Recent advances in visible light-activated radical coupling reactions triggered by (i) ruthenium, (ii) iridium and (iii) organic photoredox agents

Photoinduced B-H Arylation of N-Heterocyclic Carbene Boranes with Sulfonyl(hetero)arenes

Arylborane complexes ligated by N-heterocyclic carbenes (NHCs) can be synthesized by photoirradiation of a mixture of NHC-boranes and sulfonyl(hetero)arenes. The reaction occurs under mild and convenient conditions without any photocatalyst, which are realized by a radical chain mechanism involving NHC-boryl radicals and sulfonyl radicals. This reaction offered the opportunity to reveal the photophysical property of a 2-borylnaphtho[1,2-d]thiazole derivative.

Synthesis of Aminated C-3 Aryloylated Benzofuran, Furopyridine, Benzothiophene, and Indole Derivatives from 1,6-Enyne and N-Aminopyridinium Salt in Visible Light

We report a visible-light-assisted tandem oxidative 5-exo-dig cyclization of 1,6-enynes for the synthesis of aminated C-3 aryloylated benzofuran, furopyridine, benzothiophene, and indole derivatives. The nitrogen-centered radical generated in situ from N-aminopyridinium salt initiates the consecutive formation of C-N, C-C, and C-O bonds. The methodology exhibits good functional group tolerance and regioselectivity, furnishing products in good to excellent yields at room temperature. Preliminary biological screening of synthesized molecules reveals their potential as anticancer agents.

CeCl3-Catalyzed C-H Alkylation of N-Sulfonyl Ketimines with Alkanes and Ether via Photoinduced Ligand-to-Metal Charge Transfer

A cerium-catalyzed C-H alkylation of N-sulfonyl ketimines with low-cost and readily available alkanes as alkyl sources was developed. This transformation proceeded through the synergy of photoinitiated ligand-to-metal charge transfer (LMCT) using a chlorine radical as an HAT reagent and air as a green oxidant. A series of alkylated N-sulfonyl ketimines were synthesized with moderate to good yields in a highly atom-economic manner under chemical oxidant-free conditions.

Recent developments in the photoredox catalyzed Minisci-type reactions under continuous flow

The Minisci reaction constitutes a straightforward and convenient strategy to achieve the direct C-H functionalization of heterocyclic molecules. This radical-based reaction platform has received increasing attention due to the predictability of its outcome according to the nature of the radicals and heterocycles involved. Considering the importance of these heterocyclic scaffolds in the development and production of drug molecules, it is inevitable that scaling up this reaction manifold is of utmost importance. This review will present recent strategies to achieve the goal, which mostly involve implementing the reaction conditions under continuous flow.

Recent Applications of Sulfonium Salts in Synthesis and Catalysis

The use of sulfonium salts in organic synthesis has experienced a dramatic increase during the last years that can arguably be attributed to three main factors; the development of more direct and efficient synthetic methods that make easily available sulfonium reagents of a wide structural variety, their intrinsic thermal stability, which facilitates their structural modification, handling and purification even on large scale, and the recognition that their reactivity resembles that of hypervalent iodine compounds and therefore, they can be used as replacement of such reagents for most of their uses. This renewed interest has led to the improvement of already existing reactions, as well as to the discovery of unprecedented transformations; in particular, by the implementation of photocatalytic protocols. This review aims to summarize the most recent advancements on the area focusing on the work published during and after 2020. The scope of the methods developed will be highlighted and their limitations critically evaluated.

Photocatalytic EnT-Mediated Aminophosphorylation of Alkenes Using Oxime Esters as Bifunctional Reagents

C-P bond formation has typically been achieved by a single-electron transfer process. Herein, a novel class of oxime ester bifunctionalization reagents were first applied to the photocatalytic β-aminophosphorylation of modular olefins. The bifunctional reagents generate two distinct radical species (imine and phosphoryl radicals) that exhibit excellent regioselectivity. Subsequently, these radicals are attached to the olefins through a single-step EnT catalytic process, establishing a novel synthetic pathway. This protocol is characterized by excellent regioselectivity, broad functional group tolerance, and mild reaction conditions, which would enrich the diversity and versatility to facilitate the diversity-oriented synthesis of β-aminophosphorylated complex molecule scaffolds.

A facile access to aliphatic trifluoromethyl ketones via photocatalyzed cross-coupling of bromotrifluoroacetone and alkenes

Biological molecules incorporating trifluoromethyl ketones (TFMKs) have emerged as reversible covalent inhibitors, aiding in the management and treatment of inflammatory diseases, cancer, and respiratory conditions. TFMKs, renowned for their versatile binding properties and adaptability, are pivotal in the rational design of novel drugs for diverse diseases. The photocatalytic insertion of alkenes, abundant feedstocks, into the α-carbon of trifluoromethylacetone represents a highly effective and atom-economical method for synthesizing valuable TFMKs. However, these processes typically necessitate high-energy photoirradiation (λ > 300 nm, Hg lamp) and stoichiometric oxidants to generate the acetonyl radical from acetone. In our study, we demonstrate the visible-light photocatalytic radical addition into olefins using bromotrifluoroacetone as the trifluoroacetonyl radical precursor under mild conditions. Aliphatic trifluoromethyl ketones or the corresponding bromo-substituted products can be obtained by selecting an appropriate photocatalyst and solvent. Comprehensive experimental investigations, including cyclic voltammetry, Stern-Volmer quenching studies, and kinetic isotope effects, corroborate the synthesis of trifluoroacetonyl radical species from bromotrifluoroacetone under photoredox conditions. Further, we demonstrate the efficient synthesis of an oseltamivir derivative bearing a trifluoromethylketone moiety, which shows promising biological activity. Hence, this methodology will streamline the direct introduction of trifluoromethyl ketone into biological target molecules during drug discovery.

Photoluminescent ruthenium(II) bipyridyl complexes containing phosphonium ylide ligands

A P-ylide Ru(II) bipyridyl complex was readily synthetized and fully characterized, constituing one of the rare examples of photoluminescent metal ylide complexes. Its photophysical and redox properties have been compared with those of related NHC and cyclometalated Ru complexes and exploited in visible-light photocatalyzed SET and EnT processes.

Tunability of triplet excited states and photophysical behaviour of bis-cyclometalated iridium(III) complexes with imidazo[4,5-f][1,10]phenanthroline

This is a comprehensive study of the photophysical behaviour of heteroleptic iridium(III) complexes with imidazo[4,5-f][1,10]phenanthroline (imphen) as an ancillary ligand, represented by the general formula [Ir(N∩C)2(imphen)]PF6. As cyclometalating ligands, 2-phenylpyridine (Hppy), 2-phenylquinoline (Hpquin), 2-phenylbenzothiazole (Hpbztz), and 2-(2-pyridyl)benzothiophene (pybzthH) were used. The impact of structural modifications of cyclometalating ligands was widely explored by a combination of steady-state and time-resolved optical techniques accompanied by theoretical calculations. We evidenced that the cyclometalating ligands induce essential changes in the nature of the emissive excited state and the emission characteristics of [Ir(N∩C)2(imphen)]PF6. While the complex [Ir(ppy)2(imphen)]PF6 (1) is a typical 3MLLCT emitter, the lowest triplet states of [Ir(pquin)2(imphen)]PF6 (2), [Ir(pbztz)2(imphen)]PF6 (3) and [Ir(pybzth)2(imphen)]PF6 (4) have a predominant 3LCN∩C character. The phosphorescence colour of the investigated Ir(III) complexes changes from greenish-yellow to red, their quantum yields vary from 56 to 2%, and their triplet excited-state lifetimes fall in the 743-3840 ns range. The highest photoluminescence quantum yield was revealed for 2 in CH2Cl2, while complex 3 in MeCN shows the most pronounced increase in the lifetime. Both complexes 2 and 3 show an increased efficiency of singlet oxygen generation. The herein discussed structure-property relationships are of high significance for controlling photoinduced processes in heteroleptic iridium(III) complexes with the imphen-based ancillary ligand, and making further progress in effectively tuning the emission energies, quantum yields and excited-state lifetimes of these systems by structural modifications of cyclometalating ligands, especially the π-conjugation, the position of the N-donor and the presence of sulfur heteroatoms.

Photoredox-Catalyzed Markovnikov Hydroamination of Alkenes with Azoles

A visible-light induced intermolecular hydroamination of alkenes with azoles is reported, delivering pharmaceutically valuable N-benzyl azoles in high yields with excellent Markovnikov selectivity. Mechanistic studies suggest that the process is initiated by the energy transfer of the excited photocatalyst with alkenes, followed by the single electron reduction, protonation, and subsequent single electron oxidation to afford the key alkyl carbocation intermediate. This protocol exhibits advantages of broad functional group tolerance, excellent atom economy, high efficiency, and mild reaction conditions.

Chemoselective Cleavage and Transamidation of Tertiary p-Methoxybenzyl Amides under Metal-Free Photoredox Catalysis

A metal-free and mild cleavage of tertiary p-methoxybenzyl amides (PMB tert-amide) under photoredox conditions is developed using Mes-Acr-Ph+BF4- and Selectfluor to activate the electron-rich benzylic C-H bond of the PMB moiety. The resulting acyl fluoride intermediate is versatile and facilitates a one-pot transamidation of the PMB tert-amide. The value of this protocol is highlighted by performing the chemoselective activation of the PMB tert-amide in bifunctional molecules containing more reactive functionalities than the amide.

Photocatalytic Generation of Germyl Radicals from Digermanes Enabling the Hydro/Deuteriogermylation of Alkenes

We have developed a visible-light-induced method to photolyze digermanes through single-electron oxidation using a photocatalyst, in contrast to direct excitation, to generate germyl radicals and achieve the hydro/deuteriogermylation of alkenes. This protocol allows the previously elusive incorporation of the small trimethylgermyl group and deuterium, metabolically stable bioisosteres of tert-butyl and hydrogen, respectively, making this approach attractive in not only organic synthesis but also medicinal chemistry.

Catalytic Deprotection of Alkyne Dicobalt Hexacarbonyl Complexes using Near-Infrared Photocatalysis

Dicobalt hexacarbonyl complexes are well-known for their applications in the Nicholas reaction or simply as a protecting group for alkynes. To recover the alkyne, demetalation is necessary, which usually involves a stoichiometric amount of an oxidizing agent or a strong ligand. This article reports a demetalation methodology based on a photocatalytic process. This approach employs a photocatalyst under aerobic conditions, and the optimal results were achieved using mild near-infrared irradiation. A mechanistic investigation is also presented to elucidate how the photocatalytic system promotes this deprotection. This tool is compatible with the one-pot reaction and orthogonal deprotection of alkynes, offering new perspectives for further applications.

Ultrathin two-dimensional (2D) manganese-based metal-organic framework nanosheets for selective photocatalytic oxidation of thioether

The efficiency of photocatalysts depends largely on the accessibility of reaction species to the active centre, the electron transfer and geometric matching between the active surface of the catalyst and reaction species. In this work, we successfully synthesized and designed one two-dimensional Mn(II) MOF with [Mn2(H2L1)(H2O)2(DMF)2]n·(CH3CH2OH)n (HSTC 3) by using MnCl2·4H2O and 5,5'-(anthracene-9,10-diyl)diisophthalic acid (H4L1), in which the adjacent layers are stacked with weak interactions, and the huge gap leads to the interpenetration between layers to form a 2D + 2D → 3D interpenetration frame. Based on the particularity of the structure of HSTC 3, ultrasonic wall breaking methods were tried to successfully peel HSTC 3 into nanosheets (HSTC 3-NS), thus achieving a significant improvement in a series of optoelectronic properties due to exposure to more active centres for HSTC 3-NS. These results significantly enhance the photocatalytic selective oxidation of thioether. This study provides a new insight into the post-synthesis modification of MOF photocatalyst and their application in photocatalytic organic synthesis.

Photocatalytic Defluorinative α-Aminoalkylation of Allylic Difluorides

A photocatalytic process was devised to synthesize monofluoroalkenes via defluorinative functionalization of allylic difluorides. N-Alkylanilines are used as precursors to α-aminoalkyl radicals, which undergo regioselective addition to allylic difluorides, and subsequent SET and fluoride elimination produce monofluoroalkenes. C-C bond formation on the aniline is site-selective for the least substituted carbon α to nitrogen.

Semiheterogeneous Photoredox Catalysis: Synthesis of 3-Sulfonylquinolines via Radical Multicomponent Cascade Annulation

An efficient photocatalytic protocol combining heterogeneous semiconductors as photocatalysts and NHPI as a redox reagent was disclosed. Under this protocol, 3-sulfonylquinolines were formed in up to 92% yield via a multicomponent radical cascade annulation. Good substituent tolerance and gram-scale reaction showed the potential in fine chemicals modification and pharmaceutical synthesis.

Overestimated Halogen Atom Transfer Reactivity of α-Aminoalkyl Radicals

Halogen atom transfer (XAT) is a versatile method for generating carbon radicals. Recent interest has focused on α-aminoalkyl radicals as potential XAT reagents, previously reported to exhibit reactivity comparable to tin radicals. Utilizing an advanced time-resolved EPR technique, the XAT reactions between α-aminoalkyl radicals and organic halides were examined, allowing direct observation of the process through EPR spectroscopy and analysis of radical kinetics. Second-order rate constants for these reactions were determined, with some validated using transient absorption spectroscopy. The key finding is that the reactivity of α-aminoalkyl radicals in XAT reactions is 103 to 105 times lower than that of tin and silicon radicals and only slightly higher than alkyl radicals. This challenges the belief that α-aminoalkyl radicals are as reactive as tin radicals. The study on the solvent effect indicates that the XAT reaction of α-aminoalkyl radicals does not involve a highly polarized transition state, suggesting that the kinetic polar effect in this XAT process is not as significant as previously believed. The present study provides a reliable XAT reactivity scale for α-aminoalkyl radicals, which is crucial for designing XAT reactions and understanding their mechanisms.

Photocatalytic Asymmetric Acyl Radical Truce-Smiles Rearrangement for the Synthesis of Enantioenriched α-Aryl Amides

The radical Truce-Smiles rearrangement is a straightforward strategy for incorporating aryl groups into organic molecules for which asymmetric processes remains rare. By employing a readily available and non-expensive chiral auxiliary, we developed a highly efficient asymmetric photocatalytic acyl and alkyl radical Truce-Smiles rearrangement of α-substituted acrylamides using tetrabutylammonium decatungstate (TBADT) as a hydrogen atom-transfer photocatalyst, along with aldehydes or C-H containing precursors. The rearranged products exhibited excellent diastereoselectivities (7 : 1 to >98 : 2 d.r.) and chiral auxiliary was easily removed. Mechanistic studies allowed understanding the transformation in which density functional theory (DFT) calculations provided insights into the stereochemistry-determining step.

Ligand-to-metal charge transfer facilitates photocatalytic oxygen atom transfer (OAT) with cis-dioxo molybdenum(vi)-Schiff base complexes

Systems incorporating the cis-Mo(O)2 motif catalyse a range of important thermal homogeneous and heterogeneous oxygen atom transfer (OAT) reactions spanning biological oxidations to platform chemical synthesis. Analogous light-driven processes could offer a more sustainable approach. The cis-Mo(O)2 complexes reported here photocatalyse OAT under visible light irradiation, and operate via a non-emissive excited state with substantial ligand-to-metal charge-transfer (LMCT) character, in which a Mo[double bond, length as m-dash]O π*-orbital is populated via transfer of electron density from a chromophoric salicylidene-aminophenol (SAP) ligand. SAP ligands can be prepared from affordable commercially-available precursors. The respective cis-Mo(O)2-SAP catalysts are air stable, function in the presence of water, and do not require additional photosensitisers or redox mediators. Benchmark OAT between phosphines and sulfoxides shows that electron withdrawing groups (e.g. C(O)OMe, CF3) are necessary for photocatalytic activity. The photocatalytic system described here is mechanistically distinct from both thermally catalysed OAT by the cis-Mo(O)2 motif, as well as typical photoredox systems that operate by outer sphere electron transfer mediated by long-lived emissive states. Both photoactivated and thermally activated OAT steps are coupled to establish a catalytic cycle, offering new opportunities for the development of photocatalytic atom transfer based on readily-available, high-valent metals, such as molybdenum.

Iron-Catalyzed Electrophotochemical α-Functionalization of a Silylcyclobutanol

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

Enantioselective Relay Coupling of Perfluoroalkyl and Vinylogous Ketyl Radicals

β-Chiral carboxylic acids and their derivatives are highly valuable structural motifs in the fields of asymmetric synthesis and medicinal chemistry. However, the introduction of a sterically demanding sidechain to the β-carbon, such as an all-carbon quaternary center, remains a significant challenge in classical polar processes. Recently, N-heterocyclic carbene (NHC) mediated coupling reactions involving persistent ketyl radicals have emerged as a promising strategy to assemble highly crowded carbon-carbon bonds. Nevertheless, achieving enantioselectivity in these reactions remains highly challenging. In this work, we report our recent progress in controlling enantioselectivity for relay coupling of perfluoroalkyl and persistent vinylogous ketyl radicals. We developed a chiral bifunctional NHC-squaramide catalyst that achieves high facial selectivity in a critical bond-forming event involving the coupling of a congested tertiary carbon radical and vinylogous ketyl radical. Chiral carboxylates bearing an all-carbon quaternary center at the β-position can be prepared in good yield and excellent enantiomeric excess. Results from density functional theory (DFT) calculations and nuclear Overhauser effect (NOE) experiments indicate that the N,N'-diaryl squaramide motif adopts an unusual syn-syn conformation, enabling hydrogen bonding interactions with the enolate oxygen, thereby rigidifying the overall conformation of the transition state.

Naphthalene Diimide and Bis-Heteroleptic Ru(II) Complex-Based Hybrid Molecule with 3-in-1 Functionalities

Multipurpose applications of a newly developed homobimetallic Ru(II) complex, Ru-NDI[PF6]4, which incorporates 1,10-phenanthroline and triazole-pyridine ligands and linked via a (-CH2-)3 spacer to the reputed anion-π interacting NDI system, are described. Solution-state studies of the bimetallic complex, including EPR, PL, UV-vis, and NMR experiments, reveal two sequential one-electron transfers to the NDI unit, generating NDI⋅- and NDI2- in the presence of F- selectively. This process inhibits the primary electron transfer from Ru(II) to the NDI unit, thereby allowing the 3MLCT-based emission of the complex to be recovered, resulting in a corresponding ten-fold increase in luminescence intensity. DFT and TD-DFT computational studies further elucidate the experimentally observed absorption spectra of the complex. Secondly, CT-DNA binding studies with the complex are performed using various spectroscopic analyses such as UV-vis, PL, and CD. Comparative DNA binding studies employing EB and molecular docking reveal that the binding with CT-DNA occurs through both intercalative and groove binding modalities. Thirdly, the photocatalytic activities of the complex towards C-C, C-N, and C-O bond formation in organic cross-coupling reactions, including the amidation of α-keto acids to amines and the oxidation of alcohol to aldehydes, are also demonstrated.

Recent advances in dual photoredox/nickel catalyzed alkene carbofunctionalised reactions

Alkene carbofunctionalization reactions have great potential for synthesizing complex molecules and constructing complex structures in natural products and medicinal chemistry. Recently, dual photoredox/nickel catalysis has emerged as a novel strategy for alkene carbofunctionalization. Nickel offers numerous advantages over other transition metals, such as cost-effectiveness, abundance, and low toxicity, and moreover, it has many oxidation states. Nickel catalysts exhibit excellent catalytic activity in dual photoredox/transition metal catalysis, facilitating the formation of carbon-carbon or carbon-heteroatom bonds in organic transformations. This review highlights the latest advancements in dual photoredox/nickel-catalyzed alkene carbofunctionalizations and includes the literature published from 2020 to 2024.

Photoredox/Cu-Catalyzed Decarboxylative C(sp3)-C(sp3) Coupling to Access C(sp3)-Rich gem-Diborylalkanes

gem-Diborylalkanes are highly valuable building blocks in organic synthesis and pharmaceutical chemistry due to their ability to participate in multi-step cross-coupling transformations, allowing for the rapid generation of molecular complexity. While progress has been made in their synthetic metholodology, the construction of β-tertiary and C(sp3)-rich gem-diborylalkanes remains a synthetic challenge due to substrate limitations and steric hindrance issues. An approach is presented that utilizes synergistic photoredox and copper catalysis to achieve efficient C(sp3)-C(sp3) cross-coupling of alkyl N-hydroxyphthalimide esters, which can easily be obtained from alkyl carboxylic acids, with diborylmethyl species, providing a series of C(sp3)-rich gem-diborylalkanes with 1°, 2°, and even 3° β positions. Furthermore, this approach can also be applied to complex medicinal compounds and natural products, offering rapid access to molecular complexity and late-stage functionalization of C(sp3)-rich drug candidates. Mechanistic experiments revealed that diborylmethyl Cu(I) species participated in both the photoredox process and the key C(sp3)-C(sp3) bond-forming step.

A Photoredox Thiol-yne Reaction for the Synthesis of Vinyl Sulfide-Based Coumarins and its Effect on Fluorescence Properties

Coumarins still remain one of the most widely explored fluorescent dyes, with a broad spectrum of applications spanning various fields, such as molecular imaging, bioorganic chemistry, materials chemistry, or medical sciences. Their fluorescence is strongly based on a push-pull mechanism involving an electron-donating group (EDG), mainly located at the C7 or C8 positions of the dye core. Unfortunately, up to now, these positions have been very limited to hydroxyl or amino groups. In this study, we present in detail the synthesis of the first series of coumarins bearing a vinyl sulfide as the EDG at the C7 position. These derivatives were prepared by thiol-yne reaction, promoted by ruthenium- or porphyrin-based photoredox catalysis, enabling rapid late-stage diversification. We also functionalized coumarins with short peptides, and BSA protein as a proof-of-concept study, in a single-step process. This strategy, capable of proceeding under aqueous conditions, overcomes the protection/deprotection steps usually required by traditional methods, which also use strong bases and organic solvents. Moreover, the photophysical properties such as absorption and emission of obtained coumarins (for 3-CF3, 3-benzothiazole, 6-8-difluoro derivatives), predominantly exhibited large Stokes shifts (up to 204 nm) and maintained intramolecular charge transfer (ICT) characteristics.

NPh3-Mediated WO3-Photocatalyzed Semiheterogeneous Hydroxylation of Aryl and Alkyl Boronic Acids

With an inexpensive and commercially available WO3 semiconductor as the heterogeneous photocatalyst, a catalytic amount of NPh3 as the single-electron donor, and ambient air as the single-electron acceptor and oxygen source, the semiheterogeneous photocatalytic hydroxylation of alkyl and aryl boronic acids was developed. A broad range of hydroxylated compounds can be obtained in excellent yields.

Photocatalytic Proficiency of Cinnoline Moiety for Cross-Coupling Reactions: A Two in One Photocatalyst

Herein, we have developed a new class of organic photocatalysts that can mimic transition metals for several oxidative and reductive organic cross-coupling transformations. Due to its wide potential window in both the oxidation and reduction ranges, cinnoline exhibits dual catalytic activity under visible light illumination, acting as both a photoreductant and photooxidant.

TEMPO/O2 Synergistically Mediated BiBrO-Photocatalyzed Decarboxylative Phosphorylation of N-Arylglycines

With both TEMPO and O2 (in air) as the homogeneous redox mediators, BiBrO as the heterogeneous semiconductor photocatalyst, the first example of semi-heterogeneous photocatalytic decarboxylative phosphorylation of N-arylglycines with diarylphosphine oxides was established. A series of α-amino phosphinoxides were efficiently synthesized.

Recyclable V2O5/g-C3N4 Heterojunction-Catalyzed Visible-Light-Promoted C3-H Trifluoromethylation of Quinoxalin-2-(1H)-ones

A visible-light-initiated C-H trifluoromethylation of quinoxalin-2(1H)-ones was established using a Z-scheme V2O5/g-C3N4 heterojunction as a recyclable photocatalyst in an inert atmosphere at room temperature under additive-free and mild conditions. A variety of trifluoromethylated quinoxalin-2-(1H)-one derivatives were heterogeneously generated in moderate to high yields, exhibiting good functional group tolerance. Remarkably, the recyclable V2O5/g-C3N4 catalyst could be reused five times with a slight loss of catalytic activity.

Multielectron Reduction of Esters by a Diazabenzacenaphthenium Photoredox Catalyst

A novel diazabenzacenaphthenium photocatalyst, N-BAP, with high photoredox abilities and visible-light absorption was designed and prepared in one step. Under visible-light irradiation, N-BAP promoted the four-electron reduction of esters in the presence of ammonium oxalate as a "traceless reductant" to generate carbinol anion intermediates that underwent protonation with water to give the corresponding alcohols. The resulting carbinol anions also exhibited nucleophilic reactivity under the photocatalytic conditions to undergo a 1,2-addition to a second carbonyl compound, affording unsymmetric 1,2-diols.

Unusual Regioselective C-H Difluoroalkylation of Heteroarenes under Photoredox Catalysis

We disclose a new general strategy for the site-selective difluoroalkylation of nonprefunctionalized heteroarenes, such as quinoxaline at the C-8 position, and benzothiadiazole, benzoxadiazole, and benzothiazole at the C-4 position via consecutive organophotoredox-catalyzed radical-radical cross-coupling and base-assisted hydrogen abstraction reactions. The current methodology represents a site-selective direct difluoroalkylative strategy to allow broad functional group tolerance and a wide substrate scope in good to excellent yields. Careful experimental investigations and detailed DFT calculations revealed the exact site-selectivity of the heteroarenes and a possible mechanistic pathway.

Carbonylative cyclization of biaryl enones with aldehydes and oxamic acids

An oxidative radical-promoted carbonylative cyclization strategy for the synthesis of phenanthren-9-(10H)-one frameworks from biaryl enones using aldehydes as the carbonyl radical sources is disclosed. The reaction proceeds through a sequential addition of a carbonyl radical to the olefin followed by cyclization with an aryl ring. The method is further extended to carbamoyl radicals generated from oxamic acids to access the corresponding phenanthrenones with amide functionalities.

Hydroalkylation of styrenes enabled by boryl radical mediated halogen atom transfer

In this study, we present an inexpensive, stable, and easily available boryl radical source (BPh4Na) employed in a Halogen Atom Transfer (XAT) methodology. This mild and convenient strategy unlocks the use of not only alkyl iodides as radical precursors but also of the more challenging alkyl and aryl bromides to generate C-centered radicals. The generated radicals were further engaged in the anti-Markovnikov hydroalkylation of electronically diverse styrenes, therefore achieving the formation of C(sp3)-C(sp3) and C(sp3)-C(sp2) bonds. A series of experimental and computational studies revealed the prominent role of BPh4Na in the halogen abstraction step.

Recent advances in the application of [2 + 2] cycloaddition in the chemical synthesis of cyclobutane-containing natural products

Cyclobutanes are distributed widely in a large class of natural products featuring diverse pharmaceutical activities and intricate structural frameworks. The [2 + 2] cycloaddition is unequivocally the primary and most commonly used method for synthesizing cyclobutanes. In this review, we have summarized the application of the [2 + 2] cycloaddition with different reaction mechanisms in the chemical synthesis of selected cyclobutane-containing natural products over the past decade.

Advancements in visible-light-induced reactions via alkenyl radical intermediates

In recent years, visible-light-induced organic transformations have taken a central role driving forward the progress of modern organic synthesis. These processes typically involve the transient generation of highly reactive radical intermediates, facilitating a diverse array of chemical reactions. Despite the abundance of synthetic strategies enabling the access of aryl and alkyl-centered radicals, the exploitation of photochemistry to generate highly reactive alkenyl radicals has remained notably underdeveloped. In this review, we present recent advancements in visible-light-induced transformations that proceed through the generation of alkenyl radicals from alkenyl-containing precursors, predominantly alkenyl halides, showcasing their application in various organic transformations.

Photocatalytic C2-trifluoroethylation and perfluoroalkylation of 3-substituted indoles using fluoroalkyl halides

A photocatalytic reactivity platform for the C2-trifluoroethylation and perfluoroalkylation of 3-substituted indoles has been developed. A range of fluoroalkyl halides have been employed as radical precursors under mild, transition-metal-free conditions to access new (per)fluorinated chemical space featuring the indole substructure. This general protocol is also applicable to indole-containing peptides.

Photocatalytic Generation of Carbocation from Thiols and Application to Cross-Nucleophile Coupling

Represented herein is a simple thiol identified as an effective precursor to photochemically form a carbocation. Thanks to the thiyl radical rapid transformation to disulfide, which serves not only to stabilize the generated thiyl radical but also to allow the second electron transfer to form a carbocation. The resulting carbocations, including primary benzylic, secondary, and tertiary carbocations, can smoothly couple with nitrogen, oxygen, and carbon nucleophilic coupling partners as well as complex drug molecules, accompanied by elemental sulfur formation in air.

Photo-Induced Generation of Oxygenated Quaternary Centers via EnT Enabled Singlet O2 Addition to C3-Maleimidated Quinoxaline: A Reagent-Less Approach

Demonstrated here is an external photo-sensitizer-free (auto-sensitized) singlet oxygen-enabled solvent-dependent tertiary hydroxylation and aryl-alkyl spiro-etherification of C3-maleimidated quinoxalines. Such "reagent-less" photo-oxygenation at Csp3-H and etherification involving Csp3-H/Csp2-H are unparalleled. Possibly, the highly π-conjugated N-H tautomer allows the substrate to get excited by irradiation, and subsequently, it attains the triplet state via ISC. This excited triplet-state sensitized molecule then transfers its energy to a triplet-state oxygen (3O2) generating reactive singlet oxygen (1O2) for hydroxylation and spirocyclization depending on the solvent used. In HFIP, the generated alkoxy radical accepts a proton via HAT giving hydroxylated product. In contrast, in an aprotic PhCl it underwent a radical addition at the ortho-position of the C2 aryl to provide spiro-ether. An unprecedented orthogonal spiro-etherification was observed via the displacement of o-substitutents for ortho (-OEt, -OMe, -F, -Cl, -Br) substituted substrates. The order of ipso substitution follows the trend -OMe>-OEt>-F>-H>-Cl>-Br. Both these oxygenation reactions can be carried out with nearly equal ease using direct sunlight without the requirement of any elaborate reaction setup. Demonstration of large-scale synthesis and a few interesting transformations have also been realized. Furthermore, several insightful control experiments and quantum chemical computations were performed to unravel the mechanism.

Cyclometalated and NNN Terpyridine Ruthenium Photocatalysts and Their Cytotoxic Activity

The cyclometalated terpyridine complexes [Ru(η2-OAc)(NC-tpy)(PP)] (PP = dppb 1, (R,R)-Skewphos 4, (S,S)-Skewphos 5) are easily obtained from the acetate derivatives [Ru(η2-OAc)2(PP)] (PP = dppb, (R,R)-Skewphos 2, (S,S)-Skewphos 3) and tpy in methanol by elimination of AcOH. The precursors 2, 3 are prepared from [Ru(η2-OAc)2(PPh3)2] and Skewphos in cyclohexane. Conversely, the NNN complexes [Ru(η1-OAc)(NNN-tpy)(PP)]OAc (PP = (R,R)-Skewphos 6, (S,S)-Skewphos 7) are synthesized in a one pot reaction from [Ru(η2-OAc)2(PPh3)2], PP and tpy in methanol. The neutral NC-tpy 1, 4, 5 and cationic NNN-tpy 6, 7 complexes catalyze the transfer hydrogenation of acetophenone (S/C = 1000) in 2-propanol with NaOiPr under light irradiation at 30 °C. Formation of (S)-1-phenylethanol has been observed with 4, 6 in a MeOH/iPrOH mixture, whereas the R-enantiomer is obtained with 5, 7 (50-52% ee). The tpy complexes show cytotoxic activity against the anaplastic thyroid cancer 8505C and SW1736 cell lines (ED50 = 0.31-8.53 µM), with the cationic 7 displaying an ED50 of 0.31 µM, four times lower compared to the enantiomer 6.

3D-Printed Eosin Y-Based Heterogeneous Photocatalyst for Organic Reactions

Heterogenization of Eosin Y by 3D-printing and its application in photocatalysis are reported. The approach allows a fine tuning of the photocatalyst morphology and its rapid preparation. Photocatalytic activity was evaluated through model organic reactions involving oxidation, reduction, and photosensitization pathways. The efficiency, recyclability and stability of 3D printed EY is remarkable paving the way to new generation of heterogeneous photocatalysts with a perfect control of their shape and adaptable to any photoreactors.

Dual-Emissive Iridium(III) Complexes and Their Applications in Biological Sensing and Imaging

Phosphorescent iridium(III) complexes are widely recognized for their unique properties in the excited triplet state, making them crucial for various applications including biological sensing and imaging. Most of these complexes display single phosphorescence emission from the lowest-lying triplet state after undergoing highly efficient intersystem crossing (ISC) and ultrafast internal conversion (IC) processes. However, in cases where these excited-state processes are restricted, the less common phenomenon of dual emission has been observed. This dual emission phenomenon presents an opportunity for developing biological probes and imaging agents with multiple emission bands of different wavelengths. Compared to intensity-based biosensing, where the existence and concentration of an analyte are indicated by the brightness of the probe, the emission profile response involves modifications in emission color. This enables quantification by utilizing the intensity ratio of different wavelengths, which is self-calibrating and unaffected by the probe concentration and excitation laser power. Moreover, dual-emissive probes have the potential to demonstrate distinct responses to multiple analytes at separate wavelengths, providing orthogonal detection capabilities. In this concept, we focus on iridium(III) complexes displaying fluorescence-phosphorescence or phosphorescence-phosphorescence dual emission, along with their applications as biological probes for sensing and imaging.

Visible-light-enabled stereoselective synthesis of functionalized cyclohexylamine derivatives via [4 + 2] cycloadditions

An unprecedented intermolecular [4 + 2] cycloaddition of benzocyclobutylamines with α-substituted vinylketones, enabled by photoredox catalysis, has been developed. The current method enables facile access to highly functionalized cyclohexylamine derivatives that were otherwise inaccessible, in moderate to good yields with excellent diastereoselectivities. This protocol has some excellent features, such as full atom economy, good functional-group compatibility, mild reaction conditions, and an overall redox-neutral process. Additionally, an asymmetric version of this cycloaddition was preliminarily investigated via the incorporation of a chiral phosphoric acid (CPA), and moderate to good enantioselectivity could be effectively realized with excellent diastereoselectivity. Synthetic applications were demonstrated via a scale-up experiment and elaborations to access amino alcohol and cyclobutene derivatives. Based on the results of control experiments, a reasonable reaction mechanism was proposed to elucidate the reaction pathway.

Phosphonate-substituted porphyrins as efficient, cost-effective and reusable photocatalysts

Recent advances in visible light photocatalysis represent a significant stride towards sustainable catalytic chemistry. However, its successful implementation in fine chemical production remains challenging and requires careful optimization of available photocatalysts. Our work aims to structurally modify bioinspired porphyrin catalysts, addressing issues related to their laborious synthesis and low solubility, with the goal of increasing their efficiency and developing reusable catalytic systems. We have demonstrated the catalytic potential of readily available meso-tetrakis[4-(diethoxyphosphoryl)phenyl]porphyrins (M(TPPP)). Novel metal (Pd(II), Co(II) and In(III)) complexes with this ligand were prepared in good yields. These chromophores were characterized in solution using spectroscopic (NMR, UV-vis, fluorescence) and electrochemical methods. The introduction of phosphonate groups on the phenyl substituents of meso-tetraphenylporphyrins (M(TPP)) improves solubility in polar organic solvents without significantly altering the photophysical properties and photostability of complexes. This structural modification also leads to easier reductions and harder oxidations of the macrocycle for all investigated complexes compared to the corresponding TPP derivatives. The free base porphyrin, zinc(II), palladium(II), and indium(III) complexes were studied as photocatalysts for oxidation of sulfides to sulfoxides using molecular oxygen as a terminal oxidant. Both dialkyl and alkyl aryl sulfides were quantitatively transformed into sulfoxides under blue LED irradiation in the acetonitrile-water mixture (10 : 1 v/v) with a low loading (0.005-0.05 mol%) of porphyrin photocatalysts, where H2(TPPP) and Pd(TPPP) were found to be the most efficient. The reaction mechanism was studied using photoluminescence and EPR spectroscopies. Then, to access reusable catalysts, water-soluble derivatives bearing phosphonic acid groups, H2(TPPP-A) and Pd(TPPP-A), were prepared in high yields. These compounds were characterized using spectroscopic methods. Single-crystal X-ray diffraction analysis of Pd(TPPP-A) reveals that the complex forms a 3D hydrogen-bonded organic framework (HOF) in the solid state. Both H2(TPPP-A) and Pd(TPPP-A) were found to catalyze the photooxidation of sulfides by molecular oxygen in the acetonitrile-water mixture (1 : 1 v/v), while only Pd(TPPP-A) resulted in selective production of sulfoxides. The complex Pd(TPPP-A) was easily recovered through extraction in the aqueous phase and successfully reused in five consecutive cycles of the sulfoxidation reaction.

Revealing the Mechanism of Alcohol Dehydroxylation and C-C Bond Formation through Concerted Catalysis by Ir/Cu Bimetallic Complexes

The density functional theory (DFT) was employed to theoretically investigate the reaction mechanism of alcohol deoxygenation/trifluoromethylation. The substrate alcohol (R1) forms a complex (INT3) by binding with benzoxazole salts (NHCs). Under the influence of the photocatalyst ([IrIII]*) and quinuclidine, the C-H bond in INT3 is activated through either electron transfer-proton transfer (ETPT) or hydrogen atom transfer (HAT) mechanisms, resulting in the cleavage of C-O bonds and generation of deoxyalkyl radicals. The distribution of high-valent and low-valent states in the catalytic cycle of [Ir]-complexes is governed by the redox potential mechanism. Investigation was conducted on the source of hydrogen atom transfer reagents in the HAT reaction process under both optimal and nonoptimal conditions. The results demonstrate distinct reactivity among various radicals involved in the Cu-mediated radical capture process. Further investigations into INT3 activation modes, cycling facilitated by [Ir]-complexes, and understanding the role played by [Cu]-complexes in this reaction system provide a valuable theoretical foundation for comprehending and enhancing Ir/Cu bimetallic cooperative catalysis in alcohol deoxygenation/trifluoromethylation reactions. This provides anticipated theoretical support for future designs of more efficient and rational alcohol deoxygenation reactions.

γ-Amino Alcohols via Energy Transfer Enabled Brook Rearrangement

In the long-standing quest to synthesize fundamental building blocks with key functional group motifs, photochemistry in the recent past has comprehensively established its attractiveness. Amino alcohols are not only functionally diverse but are ubiquitous in the biologically active realm of compounds. We developed bench-stable bifunctional reagents that could then access the sparsely reported γ-amino alcohols directly from feedstock alkenes through energy transfer (EnT) photocatalysis. A designed 1,3-linkage across alkenes is made possible by the intervention of a radical Brook rearrangement that takes place downstream to the EnT-mediated homolysis of our reagent(s). A combination of experimental mechanistic investigations and detailed computational studies (DFT) indicates a radical chain propagated reaction pathway.

Total Synthesis of the Diterpenes (+)-Randainin D and (+)-Barekoxide via Photoredox-Catalyzed Deoxygenative Allylation

We report the first enantioselective total synthesis of diterpenoid randainin D, which possesses a hydroazulenone core with a β-substituted butenolide moiety on the cycloheptane ring. The trans-5/7 ring system was formed via a highly challenging ring-closing metathesis delivering the tetrasubstituted cycloheptenone. The butenolide moiety was installed via a novel deoxygenative allylation under Ir-photoredox catalysis, employing methyl oxalate as a red/ox tag. Moreover, the developed allylation was successfully utilized in the 7-step total synthesis of (+)-barekoxide. This study suggests that this deoxygenative allylation method is a promising strategy for the formation of Cq-C(sp3) bonds (Cq = quaternary center) in the context of natural product synthesis.

Red Light-Blue Light Chromoselective C(sp2)-X Bond Activation by Organic Helicenium-Based Photocatalysis

Chromoselective bond activation has been achieved in organic helicenium (nPr-DMQA+)-based photoredox catalysis. Consequently, control over chromoselective C(sp2)-X bond activation in multihalogenated aromatics has been demonstrated. nPr-DMQA+ can only initiate the halogen atom transfer (XAT) pathway under red light irradiation to activate low-energy-accessible C(sp2)-I bonds. In contrast, blue light irradiation initiates consecutive photoinduced electron transfer (conPET) to activate more challenging C(sp2)-Br bonds. Comparative reaction outcomes have been demonstrated in the α-arylation of cyclic ketones with red and blue lights. Furthermore, red-light-mediated selective C(sp2)-I bonds have been activated in iodobromoarenes to keep the bromo functional handle untouched. Finally, the strength of the chromoselective catalysis has been highlighted with two-fold functionalization using both photo-to-transition metal and photo-to-photocatalyzed transformations.

Direct, Selective α-Aryloxyalkyl Radical Cyanation and Allylation of Aryl Alkyl Ethers

We report the site-selective α-aryloxyalkyl C-H cyanation and allylation of aryl alkyl ethers using an acridinium photocatalyst with phosphate base under LED irradiation (456 nm). Oxidation of the aryl alkyl ether to its corresponding radical cation by the excited stated photocatalyst allowed facile deprotonation of the ArOC(sp3)-H bond to afford an α-aryloxyalkyl radical, which was trapped with sulfone substrates, resulting in expulsion of a sulfonyl radical and formation of allylated or cyanated products.

PIDA/I2-mediated photo-induced aerobic N-acylation of sulfoximines with methylarenes

A visible-light-promoted, PIDA/I2-mediated acylation of NH-sulfoximines with methylarenes as an acyl source has been achieved. This transition metal and photosensitizer-free approach provides easy access to N-acylsulfoximines via oxidative coupling of sulfoximines with easily available methylarenes without using any peroxide source. Mechanistic investigations suggest the intermediacy of radicals and the importance of molecular oxygen.

A free-radical design featuring an intramolecular migration for a synthetically versatile alkyl-(hetero)arylation of simple olefins

A free-radical approach has enabled the development of a synthetically versatile alkyl-(hetero)arylation of olefins. Alkyl and (hetero)aryl groups were added concurrently to a full suite of mono- to tetrasubstituted simple alkenes (i.e., without requiring directing or electronically activating groups) for the first time. Key advances also included the introduction of synthetically diversifiable alkyl groups featuring different degrees of substitution, good diastereocontrol in both cyclic and acyclic settings, the addition of biologically valuable heteroarenes featuring Lewis basic nitrogen atoms as well as simple benzenes, and the generation of either tertiary or quaternary benzylic centers. The synthetic potential of this transformation was demonstrated by leveraging it as the key step in a concise synthesis of oliceridine, a new painkiller that received FDA approval in 2020.