Photoinduced Palladium-Catalyzed 1,2-Aminoalkylation of Aromatic Alkenes with Hydroxyl as the Directing Group

The hybrid nature of Pd(I)-alkyl radical species has enabled a wide array of radical-based transformations. However, in this transformation, the secondary Pd(I)-alkyl radical species are prone to recombining into Pd(II)-alkyl species to give Heck-type products via β-H loss. Herein, we report a visible-light-induced, three-component Pd-catalyzed 1,2-aminoalkylation of alkenes with readily available alkyl halides and amines to construct C-C and C-N bonds simultaneously. Mechanistic investigation shows that the intermediate of o-quinone methide produced is the key factor in the transformation.

Photoinduced Decarboxylative Difluoroalkylation and Perfluoroalkylation of α-Fluoroacrylic Acids

Herein, a novel and practical methodology for the photoinduced decarboxylative difluoroalkylation and perfluoroalkylation of α-fluoroacrylic acids is reported. A wide range of α-fluoroacrylic acids can be used as applicable feedstocks, allowing for rapid access to structurally important difluoroalkylated and polyfluoroalkylated monofluoroalkenes with high Z-stereoselectivity under mild conditions. The protocol demonstrates excellent functional group compatibility and provides a platform for modifying complex biologically active molecules.

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.

1,4-Aminoarylation of Butadienes via Photoinduced Palladium Catalysis

A visible-light-induced, three-component palladium-catalyzed 1,4-aminoarylation of butadienes with readily available aryl halides and aliphatic amines has been developed, affording allylamines with excellent E-selectivity. The reaction exhibits exceptional control over chemo-, regio-, and stereoselectivity, a broad substrate scope, and high functional group compatibility, as demonstrated by the late-stage functionalization of bioactive molecules. Mechanistic investigations are consistent with a photoinduced radical Pd(0)-Pd(I)-Pd(II)-Pd(0) Heck-Tsuji-Trost allylation cascade.

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.

Photoexcitation of Distinct Divalent Palladium Complexes in Cross-Coupling Amination Under Air

The development of metal complexes that function as both photocatalyst and cross-coupling catalyst remains a challenging research topic. So far, progress has been shown in palladium(0) excited-state transition metal catalysis for the construction of carbon-carbon bonds where the oxidative addition of alkyl/aryl halides to zero-valent palladium (Pd0 ) is achievable at room temperature. In contrast, the analogous process with divalent palladium (PdII ) is uphill and endothermic. For the first time, we report that divalent palladium can act as a light-absorbing species that undergoes double excitation to realize carbon-nitrogen (C-N) cross-couplings under air. Differently substituted aryl halides can be applied in the mild, and selective cross-coupling amination using palladium acetate as both photocatalyst and cross-coupling catalyst at room temperature. Density functional theory studies supported by mechanistic investigations provide insight into the reaction mechanism.

Visible light-induced palladium-carbon bond weakening in catalytically relevant T-shaped complexes

Triggering one-electron redox processes during palladium catalysis holds the potential to unlock new reaction mechanisms and synthetic methods not previously accessible in the typical two-electron reaction manifolds that dominate palladium catalysis. We report that T-shaped organopalladium(ii) complexes coordinated by a bulky monophosphine, a class of organometallic intermediate featured in a range of contemporary catalytic reactions, undergo blue light-promoted bond weakening leading to mild and efficient homolytic cleavage of strong Pd(ii)-C(sp3) bonds under ambient conditions. The origin of light-triggered radical formation in these systems, which lack an obvious ligand-based chromophore (i.e., π-systems), was investigated using a combination of DFT calculations, photoactinometry, and transient absorption spectroscopy. The available data suggest T-shaped organopalladium(ii) complexes manifest unusual blue light-accessible Pd-to-C(sp3) transition. The quantum efficiency and excited state lifetime of this process were unexpectedly superior compared to a prototypical (α-diimine)Pd(ii) complex featuring a low-lying, ligand-centered LUMO (π*). These results suggest coordinatively-unsaturated organopalladium(ii) compounds, catalysts in myriad catalytic processes, have untapped potential for one-electron reactivity under visible light excitation.

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.

Merging of Light/Dark Palladium Catalytic Cycles Enables Multicomponent Tandem Alkyl Heck/Tsuji-Trost Homologative Amination Reaction toward Allylic Amines

A visible light-induced palladium-catalyzed homologative three-component synthesis of allylic amines has been developed. This protocol proceeds via a unique mechanism involving two distinct cycles enabled by the same Pd(0) catalyst: a visible light-induced hybrid radical alkyl Heck reaction between 1,1-dielectrophile and styrene, followed by the "in dark" classical Tsuji-Trost-type allylic substitution reaction. This method works well with a broad range of primary and secondary amines, aryl alkenes, dielectrophiles, and in complex settings. The regiochemistry of the obtained products is primarily governed by the structure of 1,1-dielectrophile. Involvement of π-allyl palladium intermediates allowed for the control of stereoselectivity, which has been demonstrated with up to 95:5 er.

The mechanism of visible light-induced C-C cross-coupling by Csp3-H bond activation

Csp3-C cross-coupling by activating Csp3-H bonds is a dream reaction for the chemical community, and visible light-induced transition metal-catalysis under mild reaction conditions is considered a powerful tool to achieve it. Advancement of this research area is still in its infancy because of the chemical and technical complexity of this catalysis. Mechanistic studies illuminating the operative reaction pathways can rationalize the increasing amount of experimental catalysis data and provide the knowledge allowing faster and rational advances in the field. This goal requires complementary experimental and theoretical mechanistic studies, as each of them is unfit to clarify the operative mechanisms alone. In this tutorial review we summarize representative experimental and computational mechanistic studies, highlighting weaknesses, strengths, and synergies between the two approaches.

Enhanced Excited-State Hydricity of Pd-H Allows for Unusual Head-to-Tail Hydroalkenylation of Alkenes

Photoinduced enhancement of hydricity of palladium hydride species enables unprecedented hydride addition-like ("hydridic") hydropalladation of electron-deficient alkenes, which allows for chemoselective head-to-tail cross-hydroalkenylation of electron-deficient and electron-rich alkenes. This mild and general protocol works with a wide range of densely functionalized and complex alkenes. Notably, this approach also allows for highly challenging cross-dimerization of electronically diverse vinyl arenes and heteroarenes.

Photoinduced Nitrogen-to-Alkyl Radical Relay Heck Reaction of o-Alkylbenzamides with Vinyl Arenes by Palladium Catalysis

Here, a palladium-catalyzed photoinduced N-to-alkyl radical relay Heck reaction of o-alkylbenzamides at benzylic sites with vinyl arenes is described. The reaction employs neither exogeneous photosensitizers nor external oxidants. It is proposed to proceed via a N-to-alkyl hybrid palladium-radical mechanism which occurs under mild conditions that are compatible with a wide range of functional groups. The products are easily transformed to azepinone derivatives, which are prevalent in pharmaceuticals and natural products.

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.

A Visible Light Driven Nickel Carbonylation Catalyst: The Synthesis of Acid Chlorides from Alkyl Halides

We describe here the development of a visible light driven nickel carbonylation catalyst. The combination of the large bite-angle Xantphos ligand with nickel(0) generates a catalyst capable of activating alkyl halides toward carbonylation at ambient temperature in the presence of blue light irradiation, and the reductive elimination of high energy acid chloride products. Unlike classical carbonylations, where the coordination of carbon monoxide inhibits the reactivity of earth abundant nickel catalysts, a CO-associated nickel is found to be the active catalyst in the reaction. Coupling the build-up of acid chlorides with nucleophile addition can be used to access various amides, esters and thioesters, including those of sterically encumbered substrates or with metal-reactive functionalities.

Visible-Light-Induced Metal-Free Decarboxylative Perfluoroalkylation of Aryl Acrylic Acids

Perfluoroalkylation is important for late-stage modification of biologically active molecules. Herein, we report a protocol for visible-light-induced perfluoroalkylation reactions of aryl acrylic acids. These reactions, which use perfluoroalkyl iodides as radical precursors and inexpensive eosin Y as a photocatalyst, proceed in a decarboxylative manner. The easy accessibility of perfluoroalkyl iodides and the broad substrate scope, mild conditions, and metal-free catalyst make this protocol applicable for the transformation of inexpensive raw materials to high-value chemicals.

Palladium-Catalyzed and Photoinduced Benzylic C-H Carbonylation/Annulation under Mild Conditions

A Pd-catalyzed and photoinduced benzylic cascade benzylic C-H carbonylation/annulation reaction is realized under mild conditions (35 °C, 2 bar CO). The use of a catalytic amount of base is crucial for the reaction to achieve high yields. The reaction consists of a Pd-catalyzed generation of amidyl radical from O-benzyl hydroxylamide substrates and 1,5-HAT to give a benzylic radical, followed by carbonylation and annulation. Various homophthalimides, which could be readily converted to a number of bioactive compounds, could be obtained with up to 96% yield.

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.

Accessing Illusive E Isomers of α-Ester Hydrazones via Visible-Light-Induced Pd-Catalyzed Heck-Type Alkylation

A visible-light-induced Pd-catalyzed stereoselective synthesis of alkylated ester hydrazones has been developed. This method operates via generation of a nucleophilic carbon-centered radical from alkyl bromide, iodide, or redox-active ester, followed by its addition to hydrazone, and a subsequent desaturation by palladium. The majority of products have E configuration, which are inaccessible by conventional condensation methods. In addition, a sequential C,N-alkylation protocol has been developed: a reaction between 1,3-dihalides and glyoxylate-derived hydrazone, delivering tetrahydropyridazines.

Versatile Palladium-Catalyzed Approach to Acyl Fluorides and Carbonylations by Combining Visible Light- and Ligand-Driven Operations

We describe the development of a general palladium-catalyzed carbonylative method to synthesize acyl fluorides from aryl, heteroaryl, alkyl, and functionalized organic halides. Mechanistic analysis suggests that the reaction proceeds via the synergistic combination of visible light photoexcitation of Pd(0) to induce oxidative addition with a ligand-favored reductive elimination. These together create a unidirectional catalytic cycle that is uninhibited by the classical effect of carbon monoxide coordination. Coupling the catalytic formation of acyl fluorides with their subsequent nucleophilic reactions has opened a method to perform carbonylation reactions with unprecedented breadth, including the assembly of highly functionalized carbonyl-containing products.

Stereoconvergent Synthesis of Monofluoroalkenes via Photoinduced Dual Decarboxylative Cross-Coupling of α-Fluoroacrylic Acids with Redox-Active Esters

Herein, a new strategy for the synthesis of monofluoroalkenes via employing α-fluoroacrylic acids and N-hydroxyphthalimide (NHPI) redox-active esters as coupling partners has been developed. This decarboxylative reaction enabled the formation of C(sp²)-C(sp³) bonds to provide a practical and efficient approach for the construction of a variety of monofluoroalkenes, which are key structural motifs in organic chemistry, under mild reaction conditions. The protocol exhibited excellent functional group compatibility and delivered monofluoroalkene products with excellent Z-stereoselectivity. This work also provides a platform for the modification of complex biologically active molecules containing carboxylic acids.

Introduction of Lipophilic Side Chains to NH-Sulfoximines by Palladium Catalysis Under Blue Light Irradiation

Palladium catalysis under blue (LED) light irradiation allows conversion of NH-sulfoximines into products with long lipophilic side chains attached to the S═N moiety. The three-component reactions involve both radicals and organometallic intermediates stemming from alkyl bromides and butadienes. The substrate scope is broad, and the products are formed in moderate to good yields.

Visible Light-Induced Transition Metal Catalysis

In recent years, visible light-induced transition metal catalysis has emerged as a new paradigm in organic photocatalysis, which has led to the discovery of unprecedented transformations as well as the improvement of known reactions. In this subfield of photocatalysis, a transition metal complex serves a double duty by harvesting photon energy and then enabling bond forming/breaking events mostly via a single catalytic cycle, thus contrasting the established dual photocatalysis in which an exogenous photosensitizer is employed. In addition, this approach often synergistically combines catalyst-substrate interaction with photoinduced process, a feature that is uncommon in conventional photoredox chemistry. This Review describes the early development and recent advances of this emerging field.

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.

Blue light enhanced Heck arylation at room temperature applied to allenes

A blue light enhanced synthesis of 2-vinyl pirrolidines and piperidines through a domino Heck arylation–cyclisation applied to allenyl amines is described. Essential is the role of the light in the aryl migration in the carbo-palladation step.

Copper-catalyzed direct monofluoroalkenylation of C(sp3)–H bonds via decarboxylation of α-fluoroacrylic acids

Herein, a protocol for the copper-catalyzed direct monofluoroalkenylation of C(sp3)–H bonds is reported.

Unactivated Alkyl Chloride Reactivity in Excited-State Palladium Catalysis

Excited-state palladium catalysis is an efficient process for the alkylation of diverse organic compounds via the generation of alkyl radicals from alkyl bromides and iodides. However, the generation of alkyl radicals from more stable alkyl chlorides remains challenging. Herein, we demonstrate the excited-state palladium-catalyzed synthesis of oxindoles and isoquinolinediones via alkylation/annulation reaction by overcoming inherent limitations associated with unactivated C(sp³)-Cl bond activation at room temperature.

Photoinduced and Palladium-Catalyzed Remote Desaturation of Amide Derivatives

A photoinduced and palladium-catalyzed remote desaturation of O-acyl hydroxamides to unsaturated amides under mild conditions has been achieved. The formation of the alkyl Pd(II) intermediate by the recombination of alkyl radical and Pd(I) species is critical to achieve this efficient and selective desaturation of alkanes. This reaction features good site-selectivity, is terminal oxidant-free, and produces moderate to excellent yields for a variety of unsaturated amides. Remarkably, this approach enables late-stage desaturation of complex and biologically important molecules.

Visible-Light-Induced Palladium-Catalyzed Selective Defluoroarylation of Trifluoromethylarenes with Arylboronic Acids

Selective functionalization of inactive C(sp³)-F bonds to prepare medicinally interesting aryldifluoromethylated compounds remains challenging. One promising route is the transition-metal-catalyzed cross-coupling through oxidative addition of the C(sp³)-F bond in trifluoromethylarenes (ArCF₃), which are ideal precursors for this process due to their ready availability and low cost. Here, we report an unprecedented excited-state palladium catalysis strategy for selective defluoroarylation of trifluoromethylarenes with arylboronic acids. This visible-light-induced palladium-catalyzed cross-coupling proceeds under mild reaction conditions and allows transformation of a variety of arylboronic acids and ArCF₃. Preliminary mechanistic studies reveal that the oxidative addition of the C(sp³)-F bond in ArCF₃ to excited-state palladium(0) via a single electron transfer pathway is responsible for the C(sp³)-F bond activation.

Dual Nickel/Ruthenium Strategy for Photoinduced Decarboxylative Cross-Coupling of α,β-Unsaturated Carboxylic Acids with Cycloketone Oxime Esters

Herein, a dual nickel/ruthenium strategy is developed for photoinduced decarboxylative cross-coupling between α,β-unsaturated carboxylic acids and cycloketone oxime esters. The reaction mechanism is distinct from previous photoinduced decarboxylation of α,β-unsaturated carboxylic acids. This reaction might proceed through a nickelacyclopropane intermediate. The C(sp²)-C(sp³) bond constructed by the aforementioned reaction provides an efficient approach to obtaining various cyanoalkyl alkenes, which are synthetically valuable organic skeletons in organic and medicinal chemistry, under mild reaction conditions. The protocol tolerates many critical functional groups and provides a route for the modification of complex organic molecules.

Visible-Light-Induced Palladium-Catalyzed Dehydrogenative Carbonylation of Amines to Oxalamides

The palladium-catalyzed oxidative carbonylation of amines toward the synthesis of oxalamides has been established around 30 years ago and it usually needs the presence of (over)stoichiometric amounts of oxidant. In this work, the first transformation of this type in which the oxidant was replaced by visible light is described. The new approach uses a simple robust Pd complex, which can even be partially recycled. A mechanistic reason is provided and supported by control experiments and EPR studies, showing that Pd^I was formed and Pd⁰ was the active species. Both nitrogen- and the intermediate acyl radical can be detected. Moreover, the formation of hydrogen was confirmed by gas GC.

Photoinduced Palladium-Catalyzed Dicarbofunctionalization of Terminal Alkynes

Herein, a conceptually distinct approach was developed that allowed for the dicarbofunctionalization of alkynes at room temperature using simple, bench-stable alkyl iodides and a second molecule of alkyne as coupling partner. Specifically, the photochemical activation of palladium complexes enabled this strategic dicarbofunctionalization via addition of alkyl radicals from secondary and tertiary alkyl iodides and formation of an intermediate palladium vinyl complex that could undergo subsequent Sonogashira reaction with a second alkyne molecule. This alkylation-alkynylation sequence allowed the one-step synthesis of 1,3-enynes including heteroarenes and biologically active compounds with high efficiency without exogenous photosensitizers or oxidants and now opens up pathways towards cascade reactions via photochemical palladium catalysis.

Pd-catalyzed formal Mizoroki-Heck coupling of unactivated alkyl chlorides

The use of alkyl chlorides in Pd-catalyzed Mizoroki-Heck coupling reactions remains an unsolved problem despite their significant potential for synthetic utility and applicability. The combination of the high thermodynamic barrier of alkyl chloride activation and kinetic propensity of alkylpalladium complexes to undergo undesired β-hydride elimination provides significant challenges. Herein, a variety of alkyl chlorides, even tertiary chlorides, are shown to efficiently participate in Mizoroki-Heck cross-coupling reactions with excellent functional group compatibility under mild reaction conditions via photoinduced Pd catalysis. The reaction is applied to late-stage functionalizations of diverse biologically significant scaffolds and iterative double Mizoroki-Heck annulations, affording high molecular complexity in a single step. Notably, studies on the kinetic isotope effects in combination with density functional theory (DFT)-computations completely exclude the involvement of a previously proposed β-hydride elimination in the catalytic cycle, revealing that the chlorine atom transfer process is the key catalytic turnover step. This distinctive single-electron transfer mediated reaction pathway resolves a longstanding challenge in traditional two-electron based Pd-catalyzed Mizoroki-Heck cross-coupling with alkyl electrophiles, wherein the β-hydride elimination is involved in the formation of both the desired product and undesired by-products.

Excited-State Palladium-Catalyzed 1,2-Spin-Center Shift Enables Selective C-2 Reduction, Deuteration, and Iodination of Carbohydrates

Excited-state catalysis, a process that involves one or more excited catalytic species, has emerged as a powerful tool in organic synthesis because it allows access to the excited-state reaction landscape for the discovery of novel chemical reactivity. Herein, we report the first excited-state palladium-catalyzed 1,2-spin-center shift reaction that enables site-selective functionalization of carbohydrates. The strategy features mild reaction conditions with high levels of regio- and stereoselectivity that tolerate a wide range of functional groups and complex molecular architectures. Mechanistic studies suggest a radical mechanism involving the formation of hybrid palladium species that undergoes a 1,2-spin-center shift followed by the reduction, deuteration, and iodination to afford functionalized 2-deoxy sugars. The new reactivity will provide a general approach for the rapid generation of natural and unnatural carbohydrates.

C4-arylation and domino C4-arylation/3,2-carbonyl migration of indoles by tuning Pd catalytic modes: Pd(i)-Pd(ii) catalysis vs. Pd(ii) catalysis

Efficient C4-arylation and domino C4-arylation/3,2-carbonyl migration of indoles have been developed. The former route enables C4-arylation in a highly efficient and mild manner and the latter route provides an alternative straightforward protocol for synthesis of C2/C4 disubstituted indoles. The mechanism studies imply that the different reaction pathways were tuned by the distinct acid additives, which led to either the Pd(i)-Pd(ii) pathway or Pd(ii) catalysis.

Synthesis of maleimide-based enediynes with cyclopropane moieties for enhanced cytotoxicity under normoxic and hypoxic conditions

Myers-Saito cycloaromatization (MSC) is the working mechanism of many natural enediyne antibiotics with high antitumor potency. However, the presence of the equilibrium between diradical and zwitterionic intermediates in MSC severely hinders further improvement in cytotoxicity toward tumor cells. To this end, a series of maleimide-based enediynes with cyclopropane moieties were synthesized for enhanced cytotoxicity toward tumor cells. By taking advantage of radical clock reactions, the diradical intermediates generated from MSC would rearrange to new diradicals with much longer separation and weaker interactions between two radical centers. The computational study suggested a low energy barrier (4.4 kcal mol-1) for the radical rearrangement through the cyclopropane ring-opening process. Thermolysis experiments confirmed that this radical rearrangement results in the formation of a new diradical intermediate, followed by abstracting hydrogen atoms from 1,4-cyclohexadiene. Interestingly, the DNA cleavage ability and cytotoxicity of enediynes were significantly enhanced after the introduction of cyclopropane moieties. In addition, these maleimide-based enediynes exhibited a similar cytotoxicity under hypoxic conditions to that under normoxic conditions, which is beneficial for treating solid tumors where hypoxic environments frequently lead to deteriorated efficiency of many antitumor drugs. Docking studies indicated that the diradical intermediate was located between the minor groove of DNA with a binding energy of -7.40 kcal mol-1, which is in favor of intracellular DNA damage, and thereby inducing cell death via an apoptosis pathway as suggested by immunofluorescence analysis.

Three-component three-bond forming cascade via palladium photoredox catalysis

A highly modular radical cascade strategy based upon radical cyclisation/allylic substitution sequence between alkyl/aryl bromides, 1,3-dienes and nucleophiles ranging from sulfinates to amines, phenols and 1,3-dicarbonyls is described (>80 examples). Palladium phosphine complexes – which merge properties of photo- and cross coupling-catalysts – allow to forge three bonds with complete 1,4-selectivity and stereocontrol, delivering highly value added carbocyclic and heterocyclic motifs that can feature – inter alia – vicinal quaternary centers, free protic groups, gem-difluoro motifs and strained rings. Furthermore, a flow chemistry approach was for the first time applied in palladium–photocatalysed endeavors involving radicals.

Highly modular three-bond three-component cascade featuring palladium as dual photoredox/cross coupling catalyst.

C-H functionalization reactions enabled by hydrogen atom transfer to carbon-centered radicals

Selective functionalization of ubiquitous unactivated C-H bonds is a continuous quest for synthetic organic chemists. In addition to transition metal catalysis, which typically operates under a two-electron manifold, a recent renaissance in the radical approach relying on the hydrogen atom transfer (HAT) process has led to tremendous growth in the area. Despite several challenges, protocols proceeding via HAT are highly sought after as they allow for relatively easy activation of inert C-H bonds under mild conditions leading to a broader scope and higher functional group tolerance and sometimes complementary reactivity over methods relying on traditional transition metal catalysis. A number of methods operating via heteroatom-based HAT have been extensively reported over the past few years, while methods employing more challenging carbon analogues have been less explored. Recent developments of mild methodologies for generation of various carbon-centered radical species enabled their utilization in the HAT process, which, in turn, led to the development of remote C(sp3)-H functionalization reactions of alcohols, amines, amides and related compounds. This review covers mostly recent advances in C-H functionalization reactions involving the HAT step to carbon-centered radicals.

Copper-catalyzed oxidative decarboxylative alkylation of cinnamic acids with 4-alkyl-1,4-dihydropyridines

We have developed a new oxidative decarboxylation of cinnamic acids with 4-alkyl-1,4-dihydropyridines to construct C(sp3)-C(sp2) bonds in the presence of copper catalyst and dicumyl peroxide (DCP). A variety of internal alkenes have been obtained with mild conditions, broad substrate scope and excellent functional group tolerance. This method has significant potential for application by using inexpensive and stable cinnamic acids instead of alkenyl halides and nitro-olefins.

Direct C(sp2)-H alkylation of unactivated arenes enabled by photoinduced Pd catalysis

Despite the fundamental importance of efficient and selective synthesis of widely useful alkylarenes, the direct catalytic C(sp²)–H alkylation of unactivated arenes with a readily available alkyl halide remains elusive. Here, we report the catalytic C(sp²)–H alkylation reactions of unactivated arenes with alkyl bromides via visible-light induced Pd catalysis. The reaction proceeds smoothly under mild conditions without any skeletal rearrangement of the alkyl groups. The direct syntheses of structurally diverse linear and branched alkylarenes, including the late-stage phenylation of biologically active molecules and an orthogonal one-pot sequential Pd-catalyzed C–C bond-forming reaction, are achieved with exclusive chemoselectivity and exceptional functional group tolerance. Comprehensive mechanistic investigations through a combination of experimental and computational methods reveal a distinguishable Pd(0)/Pd(I) redox catalytic cycle and the origin of the counter-intuitive reactivity differences among alkyl halides.

Direct catalytic C(sp²)–H alkylation of unactivated arenes with alkyl halides remains elusive despite the progress in C-H functionalization. Here, the authors report the catalytic C(sp²)–H alkylation of unactivated arenes with alkyl bromides via visible-light induced Pd catalysis.