Organophotoredox-catalyzed semipinacol rearrangement via radical-polar crossover

Oxidative Scission of Bicyclo[2.2.2]octenones: Untying the α-Dimethoxycarbonyl

This study explores the selective oxidative scission of bicyclo[2.2.2]octenones derived from masked o-benzoquinones (MOBs). By employing the fragmentation of ketoximes and Schmidt-type reactions, we achieved the cleavage of α-dimethoxy carbonyl groups to yield highly functionalized cyclohexene frameworks, showcasing the expanding synthetic utility of bicyclo[2.2.2]octenones in complex organic synthesis. The methodologies developed are anticipated to contribute to advancements in organic and pharmaceutical chemistry.

Enantioselective Synthesis of α-Aryl Ketones by a Cobalt-Catalyzed Semipinacol Rearrangement

A highly enantioselective cobalt-catalyzed semipinacol rearrangement of symmetric α,α-diarylallylic alcohols is disclosed. A chiral cobalt-salen catalyst generates a highly electrophilic carbocation surrogate following hydrogen atom transfer and radical-polar crossover steps. This methodology provides access to enantioenriched α-aryl ketones through invertive displacement of a cobalt(IV) complex during 1,2-aryl migration. A combination of readily available reagents, silane and N-fluoropyridinium oxidant, are used to confer this type of reactivity. An exploration into the effect of aryl substitution revealed the reaction tolerates para- and meta-halogenated, mildly electron-rich and electron-poor aromatic rings with excellent enantioselectivities and yields. The yield of the rearrangement diminished with highly electron-rich aryl rings whereas very electron-deficient and ortho-substituted arenes led to poor enantiocontrol. A Hammett analysis demonstrated the migratory preference for electron-rich aromatic rings, which is consistent with the intermediacy of a phenonium cation.

Dual-Catalytic Structural Isomerisation as a Route to α-Arylated Ketones

Isomerisation reactions provide streamlined routes to organic compounds which are otherwise hard to directly synthesise. The most common forms are positional, geometrical or stereochemical isomerisations which involve the relocation of a double bond or a change in relative location of groups in space. In contrast, far fewer examples of structural (or constitutional) isomerisation exist where the connectivity between atoms is altered. The development of platforms capable of such rearrangement poses a unique set of challenges because chemical bonds must be selectively cleaved, and new ones formed without overall addition or removal of atoms. Here, we show that a dual catalytic system can enable the structural isomerisation of readily available allylic alcohols into more challenging-to-synthesise α-arylated ketones via a H-atom transfer initiated semi-pinacol rearrangement. Key to our strategy is the combination of a cobalt catalyst and photocatalyst under reductive, protic conditions which allows intermediates to propagate catalytic turnover. By providing an unusual disconnection to structural motifs which are difficult to access through direct arylation, we anticipate inspiring other advanced catalytic isomerisation strategies that will further retrosynthetic logic for complex molecule synthesis.

(2 + 2 + 1)-Cyclization of Glycine Derivatives with Alkenes and CO2

A highly atom-economic (2 + 2 + 1)-cyclization of glycine derivatives with alkenes and CO2 facilitated by visible light catalysis is described. A range of multisubstituted pyrrolidones were synthesized under simple and mild reaction conditions. A reasonable mechanism involving redox-neutral radical-polar crossover is proposed.

A Unified Method for Oxidative and Reductive Decarboxylative Arylation with Orange Light-Driven Ir/Ni Metallaphotoredox Catalysis

Carboxylic acids and their derivatives are powerful building blocks in dual Ir/Ni metallaphotoredox methods of decarboxylative arylation due to their abundance as feedstock compounds. However, the library of accessible carboxylic acids is limited by trends in radical stability, often necessitating the development of specific systems for challenging substrates. Herein, we disclose the application of a new Ir(III) photocatalyst and low-energy orange light Ir/Ni metallaphotoredox system with broad applicability in activating both native carboxylic acids and redox-active esters (RAEs). This method represents the first known example of complementary oxidative and reductive decarboxylative paradigms with broadly similar reaction conditions, unlocking the reactivity for challenging substrates. We further show a wide scope of aryl halide and acid coupling partners in both regimes, with added advantages over blue-light-catalyzed aryl alkylation for photosensitive substrates.

Pyrene-Based Organic Photoredox Catalysts for Carbon-Carbon Bond-Forming Reactions: Reductive Coupling of Aromatic Carbonyl and Imine Compounds

Metal-free photoredox catalysts built upon a pyrene core were developed for carbon-carbon bond-forming reactions. Among these catalysts, a pyrene derivative containing a urea moiety effectively facilitated the reductive coupling of aromatic carbonyl and imine compounds under blue LED irradiation. This process provided the corresponding vicinal diols and diamines in good yields.

Fused Seven-Membered Carbocycle Construction through Electrochemical Relay Cyclization

Seven-membered carbocycles, which are frequently associated with relevant biological activities, can be found in nature and drugs in ever-increasing numbers. Radical cascade addition of 1,n-enynes-like substrates is one of the most important and efficient strategies for the synthesis of this valuable skeleton with structural diversity and complexity. Herein, we describe an electrooxidative radical-mediated synthesis of dibenzocycloheptanes from ortho-ethynyl-substituted biaryls and tertiary C(sp3)-H containing reactants. The chemo- and regioselective addition of in situ generated carbon radical onto the alkynyl moieties triggers a desired reaction cascade, resulting in the formation of three new C-C bonds. This approach provides a step-economical regime for the facile assembly of a wide range of polycyclic products containing a 6-7-5 system. This green strategy features a good substrate scope, mild conditions, and high efficiency under ferrocene-mediated electrochemical oxidation conditions.

Cobalt-Catalyzed Photo-Semipinacol Rearrangement of Unactivated Allylic Alcohols

We report a photochemical method for the semipinacol rearrangement of unactivated allylic alcohols. Aliphatic as well as aromatic groups participate as migrating groups, yielding a variety of α,α-disubstituted ketones. The reaction proceeds under mild conditions and is compatible with ethers, esters, halides, nitriles, carbamates, and substituted arenes. The operationally simple and fully catalytic conditions prescribe 1 mol % benzothiazinoquinoxaline as organophotocatalyst, 0.5 mol % Co-salen, and 10 mol % lutidinium triflate and, importantly, display reactivity complementary to procedures employing Brønsted acid. We showcase the utility of the protocol in late-stage drug diversifications.

Photocatalytic Decarboxylative Functionalization of Cyclopropenes via Cyclopropenium Cation Intermediates

A photocatalytic decarboxylative functionalization of cyclopropenes is reported. Starting from a broad range of redox-active ester-substituted cyclopropenes, cyclopropenylphthalimides can be synthesized in the absence of a nucleophile. Alternatively, different carbon and heteroatom nucleophiles can be introduced. The transformation proceeds most probably through the formation of an aromatic cyclopropenium cation, followed by trapping with the nucleophiles.

Semipinacol Rearrangement of Iododifluorohomoallyl Alcohols and Its Application in the Allylic C-H Esterification Reactions

We herein present a study on the Ag(I)-mediated semipinacol rearrangement of iododifluorohomoallyl alcohols, the resulting allylic difluoromethyl ketones underwent oxidative allylic C-H esterification under palladium catalysis in the absence of external ligand. This process yielded a range of difluoromethyl ketones derived from allyl esters in a single operation. The reaction features broad scope of o-nitrobenzoic acids and homoallylic iododifluoroalcohols affording the targeted molecules in synthetically useful yields. Control experiments illustrated that the silver salt acted as not only a Lewis acid to promote the cleavage of a C-I bond and furnish the semipinacol rearrangement but also a co-oxidant in the catalytic cycle for the allylic C-H esterification.

Synthesis of tertiary alkylphosphonate oligonucleotides through light-driven radical-polar crossover reactions

Chemical modification of nucleotides can improve the metabolic stability and target specificity of oligonucleotide therapeutics, and alkylphosphonates have been employed as charge-neutral replacements for naturally-occurring phosphodiester backbones in these compounds. However, at present, the alkyl moieties that can be attached to phosphorus atoms in these compounds are limited to methyl groups or primary/secondary alkyls, and such alkylphosphonate moieties can degrade during oligonucleotide synthesis. The present work demonstrates the tertiary alkylation of the phosphorus atoms of phosphites bearing two 2'-deoxynuclosides. This process utilizes a carbocation generated via a light-driven radical-polar crossover mechanism. This protocol provides tertiary alkylphosphonate structures that are difficult to synthesize using existing methods. The conversion of these species to oligonucleotides having charge-neutral alkylphosphonate linkages through a phosphoramidite-based approach was also confirmed in this study.

Organophotocatalytic carbo-heterofunctionalization of unactivated olefins with pendant nucleophiles

We report the difunctionalization of unactivated, terminal olefins through intermolecular addition of α-bromoketones, -esters, and -nitriles followed by formation of 4- to 6-membered heterocycles with pendant nucleophiles. The reaction can be conducted with alcohols, acids, and sulfonamides as nucleophiles furnishing products bearing 1,4 functional group relationships that offer various handles for further manipulation. Salient features of the transformations are the use of 0.5 mol% of a benzothiazinoquinoxaline organophotoredox catalyst and their robustness with respect to air and moisture. Mechanistic investigations are carried out and a catalytic cycle for the reaction is proposed.

Two Green Protocols for Halogenative Semipinacol Rearrangement

Semipinacol rearrangement is a special type of Wagner-Meerwein rearrangement that involves carbocation 1,2-rearrangement to provide carbonyl compounds with an α-quaternary carbon center. It has been strategically used for natural product synthesis and construction of highly congested quaternary carbons. Herein, we report a safe and green protocol that uses oxone/halide and Fenton bromide to achieve halogenative semipinacol rearrangement. The key feature of this method is the green in situ generation of reactive halogenating species from oxidation of halide with oxone or H₂O₂, which produces a nontoxic byproduct (potassium sulfate or water). Easy operation (insensitive to air and moisture) at room temperature without using special equipment adds additional advantage over previous methods.

Intermolecular Organophotocatalytic Cyclopropanation of Unactivated Olefins

Intermolecular cyclopropanation of mono-, di-, and trisubstituted olefins with α-bromo-β-ketoesters and α-bromomalonates under organophotocatalysis is reported. The reaction displays broad functional group tolerance, including substrates bearing acids, alcohols, halides, ethers, ketones, nitriles, esters, amides, carbamates, silanes, stannanes, boronic esters, as well as arenes, and furnishes highly substituted cyclopropanes. The transformation may be performed in the presence of air and moisture with 0.5 mol % of a benzothiazinoquinoxaline as organophotocatalyst. Mechanistic investigations, involving Stern-Volmer quenching, quantum yield determination, and deuteration experiments, are carried out, and a catalytic cycle for the transformation is discussed.

A Photoredox/Sulfide Dual Catalysis System That Uses Sulfide Radical Cations to Promote Alkene Chlorotrifluoromethylation

Sulfides and their derivatives are among the most important class of reagent in synthetic chemistry. Despite the importance of such compounds, the use of sulfide radical cations in synthetic chemistry is underdeveloped. To address this issue, herein, we describe alkene chlorotrifluoromethylation reactions promoted by photoredox/sulfide dual catalysis systems, which involves sulfide radical cations generated through the oxidation of sulfides by a photoredox catalyst. The high functional group tolerance of this chemistry was demonstrated using natural products and drug molecules as substrate alkenes.

Radical Redox Annulations: A General Light-Driven Method for the Synthesis of Saturated Heterocycles

We introduce here a two-component annulation strategy that provides access to a diverse collection of five- and six-membered saturated heterocycles from aryl alkenes and a family of redox-active radical precursors bearing tethered nucleophiles. This transformation is mediated by a combination of an Ir(III) photocatalyst and a Brønsted acid under visible-light irradiation. A reductive proton-coupled electron transfer generates a reactive radical which undergoes addition to an alkene. Then, an oxidative radical-polar crossover step leading to carbocation formation is followed by ring closure through cyclization of the tethered nucleophile. A wide range of heterocycles are easily accessible, including pyrrolidines, piperidines, tetrahydrofurans, morpholines, δ-valerolactones, and dioxanones. We demonstrate the scope of this approach through broad structural variation of both reaction components. This method is amenable to gram-scale preparation and to complex fragment coupling.

Formal Glycosylation of Quinones with exo-Glycals Enabled by Iron-Mediated Oxidative Radical-Polar Crossover

The intermolecular C-O coupling reaction of 1,4-quinones with exo-glycals under iron hydride hydrogen atom transfer (HAT) conditions is described. This method provides a direct and regioselective access to a wide range of phenolic O-ketosides related to biologically relevant natural products in diastereomeric ratios up to >98:2 in the furanose and pyranose series. No trace of the corresponding C-glycosylated products that might have resulted from the radical alkylation of 1,4-quinones was observed. The results of mechanistic experiments suggest that the key C-O bond-forming event proceeds through an oxidative radical-polar crossover process involving a single-electron transfer between the HAT-generated glycosyl radical and the electron-acceptor quinone.