Radical Homopolymerization of Linear α-Olefins Enabled by 1,4-Cyano Group Migration

α-Olefins are valued and abundant building blocks from fossil resources. They are widely used to provide small-molecule or polymeric products. Despite numerous advantages of radical polymerization, it has been well-documented as textbook knowledge that α-olefins and their functionalized derivatives cannot be radically homopolymerized because of the degradative chain transfer side reactions. Herein, we report our studies on the homopolymerization of thiocyanate functionalized α-olefins enabled by 1,4-cyano group migration under radical conditions. By this approach, a library of ABC sequence-controlled polymers with high molecular weights can be prepared. We can also extend this strategy to the homopolymerization of α-substituted styrenic and acylate monomers which are known to be challenging to achieve. Overall, the demonstrated functional group migration radical polymerization could provide new possibilities to synthesize polymers with unprecedented main chain sequences and structures. These polymers are promising candidates for novel polymeric materials.

Silylation and (Hetero)aryl/alkenylation of Unactivated Alkenes via Radical-Mediated Distal 1,4-Migration with Hydrosilanes under Organophotocatalysis

We report a novel organic photoredox catalysis to achieve unprecedented γ-(hetero)aryl/alkenyl-δ-silyl aliphatic amines via silyl-mediated distal (hetero)aryl/alkenyl migration of aromatic/alkenyl amines bearing unactivated alkenes with hydrosilanes. This protocol features mild and metal-free reaction conditions, high atom economy, excellent selectivity, and functional group compatibility. Mechanistic studies suggest that silylation and (hetero)aryl/alkenylation involve photoredox hydrogen atom transfer catalysis and subsequent 1,4-migration of a remote (hetero)aryl/alkenyl group from nitrogen to carbon.

Di-π-ethane Rearrangement of Cyano Groups via Energy-Transfer Catalysis

Molecular rearrangement occupies a pivotal position among fundamental transformations in synthetic chemistry. Radical translocation has emerged as a prevalent synthetic tool, efficiently facilitating the migration of diverse functional groups. In contrast, the development of di-π-methane rearrangement remains limited, particularly in terms of the translocation of cyano functional groups. This is primarily attributed to the energetically unfavorable three-membered-ring transition state. Herein, we introduce an unprecedented di-π-ethane rearrangement enabled by energy-transfer catalysis under visible light conditions. This innovative open-shell rearrangement boasts broad tolerance toward a range of functional groups, encompassing even complex drug and natural product derivatives. Overall, the reported di-π-ethane rearrangement represents a complementary strategy to the development of radical translocation enabled by energy-transfer catalysis.

Photoredox-catalyzed alkylarylation of activated alkenes via a ring-opening/Truce-Smiles rearrangement cascade

A photoredox-catalyzed alkylarylation of activated alkenes via a radical C-C bond cleavage/Truce-Smiles rearrangement cascade is developed. The protocol features mild and redox-neutral conditions, broad substrate scope and excellent functional group compatibility, providing a facile and efficient approach to the long-chain distal keto-amides with all-carbon quaternary centers at the alpha position.

Redox-Neutral Multicatalytic Cerium Photoredox-Enabled Cleavage of O-H Bearing Substrates

The need for synthetic methodologies capable of rapidly altering molecular structure are in high demand. Most existing methods to modify scaffolds rely on net exothermicity to drive the desired transformation. We sought to develop a general strategy for the cleavage of C-C bonds β to hydroxyl groups independent of inherent substrate strain. To this end we have applied a multicatalytic cerium photoredox-based system capable of activating O-H bonds in lactols to deliver formate esters. The same system is also capable of effecting hydrodecarboxylation and hydrodecarbonylation reactions. Initial mechanistic probes demonstrate atomic chlorine (Cl⋅) is generated under the reaction conditions, but substrate activation through cerium-alkoxides or -carboxylates cannot be ruled out.

Electrochemically-Driven 1,4-Aryl Migration via Radical Fluoromethylation of N-Allylbenzamides: a Straightforward Access to Functionalized β-Arylethylamines

An electrochemical radical Truce Smiles rearrangement of N-allylbenzamides is documented herein. The selective 1,4-aryl migration was triggered by the radical fluoromethylation of the alkene providing a direct route to fluoro derivatives of the highly privileged β-arylethylamine pharmacophore. This practical transformation utilizes readily available starting materials and employs an electrical current to drive the oxidative process under mild reaction conditions. It accommodates a variety of migratory aryl groups with different electronic properties and substitution patterns. Careful selection of the protecting group on the nitrogen atom of the N-allylbenzamide is crucial to outcompete the undesired 6-endo cyclization and achieve high level of selectivity towards the 1,4-aryl migration. DFT calculations support the reaction mechanism and unveil the origin of selectivity between the two competitive pathways.

Ultrafast Halogen Dance Reactions of Bromoarenes Enabled by Catalytic Potassium Hexamethyldisilazide

Lochmann-Schlosser base, a stoichiometric combination of nBuLi and KOtBu, is commonly used as a superbase for deprotonating a wide range of organic compounds. In the present study, we report that catalytic potassium hexamethyldisilazide (KHMDS) exhibits higher catalytic activity than KOtBu for successive bromine-metal exchanges. Accordingly, 1-10 mol% of KHMDS dramatically enhances halogen dance reactions to introduce various electrophiles to bromopyridine, bromoimidazole, bromothiophene, bromofuran, and bromobenzene derivatives with the bromo group translocated from the original position. A dual catalytic cycle is proposed to explain the ultrafast bromine transfer.

Asymmetric, Remote C(sp3)-H Arylation via Sulfinyl-Smiles Rearrangement

An efficient asymmetric remote arylation of C(sp3)-H bonds under photoredox conditions is described here. The reaction features the addition radicals to a double bond followed by a site-selective radical translocation (1,n-hydrogen atom transfer) as well as a stereocontrolled aryl migration via sulfinyl-Smiles rearrangement furnishing a wide range of chiral α-arylated amides with up to >99 : 1 er. Mechanistic studies indicate that the sulfinamide group governs the stereochemistry of the product with the aryl migration being the rate determining step preceded by a kinetically favored 1,n-HAT process.

A free-radical design featuring an intramolecular migration for a synthetically versatile alkyl-(hetero)arylation of simple olefins

A free-radical approach has enabled the development of a synthetically versatile alkyl-(hetero)arylation of olefins. Alkyl and (hetero)aryl groups were added concurrently to a full suite of mono- to tetrasubstituted simple alkenes (i.e., without requiring directing or electronically activating groups) for the first time. Key advances also included the introduction of synthetically diversifiable alkyl groups featuring different degrees of substitution, good diastereocontrol in both cyclic and acyclic settings, the addition of biologically valuable heteroarenes featuring Lewis basic nitrogen atoms as well as simple benzenes, and the generation of either tertiary or quaternary benzylic centers. The synthetic potential of this transformation was demonstrated by leveraging it as the key step in a concise synthesis of oliceridine, a new painkiller that received FDA approval in 2020.

Electrochemical cascade migratory versus ortho-cyclization of 2-alkynylbenzenesulfonamides

Efficient control over several possible reaction pathways of free radicals is the chemical basis of their highly selective transformations. Among various competing reaction pathways, sulfonimidyl radicals generated from the electrolysis of 2-alkynylbenzenesulfonamides undergo cascade migratory or ortho-cyclization cyclization selectively. It is found that the incorporation of an extra 2-methyl substituent biases the selective migration of the acyl- over vinyl-linker of the key spirocyclic cation intermediate and thus serves as an enabling handle to achieve the synthetically interesting yet under-investigated cascade migratory cyclization of spirocyclic cations.

Formal Halogen Transfer of Bromoarenes via Stepwise Reactions

A two-step halogen transfer of bromoarenes is reported. Mono-, di-, and tribromoaryllithiums generated through deprotonative lithiation were converted into organozinc species by in situ zincation, which were then subjected to bromination to provide the corresponding di-, tri-, and tetrabromoarenes, respectively, in 41-95% yields. Regioselective bromine-magnesium exchange with ethylmagnesium chloride followed by electrophilic trapping afforded benzene, pyridine, quinoline, pyrimidine, and thiazole derivatives with the bromo group translocated from the original position in 28-86% yields.

Aryl-aryl cross-coupling reactions without reagents or catalysts: photocyclization of ortho-iodoaryl ethers and related compounds via triplet aryl cation intermediates

Cyclisations of benzyl ortho-iodoaryl ethers to benzo[c]chromenes can be effected without reagents or catalysts by irradiation with UVC under flow. Reactions proceed via triplet aryl cation generation, 5-exo and 3-exo-cyclisations, and rearomatisation. They have wide scope, are easy to effect and extend to a myriad of related ring systems.

Recent Developments on Photochemical Synthesis of 1,n-Dicarbonyls

1,n-dicarbonyls are one of the most fascinating chemical feedstocks finding abundant usage in the field of pharmaceuticals. Besides, they are utilized in a plethora of synthesis in general synthetic organic chemistry. A number of 'conventional' methods are available for their synthesis, such as the Stetter reaction, Baker-Venkatraman rearrangement, oxidation of vicinal diols, and oxidation of deoxybenzoins, synonymous with unfriendly reagents and conditions. In the last 15 years or so, photocatalysis has taken the world of synthetic organic chemistry by a remarkable renaissance. It is fair to say now that everybody loves the light and photoredox chemistry has opened a new gateway to organic chemists towards milder, more simpler alternatives to the previously available methods, allowing access to many sensitive reactions and products. In this review, we present the readers with the photochemical synthesis of a variety of 1,n-dicarbonyls. Diverse photocatalytic pathways to these fascinating molecules have been discussed, placing special emphasis on the mechanisms, giving the reader an opportunity to find all these significant developments in one place.

Fluorosulfonylvinylation of Unactivated C(sp3)-H via Electron Donor-Acceptor Photoactivation

An electron donor-acceptor (EDA) complex photoactivation strategy for radical fluorosulfonylation is disclosed for the first time. Simply upon blue light irradiation, the FSO₂ radical can be generated efficiently under catalyst-free, base-free, and additive-free conditions, which enables facile access to 6-keto alkenylsulfonyl fluorides from readily available propargyl alcohols and FSO₂Cl. The 6-keto alkenylsulfonyl fluoride motif has been showcased as a versatile SuFEx hub with diverse follow-up derivatizations.

gem-Bromonitroalkane Involved Radical 1,2-Aryl Migration of α,α-Diaryl Allyl Alcohol TMS Ether via Visible-Light Photoredox Catalysis

A mild and efficient coupling method concerning the reactions of gem-bromonitroalkanes with α,α-diaryl allyl alcohol trimethylsilyl ethers was reported. A cascade consisting of visible-light-induced generation of an α-nitroalkyl radical and a subsequent neophyl-type rearrangement was key to realize the coupling reactions. Structurally diverse α-aryl-γ-nitro ketones, especially those bearing a nitrocyclobutyl structure, were prepared in moderate to high yields, which could be converted into spirocyclic nitrones and imines.

Photochemical Alkene Trifluoromethylimination Enabled by Trifluoromethylsulfonylamide as a Bifunctional Reagent

Herein, we disclose a facile and versatile trifluoromethylimination of alkene with a rationally designed N-(diphenylmethylene)-1,1,1-trifluoromethanesulfonamide as a bench-stable and readily accessible carboamination reagent. Enabled by an energy transfer (EnT) process, an array of alkenes were able to be facilely CF₃-iminated under metal-free photocatalytic conditions. The mild reaction conditions and good functional group compatibility render this protocol highly valuable for the difunctionalization of olefins with structural complexity and diversity.

Visible-Light-Induced Diradical-Mediated ipso-Cyclization towards Double Dearomative [2+2]-Cycloaddition or Smiles-Type Rearrangement

When mono-radical ipso-cyclization of aryl sulfonamides tend to undergo Smiles-type rearrangement through aromatization-driven C-S bond cleavage, diradical-mediated cyclization must perform in a distinct reaction pathway. It is interesting meanwhile challenging to tune the rate of C-S bond cleavage to achieve a chemically divergent reaction of (hetero) aryl sulfonamides in a visible-light induced energy transfer (EnT) reaction pathway involving diradical species. Herein a chemically divergent reaction based on the designed indole-tethered (hetero)arylsulfonamides is reported which involves a diradical-mediated ipso-cyclization and a controllable cleavage of an inherent C-S bond. The combined experimental and computational results have revealed that the cleavage of the C-S bond in these substrates can be controlled by tuning the heteroaryl moieties: a) If the (hetero)aryl is thienyl, furyl, phenanthryl, etc., the radical coupling of double dearomative diradicals (DDDR) precedes over C-S bond cleavage to afford cyclobutene fused indolines by double dearomative [2+2]-cycloaddition; b) if the (hetero)aryl is phenyl, naphthyl, pyridyl, indolyl etc., the cleavage of C-S bond in DDDR is favored over radical coupling to afford biaryl products.

Photoinduced Disulfide-Catalyzed Intramolecular Anti-Markovnikov Hydroamination through in Situ N-S Species

The photoinduced anti-Markovnikov hydroamination of olefins typically required photocatalysts with a high oxidative ability to initiate the single-electron process. Herein, we alternatively utilize bis(2,4,6-triisopropylphenyl) disulfide, an inexpensive reagent with relatively low oxidative ability, as a photo and hydrogen atom transfer catalyst to achieve intramolecular hydroamination. The mechanistic studies as well as the DFT calculations are consistent with a novel process involving N-centered radical generation through the homolysis of the in situ formed N-S species and subsequent cyclization. An array of diverse nitrogen-containing cycles could be obtained.

Real-space imaging of a phenyl group migration reaction on metal surfaces

The explorations to extend present chemical synthetic methods are of great importance to simplify synthetic routes of chemical species. Additionally, understanding the chemical reaction mechanisms is critical to achieve controllable synthesis for applications. Here, we report the on-surface visualization and identification of a phenyl group migration reaction of 1,4-dimethyl-2,3,5,6-tetraphenyl benzene (DMTPB) precursor on Au(111), Cu(111) and Ag(110) substrates. With the combination of bond-resolved scanning tunneling microscopy (BR-STM), noncontact atomic force microscopy (nc-AFM) and density functional theory (DFT) calculations, the phenyl group migration reaction of DMTPB precursor is observed, forming various polycyclic aromatic hydrocarbons on the substrates. DFT calculations reveal that the multiple-step migrations are facilitated by the hydrogen radical attack, inducing cleavage of phenyl groups and subsequent rearomatization of the intermediates. This study provides insights into complex surface reaction mechanisms at the single molecule level, which may guide the design of chemical species.

Visible light-initiated radical 1,3-difunctionalization of β,γ-unsaturated ketones

Radical-mediated 1,2-difunctionalization of olefins is a well-established synthetic technique widely used in the rapid construction of structurally diverse molecular entities. However, radical-mediated 1,3-difunctionalization reactions are rare, and the substrates are generally limited to strained skeletons. Here, we report a practical approach for 1,3-difunctionalization of available β,γ-unsaturated ketones via a radical cascade process including visible light-irradiated radical addition, thermodynamic stability-driven 1,2-carbonyl migration from unactivated all-carbon quaternary center, and terminal C-radical varied transformations. Various highly functionalized alkyl skeletons with different valuable functional groups at positions 1 and 3 and the carbonyl group at position 2 have been synthesized through a radical chain pathway or Cu-catalyzed Ritter-type reaction. Moreover, this protocol provides a real case of diversity-oriented radical rearrangement for drug discovery. We identified a previously unknown chemotype of dual inhibitors for hypoxia-inducible factor (HIF) and WNT signaling pathways from products. These small-molecule inhibitors could suppress HIF and WNT signaling-dependent HCT116 cell growth in 2D and 3D culture systems.

Designing a Nitro-Induced Sutured Biomacromolecule to Engineer Electroconductive Adhesive Hydrogels

Nitro-functionality, with a large deficit of negative charge, embraces biological importance and has proven its therapeutic essence even in chemotherapy. Functionally, with its strong electron-withdrawing capability, nitro can manipulate the electron density of organic moieties and regulates cellular-biochemical reactions. However, the chemistry of nitro-functionality to introduce physiologically relevant macroscopic properties from the molecular skeleton is unknown. Therefore, herein, a neurotransmitter moiety, dopamine, was chemically modified with a nitro-group to explore its influence on synthesizing a multifunctional biomaterial for therapeutic applications. Chemically, while the nitro-group perturbed the aromatic electron density of nitrocatecholic domain, it facilitated the suturing of nitrocatechol moieties to regain its aromaticity through a radical transfer mechanism, forming a novel macromolecular structure. Incorporation of the sutured-nitrocatecholic strand (S-nCAT) in a gelatin-based hydrogel introduced an electroconductive microenvironment through the delocalization of π-electrons in S-nCAT, while maintaining its catechol-mediated adhesive property for tissue repairing/sealing. Meanwhile, the engineered hydrogel enriched with noncovalent interactions, demonstrated excellent mechano-physical properties to support tissue functions. Cytocompatibility of the bioadhesive was assessed with in vitro and in vivo studies, confirming its potential usage for biomedical applications. In conclusion, this novel chemical approach enabled designing a multifunctional biomaterial by manipulating the electronic properties of small bioactive molecules for various biomedical applications.

Sliver-Catalyzed 1,3-Aza-Benzyl Migration of Allyl Alcohol

Carbon migration of alkenyl alcohols has been recognized as an increasingly viable methodology in organic synthesis. Herein, we disclose a silver-catalyzed 1,3-aza-benzyl migration of allyl alcohols by utilizing chelation-assisted selective cleavage of an unstrained C(sp³)-C(sp³) bond. This approach provides an available, efficient, high atom-economic, and environmentally benign procedure, leading to alkylation products with broad substrate scopes and excellent yields. The migration proceeds via a one-pot, two-step process involving a free-state alkyl metal species.

Selective C(sp2)-H bond functionalization of olefins via visible-light-induced photoredox-quinuclidine dual catalysis

The site selective C(sp²)-H bond functionalization of olefins has been achieved through a visible-light-induced photoredox-quinuclidine dual catalysis upon merging the quinuclidinium radical cation addition to alkene strategy and the distal heteroaryl ipso-migration strategy. This synthetic protocol features a simple operation with readily available starting materials in good step-economy to access alkenylheteroaromatic products in moderate to good yields under mild conditions. A plausible cascade catalytic reaction mechanism is also proposed.

Photocatalytic difluoromethylarylation of unactivated alkenes via a (hetero)aryl neophyl-like radical migration

Photoredox-catalyzed addition of the difluoromethylradical to unactivated alkenes has been found to trigger neophyl-like aryl and heteroaryl migrations which allowed the construction of a diverse series of difluoromethyl ketones. The reaction featured mild reaction conditions and broad substrate scope.

Electrochemical Migratory Cyclization of N-Acylsulfonamides

Benzoxathiazine dioxide, as a bioisostere of the clinically widely used diazoxide, exhibits interesting biological activity. However, limited success has been achieved in terms of its concise and direct synthesis. We report herein a facile electrochemical migratory cyclization of N-acylsulfonamides to access a diverse array of benzoxathiazine dioxides. The inclusion of electrochemistry is crucial for realizing such a novel transformation, which is substantiated both by the experiments and density-functional-theory calculations.

Synthesis of Benzimidazo[2,1-a]isoquinoline and Indolo[2,1-a]isoquinoline Derivatives via Copper-Catalyzed Silylation/Methylation of 2-Arylindoles and 2-Arylbenzimidazoles

A one-pot method for the synthesis of silylsubstituted/methylsubstituted indolo[2,1-a]isoquinolin-6(5H)-ones and benzimidazo[2,1-a]isoquinoline-6(5H)-ones via copper(II)-initiated silylation/methylation of 2-arylindoles and 2-arylbenzimidazoles was developed. In this procedure, the C-Si bond and C-C bond were constructed by radical addition and cyclization. A series of 2-arylindole and 2-arylbenzimidazole derivatives were facilely transformed to indolo[2,1-a]isoquinolines and benzimidazo[2,1-a]isoquinolines in 39-83% yields.

1,4-Aryl migration in ketene-derived enolates by a polar-radical-crossover cascade

The arylation of carboxylic acid derivatives via Smiles rearrangement has gained great interest in recent years. Both radical and ionic approaches, as well as radical-polar crossover concepts, have been developed. In contrast, a reversed polar-radical crossover approach remains underexplored. Here we report a simple, efficient and scalable method for the preparation of sterically hindered and valuable α-quaternary amides via a polar-radical crossover-enolate oxidation-aryl migration pathway. A variety of easily accessible N-alkyl and N-arylsulfonamides are reacted with disubstituted ketenes to give the corresponding amide enolates, which undergo upon single electron transfer oxidation, a 1,4-aryl migration, desulfonylation, hydrogen atom transfer cascade to provide α-quaternary amides in good to excellent yields. Various mono- and di-substituted heteroatom-containing and polycyclic arenes engage in the aryl migration reaction. Functional group tolerance is excellent and substrates as well as reagents are readily available rendering the method broadly applicable.

The α-arylation of amides via aryl migration has attracted considerable interest in recent years. Here, the authors report a method for the preparation of bulky α-quaternary amides via a polar-radical crossover enolate oxidation-aryl migration cascade.

Microwave-accelerated and benzoyl peroxide (BPO)-initiated cyclization of 1,5-enynes having cyano groups with cyclic alkanes under metal-free conditions

A novel and efficient method for preparing exocyclic indan derivatives, with this method involving benzoyl peroxide (BPO)-initiated cyclization of 1,5-enynes having cyano groups with simple cyclic alkanes under microwave irradiation, has been developed. The presented approach showed advantages of simple conditions, an environmentally friendly protocol, good functional-group tolerance, and high yields of products.

Visible-Light-Induced Alkylarylation of Unactivated Alkenes via Radical Addition/Truce-Smiles Rearrangement Cascade

We disclosed a visible-light-induced alkylarylation reaction of unactivated alkenes via a metal-free radical addition/aryl translocation cascade sequence. Distal olefinic sulfonate was designed as a unique molecular scaffold allowing for a domino process to synthesize valuable alkylarylated alcohols in good yields with excellent diastereoselectivity, featuring mild reaction conditions, broad substrate scope, and excellent functional group tolerance. The mechanism investigation suggests that a visible-light-induced radical chain process dominates the cascade transformation.

Switchable Regioselective 6-endo or 5-exo Radical Cyclization via Photoredox Catalysis

Controlling the regioselectivity of radical cyclizations to favor the 6-endo mode over its kinetically preferred 5-exo counterpart is difficult without introducing substrate prefunctionalization. To address this challenge, we have developed a simple method for reagent controlled regioselective radical cyclization of halogenated N-heterocycles onto pendant olefins. Radical generation occurs under mild photoredox conditions with control of the regioselectivity governed by the rate of hydrogen atom transfer (HAT). Utilizing a polarity-matched thiol-based HAT agent promotes the highly selective formation of the 5-exo cyclization product. Conversely, limiting the solubility of the HAT reagent Hantzsch ester (HEH) leads to selective formation of the thermodynamically favored 6-endo product. This occurs through an initial 5-exo cyclization, with the resulting alkyl radical intermediate undergoing neophyl rearrangement to form the 6-endo product. Development of this switchable catalysis strategy allows for two modes of divergent reactivity to form either the 6-endo or 5-exo product, generating fused N-heteroaromatic/saturated ring systems.

Passerini-Smiles Reaction of α-Ketophosphonates: Platform for Phospha-Brook/Smiles Embedded Cascades

The Passerini-Smiles reactions of α-ketophosphonates with nitrophenols has been used as a platform to observe complex cascades involving multiple Smiles transfers coupled with phospha-Brook rearrangement. When using 4-nitrophenols a rare 1,3-Truce-Smiles rearrangement is observed leading to diarylacetamide derivatives. 2-Nitro-derivatives lead to a completely different reactivity pattern that may be explained by a nitro to nitroso conversion followed by a σ-π metathesis. All mechanistic assumptions are confirmed by DFT calculations performed on both families of adducts. The potential of this work has been further demonstrated by the use of N-aryl α-ketoamides as alternative starting materials for these cascades as well as the disclosure of new aza-Nazarov access to hydroxy-indolones.

Strategies to Generate Nitrogen-centered Radicals That May Rely on Photoredox Catalysis: Development in Reaction Methodology and Applications in Organic Synthesis

For more than 70 years, nitrogen-centered radicals have been recognized as potent synthetic intermediates. This review is a survey designed for use by chemists engaged in target-oriented synthesis. This review summarizes the recent paradigm shift in access to and application of N-centered radicals enabled by visible-light photocatalysis. This shift broadens and streamlines approaches to many small molecules because visible-light photocatalysis conditions are mild. Explicit attention is paid to innovative advances in N-X bonds as radical precursors, where X = Cl, N, S, O, and H. For clarity, key mechanistic data is noted, where available. Synthetic applications and limitations are summarized to illuminate the tremendous utility of photocatalytically generated nitrogen-centered radicals.

Aryl Transfer Strategies Mediated by Photoinduced Electron Transfer

Radical aryl migrations are powerful techniques to forge new bonds in aromatic compounds. The growing popularity of photoredox catalysis has led to an influx of novel strategies to initiate and control aryl migration starting from widely available radical precursors. This review encapsulates progress in radical aryl migration enabled by photochemical methods─particularly photoredox catalysis─since 2015. Special attention is paid to descriptions of scope, mechanism, and synthetic applications of each method.

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner's guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N-H, O-H, S-H, and C-H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X═Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.

Electrochemically-promoted synthesis of benzo[b]thiophene-1,1-dioxides via strained quaternary spirocyclization

We report an approach for the synthesis of benzothiophene motifs under electrochemical conditions by the reaction of sulfonhydrazides with internal alkynes. Upon the formation of a quaternary spirocyclization intermediate by the selective ipso-addition instead of an ortho-attack, the S-migration process was rationalized to lead to the products. Computational studies revealed the selectivity and the compatibility of drug molecules showcased the potential application of the protocols.

We report an approach for the synthesis of benzothiophene motifs under electrochemical conditions by the reaction of sulfonhydrazides with internal alkynes.

2-Pyridinylmethyl borrowing: base-promoted C-alkylation of (pyridin-2-yl)-methyl alcohols with ketones via cleavage of unstrained C(sp3)–C(sp3) bonds

2-Pyridinylmethyl Borrowing: Transition-metal-free 2-pyridinylmethyl borrowing C-alkylation of alcohols access to ketones is developed. This unstrained C(sp3)–C(sp3) bonds cleavage of unactivated alcohols avoids the use of transition metals.

A two-phase bromination process using tetraalkylammonium hydroxide for the practical synthesis of α-bromolactones from lactones

A simple and efficient method for α-brominating lactones that affords α-bromolactones under mild conditions using tetraalkylammonium hydroxide (R₄N⁺OH^-) as a base was developed. Lactones are ring-opened with Br₂ and a substoichiometric amount of PBr₃, leading to good yields of the corresponding α-bromocarboxylic acids. Subsequent intramolecular cyclization over 1 h using a two-phase system (H₂O/CHCl₃) containing R₄N⁺OH^- afforded α-bromo lactones in good yields. This method can be applied at the 10 mmol scale using simple operations. α-Bromo-δ-valerolactone, which is extremely reactive and difficult to isolate, could be isolated and stored in a freezer for about one week using the developed method. Optimizing the solvent for environmentally friendly large-scale syntheses revealed that methyl ethyl ketone (MEK) was as effective. In addition, in situ-generated α-bromo-δ-valerolactone was directly converted into a sulfur-substituted functional lactone without difficulty by reacting it with a sulfur nucleophile in one pot without isolation. This new bromination system is expected to facilitate the industrial use of α-bromolactones as important intermediates.

Copper-Catalyzed Aminoarylation of Alkenes via Aminyl Radical Addition and Aryl Migration

We describe a new strategy for aminoarylation of alkenes by copper-catalyzed smiles rearrangement using O-benzoylhydroxylamines as the amine reagent. This method affords various β-amino amide derivatives possessing a quaternary carbon center with wide functional group tolerance and high regioselectivity. The mechanistic studies indicate that the transformation can involve aminyl radical intermediates under acid-free condition.

Radical 1,4/5-Amino Shift Enables Access to Fluoroalkyl-Containing Primary β(γ)-Aminoketones under Metal-Free Conditions

A novel radical 1,4/5-amino shift from the oxygen center of alkene-tethered diphenyl ketoxime ethers to the carbon center to achieve high value-added fluoroalkyl-containing primary β(γ)-amino-ketones is reported. Mechanism studies reveal that the migration is triggered by the alkene addition of fluoroalkyl radical derived from the electron donor-acceptor (EDA) complex of Togni's reagent II or fluoroalkyl iodides and quinuclidine, and involves a unique 5(6)-exo-trig cyclization of the carbon-centered radical onto the N-atom of ketoxime ethers followed by a cascade sequence of N-O bond cleavage and dehydrogenation. Notably, besides Togni's reagent II and fluoroalkyl iodides, this protocol is also compatible with other radical precursors to provide various functionalized primary aminoketones.

Visible-Light Induced C(sp2 )-H Amidation with an Aryl-Alkyl σ-Bond Relocation via Redox-Neutral Radical-Polar Crossover

We report an approach for the intramolecular C(sp² )-H amidation of N-acyloxyamides under photoredox conditions to produce δ-benzolactams with an aryl-alkyl σ-bond relocation. Computational studies on the designed reductive single electron transfer strategy led us to identify N-[3,5-bis(trifluoromethyl)benzoyl] group as the most effective amidyl radical precursor. Upon the formation of an azaspirocyclic radical intermediate by the selective ipso-addition with outcompeting an ortho-attack, radical-polar crossover was then rationalized to lead to the rearomative ring-expansion with preferential C-C bond migration.