Electrochemical C-H functionalization reaction of N-heterocycles with alkyl iodides

Herein, we report on an electrochemical protocol for the C-H alkylation of N-heterocycles with easily accessible alkyl halides. A wide range of azauracil derivatives including bioactive tethered azauracil, pyrazinone and quinoxalinone were well accommodated and delivered the alkylated products in good to excellent yield.

Visible Light-Promoted Ir(III)-Catalyzed Stereoselective Synthesis of Azauracil-C-Nucleosides from 1-Bromosugar

A visible light-promoted, mild, and efficient Ir(III)-catalyzed synthesis of C-nucleosides is reported, utilizing 1-bromosugar as the glycosyl radical precursor and 6-azauracil as the nucleobase. The method exhibits high α-selectivity and excellent functional group tolerance. Spectroscopic evidence shows that the coupling reaction happens via initial reductive quenching of the Ir(III) catalyst under visible light. Density functional theory calculation reveals the reason for complete α-selectivity. Finally, biologically active 6-aza pseudouridine analogues were synthesized, making the process a valuable platform for C-nucleosides.

Organic Photocatalyst Utilizing Triplet Excited States for Highly Efficient Visible-Light-Driven Polymerizations

ConspectusUltraviolet (UV) light has traditionally been used to drive photochemical organic transformations, mainly due to the limited visible-light absorption of most organic molecules. However, the high energy associated with UV light often causes undesirable side reactions. In the late 2000s, MacMillan, Yoon, and Stephenson pioneered the use of visible light in conjunction with photocatalysts (PCs) to initiate organic transformations. This innovative approach overcame the limitations of UV light by utilizing visible-light-absorbing PCs in their photoexcited states for electron or energy transfer, generating reactive radical species and promoting the photoreactions. Furthermore, while the photocatalysis has predominantly relied on transition-metal complexes, concerns over the potential toxicity, cost, and sustainability of these metals have driven the development of organic PCs. These organic PCs eliminate the need for metal removal, offer structural diversity, and enable tuning of their properties, thus paving the way for the creation of a tailored library of PCs.In recent decades, significant advancements have been made in the development of novel organic PCs with diverse scaffolds, with a notable example being the work of Zhang et al. in 2016. They demonstrated that cyanoarene analogues, originally developed by Adachi et al. for thermally activated delayed fluorescence (TADF) in organic light-emitting diodes, could function effectively as PCs. Building on these insights, we developed a PC design platform featuring TADF compounds with twisted donor-acceptor structures, which paved the way for new PC discoveries. We showcased these PCs' ability (i) to generate long-lived lowest triplet excited (T1) states and (ii) to tune redox potentials by independently modifying donor and acceptor moieties. Through this platform, we discovered PCs with varying redox potentials and the capability to effectively populate T1 states, establishing structure-property relationships within our PC library and creating PC selection criteria for targeted reactions. Specifically, we tailored PCs for highly efficient reversible-deactivation radical polymerizations, including organocatalyzed atom transfer radical polymerization, photoinduced electron/energy transfer reversible addition-fragmentation chain transfer polymerization, and atom transfer radical polymerization with dual photoredox/copper catalysis as well as rapid free radical polymerizations. These advancements have also facilitated the development of functionalized, visible-light-cured adhesives for advanced display technologies. Furthermore, we investigated the origins of the exceptional catalytic performance of these PCs through comprehensive mechanistic studies, including electrochemical and photophysical measurements, quantum chemical calculations, and kinetics simulations. Specifically, we studied the formation and degradation of key PC intermediates in photocatalytic dehalogenations of alkyl and aryl halides. Our findings revealed a distinctive photodegradation pattern in the cyanoarene-based PCs, which significantly impact their catalytic efficiency in the reaction. Additionally, this discovery led us to introduce a concept of beneficial PC degradation for the first time.Over the past decades, organic photocatalysis based on the T1 state has become central to polymerization and organic synthesis. Utilizing our PC design platform, we have developed novel PCs and catalytic systems that enhance the overall efficiency of various organic transformations. In this overview of our contributions to visible-light-driven organic photocatalysis, we highlight the role of the T1 state in broadening applications through mechanistic analysis, enabling more sustainable transformations.

Site-Selective C─H Bond Functionalization of Sugars

Non-typical C-functionalized sugars represent a prominent yet hardly accessible class of biologically-active compounds. The available synthetic methodologies toward such sugar derivatives suffer either from an extensive use of protecting groups, requiring long and laborious synthetic manipulations, or from limited predictability and noncontrollable site-selectivity of the employed C-functionalization reactions. In this work, we disclose an alternative synthetic methodology toward nontypical sugars that allows facile, site-selective, and stereocontrolled C-functionalization of sugars through a traceless tethering approach. The described silyl-based redox-active tethering group appends directly to the unprotected sugar substrate and mediates the C-functionalization reaction through a photochemically-promoted 1,6-hydrogen atom transfer (HAT) mechanism, while transforming into a readily-removable silyl protecting group. The protocol is compatible with a variety of unprotected carbohydrate substrates featuring sensitive aglycons and a diverse set of coupling partners, providing a straightforward and scalable route to pharmaceutically relevant C-functionalized carbohydrate conjugates.

Photocatalytic hydroalkylation of 3-methyleneisoindolin-1-ones with unactivated alkyl iodides

We report herein a simple method for hydroalkylation of 3-methyleneisoindolin-1-ones with unactivated iodoalkanes using visible light photocatalysis and a halogen atom transfer (XAT) process. This operationally simple method exhibits broad substrate scope and allows late-stage modifications of iodoalkanes derived from either active pharmaceutical ingredients or natural products, producing a range of structurally diverse and valuable corresponding hydroalkylation products in decent yields. The generation of alkyl radicals and carbanion intermediates was directly proven in the catalytic cycle through radical trapping/radical clock and isotope labeling studies, respectively.

Photoredox-Catalyzed Decarboxylative Elimination via Halogen Atom Transfer

Enamides and enecarbamates offer an excellent balance between stability and reactivity. Decarboxylation of widely available amino acids offers a green and efficient alternative to accessing these reagents. The present study describes a photocatalytic approach for the direct decarboxylative synthesis of enamides via sequential radical decarboxylation and putative halogen-atom transfer (XAT). This operationally simple, economical protocol is scalable and allows for mild reaction conditions and short reaction times. In addition, it obviates the need for transition metals and preactivation of carboxylic acids.

Unraveling the Roles of Amines in Atom Transfer Radical Polymerization in the Dark

Multidentate amines have been widely used as ligands (L) for Cu-catalysts in atom transfer radical polymerization (ATRP) and as electron donors in photochemically induced polymerizations. However, mechanistic aspects of the role of amines in ATRP in the dark have remained elusive. Herein, the structure-activity relationship and the related electron transfer reactions with Br-CuII/L complexes and/or with alkyl bromides (R-Br) were investigated for 25 amines. Amines function as electron donors and reducing agents for Br-CuII/L complexes via an outer sphere electron transfer (OSET) mechanism, enabling slow but continuous generation of CuI/L activators and inducing controlled ATRP. However, two amines, diazabicyclo(5.4.0)undec-7-ene (DBU) and 1,1,3,3-tetramethylguanidine (TMG), reduced Br-CuII/L faster, suggesting an inner sphere electron transfer (ISET) process. ATRP, starting with initial deactivators (Br-CuII/L) species, proceeded in the dark in the presence of an excess of tertiary amines, such as tris[2-(dimethylamino)ethyl]amine (Me6TREN), 1,4-diazabicyclo[2.2.2]octane (DABCO), and TMG at room temperature and afforded polymers with low dispersities (Đ ≤ 1.15). With copper(II) triflate complex (CuII/L+2, -(OTf)2), which has a more positive reduction potential, ATRP proceeded at room temperature with several inexpensive secondary and tertiary amines including triethylamine (TEA) and dimethylethanolamine (DMAE). Interestingly, multidentate amines also served as direct R-Br activators at elevated temperatures (60 °C). In all cases, chains were initiated with R-Br and not by the amine radical cations as byproducts of electron transfer. Amines also enabled ATRP in the presence of residual air in flasks with a large headspace, underpinning them as a robust and accessible reducing agent for practical applications.

Haloalkane-driven dichotomous reactivity of aryl radicals as halogen and hydrogen atom transfer agents: photocatalytic olefin and alkyne functionalization cascades

A visible-light mediated protocol for the annulative trifluoroethylation, perfluoroalkylation and di/tri-chloromethylation of olefins and alkynes tethered to aromatic rings is described. Both PIDA and diazonium salts can independently promote this transformation by simultaneously acting as both an oxidant and halogen atom transfer reagent. The reaction is applicable to multiple precursor classes, leading to the synthesis of various N- and O-containing ring systems including dihydroisoquinolinones, coumarins, fused benzimidazoles, and quinazolinone. The activation of chloroform and dichloromethane was also achieved by hydrogen atom transfer, displaying a mechanistic dichotomy of aryl radicals.

Facile, general allylation of unactivated alkyl halides via electrochemically enabled radical-polar crossover

Electrochemically driven carbon-carbon formation is receiving considerable interest in organic synthesis. In this study, we present an electrochemically driven method for the formation of C(sp3)-C(sp3) bonds using readily available allylic carbonates, as well as primary, secondary, and tertiary alkyl bromides as electrophiles. This approach offers a highly selective route for synthesizing a broad range of allylic products with excellent functional group tolerance, all without the need for transition metal catalysts. Remarkably, this method also enables the smooth late-stage functionalization of various natural product- and drug-derived substrates, yielding the corresponding complex allylalkanes.

Base-Promoted Homolytic Aromatic Substitution (BHAS) Reactions and Hydrodehalogenations Driven by Green Light and an Iron(III)-NHC Photoredox Catalyst

An Fe(III)-NHC complex has been employed for the green light driven catalysis of base-promoted homolytic aromatic substitution (BHAS) reactions. Tributylamine was used as a sacrificial electron donor, together with potassium carbonate as base in dimethyl sulfoxide as solvent. In contrast to previously studied photocatalysts, the excited Fe(III)-NHC complex is not reducing the arylhalide substrates. Instead, the latter are activated by α-aminoalkyl radicals formed upon reductive quenching of the photocatalyst by tributylamine. Avoiding strongly reducing photocatalysts as well as strong base, these mild reaction conditions allowed for the expansion of the substrate scope to accommodate also aldehyde and ester substituents. 100 % conversion was obtained after 48 h of irradiation. In this way a wide variety of cyclized products and their corresponding hydrodehalogenated products were obtained as isolated and pure compounds, in the vast majority of cases.

Switchable Closed-Shell and Open-Shell Biradical States in Bis-Palladium Complexes of Tetrathiadodecaphyrin via Coordination Rearrangement

A figure-eight tetrathiadodecaphyrin (1), featuring two porphyrin-like sub-pockets separated by central carbazolylenes was synthesized. Metalation of the thiaporphyrinoid ligand with Pd(OAc)2 produces two distinct bis-Pd(II) complexes with different coordination environments. Complex 2, adopting an {NNCS} metalation mode, exhibits a closed-shell electronic structure, whereas complex 3, with an {NNCC} coordination environment, exists as a ligand-centered organic biradicaloid with two magnetically independent spins (S = 1/2). Biradical formation is attributed to single-electron transfer from each ligand sub-pocket to the Pd(II) center accommodated in a d8 square-planner coordination geometry. Notably, the complexes are interconvertible through doubly one-electron redox processes, demonstrating a reversible metal coordination rearrangement via thiophene ring flipping within a porphyrinoid framework. This work establishes the first example of such tunable metal coordination, offering a precise strategy for modulating closed-shell and open-shell biradical states. In addition, while complex 2 displays intense absorption and photoacoustic responses to the first near-infrared (NIR-I) light in water after encapsulation within nanoparticles, the nanocomposites encapsulating biradicaloid 3 exhibits enhanced responsiveness in the second near-infrared (NIR-II) region.

Enantioselective Hydrodifluoroalkylation of Alkenes with Conformationally Tuned Peptidyl Hydrogen Atom Transfer Catalysts

We report the enantioselective hydrodifluoroalkylation of alkenes proceeding via an asymmetric hydrogen atom transfer (HAT) event catalyzed by thiol-containing tetrapeptides. Photocatalytic generation of a difluoroacetyl radical followed by carbon-carbon bond formation results in a prochiral carbon-centered radical that engages with the chiral catalyst. A trialkylamine reductant is proposed to turn over the catalyst in this net-reductive transformation. Notably, incorporating an (S)-β-methyl-substituted cysteine as the N-terminal residue improved selectivity relative to that of the native N-terminal cysteine (Cys) residue, and X-ray crystallographic analysis supports the conformational underpinning of this effect. A range of enantioenriched γ-substituted amides were synthesized in up to a 96:4 enantiomeric ratio, demonstrating the broad functional group tolerance of this method. Models accounting for asymmetric induction are proposed with supporting DFT calculations.

Electrochemical alkylation of C(sp2)-H bonds via halogen-atom transfer (XAT) from alkyl iodides

Here, we present an electrochemical C(sp2)-H bond alkylation of unactivated alkyl iodides via a halogen-atom transfer (XAT) process under mild conditions. This strategy avoids the drawbacks associated with sacrificing reactive metal anodes in electrochemical direct reduction and demonstrates excellent functional group tolerance.

Photo-induced dehalogenative deuteration and elimination of alkyl halides enabled by phosphine-mediated halogen-atom transfer

Dehalogenative deuteration of organic halides is an efficient and straightforward method for incorporating deuterium atoms at specific locations within target molecules. However, utilizing organic halides in photoredox chemistry, particularly unactivated alkyl halides, presents challenges due to their low reduction potentials. In this work, we present a general and effective photoinduced dehalogenative deuteration method for a diverse array of alkyl halides, employing D2O as an economical source of deuterium. The use of Cy3P as a halogen-atom transfer reagent facilitates the dehalogenation of alkyl halides. This method demonstrates a broad scope, with over 70 examples, and shows excellent tolerance for various alkyl halides. The precise dehalogenation of complex alkyl halides highlights the potential of this protocol for late-stage dehalogenative deuteration of natural product derivatives and pharmaceutical compounds. Additionally, the dehalogenative elimination of unactivated alkyl halides can also be achieved by integrating photoredox and cobalt catalysis using the same halogen-atom transfer agents.

Photoredox cobalt-catalyzed asymmetric desymmetric reductive coupling of cyclobutenes with alkynes

Catalytic methods to couple alkynes and alkenes are highly valuable in synthetic chemistry. The cobalt-catalyzed intermolecular reductive coupling of alkenes and alkynes is particularly attractive due to the unique reactivity and cost-effectiveness of cobalt catalysts. However, the enantioselective transformations of this kind are less developed. The limited successful enantioselective examples are restricted to the use of electronically biased activated olefins as the coupling partners. Herein, we report an asymmetric desymmetric reductive coupling of electronically unbiased succinimide-containing cyclobutenes with alkynes to synthesize enantioenriched, synthetically important vinyl cyclobutanes via photoredox and cobalt dual catalysis. Excellent enantioselectivities, good diastereoselectivities and regioselectivities are obtained. Preliminary mechanistic studies suggest that Hantzsch ester is a better reducing reagent when used in combination with Et3N. Density functional theory calculations reveal that the reaction proceeds more likely through a Co(III)-H migratory insertion mechanism.

MeOH-Triggered Halogen-Atom Transfer of Unactivated Alkyl Bromides Enabling the Photoredox Giese Addition

Herein, a nickel-catalyzed, photoredox Giese addition reaction with readily accessible alkyl bromides, driven by readily available feedstock MeOH as the halogen-atom transfer (XAT) reagent, was successfully achieved under mild conditions. The versatility of this protocol was demonstrated through a range of structurally varied alkyl bromides and Giese-type acceptors with moderate to good yields. Mechanistic investigation highlights that the formation of alkyl radicals through the XAT of alkyl bromides was tentatively prompted by •CH2OH, which was derived from the sequential photo-oxidation/1,2-hydrogen-atom transfer of MeOH.

Iodoarene Activation: Take a Leap Forward toward Green and Sustainable Transformations

Constructing chemical bonds under green sustainable conditions has drawn attention from environmental and economic perspectives. The dissociation of (hetero)aryl-halide bonds is a crucial step of most arylations affording (hetero)arene derivatives. Herein, we summarize the (hetero)aryl halides activation enabling the direct (hetero)arylation of trapping reagents and construction of highly functionalized (hetero)arenes under benign conditions. The strategies for the activation of aryl iodides are classified into (a) hypervalent iodoarene activation followed by functionalization under thermal/photochemical conditions, (b) aryl-I bond dissociation in the presence of bases with/without organic catalysts and promoters, (c) photoinduced aryl-I bond dissociation in the presence/absence of organophotocatalysts, (d) electrochemical activation of aryl iodides by direct/indirect electrolysis mediated by organocatalysts and mediators acting as electron shuttles, and (e) electrophotochemical activation of aryl iodides mediated by redox-active organocatalysts. These activation modes result in aryl iodides exhibiting diverse reactivity as formal aryl cations/radicals/anions and aryne precursors. The coupling of these reactive intermediates with trapping reagents leads to the facile and selective formation of C-C and C-heteroatom bonds. These ecofriendly, inexpensive, and functional group-tolerant activation strategies offer green alternatives to transition metal-based catalysis.

Visible-Light-Mediated Radical α-C(sp3)─H gem-Difluoroallylation of Amides with Trifluoromethyl Alkenes via Halogen Atom Transfer and 1,5-Hydrogen Atom Transfer

Direct gem-difluoroallylation at the α-carbonyl position is a challenging process by conventional methods. Herein we report the photocatalytic radical α-C(sp3)─H gem-difluoroallylation of amides with trifluoromethyl alkenes to access the target compounds with good yields and functional group tolerance. The mild and effective conditions allow gem-difluoroalkene motifs as carbonyl bioisosteres incorporated concisely to some complex molecules, including gemfibrozil and estrone derivatives, presenting great potential for late-stage functionalization of drugs, natural products, and bioactive intermediates. Mechanistic investigations suggest a radical pathway combining XAT and 1,5-HAT.

Minisci C-H Alkylation of Heterocycles with Unactivated Alkyl Iodides Enabled by Visible Light Photocatalysis

In this work, we developed a general catalytic strategy that allows Minisci C-H alkylation of a variety of heterocycles using unactivated alkyl halide as an alkyl radical source under visible light photocatalysis. Mild reaction conditions, employing 4CzIPN as an organophotocatalyst and aerial oxygen as a green terminal oxidant, a broad scope, good functional group tolerance, and late-stage C-H alkylation of bioactive and pharmaceutically relevant molecules are advantages of the protocol. Preliminary mechanistic studies indicate the involvement of the α-amino alkyl radical and the alkyl radical and further involvement of aerial oxygen under our reaction conditions.

Nickel-Catalyzed Asymmetric Homobenzylic Hydroamidation of Aryl Alkenes to Access Chiral β-Arylamides

Herein, we introduce a Ni-catalyzed asymmetric homobenzylic hydroamidation reaction that efficiently addresses the dual challenges of achieving regio- and enantioselectivity in the synthesis of β-(hetero)arylethylamides. By employing a transposed NiH catalysis approach, this method facilitates the formation of key chiral nickel-amido intermediates, enabling asymmetric insertion into alkenes to produce the desired β-arylamide products with excellent enantioselectivity. The reaction exhibits a high functional group tolerance and utilizes readily available starting materials of vinylarenes to react with dioxazolone as a robust amidating source. Notably, this approach was successfully applied to the synthesis of pharmaceutical compounds and natural products, such as Clobenzorex, Direx, Selegiline, Sacubitril, and Cipargamin.

Isolated Neutral Organic Radical Unveiled Solvent-Radical Interaction in Highly Reducing Photocatalysis

Diffusion-limited kinetics is a key mechanistic debate when consecutive photoelectron transfer (conPET) is discussed in photoredox catalysis. In situ generated organic photoactive radicals can access catalytic systems as reducing as alkaline metals that can activate remarkably stable bonds. However, in many cases, the extremely short-lived transient nature of these doublet state open-shell species has led to debatable mechanistic studies, hindering adoption and development. Herein, we document the use of an isolated and stable neutral organic nPrDMQA radical as a highly photoreducing species. The isolated radical offers a unique platform to investigate the mechanism behind the photocatalytic activity of organic photocatalyst radicals. The involvement of reduced solvent is observed, formed by single electron transfer (SET) between the short-lived excited state nPrDMQA radical and the solvent. In our detailed mechanistic studies, spectroscopic and chemical affirmation of solvent reduction is strongly evident. Reduction of aryl halides, including difluoroarenes is presented as a model study of the conPET method. Further, the activation of N2O, a greenhouse gas that is yet to be activated by photoredox catalysis, is showcased in the absence of a transition metal.

Modular assembly of amines and diborons with photocatalysis enabled halogen atom transfer of organohalides for C(sp3)-C(sp3) bond formation

In the past few years, the direct activation of organohalides by ligated boryl radicals has emerged as a potential synthetic tool for cross-coupling reactions. In most existing methods, ligated boryl radicals are accessed from NHC-boranes or amine-boranes. In this work, we report a new photocatalytic platform by modular assembly of readily available amines and diboron esters to access a library of ligated boryl radicals for reaction screening, thus enabling the cross-coupling of organohalides and alkenes including both activated and unactivated ones for C(sp3)-C(sp3) bond formation by using the assembly of DABCO A1 and B2Nep2B1. The strategy features operational simplicity, mild conditions and good functional group tolerance. A range of organohalides including activated alkyl chlorides, alkyl bromides (1°, 2° and 3° C-Br) as well as aromatic bromides are applicable in the strategy. Experimental and computational studies rationalize the proposed mechanism.

Co-Catalytic Coupling of Alkyl Halides and Alkenes: the Curious Role of Lutidine

Continuous pressure to shorten synthetic sequences along with the concomitant expansion of scope makes the use of alkyl bromides, chlorides, and oxygen based leaving groups- which are abundant and readily available feedstocks, highly attractive for C-C bond synthesis. However, selective activation of these bonds to generate radical intermediates remains challenging and is generally unfeasible using traditional activation strategies. Herein, we report a dual catalytic activation strategy to access primary, secondary, and tertiary alkyl radicals from respective alkyl chlorides and bromides, as well as primary tosylates and trifluoroacetates. While the method relies on visible light and a photocatalyst to facilitate electron transfer, based on reduction potentials, the substrates are not expected to be reduceable, and yet they are reduced in the presence of lutidine. Ultimately, our investigation revealed that lutidine was a precatalyst and ultimately led to the use of lutidinium iodide salt which served as a critical cocatalyst that resulted in improved reaction profiles. Our studies revealed two critical roles that lutidinium iodide salts play which made it possible to engage otherwise unreactive substrates: nucleophilic exchange and halogen atom transfer by the lutidinium radical. In short, this work converts unactivated alkyl chlorides, bromides, tosylates, and trifluoroacetates to radicals that can be used for C-C bond formation without the need for preactivation─effectively expediting synthesis.

Redox-Neutral Carboxylation of Benzylic Tertiary C-H Bonds with Carbon Dioxide

The direct catalytic carboxylation of benzylic tertiary C-H bonds with CO2 for the synthesis of all-carbon quaternary carboxylic acids represents a significant challenge. Here, we present a redox-neutral approach to address this difficulty by leveraging the synergistic interplay between photocatalysis and cascade hydrogen abstraction cycles. Remarkably, this strategy eliminates the need for sacrificial electron donors, electron acceptors, or stoichiometric additives, offering enhanced atom economy and environmental sustainability. It is particular that the combination of α-amino alkyl radicals with sulfur radicals generated in situ from the decomposition of DMSO was employed to realize the abstraction of benzylic tertiary C-H bonds. Our method enables the direct synthesis of a diverse array of benzylic quaternary carboxylic acids with excellent functional group tolerance as well as the derivatization of bioactive molecules and the gram-scale synthesis of pharmaceuticals under mild reaction conditions.

N-Protection Dependent Phosphorylation of Dehydroamino Acids to Build Unusual Phosphono-Peptides

An efficient Mn(III)-promoted phosphorylation of dehydroalanine (Dha) has been developed to give unusual α-amino acids bearing phosphonates/phosphine oxides and β-vinyl phosphonates/phosphinates depending on N-protection of amino acid. N,N-diprotected dehydroalanine reacted with H-phosphonates and H-phosphine oxides to give structurally diverse phosphorylated α-amino acids through conjugate addition of phosphorous radical generated by Mn(OAc)3.2H2O. Whereas, a highly Z-selective phosphorylation was observed in the case of mono N-Boc protected dehydroalanine via cross dehydrogenative coupling to give (Z)- β -vinyl phosphono amino acid. The method is successfully applied to short peptides to derive unusual phosphono-peptides under mild conditions.

Catalytic Reductive Homocoupling of Benzyl Chlorides Enabled by Zirconocene and Photoredox Catalysis

The bibenzyl skeleton is prevalent in numerous natural products and other biologically active compounds. Radical homocoupling provides a straightforward approach for synthesizing bibenzyls in a single step with the reductive homocoupling of benzyl halides undergoing extensive development. Unlike benzyl bromides and other tailored precursors used in visible-light-mediated homocoupling, benzyl chlorides offer greater abundance and chemical stability. Nevertheless, achieving chemoselective cleavage of the C-Cl bond poses significant challenges, with only a limited number of studies reported to date. Herein, we demonstrate a catalytic reductive homocoupling of benzyl chlorides facilitated by zirconocene and photoredox catalysis. This cooperative catalytic system promotes C-Cl bond cleavage in benzyl chlorides under mild conditions and supports the homocoupling of a wide range of benzyl chlorides, including those derived from pharmaceutical agents. Our preliminary mechanistic investigations highlight the pivotal role of hydrosilane in the catalytic cycle.

Activation and C-C Coupling of Aryl Iodides via Bismuth Photocatalysis

Within the emerging field of bismuth redox catalysis, the catalytic formation of C-C bonds using aryl halides would be highly desirable; yet such a process remains a synthetic challenge. Herein, we present a chemoselective bismuth-photocatalyzed activation and subsequent coupling of (hetero)aryl iodides with pyrrole derivatives to access C(sp2)-C(sp2) linkages through C-H functionalization. This unique reactivity is the result of the bismuth complex featuring two redox state-dependent interactions with light, which 1) activates the Bi(I) complex for oxidative addition via MLCT, and 2) promotes the homolytic cleavage of aryl Bi(III) intermediates through a LLCT process.

Photoredox-Catalyzed Site-Selective Intermolecular C(sp3)-H Alkylation of Tetrahydrofurfuryl Alcohol Derivatives

4'-Selective alkylation of nucleosides has been recognized as one of the ideal and straightforward approaches to chemically modified nucleosides, but such a transformation has been scarce and less explored. In this Letter, we combine a visible-light-mediated photoredox catalysis and hydrogen atom transfer (HAT) auxiliary to achieve β-C(sp3)-H alkylation of alcohol on tetrahydrofurfuryl alcohol scaffolds and exploit it for 4'-selective alkylation of nucleosides. The reaction involves an intramolecular 1,5-HAT process and stereocontrolled Giese addition of the resultant radicals.

Flavin-Catalyzed, Photochemical Conversion of Dehydroalanine into 4,5-Dihydroxynorvaline

The chemical synthesis of unnatural amino acids (UAA) is a key strategy for preparing designed peptides, including pharmaceutically active compounds. Alterations of existing amino acid residues such as dehydroalanine (Dha) are particularly important since selected positions can be addressed without the necessity of a complete de novo synthesis. The intriguing UAA 4,5-dihydroxynorvaline (Dnv) is found in a variety of naturally occurring peptides and biologically active compounds. However, no method is currently available to modify an existing peptide with this residue. We report the use of flavin catalysts and visible light irradiation for this challenge, which serves as a versatile strategy for converting Dha into Dnv. Our study shows that excited flavins are competent hydrogen atom abstraction catalysts for ethers and acetals, which allows masked 1,2-dihydroxyethylene functionalization from 2,2-dimethyl-1,3-dioxolane. The masked diol was successfully coupled to Dha residues, and a series of Dnv-containing products is reported. A mild and orthogonal protocol for deprotection of the acetal group was also identified, allowing free Dnv-modified peptides to be obtained. This method provides a straightforward strategy for Dnv functionalization, which is envisioned to be crucial for accessing natural products and synthetic analogues with pharmaceutical activity.

Better Together: Photoredox/Copper Dual Catalysis in Atom Transfer Radical Polymerization

Photomediated Atom Transfer Radical Polymerization (photoATRP) is an activator regeneration method, which allows for the controlled synthesis of well-defined polymers via light irradiation. Traditional photoATRP is often limited by the need for high-energy ultraviolet or violet light. These could negatively affect the control and selectivity of the polymerization, promote side reactions, and may not be applicable to biologically relevant systems. This drawback can be circumvented by an introduction of the catalytic amount of photocatalysts, which absorb visible and/or NIR light and, therefore, controlled, regenerative ATRP can be performed with the dual-catalytic cycle. Herein, a critical summary of recent developments in the field of dual-catalysis concerning Cu-catalyzed ATRP is provided. Contributions of involved species are examined mechanistically, followed by challenges and future directions towards the next generation of advanced functional macromolecular materials.

Photoredox-Catalyzed 1,4-Dichloromethyldimerization of Alkenes with Chloroform: Access to Polychlorinated Vicinal Diaryl Alkanes

A visible-light-mediated strategy is reported for the direct synthesis of polychlorinated vicinal diaryl alkanes from aryl alkenes and chloroform. In this approach, two haloalkyl radicals generated from chloroform via halogen atom transfer (XAT) and direct single electron transfer (SET) within the same photoredox catalysis cycle enable the 1,4-dichloromethyldimerization of alkenes. Besides chloroform, this strategy is applicable to carbon tetrachloride, bromotrichloromethane, and α-bromo carboxylic esters, yielding corresponding 1,4-disubstituted vicinal diaryl alkanes. Diverse polychlorinated structures containing highly congested vicinal quaternary carbon centers are effectively synthesized by this method. The potential of this reaction in late-stage drug modification is highlighted by the successful transformation of olefins with pharmaceutical structures.

Halide Perovskite Induces Halogen/Hydrogen Atom Transfer (XAT/HAT) for Allylic C-H Amination

Allylic C-H amination has emerged as a powerful tool to construct allylamines, common motifs in molecular therapeutics. Such reaction implies an oxidative path for C-H activation but furnishes reductive amines, inferring mild oxidants' inactivity for C-H oxidation but strong oxidants' detriment to products. Herein we report a heterogeneous catalytic approach that manipulates halogen-vacancies of perovskite photocatalyst and exploits halogenated-solvents (i.e. CH2Cl2, CH2Br2) as mild oxidants for selective C-H allyl amination with 19,376 turnovers. CsPbBr3 nanocrystals induce cooperative hydrogen-atom-transfer (HAT, C-H oxidation, and halogen-vacancy CsPbBr3-x formation) and halogen-atom-transfer (XAT, CsPbBr3-x-induced solvent reduction) under a radical chain mechanism. Terminal/internal olefins are amenable to forge aromatic/aliphatic, cyclic/acyclic, secondary/tertiary allylamines (70 examples), including drugs or their derivatives.

Light-assisted functionalization of aryl radicals towards metal-free cross-coupling

The many synthetic possibilities that arise when using radical intermediates, in place of their polar counterparts, make contemporary radical chemistry research an exhilarating field. The introduction of photocatalysis has helped tame aryl radicals, leading to a resurgence of interest in their chemistry, and an expansion of viable coupling partners and attainable transformations. These methods are more selective and safer than classical approaches, and they utilize new radical precursors. Given the importance of sustainability in current organic synthesis and our interest in light-assisted metal-free transformations, this Review focuses on recent advances in the use of aryl radicals in photoinduced cross-couplings that do not rely on metals for the crucial bond-forming event, and it is structured according to the key step that the aryl radicals engage in.

Giese-type alkylation of dehydroalanine derivatives via silane-mediated alkyl bromide activation

The rising popularity of bioconjugate therapeutics has led to growing interest in late-stage functionalization (LSF) of peptide scaffolds. α,β-Unsaturated amino acids like dehydroalanine (Dha) derivatives have emerged as particularly useful structures, as the electron-deficient olefin moiety can engage in late-stage functionalization reactions, like a Giese-type reaction. Cheap and widely available building blocks like organohalides can be converted into alkyl radicals by means of photoinduced silane-mediated halogen-atom transfer (XAT) to offer a mild and straightforward methodology of alkylation. In this research, we present a metal-free strategy for the photochemical alkylation of dehydroalanine derivatives. Upon abstraction of a hydride from tris(trimethylsilyl)silane (TTMS) by an excited benzophenone derivative, the formed silane radical can undergo a XAT with an alkyl bromide to generate an alkyl radical. Consequently, the alkyl radical undergoes a Giese-type reaction with the Dha derivative, forming a new C(sp3)-C(sp3) bond. The reaction can be performed in a phosphate-buffered saline (PBS) solution and shows post-functionalization prospects through pathways involving classical peptide chemistry.

Carbonylation Reactions at Carbon-Centered Radicals with an Adjacent Heteroatom

Heteroatoms are essential to living organisms and present in almost all molecules with medicinal usage. The catalytic functionalization at the carbon-centered radical with an adjacent heteroatom provides an effective way to value added moiety while retaining the unique physicochemical and pharmacological properties of heteroatoms, which can promote the development of pharmaceutical and fine chemical production. Carbonylative transformation was discovered nearly a century ago which is an efficient method for the synthesis of carbonyl-containing molecules with potent applications in both industry and academia. Despite numerous advances in new reaction development, carbonylative transformation involving adjacent heteroatom carbon radical remain a subject that deserves to be discussed. In this minireview, we systematically summarized and discussed the recent advances in carbonylative transformations involving carbon-centered radicals with an adjacent heteroatom, including oxygen (O), nitrogen (N), phosphorus (P), silicon (Si), sulfur (S), boron (B), fluorine (F), and chlorine (Cl). The related reaction mechanism was also discussed.

Photo-Induced Three-Component Reaction for the Construction Of α-Tertiary Amino Acid Derivatives

The synthesis of α-tertiary amino acids (ATAAs), which are pivotal components in natural metabolism and pharmaceutical innovation, continues to attract significant research interest. Despite substantial advancements, the pursuit of a facile, versatile, and resource-efficient methodology remains an area of active development. In this work, we introduce a visible light-triggered three-component reaction involving readily available nitrosoarenes, N-acyl pyrazoles, and allyl or (bromomethyl)benzenes under mild conditions. This approach enables the straightforward assembly of a wide array of ATAA derivatives (42 examples) in commendably high yields (up to 89 %). Mechanistic investigations elucidate that the reaction proceeds through a dehydration condensation between nitrosoarenes and N-acyl pyrazoles to generate ketimine intermediates. This is followed by a light-driven halogen atom transfer (XAT) process and a radical addition, culminating in the formation of the desired products. The approach showcases excellent functional group compatibility and late-stage derivatization potential, offering new insights and avenues for the synthesis of ATAA analogs.

Cross-Electrophile Coupling: Principles, Methods, and Applications in Synthesis

Cross-electrophile coupling (XEC), defined by us as the cross-coupling of two different σ-electrophiles that is driven by catalyst reduction, has seen rapid progression in recent years. As such, this review aims to summarize the field from its beginnings up until mid-2023 and to provide comprehensive coverage on synthetic methods and current state of mechanistic understanding. Chapters are split by type of bond formed, which include C(sp3)-C(sp3), C(sp2)-C(sp2), C(sp2)-C(sp3), and C(sp2)-C(sp) bond formation. Additional chapters include alkene difunctionalization, alkyne difunctionalization, and formation of carbon-heteroatom bonds. Each chapter is generally organized with an initial summary of mechanisms followed by detailed figures and notes on methodological developments and ending with application notes in synthesis. While XEC is becoming an increasingly utilized approach in synthesis, its early stage of development means that optimal catalysts, ligands, additives, and reductants are still in flux. This review has collected data on these and various other aspects of the reactions to capture the state of the field. Finally, the data collected on the papers in this review is offered as Supporting Information for readers.

Photoredox-Catalyzed C-H Methylation of N-Heteroarenes Enabled by N,N-Dimethylethanolamine

A visible-light-driven radical C-H methylation of N-heteroarenes that is efficient and additive- and catalyst-free and employs readily available N,N-dimethylethanolamine as the methyl source has been developed. The transformation offers the benefits of broad substrate scope, mild reaction conditions, and operational simplicity. A photoactive electron donor-acceptor (EDA) complex between N-heteroarenes and N,N-dimethylethanolamine is essential for this transformation, as revealed by mechanistic investigations.

Iron-Catalyzed Perfluoroalkylarylation of Styrenes with Arenes and Alkyl Iodides Enabled by Halogen Atom Transfer

A new iron-catalyzed three-component perfluoroalkylarylation of styrenes with alkyl halides and arenes has been established. Alkyl halides undergo halogen atom transfer with methyl radicals to form alkyl radicals in reactions initiated by a combination of tert-butyl peroxybenzoate and an iron catalyst, thus adducting to the olefins, which results in alkylarylation products. The protocol is compatible with a wide range of perfluoroalkyl and non-perfluoroalkyl halides, features excellent functional group tolerance, and enables the synthesis of structurally diverse 1,1-diaryl fluoro-substituted alkanes.

Divergent Synthesis of (E)- and (Z)-Alkenones via Photoredox C(sp3)-H Alkenylation-Dehydrogenation of o-Iodoarylalkanols with Alkynes

A photoredox C(sp3)-H alkenylation-dehydrogenation of o-iodoarylalkanols with terminal alkynes for the synthesis of (E)- and (Z)-quaternary carbon center-containing pent-4-en-1-ones is described. The stereoselectivity depends on the utilization of alkynes and photocatalysts. While using an organic photocatalyst like 4-DPAIPN manipulates the C(sp3)-H alkenylation-dehydrogenation of o-iodoarylalkanols with arylalkynes to assemble (E)-pent-4-en-1-ones, in the case of an Ir potocatalyst such as Ir(ppy)2(dtbbpy)PF6 the reaction with arylalkynes delivers (Z)-pent-4-en-1-ones. For alkylalkynes, the reaction furnishes (E)-pent-4-en-1-ones exclusively in the presence of 4-DPAIPN or Ir(ppy)2(dtbbpy)PF6.

Hydro- and deutero-deamination of primary amines using O-diphenylphosphinylhydroxylamine

While selective defunctionalizations are valuable in organic synthesis, hydrodeamination of primary amines poses challenges. Deuterodeamination, analogous to hydrodeamination, presents even greater difficulties due to its frequently slower deuteration rate, interference by hydrogenation and constraints in deuterated sources. This study introduces a reliable, robust, and scalable hydro- and deuterodeamination method capable of handling various primary amines. Defined by its mild reaction conditions, rapid completion, simplified purification facilitated by water-soluble byproducts, the method leverages deuterium oxide as a deuterium source and employs commercialized O-diphenylphosphinylhydroxylamine for deamination. Applied to a diverse range of biologically active molecules, it has consistently achieved high yields and efficient deuterium incorporation. By synergizing with site-selective C-H functionalization of primary aliphatic amines, our method reveals synthetic strategies utilizing nitrogen atom as a traceless directing group, encompassing deaminative alkylation, 1,1-deuteroalkylation, 1,1-dialkylation, 1,1,1-deuterodialkylation, C-H arylation, and 1,3-deuteroarylation. Emphasizing this innovation, the processes of deaminative degree-controlled deuteration have been developed.

Catalytic Aldehyde-Alkyne Couplings Triggered by Ketyl Radicals

A general and flexible platform for catalytic aldehyde-alkyne couplings triggered by ketyl radicals is described. This open-shell strategy necessitates only a catalytic quantity of a photoredox catalyst, along with Hünig's base (DIPEA) as a halogen atom transfer reagent. The reaction proceeds through sequential steps involving activation, halogen atom transfer, and radical addition. This carbonyl-alkyne coupling exhibits a wide substrate scope and functional group compatibility and has been successfully applied to the late-stage modification of complex architectures.

Overcoming Copper Reduction Limitation in Asymmetric Substitution: Aryl-Radical-Enabled Enantioconvergent Cyanation of Alkyl Iodides

Cu-catalyzed enantioconvergent cross-coupling of alkyl halides has emerged as a powerful strategy for synthesizing enantioenriched molecules. However, this approach is intrinsically limited by the weak reducing power of copper(I) species, which restricts the scope of compatible nucleophiles and necessitates extensive ligand optimization or the use of complex chiral scaffolds. To overcome these challenges, we introduce an aryl-radical-enabled strategy that decouples the alkyl halide activation step from the chiral Cu center. We demonstrate that merging aryl-radical-enabled iodine abstraction with Cu-catalyzed asymmetric radical functionalization enables the conversion of racemic α-iodoamides to enantioenriched alkyl nitrile products with good yield and enantioselectivity. The rational design of chiral ligands identified a new class of carboxamide-containing BOX ligands. Mechanistic studies support an aryl-radical-enabled pathway and the unique hydrogen-bonding ability in the newly designed BOX ligands. This aryl-radical-enabled asymmetric substitution reaction has the potential to significantly expand the scope of Cu-catalyzed enantioconvergent cross-coupling reactions.

Light-Induced Difunctionalization of Alkenes with Polyhaloalkanes and Quinoxalin-2(1H)-ones

Herein, we report a metal-free light-induced three-component reaction for the synthesis of polychloroalkyl-substituted quinoxalin-2(1H)-ones using commercially available alkenes, polyhalo alkanes, and quinoxalin-2(1H)-ones. Preliminary mechanistic studies suggested the generation of radical intermediates via an EDA-complex, single electron transfer, or halogen atom transfer pathway. Under mild reaction conditions, various alkenes and quinoxalin-2(1H)-ones containing different functional groups are compatible, providing the corresponding polychloroalkyl-substituted quinoxalin-2(1H)-ones in moderate to good yields.

Free radical mechanisms of ammonium sulfate as intensively used industrial materials on suppressing organic pollutants

Organic free radicals are critical intermediates for the generation and inhibition of organic pollutants during industrial processes. Clarifying the free radical mechanism of pollutant inhibition is significant for their efficient control. Ammonium sulfate is intensively used in industrial materials to suppress organic pollutants. In this study, organic free radical intermediate species in metal-catalyzed reactions inhibited by ammonium sulfate were identified using continuous-wave electron paramagnetic resonance (EPR) spectroscopy, providing direct evidence for the free radical mechanisms of organic pollutants inhibition. The transverse (T2) and longitudinal (T1) relaxation time variations catalyzed by different metal catalysts in the presence of ammonium sulfate were compared using pulsed-wave EPR. Consequently, after the addition of ammonium sulfate, the observed increase in T2 suggests that ammonium sulfate leads to radical concentration reduction. A decrease in the T1 relaxation time suggests the enhanced interaction between organic radicals and metals, which is an obstacle to subsequent radical reactions. Therefore, ammonium sulfate dominantly changed the free radical intermediates species, concentrations, and their reactivity, and then inhibited the organic pollutants formations. The inhibition mechanisms of ammonium sulfate on metal-catalyzed pollutants were then proposed combining EPR analysis, X-ray characterization, and high-resolution mass spectrometry screening. As a result, (1) occupying the active sites of metal catalysis and (2) inhibiting free radical intermediates are the two main intrinsic inhibition mechanisms of ammonium sulfate. The findings provide new perspectives on the efficient inhibition of organic pollutants in industrial processes involving various metal catalysts.

Exploring Photoredox Catalytic Reactions as an Entry to Glycosyl-α-amino Acids

The synthesis of glycosyl-α-amino acids presents a significant challenge due to the need for precise glycosidic linkages connecting carbohydrate moieties to amino acids while maintaining stereo- and regiochemical fidelity. Classical methods relying on ionic intermediates (2e-) often involve intricate synthetic procedures, particularly when dealing with 2-N-acetamido-2-deoxyglycosides linked to α-amino acids-a crucial class of glycoconjugates that play important biological roles. Considering the growing prominence of photocatalysis, this study explores various photoredox catalytic approaches to achieving glycosylation reactions. Our focus lies on the notoriously difficult case of 2-N-acetamido-2-deoxyglycosyl-α-amino acids, which could be obtained efficiently by two methodologies that involved, on the one hand, photoredox Giese reactions using a chiral dehydroalanine (Dha) as an electron density-deficient alkene in these radical 1,4-additions and, on the other hand, photoredox glycosylations using selenoglycosides as glycosyl donors and hydroxyl groups of protected amino acids as acceptors.

Transition-Metal-Free Radical Relay Cascade Annulation of Amides: Access to Antitumor Active Benzo[b]azepine and Oxindole Derivatives

Efficient transition-metal-free synthesis of benzo[b]azepines and oxindoles is achieved via a radical relay cascade strategy employing halogen atom transfer (XAT) for aryl radical generation followed by intramolecular hydrogen atom transfer (HAT). Optimization yielded moderate to substantial yields under visible light irradiation. Preliminary biological assessments revealed promising anti-tumor activity for select compounds. This study underscores the potential of XAT-mediated radical relay cascades in medicinal chemistry and anticancer drug discovery.

Selective Functionalization of White Phosphorus with Alkyl Bromides under Photocatalytic Conditions: A Chlorine-Free Protocol to Dialkyl and Trialkyl Phosphine Oxides

A novel and efficient method for the direct selective alkylation of white phosphorus (P4) with alkyl bromides has been developed, utilizing 4DPAIPN as the photocatalyst and Hantzsch ester as the reductant. This method facilitates the synthesis of structurally diverse dialkyl phosphine oxides in good yields, offering a streamlined alternative to the traditional stepwise approach of chlorinating P4 with Cl2 and subsequently displacing the chlorine atom. Noteworthy features of this reaction include excellent product selectivity, remarkable functional group tolerance, and a broad substrate scope. Additionally, this method is effective for the synthesis of trialkyl phosphine oxides.

Visible-Light-Induced α-Arylation of Ketones with (Hetero)aryl Halides

An enamine-mediated photoredox catalyzed C(sp2)-C(sp3) cross-coupling of dual radical precursors for the arylation of ketone is presented in this Letter. These reactions led to the formation of an enamine by using pyrrolidine to functionalize the C(sp3)-H bond in ketone substrates, which could be smoothly converted to α-arylated ketones with inert aryl bromides and even aryl chlorides in moderate to good yields under mild reaction conditions. The photocatalytically induced C(sp2)-C(sp3) cross-coupling between unactivated noncyclic ketones and aryl halides was achieved, and multiple carbonyl α-arylated backbones containing various natural products and drug molecules were successfully constructed.

Shuttle HAT for mild alkene transfer hydrofunctionalization

Hydrogen atom transfer (HAT) from a metal-hydride is a reliable and powerful method for functionalizing unsaturated C-C bonds in organic synthesis. Cobalt hydrides (Co-H) have garnered significant attention in this field, where the weak Co-H bonds are most commonly generated in a catalytic fashion through a mixture of stoichiometric amounts of peroxide oxidant and silane reductant. Here we show that the reverse process of HAT to an alkene, i.e. hydrogen atom abstraction of a C-H adjacent to a radical, can be leveraged to generate catalytically active Co-H species in an application of shuttle catalysis coined shuttle HAT. This method obviates the need for stoichiometric reductant/oxidant mixtures thereby greatly simplifying the generation of Co-H. To demonstrate the generality of this shuttle HAT platform, five different reaction manifolds are shown, and the reaction can easily be scaled up to 100 mmol.