Organophotoredox-Catalyzed Three-Component Coupling of Heteroatom Nucleophiles, Alkenes, and Aliphatic Redox Active Esters

Defluorinative Haloalkylation of Unactivated Alkenes Enabled by Dual Photoredox and Copper Catalysis

A three-component defluorinative haloalkylation of alkenes with trifluoromethyl compounds and TBAX (X = Cl, Br) via dual photoredox/copper catalysis is reported. The mild conditions are compatible with a wide array of activated trifluoromethyl aromatics bearing diverse substituents, and various nonactivated terminal and internal alkenes, enabling straightforward access to synthetically valuable γ-gem-difluoroalkyl halides with high efficiency. Mechanistic studies indicate that the [Cu] complexes not only serve as XAT catalysts but also facilitate the SET reduction of trifluoromethyl groups by photocatalysts. Additionally, the resulting alkyl halide products can serve as versatile conversion intermediates for the synthesis of a diverse range of γ-gem-difluoroalkyl compounds.

Diverse Synthesis of (Thio)ethers and (Thio)esters Using Halodiborylmethane as a Transformable C1 Building Block

The development of effective strategies to forge C-O and C-S bonds in diverse chemical spaces is of considerable interest in synthetic organic chemistry. Herein we report a versatile approach for the modular synthesis of structurally diverse (thio)ethers and (thio)esters via homologative coupling of α-halodiborylmethane followed by transformation of the introduced diborylmethyl group. This method accommodates a wide array of oxygen- and sulfur-containing molecules, including biologically active compounds. The initial coupling exhibits a broad substrate scope, while subsequent diversification of the diborylmethyl moiety enables access to various structural motifs through deborylative alkylation, Zweifel olefination, and boron-Wittig reaction. This protocol efficiently generates diversely functionalized (thio)ethers and (thio)esters, expanding the toolkit for accessing biologically relevant scaffolds.

Mechanisms for radical reactions initiating from N-hydroxyphthalimide esters

Due to their ease of preparation, stability, and diverse reactivity, N-hydroxyphthalimide (NHPI) esters have found many applications as radical precursors. Mechanistically, NHPI esters undergo a reductive decarboxylative fragmentation to provide a substrate radical capable of engaging in diverse transformations. Their reduction via single-electron transfer (SET) can occur under thermal, photochemical, or electrochemical conditions and can be influenced by a number of factors, including the nature of the electron donor, the use of Brønsted and Lewis acids, and the possibility of forming charge-transfer complexes. Such versatility creates many opportunities to influence the reaction conditions, providing a number of parameters with which to control reactivity. In this perspective, we provide an overview of the different mechanisms for radical reactions involving NHPI esters, with an emphasis on recent applications in radical additions, cyclizations and decarboxylative cross-coupling reactions. Within these reaction classes, we discuss the utility of the NHPI esters, with an eye towards their continued development in complexity-generating transformations.

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.

Super-Reducing Behavior of Benzo[b]phenothiazine Anion Under Visible-Light Photoredox Condition

Herein we describe the anion of benzo[b]phenothiazine as a super reductant species upon excitation by visible light. In contrary to N-substituted phenothiazines or benzophenothiazines, this molecule holds extreme reducing power to promote single electron transfer-based reductive cleavage at a potential of -3.51 V vs SCE. As a proof, a plethora of aryl chloride substrates have been reductively cleaved to fabricate molecules of the class isoindolinone and oxindole. Moreover, an aryl-chloride bond has been homolytically cleaved to generate aryl radicals that have been utilized for C-C cross-coupling or C-P bond formation reactions. To prove its extreme reducing ability, some of the aryl fluoride bonds have been cleaved to generate aryl radicals. A detailed photophysical study including steady-state and time-resolved spectroscopic techniques explain the molecule's behavior upon light excitation, and that correlates with its reactivity pattern. Theoretical calculations disclose the benzophenothiazine anion to be slightly puckered at the ground state as the molecule is antiaromatic in nature. In contrast, the excited-state geometry is planar, which is also close to that of the intermediate after one electron transfer. Abating the antiaromaticity of the anionic species is partially responsible for its highly reducing behavior.

One-Pot Formal Carboradiofluorination of Alkenes: A Toolkit for Positron Emission Tomography Imaging Probe Development

We report the first one-pot formal alkene carboradiofluorination reaction employing easily accessible alkenes as both prosthetic group precursors and coupling partners. The methodology features rapid sequential Markovnikov-selective iodofluorination and photoinduced Pd(0/I/II)-catalyzed alkyl Heck reaction as a mild and robust fluorine-18 (18F) radiochemical approach for positron emission tomography (PET) imaging probe development. A new class of prosthetic groups for PET imaging probe synthesis was isolated as iodofluorinated intermediates in moderate to excellent yields. The one-pot formal alkenylfluorination reaction was carried out to produce over 30 analogues of a wide range of bioactive molecules. Further application of the Pd(0/I/II) manifold in PET probe development was illustrated by the direct carbo(radio)fluorination of electron-rich alkenes. The methods were successfully translated to radiolabel a broad scope of medicinally relevant small molecules in generally good radiochemical conversion. The protocol was further optimized to accommodate no-carrier-added conditions with similar efficiency for future (pre)clinical translation. Moreover, the radiosynthesis of prosthetic groups was automated in a radiochemistry module to facilitate its practical use in multistep radiochemical reactions.

Visible-Light-Induced Synthesis of Branched Ethers via Multicomponent Reactions

The Spin-Center Shift (SCS) elimination is a specific way for the generation of radicals with relevance in synthetic and biochemical pathways. The combination of SCS-mediated radical chemistry and atom-transfer radical addition (ATRA) offers new directions in diversity-oriented chemical synthesis. Herein, we report a photoredox three-component reaction of α-acyloxy-N-heterocycles as radical precursors, styrene derivatives as radical trapping agents, and alcohols as nucleophilic quenchers. The novel radical-polar crossover reaction provides access to a diverse set of branched ethers possessing high structural complexity. The utility of the transformation was also demonstrated by the synthesis of a complex drug derivative and it was easily scalable to the multigram level. The scope and limitations were also explored and a plausible mechanism was proposed.

Strategies for Nucleophilic C(sp3)-(Radio)Fluorination

This Perspective surveys the progress and current limitations of nucleophilic fluorination methodologies. Despite the long and rich history of C(sp³)-F bond construction in chemical research, the inherent challenges associated with this transformation have largely constrained nucleophilic fluorination to a privileged reaction platform. In recent years, the Doyle group─along with many others─has pursued the study and development of this transformation with the intent of generating deeper mechanistic understanding, developing user-friendly fluorination reagents, and contributing to the invention of synthetic methods capable of enabling radiofluorination. Studies from our laboratory are discussed along with recent developments from others in this field. Fluoride reagent development and the mechanistic implications of reagent identity are highlighted. We also outline the chemical space inaccessible by current synthetic technologies and a series of future directions in the field that can potentially fill the existing dark spaces.

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.

Photoinduced Remote C(sp3)-H Cyanation and Oxidation Enabled by a Vinyl Radical-Mediated 1,5-HAT Strategy

We report a vinyl radical-mediated 1,5-hydrogen atom transfer (1,5-HAT) strategy for the remote C(sp³)-H functionalization reaction, which includes cyanation, oxidation, and etherification under visible-light-induced photochemical conditions. This reaction is achieved using readily available alkyl N-hydroxyphthalimide esters as radical precursors, which can efficiently react with diverse alkynes to form key vinyl radical intermediates followed by a 1,5-HAT process. A series of structurally diverse γ-cyano, γ-carbonyl, and γ-oxygenated alkenes with excellent stereoselectivity can be efficiently constructed by this synthetic protocol.

A three-component difunctionalization of N-alkenyl amides via organophotoredox radical-polar crossover

Herein, we report a three-component organophotoredox coupling of N-alkenyl amides with α-bromocarbonyls and various nucleophiles. This transition metal-free difunctionalization protocol installs sequential C-C and C-Y (Y = S/O/N) bonds in alkenes. This reaction works with terminal and internal alkenes containing both cyclic and acyclic amides via radical-polar crossover.

Gram-Scale Synthesis of the N-Phenyl Phenothiazine Photocatalyst by Benzyne Addition

N-phenyl phenothiazine is one of the most reducing photoredox catalysts. Its synthesis commonly requires transition metal catalyzed cross-coupling reactions. Here we show the syntheses of four aryl phenothiazines via a benzyne route, including a multi-gram scale synthesis of N-phenyl phenothiazine. While yields are modest, the simplicity, low cost, and lack of requirement for cross-coupling catalysts in this synthesis will be attractive to users of this photocatalyst.

Recent Advances in Employing Catalytic Donors and Acceptors in Electron Donor-Acceptor Complex Photochemistry

Electron donor-acceptor (EDA) complexes provide a means to initiate radical reactions under visible light irradiation using substrates that do not absorb visible light individually. Catalytic approaches to complex formation are vital for advancing this synthetic strategy as it decouples the complexation and photogeneration of radicals from substrate functionalization, a limitation inherent to stoichiometric approaches that restricts structural diversity. This Synopsis highlights recent developments in EDA complex photochemistry in which either the donor or acceptor are employed catalytically.

Branching out: redox strategies towards the synthesis of acyclic α-tertiary ethers

Acyclic α-tertiary ethers represent a highly prevalent functionality, common to high-value bioactive molecules, such as pharmaceuticals and natural products, and feature as crucial synthetic handles in their construction. As such their synthesis has become an ever-more important goal in synthetic chemistry as the drawbacks of traditional strong base- and acid-mediated etherifications have become more limiting. In recent years, the generation of highly reactive intermediates via redox approaches has facilitated the synthesis of highly sterically-encumbered ethers and accordingly these strategies have been widely applied in α-tertiary ether synthesis. This review summarises and appraises the state-of-the-art in the application of redox strategies enabling acyclic α-tertiary ether synthesis.

Organophotoredox-catalyzed semipinacol rearrangement via radical-polar crossover

Over the past century, significant progress in semipinacol rearrangement involving 1,2-migration of α-hydroxy carbocations has been made in the areas of catalysis and total synthesis of natural products. To access the α-hydroxy carbocation intermediate, conventional acid-mediated or electrochemical approaches have been employed. However, the photochemical semipinacol rearrangement has been underdeveloped. Herein, we report the organophotoredox-catalyzed semipinacol rearrangement via radical-polar crossover (RPC). A phenothiazine-based organophotoredox catalyst facilitates the generation of an α-hydroxy non-benzylic alkyl radical followed by oxidation to the corresponding carbocation, which can be exploited to undergo the semipinacol rearrangement. As a result, the photochemical approach enables decarboxylative semipinacol rearrangement of β-hydroxycarboxylic acid derivatives and alkylative semipinacol type rearrangement of allyl alcohols with carbon electrophiles, producing α-quaternary or α-tertiary carbonyls bearing sp3-rich scaffolds.

A Triple Photoredox/Cobalt/Brønsted Acid Catalysis Enabling Markovnikov Hydroalkoxylation of Unactivated Alkenes

We demonstrate Markovnikov hydroalkoxylation of unactivated alkenes using alcohols through a triple catalysis consisting of photoredox, cobalt, and Brønsted acid catalysts under visible light irradiation. The triple catalysis realizes three key elementary steps in a single catalytic cycle: (1) Co(III) hydride generation by photochemical reduction of Co(II) followed by protonation, (2) metal hydride hydrogen atom transfer (MHAT) of alkenes by Co(III) hydride, and (3) oxidation of the alkyl Co(III) complex to alkyl Co(IV). The precise control of protons and electrons by the three catalysts allows the elimination of strong acids and external reductants/oxidants that are required in the conventional methods.

Photoredox/Copper Dual-Catalyzed Benzylic C-H Esterification via Radical-Polar Crossover

Herein, we report a distinctive photoredox/copper dual-catalyzed esterification of benzylic C-H bonds through the combination of photoredox-mediated hydrogen atom transfer and Cu(II)-mediated radical-polar crossover. This methodology demonstrates a high functional group tolerance (>40 examples) and moderate to good yields with structurally diverse benzylic C-H substrates. Notably, stoichiometric amounts of carboxylic acids are used as coupling partners, which allows the synthesis of structurally diverse benzylic esters and the late-stage functionalization of pharmaceuticals.

Photons or Electrons? A Critical Comparison of Electrochemistry and Photoredox Catalysis for Organic Synthesis

Redox processes are at the heart of synthetic methods that rely on either electrochemistry or photoredox catalysis, but how do electrochemistry and photoredox catalysis compare? Both approaches provide access to high energy intermediates (e.g., radicals) that enable bond formations not constrained by the rules of ionic or 2 electron (e) mechanisms. Instead, they enable 1e mechanisms capable of bypassing electronic or steric limitations and protecting group requirements, thus enabling synthetic chemists to disconnect molecules in new and different ways. However, while providing access to similar intermediates, electrochemistry and photoredox catalysis differ in several physical chemistry principles. Understanding those differences can be key to designing new transformations and forging new bond disconnections. This review aims to highlight these differences and similarities between electrochemistry and photoredox catalysis by comparing their underlying physical chemistry principles and describing their impact on electrochemical and photochemical methods.

External oxidant-free and selective thiofunctionalization of alkenes enabled by photoredox-neutral catalysis

A photoredox approach was reported to realize a highly selective three-component thiohydroxylation, thioalkoxylation and thioamination of vinylarenes towards valuable vicinal S,O- and S,N-disubstituted molecules under mild conditions.

Extended phenothiazines: synthesis, photophysical and redox properties, and efficient photocatalytic oxidative coupling of amines

Herein, we successfully developed a ring-fusion approach to extend the conjugation length of phenothiazines that were demonstrated to be efficient photocatalysts for visible-light-driven oxidative coupling reactions of amines under an air atmosphere.

Dual Photoredox/Nickel-Promoted Alkylation of Heteroaryl Halides with Redox-Active Esters

Herein a method for the radical alkylation of heteroaryl halides that relies upon the combination of photoredox and nickel catalysis is described. The use of aliphatic N-(acyloxy)phthalimides as redox-active esters affords primary and secondary radicals for the decarboxylative dual cross-coupling with pyrimidine and pyridine heteroaryl chlorides, bromides, and iodides. The method provides an additional synthetic tool for the incorporation of medicinally relevant heterocyclic motifs.

Electrochemical phenothiazination of naphthylamines and its application in photocatalysis

N-Phenylphenothiazine as an inexpensive, highly reductive and oxygen tolerant organophotocatalyst has exhibited potential in various challenging photochemical transformations. Here we report a general and straightforward method to access structurally diverse N-phenylphenothiazine derivatives by means of a novel electrochemical tool. The introduction of a 2-naphthylamine moiety with an extended π-system and an amine group led to the variation of spectral characterization. Photochemical verification experiments demonstrated that the formed N-arylation products with good efficacy and chemo/site-control displayed competitive catalytic activity in challenging transformations.

Visible-Light-Driven Photoredox-Catalyzed Three-Component Radical Cyanoalkylfluorination of Alkenes with Oxime Esters and a Fluoride Ion

A metal-free, photoredox-catalyzed three-component cyanoalkylfluorination of alkenes under mild and redox-neutral conditions is reported. This protocol features use of readily available alkenes, oxime esters, and cost-effective nucleophilic fluoride reagents, giving diverse cyanoalkylfluorinated products with generally good yields. Excellent functional group tolerance and mild reaction conditions also render this protocol suitable for cyanoalkylfluorination of pharmaceutically relevant molecule-derived alkene.

Visible-light synthesis of 4-substituted-chroman-2-ones and 2-substituted-chroman-4-ones via doubly decarboxylative Giese reaction

Doubly decarboxylative, photoredox synthesis of 4-substituted-chroman-2-ones and 2-substituted-chroman-4-ones is described. The reaction involves two independent decarboxylation processes: the first one initiating the cycle and the second completing the process. Visible light, photoredox catalyst, base, anhydrous solvent and inert atmosphere constitute the key parameters for the success of the developed transformation. The protocol proved applicable for coumarin-3-carboxylic acids and chromone-3-carboxylic acids as well as N-(acyloxy)phthalimide which served as precursors of the corresponding alkyl radicals.

The manuscript describes the doubly decarboxylative Giese reaction between N-(acyloxy)phthalimides and coumarin-3-carboxylic acids or chromone-3-carboxylic acids.

Decarboxylative N-Alkylation of Azoles through Visible-Light-Mediated Organophotoredox Catalysis

An organophotoredox-catalyzed decarboxylative cross-coupling between azole nucleophiles and aliphatic carboxylic acid-derived redox-active esters is demonstrated. This protocol efficiently installs various tertiary or secondary alkyl fragments onto the nitrogen atom of azole nucleophiles under mild and transition-metal-free conditions. The pyridinium additive successfully inhibits the formation of elimination byproducts from the carbocation intermediate. This reaction is applicable to the synthesis of a protein-degrader-like molecule containing an azole and a thalidomide.

Photoinduced 1,2-dicarbofunctionalization of alkenes with organotrifluoroborate nucleophiles via radical/polar crossover

Alkene 1,2-dicarbofunctionalizations are highly sought-after transformations as they enable a rapid increase of molecular complexity in one synthetic step. Traditionally, these conjunctive couplings proceed through the intermediacy of alkylmetal species susceptible to deleterious pathways including β-hydride elimination and protodemetalation. Herein, an intermolecular 1,2-dicarbofunctionalization using alkyl N-(acyloxy)phthalimide redox-active esters as radical progenitors and organotrifluoroborates as carbon-centered nucleophiles is reported. This redox-neutral, multicomponent reaction is postulated to proceed through photochemical radical/polar crossover to afford a key carbocation species that undergoes subsequent trapping with organoboron nucleophiles to accomplish the carboallylation, carboalkenylation, carboalkynylation, and carboarylation of alkenes with regio- and chemoselective control. The mechanistic intricacies of this difunctionalization were elucidated through Stern-Volmer quenching studies, photochemical quantum yield measurements, and trapping experiments of radical and ionic intermediates.

Photoredox-Catalyzed Benzylic Esterification via Radical-Polar Crossover

Photoredox-catalyzed C-O bond formation reactions are reported. The decarboxylative esterification reaction allows the conversion of a variety of arylacetic acids into the corresponding benzyl carboxylates. Furthermore, the use of (diacetoxyiodo)benzene allows the conversion of the benzylic C-H bond through hydrogen atom transfer. The reactions were applied to the divergent transformation of pharmaceuticals via decarboxylative or C-H esterification reactions.

Direct excitation strategy for radical generation in organic synthesis

This tutorial review encompasses the radical generation based on classical methods and photoredox catalysis. It will also focus on radical generation only demanding visible-light, which involves EDA complex and direct photo-excitation strategy.