Direct Allylic C(sp3)–H Thiolation with Disulfides via Visible Light Photoredox Catalysis

Stabilized Carbon-Centered Radical-Mediated Carbosulfenylation of Styrenes: Modular Synthesis of Sulfur-Containing Glycine and Peptide Derivatives

Sulfur-containing amino acids and peptides play critical roles in organisms. Thiol-ene reactions between the thiol residues of L-cysteine and the alkenyl fragments in the designed coupling partners serve as primary tools for constructing C─S bonds in the synthesis of unnatural sulfur-containing amino acid derivatives. These reactions are favored due to the preference for hydrogen transfer from thiol to β-sulfanyl carbon radical intermediates. In this paper, the study proposes utilizing carbon-centered radicals stabilized by the capto-dative effect, generated under photocatalytic conditions from N-aryl glycine derivatives. The aim is to compete with the thiol hydrogen, enabling radical C─C bond formation with β-sulfanyl carbon radicals. This protocol is robust in the presence of air and water, offers significant potential as a modular and efficient platform for synthesizing sulfur-containing amino acids and modifying peptides, particularly with abundant disulfides and styrenes.

Electrochemical Allylic C(sp3)-H Isothiocyanation via [3,3]-Sigmatropic Rearrangement

The direct allylic C(sp3)-H functionalization provides a straightforward protocol for the synthesis of valuable molecules. We report herein the first chemo- and site-selective method for allylic C(sp3)-H isothiocyanation of various internal alkenes under mild electrochemical conditions. This method exhibits broad functional group tolerance and excellent selectivity and can be applied for late-stage isothiocyanation of bioactive molecules. Combined experimental and computational studies indicate that the reaction proceeds via an unexpected [3,3]-sigmatropic rearrangement.

Manganese-promoted reductive cross-coupling of disulfides with dialkyl carbonates

Manganese is a cheap and environmentally friendly metal on Earth. Herein, we report a manganese-promoted reductive cross-coupling using easily available and odorless disulfides as thiolating agents in an excellent 100% sulfur atom economy. The protocol featured a broad substrate scope, including various alkyl disulfides and excellent functional group compatibility, constructing diverse thioethers under simple conditions. Ultimately, thioethers can be prepared in gram-scale reactions and further transformed into structurally complex molecules.

Photo-induced 1,2-thiohydroxylation of maleimide involving disulfide and singlet oxygen

A visible light-driven di-functionalization of maleimide with disulfide and in situ-generated singlet oxygen offers selective 1,2-thiohydroxylation under additive-free conditions. Here the disulfide plays the dual role of photosensitizer and the coupling reagent. Notably, the hydroxyl functionality originates from the in situ generated singlet oxygen followed by HAT from H2O (moisture).

Transition-metal-catalyzed synthesis of quinazolines: A review

Quinazolines are a class of nitrogen-containing heterocyclic compounds with broad-spectrum of pharmacological activities. Transition-metal-catalyzed reactions have emerged as reliable and indispensable tools for the synthesis of pharmaceuticals. These reactions provide new entries into pharmaceutical ingredients of continuously increasing complexity, and catalysis with these metals has streamlined the synthesis of several marketed drugs. The last few decades have witnessed a tremendous outburst of transition-metal-catalyzed reactions for the construction of quinazoline scaffolds. In this review, the progress achieved in the synthesis of quinazolines under transition metal-catalyzed conditions are summarized and reports from 2010 to date are covered. This is presented along with the mechanistic insights of each representative methodology. The advantages, limitations, and future perspectives of synthesis of quinazolines through such reactions are also discussed.

Lighting Up the Organochalcogen Synthesis: A Concise Update of Recent Photocatalyzed Approaches

This review describes the recent advances in photocatalyzed reactions to form new carbon–sulfur and carbon–selenium bonds. With a total of 136 references, of which 81 articles are presented, the authors introduce in five sections an updated picture of the state of the art in the light-promoted synthesis of organochalcogen compounds (from 2019 to present). The light-promoted synthesis of sulfides by direct sulfenylation of C–C π-bonds; synthesis of sulfones; the activation of Csp2–N bond in the formation of Csp2–S bonds; synthesis of thiol ester, thioether and thioacetal; and the synthesis of organoselenium compounds are discussed, with detailed reaction conditions and selected examples for each protocol.

Synthesis and Application Dichalcogenides as Radical Reagents with Photochemical Technology

Dichalcogenides (disulfides and diselenides), as reactants for organic transformations, are important and widely used because of their potential to react with nucleophiles, electrophilic reagents, and radical precursors. In recent years, in combination with photochemical technology, the application of dichalcogenides as stable radical reagents has opened up a new route to the synthesis of various sulfur- and selenium-containing compounds. In this paper, synthetic strategies for disulfides and diselenides and their applications with photochemical technology are reviewed: (i) Cyclization of dichalcogenides with alkenes and alkynes; (ii) direct selenylation/sulfuration of C-H/C-C/C-N bonds; (iii) visible-light-enabled seleno- and sulfur-bifunctionalization of alkenes/alkynes; and (iv) Direct construction of the C(sp)-S bond. In addition, the scopes, limitations, and mechanisms of some reactions are also described.

Photocatalytic Late-Stage C-H Functionalization

The emergence of modern photocatalysis, characterized by mildness and selectivity, has significantly spurred innovative late-stage C-H functionalization approaches that make use of low energy photons as a controllable energy source. Compared to traditional late-stage functionalization strategies, photocatalysis paves the way toward complementary and/or previously unattainable regio- and chemoselectivities. Merging the compelling benefits of photocatalysis with the late-stage functionalization workflow offers a potentially unmatched arsenal to tackle drug development campaigns and beyond. This Review highlights the photocatalytic late-stage C-H functionalization strategies of small-molecule drugs, agrochemicals, and natural products, classified according to the targeted C-H bond and the newly formed one. Emphasis is devoted to identifying, describing, and comparing the main mechanistic scenarios. The Review draws a critical comparison between established ionic chemistry and photocatalyzed radical-based manifolds. The Review aims to establish the current state-of-the-art and illustrate the key unsolved challenges to be addressed in the future. The authors aim to introduce the general readership to the main approaches toward photocatalytic late-stage C-H functionalization, and specialist practitioners to the critical evaluation of the current methodologies, potential for improvement, and future uncharted directions.

Redox-Neutral 1,4-Dicarbonfunctionalization of 1,3-Butadiene by Merging Photoredox and Nickel Catalysis

The diverse functionalization of 1,3-butadiene provides wide applicability toward the synthesis of abundant and useful allylic compounds. Here, we describe a three-component and redox-neutral assembly of readily available C═X compounds, 1,3-butadiene, and various nucleophiles by merging photoredox and nickel catalysis, enabling the rapid synthesis of structurally diverse homoallyl amines and homoallylic alcohols.

Aliphatic C-H Functionalization Using Pyridine N-Oxides as H-Atom Abstraction Agents

The alkylation and heteroarylation of unactivated tertiary, secondary, and primary C(sp3)-H bonds was achieved by employing an acridinium photoredox catalyst along with readily available pyridine Noxides as hydrogen atom transfer (HAT) precursors under visible light. Oxygen-centered radicals, generated by single-electron oxidation of the Noxides, are the proposed key intermediates whose reactivity can be easily modified by structural adjustments. A broad range of aliphatic C-H substrates with electron-donating or -withdrawing groups as well as various olefinic radical acceptors and heteroarenes were well tolerated.

Sn(OTf)2-Catalyzed Allylic Substitution of Thiols to Allyl Alcohols: Access to Allyl Sulfides

A novel method for the mild and efficient synthesis of allyl sulfides has been developed with allyl alcohols and thiols as substrates. The desired allyl sulfide was obtained using a catalytic amount of Sn(OTf)2 in dichloromethane at room temperature after a reaction time of 12 hours. A diverse range of allyl sulfides have been obtained with good to excellent yields, including both linear and cyclic derivatives (27 products). Additionally, gram-scale reactions can be easily carried out with only 1 mol% catalyst, giving over 90% yields, which further proves the efficiency of our approach in synthesis. This methodology has both deep research significance and application value, providing a new pathway to access sulfide compounds. We strongly believe our method would be attractive to synthetic chemists and would be widely used in synthetic chemistry.

A Water-Soluble Organic Photocatalyst Discovered for Highly Efficient Additive-Free Visible-Light-Driven Grafting of Polymers from Proteins at Ambient and Aqueous Environments

Since the pioneering discovery of a protein bound to poly(ethylene glycol), the utility of protein-polymer conjugates (PPCs) is rapidly expanding to currently emerging applications. Photoinduced energy/electron-transfer reversible addition-fragmentation chain-transfer (PET-RAFT) polymerization is a very promising method to prepare structurally well-defined PPCs, as it eliminates high-cost and time-consuming deoxygenation processes due to its oxygen tolerance. However, the oxygen-tolerance behavior of PET-RAFT polymerization is not well-investigated in aqueous environments, and thereby the preparation of PPCs using PET-RAFT polymerization needs a substantial amount of sacrificial reducing agents or inert-gas purging processes. Herein a novel water-soluble and biocompatible organic photocatalyst (PC) is reported, which enables visible-light-driven additive-free "grafting-from" polymerizations of a protein in ambient and aqueous environments. Interestingly, the developed PC shows unconventional "oxygen-acceleration" behavior for a variety of acrylic and acrylamide monomers in aqueous conditions without any additives, which are apparently distinct from previously reported systems. With such a PC, "grafting-from" polymerizations are successfully performed from protein in ambient buffer conditions under green light-emitting diode (LED) irradiation, which result in various PPCs that have neutral, anionic, cationic, and zwitterionic polyacrylates, and polyacrylamides. It is believed that this PC will be widely employed for a variety of photocatalysis processes in aqueous environments, including the living cell system.

Photoredox-Catalyzed C-H Functionalization Reactions

The fields of C-H functionalization and photoredox catalysis have garnered enormous interest and utility in the past several decades. Many different scientific disciplines have relied on C-H functionalization and photoredox strategies including natural product synthesis, drug discovery, radiolabeling, bioconjugation, materials, and fine chemical synthesis. In this Review, we highlight the use of photoredox catalysis in C-H functionalization reactions. We separate the review into inorganic/organometallic photoredox catalysts and organic-based photoredox catalytic systems. Further subdivision by reaction class─either sp2 or sp3 C-H functionalization─lends perspective and tactical strategies for use of these methods in synthetic applications.

Photochemical Disulfide-Ene Modification Enhances Protein Sequencing and Disulfide Mapping by Mass Spectrometry

A new photochemical disulfide-ene reaction system capable of alkylating protein disulfide bonds in seconds has been established. The system is simple, containing acetone and isopropanol for disulfide reduction under 254 nm UV irradiation and norbornene as a highly efficient alkylation reagent. Enhanced characterization of disulfide-rich proteins with significantly shortened analysis time is demonstrated by coupling the reaction online with mass spectrometry.

Light-Promoted and Tertiary-Amine-Assisted Hydroxysulfenylation of Alkenes: Selective and Direct One-Pot Synthesis of β-Hydroxysulfides

A light-promoted and tertiary-amine-assisted strategy for efficient hydroxysulfenylation of both electron-rich and electron-deficient alkenes with thiophenols to selectively and directly access β-hydroxysulfides in one pot is described herein. In contrast to the previously reported thiol-oxygen co-oxidation reactions, this simple and sustainable approach features mild reaction conditions, high efficiency, and excellent functional group tolerance.

Iron-Catalyzed Thiolation and Selenylation of Cycloalkyl Hydroperoxides via C-C Bond Cleavage

A cheap iron-catalyzed C-C bond cleavage/thiolation and selenylation of cycloalkyl hydroperoxides are presented. This redox-neutral protocol provides efficient access to diverse distal keto-functionalized thioethers and selenium compounds. Remarkably, only some amounts of disulfides are required for this transformation.

Direct Allylic C(sp3 )-H and Vinylic C(sp2 )-H Thiolation with Hydrogen Evolution by Quantum Dots and Visible Light

Direct allylic C-H thiolation is straightforward for allylic C(sp³ )-S bond formation. However, strong interactions between thiol and transition metal catalysts lead to deactivation of the catalytic cycle or oxidation of sulfur atom under oxidative condition. Thus, direct allylic C(sp³ )-H thiolation has proved difficult. Represented herein is an exceptional for direct, efficient, atom- and step-economic thiolation of allylic C(sp³ )-H and thiol S-H under visible light irradiation. Radical trapping experiments and electron paramagnetic resonance (EPR) spectroscopy identified the allylic radical and thiyl radical generated on the surface of photocatalyst quantum dots (QDs). The C-S bond formation does not require external oxidants and radical initiators, and hydrogen (H₂ ) is produced as byproduct. When vinylic C(sp² )-H was used instead of allylic C(sp³ )-H bond, the radical-radical cross-coupling of C(sp² )-H and S-H was achieved with liberation of H₂ . Such a unique transformation opens up a door toward direct C-H and S-H coupling for valuable organosulfur chemistry.

Visible-light photocatalytic preparation of alkenyl thioethers from 1,2,3-thiadiazoles and Hantzsch esters: synthetic and mechanistic investigations

A visible-light-promoted S-alkylation of 1,2,3-thiadiazoles with C-radical precursors, 4-alkyl-1,4-dihydropyridines (DHPs), to produce alkenyl thioethers is disclosed.

Superiority of Iridium Photocatalyst and Role of Quinuclidine in Selective α-C(sp3)-H Alkylation: Theoretical Insights

Recent experimental work reported that visible-light photoredox catalysis coupled with primary sulfonamides and electron-deficient alkenes could efficiently construct C-C bonds at the α-position of primary amine derivatives under mild conditions. Here, a systematic study was conducted to explore the non-negligible excited-state single-electron-transfer (SET) processes and the catalytic cycle. Hydrogen atom transfer (HAT) catalysis containing different site-selective functionalization, involved as a critical process during the reaction, was computationally characterized. The superiorities of iridium-based photoredox catalysts in terms of photoabsorption properties, phosphorescence rates, and electron-transfer rates for SET processes were focused on. In addition, the function of quinuclidine in the entire photocatalytic reaction was also probed. These intrinsic properties and detailed insights into the mechanism are supposed to be helpful to the understanding of the C-C bond functionalization reaction and the future application of the iridium-based photoredox catalyst.

Recent advances in the transition metal catalyzed synthesis of quinoxalines: a review

This review summarizes the recent developments in the synthesis of a variety of substituted quinoxalines using transition metal catalysts.

K2S2O8 promoted dehydrative cross-coupling between α,α-disubstituted allylic alcohols and thiophenols/thiols

K₂S₂O₈ promoted dehydrative cross-coupling between α,α-disubstituted allylic alcohols and thiophenols/thiols is demonstrated for the first time, leading to a wide range of allyl sulfides in good to high yields.

A photoexcited halogen-bonded EDA complex of the thiophenolate anion with iodobenzene for C(sp3)–H activation and thiolation

A direct photochemical thiolation of C(sp3)–H bond-containing substrates with thiophenol was developed. A halogen bonding-type EDA complex was found to trigger the downstream single electron transfer and hydrogen atom transfer process.

General Light-Mediated, Highly Diastereoselective Piperidine Epimerization: From Most Accessible to Most Stable Stereoisomer

We report a combined photocatalytic and hydrogen atom transfer (HAT) approach for the light-mediated epimerization of readily accessible piperidines to provide the more stable diastereomer with high selectivity. The generality of the transformation was explored for a large variety of di- to tetrasubstituted piperidines with aryl, alkyl, and carboxylic acid derivatives at multiple different sites. Piperidines without substitution on nitrogen as well as N-alkyl and aryl derivatives were effective epimerization substrates. The observed diastereoselectivities correlate with the calculated relative stabilities of the isomers. Demonstration of reaction reversibility, luminescence quenching, deuterium labeling studies, and quantum yield measurements provide information about the mechanism.

Visible light photocatalysis in the late-stage functionalization of pharmaceutically relevant compounds

The late stage functionalization (LSF) of complex biorelevant compounds is a powerful tool to speed up the identification of structure-activity relationships (SARs) and to optimize ADME profiles. To this end, visible-light photocatalysis offers unique opportunities to achieve smooth and clean functionalization of drugs by unlocking site-specific reactivities under generally mild reaction conditions. This review offers a critical assessment of current literature, pointing out the recent developments in the field while emphasizing the expected future progress and potential applications. Along with paragraphs discussing the visible-light photocatalytic synthetic protocols so far available for LSF of drugs and drug candidates, useful and readily accessible synoptic tables of such transformations, divided by functional groups, will be provided, thus enabling a useful, fast, and easy reference to them.

BF3·OEt2-mediated cyclization of β,γ-unsaturated oximes and hydrazones with N-(arylthio/arylseleno)succinimides: an efficient approach to synthesize isoxazoles or dihydropyrazoles

A highly efficient BF3·OEt2-mediated cyclization of β,γ-unsaturated oximes and tosylhydrazones with N-(arylthio/arylseleno)succinimides has been established for the construction of N-heterocycles in a one-step manner. This metal-free cyclization provides direct access to isoxazoles and dihydropyrazoles in good to excellent yields at room temperature. The mechanistic experiments support the formation of a cationic species PhS+ which plays a critical role in this cyclization process.

Pyridine hydrochloride-catalyzed thiolation of alkenes: divergent synthesis of allyl and vinyl sulfides

Divergent synthesis of allyl and vinyl sulfides by a Py-HCl catalyzed tandem thiolation–elimination reaction between N-thiosuccinimides and alkenes.