Selective Difunctionalization of Unactivated Aliphatic Alkenes Enabled by a Metal–Metallaaromatic Catalytic System

Green-light-induced selective hydroselenation of olefins with diselenides

A selective green-light-induced hydroselenation of alkenes with diselenides using Hantzsch ester as the hydrogen donor has been developed. In the case of electron-neutral diaryl diselenides and diacyl ones, alkenes undergo anti-Markovnikov-selective hydroselenation. When switching to electron-deficient diaryl diselenides and dialkyl ones, Markovnikov-selective hydroselenation occurs.

Amine-Promoted Three-Component Radical Selenofunctionalization for the Construction of β-Hydroxy Selenide Derivatives

An amine-promoted three-component radical selenofunctionalization reaction of alkenes with TBHP and diselenide is disclosed. The reaction conditions are mild and suitable for a wide range of substrates (29 examples), and all give the corresponding hydroxyselenenylation products in moderate to excellent yields. In addition, preliminary studies on the mechanism reveal that the current method might proceed via a radical pathway. TBHP serves as both radical initiator and the source of hydroxyl group.

Hydroselenation of olefins: elucidating the β-selenium effect

We report a light-promoted hydroselenation of alkenes with high anti-Markovnikov selectivity. Blue light activates an aryl diselenide to generate a seleno radical with subsequent addition into an alkene to form a β-seleno carbon radical. Hydrogen atom transfer (HAT) from the selenol to the carbon radical generates the linear selenide with high selectivity in preference to the branched isomer. These studies reveal a unique β-selenium effect, where a selenide β to a carbon radical imparts high anti-selectivity for radical addition through delocalization of the HAT transition state.

Osmapentalenofurans Constructed by Reacting Os≡C1 of Osmapentalyne with Phenols

Over the past decade, significant research efforts have focused on osmapentalyne, characterized by the more reactive Os≡C7 (Carbon atoms numbered in a clockwise direction on the osmapentalyne skeleton), across areas encompassing electrophilic, nucleophilic, and addition reactions. Nevertheless, the reactivity of osmapentalyne featuring Os≡C1 remains ripe for further exploration. In this investigation, we effectively synthesized a lineage of osmapentalenofurans through the nucleophilic reaction of osmapentalyne incorporating Os≡C1 with phenols. These resulting complexes demonstrate near-infrared luminescence traits in both solid and liquid states. Particularly noteworthy is the osmapentalenofuran derived from tetraphenylethane (TPE) unit, which showcases remarkable aggregation-induced emission (AIE) property in the aggregated state. These osmapentalenofurans are also able to further extend their range of reactions, including reactions with base and isonitrile. This study not only broadens the scope of applications for metal aromatics but also furnishes valuable insights into the realm of specialized functional materials.

Calcium(II)-Mediated Three-Component Selenylation of gem-Difluoroalkenes: Access to α,α-Difluoroalkyl-β-selenides

A calcium-mediated three-component selenylation of gem-difluoroalkenes using alcohols as nucleophiles and N-(phenylseleno)phthalimide as the selenylation agent has been developed for the efficient synthesis of various α,α-difluoroalkyl-β-selenides. This selenylation reaction exhibits broad substrate and functional group tolerance, along with high levels of chemo- and regioselectivity. Additionally, the synthetic utility of the developed transformation in the late-stage functionalization of drug molecules was demonstrated.

Zn(OTf)2-catalyzed intra- and intermolecular selenofunctionalization of alkenes under mild conditions

Zn(OTf)2-catalyzed intra- and intermolecular selenofunctionalization of alkenes was achieved with electrophilic N-phenylselenophthalimide. This method provides straightforward and efficient access to various seleno-substituted heterocycles and vicinal Se heteroatom-disubstituted molecules under mild conditions. This reaction is compatible with various substrates/functional groups, and preliminary studies on the reaction mechanistic were also conducted.

Recent Progress in Synthetic and Biological Application of Diorganyl Diselenides

Diorganyl diselenides have emerged as privileged structures because they are easy to prepare, have distinct reactivity, and have broad biological activity. They have also been used in the synthesis of natural products as an electrophile in the organoselenylation of aromatic systems and peptides, reductions of alkenes, and nucleophilic substitution. This review summarizes the advancements in methods for the transformations promoted by diorganyl diselenides in the main functions of organic chemistry. Parallel, it will also describe the main findings on pharmacology and toxicology of diorganyl diselenides, emphasizing anti-inflammatory, hypoglycemic, chemotherapeutic, and antimicrobial activities. Therefore, an examination detailing the reactivity and biological characteristics of diorganyl diselenides provides valuable insights for academic researchers and industrial professionals.

Electrophotocatalysis Versus Indirect Electrolysis: Electrochemical Selenocyclization of 3-Aza-1,5-dienes Facilitated by Energy Transfer, Direct Photolysis or N-Hydroxyphthalimide

Three hybrid electrochemical protocols, which involve the energy transfer, direct photolysis and N-hydroxyphthalimide catalyst, respectively, are presented for the selenylation/cyclization of the fragile substrates of 3-aza-1,5-dienes with diorganyl diselenides to afford 3-selenomethyl-4-pyrrolin-2-ones. The two electrophotocatalytic reactions and the indirect electrolysis one are both regioselective and external-oxidant- and transition-metal-free, and are associated with a broad substrate scope and high Se-economy, and all three methods are amenable to gram-scale syntheses, late-stage functionalizations, sunlight-induced experiments and all-solar-driven syntheses.

Syntheses and reactivities of strained fused-ring metallaaromatics containing planar eleven-carbon chains

Carbolong complexes are one of the primary types of metallaaromatics, and they include metallapentalynes and metallapentalenes. A series of 7C-10C and 12C-carbolong complexes with planar ligand skeletons respectively containing 7-10 and 12 carbon atoms in their backbones, have been previously reported. Herein, two classes of strained substances, metallabenzyne-fused metallapentalenes and metallabenzene-fused metallapentalynes, were prepared, both representing 11C-carbolong complexes with a planar carbon-chain ligand. Furthermore, the former type is also the carbolong derivatives containing a metallabenzyne skeleton, another primary metallaaromatic framework. Metallabenzyne-fused metallapentalenes show versatile reactivities, and the most interesting one is the metal carbyne bond shift from a 6-membered to a more strained 5-membered ring, affording the above-mentioned metallabenzene-fused metallapentalyne. This work makes carbolong chemistry more complete, and provides a method to achieve metallabenzynes, which is anticipated to concurrently advance the development of these two types of metallaaromatics.

Visible-Light [4+2] Homodimerization of Decomposition-Prone Styrenes via Electron Transfer Catalysis of Diaryl Diselenides

The facile electron transfer catalysis of diaryl diselenides was utilized for the visible-light [4+2] homodimerization of decomposition-prone styrenes. The reaction required only 0.5 mol % TPT+BF4- photocatalyst and 1.5 mol % electron transfer catalyst (ArSe)2. The spontaneous electron transfer capability of diaryl diselenides was demonstrated for the first time, leading to the sequestration of redox-prone radical cation intermediates via electron transfer processes. A variety of polymerization-prone styrenes smoothly underwent the visible-light-promoted [4+2] homodimerization to tetralin derivatives.

Electrochemical selenofunctionalization of unactivated alkenes: access to β-hydroxy-selenides

This work demonstrates the electrochemical construction of 2-methyl-1-aryloxy-3-(arylselanyl)propan-2-ol/2-hydroxy-2-methyl-3-(arylselanyl)propyl 2-(2-hydroxy-2-methyl-3-(arylselanyl)propoxy)benzoate starting from aryl allyl ethers/allyl benzoates and diaryl diselenides under additive-free electrochemical conditions. This environmentally friendly method was achieved through constant current electrolysis in an undivided cell setup under acid, oxidant, or catalyst-free conditions. Additionally, this technique enabled the synthesis of a variety of β-hydroxy selenides including late-stage functionalization of drug derivatives in good to exceptional yields across various substrates under mild reaction conditions.

Enhancing Precatalyst Performance and Robustness through Aromaticity: Insights from Iridaheteroaromatics

Despite the inherent stability-enhancing benefits of dπ-pπ conjugation-induced aromaticity, metallaaromatic catalysts remain underutilized in this context, despite their reactivity with organic functionalities in stoichiometric reactions. We present a strategy for synthesizing a diverse range of iridaheteroaromatics, (L^L)IrIII(Cp*)I, including iridapyridylidene-indole, iridapyridene-indole, and iridaimidazole, via in situ deprotonation/metalation reactions utilizing [Cp*IrCl2]2 and the respective ligands. These catalysts exhibit enhanced σ-donor and π-acceptor properties, intrinsic σ-π continuum attributes, and versatile binding sites, contributing to stability through enhanced dπ-pπ conjugation-induced aromaticity. Spectroscopic data, X-ray crystallographic data, and density functional theory calculations confirm their aromaticity. These iridaheteroaromatics exhibit formidable catalytic ability across a spectrum of transformations under industrially viable conditions, notably excelling in highly selective cross alkylation and β-alkylation of alcohols and an eco-friendly avenue for quinolone synthesis, achieving remarkably high turnover frequencies (TOFs). Additionally, this method extends to the self-condensation of bioalcohols like ethanol, n-butanol, and n-hexanol in water, replicating conditions frequently encountered in primary fermentation solutions. These iridaheteroaromatics exhibit strong catalytic activity with fast reaction rates, high TOFs, broad substrate compatibility, and remarkable selectivity, displaying their potential as robust catalysts in large-scale applications and emphasizing their practical significance beyond their structural and theoretical importance.

Photoinduced Copper-Catalyzed 1,2-Difunctionalization of 1,3-Dienes with Aryl Diselenides

Described herein is a photoinduced copper-catalyzed 1,2-difunctionalization of 1,3-dienes. The selenium atom radical was generated by the visible light irradiation of diselenides, triggering radical addition with 1,3-dienes to form allyl radical intermediate. Subsequent rapid Z/E isomerization allowed for thermodynamically favorable intermediate formation and enabled copper catalyzed stereoselective functionalization with various nucleophiles.

Calcium(II)-Mediated Three-Component Selenofunctionalization of Alkenes under Mild Conditions

A mild and general protocol involving amnio- and oxyselenation of diverse alkenes for the efficient synthesis of organo-Se compounds is achieved via an environmentally benign calcium-catalyzed three-component reaction. This selenofunctionalization reaction exhibits excellent substrate/functional group tolerance and high levels of chemo- and regioselectivity. Its utility was exemplified in the late-stage functionalization and even aggregation-induced emission luminogen labeling of organo-Se compounds.

The application of aromaticity and antiaromaticity to reaction mechanisms

Aromaticity, in general, can promote a given reaction by stabilizing a transition state or a product via a mobility of π electrons in a cyclic structure. Similarly, such a promotion could be also achieved by destabilizing an antiaromatic reactant. However, both aromaticity and transition states cannot be directly measured in experiment. Thus, computational chemistry has been becoming a key tool to understand the aromaticity-driven reaction mechanisms. In this review, we will analyze the relationship between aromaticity and reaction mechanism to highlight the importance of density functional theory calculations and present it according to an approach via either aromatizing a transition state/product or destabilizing a reactant by antiaromaticity. Specifically, we will start with a particularly challenging example of dinitrogen activation followed by other small-molecule activation, C-F bond activation, rearrangement, as well as metathesis reactions. In addition, antiaromaticity-promoted dihydrogen activation, CO2 capture, and oxygen reduction reactions will be also briefly discussed. Finally, caution must be cast as the magnitude of the aromaticity in the transition states is not particularly high in most cases. Thus, a proof of an adequate electron delocalization rather than a complete ring current is recommended to support the relatively weak aromaticity in these transition states.

Synthesis of Tertiary Fluorides through an Acid-Mediated Deoxyfluorination of Tertiary Alcohols

The combination of methanesulfonic acid and potassium bifluoride is reported for the deoxyfluorination of tertiary alcohols. Under metal-free conditions that use readily available, cheap, and easy-to-handle reagents, a range of tertiary alcohols could be converted into the corresponding fluorides in excellent yields (average yields of 85% for 23 examples). Mechanistic investigation showed that the reaction proceeds at 0 °C, in part, through an elimination/hydrofluorination pathway, but no residual alkenes are observed. The application of these conditions for the fluorination of ether and ester is also demonstrated.

Reshaping aromatic frameworks: expansion of aromatic system drives metallabenzenoids to metallapentalenes

Reshaping an aromatic framework to generate other skeletons is a challenging issue due to the stabilization energy of aromaticity. Such reconfigurations of aromatics commonly generate non-aromatic products and hardly ever reshape to a different aromatic framework. Herein, we present the transformation of metallaindenols to metallapentalenes and metallaindenes in divergent pathways, converting one aromatic framework to another with an extension of the conjugation framework. The mechanistic study of this transformation shows that phosphorus ligands play different roles in the divergent processes. Further theoretical studies indicate that the expansion of the aromatic system is the driving force promoting this skeletal rearrangement. Our findings offer a new concept and strategy to reshape and construct aromatic compounds.

Hetero-carbolong chemistry: experimental and theoretical studies of diaza-metallapentalenes

Four diaza-osmapentalenes were prepared by two-step reactions, through the treatment of an alkyne-coordinated osmium complex with azo compounds, followed by the addition of AgSbF₆/CO. Their aromaticity was confirmed by crystal parameters, NMR spectra and theoretical calculations. These complexes are the first diaza-metallapentalenes representing a new class of metallaaromatics.

Heterocyclic Suzuki-Miyaura coupling reaction of metalla-aromatics and mechanistic analysis of site selectivity

Pd-catalyzed Suzuki-Miyaura cross-coupling is one of the most straightforward and versatile methods for the construction of functionalized arenes and heteroarenes but site-selective cross-coupling of polyhalogenated (hetero)arenes containing identical halogen substituents remains a challenging problem. Herein, we report a new candidate for heterocyclic Suzuki-Miyaura coupling reaction. This candidate has been applied in organometallic systems by combining classical aryl boronic acid reagents with non-classical heteroarenes. Experimental and computational studies of the mechanism of the reactions were performed, with an emphasis on the identity of the reactive species in the oxidative addition step and the nature of the precise site selectivity. The influence of both the aromaticity of the metalla-aromatic substrates and the steric and electronic properties of the halogenated sites are studied in detail.

Unique Properties and Emerging Applications of Carbolong Metallaaromatics

ConspectusAromatic compounds are important in synthetic chemistry, biomedicines, and materials science. As a special type of aromatic complex, transition-metal-based metallaaromatics contain at least one transition metal in an aromatic framework. The chemistry of metallaaromatics has seen much progress in computational studies and synthetic methods, but their properties and applications are still emerging. In recent years, we have disclosed a series of metal-centered conjugated polycyclic metallacycles in which a carbon chain is chelated to a metal center through at least three metal-carbon bonds. These are termed carbolong complexes and exhibit good stability to water, oxygen, light, and heat on account of their polydentate chelation and aromaticity, making them easy to handle. Carbolong complexes are not only special π-conjugated aromatics but also organometallics; therefore, they have the properties of both species. In this Account, we showcase the recent advances in their applications based on their different properties.First, carbolong complexes are a special kind of π-conjugated aromatic, with the ability to transmit electrons, allowing them to function as single-molecule conductors and candidates for electron transporting layer materials (ETLs) in solar cells. A series of carbolong complexes have been proved to be useful as achievable ETLs which enhance device performance in both organic solar cells and perovskite solar cells.Second, due to the involvement of d orbitals in the conjugation, carbolong complexes normally exhibit strong and broad absorption, even in some cases extending to the near-infrared region (NIR). The absorbed optical energy can be converted into light, heat, and ultrasound; consequently, carbolong compounds can be used as core moieties in smart materials. For example, 7C carbolong complexes were found to exhibit aggregation-enhanced near-infrared emission (AIEE). Some 12C carbolong complexes have been designed into the core moieties of NIR-responsive polymers, such as cylindrical NIR-responsive materials, self-healing materials, and shape memory materials. In contrast to the stereotypically toxic osmium compounds such as the highly toxic OsO₄, some osmium carbolong complexes exhibit low cell cytotoxicity and good biocompatibility; consequently, they also have potential applications in the biomedical area. For example, benefiting from broad absorption in the NIR, 9C and 12C carbolong complexes have been used in photoacoustic imaging and photothermal therapy, respectively. In addition, photodynamic therapeutic applications which take advantage of a carbolong peroxo complex are discussed.Third, as special transition-metal complexes chelated by carbon-based ligands, a carbolong peroxo complex has displayed catalytic activity in the dehydrogenation of alcohols and a bimetallic carbolong complex has been used to catalyze difunctionalization reactions of unactivated alkenes.Overall, aromatic carbolong complexes have been applied to photovoltaics, smart materials, phototherapy, and catalytic reactions. Moving forward, we hope that this Account will shed light on future studies and theoretical research and encourage more discoveries of the properties of other metallaaromatics.

Enantioselective Selenol-ene Using Rh-Hydride Catalysis

This study showcases the first enantioselective hydroselenation of styrenes. Organoselenium building blocks are accessed with selectivity for the branched isomer. Through a Rh-hydride pathway, C-Se bonds can be forged with excellent regio- and enantiocontrol.

Cu-catalyzed oxidative denitrogenation of 3-aminoindazoles for the synthesis of isoquinolinones

A Cu-catalyzed oxidative dual arylation of active alkenes via the cleavage of two C-N bonds of 3-aminoindazoles is presented for constructing isoquinolinones. Importantly, 3-aminoindazoles are used as efficient arylating agents through a radical process. This method has a good substrate scope and functional group compatibility.

Conjugated polymers based on metalla-aromatic building blocks

Conjugated polymers usually require strategies to expand the range of wavelengths absorbed and increase solubility. Developing effective strategies to enhance both properties remains challenging. Herein, we report syntheses of conjugated polymers based on a family of metalla-aromatic building blocks via a polymerization method involving consecutive carbyne shuttling processes. The involvement of metal d orbitals in aromatic systems efficiently reduces band gaps and enriches the electron transition pathways of the chromogenic repeat unit. These enable metalla-aromatic conjugated polymers to exhibit broad and strong ultraviolet-visible (UV-Vis) absorption bands. Bulky ligands on the metal suppress π-π stacking of polymer chains and thus increase solubility. These conjugated polymers show robust stability toward light, heat, water, and air. Kinetic studies using NMR experiments and UV-Vis spectroscopy, coupled with the isolation of well-defined model oligomers, revealed the polymerization mechanism.

Branching out: redox strategies towards the synthesis of acyclic α-tertiary ethers

Acyclic α-tertiary ethers represent a highly prevalent functionality, common to high-value bioactive molecules, such as pharmaceuticals and natural products, and feature as crucial synthetic handles in their construction. As such their synthesis has become an ever-more important goal in synthetic chemistry as the drawbacks of traditional strong base- and acid-mediated etherifications have become more limiting. In recent years, the generation of highly reactive intermediates via redox approaches has facilitated the synthesis of highly sterically-encumbered ethers and accordingly these strategies have been widely applied in α-tertiary ether synthesis. This review summarises and appraises the state-of-the-art in the application of redox strategies enabling acyclic α-tertiary ether synthesis.

A Convenient Synthesis towards 2-Bromo-2-(methoxy(phenyl)methyl)malononitrile and 2-Iodo-2-(methoxy(phenyl)methyl)malononitrile

This short note elaborates a concise protocol for the synthesis of two novel vicinal haloethers bearing a malonontrile group, 2-bromo-2-(methoxy(phenyl)methyl)malononitrile (1) and 2-iodo-2-(methoxy(phenyl)methyl)malononitrile (2). The structures of the synthesized compounds were confirmed by 1H, 13C-NMR spectroscopy. The final products indicate that methanol not only acts as solvent but also participates in and dominates the reaction result.

An aromatic dimetallapolycyclic complex with two rhenapyrylium rings

Extension of the polycyclic benzo-rhenapyrylium structure by a fused metallaaromatic ring and a benzene unit are reported. The dirhena-aromatic complex 4 shows strong absorption in the visible region and a significant absorption in the near-infrared (NIR) region (λ_max = 842 nm). DFT calculations are performed to understand its aromatic nature and electronic behavior.