Visible-Light Photoredox Catalysis

Photochemical α-selective radical ring-opening reactions of 1,3-disubstituted acyl bicyclobutanes with alkyl halides: modular access to functionalized cyclobutenes

Although ring-opening reactions of bicyclobutanes bearing electron-withdrawing groups, typically with β-selectivity, have evolved as a powerful platform for synthesis of cyclobutanes, their application in the synthesis of cyclobutenes remains underdeveloped. Here, a novel visible light induced α-selective radical ring-opening reaction of 1,3-disubstituted acyl bicyclobutanes with alkyl radical precursors for the synthesis of functionalized cyclobutenes is described. In particular, primary, secondary, and tertiary alkyl halides are all suitable substrates for this photocatalytic transformation, providing ready access to cyclobutenes with a single all-carbon quaternary center, or with two contiguous centers under mild reaction conditions.

Visible light-mediated halogenation of organic compounds

The use of visible light and photoredox catalysis emerged as a powerful and sustainable tool for organic synthesis, showing high value for distinctly different ways of bond creation. Halogenated compounds are the cornerstone of contemporary organic synthesis: it is almost impossible to develop a route towards a pharmaceutical reagent, agrochemical, natural product, etc. without the involvement of halogen-containing intermediates. Moreover, the halogenated derivatives as final products became indispensable for drug discovery and materials science. The idea of this review is to understand and summarise the impact of visible light-promoted chemistry on halogenation and halofunctionalisation reactions.

Exploring Solvent Polarity-Dependent Photocatalysis Mechanism of Organic Photoredox Catalysts

Organic dyads with intramolecular charge-transfer (ICT) character are emerging as viable and more sustainable photocatalysts than metal-based complexes. Herein, a carbazole- and naphthalimide-based organic dyad (Cz-NI) was designed as an efficient organic photocatalyst for the direct C(sp3)-H carbamoylation of saturated aza-heterocycles. Aiming at understanding the effect of environment, especially the solvent polarity on photocatalysis performance, the excited-state dynamics of Cz-NI in different polar solvents were studied by femtosecond (fs) and nanosecond (ns) time-resolved transient absorption (TA) spectroscopy. Fs-TA measurements indicate that the formation of an intramolecular charge separation (ICS) state with twisted structural feature in polar solvents is driven and stabilized by solvation dynamics. Combined with chemical calculations, we found that orbital decoupling, poor conjugation between Cz and NI groups due to intramolecular torsional motion and transition moments associated with ICT emission, limits excited-state deactivation through radiation and nonradiation transition to the ground state. In addition, the orthogonal π-system of the ICS state enables the efficient spin-orbit, charge-transfer intersystem crossing to a triplet state, which is localized on the NI group. Spectroscopic and computational results reveal the formation of an ICS state at an appropriate energy that enables the population of the triplet state with high quantum yield, and the localized triplet state has long lifetime and high reduction potential for subsequent reactions. Therefore, solvent-solute interaction, especially the solvation-coupled excited-state structural relaxation, is the main factor that the photocatalysis efficiency of Cz-NI has a significant polarity correlation.

Sustainable Photo- and Electrochemical Transformation of White Phosphorous (P4 ) into P1 Organo-Compounds

Elemental white phosphorous (P4 ) is a crucial feedstock for the entire phosphorus-derived chemical industry, serving as a common precursor for the ultimate preparation of high-grade monophosphorus (P1 ) fine chemicals. However, the corresponding manufacturing processes generally suffer from a deep reliance on hazardous reagents, inputs of immense energy, emissions of toxic pollutants, and the generation of substantial waste, which have negative impacts on the environment. In this context, sustainability and safety concerns provide a consistent impetus for the urgent overall improvement of phosphorus cycles. In this Concept, we present an overview of the most recent growth in photo- and electrochemical synthesis of P1 organo-compounds from P4 , with special emphasis on sustainable features. The key aspects of innovations regarding activation mode and mechanism have been comprehensively analyzed. A preliminary look at the possible future direction of development is also provided.

Diastereoselective Hydroacylation of Cyclopropenes by Visible-Light Photocatalysis

An efficient and general strategy for the hydroacylation of cyclopropene is disclosed for synthesizing various 2-acylcyclopropane derivatives under mild reaction conditions. High functional group tolerance of this protocol features a novel route to access a divergent synthesis of acylated cyclopropane in a diastereoselective manner by photoinduced decarboxylation of α-ketoacid followed by acyl radical addition to cyclopropene. Additionally, the regioselective addition of acyl radical at the least substituted olefinic carbon center with trans-selective fashion makes this protocol more appealing toward natural product development.

Photo-Induced Carbonylation of Aryl Bromides for the Synthesis of Aryl Esters and Amides Under Transition Metal-Free Conditions

In this work, we developed a photo-induced carbonylation of aryl bromides under transition metal-free conditions. The reaction shows good activity with alcohol and amine nucleophiles. Various esters and amides were formed from aryl halides and alcohols and amines under mild conditions in moderate to good yields.

Photoredox Metal-Free Synthesis of Unnatural β-Silyl-α-Amino Acids via Hydrosilylation

An efficient, practical and metal-free methodology for the synthesis of β-silyl-α-amino acid motifs via photoredox and hydrogen atom transfer (HAT) process is described. This protocol enables the direct hydrosilylation of dehydroalanine derivatives and tolerates a wide array of functional groups and synthetic handles, leading to valuable β-silyl-α-amino acids with moderate to good yields.

Visible Light-Promoted β-Functionalization of Carbonyl Compounds in the Presence of Organic Dyes

Herein, we investigate the use of organic photocatalysts in the visible light-promoted β-functionalization of carbonyl compounds. In particular, we studied the addition of aliphatic aldehydes to α,β-unsaturated compounds (β-Michael addition), and the reaction of cyclic ketones with either ketones (β-aldol condensation) or imines (β-Mannich reaction). Among the dyes tested, donor-acceptor cyanoarenes gave the best results, promoting the transformations of interest in moderate to good yields. The reaction scope was investigated on substrates with different steric and electronic properties. Fluorescence quenching analysis (Stern-Volmer experiments) led us to propose for these reactions a reductive quenching mechanism involving a transient 5πe- activation mode.

Red-light-mediated Barton decarboxylation reaction and one-pot wavelength-selective transformations

In organic chemistry, selecting mild conditions for transformations and saving energy are increasingly important for achieving sustainable development goals. Herein, we describe a red-light-mediated Barton decarboxylation using readily available red-light-emitting diodes as the energy source and zinc tetraphenylporphyrin as the catalyst, avoiding explosive or hazardous reagents or external heating. Mechanistic studies suggest that the reaction probably proceeds via Dexter energy transfer between the activated catalyst and the Barton ester. Furthermore, a one-pot wavelength-selective reaction within the visible light range is developed in combination with a blue-light-mediated photoredox reaction, demonstrating the compatibility of two photochemical transformations based on mechanistic differences. This one-pot process expands the limits of the decarboxylative Giese reaction beyond polarity matching.

Visible-Light-Driven α-Substituted Amines Enabled by In Situ Formation of Amine Substrate Aggregates

A visible-light-driven, photocatalyst-free, air-promoted, α-substituted reaction of amines with varying nucleophiles is described. The amine substrate aggregates formed in situ through physical π-π stacking by H2O regulation in organic solvent can absorb visible light and then generate iminium ion intermediates, which undergo nucleophilic substitution reactions with varying nucleophiles to afford α-substituted amines. This reaction features catalyst-free, good functional group tolerance, simple operation procedure, and green reaction conditions.

Visible-Light-Induced Synthesis of 3-Alkyl Chromones under Catalyst- and Additive-Free Conditions

Herein, we reported an efficient and facile visible-light-induced 3-alkyl chromone synthesis from easily accessible o-hydroxyaryl enaminones and α-diazo esters. In this protocol, excellent yields were obtained with a broad substrate scope at room temperature, tolerating various functional groups. Of note is that this eco-friendly methodology features catalyst- and additive-free, mild reaction conditions, simple operation procedure, and easy scale-up, which affords a convenient pathway for the preparation of 3-alkyl chromones. Experimental results and density functional theory (DFT) computation analyses confirm the participation of carbene species and active cyclopropane intermediate.

Visible-light-induced [3+2] cycloadditions of donor/donor diazo intermediates with alkenes to achieve (spiro)-pyrazolines and pyrazoles

To date, [3 + 2] cycloadditions of diazo esters with alkynes or alkenes have been a robust tool to generate pyrazoles and pyrazolines. However, methods capable of generating donor/donor diazo species from readily available N-tosylhydrazones to furnish [3 + 2] cycloadditions, remain elusive. Herein, we describe the first visible-light-induced [3 + 2] cycloadditions of donor/donor diazo precursors with alkenes to afford pyrazoles and novel (spiro)pyrazolines bearing a quaternary center. This protocol shows a tolerable substrate scope covering versatile carbonyl compounds and alkenes. Late-stage functionalization of bioactive molecules, one-pot approach, and gram-scale synthesis have also been introduced successfully to prove the practicability. At last, mechanistic experiments and DFT studies suggested the formation of non-covalent interactions enabling the activation of N-tosylhydrazones and the formation of the donor/donor diazo intermediates.

Photochemical Sonogashira coupling reactions: beyond traditional palladium-copper catalysis

Sonogashira coupling is one of the Nobel reactions discovered in 1975 to form a C-C bond using palladium and copper as co-catalysts. Incorporating alkyne functionalities either in macro or micro molecules by using this Sonogashira reaction has already proven its viability and relevance in the sphere of synthetic chemistry. While aiming for sustainable chemistry, in recent years, visible light photoredox catalysts have appeared as an advanced tool in this regard. In this review, we aim to portray a comprehensive summary of modern visible light photo redox catalyzed Sonogashira reaction, which will leave space for the readers to rethink alternative strategies to conduct the Sonogashira reaction. This review briefly describes the implementation of various metal-based nanomaterial photocatalysts, developing either copper or palladium-free photocatalytic methods, and organo-photolytic and bioinspired photocatalysts for the Sonogashira coupling reactions. Besides, this review also gives a concise overview of the mechanistic aspects and highlights selective examples for substrate scope.

Photocatalytic Decarboxylative Minisci Reaction Catalyzed by Palladium-Loaded Gallium Nitride

The decarboxylative Minisci reaction is a versatile tool for the direct C-H alkylation of heteroarenes, where stoichiometric amounts of oxidants or expensive, precious metal reagents are commonly used. Herein, we reported a photodriven decarboxylative Minisci reaction enabled by a gallium nitride-based heterogeneous photocatalyst under mild conditions. This method can be effectively applied to a broad substrate scope of acids, including primary, secondary, and tertiary carboxylic acids and N-heteroarenes effectively. The practicability and robustness of the approach are demonstrated for the functionalization of biologically active compounds.

A beginners guide to understanding the mechanisms of photochemical reactions: things you should know if light is one of your reagents

The increasing popularity of applied photochemistry has changed the composition of the practitioners of photochemistry, from traditional specialists, to users whose expertise lies elsewhere, yet they find light as a useful and powerful reagent. I introduce Kasha's rule very early in this tutorial; this unconventional approach allows me to bypass information about high electronic states in favor of the lowest singlet and triplet excited states. Doing this I try to provide a fast entry enabling newcomers in the field of applied photochemistry to have a taste of what the field has to offer, in the hope that they will like what they see, and venture further into the many resources available to go deeper into the fascinating field of organic photochemistry.

Selectfluor-mediated tandem cyclization of enaminones with diselenides toward the synthesis of 3-selenylated chromones

A practical and metal-free approach for the regioselective selenation of chromones employing Selectfluor reagent under mild conditions is described. The developed method is suitable for a wide substrate scope and affords 3-selenylated chromones in good to excellent yield with high selectivity. An ionic mechanism is proposed for this transformation. Furthermore, the application of potassium thiocyanate with enaminones for the synthesis of thiocyano chromones in this transformation is also successful.

Visible-light induced cross-electrophile coupling of imines and anhydrides to synthesize α-amino ketones

α-Amino ketones are important motifs in synthetic and medicinal chemistry. Efficient methods to directly access these motifs from feasible precursors are, however, limited. Herein, a visible-light mediated reductive cross-electrophile coupling of readily available imines and anhydrides was developed. Under mild reaction conditions, the umpolung reactivity of diverse imines engaged with anhydrides gives a variety of α-amino ketones with good yields and a broad functional group compatibility. Primary mechanistic studies revealed that this transformation might proceed through a radical-radical cross coupling pathway dominantly.

Visible-Light-Induced Intramolecular Tandem Cyclization of Unactivated Indoloalkynes for the Synthesis of Sulfonylated and Selenylated Indolo[1,2-a]quinolines

A convenient and efficient visible-light-induced method has been developed for the construction of sulfonated and selenylated indolo[1,2-a]quinolines through sulfonyl or selenyl radical-initiated tandem cyclization of unactivated alkynes with sodium sulfinates or diaryl diselenides under mild conditions. This protocol, which simply utilizes visible light as the safe and eco-friendly energy source and an inexpensive and nontoxic organic dye as a photocatalyst without the aid of an external photocatalyst, provides various sulfonyl- and selenyl-containing indolo[1,2-a]quinolines in moderate to good yields.

Visible Light Promoted Brominative Dearomatization of Biaryl Ynones to Spirocycles

Bromine induced spiro cyclization of biaryl ynones facilitated the synthesis of spiro[5,5]trienones suitable for extended functionality at the C(3') position. Herein, a step-economic photo-oxidative brominative carbannulation of biaryl ynones employing ammonium bromide and riboflavin tetraacetate (RFTA) has been developed. The reactivity between distal phenyl C-H activated ortho-annulation and dearomative ipso-annulation is well exemplified. The eminent features of the methodology include metal-free, external additive free, low-loading photocatalyst (0.1 mol %), and use of a simple precursor.

Photoredox-Catalyzed Synthesis of 3-Sulfonylated Pyrrolin-2-ones via a Regioselective Tandem Sulfonylation Cyclization of 1,5-Dienes

A mild, visible-light-induced, regioselective cascade sulfonylation-cyclization of 1,5-dienes with sulfonyl chlorides through the intermolecular radical addition/cyclization of alkenes C(sp²)-H was developed. This procedure proceeds well and affords a mild and efficient route to a range of monosulfonylated pyrrolin-2-ones at room temperatures.

Photoredox catalysis enabling decarboxylative radical cyclization of γ,γ-dimethylallyltryptophan (DMAT) derivatives: formal synthesis of 6,7-secoagroclavine

An unusual photoredox-catalyzed radical decarboxylative cyclization cascade reaction of γ,γ-dimethylallyltryptophan (DMAT) derivatives containing unactivated alkene moieties has been developed, providing green and efficient access to various six-, seven-, and eight-membered ring 3,4-fused tricyclic indoles. This type of cyclization, which was hitherto very difficult to comprehend in ergot biosynthesis and to accomplish by more conventional procedures, enables the synthesis of ergot alkaloid precursors. In addition, this work describes a mild, environmentally friendly method to activate, reductively and oxidatively, natural carboxylic acids for decarboxylative C-C bond formation by exploiting the same photocatalyst.

Organometallic Intermediates in the Synthesis of Photoluminescent Zirconium and Hafnium Complexes with Pyridine Dipyrrolide Ligands

The two commercially available zirconium complexes tetrakis(dimethylamido)zirconium, Zr(NMe2)4, and tetrabenzylzirconium, ZrBn4, were investigated for their utility as starting materials in the synthesis of bis(pyridine dipyrrolide)zirconium photosensitizers, Zr(PDP)2. Reaction with one equivalent of the ligand precursor 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine, H2MePDPPh, resulted in the isolation and structural characterization of the complexes (MePDPPh)Zr(NMe2)2thf and (MePDPPh)ZrBn2, which could be converted to the desired photosensitizer Zr(MePDPPh)2 upon addition of a second equivalent of H2MePDPPh. Using the more sterically encumbered ligand precursor 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine, H2MesPDPPh, only ZrBn4 yielded the desired bis-ligand complex Zr(MesPDPPh)2. Careful monitoring of the reaction at different temperatures revealed the importance of the organometallic intermediate (cyclo-MesPDPPh)ZrBn, which was characterized by X-ray diffraction analysis and 1H NMR spectroscopy and shown to contain a cyclometalated MesPDPPh unit. Taking inspiration from the results for zirconium, syntheses for two hafnium photosensitizers, Hf(MePDPPh)2 and Hf(MesPDPPh)2, were established and shown to proceed through similar intermediates starting from tetrabenzylhafnium, HfBn4. Initial studies of the photophysical properties of the photoluminescent hafnium complexes indicate similar optical properties compared to their zirconium analogues.

Direct allylic acylation via cross-coupling involving cooperative N‑heterocyclic carbene, hydrogen atom transfer, and photoredox catalysis

Herein, we report a mild, operationally simple, multicatalytic method for the synthesis of β,γ-unsaturated ketones via allylic acylation of alkenes. Specifically, the method combines N‑heterocyclic carbene catalysis, hydrogen atom transfer catalysis, and photoredox catalysis for cross-coupling reactions between a wide range of feedstock carboxylic acids and readily available olefins to afford structurally diverse β,γ-unsaturated ketones without olefin transposition. The method could be used to install acyl groups on highly functionalized natural-product-derived compounds with no need for substrate pre-activation, and C-H functionalization proceed with excellent site selectivity. To demonstrate the potential applications of the method, we convert a representative coupling product into various useful olefin synthons.

Organic Covalent Interaction-based Frameworks as Emerging Catalysts for Environment and Energy Applications: Current Scenario and Opportunities

The term "covalent organic framework" (COF) refers to a class of porous organic polymeric materials made from organic building blocks that have been covalently bonded. The preplanned and predetermined bonding of the monomer linkers allow them to demonstrate directional flexibility in two- or three-dimensional spaces. COFs are modern materials, and the discovery of new synthesis and linking techniques has made it possible to prepare them with a variety of favorable features and use them in a range of applications. Additionally, they can be post-synthetically altered or transformed into other materials of particular interest to produce compounds with enhanced chemical and physical properties. Because of its tunability in different chemical and physical states, post-synthetic modifications, high stability, functionality, high porosity and ordered geometry, COFs are regarded as one of the most promising materials for catalysis and environmental applications. This study highlights the basic advancements in establishing the stable COFs structures and various post-synthetic modification approaches. Further, the photocatalytic applications, such as organic transformations, degradation of emerging pollutants and removal of heavy metals, production of hydrogen and Conversion of carbon dioxide (CO₂ ) to useful products have also been presented. Finally, the future research directions and probable outcomes have also been summarized, by focusing their promises for specialists in a variety of research fields.

Blending Aryl Ketone in Covalent Organic Frameworks to Promote Photoinduced Electron Transfer

Aryl-ketone derivatives have been acknowledged as promising organic photocatalysts for photosynthesis. However, they are limited by their photostability and have been less explored for photoinduced electron transfer (PET) applications. Herein we demonstrate a novel strategy to cover the shortage of aryl-ketone photocatalysts and control the photoreactivity by implanting symmetric aryl ketones into the conjugated covalent organic frameworks (COFs). To prove the concept, three comparative materials with the same topology and varied electronic structures were built, adopting truxenone knot and functionalized terephthalaldehyde linkers. Spectroscopic investigation and excited carrier dynamics analysis disclosed improvements in the photostability and electronic transfer efficiency as well as the structure-performance relationships toward N-aryl tetrahydroisoquinoline oxidation. This system provides a robust rule of thumb for designing new-generation aryl-ketone photocatalysts.

Site- and Stereoselective Synthesis of Alkenyl Chlorides by Dual Functionalization of Internal Alkynes via Photoredox/Nickel Catalysis

Herein, we report a redox-neutral and atom-economical protocol to synthesize valuable alkenyl chlorides from unactivated internal alkynes and abundant organochlorides via photoredox and nickel catalysis. This protocol enables the site- and stereoselective addition of organochlorides to alkynes via chlorine photoelimination-initiated sequential hydrochlorination/remote C-H functionalization. The protocol is compatible with a wide range of medicinally relevant heteroaryl, aryl, acid, and alkyl chlorides for efficiently producing γ-functionalized alkenyl chlorides, exhibiting excellent regioselectivities and stereoselectivities. Late-stage modifications and synthetic manipulations of the products and preliminary mechanistic studies are also presented.

Photoinduced Radical Sulfinylation of C(sp3)-H Bonds with Sulfinyl Sulfones

A direct C(sp³)-H sulfinylation reaction of alkanes with sulfinyl sulfones via decatungstate photocatalysis is reported. The sulfinyl sulfones generated in situ from sulfinates in the presence of an acylating reagent were able to trap the alkyl radicals that were produced via the photoinduced direct hydrogen atom transfer of alkanes, leading to a range of sulfoxides. This radical sulfinylation process provides an efficient and concise method for the synthesis of sulfoxides from abundant alkanes under mild conditions. Using the same strategy, aldehydes can also be transferred to the corresponding sulfoxides via decarbonylative sulfinylation.

Visible-Light-Promoted Direct C3-H Cyanomethylation of 2H-Indazoles

The efficient visible-light-promoted cyanomethylation of 2H-indazoles in the presence of Ir(ppy)₃ as the photocatalyst and bromoacetonitrile as the cyanomethyl radical source was achieved under mild conditions, providing a series of C3-cyanomethylated derivatives in good yields.

Photoinduced copper-catalyzed enantioselective coupling reactions

Copper-catalyzed enantioselective coupling has been widely investigated, which allows rapid construction of various chiral molecules. Despite important advances via polar and radical mechanisms, exploring general and practical strategies for the regio-, enantio- and diastereoselective assembly of stereogenic centers is of significant value but remains highly problematic. The integration of photocatalysis with asymmetric copper catalysis could provide appealing access to the development of new reaction pathways and structurally diverse chiral compounds, and extend the boundaries of radical chemistry. This review summarizes recent advances in photoinduced copper-catalyzed enantioselective coupling reactions, and discusses the mechanistic aspects.

Recent advances on catalysts for photocatalytic selective hydrogenation of nitrobenzene to aniline

Selective hydrogenation of nitrobenzene (SHN) is an important approach to synthesize aniline, an essential intermediate with extremely high research significance and value in the fields of textiles, pharmaceuticals and dyes. SHN reaction requires high temperature and high hydrogen pressure via the conventional thermal-driven catalytic process. On the contrary, photocatalysis provides an avenue to achieve high nitrobenzene conversion and high selectivity towards aniline at room temperature and low hydrogen pressure, which is in line with the sustainable development strategies. Designing efficient photocatalysts is a crucial step in SHN. Up to now, several photocatalysts have been explored for photocatalytic SHN, such as TiO2, CdS, Cu/graphene and Eosin Y. In this review, we divide the photocatalysts into three categories based on the characteristics of the light harvesting units, including semiconductors, plasmonic metal-based catalysts and dyes. The recent progress of the three categories of photocatalysts is summarized, the challenges and opportunities are pointed out and the future development prospects are described. It aims to give a clear picture to the catalysis community and stimulate more efforts in this research area.

Access to Azetidines via Gold Mediated Energy Transfer Photocatalysis

The area of energy transfer photocatalysis to generate four-membered rings is experiencing an unprecedented level of activity. Here, we report an operationally simple method toward azetidines from 2-isoxasoline-3-carboxylates and alkenes, using [Au(cbz)(NHC)] complexes as photocatalysts. The procedure enables the reaction for a wide range of substrates. Mechanistic studies confirm the energy transfer pathway. This contribution adds to the earlier reported use of these gold catalysts as a potentially versatile tool in energy transfer chemistry and catalysis.

Construction of Covalent Organic Frameworks via a Visible-Light-Activated Photocatalytic Multicomponent Reaction

Multicomponent reactions (MCRs), as a powerful one-pot combinatorial synthesis tool, have been recently applied to the synthesis of covalent organic frameworks (COFs). Compared with the thermally driven MCRs, the photocatalytic MCR-based COF synthesis has not yet been investigated. Herein, we first report the construction of COFs by a photocatalytic multicomponent reaction. Upon visible-light irradiation, a series of COFs with excellent crystallinity, stability, and permanent porosity are successfully synthesized via photoredox-catalyzed multicomponent Petasis reaction under ambient conditions. Additionally, the obtained Cy-N₃-COF exhibits excellent photoactivity and recyclability for the visible-light-driven oxidative hydroxylation of arylboronic acids. The concept of photocatalytic multicomponent polymerization not only enriches the methodology for COF synthesis but also opens a new avenue for the construction of COFs that might not be possible with the existing synthetic methods based on thermally driven MCRs.

Recent Advancements on Metal-Free Vicinal Diamination of Alkenes: Synthetic Strategies and Mechanistic Insights

The oxidative aminative vicinal difunctionalization of alkenes or related chemical feedstocks has emerged as sustainable and multipurpose strategies that can efficiently construct two -N bonds, and simultaneously prepare the synthetically fascinating molecules and catalysis in organic synthesis that typically required multi-step reactions. This review summarized the impressive breakthroughs on synthetic methodologies (2015-2022) documented especially over inter/intra-molecular vicinal diamination of alkenes with electron-rich or deficient diverse nitrogen sources. These unprecedented strategies predominantly involved iodine-based reagents/catalysts, which resent the interest of organic chemists due to their impressive role as flexible, non-toxic, and environmentally friendly reagents, resulting in a wide variety of synthetically useful organic molecules. Moreover, the information collected also describes the significant role of catalyst, terminal oxidant, substrate scope, synthetic applications, and their unsuccessful results to highlight the limitations. Special emphasis has been given to proposed mechanistic pathways to determine the key factors governing the issues of regioselectivity, enantioselectivity, and diastereoselectivity ratios.

Visible-light-induced reactions of methylenecyclopropanes (MCPs)

Diverse, visible-light-induced transformations of methylenecyclopropanes (MCPs) have been reported in recent years, attracting significant attention from synthetic chemists. As readily accessible strained molecules, MCPs have sufficient reactivity to selectively generate different target products, through reactions with various radical species upon visible-light irradiation under regulated reaction conditions. These transformations can be classified into three subcategories of reaction pathway, forming ring-opened products, cyclopropane derivatives, and alkynes. These products include pharmaceutical intermediates and polycyclic/heterocyclic compounds that are challenging to obtain using traditional methods. This review summarizes the recent advancements in this field.

Effects of Heavy Iodine Atoms and π-Expanded Conjugation on Charge Transfer Dynamics in Carboxylic Acid BODIPY Derivatives as Triplet Photosensitizers

Borondipyrromethene (BODIPY) chromophores are composed of a functional-COOH group at meso position with or without a biphenyl ring, and their compounds with heavy iodine atoms at -2, -6 positions of the BODIPY indacene core were synthesized. The photophysical properties of the compounds were studied with steady-state absorption and fluorescence measurements. It was observed that the absorption band is significantly red-shifted, and fluorescence signals are quenched in the presence of iodine atoms. In addition to that, it was indicated that the biphenyl ring does not affect the spectral shifting in the absorption as well as fluorescence spectra. In an attempt to investigate the effect of π-expanded biphenyl moieties and heavy iodine atoms on charge transfer dynamics, femtosecond transient absorption spectroscopy measurements were carried out in the environment of the tetrahydrofuran (THF) solution. Based on the performed ultrafast pump-probe spectroscopy, BODIPY compounds with iodine atoms lead to intersystem crossing (ISC) and ISC rates were determined as 150 ps and 180 ps for iodine BODIPY compounds with and without π-expanded biphenyl moieties, respectively. According to the theoretical results, the charge transfer in the investigated compounds mostly appears to be intrinsic local excitations, corresponding to high photoluminescence efficiency. These experimental findings are useful for the design and study of the fundamental photochemistry of organic triplet photosensitizers.

Metal-free photosensitized radical relay 1,4-carboimination across two distinct olefins

Intermolecular carboamination of olefins offers a powerful platform for the rapid construction of structurally complex amines from abundant feedstocks. However, these reactions often require transition-metal catalysis, and are mainly limited to 1,2-carboamination. Herein, we report a novel radical relay 1,4-carboimination across two distinct olefins with alkyl carboxylic acid-derived bifunctional oxime esters via energy transfer catalysis. The reaction is highly chemo- and regioselective, and multiple C-C and C-N bonds were formed in a single orchestrated operation. This mild and metal-free method features a remarkably broad substrate scope with excellent tolerance of sensitive functional groups, therefore providing easy access to structurally diverse 1,4-carboiminated products. Moreover, the obtained imines could be easily converted into valuable biologically relevant free γ-amino acids.

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.

Organic photoredox catalyzed dealkylation/acylation of tertiary amines to access amides

A mild metal-free C-N bond activation strategy for the direct conversion of inert tertiary amines with acyl chlorides into tertiary amides via organic photoredox catalysis is presented. In this protocol, a novel organic photocatalyst (Cz-NI-Ph) that showed excellent catalytic performance during C-N bond cleavage is developed. Moreover, this reaction features green and mild conditions, broad substrate scope, and readily available raw materials.

Mechanistic explorations on the decarboxylative allylation of amino esters via dual photoredox and palladium catalysis

Mechanistic studies reveal that the decarboxylative allylation of amino esters via dual photoredox and palladium catalysis occurs via oxidation giving π-allyl-Pd(II) species and carboxylate, which is oxidized by *Ir(III)-catalyst offering benzyl radicals. The alkylated product is formed via an S_N2 pathway. Single-electron transfer between Pd(I)-species and Ir(II)-catalysis restores both catalysts.

Photocatalyst-free visible light driven synthesis of gem-dihaloenones from alkynes, tetrahalomethanes and water

Photocatalytic reactions, in particular, processes without photosensitisers, have attracted increased attention due to their green aspect and high economic value and are considered valuable tools in organic synthesis. A new practical photocatalytic system was investigated in this study, and it can efficiently produce gem-dihaloenones by combining terminal alkynes with tetrahalomethanes (BrCCl₃ and CBr₄) and water without a photocatalyst, and the yield can reach up to 87%. The catalytic system is straightforward, the raw materials are inexpensive and easy to obtain, and the operation is simple.

Visible-light-induced selective defluoroalkylations of polyfluoroarenes with alcohols

To provide α-polyfluoroarylalcohols, a novel protocol for the selective defluoroalkylation of polyfluoroarenes with easily accessible alcohols was reported via the cooperation of photoredox and hydrogen atom transfer (HAT) strategies with the assistance of Lewis acids under visible light irradiation. The protocol featured broad scope, excellent regioselectivity for both C-H and C-F bond cleavages, and mild conditions. Mechanistic studies suggested that the reaction occurred through Lewis acid-promoted HAT to provide an alkyl radical and sequential addition to polyfluoroarenes. Impressively, the regioselectivity for C-F cleavage was verified with the Fukui function. The feasibility and application of this protocol on fluoroarene synthesis were well illustrated by gram-scale synthesis under both batch and flow conditions, late-stage decoration of bioactive compounds, and further transformations of the fluoroarylalcohols.

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.

Radical-Based Route to Functionalized Tetralin: Formal Total Synthesis of (±)-Hamigeran B

A formal synthetic route to hamigeran B, an antiviral marine natural product with a unique tricyclic molecular architecture, has been developed. The key chemical transformations in the present route include a novel zinc(II)porphyrin-catalyzed photoredox radical cascade cyclization to access a functionalized tetralin, a catalyst-free benzylic radical bromination with NBS by visible-light irradiation, and a samarium(II)-induced cyclization of brominated tetralone possibly via an orthoquinodimethane-like intermediate.

Metal complexes for catalytic and photocatalytic reactions in living cells and organisms

The development of organometallic catalysis has greatly expanded the synthetic chemist toolbox compared to only exploiting "classical" organic chemistry. Although more widely used in organic solvents, metal-based catalysts have also emerged as efficient tools for developing organic transformations in water, thus paving the way for further development of bio-compatible reactions. However, performing metal-catalysed reactions within living cells or organisms induces additional constraints to the design of reactions and catalysts. In particular, metal complexes must exhibit good efficiency in complex aqueous media at low concentrations, good cell specificity, good cellular uptake and low toxicity. In this review, we focus on the presentation of discrete metal complexes that catalyse or photocatalyse reactions within living cells or living organisms. We describe the different reaction designs that have proved to be successful under these conditions, which involve very few metals (Ir, Pd, Ru, Pt, Cu, Au, and Fe) and range from in cellulo deprotection/decaging/activation of fluorophores, drugs, proteins and DNA to in cellulo synthesis of active molecules, and protein and organelle labelling. We also present developments in bio-compatible photo-activatable catalysts, which represent a very recent emerging area of research and some prospects in the field.

Photoredox-Catalyzed Synthesis of β-Amino Alcohols: Hydroxymethylation of Imines with α-Silyl Ether as Hydroxymethyl Radical Precursor

Carbon-carbon bond formation is an efficient approach for the synthesis of amino alcohols using two simple starting materials. Herein, we present a novel method for a divergent synthesis of β-amino ethers and β-amino alcohols in a sequential one-pot protocol under high-efficiency, mild, and metal- or metal-free conditions. Especially, TMSCH₂OPMP was developed as a synthetic equivalent of α-hydroxymethyl radical in an in situ photocatalyzed oxidative PMP group deprotection strategy under air. A preliminary mechanistic investigation provides evidence for reaction mechanism involving a photoinduced α-alkoxy methyl radical and superoxide.