Identification of Alkoxy Radicals as Hydrogen Atom Transfer Agents in Ce-Catalyzed C-H Functionalization

Synergistic LMCT and Ni Catalysis for Methylative Cross-Coupling Using tert-Butanol: Modulating Radical Pathways via Selective Bond Homolysis

Ligand-to-metal charge transfer (LMCT) excitation has emerged as a potent strategy for the selective generation of heteroatom-centered radicals, yet its full potential in modulating open-shell radical pathways remains underexplored. Here, we present a photocatalytic methylative cross-coupling reaction that capitalizes on the synergistic interplay between LMCT and Ni catalysis, enabling the use of tert-butanol as an efficient and benign methylating reagent. The electron-deficient ligand 2,6-ditrifluoromethyl benzoate facilitates Ce(IV)-mediated bond scission of tert-butanol, generating a methyl radical that is subsequently captured by the Ni catalytic cycle to form C-CH3 bonds. Under mild reaction conditions, this strategy affords efficient methylation of sp3 carbons adjacent to carbonyls and sp2 centers, demonstrating broad functional group tolerance and applicability in late-stage functionalization of bioactive molecules. Additionally, trideuteromethylative coupling can be facilely achieved using commercial tert-butanol-d10. This approach circumvents the need for traditional tert-butoxy radical precursors, such as peroxides, while strategically modulating the radical pathway to favor β-scission and suppress unwanted tert-butoxy radical formation in solution. Mechanistic studies reveal that the benzoate ligand plays a crucial role in enabling LMCT excitation and facilitating methyl radical generation, supporting a concerted Ce-OR and β-C-C bond homolysis mechanism, further evidenced by the modulation of regioselectivity in alkoxy radical-mediated β-scission.

Iron-Cobalt Dual Catalysis for the Synthesis of Alkenyl Amino Acids and Modification of Peptides

Herein, we report an Fe/Co dual-catalyzed strategy for synthesizing alkenyl unnatural amino acids and peptide modifications. This approach utilizes aspartic acid and glutamic acid derivatives as alkyl radical precursors. It avoids the use of expensive photoredox catalysts and substrate preactivation while preserving the chirality of the amino acids. Furthermore, this strategy enables both modification of peptides and the synthesis of amino-acid-based drug candidates for boron-neutron capture therapy (BNCT).

Supramolecular assemblies of tetravalent terbium complex units: syntheses, structure, and materials properties

There is a growing interest in lanthanide complexes exhibiting unconventional oxidation states, primarily due to their unique electronic structures and accompanying physicochemical properties. Herein, likely the first examples of supramolecular assemblies of non-Ce(iv) tetravalent lanthanide complexes, with the general formula [Tb(OSiPh3)4Lx] n [1 (n = 2, L1 = 1,2-bis(4-pyridyl)ethane); 2 (L2 = 4,4'-bipyridine), 3 (L3 = 1,2-bis(4-pyridyl)acetylene), 4 (L4 = 1,2-bis(4-pyridyl)ethylene), and 5 (L5 = 1,4-bis(4-pyridyl)benzene)], are reported. Cyclic voltammetry studies show two successive redox events, indicating electronic interactions between the two Tb(iv) centers in the dimeric metallomacrocycle 1. Compounds 2-5 are zig-zag structured coordination polymers featuring complex units of Tb(OSiPh3)4 bridged by their respective pyridyl-based ditopic ligands. These tetravalent lanthanide species display impressive stability in air, which is believed to result from the stabilization effect of ligand Lx and the extensive multifarious interactions involving the aromatic rings of the anionic (Ph3SiO-) and bridging ligands. UV-vis absorption spectroscopic studies show that 2-5 are semiconducting, each with a narrow bandgap of ca. 1.7 eV. Magnetic property studies yielded magnetic entropy changes of ca. 8.0 J (kg K)-1 at 2.5 K and 7T, which is reasonable for a complex with high-molecular-weight ligands, suggesting the potential development of Tb(iv) complexes as molecular refrigerants due to their f7 electronic configuration.

Cerium Ammonium Nitrate (CAN)-Promoted C(sp2)-N Coupling of Secondary Amides with Aryl Boronic Acids: Entries to Tertiary Aryl Amides

A cerium ammonium nitrate (CAN)-promoted C(sp2)-N coupling reaction of secondary amides with aryl boronic acids has been realized, providing a new entry for the construction of structurally diverse tertiary aryl amides, which are widely found in various biologically active molecules. Preliminary mechanistic studies indicated that a radical pathway may be involved in the C(sp2)-N coupling process. Compared with other metal-catalyzed methods, which in some cases require well-designed catalysts, preassembled directing groups, and/or complicated operations, this methodology features a ligand- and directing group-free process, as well as ease of handling.

Pd-Catalyzed Aerobic C-H Carbonylative Esterification of Imidazo[1,2-a]pyridines with Alcohols as the Carbonyl Source

A simple and practical method has been developed for the carbonylative esterification of imidazo[1,2-a]pyridines via C(sp2)-H bond functionalization using alkyl alcohols under mild reaction conditions. The carbonyl fragment is sourced from radical-mediated C-C cleavage of the alcohols, providing a green, safe, and economic alternative to traditional carbonyl sources like carbon monoxide. Through this strategy, a number of imidazo[1,2-a]pyridine-3-carboxylates were obtained from simple substrates by a single step.

N-Hydroxyphthalimide/TiO2 Catalyzed Addition of Ethers, Alkylarenes and Aldehydes to Azodicarboxylates under Visible Light

The addition of carbon-centered radicals to double bonds is one of the most atom-efficient approaches to the formation of new C-C or C-heteroatom bonds. Existing approaches for the generation of carbon-centered radicals often require elevated temperatures, UV radiation or expensive transition metal catalysts. In this work, a photocatalytic system based on a heterogeneous TiO2 catalyst and a redox organocatalyst N-hydroxyphthalimide is proposed for the generation of carbon-centered radicals from C(sp3)-H substrates or aldehydes at room temperature under visible light irradiation. The developed approach was successfully applied to the addition of ethers, alkylarenes and aldehydes to azodicarboxylates. Titanium oxide acts as a photocatalyst, producing phthalimide-N-oxyl radicals from N-hydroxyphthalimide, thereby enabling the organocatalytic process in solution. Phthalimide-N-oxyl radicals act as catalytically active species that cleave C-H bonds to form carbon-centered radicals.

Photocatalytic CeCl3-Promoted C-H Alkenylation and Alkynylation of Alkanes

The reemerging field of photoredox catalysis offers numerous advantages towards the development of novel, sustainable and easy-to-execute organic transformations. Herein, we report a light-triggered application of cerium complexes towards the C-H alkenylation and alkynylation of alkanes. An indirect HAT-mediated photocatalytic protocol was developed, using a cerium salt (CeCl3 ⋅ 7H2O) and a chlorine source (TBACl) as the catalytic system. A variety of cyclic and linear hydrocarbons were utilized, delivering the corresponding alkenylation or alkynylation products in good to high yields, displaying high regioselectivity. A series of mechanistic experiments were conducted.

Mechanistic Investigation of the Ce(III) Chloride Photoredox Catalysis System: Understanding the Role of Alcohols as Additives

Photocatalytic C-H activation is an emerging area of research. While cerium chloride photocatalysts have been extensively studied, the role of alcohol additives in these systems remains a subject of ongoing discussion. It was demonstrated that the photocatalyst [NEt4]2[CeIVCl6] (1) produces •Cl and added alcohols exhibit zero-order kinetics. Prior studies by other researchers suggested that 1 and alcohols lead to cerium alkoxide [Ce-OR] and alkoxy radical intermediates. To understand these seemingly divergent mechanistic proposals, an expanded investigation comparing cerium(IV) catalyst 1 and cerium(III) complex [NEt4]3[CeIIICl6] (2), which exhibit markedly different reactivity and C-H selectivity, is disclosed. Our findings reveal that alcohol additives accelerate the conversion of cerium(III) to cerium(IV) catalysts, forming key intermediates such as [NEt4]2[CeIIICl5(HOCH3)] (5) and [NEt4]2[CeIVCl5(OCH3)] (6), driven by excited-state di-tert-butyl azodicarboxylate under blue light irradiation. The active complex 6 releases the •OCH3 radical, in sharp contrast to •Cl radicals initiated by cerium(IV) photoredox catalyst 1. These different reactivity and selectivity profiles can be understood in the context of complex 5 generation and in situ formation of base to afford complex 6. Experimental validation shows enhanced selectivity toward C-H bonds with different reactivity with catalyst 1 and methanol upon the addition of base and decreased selectivity with catalyst 2 and methanol upon the addition of acid. These findings unify the previously contrasting observations of cerium halide/alkoxide photocatalytic systems and provide a comprehensive understanding on the essential role of base/acid and alcohol in selectivity and reactivity.

Lewis Acidic Ionic-Liquid-Catalyzed Radical-Cascade Alkylation/Cyclization of N-Alkyl-N-methacryloyl Benzamides with Alkanes via Visible-Light-Induced Ligand-to-Metal Charge Transfer: Access to Alkylated Isoquinoline-1,3(2H,4H)-diones

With the flourishing progress of ligand-to-metal charge transfer (LMCT) photocatalysis, various metals were developed as catalysts to activate abundant alkane feedstocks for the synthesis of functionalized organic compounds. However, to the best of our knowledge, most of the LMCT catalysts are difficult to recover and reuse for the next cycles. Herein, we report a reusable Lewis acidic ionic liquid (LAIL)-catalyzed radical-cascade alkylation/cyclization of N-alkyl-N-methacryloyl benzamides with unactivated alkanes for the synthesis of alkylated isoquinoline-1,3-(2H,4H)-diketones. The protocol features mild reaction conditions, high atom utilization efficiency, scale-up synthesis, simple operation, and recycling of catalysts.

Site-Selective Access to Functionalized Pyrroloquinoxalinones via H-Atom Transfer from N═Csp2-H Bonds of Quinoxalinones

Site-selective hydrogen atom transfer (HAT) from the N═Csp2-H bonds of quinoxaline-2(1H)-ones is a highly attractive but underdeveloped domain. Reported herein is a highly selective, practical, and economically efficient approach for facile assembly of pyrroloquinoxalinones by synergistic photocatalysis and HAT catalysis. The reaction proceeds through bromine radical-mediated HAT of quinoxalinones and imine radical addition to α-cyano-α,β-unsaturated ketones that establishes a cross-coupling/annulation cascade process, resulting in the synthesis of a series of functionalized pyrroloquinoxalinones. This protocol does not require transition metals or excess oxidants and uses easy-to-synthesize starting materials with excellent scalability and broad substrate scope. The establishment of N═Csp2 radical chemistry illustrates great potential for the synthesis of imine-containing molecules that are not possible with some traditional methods.

Dual transition metal electrocatalysis enables selective C(sp3)-C(sp3) bond cleavage and arylation of cyclic alcohols

We report a dual transition metal electrocatalytic approach for C(sp3)-C(sp3) bond cleavage and arylation of cyclic alcohols, providing an efficient and sustainable method for site-specific arylation of ketones. The reaction involves electrophotochemical cerium-catalysed generation of alkoxyl radicals from readily accessible alcohols. Subsequently, homolytic cleavage of the β-C-C bond leads to the generation of carbon-centered radicals that could be effectively utilized by nickel catalysis powered by cathode reduction to deliver the remote arylated ketone products.

Ligand-to-Metal Charge Transfer (LMCT) Catalysis: Harnessing Simple Cerium Catalysts for Selective Functionalization of Inert C-H and C-C Bonds

ConspectusChemists have long pursued harnessing light energy and photoexcitation processes for synthetic transformations. Ligand-to-metal charge transfer (LMCT) in high-valent metal complexes often triggers bond homolysis, generating oxidized ligand-centered radicals and reduced metal centers. While photoinduced oxidative activations can be enabled, this process, typically seen as photochemical decomposition, remains underexplored in catalytic applications. To mitigate decomposition during LMCT excitation, we developed a catalytic cycle integrating in situ coordination, LMCT, and ligand homolysis to activate ligated alcohols transiently into alkoxy radicals. This catalytic approach leverages Ce(IV) LMCT excitation and highly reactive alkoxy radical intermediates for selective functionalizations of C(sp3)-H and C(sp3)-C(sp3) bonds under mild conditions. In this Account, we discuss these advancements, highlighting the practical utility of cost-effective cerium salts as catalysts and their potential to develop innovative transformations, addressing long-standing synthetic challenges.Selective functionalization of chemically inert C(sp3)-H bonds has long posed a significant challenge. We first detail our research using LMCT-enabled alkoxy radical-mediated hydrogen atom transfer (HAT) processes for selective C(sp3)-H functionalizations. Using readily available CeCl3, we established a general protocol for employing free alcohols in the Barton reaction. By integrating LMCT and HAT catalysis, we introduced a selective photocatalytic strategy for functionalizing feedstock alkanes, converting gaseous hydrocarbons into valuable products. Employing simple cerium salts like Ce(OTf)3 and CeCl3, we achieved selective C-H amination of methane and ethane at ambient temperature, achieving turnover numbers of 2900 and 9700, respectively. This catalytic manifold has been further exploited to address the site-selectivity challenge in the C-H functionalization of linear alkanes. The use of methanol as a cocatalyst enabled preferential functionalization of the most electron-rich sites, achieving a high intrinsic selectivity over 12:1 of secondary vs primary sites in pentane and hexane.Next, we discuss the catalytic utilization of alkoxy-radical-mediated β-scission, a frequently encountered side reaction in HAT transformations, for selective cleavage and functionalization of C-C bonds. The versatility of the LMCT catalytic platform facilitates the generation of alkoxy radicals from various free alcohols. In our initial demonstration of LMCT-enabled C(sp3)-C(sp3) bond activation, we developed a cerium-catalyzed ring-opening and amination of cycloalkanols, providing an effective protocol for cleaving unstrained C-C bonds. This strategy has been successfully applied to various radical cross-coupling processes, leading to innovative transformations such as ring expansions of cycloalkanols, dehydroxymethylative alkylation, amination, alkenylation, and ring expansions of cyclic ketones. These results highlight the synthetic potential of employing LMCT-mediated β-scission and ubiquitous C-C bonds as unconventional functional handles for generating molecular complexity.Lastly, we delve into our mechanistic investigations. Beyond the catalytic application of Ce(IV) LMCT in various transformations, we have undertaken comprehensive mechanistic studies. These investigations encompass characterization of Ce(IV) alkoxide complexes to elucidate their structures, evaluation of their photoactivity and selectivity in radical generation, and elucidation of kinetic pathways associated with transient LMCT excited states. Our research has revealed ultrafast bond homolysis, back electron transfer, and the selectivity of heteroleptic complexes in homolysis, providing crucial insights for advancing LMCT catalysis.

A De Novo Metalloenzyme for Cerium Photoredox Catalysis

Cerium photoredox catalysis has emerged as a powerful strategy to activate molecules under mild conditions. Radical intermediates are formed using visible light and simple complexes of the earth-abundant lanthanide. Here, we report an artificial photoenzyme enabling this chemistry inside a protein. We utilize a de novo designed protein scaffold that tightly binds lanthanide ions in its central cavity. Upon visible-light irradiation, the cerium-dependent enzyme catalyzes the radical C-C bond cleavage of 1,2-diols in aqueous solution. Protein engineering led to variants with improved photostability and metal binding behavior. The photoenzyme cleaves a range of aromatic and aliphatic substrates, including lignin surrogates. Surface display of the protein scaffold on Escherichia coli facilitates whole-cell photobiocatalysis. Furthermore, we show that also natural lanthanide-binding proteins are suitable for this approach. Our study thus demonstrates a new-to-nature enzymatic photoredox activity with broad catalytic potential.

Photocatalyzed Direct C(sp3)-H Alkenylation of Unactivated Alkanes via Tandem C-C Activation of Cyclopropenes

A highly adaptable method has been developed for the alkenylation of a broad spectrum of inert alkanes, employing milder reaction conditions. Tetrabutylammonium decatungstate (TBADT) serves as a photocatalyst for hydrogen atom transfer (HAT), instigating the formation of transient alkyl radicals through C(sp3)-H functionalization. These radicals exhibit regioselective addition to cyclopropenes, followed by the subsequent activation of C-C bonds, forming the corresponding vinylated derivatives. This methodology accommodates diverse unreactive C(sp3)-H bond motifs and multisubstituted cyclopropenes, enabling the efficient synthesis of highly functionalized olefins with high diastereoselectivity.

Iron-Catalyzed Electrophotochemical α-Functionalization of a Silylcyclobutanol

Four-membered ring structure is important in organic chemistry, and selective cleavage and functionalization of these strained rings are of great interest. However, direct α-functionalization of cyclobutanols is rarely reported because of the high O-H bond dissociation energy and the occurrence of β-scission of C-C bonds in these alcohols. Recently, transition-metal catalysis has facilitated alkoxy radical generation. Herein, we report a method for electrophotochemical α-functionalization of a silylcyclobutanol via visible-light-induced LMCT reactions of M-alkoxy complexes. Introduction of the silyl group into the cyclobutanol structure favored fast [1,2]-silyl transfer over ring opening, thus allowing the generation of α-functionalized products.

Site-selective α-C(sp3)-H arylation of dialkylamines via hydrogen atom transfer catalysis-enabled radical aryl migration

Site-selective C(sp3)-H arylation is an appealing strategy to synthesize complex arene structures but remains a challenge facing synthetic chemists. Here we report the use of photoredox-mediated hydrogen atom transfer (HAT) catalysis to accomplish the site-selective α-C(sp3)-H arylation of dialkylamine-derived ureas through 1,4-radical aryl migration, by which a wide array of benzylamine motifs can be incorporated to the medicinally relevant systems in the late-stage installation steps. In contrast to previous efforts, this C-H arylation protocol exhibits specific site-selectivity, proforming predominantly on sterically more-hindered secondary and tertiary α-amino carbon centers, while the C-H functionalization of sterically less-hindered N-methyl group can be effectively circumvented in most cases. Moreover, a diverse range of multi-substituted piperidine derivatives can be obtained with excellent diastereoselectivity. Mechanistic and computational studies demonstrate that the rate-determining step for methylene C-H arylation is the initial H atom abstraction, whereas the radical ipso cyclization step bears the highest energy barrier for N-methyl functionalization. The relatively lower activation free energies for secondary and tertiary α-amino C-H arylation compared with the functionalization of methylic C-H bond lead to the exceptional site-selectivity.

Aerobic mechanochemical reversible-deactivation radical polymerization

Polymer materials suffer mechano-oxidative deterioration or degradation in the presence of molecular oxygen and mechanical forces. In contrast, aerobic biological activities combined with mechanical stimulus promote tissue regeneration and repair in various organs. A synthetic approach in which molecular oxygen and mechanical energy synergistically initiate polymerization will afford similar robustness in polymeric materials. Herein, aerobic mechanochemical reversible-deactivation radical polymerization was developed by the design of an organic mechano-labile initiator which converts oxygen into activators in response to ball milling, enabling the reaction to proceed in the air with low-energy input, operative simplicity, and the avoidance of potentially harmful organic solvents. In addition, this approach not only complements the existing methods to access well-defined polymers but also has been successfully employed for the controlled polymerization of (meth)acrylates, styrenic monomers and solid acrylamides as well as the synthesis of polymer/perovskite hybrids without solvent at room temperature which are inaccessible by other means.

Light-induced ligand-to-metal charge transfer of Fe(III)-OR species in organic synthesis

Light-induced ligand-to-metal charge transfer (LMCT) has been utilized as a powerful strategy in various organic reactions. First-row transition metals, especially iron complexes, show good applications in this process. Fe(III)-Cl and Fe(III)-OR species are two key intermediates involved in the LMCT of iron complexes. This review highlights studies on LMCT of Fe(III)-OR species, including carboxylate-iron and alkoxy-iron species, in organic transformations. Reaction conditions, substrate scope and related mechanisms are discussed.

Photoinduced Ligand-to-Metal Charge Transfer in Base-Metal Catalysis

The absorption of light by photosensitizers has been shown to offer novel reactive pathways through electronic excited state intermediates, complementing ground state mechanisms. Such strategies have been applied in both photocatalysis and photoredox catalysis, driven by generating reactive intermediates from their long-lived excited states. One developing area is photoinduced ligand-to-metal charge transfer (LMCT) catalysis, in which coordination of a ligand to a metal center and subsequent excitation with light results in the formation of a reactive radical and a reduced metal center. This mini review concerns the foundations and recent developments in ligand-to-metal charge transfer in transition metal catalysis focusing on the organic transformations made possible through this mechanism.

Synergistic use of photocatalysis and convergent paired electrolysis for nickel-catalyzed arylation of cyclic alcohols

The merging of transition metal catalysis with electrochemistry has become a powerful tool for organic synthesis because catalysts can govern the reactivity and selectivity. However, coupling catalysts with alkyl radical species generated by anodic oxidation remains challenging because of electrode passivation, dimerization, and overoxidation. In this study, we developed convergent paired electrolysis for the coupling of nickel catalysts with alkyl radicals derived from photoinduced ligand-to-metal charge-transfer of cyclic alcohols and iron catalysts, providing a practical method for site-specific and remote arylation of ketones. The synergistic use of photocatalysis with convergent paired electrolysis can provide alternative avenues for metal-catalyzed radical coupling reactions.

Photogeneration of Chlorine Radical from a Self-Assembled Fluorous 4CzIPN•Chloride Complex: Application in C-H Bond Functionalization

The chlorine radical is a strong HAT (Hydrogen Atom Transfer) agent that is very useful for the functionalization of C(sp3)-H bonds. Albeit highly attractive, its generation from the poorly oxidizable chloride ion mediated by an excited photoredox catalyst is a difficult task. We now report that 8Rf8-4CzIPN, an electron-deficient fluorous derivative of the benchmark 4CzIPN photoredox catalyst belonging to the donor-acceptor carbazole-cyanoarene family, is not only a better photooxidant than 4CzIPN, but also becomes an excellent host for the chloride ion. Combining these two properties ultimately makes the self-assembled 8Rf8-4CzIPN•Cl- dual catalyst highly reactive in redox-neutral Giese-type C(sp3)-H bond alkylation reactions promoted by the chlorine radical. Additionally, because of its fluorous character, the efficient separation/recovery of 8Rf8-4CzIPN could be envisioned.

Photoelectrochemical Fe/Ni cocatalyzed C-C functionalization of alcohols

The simultaneous activation of reactants on the anode and cathode via paired electrocatalysis has not been extensively demonstrated. This report presents a paired oxidative and reductive catalysis based on earth-abundant iron/nickel cocatalyzed C-C functionalization of ubiquitous alcohols. A variety of alcohols (i.e., primary, secondary, tertiary, or unstrained cyclic alcohols) can be activated at very low oxidation potential of (~0.30 V vs. Ag/AgCl) via photoelectrocatalysis coupled with versatile electrophiles. This reactivity yields a wide range of structurally diverse molecules with broad functional group compatibility (more than 50 examples).

Alkoxy Radical Generation Mediated by Sulfoxide Cation Radicals for Alcohol-Directed Aliphatic C-H Functionalization

The C-H functionalization of remote, unactivated C-H bonds offers a unique method of garnering structural complexity in a synthesis. The use of directing groups has provided a means of enacting C-H functionalization on these difficult-to-access bonds; however, the installation and removal of directing groups on a substrate require additional synthetic manipulations, detracting from both the efficiency and economic feasibility of a transformation. The use of alkoxy radicals as transient directing groups for the functionalization of remote C-H bonds allows access to the synthesis of complex molecules without the need for additional functionality. Herein, we report a method for alkoxy radical formation from unactivated alcohols and reactivity mediated by photoredox-generated sulfoxide cation radicals. This protocol leverages the unique reactivity of alkoxy radicals to implement different reaction manifolds: 1,5-hydrogen atom transfer (HAT), cyclization, and β-scission. Furthermore, it was discovered that this methodology could be utilized to impose radical group transfer reactions via the β-scission pathway. Stern-Volmer analysis supports the formation of an alkoxy radical via the intermediacy of a sulfurane radical rather than a proton-coupled electron transfer (PCET) mechanism.

Iron-Catalyzed Photochemical Synthesis of Sulfinamides from Aliphatic Hydrocarbons and Sulfinylamines

An iron-catalyzed photochemical sulfinamidation of hydrocarbons with N-sulfinylamines has been developed. The merger of ligand-to-metal charge transfer (LMCT) of FeCl3 with hydrogen atom transfer (HAT) process is the key for the generation of alkyl radicals from hydrocarbons, and the resultant alkyl radicals were readily trapped by N-sulfinylamines to produce structurally diverse sulfinamides. Contrary to traditional methods that inevitably use sensitive organometallic reagents and prefunctionalized substrates, our approach features simple operation and the wide availability of starting materials. Gratifyingly, the reaction is scalable, and the obtained sulfinamides can be conveniently converted to highly functionalized sulfur(VI) derivatives.

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

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

Tetravalent Cerium Alkyl and Benzyl Complexes

High-valent cerium complexes of alkyl and benzyl ligands are unprecedented due to the incompatibility of the typically highly oxidizing Ce4+ ion and the reducing alkyl or benzyl ligand. Herein we report the synthesis and isolation of the first tetravalent cerium alkyl and benzyl complexes supported by the tri-tert-butyl imidophosphorane ligand, [NP(tBu)3]1-. The Ce4+ monoiodide complex, [Ce4+I(NP(tert-butyl)3)3] (1-CeI), serves as a precursor to the alkyl and benzyl complexes, [Ce4+(Npt)(NP(tert-butyl)3)3] (2-CeNpt) (Npt = neopentyl, CH2C(CH3)3) and [Ce4+(Bn)(NP(tert-butyl)3)3] (2-CeBn) (Bn = benzyl, CH2Ph). The bonding and structure of these complexes are characterized by single-crystal XRD, NMR and UV-vis-NIR spectroscopy, cyclic voltammetry, and DFT studies.

Unraveling the Prominent Existence of Trace Metals in Photocatalysis: Exploring Iron Impurity Effects

Metal impurities can complicate the identification of active catalyst species in transition metal catalysis and electrocatalysis, potentially leading to misleading findings. This study investigates the influence of metal impurities on photocatalysis. Specifically, the photocatalytic reaction of inert alkanes using chlorides without the use of an external photocatalyst was studied, achieving successful C(sp3)-H functionalization. The observations reveal that Fe and Cu impurities are challenging to avoid in a typical laboratory environment and are prominently present in normal reaction systems, and iron impurities play a dominant role in the aforementioned apparent 'metal-free' reaction. Additionally, iron exhibits significantly higher catalytic activity compared to Cu, Ce, and Ni at low metal concentrations in the photocatalytic C(sp3)-H functionalization using chlorides. Considering the widespread presence of Fe and Cu impurities in typical laboratory environments, this study serves as a reminder of their involvement in reaction processes.

Tethering Three Radical Cascades for Controlled Termination of Radical Alkyne peri-Annulations: Making Phenalenyl Ketones without Oxidants

Although Bu3Sn-mediated radical alkyne peri-annulations allow access to phenalenyl ring systems, the oxidative termination of these cascades provides only a limited selection of the possible isomeric phenalenone products with product selectivity controlled by the intrinsic properties of the new cyclic systems. In this work, we report an oxidant-free termination strategy that can overcome this limitation and enable selective access to the full set of isomerically functionalized phenalenones. The key to preferential termination is the preinstallation of a "weak link" that undergoes C-O fragmentation in the final cascade step. Breaking a C-O bond is assisted by entropy, gain of conjugation in the product, and release of stabilized radical fragments. This strategy is expanded to radical exo-dig cyclization cascades of oligoalkynes, which provide access to isomeric π-extended phenalenones. Conveniently, these cascades introduce functionalities (i.e., Bu3Sn and iodide moieties) amenable to further cross-coupling reactions. Consequently, a variety of polyaromatic diones, which could serve as phenalenyl-based open-shell precursors, can be synthesized.

Terminal C(sp3)-H borylation through intermolecular radical sampling

Hydrogen atom transfer (HAT) processes can overcome the strong bond dissociation energies (BDEs) of inert C(sp3)-H bonds and thereby convert feedstock alkanes into value-added fine chemicals. Nevertheless, the high reactivity of HAT reagents, coupled with the small differences among various C(sp3)-H bond strengths, renders site-selective transformations of straight-chain alkanes a great challenge. Here, we present a photocatalytic intermolecular radical sampling process for the iron-catalyzed borylation of terminal C(sp3)-H bonds in substrates with small steric hindrance, including unbranched alkanes. Mechanistic investigations have revealed that the reaction proceeds through a reversible HAT process, followed by a selective borylation of carbon radicals. A boron-sulfoxide complex may contribute to the high terminal regioselectivity observed.

Rare Earths-The Answer to Everything

Rare earths, scandium, yttrium, and the fifteen lanthanoids from lanthanum to lutetium, are classified as critical metals because of their ubiquity in daily life. They are present in magnets in cars, especially electric cars; green electricity generating systems and computers; in steel manufacturing; in glass and light emission materials especially for safety lighting and lasers; in exhaust emission catalysts and supports; catalysts in artificial rubber production; in agriculture and animal husbandry; in health and especially cancer diagnosis and treatment; and in a variety of materials and electronic products essential to modern living. They have the potential to replace toxic chromates for corrosion inhibition, in magnetic refrigeration, a variety of new materials, and their role in agriculture may expand. This review examines their role in sustainability, the environment, recycling, corrosion inhibition, crop production, animal feedstocks, catalysis, health, and materials, as well as considering future uses.

C-H Alkylation of Cubanes via Catalytic Generation of Cubyl Radicals

A catalytic method for the C-H alkylation of cubanes is described. Some hydrogen atom transfer catalysts enable the direct abstraction of a hydrogen atom from the C-H bond of cubanes, followed by conjugate addition of the generated cubyl radicals to electron-deficient alkenes. Synthetic applications of the functionalization method developed are also described.

O-H bond activation of β,γ-unsaturated oximes via hydrogen atom transfer (HAT) and photoredox dual catalysis

Hydrogen atom transfer (HAT) and photoredox dual catalysis provides a unique opportunity in organic synthesis, enabling the direct activation of C/Si/S-H bonds. However, the activation of O-H bonds of β,γ-unsaturated oximes poses a challenge due to their relatively high redox potential, which exceeds the oxidizing capacity of most currently developed photocatalysts. We here demonstrate that the combination of HAT and photoredox catalysis allows the activation of O-H bond of β,γ-unsaturated oximes. The strategy effectively addresses the oxime's high redox potential and offers a universal pathway for iminoxyl radical formation. Leveraging the versatility of this approach, a diverse array of valuable heterocycles have been synthesized with the use of different radical acceptors. Mechanistic studies confirm a HAT process for the O-H bond activation.

Cerium Photocatalyst in Action: Structural Dynamics in the Presence of Substrate Visualized via Time-Resolved X-ray Liquidography

[Ce(III)Cl6]3-, with its earth-abundant metal element, is a promising photocatalyst facilitating carbon-halogen bond activation. Still, the structure of the reaction intermediate has yet to be explored. Here, we applied time-resolved X-ray liquidography (TRXL), which allows for direct observation of the structural details of reaction intermediates, to investigate the photocatalytic reaction of [Ce(III)Cl6]3-. Structural analysis of the TRXL data revealed that the excited state of [Ce(III)Cl6]3- has Ce-Cl bonds that are shorter than those of the ground state and that the Ce-Cl bond further contracts upon oxidation. In addition, this study represents the first application of TRXL to both photocatalyst-only and photocatalyst-and-substrate samples, providing insights into the substrate's influence on the photocatalyst's reaction dynamics. This study demonstrates the capability of TRXL in elucidating the reaction dynamics of photocatalysts under various conditions and highlights the importance of experimental determination of the structures of reaction intermediates to advance our understanding of photocatalytic mechanisms.

Scalability of advanced oxidation processes (AOPs) in industrial applications: A review

Disinfection and decontamination of water by application of oxidisers is an essential treatment step across numerous industrial sectors including potable supply and industry waste management, however, could be greatly enhanced if operated as advanced oxidation processes (AOPs). AOPs destroy contaminants including pathogens by uniquely harnessing radical chemistry. Despite AOPs offer great practical opportunities, no reviews to date have highlighted the critical AOP virtues that facilitate AOPs' scale up under growing industrial demand. Hence, this review analyses the critical AOP parameters such as oxidant conversion efficiency, batch mode vs continuous-flow systems, location of radical production, radical delivery by advanced micro-/mesoporous structures and AOP process costs to assist the translation of progressing developments of AOPs into their large-scale applications. Additionally, the state of the art is analysed for various AOP inducing radical/oxidiser measurement techniques and their half-lives with a view to identify radicals/oxidisers that are suitable for in-situ production. It is concluded that radicals with short half-lives such as hydroxyl (10-4 μsec) and sulfate (30-40 μsec) need to be produced in-situ via continuous-flow reactors for their effective transport and dosing. Meanwhile, radicals/oxidisers with longer half-lives such as ozone (7-10 min), hydrogen peroxide (stable for several hours), and hypochlorous acid (10 min -17 h) need to be applied through batch reactor systems due to their relatively longer stability during transportation and dosing. Complex and costly synthesis as well as cytotoxicity of many micro-/mesoporous structures limit their use in scaling up AOPs, particularly to immobilising and delivering the short-lived hydroxyl and sulfate radicals to their point of applications. Overall, radical delivery using safe and advanced biocompatible micro-/mesoporous structures, radical conversion efficiency using advanced reactor design and portability of AOPs are priority areas of development for scaling up to industry.

Multiplicative enhancement of stereoenrichment by a single catalyst for deracemization of alcohols

Stereochemical enrichment of a racemic mixture by deracemization must overcome unfavorable entropic effects as well as the principle of microscopic reversibility; recently, photochemical reaction pathways unveiled by the energetic input of light have led to innovations toward this end, most often by ablation of a stereogenic C(sp3)-H bond. We report a photochemically driven deracemization protocol in which a single chiral catalyst effects two mechanistically different steps, C-C bond cleavage and C-C bond formation, to achieve multiplicative enhancement of stereoinduction, which leads to high levels of stereoselectivity. Ligand-to-metal charge transfer excitation of a titanium catalyst coordinated by a chiral phosphoric acid or bisoxazoline efficiently enriches racemic alcohols that feature adjacent and fully substituted stereogenic centers to enantiomeric ratios up to 99:1. Mechanistic investigations support a pathway of sequential radical-mediated bond scission and bond formation through a common prochiral intermediate and reveal that, although the overall stereoenrichment is high, the selectivity in each individual step is moderate.

Visible-light induced C(sp3)-H arylation of glycine derivatives by cerium catalysis

A Ce(III)-catalyzed, visible-light induced aerobic oxidative dehydrogenative coupling reaction between glycine derivatives and electron-rich arenes is disclosed. The protocol proceeds efficiently under mild conditions, providing an efficient method for the rapid synthesis of α-arylglycine derivatives without the need for an external photosensitizer and additional oxidant. Moreover, this protocol could be performed on a 5 mmol scale, without obvious reduction of the efficiency.

Photoelectrochemical oxidative C(sp3)-H borylation of unactivated hydrocarbons

Organoboron compounds are of high significance in organic synthesis due to the unique versatility of boryl substituents to access further modifications. The high demand for the incorporation of boryl moieties into molecular structures has witnessed significant progress, particularly in the C(sp3)-H borylation of hydrocarbons. Taking advantage of special characteristics of photo/electrochemistry, we herein describe the development of an oxidative C(sp3)-H borylation reaction under metal- and oxidant-free conditions, enabled by photoelectrochemical strategy. The reaction exhibits broad substrate scope (>57 examples), and includes the use of simple alkanes, halides, silanes, ketones, esters and nitriles as viable substrates. Notably, unconventional regioselectivity of C(sp3)-H borylation is achieved, with the coupling site of C(sp3)-H borylation selectively located in the distal methyl group. Our method is operationally simple and easily scalable, and offers a feasible approach for the one-step synthesis of high-value organoboron building blocks from simple hydrocarbons, which would provide ample opportunities for drug discovery.

Visible-Light-Enabled Lanthanum-Mediated Intramolecular Epoxy-Ring Opening/Dehydrogenative Lactonization

Catalytic C(sp3)-H functionalization has afforded great opportunities to prepare organic substances, facilitating the derivatization of complex drugs and natural molecules. This letter describes an efficient and practical protocol for lanthanum-catalyzed continuous epoxy-ring opening and oxidative dehydrogenative lactonization under visible-light irradiation. Notably, the lanthanum catalyst also acts as a photocatalyst while acting as a Lewis acid in this reaction; therefore, no additional photocatalyst is required. We can conveniently prepare a series of diverse isochromanones with oxygen-containing spirocyclic structural units under a balloon-oxygen atmosphere at room temperature. Mechanistic studies and control experiments reveal that the in situ-generated lanthanum bromide should be crucial in the reaction.

Efficient C(sp3 )-H Carbonylation of Light and Heavy Hydrocarbons with Carbon Monoxide via Hydrogen Atom Transfer Photocatalysis in Flow

Despite their abundance in organic molecules, considerable limitations still exist in synthetic methods that target the direct C-H functionalization at sp3 -hybridized carbon atoms. This is even more the case for light alkanes, which bear some of the strongest C-H bonds known in Nature, requiring extreme activation conditions that are not tolerant to most organic molecules. To bypass these issues, synthetic chemists rely on prefunctionalized alkyl halides or organometallic coupling partners. However, new synthetic methods that target regioselectively C-H bonds in a variety of different organic scaffolds would be of great added value, not only for the late-stage functionalization of biologically active molecules but also for the catalytic upgrading of cheap and abundant hydrocarbon feedstocks. Here, we describe a general, mild and scalable protocol which enables the direct C(sp3 )-H carbonylation of saturated hydrocarbons, including natural products and light alkanes, using photocatalytic hydrogen atom transfer (HAT) and gaseous carbon monoxide (CO). Flow technology was deemed crucial to enable high gas-liquid mass transfer rates and fast reaction kinetics, needed to outpace deleterious reaction pathways, but also to leverage a scalable and safe process.

C-C bond formation via photocatalytic direct functionalization of simple alkanes

The direct functionalization of alkanes represents a very important challenge in the goal to develop more atom-efficient and clean C-C bond forming reactions. These processes, however, are hampered by the low reactivity of the aliphatic C-H bonds. Photocatalytic processes based on hydrogen atom transfer C-H bond activation strategies have become a useful tool to activate and functionalize these inert compounds. In this article, we summarize the main achievements in this field applied to the development of C-C bond forming reactions, and we discuss the key mechanistic features that enable these transformations.

Co-function Mechanisms of Chlorine and Alkoxy Radicals in Cerium-Catalyzed C-H Functionalization of Alkane Mediated by Visible Light

Identification of radical intermediates for the catalytic functionalization of alkanes offers a number of unique challenges and has recently raised a controversial issue concerning the subtle role of chlorine versus alkoxy radicals in cerium photocatalysis. This study is an attempt to settle the controversy within the theoretical frameworks of Marcus electron transfer and transition state theory. Co-function mechanisms were proposed together with a scheme of kinetic evaluations to account for ternary dynamic competition among photolysis, back electron transfer, and hydrogen atom transfer (HAT). Cl•-based HAT has been proven to initially control the early dynamics of the photocatalytic transformation on the picosecond to nanosecond time scale, which is subsequently taken over by a postnanosecond event of alkoxy radical-mediated HAT. The theoretical models developed herein provide a uniform understanding of the continuous time dynamics of photogenerated radicals to address some paradoxical arguments in lanthanide photocatalysis.

Resurgence and advancement of photochemical hydrogen atom transfer processes in selective alkane functionalizations

The selective functionalization of alkanes has long been recognized as a prominent challenge and an arduous task in organic synthesis. Hydrogen atom transfer (HAT) processes enable the direct generation of reactive alkyl radicals from feedstock alkanes and have been successfully employed in industrial applications such as the methane chlorination process, etc. Nevertheless, challenges in the regulation of radical generation and reaction pathways have created substantial obstacles in the development of diversified alkane functionalizations. In recent years, the application of photoredox catalysis has provided exciting opportunities for alkane C-H functionalization under extremely mild conditions to trigger HAT processes and achieve radical-mediated functionalizations in a more selective manner. Considerable efforts have been devoted to building more efficient and cost-effective photocatalytic systems for sustainable transformations. In this perspective, we highlight the recent development of photocatalytic systems and provide our views on current challenges and future opportunities in this field.

Iron-Catalyzed C(Sp3)-H Borylation, Thiolation, and Sulfinylation Enabled by Photoinduced Ligand-to-Metal Charge Transfer

Catalytic C(sp³)-H functionalization has provided enormous opportunities to construct organic molecules, facilitating the derivatization of complex pharmaceutical compounds. Within this framework, direct hydrogen atom transfer (HAT) photocatalysis becomes an appealing approach to this goal. However, the viable substrates utilized in these protocols are limited, and the site selectivity shows preference to activated and thermodynamically favored C(sp³)-H bonds. Herein, we describe the development of undirected iron-catalyzed C(sp³)-H borylation, thiolation, and sulfinylation reactions enabled by the photoinduced ligand-to-metal charge transfer (LMCT) process. These reactions exhibit remarkably broad substrate scope (>150 examples in total), and most importantly, all of these three reactions show unconventional regioselectivity, with the occurrence of C(sp³)-H borylation, thiolation, and sulfinylation preferentially at the distal methyl position. The procedures are operationally simple and readily scalable and provide access to high-value products from simple hydrocarbons in one step. Mechanistic studies and control experiments indicate that the afforded site selectivity is not only relevant to the HAT species but also largely affected by the use of boron- and sulfone-based radical acceptors.

Recent advances in the synthesis of chiral α-tertiary amines via transition-metal catalysis

The significance of chiral α-tertiary amines in medicinal chemistry and drug development has been unquestionably established in the last few decades. α-Tertiary amines are attractive structural motifs for natural products, bioactive molecules and pharmaceuticals and are preclinical candidates. Their syntheses have been the focus of intensive research, and the development of new methods has continued to attract more and more attention. In this review, we present the progress in the last decade in the development of synthetic methods for the assembly of chiral ATAs via transition-metal catalysis. To date, the effective approaches in this area could be categorized into three strategies: enantioselective direct and indirect Mannich addition to ketimines; umpolung asymmetric alkylation of imine derivatives; and asymmetric C-N cross-coupling of tertiary alkyl electrophiles. Several related developing strategies for the synthesis of ATAs, such as hydroamination of alkenes, HAT amination approaches and the C-C coupling of α-aminoalkyl fragments, are also described in this article. These strategies have emerged as attractive C-C and C-N bond-forming protocols for enantioselective construction of chiral α-tertiary amines, and to some extent are complementary to each other, showing the prospect of application in medicinal chemistry and chemical biology.

Visible-light-induced Csp3-H functionalization of glycine derivatives by cerium catalysis

A Ce(III)-catalyzed, visible-light-induced aerobic oxidative dehydrogenative coupling/aromatization reaction between glycine derivatives and alkenes has been developed, which provides an efficient approach for the synthesis of quinoline derivatives and post-modification of oligopeptides containing glycine residues under mild conditions without the need for external photosensitizers.

Electrophotochemical Metal-Catalyzed Enantioselective Decarboxylative Cyanation

In contrast to the rapid growth of electrophotocatalysis in recent years, enantioselective catalytic reactions powered by this unique methodology remain rare. In this work, we report an electrophotochemical metal-catalyzed protocol for direct asymmetric decarboxylative cyanation of aliphatic carboxylic acids. The synergistic merging of electrophotochemical cerium catalysis and asymmetric electrochemical copper catalysis permits mild reaction conditions for the formation and utilization of the key carbon centered radicals by combining the power of light and electrical energy. Electrophotochemical cerium catalysis enables radical decarboxylation to produce alkyl radicals, which could be effectively intercepted by asymmetric electrochemical copper catalysis for the construction of C-CN bonds in a highly stereoselective fashion. This environmentally benign method smoothly converts a diverse array of arylacetic acids into the corresponding alkyl nitriles in good yields and enantioselectivities without using chemical oxidants or pre-functionalization of the acid substrates and can be readily scaled up.