Unactivated Alkyl Chloride Reactivity in Excited-State Palladium Catalysis

Photoredox Iron-Catalyzed Decarboxylative Radical Cyclization for the Synthesis of Oxindoles and Chroman-4-ones

A sustainable, photocatalytic approach for the synthesis of oxindoles and chroman-4-ones was developed using carboxylate salts as radical precursors and FeCl3 as a catalyst. The reaction proceeds via a decarboxylative radical cyclization mechanism triggered by ligand-to-metal charge transfer under visible light irradiation, operating efficiently at room temperature. This method demonstrates excellent substrate scope, including the use of various alkyl carboxylates, and functional group tolerance and offers a scalable pathway for gram-scale synthesis, highlighting its practical application.

Photoredox nickel-catalyzed radical cyclization of N-arylacrylamides with alkyl bromides

3,3-Disubstituted oxindoles constitute a significant class of biologically active molecules, often found in a wide range of bioactive compounds. In this work, we present a photoredox nickel-catalyzed intermolecular cyclization between N-arylacrylamides and readily accessible alkyl bromides, which affords a diverse range of 3,3-disubstituted oxindoles in moderate to high yields. Our mechanistic studies reveal that the reduction of alkyl bromides via single-electron transfer from a reactive Ni(I) species is a critical step in driving this radical cascade transformation. This approach offers several advantages, including mild reaction conditions, broad functional group tolerance, and a simple workup procedure.

Visible-light-induced radical-cascade alkylation/cyclization of acrylamides: access to 3,3-dialkylated oxindoles

A visible-light-induced deoxygenative alkylation/cyclization of acrylamides with alcohols activated by CS2 has been developed by using xanthate salts as alkyl radical precursors in the presence of tricyclohexylphosphine. It proceeds through a tandem radical addition/cyclization process, and this protocol provides a reliable and practical approach to building the skeleton of 3,3-disubstituted oxindoles in moderate to good yields. Notable features of this reaction include readily available starting reagents, broad substrate scope and mild reaction 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.

C-H Alkylation of Heterocycles via Light-Mediated Palladium Catalysis

Methods enabling direct C-H alkylation of heterocycles are of fundamental importance in the late-stage modification of natural products, bioactive molecules, and medicinally relevant compounds. However, there is a scarcity of a general strategy for the direct C-H alkylation of a variety of heterocycles using commercially available alkyl surrogates. We report an operationally simple palladium-catalyzed direct C-H alkylation of heterocycles using alkyl halides under the visible light irradiation with good scalability and functional group tolerance. Our studies suggest that the photoinduced alkylation proceeds through a cascade of events comprising, site-selective alkyl radical addition, base-assisted deprotonation, and oxidation. A combination of experiments and computations was employed for the generalization of this strategy, which was successfully translated towards the modification of natural products and pharmaceuticals.

Highly Dispersed Pd@ZIF-8 for Photo-Assisted Cross-Couplings and CO2 to Methanol: Activity and Selectivity Insights

Our study unveils a pioneering methodology that effectively distributes Pd species within a zeolitic imidazolate framework-8 (ZIF-8). We demonstrate that Pd can be encapsulated within ZIF-8 as atomically dispersed Pd species that function as an excited-state transition metal catalyst for promoting carbon-carbon (C-C) cross-couplings at room temperature using visible light as the driving force. Furthermore, the same material can be reduced at 250 °C, forming Pd metal nanoparticles encapsulated in ZIF-8. This catalyst shows high rates and selectivity for carbon dioxide hydrogenation to methanol under industrially relevant conditions (250 °C, 50 bar): 7.46 molmethanol molmetal -1 h-1 and >99 %. Our results demonstrate the correlations of the catalyst structure with the performances at experimental and theoretical levels.

Feedstock chemical dichloromethane as the C1 source for the chemoselective multicomponent synthesis of valuable 1,4,2-dioxazoles

The development of mild and practical strategies to produce value-added fine chemicals directly from inexpensive and readily available commodity chemicals is actively pursued by chemists. However, the application of feedstock chemical dichloromethane (DCM) as the C1 source in organic synthesis is still in its infancy. Herein, we describe a multicomponent strategy for the chemoselective synthesis of valuable 1,4,2-dioxazoles by using DCM as a C1 source. Critical to the success of this process is tuning of the type of nucleophiles to inhibit the easily-occurring side reactions. This approach features mild and simple conditions, excellent chemoselectivity, metal free, and broad substrate scope covering different types of nucleophiles. Furthermore, its synthetic utility is further demonstrated by the preparation of deuterated 1,4,2-dioxazoles, the late-stage functionalization of complex molecules and large-scale synthesis. Preliminary mechanistic studies indicate the dual roles of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as both a proton scavenger and a nucleophilic catalyst. This work provides not only a platform for DCM application, but also an excellent complementary strategy to the established 1,4,2-dioxazoles synthesis.

Pd-Catalyzed 1,4-Difluoromethylative Functionalization of 1,3-Dienes Using Freon-22

We report a visible-light-driven, palladium-catalyzed 1,4-difluoromethylative functionalization of conjugated dienes using chlorodifluoromethane (ClCF2H, Freon-22) as a cost-effective difluoromethyl source. The excited palladium catalyst efficiently reduces the C-Cl bond, which generates a CF2H radical, followed by regioselective SN2' substitution to afford 1,4-difunctionalized products. This versatile, redox-neutral method accommodates diverse nucleophiles and exhibits broad functional group compatibility, making it suitable for late-stage functionalization in drug discovery and offering a direct route to difluoromethylated molecules.

Alcohols as Alkyl Electrophiles for Deoxygenative Heck Reaction Enabled by Excited State Pd Catalysis

Here, we present a general method for the photoinduced Pd-catalyzed deoxygenative Heck reaction of vinyl arenes with ortho-iodophenyl-thionocarbonate derived from alcohols. Mechanistic studies reveal that the deoxygenation involves a 5-endo-trig cyclization and fragmentation process, with radical addition identified as the rate-determining step in this transformation. This one-pot procedure demonstrates excellent selectivity for less hindered hydroxyl groups in diols, facilitating late-stage functionalization of complex molecules and scalability to gram-scale synthesis. The protocol highlights significant synthetic potential and can be extended to the cascade 1,1-difunctionalization of isocyanides and the intermolecular radical cascade cyclization of N-arylacrylamides.

Radical Replacement Process for Ligated Boryl Radical-Mediated Activation of Unactivated Alkyl Chlorides for C(sp3)-C(sp3) Bond Formation

The ligated boryl radical (LBR) has emerged as a potent tool for activating alkyl halides in radical transformations through halogen-atom transfer (XAT). However, unactivated alkyl chlorides still present an open challenge for this strategy. We herein describe a new activation mode of the LBR for the activation of unactivated alkyl chlorides to construct a C(sp3)-C(sp3) bond. Mechanistic studies reveal that the success of the protocol relies on a radical replacement process between the LBR and unactivated alkyl chloride, forming an alkyl borane intermediate as the alkyl radical precursor. Aided with the additive K3PO4, the alkyl borane then undergoes one-electron oxidation, generating an alkyl radical. The incorporation of the radical replacement activation model to activate unactivated alkyl chlorides significantly enriches LBR chemistry, which has been applied to activate alkyl iodides, alkyl bromides, and activated alkyl chlorides via XAT.

Photoinduced Palladium-Catalyzed Radical Germylative Arylation of Alkenes with Chlorogermanes

We describe a visible light-induced palladium-catalyzed radical germylative arylation of alkenes with easily accessible chlorogermanes. This protocol provides expedient access to germanium-substituted indolin-2-ones in good to excellent yields under mild reaction conditions. The key step for this strategy lies in the reductive activation of germanium-chloride bonds with an excited palladium complex under visible light irradiation. The involvement of germanium radicals was evidenced by electron paramagnetic resonance spectroscopy experiments.

Visible-light-induced alkyl-arylation of olefins via a halogen-atom transfer process

Visible-light-induced three-component 1,2-alkyl-arylation of alkenes and alkyl radical addition/cyclization of acrylamides have been realized via a photocatalytic halogen-atom transfer (XAT) process. This metal-free protocol utilizes readily available tertiary alkylamine as both an electron donor and an XAT reagent for the activation of alkyl halides using naphthalimide (NI)-based organic photocatalysts. This process features broad substrate scope and good functional group tolerance under mild conditions, and could be effectively applied to a variety of medicinally relevant substrates.

Accessing Benzoazepine Derivatives via Photoinduced Radical Relay Formal [5 + 2] Reaction of Amide/Alkyne Enabled by Palladium Catalysis

A novel class of alkyne-tethered amides facilitates an unprecedented photoinduced palladium-catalyzed radical relay formal [5 + 2] reaction. This innovative strategy allows for the rapid construction of diverse fused benzoazepine structures, yielding structurally novel and compelling compounds. With a broad substrate scope and excellent functional group tolerance, the methodology synthesizes biologically active compounds. Notably, the resulting tricyclic benzo[b]azepines offer diversification opportunities through simple transformations. DFT calculations elucidate a seven-membered ring closure mechanism involving the alkenyl radical and Pd(I) rebound alongside a concerted metalation-deprotonation (CMD) process.

Photochemical C(sp3)-H Activation for Diversity-Oriented Synthesis of 3-Functionalized Oxindoles

Heteroatom-adjacent C(sp3) radical cyclization of N-arylacrylamides provides a straightforward pathway to synthesize valuable 3-functionalized oxindoles. Traditional cyclization reactions normally require harsh conditions or transition-metal catalysts. Here, we developed a metal-free, diversity-oriented synthesis of 3-functionalized oxindoles via photochemically induced selective cleavage of C(sp3)-H bonds. A variety of 3-substituted oxindoles with functionalities such as ethers, polyhalogens, benzyl, and formyl groups can be obtained by a rational design. This strategy is characterized by its simple operation and mild conditions, aligning well with the developmental requirements for sustainable chemistry. The gram-scale continuous-flow synthesis and efficient construction of bioactive molecules highlight its practical utility.

Recent Advances in Visible Light Induced Palladium Catalysis

Visible light-induced Pd catalysis has emerged as a promising subfield of photocatalysis. The hybrid nature of Pd radical species has enabled a wide array of radical-based transformations otherwise challenging or unknown via conventional Pd chemistry. In parallel to the ongoing pursuit of alternative, readily available radical precursors, notable discoveries have demonstrated that photoexcitation can alter not only oxidative addition but also other elementary steps. This Minireview highlights the recent progress in this area.

Photo- and electro-chemical strategies for the activations of strong chemical bonds

The employment of light and/or electricity - alternatively to conventional thermal energy - unlocks new reactivity paradigms as tools for chemical substrate activations. This leads to the development of new synthetic reactions and a vast expansion of chemical spaces. This review summarizes recent developments in photo- and/or electrochemical activation strategies for the functionalization of strong bonds - particularly carbon-heteroatom (C-X) bonds - via: (1) direct photoexcitation by high energy UV light; (2) activation via photoredox catalysis under irradiation with relatively lower energy UVA or blue light; (3) electrochemical reduction; (4) combination of photocatalysis and electrochemistry. Based on the types of the targeted C-X bonds, various transformations ranging from hydrodefunctionalization to cross-coupling are covered with detailed discussions of their reaction mechanisms.

Cobalt-Catalyzed Electroreductive Alkylation of Unactivated Alkyl Chlorides with Conjugated Olefins

Reactions of unactivated alkyl chlorides under mild and sustainable conditions are rare compared to those of alkyl bromides or iodides. As a result, synthetic methods capable of modifying the vast chemical space of chloroalkane reagents, wastes, and materials are limited. We report the cobalt-catalyzed reductive addition of unactivated alkyl chlorides to conjugated alkenes. Co-catalyzed activation of alkyl chlorides is performed under electroreductive conditions, and the resulting reactions constitute formal alkyl-alkyl bond formation. In addition to developing an operationally simple methodology, detailed mechanistic studies provide insights into the elementary steps of a proposed catalytic cycle. In particular, we propose a switch in the mechanism of C-Cl bond activation from nucleophilic substitution to halogen atom abstraction, which is critical for efficiently generating alkyl radicals. These mechanistic insights were leveraged in designing ligands that enable couplings of primary, secondary, and tertiary alkyl chlorides.

Redox-Neutral Radical Cascade Cyclization of N-Arylacrylamides with Unactivated Alkyl and Aryl Chlorides

Herein, we report the first metal-free, redox-neutral strategy for radical cascade alkylative radical addition, cyclization of N-arylacrylamides with unactivated alkyl chlorides to give corresponding 3,3-disubstituted oxindoles in moderate to good yields. This transformation's salient features are the utilization of an organo photocatalyst, mild reaction conditions, and broad substrate scope. Moreover, this methodology is suitable for hetero cycle derived acrylamides and further allowed to utilize aryl chlorides for radical cyclization reaction. Finally, DFT studies allow us to shed light on the reaction mechanism.

Mechanistic insights into excited-state palladium catalysis for C-S bond formations and dehydrogenative sulfonylation of amines

Photocatalytic selective C(sp3)-H activation/cross-coupling reactions are appealing in organic synthesis. In this manuscript, we describe the development of photoexcited-state Pd-catalyzed dehydrogenative β-sulfonylation reactions using amines and aryl sulfonyl chlorides via intermolecular hydrogen atom transfer and C-S cross-coupling processes at room temperature. The transformation can be achieved by the direct generation of two distinct Pd-radical hybrid species and their capability to promote two different reactivities from Pd(0) and aryl sulfonyl chlorides, allowing for the efficient conversion of readily available amines into stable sulfonyl-substituted enamines at room temperature. The in-depth experimental, computational, and transient optical spectroscopic study and catalytic applications of a dehydrogenative functionalization event provide evidence for both static and dynamic quenching, as well as inner-sphere and outer-sphere mechanisms.

Persulfate promoted carbamoylation of N-arylacrylamides and N-arylcinnamamides with 4-carbamoyl-Hantzsch esters

Carbamoyl-Hantzsch esters were used as carbamoyl radical precursors for oxidative carbamoylation of N-arylacrylamides and N-arylcinnamamides in the presence of inexpensive persulfates. This protocol can be applied to a broad range of substrates with various functional groups, providing a variety of 3,3-disubstituted oxindoles and 3,4-disubstituted dihydroquinolin-2(1H)-ones in moderate to good yields via an intermolecular addition/cyclization process.

Photoinduced activation of alkyl chlorides

In recent years, the activation of unactivated alkyl chlorides through light-induced processes has emerged as a promising field in radical chemistry, and has led to new transformations in organic synthesis. Direct utilization of alkyl chlorides as C(sp3)-hybridized electrophiles enables the facile construction of carbon-carbon and carbon-heteroatom bonds. Furthermore, recent studies in medicinal chemistry indicate that their presence is associated with high levels of success in clinical trials. This review summarizes the recent advances in the photoinduced activation of unactivated alkyl chlorides and discusses the mechanistic aspects underlying these reactions. We anticipate that this review will serve as a valuable resource for researchers in the field of unactivated chemical bond functionalization, and inspire considerable developments in organic chemistry, drug synthesis, materials science and other related disciplines.

Photocatalytic Alkylation/Arylative Cyclization of N-Acrylamides of N-Heteroarenes and Arylamines with Dihydroquinazolinones from Unactivated Ketones

We describe a visible-light photoredox-catalyzed alkylation/arylative cyclization of N-acrylamides─from 2-arylindoles, 2-arylbenzimidazoles, or N-substituted anilines─with ketone-derived dihydroquinazolinones, accessing indolo- and benzimidazolo[2,1-a]isoquinolines or 2-oxindoles. The consecutive incorporation of alkyl- and aryl-carbogenic motifs across a C=C bond via formal cleavage of ketone α-C-C and arene C-H bonds leads to the formation of five- and six-membered rings, with an all-carbon quaternary stereocenter. This dicarbofunctionalization elaborates aromatization-driven radical C-C functionalization of unactivated aliphatic ketones to construct diverse cyclic structures with functionality tolerance.

Photoinduced Alkyl/Aryl Radical Cascade for the Synthesis of Quaternary CF3-Containing Oxindoles and Indoline Alkaloids

Metal- and additive-free, photoinduced decarboxylative radical alkylation-cyclization of CF₃-acrylamides with alkyl redox-active esters provided the corresponding quaternary CF₃-oxindole derivatives in good yields. Notably, diaryliodonium salts also efficiently participated in the arylation-cyclization of CF₃-acrylamides in environmentally benign H₂O as a solvent. The present approach has been extended for the concise synthesis of CF₃-attached indoline alkaloid analogues, i.e., CF₃-(±)-desoxyeseroline, CF₃-(±)-esermethole, and CF₃-(±) progesterone receptor antagonists. The preliminary mechanistic studies revealed that the reaction is likely to proceed through initial photoexcitation of redox-active ester/diaryliodonium salts followed by the SET process with acrylamide.

Pd-Catalyzed Photochemical Alkylative Functionalization of C═C and C═N Bonds

The development of excited-state palladium-catalyzed alkylative cyclization of acrylamides and the alkylation of quinoxalinones is described. The application of a variety of primary, secondary, and tertiary unactivated alkyl halides as alkyl radical precursors and the use of a simple catalyst system are the highlights of this reactivity manifold. The reactions exhibit wide scope, occur under mild conditions, and furnish the products in excellent yields.

Photoexcited cobalt catalysed endo-selective alkyl Heck reaction

Herein, we report an intramolecular endo-selective Heck reaction of iodomethylsilyl ethers of phenols and alkenols. The reaction leads to the formation of seven- and eight-membered siloxycycles in excellent yields, which could be further converted into the corresponding allylic alcohols upon oxidation. Thus, this method could be used for the selective (Z)-hydroxymethylation of o-hydroxystyrenes and alkenols. Rapid scan EPR experiments and DFT calculations suggest a concerted β-hydrogen elimination event to take place in the triplet state.

NaI/PPh3-catalyzed visible-light-mediated decarboxylative radical cascade cyclization of N-arylacrylamides for the efficient synthesis of quaternary oxindoles

A practical NaI/PPh3-catalyzed decarboxylative radical cascade cyclization of N-arylacrylamides with redox-active esters is described, which is mediated by visible light irradiation. A wide range of substrates bearing different substituents and derived from ubiquitous carboxylic acids, including α-amino acids, were synthesized and examined under this very mild, efficient, and cost effective transition-metal-free synthetic method. These afforded various functionalized oxindoles featuring a C3 quaternary stereogenic center. Mechanistic experiments suggest a radical mechanism.

Photoinduced Desaturation of Amides by Palladium Catalysis

A photoinduced palladium-catalyzed desaturation method that is suitable for converting the linear amides to their α,β-unsaturated counterparts is reported. The reaction does not require strong base/acid or sulfur/selenium and oxidant reagents and can be carried out at room temperature through a simple one-step operation. The protocol exhibits great scalability and functional group tolerance. The reaction mechanism has been investigated through deuterium labeling experiments, radical clock, radical capture, and kinetic studies. Mechanistic studies suggested a radical pathway involving aryl/alkyl Pd-radical intermediates.

Photoassisted Cross-Coupling Reaction of α-Chlorocarbonyl Compounds with Arylboronic Acids

A Suzuki-Miyaura cross-coupling reaction of α-chloroacetates or α-chloroacetamides with arylboronic acids is made possible by visible-light irradiation. This reaction provides a useful method for the synthesis of α-arylacetates and α-arylacetamides from chlorides under mild reaction conditions. An indole-3-acetic acid derivative that is the key intermediate of the plant hormone auxin can be synthesized from 1-Boc-indole in two steps by combining an iridium-catalyzed C-H borylation and a palladium-catalyzed cross-coupling reaction.

Visible-Light-Driven Palladium-Catalyzed Radical Tandem Dearomatization of Indoles with Unactivated Alkenes

A mild visible-light-driven palladium-catalyzed radical tandem dearomatization of indoles with unactivated alkenes is described with moderate to good yields and good to excellent diastereoselectivities. Under visible-light irradiation, the photoexcited state of the palladium complex was formed, which could transfer a single electron to N-(2-bromobenzoyl)indoles, leading to a hybrid palladium radical chemistry. This provides efficient and atom-economical access to diverse 2,3-disubstituted indoline derivatives.

Excited-state palladium-catalysed reductive alkylation of imines: scope and mechanism

Palladium catalysis induced by visible-light irradiation is a promising tool for promoting unusual chemical reactivity. Here, the hybrid alkyl radical/Pd(i) species generated is used to promote the reductive alkylation of imines.

Visible light-mediated radical-cascade addition/cyclization of arylacrylamides with aldehydes to form quaternary oxindoles at room temperature

The visible light-induced oxidative radical cascade coupling of N-arylacrylamides with aldehydes using bromide as the hydrogen atom transfer agent to synthesize functional oxindoles is described.

Phenolate anion-catalyzed direct activation of inert alkyl chlorides driven by visible light

A photochemical activation of inert alkyl chlorides catalyzed by phenolate anions was developed for C–O bond formation, dehalogenation, and cyclization under mild conditions.