chiral C(sp³ )-C(sp) bonds because of the high atom and step economy, but remains underexplored. Here, we report a copper-catalyzed asymmetric C(sp³ )-C(sp) cross-coupling of (hetero)benzylic and (cyclic)allylic C-H bonds with terminal alkynes that occurs with high to excellent enantioselectivity. Critical to the success is the rational design of chiral oxazoline-derived N,N,P(O)-ligands that not only tolerate the strong oxidative conditions which are requisite for intermolecular hydrogen atom abstraction (HAA) processes but also induce the challenging enantiocontrol. Direct access to a range of synthetically useful chiral benzylic alkynes and 1,4-enynes, high site-selectivity among similar C(sp³ )-H bonds, and facile synthesis of enantioenriched medicinally relevant compounds make this approach very attractive.

Stable and reactive diacetyliminoxyl radical in oxidative C-O coupling with β-dicarbonyl compounds and their complexes

As a rule, reactive free radicals used in organic synthesis are too labile to be isolated, whereas persistent radicals are inert and find limited synthetic application. In the present study, the unusually stable diacetyliminoxyl radical was presented as a "golden mean" between transient and stable unreactive radicals. It was successfully employed as a reagent for oxidative C-O coupling with β-dicarbonyl compounds. Using this model radical the catalytic activity of acids, bases and transition metal ions in free-radical coupling was revealed.

Copper-Catalyzed Cross-Nucleophile Coupling of β-Allenyl Silanes with Tertiary C-H Bonds: A Radical Approach to Branched 1,3-Dienes

Described herein is a distinctive approach to branched 1,3-dienes through oxidative coupling of two nucleophilic substrates, β-allenyl silanes, and hydrocarbons appending latent functionality by copper catalysis. Notably, C(sp³)-H dienylation proceeded in a regiospecific manner, even in the presence of competitive C-H bonds that are capable of occurring hydrogen atom transfer process, such as those located at benzylic and other tertiary sites, or adjacent to an oxygen atom. Control experiments support the intermediacy of functionalized alkyl radicals.

Synthesis of Desacyl Furanmonogones A and B

A strategy for the stereoselective synthesis of desacyl furanmonogones A and B has been achieved. The key steps in this synthesis are (1) an Fe(ClO₄)₃-mediated oxidative radical cyclization for construction of a cis-fused [5-6]-bicyclic core with a bridged lactone substitute, (2) a phosphorane-mediated rearrangement to convert the cis-fused [5-6]-bicyclic core to the corresponding trans-fused [5-6]-bicyclic core, and (3) a Au-catalyzed cascade reaction for formation of the 4,5-seco-3(2H)-furanone motif.

Mn(OAc)3-Mediated Addition Reactions of NaSO2CF3 and Perhalogenated Carboxylic Acids with Unactivated Alkenes Conjectured by a Single Electron Transfer and Halogen Abstraction Mechanism

A free-radical halotrifluoromethylation of olefins by using Mn(OAc)₃·2H₂O, CF₃SO₂Na, and perhalogenated carboxylic acids has been achieved. Perhalogenated carboxylic acids act as a halogen source and CF₃SO₂Na acts as a CF₃ source. The reaction displayed good tolerance of functional groups in the substrates under mild conditions. The radical clock experiment and TEMPO inhibition experiment support a radical process. The halogen reagent competition experiment shows that the last step of halogenation process is mainly through a halogen abstraction mechanism.

Direct transfer of tri- and di-fluoroethanol units enabled by radical activation of organosilicon reagents

Trifluoroethanol and difluoroethanol units are important motifs in bioactive molecules, but the methods to direct incorporate these units are limited. Herein, we report two organosilicon reagents for the transfer of trifluoroethanol and difluoroethanol units into molecules. Through intramolecular C-Si bond activation by alkoxyl radicals, these reagents were applied in allylation, alkylation and alkenylation reactions, enabling efficient synthesis of various tri(di)fluoromethyl group substituted alcohols. The broad applicability and general utility of the approach are highlighted by late-stage introduction of these fluoroalkyl groups to complex molecules, and the synthesis of antitumor agent Z and its difluoromethyl analog Z'.

Enantioselective Radical-Polar Crossover Reactions of Indanonecarboxamides with Alkenes

Highly efficient asymmetric intermolecular radical-polar crossover reactions were realized by combining a chiral N,N'-dioxide/Ni^II complex catalyst with Ag₂ O under mild reaction conditions. Various terminal alkenes and indanonecarboxamides/esters underwent radical addition/cyclization reactions to afford spiro-iminolactones and spirolactones with good to excellent yields (up to 99 %) and enantioselectivities (up to 97 % ee). Furthermore, a range of different radical-mediated oxidation/elimination or epoxide ring-opening products were obtained under mild reaction conditions. The Lewis acid catalysts exhibited excellent performance and precluded the strong background reaction.

Copper(I)-Catalyzed Asymmetric Reactions Involving Radicals

Asymmetric functionalization of alkyl radicals represents a robust yet underdeveloped method for efficient construction and decoration of carbon skeletons in chiral organic molecules. In this field, we have been inspired by the excellent redox, alkyl radical trapping, and Lewis acidic properties of copper to develop several catalytic modes for asymmetric reactions involving alkyl radicals. At the beginning, we discovered tandem radical hydrotrifluoromethylation of unactivated alkenes and enantioselective alkoxylation of remote C(sp³)-H bonds by copper/chiral phosphate relay catalysis. This success has stimulated us to develop an asymmetric three-component 1,2-dicarbofunctionalization of 1,1-diarylalkenes using a similar strategy via radical intermediates. Meanwhile, we also discovered a copper/chiral secondary amine cooperative catalyst for asymmetric radical intramolecular cyclopropanation of alkenes using α-aldehyde methylene groups as C1 sources. The trapping of alkyl radical intermediates by Cu^II species during the reaction was essential for the chemoselectivity toward cyclopropanation. Encouraged by the efficient enantiocontrol with chiral phosphate and the effective trapping of alkyl radicals with Cu^II species, we then sought to develop copper/chiral phosphate as a single-electron-transfer catalyst for asymmetric reactions involving alkyl radicals. Subsequently, we successfully achieved a series of highly enantioselective 1,2-aminofluoroalkylation, -aminoarylation, -diamination, -aminosilylation, and -oxytrifluoromethylation of unactivated alkenes. The key for high enantioinduction was believed to be the effective trapping of alkyl radicals by Cu^II/chiral phosphate complexes. Besides, an achiral pyridine ligand was found to be indispensable for achieving high enantioselectivity, presumably via stabilization of Cu^III species in the 1,2-alkoxytrifluoromethylation reaction. This discovery reminded us of tuning the redox properties and chemoreactivity of copper centers with an ancillary ligand. As a result, we subsequently identified cinchona alkaloid-derived sulfonamides as novel neutral-anionic hybrid ligands for simultaneous chemo- and enantiocontrol. We thus accomplished highly enantioselective 1,2-iminoxytrifluoromethylation of unactivated alkenes under the catalysis of copper/cinchona alkaloid-derived sulfonamide ligand, affording trifluoromethylated isoxazolines in high enantiomeric excess. Our copper-catalyzed asymmetric reactions with alkyl radicals provide expedient access to a diverse range of valuable chiral molecules with broad application potential in areas of organic synthesis, medicine, agrochemical, and material sciences.

Synthesis of polysubstituted quinolines through promoter-regulated selective annulation and C–C bond cleavage from 2-styrylanilines and β-keto esters

I₂ and Mn(OAc)₃ regulated annulation of 2-styrylanilines and β-keto esters is accomplished for chemoselective synthesis of 2-alkylquinolines and quinoline-2-carboxylates.

Treasures old and new: what we can learn regarding the macrocyclic problem from past and present efforts in natural product total synthesis

In this review the strategies leading to successful macrocyclization, in the context of total synthesis are discussed.

Synthesis of the Tricyclic Picrotoxane Motif by an Oxidative Cascade Cyclization

An oxidative cascade cyclization of β-keto esters has been developed for the construction of the tricyclic picrotoxane motif in a single step, and DFT calculations suggested a possible cationic cyclization mechanism. This cascade cyclization can be operated on a 20 g scale to obtain a 77% total yield of the tricyclic products, which in turn can be converted to versatile intermediates for further elaboration to picrotoxanes and their structurally related compounds.

Mn-Catalyzed Electrochemical Chloroalkylation of Alkenes

The heterodifunctionalization of alkenes is an efficient method for synthesizing highly functionalized organic molecules. In this report, we describe the use of anodically coupled electrolysis for the catalytic chloroalkylation of alkenes-a reaction that constructs vicinal C-C and C-Cl bonds in a single synthetic operation-from malononitriles or cyanoacetates and NaCl. Knowledge of the persistent radical effect guided the reaction design and development. A series of controlled experiments, including divided-cell electrolysis that compartmentalized the anodic and cathodic events, allowed us to identify the key radical intermediates and the pathway to their electrocatalytic formation. Cyclic voltammetry data further support the proposed mechanism entailing the parallel, Mn-mediated generation of two radical intermediates in an anodically coupled electrolysis followed by their selective addition to the alkene.

Radical addition of ketones and cyanide to olefinsviaacid catalyzed formation of intermediate alkenyl peroxides

γ-Cyanoketones are formed by double radical addition from olefins, ketones and sulfonyl cyanidesviareactive alkenyl peroxide intermediates.

Synthesis of Chlorotrifluoromethylated Pyrrolidines by Electrocatalytic Radical Ene-Yne Cyclization

The stereoselective synthesis of chlorotrifluoromethylated pyrrolidines was achieved using anodically coupled electrolysis, an electrochemical process that combines two parallel oxidative events in a convergent and productive manner. The bench-stable and commercially available solids CF₃ SO₂ Na and MgCl₂ were used as the functional group sources to generate CF₃^. and Cl^. , respectively, via electrochemical oxidation, and the subsequent reaction of these radicals with the 1,6-enyne substrate was controlled with an earth-abundant Mn catalyst. In particular, the introduction of a chelating ligand allowed for the ene-yne cyclization to take place with high stereochemical control over the geometry of the alkene group in the pyrrolidine product.

A Total Synthesis of Salinosporamide A

Salinosporamide A is a β-lactone proteasome inhibitor currently in clinical trials for the treatment of multiple-myeloma. Herein we report a short synthesis of this small, highly functionalized, biologically important natural product that uses an oxidative radical cyclization as a key step and allows for the preparation of gram quantities of advanced synthetic intermediates.

Anodically Coupled Electrolysis for the Heterodifunctionalization of Alkenes

The emergence of new catalytic strategies that cleverly adopt concepts and techniques frequently used in areas such as photochemistry and electrochemistry has yielded a myriad of new organic reactions that would be challenging to achieve using orthodox methods. Herein, we discuss the strategic use of anodically coupled electrolysis, an electrochemical process that combines two parallel oxidative events, as a complementary approach to existing methods for redox organic transformations. Specifically, we demonstrate anodically coupled electrolysis in the regio- and chemoselective chlorotrifluoromethylation of alkenes.

Metal-free oxidative radical cascade addition/oxobutylation of unactivated alkenes with acetone towards 3-(3-oxobutyl)indolines

A direct oxobutylation of N-allyl anilines with acetone was developed under metal-free and acid-free conditions.

Fe-Catalyzed three-component carboazidation of alkenes with alkanes and trimethylsilyl azide

Iron out the difference. Carboazidation of alkenes using cycloalkanes, CH₂Cl₂, CHCl₃ and CCl₄ as alkylating reagents proceeded smoothly in the presence of TMSN₃, DTBP and a catalytic amount of Fe(acac)₃ to afford chain extended alkyl azides and γ-azido chloroalkanes in good to high yields.