Asymmetric counteranion-directed photoredox catalysis

Expedient access to bora-butenolide bioisosteres by counteranion-mediated trans-hydroboration of alkynes

The hydroboration of alkynes is a textbook example of a syn-selective concerted addition reaction, while trans-selective additions of borane to alkynes remain to be developed. We herein report a transition metal-free anti-addition of pinacolborane to alkynes, facilitated by the counteranion effect. This work further develops Chan alkyne reduction by utilizing the borane instead of aluminohydride reagents, enabling the facile synthesis of valuable five-membered boracycles that constitute isosteric alternatives to bioactive butenolides and a versatile platform for abundant downstream transformations. The practical method is distinguished by excellent regioselectivity, a broad substrate scope, and high compatibility with a variety of functional groups. The exploration of trans-selective patterns affords not only a stereo-complementary approach to traditional organic synthesis, but also mandates a new perspective on the noncanonical trans-hydroboration mechanism. A combination of control experiments and computational studies at the DFT level of theory reveal the previously unrecognized role of the HMDS counteranion in a stepwise intermolecular hydrogen transfer process.

Nitrogen-Based Organofluorine Functional Molecules: Synthesis and Applications

Fluorine and nitrogen form a successful partnership in organic synthesis, medicinal chemistry, and material sciences. Although fluorine-nitrogen chemistry has a long and rich history, this field has received increasing interest and made remarkable progress over the past two decades, driven by recent advancements in transition metal and organocatalysis and photochemistry. This review, emphasizing contributions from 2015 to 2023, aims to update the state of the art of the synthesis and applications of nitrogen-based organofluorine functional molecules in organic synthesis and medicinal chemistry. In dedicated sections, we first focus on fluorine-containing reagents organized according to the type of fluorine-containing groups attached to nitrogen, including N-F, N-RF, N-SRF, and N-ORF. This review also covers nitrogen-linked fluorine-containing building blocks, catalysts, pharmaceuticals, and agrochemicals, underlining these components' broad applicability and growing importance in modern chemistry.

Synthesis of polysubstituted cyclobutanes through a photoredox strain-release/[3,3]-rearrangement cascade

Small saturated carbocycles, such as cyclobutanes, with elevated three-dimensionality and rich Csp3 centers are privileged scaffolds in naturally occurring molecules and drug discovery. It remains highly desirable and challenging to develop modular and straightforward strategies to craft densely substituted cyclobutanes. Herein, a photoredox-catalyzed radical strain-release/[3,3]-rearrangement cascade (SRRC) strategy for efficient synthesis of polysubstituted cyclobutanes is disclosed. This protocol operates with readily available α-silylamines as radical precursors, and strained bicyclo[1.1.0]butanes (BCBs) and cyclobutenes as radical acceptors, to access an array of structurally diverse 1,1,3- and 1,1,2-trisubstituted cyclobutanes containing a unique non-natural amino acid scaffold. Mechanistic studies reveal the pivotal reactivity of the silylketene acetal intermediate and the origin of diastereoselectivity. The power and utility of this method are illustrated with diverse transformations and preliminary anticancer assessment.

Deracemization of C(sp3)-H Arylated Carbonyl Compounds via Asymmetric Ion-Pairing Photoredox Catalysis

Deracemization of C(sp3)-H arylated carbonyl compounds faces limitations in terms of substrate scope. Through the photoactivation of the aryl group and the stereocontrol of the generated arene radical cation via asymmetric ion-pairing catalysis, we are able to achieve deracemization of carbonyl compounds arylated at both enolizable and unenolizable stereocenters. A diverse range of α-, β-, and γ-aryl ketones and esters, including natural products and medicinal derivatives, can be effectively converted into their enantiomers with high enantioselectivity. Mechanistic investigations through combined experimental and computational studies suggest that the reaction involves single-electron oxidation of electron-rich aryl groups, followed by a kinetic resolution of the resulting radical cation intermediates by the chiral phosphate anion. Deprotonation is identified as the stereodetermining step, while stereoselective back electron transfer and triplet-state quenching of 3 Mes-Acr1+* may also affect the enantioselectivity at the photostationary state.

Asymmetric Synthesis of Strained Multichiral Spirocyclobutanes through Cage-Confined Cross [2 + 2] Photocycloaddition

Chiral spirocycles possess the ability to undergo diverse modifications in three-dimensional space, offering advantages in terms of physicochemical property and structural variability over conventional organic scaffolds and holding promising potential for the design of biologically active molecules and drugs. Among them, highly strained spirocyclobutanes with multiple chiral center-containing four-membered rings have attracted significant attention, but their viable and efficient synthesis poses a great challenge. By virtue of cage-confined asymmetric photocatalysis, we successfully construct spirocycle and bispirocycle compounds containing multiple quaternary and tertiary chiral carbon centers in cyclobutane rings through cross [2 + 2] photocycloaddition with visible-light-induced and mild reactions. The mechanistic studies unveil that the chiral open pockets of a cage photoreactor facilitate dynamic bimolecular recognition to render preferential heteromolecular cross-cycloaddition with enhanced reactivity, unconventional enantioselectivity, and good substrate tolerance, providing a promising direction for enzyme-mimetic catalyst design for challenging asymmetric photochemical transformations.

Highly Diastereoselective Synthesis of 5/6-Fused Bicyclic Ring Systems via Radical Cyano Group Migration

Here we report a highly diastereoselective cyano group transfer radical cyclization reaction to construct 5/6-fused bicyclic ring systems that bear three contiguous and congested stereogenic elements, with 100% atom economy under catalyst-free and near-ultraviolet light irradiation conditions. Mechanistic investigations and density functional theory calculations suggest that the diastereoselectivity is governed by the conformational distribution of the triplet diradical intermediate and the rate of reverse intersystem crossing (RISC) before radical coupling.

Phosphoric Acid-Catalyzed Enantioselective Synthesis of Axially Chiral Cyclobutanamides

Chiral cyclobutanamide is a privileged scaffold in drug discovery. Here, we describe, for the first time, the synthesis of axially chiral cyclobutanamides via phosphoric acid-catalyzed enantioselective condensation between N-arylcarbamyl cyclobutanones and hydroxylamines. Rational substrate design, incorporating an amide moiety (CONHR) into the cyclobutanone backbone and the formation of a multi-hydrogen bonding network involving the N-H of this amide portion, is responsible for the excellent enantioselectivity achieved in the stereodetermining dehydration process, which is supported by a detailed mechanistic study.

Asymmetric Counteranion-Directed Halogen Bonding Catalysis

Halogen bonding has been established as a promising tool in organocatalysis. Asymmetric processes are nevertheless scarce, and their applications are limited to a few studies applying chiral halogen bond donors. Herein, we combine halogen bonding with asymmetric counteranion-directed catalysis, providing the first highly enantioselective example of such an approach. A strong bidentate iodine(III)-based catalyst with chiral disulfonimides as counteranions is applied in the first asymmetric organocatalysis of the Diels-Alder reaction between cyclopentadiene and trans-β-nitrostyrene, the key step in the synthesis of the drug fencamfamine, which was prepared with high enantioselectivity.

Quantitative Formation of Octa-substituted Cyclobutanes by the [2+2] Photocycloaddition of Stiff-stilbenes

The creation of new molecules with specific structural motifs is a primary goal in synthetic chemistry. In this context, cyclobutanes (CBs), the highly strained ring systems, are of great interest because of their wide applications from pharmaceutical chemistry to materials science. The [2+2] photocycloaddition is the most straightforward approach for CBs; however, access to fully substituted cyclobutanes presents a significant challenge due to the steric hindrance imposed by eight substituents. Here, we report an efficient synthesis of structurally unique, octa-substituted cyclobutanes from stiff-stilbene precursors, which is enabled by adaptable anion coordination through hydrogen bonding networks. The synthesized spiro-benzocyclopentyl-substituted cyclobutanes are confirmed by single-crystal structures. DFT calculations indicate that these cyclobutanes have high ring-strain energies (~20.1 kcal/mol) and can be completely converted back to stiff-stilbenes. Such a reversible cycloaddition/reversion process offers a promising platform for applications in responsive and energy storage materials.

Anionic Stereogenic-at-Cobalt(III) Complex-Enabled Asymmetric Oxidation of N,N-Dialkyl Sulfenamides

An asymmetric oxidation of N,N-dialkyl sulfenamides is exhibited by using anionic stereogenic-at-cobalt(III) complexes as catalysts. This protocol provides an alternative approach to access a diverse set of chiral tertiary sulfinamides with high enantioselectivities (24 examples, up to 94:6 e.r.). Additionally, control experiments suggest that this protocol could be accomplished through a chiral cationic S(IV) intermediate.

Catalytic asymmetric photocycloaddition reactions mediated by enantioselective radical approaches

The use of olefins in the construction of cyclic compounds represents a powerful strategy for advancing the pharmaceutical industry. Photocycloaddition has attracted significant interest from chemists due to its ability to exploit simple and readily available olefins along with their reaction patterns under mild conditions. Moreover, the sustainable and versatile pathways for generating highly reactive intermediates can greatly enrich both substrate diversity and reaction patterns. As a result, numerous photocycloaddition reactions have been successfully developed, particularly asymmetric [2+2], [3+2], and [4+2] photocycloadditions mediated by enantioselective radical approaches, achieving remarkable enantioselectivities. This review offers a comprehensive overview of this rapidly evolving field, organizing the discussion into three distinct reaction types that facilitate the construction of enantioenriched derivatives of cyclobutanes, cyclopentanes, and cyclohexanes. Emphasis is placed on analyzing and summarizing established strategies aimed at circumventing the challenges posed by racemic background transformations. Additionally, the exploration of asymmetric [3+2] and [4+2] photocycloaddition reactions will be interwoven with a detailed discussion of the various substrate types involved. This systematic framework seeks to enhance understanding of the strategies employed to manage the high reactivity of radicals while achieving high enantioselectivity. Importantly, it aims to guide readers in identifying uncharted radical-based cycloaddition pathways, which possess significant potential to broaden the diversity of complex cyclic molecules.

Peroxycarbenium-Mediated Asymmetric Synthesis of 1,2-Dioxanes and 1,2-Dioxolanes

The endoperoxide scaffold is found in numerous natural products and synthetic substances of pharmaceutical interest. The main challenge to their synthetic access remains the preparation of chiral compounds due to the weakness of the peroxide bond, which limits the scope of available or applicable methods. Here, we demonstrate how peroxycarbenium species can be trapped by silylated nucleophiles with high enantioselectivities and diastereoselectivities when applicable, using a chiral imidophosphorimidate (IDPi) as a catalyst. The scope of the methodology is broad, encompassing a large variety of enoxysilanes and yielding 1,2-dioxanes or 1,2-dioxolanes. Peroxides can be converted into alcohols or trans-epoxides, and the methodology was applied in a key step of the total synthesis of ethyl plakortide Z, enhancing the selectivity compared to a conventional Lewis acid-catalyzed transformation. Kinetic studies have shown that the reaction necessitates an induction period, indicating the formation of a silylium species that behaves as a true catalyst.

Asymmetric Counteranion-Directed Electrocatalysis for Enantioselective Control of Radical Cation

The control of enantioselectivity in radical cation reactions presents long-standing challenges, despite a few successful examples. We introduce a novel strategy of asymmetric counteranion-directed electrocatalysis to address enantioselectivity in radical cation chemistry. This concept has been successfully demonstrated in two reactions: an asymmetric dehydrogenative indole-phenol [3+2] coupling and an atroposelective C-H/N-H dehydrogenative coupling. These reactions have enabled the synthesis of benzofuroindolines and C-N axially chiral indoles with high yields and excellent enantiomeric excesses. Detailed mechanistic studies confirmed a radical-radical coupling mechanism. Moreover, density functional theory (DFT) calculations supported the indole radical cation as the pivotal intermediate, rather than a neutral indolyl radical, shedding new light on the underlying processes driving these reactions.

Dual ligand-enabled iron and halogen-containing carboxylate-based photocatalysis for chloro/fluoro-polyhaloalkylation of alkenes

Herein, we demonstrate a practical dual ligand-enabled iron photocatalysis paradigm-converting all kinds of halogen-containing carboxylates (C n X m COO-, X: F, Cl, Br) into C n X m radicals for the valuable chloro/fluoro-polyhaloalkylation of non-activated alkenes with easily available trichloroacetonitrile/Selectfluor as the electrophilic halogenation reagent. The modular in situ assembly of the effective iron and C n X m COO--based light-harvesting species using the two ligands-OMe/CF3-substituted bipyridine and acetonitrile/trichloroacetonitrile is evidenced by detailed mechanistic studies. The late-stage modification, low loading amount of iron (TON: 257) and feasible gram-scale synthesis show the utility of this protocol. We thus anticipate that the dual ligand-enabled iron photocatalysis paradigm may facilitate activation and transformation of inert bulk chemicals.

Chiral Brønsted Acid-Catalyzed Intramolecular Asymmetric Dearomatization Reaction of Indoles with Cyclobutanones via Cascade Friedel-Crafts/Semipinacol Rearrangement

The highly efficient synthesis of chiral indolines fused with an azabicyclo[2.2.1]heptanone moiety is achieved by an asymmetric dearomatization reaction of indoles with cyclobutanones. A new chiral imidodiphosphorimidate (IDPi) catalyst is synthesized and exhibits extraordinary activity in promoting a cascade Friedel-Crafts/semipinacol rearrangement. Target molecules are prepared in good yields (up to 95%) with excellent enantioselectivity (up to 98% ee) with operational convenience. Combined experimental and computational studies provide detailed mechanistic insights into the energy landscape and origin of the stereochemical induction of the reaction.

Efficient Energy and Electron Transfer Photocatalysis with a Coulombic Dyad

Photocatalysis holds great promise for changing the way value-added molecules are currently prepared. However, many photocatalytic reactions suffer from quantum yields well below 10%, hampering the transition from lab-scale reactions to large-scale or even industrial applications. Molecular dyads can be designed such that the beneficial properties of inorganic and organic chromophores are combined, resulting in milder reaction conditions and improved reaction quantum yields of photocatalytic reactions. We have developed a novel approach for obtaining the advantages of molecular dyads without the time- and resource-consuming synthesis of these tailored photocatalysts. Simply by mixing a cationic ruthenium complex with an anionic pyrene derivative in water a salt bichromophore is produced owing to electrostatic interactions. The long-lived organic triplet state is obtained by static and quantitative energy transfer from the preorganized ruthenium complex. We exploited this so-called Coulombic dyad for energy transfer catalysis with similar reactivity and even higher photostability compared to a molecular dyad and reference photosensitizers in several photooxygenations. In addition, it was shown that this system can also be used to maximize the quantum yield of photoredox reactions. This is due to an intrinsically higher cage escape quantum yield after photoinduced electron transfer for purely organic compounds compared to heavy atom-containing molecules. The combination of laboratory-scale as well as mechanistic irradiation experiments with detailed spectroscopic investigations provided deep mechanistic insights into this easy-to-use photocatalyst class.

Challenges and Future Perspectives in Photocatalysis: Conclusions from an Interdisciplinary Workshop

Photocatalysis is a versatile and rapidly developing field with applications spanning artificial photosynthesis, photo-biocatalysis, photoredox catalysis in solution or supramolecular structures, utilization of abundant metals and organocatalysts, sustainable synthesis, and plastic degradation. In this Perspective, we summarize conclusions from an interdisciplinary workshop of young principal investigators held at the Lorentz Center in Leiden in March 2023. We explore how diverse fields within photocatalysis can benefit from one another. We delve into the intricate interplay between these subdisciplines, by highlighting the unique challenges and opportunities presented by each field and how a multidisciplinary approach can drive innovation and lead to sustainable solutions for the future. Advanced collaboration and knowledge exchange across these domains can further enhance the potential of photocatalysis. Artificial photosynthesis has become a promising technology for solar fuel generation, for instance, via water splitting or CO2 reduction, while photocatalysis has revolutionized the way we think about assembling molecular building blocks. Merging such powerful disciplines may give rise to efficient and sustainable protocols across different technologies. While photocatalysis has matured and can be applied in industrial processes, a deeper understanding of complex mechanisms is of great importance to improve reaction quantum yields and to sustain continuous development. Photocatalysis is in the perfect position to play an important role in the synthesis, deconstruction, and reuse of molecules and materials impacting a sustainable future. To exploit the full potential of photocatalysis, a fundamental understanding of underlying processes within different subfields is necessary to close the cycle of use and reuse most efficiently. Following the initial interactions at the Lorentz Center Workshop in 2023, we aim to stimulate discussions and interdisciplinary approaches to tackle these challenges in diverse future teams.

Unlocking Photocatalytic Activity of Acridinium Salts by Anion-Binding Co-Catalysis

The in situ generation of active photoredox organic catalysts upon anion-binding co-catalysis by making use of the ionic nature of common photosensitizers is reported. Hence, the merge of anion-binding and photocatalysis permitted the modulation of the photocatalytic activity of simple acridinium halide salts, building an effective anion-binding - photoredox ion pair complex able to promote a variety of visible light driven transformations, such as anti-Markovnikov addition to olefins, Diels-Alder and the desilylative C-C bond forming reactions. Anion-binding studies, together with steady-state and time-resolved spectroscopy analysis, supported the postulated ion pair formation between the thiourea hydrogen-bond donor organocatalyst and the acridinium salt, which proved essential for unlocking the photocatalytic activity of the photosensitizer.

Data-science-guided calibration curve prediction of an MLCT-based ee determination assay for chiral amines

Circular dichroism (CD) based enantiomeric excess (ee) determination assays are optical alternatives to chromatographic ee determination in high-throughput screening (HTS) applications. However, the implementation of these assays requires calibration experiments using enantioenriched materials. We present a data-driven approach that circumvents the need for chiral resolution and calibration experiments for an octahedral Fe(II) complex (1) used for the ee determination of α-chiral primary amines. By computationally parameterizing the imine ligands formed in the assay conditions, a model of the circular dichroism (CD) response of the Fe(II) assembly was developed. Using this model, calibration curves were generated for four analytes and compared to experimentally generated curves. In a single-blind ee determination study, the ee values of unknown samples were determined within 9% mean absolute error, which rivals the error using experimentally generated calibration curves.

Solar overall water-splitting by a spin-hybrid all-organic semiconductor

Direct solar-to-hydrogen conversion from pure water using all-organic heterogeneous catalysts remains elusive. The challenges are twofold: (i) full-band low-frequent photons in the solar spectrum cannot be harnessed into a unified S1 excited state for water-splitting based on the common Kasha-allowed S0 → S1 excitation; (ii) the H+ → H2 evolution suffers the high overpotential on pristine organic surfaces. Here, we report an organic molecular crystal nanobelt through the self-assembly of spin-one open-shell perylene diimide diradical anions (:PDI2-) and their tautomeric spin-zero closed-shell quinoid isomers (PDI2-). The self-assembled :PDI2-/PDI2- crystal nanobelt alters the spin-dependent excitation evolution, leading to spin-allowed S0S1 → 1(TT) → T1 + T1 singlet fission under visible-light (420 nm~700 nm) and a spin-forbidden S0 → T1 transition under near-infrared (700 nm~1100 nm) within spin-hybrid chromophores. With a triplet-triplet annihilation upconversion, a newly formed S1 excited state on the diradical-quinoid hybrid induces the H+ reduction through a favorable hydrophilic diradical-mediated electron transfer, which enables simultaneous H2 and O2 production from pure water with an average apparent quantum yield over 1.5% under the visible to near-infrared solar spectrum.

Chiral Au-Pd Alloy Nanorods with Tunable Optical Chirality and Catalytically Active Surfaces

Integrating the plasmonic chirality with excellent catalytic activities in plasmonic hybrid nanostructures provides a promising strategy to realize the chiral nanocatalysis toward many chemical reactions. However, the controllable synthesis of catalytically active chiral plasmonic nanoparticles with tailored geometries and compositions remains a significant challenge. Here it is demonstrated that chiral Au-Pd alloy nanorods with tunable optical chirality and catalytically active surfaces can be achieved by a seed-mediated coreduction growth method. Through manipulating the chiral inducers, Au nanorods selectively transform into two different intrinsically chiral Au-Pd alloy nanorods with distinct geometric chirality and tunable optical chirality. By further adjusting several key synthetic parameters, the optical chirality, composition, and geometry of the chiral Au-Pd nanorods are fine-tailored. More importantly, the chiral Au-Pd alloy nanorods exhibit appealing chiral catalytic activities as well as polarization-dependent plasmon-enhanced nanozyme catalytic activity, which has great potential for chiral nanocatalysis and plasmon-induced chiral photochemistry.

Recent advances in catalytic asymmetric synthesis

Asymmetric catalysis stands at the forefront of modern chemistry, serving as a cornerstone for the efficient creation of enantiopure chiral molecules characterized by their high selectivity. In this review, we delve into the realm of asymmetric catalytic reactions, which spans various methodologies, each contributing to the broader landscape of the enantioselective synthesis of chiral molecules. Transition metals play a central role as catalysts for a wide range of transformations with chiral ligands such as phosphines, N-heterocyclic carbenes (NHCs), etc., facilitating the formation of chiral C-C and C-X bonds, enabling precise control over stereochemistry. Enantioselective photocatalytic reactions leverage the power of light as a driving force for the synthesis of chiral molecules. Asymmetric electrocatalysis has emerged as a sustainable approach, being both atom-efficient and environmentally friendly, while offering a versatile toolkit for enantioselective reductions and oxidations. Biocatalysis relies on nature's most efficient catalysts, i.e., enzymes, to provide exquisite selectivity, as well as a high tolerance for diverse functional groups under mild conditions. Thus, enzymatic optical resolution, kinetic resolution and dynamic kinetic resolution have revolutionized the production of enantiopure compounds. Enantioselective organocatalysis uses metal-free organocatalysts, consisting of modular chiral phosphorus, sulfur and nitrogen components, facilitating remarkably efficient and diverse enantioselective transformations. Additionally, unlocking traditionally unreactive C-H bonds through selective functionalization has expanded the arsenal of catalytic asymmetric synthesis, enabling the efficient and atom-economical construction of enantiopure chiral molecules. Incorporating flow chemistry into asymmetric catalysis has been transformative, as continuous flow systems provide precise control over reaction conditions, enhancing the efficiency and facilitating optimization. Researchers are increasingly adopting hybrid approaches that combine multiple strategies synergistically to tackle complex synthetic challenges. This convergence holds great promise, propelling the field of asymmetric catalysis forward and facilitating the efficient construction of complex molecules in enantiopure form. As these methodologies evolve and complement one another, they push the boundaries of what can be accomplished in catalytic asymmetric synthesis, leading to the discovery of novel, highly selective transformations which may lead to groundbreaking applications across various industries.

Palladium-Catalyzed Divergent Enantioselective Functionalization of Cyclobutenes

Aliphatic strained rings have been increasingly applied in medicinal chemistry due to their beneficial physicochemical and pharmacokinetic properties. However, the divergent synthesis of enantioenriched cyclobutane derivatives with various structural patterns continues to be a significant challenge. Here, we disclose a palladium-catalyzed enantioselective desymmetrization of cyclobutenes, resulting in a series of hydroarylation and 1,2- and 1,3-diarylation products via the interceptions of a common Heck intermediate. Mechanistic investigations provide valuable insights into understanding the catalytic mode of the palladium catalysts and the observed variations in the deuterium-responsive behavior during reactions. Furthermore, the synthetic utility is demonstrated in the syntheses of deuterated drug candidate belaperidone skeletons and pseudosymmetrical truxinic acid-type derivatives.

Investigating the diastereoselective synthesis of a macrocycle under Curtin-Hammett control

This work sheds new light on the stereoselective synthesis of chiral macrocycles containing twisted aromatic units, valuable π-conjugated materials for recognition, sensing, and optoelectronics. For the first time, we use the Curtin-Hammett principle to investigate a chiral macrocyclisation reaction, revealing the potential for supramolecular π-π interactions to direct the outcome of a dynamic kinetic resolution, favouring the opposite macrocyclic product to that expected under reversible, thermodynamically controlled conditions. Specifically, a dynamic, racemic perylene diimide dye (1 : 1 P : M) is strapped with an enantiopure (S)-1,1'-bi-2-naphthol group (P-BINOL) to form two diastereomeric macrocyclic products, the homochiral macrocycle (PP) and the heterochiral species (PM). We find there is notable selectivity for the PM macrocycle (dr = 4 : 1), which is rationalised by kinetic templation from intramolecular aromatic non-covalent interactions between the P-BINOL π-donor and the M-PDI π-acceptor during the macrocyclisation reaction.

Enantioselective and Band-Gap Modulation in Photocatalysis of Metal-Free Chiral Carbon Dots

Photodriven chiral catalysis is the combination of photocatalysis and chiral catalysis and is considered one of the cleanest and most efficient methods for the synthesis of chiral compounds or drugs. Furthermore, due to the potential metal contamination associated with most metal-based catalysts, metal-free chiral photocatalysts are ideal candidates. In this work, we demonstrate that metal-free chiral carbon dots (CDs) exhibit size-dependent enantioselective photocatalytic activity. Using serine as the raw material, chiral CDs with well-defined structures and average sizes of 2.22, 3.01, 3.70, 4.77, and 7.21 nm were synthesized using the electrochemical method. These chiral CDs possess size-dependent band gaps and exhibit photoresponsive enantioselective catalytic activity toward the oxidation of dihydroxyphenylalanine (DOPA). Under light-assisted conditions, chiral CDs (L72, 500 μg/mL) exhibit high selectivity (selectivity factor: 2.07) and maintain a certain level of catalytic activity (1.34 μM/min) even at a low temperature of 5 °C. The high catalytic activity of the chiral CDs arises from their photoelectrons reducing O2 to generate O2-, as the active oxygen species for DOPA oxidation. The high enantioselectivity of the chiral CDs is attributed to their differential adsorption capabilities toward DOPA enantiomers. This study provides a new approach for designing metal-free chiral photocatalysts with high enantioselectivity.

Valence-isomer selective cycloaddition reaction of cycloheptatrienes-norcaradienes

The rapid and precise creation of complex molecules while controlling multiple selectivities is the principal objective in synthetic chemistry. Combining data science and organic synthesis to achieve this goal is an emerging trend, but few examples of successful reaction designs are reported. We develop an artificial neural network regression model using bond orbital data to predict chemical reactivities. Actual experimental verification confirms cycloheptatriene-selective [6 + 2]-cycloaddition utilizing nitroso compounds and norcaradiene-selective [4 + 2]-cycloaddition reactions employing benzynes. Additionally, a one-pot asymmetric synthesis is achieved by telescoping the enantioselective dearomatization of non-activated benzenes and cycloadditions. Computational studies provide a rational explanation for the seemingly anomalous occurrence of thermally prohibited suprafacial [6 + 2]-cycloaddition without photoirradiation.

Asymmetric Ion-Pairing Photoredox Catalysis for Stereoselective Cationic Polymerization under Light Control

By virtue of noninvasive regulations by light, photocontrolled polymerizations have attracted considerable attention for the precision synthesis of macromolecules. However, a cationic polymerization with simultaneous photocontrol and tacticity-regulation remains elusive so far. Herein, we introduce an asymmetric ion-pairing photoredox catalysis strategy that allows for the development of a stereoselective cationic polymerization with concurrent light regulation for the first time. By employing an ion pair catalyst (PC+/*A-) consisting of a photoredox active cation (PC+) and a sterically confined chiral anion (*A-) to deliver the stereochemical control, the cationic polymerization of vinyl ethers can be achieved with photocontrol and high isotactic selectivity (up to 91% m) at a remarkable low catalyst loading (50 ppm).

Unveiling Mechanistic Insights and Photocatalytic Advancements in Intramolecular Photo-(3 + 2)-Cycloaddition: A Comparative Assessment of Two Paradigmatic Single-Electron-Transfer Models

The synthesis of 1-aminonorbornane (1-aminoNB), a potential aniline bioisostere, through photochemistry or photoredox catalysis signifies a remarkable breakthrough with implications in organic chemistry, pharmaceutical chemistry, and sustainable chemistry. However, an understanding of the underlying mechanisms involved in these reactions remains limited and ambiguous. Herein, we employ high-precision CASPT2//CASSCF calculations to elucidate the intricate mechanisms regulating the intramolecular photo-(3 + 2)-cycloaddition reactions for the synthesis of 1-aminoNB in the presence or absence of the Ir-complex-based photocatalyst. Our investigations delve into radical cascades, stereoselectivity, particularly single-electron-transfer (SET) events, etc. Furthermore, we innovatively introduce and compare two SET models integrating Marcus electron-transfer theory and transition-state theory. These models combined with kinetic data contribute to recognizing the critical control factors in diverse photocatalysis, thereby guiding the design and manipulation of photoredox catalysis as well as the improvement and modification of photocatalysts.

Red-light operable photosensitizer with symmetry-breaking charge transfer induced intersystem crossing for polymerization of methyl methacrylate

We present a red light-activated zincII bis(dipyrrin) symmetry breaking charge transfer (SBCT) architecture, showing a large molar absorption coefficient (ε = 15.4 × 104 M-1 cm-1), high reactive singlet oxygen generation efficiency (ΦΔ ≈ 0.8) and long-lived triplet state (τT = 150 μs) compared to the donor-acceptor analogue dipyrrin-BF2 complex, highlighting the superiority of the SBCT approach. For the first time, we demonstrated the potential of a SBCT scaffold in red-light-induced methyl methacrylate (MMA) polymerization, using a dual photocatalyst excitation approach.

Asymmetric Transfer Hydrogenation of Cyclobutenediones

Four-membered carbocycles are fundamental substructures in bioactive molecules and approved drugs and serve as irreplaceable building blocks in organic synthesis. However, developing efficient protocols furnishing diversified four-membered ring compounds in a highly regio-, diastereo-, and enantioselective fashion remains challenging but very desirable. Here, we report the unprecedented asymmetric transfer hydrogenation of cyclobutenediones. The reaction can selectively afford three types of four-membered products in high yields with high stereoselectivities, and the highly functionalized products enable a series of further transformations to form more diversified four-membered compounds. Asymmetric synthesis of di-, tri-, and tetrasubstituted bioactive molecules has also been achieved. Systematic mechanistic studies and theoretical calculations have revealed the origin of the regioselectivity, the key hydrogenation transition state models, and the sequence of the double and triple hydrogenation processes. The work provides a new choice for the catalytic asymmetric synthesis of cyclobutanes and related structures and demonstrates the robustness of asymmetric transfer hydrogenation in the accurate selectivity control of highly functionalized substrates.

Ag3PO4 enables the generation of long-lived radical cations for visible light-driven [2 + 2] and [4 + 2] pericyclic reactions

Photocatalytic redox reactions are important for synthesizing fine chemicals from olefins, but the limited lifetime of radical cation intermediates severely restricts semiconductor photocatalysis efficiency. Here, we report that Ag3PO4 can efficiently catalyze intramolecular and intermolecular [2 + 2] and Diels-Alder cycloadditions under visible-light irradiation. The approach is additive-free, catalyst-recyclable. Mechanistic studies indicate that visible-light irradiation on Ag3PO4 generates holes with high oxidation power, which oxidize aromatic alkene adsorbates into radical cations. In photoreduced Ag3PO4, the conduction band electron (eCB-) has low reduction power due to the delocalization among the Ag+-lattices, while the particle surfaces have a strong electrostatic interaction with the radical cations, which considerably stabilize the radical cations against recombination with eCB-. The radical cation on the particle's surfaces has a lifetime of more than 2 ms, 75 times longer than homogeneous systems. Our findings highlight the effectiveness of inorganic semiconductors for challenging radical cation-mediated synthesis driven by sunlight.

Asymmetric Photoaerobic Lactonization and Aza-Wacker Cyclization of Alkenes Enabled by Ternary Selenium-Sulfur Multicatalysis

An adaptable, sulfur-accelerated photoaerobic selenium-π-acid ternary catalyst system for the enantioselective allylic redox functionalization of simple, nondirecting alkenes is reported. In contrast to related photoredox catalytic methods, which largely depend on olefinic substrates with heteroatomic directing groups to unfold high degrees of stereoinduction, the current protocol relies on chiral, spirocyclic selenium-π-acids that covalently bind to the alkene moiety. The performance of this ternary catalytic method is demonstrated in the asymmetric, photoaerobic lactonization and cycloamination of enoic acids and unsaturated sulfonamides, respectively, leading to an averaged enantiomeric ratio (er) of 92:8. Notably, this protocol provides for the first time an asymmetric, catalytic entryway to pharmaceutically relevant 3-pyrroline motifs, which was used as a platform to access a 3,4-dihydroxyproline derivative in only seven steps with a 92:8 er.

Photo-Catalyzed α-Arylation of Enol Acetate Using Recyclable Silica-Supported Heteroleptic and Homoleptic Copper(I) Photosensitizers

Earth-abundant photosensitizers are highly sought after for light-mediated applications, such as photoredox catalysis, depollution and energy conversion schemes. Homoleptic and heteroleptic copper(I) complexes are promising candidates in this field, as copper is abundant and the corresponding complexes are easily obtained in smooth conditions. However, some heteroleptic copper(I) complexes suffer from low (photo)stability that leads to the gradual formation of the corresponding homoleptic complex. Such degradation pathways are detrimental, especially when recyclability is desired. This study reports a novel approach for the heterogenization of homoleptic and heteroleptic Cu complexes on silica nanoparticles. In both cases, the photophysical properties upon surface immobilization were only slightly affected. Excited-state quenching with aryl diazonium derivatives occurred efficiently (108 -1010  M-1  s-1 ) with heterogeneous and homogeneous photosensitizers. Moderate but almost identical yields were obtained for the α-arylation of enol acetate using the homoleptic complex in homogeneous or heterogeneous conditions. Importantly, the silica-supported photocatalysts were recycled with moderate loss in photoactivity over multiple experiments. Transient absorption spectroscopy confirmed that excited-state electron transfer occurred from the homogeneous and heterogeneous homoleptic copper(I) complexes to aryl diazonium derivatives, generating the corresponding copper(II) center that persisted for several hundreds of microseconds, compatible with photoredox catalysis applications.

Metal Modulation: An Effortless Tactic for Refining Photoredox Catalysis in Living Cells

The remarkable impact of photoredox catalytic chemistries has sparked a wave of innovation, opening doors to novel biotechnologies in the realm of catalytic antitumor therapy. Yet, the quest for novel photoredox catalysts (PCs) suitable for living systems, or the enhancement of catalytic efficacy in existing biocompatible PC systems, persists as a formidable challenge. Within this context, we introduce a readily applicable metal modulation strategy that significantly augments photoredox catalysis within living cells, exemplified by a set of metalloporphyrin complexes termed M-TCPPs (M = Zn, Mn, Ni, Co, Cu). Among these complexes, Zn-TCPP emerges as an exceptional catalyst, displaying remarkable photocatalytic activity in the oxidation of nicotinamide adenine dinucleotide (NADH), nicotinamide adenine dinucleotide phosphate (NADPH), and specific amino acids. Notably, comprehensive investigations reveal that Zn-TCPP's superior catalytic prowess primarily arises from the establishment of an efficient oxidative cycle for PC, in contrast to previously reported PCs engaged in reductive cycles. Moreover, theoretical calculations illuminate that amplified intersystem crossing rates and geometry alterations in Zn-TCPP contribute to its heightened photocatalytic performance. In vitro studies demonstrated that Zn-TCPP exhibits therapeutic potential and is found to be effective for photocatalytic antitumor therapy in both glioblastoma G98T cells and 3D multicellular spheroids. This study underscores the transformative role of "metal modulation" in advancing high-performance PCs for catalytic antitumor therapy, marking a significant stride toward the realization of this innovative therapeutic approach.

syn-Selective Difunctionalization of Bicyclobutanes Enabled by Photoredox-Mediated C-S σ-Bond Scission

Given the importance of cyclic frameworks in molecular scaffolds and drug discovery, it is intriguing to precisely forge and manipulate ring systems in synthetic chemistry. In this field, the intermolecular synthesis of densely substituted cyclobutanes with precise diastereocontrol under simple reaction conditions remains a challenge. Herein, a photoredox strategy for the difunctionalization of bicyclo[1.1.0]butanes (BCBs) under high regio- and syn-selectivity is disclosed. C-S σ-bond cleavage of partially unsaturated sulfur-containing bifunctional reagents in an overall strain-release-driven process enables the thio-alkynylation, -alkenylation, and -allylation of BCBs under mild conditions and demonstrates the generality of this protocol. Mechanistic studies suggest that the intermediacy of cyclic distonic radical cations might be key for the efficient scission of C-S σ-bonds and the origin of diastereoselectivity.

Efficient Intersystem Crossing and Long-lived Charge-Separated State Induced by Through-Space Intramolecular Charge Transfer in a Parallel Geometry Carbazole-Bodipy Dyad

The design of efficient heavy atom-free triplet photosensitizers (PSs) based on through bond charge transfer (TBCT) features is a formidable challenge due to the criteria of orthogonal donor-acceptor geometry. Herein, we propose using parallel (face-to-face) conformation carbazole-bodipy donor-acceptor dyads (BCZ-1 and BCZ-2) featuring through space intramolecular charge transfer (TSCT) process as efficient triplet PS. Efficient intersystem crossing (ΦΔ =61 %) and long-lived triplet excited state (τT =186 μs) were observed in the TSCT dyad BCZ-1 compared to BCZ-3 (ΦΔ =0.4 %), the dyad involving TBCT, demonstrating the superiority of the TSCT approach over conventional donor-acceptor system. Moreover, the transient absorption study revealed that TSCT dyads have a faster charge separation and slower intersystem crossing process induced by charge recombination compared to TBCT dyad. A long-lived charge-separated state (CSS) was observed in the BCZ-1 (τCSS =24 ns). For the first time, the TSCT dyad was explored for the triplet-triplet annihilation upconversion, and a high upconversion quantum yield of 11 % was observed. Our results demonstrate a new avenue for designing efficient PSs and open up exciting opportunities for future research in this field.

Highly Enantioselective Radical Cation [2 + 2] and [4 + 2] Cycloadditions by Chiral Iron(III) Photoredox Catalysis

Radical cations show a unique reactivity that is fundamentally different from that of conventional cations and have thus attracted considerable attention as alternative cationic intermediates for novel types of organic reactions. However, asymmetric catalysis to promote enantioselective radical cation reactions remains a major challenge in contemporary organic synthesis. Here, we report that the judicious design of an ion pair consisting of a radical cation and a chiral counteranion induces an excellent level of enantioselectivity. This strategy was applied to enantio-, diastereo-, and regioselective [2 + 2] cycloadditions, as well as enantio-, diastereo-, and regioselective [4 + 2] cycloadditions, by using chiral iron(III) photoredox catalysis. We anticipate that this strategy has the potential to expand the use of several mature chiral anions to develop numerous unprecedented enantioselective radical cation reactions.