Synthesis of pyridino[2,1-b]quinazolinones via a visible light-induced functionalization of alkynes/nitrile insertion/cyclization tandem sequences in continuous-flow technology

We successfully developed an unprecedented route to pyridino[2,1-b]quinazolinone synthesis through a visible light-induced functionalization of alkynes/nitrile insertion/cyclization tandem sequences in a microchannel reactor. The yield of the template reaction under optimized conditions is 91%. This photocatalytic reaction provides rapid, facile, and practical access to valuable polycyclic quinazolinone, and is amenable to the gram scale. In addition, a reasonable reaction mechanism is proposed based on controlled experiments.

Factors Controlling Diastereoselectivity and Reactivity in the Catalytic Aerobic Carbooxygenation of (E)-2-Fluoro-3-aryl-allyl Nitroacetates

Radical cyclizations are powerful tools for complexity building, providing facile access to functionalized cyclic adducts. However, forging two contiguous tetrasubstituted carbons via radical cyclization through the addition of tertiary radicals to geminally disubstituted sp2 carbons has rarely been investigated. Furthermore, the effect of double-bond geometry at the geminally disubstituted sp2 carbon on reactivity and stereochemical outcomes remains underexplored. In this study, we present experimental and computational studies on the carbooxygenation of 2-fluoro-3-aryl-allyl nitroacetates to investigate reactivity and selectivity differences between (E)- and (Z)-isomers. First, we identify that (E)-isomers are less reactive than (Z)-isomers. Second, both (E)- and (Z)-isomers undergo conversion to α,α,β,β-tetrasubstituted γ-lactones with high syn-selectivity, despite the absence of putative E/Z isomerization of the alkene unit. Third, incorporating an electron-donating group at the radical acceptor enhances the reactivity of (E)-isomers in catalytic aerobic carbooxygenation. Fourth, computational studies show that syn-selectivity is mainly governed by the fluorine-induced gauche effect, whereas SOMO-HOMO level inversion induced by the electron-donating group at the radical acceptor enhances (E)-isomer reactivity. Based on these mechanistic insights, we develop a diastereoconvergent protocol using the E/Z mixture as a starting material to synthesize a potent antifungal agent.

Photocatalytic Radical Cascade Cyclization of N-(o-Cyanobiaryl)acrylamides with Sulfonyl Chlorides

A photoredox-catalyzed radical cascade cyclization of N-(o-cyanobiaryl)acrylamides with sulfonyl chlorides for the construction of sulfonyl-containing pyrido[4,3,2-gh]phenanthridines has been disclosed. The developed synthetic tool tolerates a broad range of sulfonyl chlorides to undergo a cascade sequence, including sulfonyl radical addition, nitrile insertion, and cyclization.

Through Space π-Electrons Communication in [2,2]-Paracyclophanes: Unprecendented Stabilization of Radicals

Efforts to understand radical stability have led to considerable progress in radical chemistry. In this article, we investigated a novel approach to enhancing the radical stability of carbon-centered radicals through space electron delocalization within [2,2]-paracyclophanes. Alkoxyamines possessing a paracyclophane scaffold exploit face-to-face π-π-interactions between the aromatic rings to effectively lower bond dissociation energy (BDE) for NO-C bond homolysis. Electron spin resonance (ESR) experiments and computational modeling have confirmed a better stability compared to the analogues without the paracyclophane core. Theoretical analyses further elucidate the role of through-space electron communication in enhancing radical stability. This study highlights promising applications in fields such as organic synthesis, material science, and drug design. By achieving a low BDE for homolysis, the alkoxyamines efficiently release radicals, enabling successful application in nitroxide-mediated polymerization (NMP) of styrene, which provides high control over polymer architecture. Additionally, preliminary anti-proliferative assays reveal that the alkoxyamines exhibit promising anti-cancer activities against lung, breast, and prostate cells, which is correlated to their ability to release radicals upon homolysis.

Bioinspired Copper/Amine Cooperative Catalysis Enables Asymmetric Radical Azidation

Asymmetric radical transformations (ARTs) are vital for constructing chiral drugs and materials, while the highly reactive nature of radicals often imposes a challenge in selectivity control of these processes. Inspired by the principles of enzyme-cofactor cooperation to enhance stereochemical induction in enantioselective radical transformations, we developed an enantioselective asymmetric radical azidation via cooperative organo- and transition metal catalysis. This approach enables the efficient synthesis of heavily functionalized tertiary azides from readily available aldehydes. The key to this enantioselective process is the use of both chiral organocatalysts to transiently convert aldehydes to the corresponding chiral radical cationic species upon oxidation along with a detailed screening of chiral metal-azide catalysts to cooperatively enhance stereoinduction in carbon-azide bond formation. DFT studies suggest a favorable stereocontrol model and validate the crucial roles of chirality pairing of both catalytic schemes. We envision that this copper/amine cooperative catalysis could offer a useful strategy of constructing tetrasubstituted stereogenic carbon in asymmetric radical transformation development.

Stereoselective synthesis of atropisomeric amides enabled by intramolecular acyl transfer

C-N atropisomeric amides are important compounds in medicinal chemistry and agrochemistry. Atropselective methods for their synthesis are therefore important. In this study, a novel strategy to make C-N atropisomeric amides based on intramolecular acyl transfer via a tethered Lewis basic pyridine or tertiary amine group is reported. The reactions operate under kinetic control and in most cases are highly atropselective, with the products isolable as pure, single diastereoisomers following chromatography. The kinetically favored atropisomer can also be isomerised into the alternative thermodynamically favored atropisomer upon heating. The kinetic and thermodynamic outcomes are supported by computational studies, while additional mechanistic studies support operation via initial fast acylation of the Lewis basic group, followed by rate-determining acyl transfer, which also enables control over the atropisomer formed.

A General Three-Component Nozaki-Hiyama-Kishi-Type Reaction Enabled by Delayed Radical-Polar Crossover

Nozaki-Hiyama-Kishi (NHK) reactions offer a mild approach for the formation of alcohol motifs through radical-polar crossover-based pathways from various radical precursors. However, the application of multicomponent NHK-type reactions, which allow the formation of multiple bonds in a single step, has been largely restricted to bulky alkyl radical precursors, thus limiting their expanded utilization. Herein, we disclose a general three-component NHK-type reaction enabled by delayed radical-polar crossover, which efficiently tolerates a plethora of radical precursors that were previously unavailable. This method enables the modular assembly of versatile homoallylic alcohols from feedstock chemicals with excellent chemo-, regio-, diastereo-, and enantioselectivities in a single step. Experimental studies and density functional theory (DFT) calculations reveal that the kinetically favored formation of an allylchromium(III) species is paramount for enforcing the delayed radical-polar crossover over direct radical addition. Finally, straightforward transformations and applications of the homoallylic alcohol products were demonstrated, showcasing the synthetic utility of this method.

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.

Radical Alkylcyanation of 1,6-Enynes with Isonitriles as Bifunctional Reagents

We report a radical cyano-cyclization of 1,6-enynes with isonitriles via photochemically driven nickel catalysis, forging alkenyl nitrile-tethered γ-lactams under mild conditions. This reaction leverages the photoexcitation of in situ generated nickel (isonitrile) species to facilitate isonitriles serving as alkyl radical precursors and cyanide sources. The reaction accommodates a wide range of substrates, exhibiting excellent regioselectivity and Z/E stereoselectivity.

Synthesis of benzosultams via Ag(I)-catalyzed alkylative cyclization of vinyl sulfonamides

A convenient method to access benzo-fused-γ-sultams via alkyl radical induced cyclization of vinyl sulfonamides is presented. A wide range of carboxylic acids including sterically hindered adamantanes participated as alkyl donors in this Ag(I)-catalyzed decarboxylative alkylation. The reaction utilizes readily available starting materials and demonstrates a broad substrate scope.

Mn-Catalyzed Electrooxidative Radical Cascade Cyclization for the Synthesis of 6-Phosphorylated Quinoxalino[2,1-b]quinazolin-12-ones

Due to their important potential medicinal value, chemists are pursuing mild and efficient methods to synthesize structurally diverse quinazolinone derivatives. In this paper, a series of isocyano-tethered N-aryl quinazolinones were designed and synthesized to conduct electrocatalytic radical cascade cyclization reactions with phosphine oxides by utilizing inexpensive MnII salt as the catalyst. The desired 6-phosphorylated quinoxalino[2,1-b]quinazolin-12-ones were obtained in moderate to good yields.

Photo-Driven Regiodivergent Arylation/Cyclization and Arylation/Hydroxylation of N-Aryl Methacrylamides with Aryltriazenes: Access to Functionalized 3,3-Disubstituted Oxindoles and α-Hydroxylamides

A metal-free, light-induced regiodivergent functionalization of α,β-unsaturated amides with aryltriazenes under ambient conditions was developed. The visible light and B(C6F5)3 cocatalyzed radical cascade arylation/cyclization of N-alkyl-N-arylmethacrylamides can obtain functionalized 3,3-disubstituted oxindoles with the assistance of photocatalyst eosin Y-Na2. In the absence of any catalyst, with purple light irradiation and electron-donor-acceptor (EDA) complex initiation, the radical cascade arylation/hydroxylation of N-arylmethacrylamides affords α-hydroxylamides. This methodology highlights the arts in accessing different regioisomers by altering the substrates and photocatalytic strategies.

Visible-Light-Mediated Activation of Remote C(sp3)-H Bonds by Carbon-Centered Biradical via Intramolecular 1,5- or 1,6-Hydrogen Atom Transfer

In this study, we introduce a novel intramolecular hydrogen atom transfer (HAT) reaction that efficiently yields azetidine, oxetane, and indoline derivatives through a mechanism resembling the carbon analogue of the Norrish-Yang reaction. This process is facilitated by excited triplet-state carbon-centered biradicals, enabling the 1,5-HAT reaction by suppressing the critical 1,4-biradical intermediates from undergoing the Norrish Type II cleavage reaction, and pioneering unprecedented 1,6-HAT reactions initiated by excited triplet-state alkenes. We demonstrate the synthetic utility and compatibility of this method across various functional groups, validated through scope evaluation, large-scale synthesis, and derivatization. Our findings are supported by control experiments, deuterium labeling, kinetic studies, cyclic voltammetry, Stern-Volmer experiments, and density functional theory (DFT) calculations.

Visible light-induced chemoselective 1,2-diheteroarylation of alkenes

Visible-light photocatalysis has evolved as a powerful technique to enable controllable radical reactions. Exploring unique photocatalytic mode for obtaining new chemoselectivity and product diversity is of great significance. Herein, we present a photo-induced chemoselective 1,2-diheteroarylation of unactivated alkenes utilizing halopyridines and quinolines. The ring-fused azaarenes serve as not only substrate, but also potential precursors for halogen-atom abstraction for pyridyl radical generation in this photocatalysis. As a complement to metal catalysis, this photo-induced radical process with mild and redox neutral conditions assembles two different heteroaryl groups into alkenes regioselectively and contribute to broad substrates scope. The obtained products containing aza-arene units permit various further diversifications, demonstrating the synthetic utility of this protocol. We anticipate that this protocol will trigger the further advancement of photo-induced alkyl/aryl halides activation.

Dynamic stereomutation of vinylcyclopropanes with metalloradicals

The ever increasing demands for greater sustainability and lower energy usage in chemical processes call for fundamentally new approaches and reactivity principles. In this context, the pronounced prevalence of odd-oxidation states in less precious metals bears untapped potential for fundamentally distinct reactivity modes via metalloradical catalysis1-3. Contrary to the well-established reactivity paradigm that organic free radicals, upon addition to a vinylcyclopropane, lead to rapid ring opening under strain release-a transformation that serves widely as a mechanistic probe (radical clock)4 for the intermediacy of radicals5-we herein show that a metal-based radical, that is, a Ni(I) metalloradical, triggers reversible cis/trans isomerization instead of opening. The isomerization proceeds under chiral inversion and, depending on the substitution pattern, occurs at room temperature in less than 5 min, requiring solely the addition of the non-precious catalyst. Our combined computational and experimental mechanistic studies support metalloradical catalysis as origin of this profound reactivity, rationalize the observed stereoinversion and reveal key reactivity features of the process, including its reversibility. These insights enabled the iterative thermodynamic enrichment of enantiopure cis/trans mixtures towards a single diastereomer through multiple Ni(I) catalysis rounds and also extensions to divinylcyclopropanes, which constitute strategic motifs in natural product- and total syntheses6. While the trans-isomer usually requires heating at approximately 200 °C to trigger thermal isomerization under racemization to cis-divinylcyclopropane, which then undergoes facile Cope-type rearrangement, the analogous contra-thermodynamic process is herein shown to proceed under Ni(I) metalloradical catalysis under mild conditions without any loss of stereochemical integrity, enabling a mild and stereochemically pure access to seven-membered rings, fused ring systems and spirocycles.

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.

Boryl Radical as a Catalyst in Enabling Intra- and Intermolecular Cascade Radical Cyclization Reactions: Construction of Polycyclic Molecules

Cascade radical cyclization constitutes an atom- and step-economic route for rapid assembly of polycyclic molecular skeletons. Although an array of redox-active metal catalysts has recently shown robust applications in enabling various catalytic cascade radical processes, the use of free organic radical as the catalyst, which is capable of triggering strategically distinct cascades, has rarely been developed. Here, we disclosed that the benzimidazolium-based N-heterocyclic carbene (NHC)-boryl radical is capable of catalyzing cascade cyclization reactions in both intra- and intermolecular pathways, assembling [5,5] fused bicyclic and [6,6,6] fused tricyclic molecules, respectively. The catalytic reactions start with the chemo- and regioselective addition of the boryl radical catalyst to a tethered alkene or alkyne moiety, followed by either an intramolecular formal [3+2] or an intermolecular [2+2+2] cycloaddition process to construct bicyclo[3.3.0]octane or tetrahydrophenanthridine skeletons, respectively. Eventually, a β-elimination occurs to release the boryl radical catalyst, completing a catalytic cycle. High to excellent diastereoselectivity is achieved in both catalytic reactions under substrate control.

Total Synthesis of Strigolactones via Palladium-Catalyzed Cascade Carbonylative Carbocyclization of Enallenes

Here we report an efficient route for synthesizing strigolactones (SLs) and their derivatives. Our method relies on a palladium-catalyzed oxidative carbonylation/carbocyclization/carbonylation/alkoxylation cascade reaction, which involves the formation of three new C-C bonds and a new C-O bond while cleaving one C(sp3)-H bond in a single step. With our versatile synthetic strategy, both naturally occurring and artificial SLs were prepared.

Visible-Light-Promoted Radical Cascade Cyclization of 2-Vinyl Benzimidazoles: Access to Benzo[4,5]imidazo[1,2-b]isoquinolin- 11(6H)-ones

A visible-light-enabled photoredox radical cascade cyclization of 2-vinyl benzimidazole derivatives is developed. This chemistry is applicable to a wide range of N-aroyl 2-vinyl benzimidazoles as acceptors, and halo compounds, including alkyl halides, acyl chlorides and sulfonyl chlorides, as radical precursors. The Langlois reagent also serves as an effective partner in this photocatalytic oxidative cascade process. This protocol provides a robust alternative for rendering highly functionalized benzo[4,5]imidazo[1,2-b]isoquinolin-11(6H)-ones.

Cascade ring expansion reactions for the synthesis of medium-sized rings and macrocycles

This Feature Article discusses recent advances in the development of cascade ring expansion reactions for the synthesis of medium-sized rings and macrocycles. Cascade ring expansion reactions have much potential for use in the synthesis of biologically important medium-sized rings and macrocycles, most notably as they don't require high dilution conditions, which are commonly used in established end-to-end macrocyclisation methods. Operation by cascade ring expansion method can allow large ring products to be accessed via rearrangements that proceed exclusively by normal-sized ring cyclisation steps. Ensuring that there is adequate thermodynamic driving force for ring expansion is a key challenge when designing such methods, especially for the expansion of normal-sized rings into medium-sized rings. This Article is predominantly focused on methods developed in our own laboratory, with selected works by other groups also discussed. Thermodynamic considerations, mechanism, reaction design, route planning and future perspective for this field are all covered.

Copper-Catalyzed Radical Relay 1,3-Carbocarbonylation across Two Distinct C═C Bonds

The radical relay provides an effective paradigm for intermolecular assembly to achieve functionalization across remote chemical bonds. Herein, we report the first radical relay 1,3-carbocarbonylation of α-carbonyl alkyl bromides across two separate C═C bonds. The reaction is highly chemo- and regioselective, with two C(sp3)-C(sp3) bonds and one C═O bond formed in a single orchestrated operation. In addition, the synthesis method under mild conditions and using inexpensive copper as the catalyst allows facile access to structurally diverse 1,3-carbocarbonylation products. The plausible mechanism is investigated through a series of control experiments, including radical trapping, radical clock experiments, critical intermediate trapping, and 18O labeling experiment.

Radical Addition-Enabled C-C σ-Bond Cleavage/Reconstruction to Access Functional Indanones: Total Synthesis of Carexane L

C-C σ-bond cleavage and reconstruction is a significant tool for structural modification in synthetic chemistry but it remains a formidable challenge to perform on unstrained skeletons. Herein, we describe a radical addition-enabled C-C σ-bond cleavage/reconstruction reaction of unstrained allyl ketones to access various functional indanones bearing a benzylic quaternary center. The synthetic utility of this method has been showcased by the first total synthesis of carexane L, an indanone-based natural product.

Access Polyarylbipyrazoles via Palladium-Catalysis and Visible-Light-Driven C(sp3)-P(V) Cleavage Relay Strategy

An unprecedented palladium-catalyzed and visible-light-driven relay reaction of allenylphosphine oxide with in situ generated nitrile imines is presented for the direct synthesis of highly valuable polyarylbipyrazole skeletons. This one-pot strategy involves double 1,3-dipolar cycloaddition and C(sp3)-P(V) bond cleavage under photocatalyst-free and mild reaction conditions. The approach features simple operation, a high step economy, and a broad substrate scope, affording the corresponding products in moderate to excellent yields.

Ru(II)-Catalyzed Divergent C-H Alkynylation Cascade with Bifunctional α-Alcohol Haloalkynes

Native functionality directed the C-H activation cascade to enable rapid construction of molecular complexity, featuring step-economy and synthetic efficiency. Herein, by exploiting bifunctional α-alcohol haloalkynes, we developed Ru(II)-catalyzed carboxylic acid, amine, and amide assisted divergent C-H alkynylation and annulation cascade, affording polyfunctional heterocycles. Significantly, a bilateral aryl C-H polycyclization cascade of azobenzenes was achieved using the versatile haloalkynes.

Copper-Catalyzed, Interrupted Remote Fluoromethylthiolation of Unactivated C(sp3)-H Bonds

An efficient copper-catalyzed selective fluoromethylthiolation of an inert δ-C(sp3)-H bond in sulfonamides was reported. In the presence of a copper catalyst and PhSO2SRf, the radical generated through 1,5-hydrogen atom transfer (HAT) was sufficiently trapped by PhSO2SRf, instead of copper, which was prevalent in metal-catalyzed radical-relay processes, incorporating a fluoromethylthio group into molecules. The general substrate scope and mild conditions endowed the method with wide potential applications in pharmaceuticals and agrochemicals.

Bioinspired Divergent Synthesis of Aspersteroids A and B

Aspersteroids A and B are novel ergostane-type 18,22-cyclosterols with immunosuppressive and antimicrobial activities. Herein, we report the first synthesis of these two natural products, which was accomplished in 15 and 14 steps, respectively, from commercially available ergosterol by means of a bioinspired divergent approach. Key features of this synthesis include an unprecedented radical relay cyclization that was initiated by iron(II)-mediated decomposition of an alkyl hydroperoxide to construct the E ring cyclopentane motif; a titanium(III)-mediated diastereoselective radical reduction of an epoxide to install the challenging C22 stereocenter; and highly regioselective, divergent late-stage oxidations to access the highly oxidized core framework.

Selectivity Control by Relay Catalysis in CO and CO2 Hydrogenation to Multicarbon Compounds

ConspectusThe hydrogenative conversion of both CO and CO2 into high-value multicarbon (C2+) compounds, such as olefins, aromatic hydrocarbons, ethanol, and liquid fuels, has attracted much recent attention. The hydrogenation of CO is related to the chemical utilization of various carbon resources including shale gas, biomass, coal, and carbon-containing wastes via syngas (a mixture of H2 and CO), while the hydrogenation of CO2 by green H2 to chemicals and liquid fuels would contribute to recycling CO2 for carbon neutrality. The state-of-the-art technologies for the hydrogenation of CO/CO2 to C2+ compounds primarily rely on a direct route via Fischer-Tropsch (FT) synthesis and an indirect route via two methanol-mediated processes, i.e., methanol synthesis from CO/CO2 and methanol to C2+ compounds. The direct route would be more energy- and cost-efficient owing to the reduced operation units, but the product selectivity of the direct route via FT synthesis is limited by the Anderson-Schulz-Flory (ASF) distribution. Selectivity control for the direct hydrogenation of CO/CO2 to a high-value C2+ compound is one of the most challenging goals in the field of C1 chemistry, i.e., chemistry for the transformation of one-carbon (C1) molecules.We have developed a relay-catalysis strategy to solve the selectivity challenge arising from the complicated reaction network in the hydrogenation of CO/CO2 to C2+ compounds involving multiple intermediates and reaction channels, which inevitably lead to side reactions and byproducts over a conventional heterogeneous catalyst. The core of relay catalysis is to design a single tandem-reaction channel, which can direct the reaction to the target product controllably, by choosing appropriate intermediates (or intermediate products) and reaction steps connecting these intermediates, and arranging optimized yet matched catalysts to implement these steps like a relay. This Account showcases representative relay-catalysis systems developed by our group in the past decade for the synthesis of liquid fuels, lower (C2-C4) olefins, aromatics, and C2+ oxygenates from CO/CO2 with selectivity breaking the limitation of conventional catalysts. These relay systems are typically composed of a metal or metal oxide for CO/CO2/H2 activation and a zeolite for C-C coupling or reconstruction, as well as a third or even a fourth catalyst component with other functions if necessary. The mechanisms for the activation of H2 and CO/CO2 on metal oxides, which are distinct from that on the conventional transition or noble metal surfaces, are discussed with emphasis on the role of oxygen vacancies. Zeolites catalyze the conversion of intermediates (including hydrocracking/isomerization of heavier hydrocarbons, methanol-to-hydrocarbon reactions, and carbonylation of methanol/dimethyl ether) in the relay system, and the selectivity is mainly controlled by the Brønsted acidity and the shape-selectivity or the confinement effect of zeolites. We demonstrate that the thermodynamic/kinetic matching of the relay steps, the proximity and spatial arrangement of the catalyst components, and the transportation of intermediates/products in sequence are the key issues guiding the selection of each catalyst component and the construction of an efficient relay-catalysis system. Our methodology would also be useful for the transformation of other C1 molecules via controlled C-C coupling, inspiring more efforts toward precision catalysis.

Photocatalytic Cyclization Cascades by Radical Relay toward Pyrrolo[1,2-a]indoles: Synthesis, Mechanism, and Application

A photocatalytic annulation cascade of unactivated N-alkene-linked indoles with Langlois' reagent by a radical relay is developed at room temperature under blue LED irradiation. The reaction afforded a series of tri/difluoromethylated pyrrolo[1,2-a]indoles in moderate to good yields. The DFT study suggests that the reaction is ascribed to a rhodamine 6G-induced cyclization cascade involving vinyl addition-radical relay and hydrogen-atom-abstraction (HAA) processes, and interestingly, pyrrolo[1,2-a]indoles are applied as fluorescent dyes into the fluorescence spectrum and live-cell imaging. This paper represents an initial example on photocatalytic cyclization cascades by radical relay and the HAA process.

Divergent Cascade Ring-Expansion Reactions of Acryloyl Imides

Macrocyclic and medium-sized ring ketones, lactones and lactams can all be made from common acryloyl imide starting materials through divergent, one-pot cascade ring-expansion reactions. Following either conjugate addition with an amine or nitromethane, or osmium(VIII)-catalysed dihydoxylation, rearrangement through a four-atom ring expansion takes place spontaneously to form the ring expanded products. A second ring expansion can also be performed following a second iteration of imide formation and alkene functionalisation/ring expansion. In the dihydroxylation series, three- or four-atom ring expansion can be performed selectively, depending on whether the reaction is under kinetic or thermodynamic control.

Carbonyl Olefin Metathesis and Dehydrogenative Cyclization of Aromatic Ketones and gem-Difluoroalkenes

The beauty of one-pot cascade reaction lies in the efficient disconnection and construction of several bonds in a single reaction flask, without the isolation of any intermediates. Herein, we report the first photoinduced thermally promoted cascade reactions of readily available aromatic ketones and aromatic gem-difluoroalkenes for the synthesis of phenanthrenes which possess potential utility in drug design and materials science. The reaction combines carbonyl-olefin metathesis (cascade photoinduced [2+2] cyclization and thermally controlled retro [2+2] cyclization) and dehydrogenative cyclization (cascade photoinduced conrotatory 6π electrocyclization and collidine-promoted dehydrogenative aromatization) together in one pot. The oxidant-free, acid-free and metal-free reaction shows broad substrate scope and wide functional group tolerance.

P/N-heteroleptic Cu(I)-photosensitizer-catalyzed domino radical relay annulation of 1,6-enynes with aryldiazonium salts

A visible-light driven photocatalytic construction of benzo[b]fluorenones from 1,6-enynes and aryldiazonium salts has been achieved via a P/N-heteroleptic Cu(I)-photosensitizer-catalyzed domino radical relay annulation process. Preliminary mechanistic studies revealed that the aryl radicals in situ generated from aryldiazonium salts with the excited state of the Cu(I)-photosensitizer played a dual role of a radical initiator and a radical terminator in the concise construction of the highly fused benzo[b]fluorenone scaffold.

Opportunities and Challenges of Metal-Organic Framework Micro/Nano Reactors for Cascade Reactions

Building bridges among different types of catalysts to construct cascades is a highly worthwhile pursuit, such as chemo-, bio-, and chemo-bio cascade reactions. Cascade reactions can improve the reaction efficiency and selectivity while reducing steps of separation and purification, thereby promoting the development of "green chemistry". However, compatibility issues in cascade reactions pose significant constraints on the development of this field, particularly concerning the compatibility of diverse catalyst types, reaction conditions, and reaction rates. Metal-organic framework micro/nano reactors (MOF-MNRs) are porous crystalline materials formed by the self-assembly coordination of metal sites and organic ligands, possessing a periodic network structure. Due to the uniform pore size with the capability of controlling selective transfer of substances as well as protecting active substances and the organic-inorganic parts providing reactive microenvironment, MOF-MNRs have attracted significant attention in cascade reactions in recent years. In this Perspective, we first discuss how to address compatibility issues in cascade reactions using MOF-MNRs, including structural design and synthetic strategies. Then we summarize the research progress on MOF-MNRs in various cascade reactions. Finally, we analyze the challenges facing MOF-MNRs and potential breakthrough directions and opportunities for the future.

Approach to Access Benzo[j]phenanthridinones from 1,7-Enynes and Aryldiazonium Salts via a Domino Radical Relay Process Enabled by a P/N-Heteroleptic Cu(I)-Photosensitizer

A mild approach for the synthesis of benzo[j]phenanthridin-6(5H)-one derivatives from 1,7-enynes and aryldiazonium salts has been successfully developed involving a domino radical relay process enabled by a heteroleptic Cu(I)-photosensitizer under visible-light-driven photocatalytic conditions. Mechanistic studies disclosed that the oxidative quenching of the excited state of PS 4 with aryldiazonium salts via an SET process generated aryl radicals, which could play a radical initiator-terminator dual role within the whole radical relay process, namely, at the initial step acting as a radical donor to trigger the radical addition to the olefin moieties of 1,7-enynes while at the final stage serving as a radical acceptor to complete the cyclization.

Enantioselective catalytic radical decarbonylative azidation and cyanation of aldehydes

Empowered by the ubiquity of carbonyl functional groups in organic compounds, decarbonylative functionalization was prevalent in the construction of complex molecules. Under this context, asymmetric decarbonylative functionalization has emerged as an efficient pathway to accessing chiral motifs. However, ablation of enantiomeric control in a conventional 2e transition metal-catalyzed process was notable because of harsh conditions (high temperatures, etc.) that are usually required. To address this challenge and use readily accessible aldehyde directly, we report the asymmetric radical decarbonylative azidation and cyanation. Diverse aldehydes were directly used as alkyl radical precursor, engaging in the subsequent inner-sphere or outer-sphere ligand transfer where functional motifs (CN and N3) could be incorporated in excellent site- and enantioselectivity. Mild conditions, broad scope, excellent regioselectivity (driven by polarity-matching strategy), and enantioselectivity were shown for both transformations. This radical decarbonylative strategy using aldehydes as alkyl radical precursor has offered a powerful reaction manifold in asymmetric radical transformations to construct functional motifs regio- and stereoselectively.

Rhodium-Catalyzed Selective Nucleophilic Cyclization/Cross-Coupling of Two ortho-Alkynylanilines Bearing Differential N-Substituents

Herein, we reported an effective selective nucleophilic cyclization/cross-coupling cascade reaction of N-tosyl ortho-alkynylanilines and N-acyl ortho-alkynylanilines using Rh(COD)2BF4/tBuXantPhos as a catalyst. The present protocol features excellent chemo- and regioselectivity, high atom-economy, and a broad range of substrates. The mechanism studies indicated that the key to the success of this reaction is the powerful capacity of the rhodium catalyst to recognize the N-substituent group in the selective nucleophilic cyclization and selective alkyne insertion.

ipso-Cyclization of unactivated biaryl ynones leading to thio-functionalized spirocyclic enones

Ceric ammonium nitrate (CAN)-promoted oxidative ipso-cyclization of unactivated biaryl ynones with S-centered radicals (SCN/SCF3) to access spiro[5,5]trienones has been established. This approach displayed excellent regioselectivity towards spirocyclization and tolerated a variety of functional groups. Dearomatization of hitherto unknown aryl/heteroaryl groups is also disclosed. DMSO is employed as a low-toxicity, inexpensive solvent as well as a source of oxygen.

Enantioselective radical cascade cyclization via Ti-catalyzed redox relay

Radical cascade cyclization reactions provide an efficient method for the construction of polycyclic architectures with multiple stereogenic centers. However, achieving enantioselectivity control of this type of reaction is a challenging task. Here, we report an enantioselective cyclization of polyfunctional aryl cyclopropyl ketone and alkyne units, wherein the stereochemical outcome is directed by a chiral Ti(salen) catalyst. This transformation was proposed to proceed via a radical cascade process involving the reductive ring-opening of the cyclopropyl ketone followed by two annulation events entailing cyclization of the ensuing alkyl radical onto the alkyne and subsequent addition of the incipient vinyl radical to the Ti(IV)-enolate.

Asymmetric Radical Bicyclization for Stereoselective Construction of Tricyclic Chromanones and Chromanes with Fused Cyclopropanes

Asymmetric radical bicyclization processes have been developed via metalloradical catalysis (MRC) to stereoselectively construct chiral chromanones and chromanes bearing fused cyclopropanes. Through optimization of a versatile D2-symmetric chiral amidoporphyrin ligand platform, a Co(II)-metalloradical system can homolytically activate both diazomalonates and α-aryldiazomethanes containing different alkene functionalities under mild conditions for effective radical bicyclization, delivering cyclopropane-fused tricyclic chromanones and chromanes, respectively, in high yields with excellent control of both diastereoselectivities and enantioselectivities. Combined computational and experimental studies, including the electron paramagnetic resonance (EPR) detection and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) trapping of key radical intermediates, shed light on the working details of the underlying stepwise radical mechanisms of the Co(II)-catalyzed bicyclization processes. The two catalytic radical processes provide effective synthetic tools for stereoselective construction of valuable cyclopropane-fused chromanones and chromanes with newly generated contiguous stereogenic centers. As a specific demonstration of synthetic application, the Co(II)-catalyzed radical bicyclization has been employed as a key step for the first asymmetric total synthesis of the natural product (+)-Radulanin J.

Radical bicyclization of 1,6-enynes with sulfonyl hydrazides by the use of TBAI/TBHP in the aqueous phase

A novel 5-exo-dig/6-endo-trig bicyclization of 1,6-enynes with sulfonyl hydrazides in the aqueous phase using the cheap and available tetrabutylammonium iodide (TBAI)-tert-butyl hydroperoxide (TBHP) combined system is reported. The resulting reaction of diverse nitrogen- and oxygen-polyheterocycles displays high chemical selectivity, high step-economy, and a moderate substrate scope. Moreover, iodosulfonylation can be realized by modulating the structure of the 1,6-enynes.

Combined Catalysis: A Powerful Strategy for Engineering Multifunctional Sustainable Lignin-Based Materials

The production and engineering of sustainable materials through green chemistry will have a major role in our mission of transitioning to a more sustainable society. Here, combined catalysis, which is the integration of two or more catalytic cycles or activation modes, provides innovative chemical reactions and material properties efficiently, whereas the single catalytic cycle or activation mode alone fails in promoting a successful reaction. Polyphenolic lignin with its distinctive structural functions acts as an important template to create materials with versatile properties, such as being tough, antimicrobial, self-healing, adhesive, and environmentally adaptable. Sustainable lignin-based materials are generated by merging the catalytic cycle of the quinone-catechol redox reaction with free radical polymerization or oxidative decarboxylation reaction, which explores a wide range of metallic nanoparticles and metal ions as the catalysts. In this review, we present the recent work on engineering lignin-based multifunctional materials devised through combined catalysis. Despite the fruitful employment of this concept to material design and the fact that engineering has provided multifaceted materials able to solve a broad spectrum of challenges, we envision further exploration and expansion of this important concept in material science beyond the catalytic processes mentioned above. This could be accomplished by taking inspiration from organic synthesis where this concept has been successfully developed and implemented.

Rhodium(II)-Catalyzed Desaturative [3+2] Tandem Cyclization of Arylcycloalkanes with β-Dicarbonyls

Synthetically important scaffolds, fused tricyclic frameworks containing a 2,3-cyclo[b]dihydrofuran unit, play a crucial role in drug discovery. In this study, we demonstrate that rhodium(II)/N-fluorobenzenesulfonimide can catalyze the in situ generation of highly reactive alkene intermediates from commonly accessible alkanes, which undergo intermolecular [3+2] tandem cyclization with the simultaneously generated β-dicarbonyl radical to synthesize a series of fused tricyclic frameworks containing a 2,3-cyclo[b]dihydrofuran unit with a quaternary carbon center.

Chitosan bead containing metal-organic framework encapsulated heteropolyacid as an efficient catalyst for cascade condensation reaction

Using cyclodextrin and chitosan that are bio-based compounds, a novel bi-functional catalytic composite is designed, in which metal-organic framework encapsulated phosphomolybdic acid was incorporated in a dual chitosan-cyclodextrin nanosponge bead. The composite was characterized via XRD, TGA, ICP, BET, NH₃-TPD, FTIR, FE-SEM/EDS, elemental mapping analysis and its catalytic activity was examined in alcohol oxidation and cascade alcohol oxidation-Knoevenagel condensation reaction. It was found that the designed catalyst that possess both acidic feature and redox potential could promote both reactions in aqueous media at 55 °C and various substrates with different electronic features could tolerate the aforementioned reactions to furnish the products in 75-95% yield. Furthermore, the catalyst could be readily recovered and recycled for five runs with slight loss of the catalytic activity. Notably, in this composite the synergism between the components led to high catalytic activity, which was superior to each component. In fact, the amino groups on the chitosan served as catalysts, while cyclodextrin nanosponge mainly acted as a phase transfer agent. Moreover, measurement of phosphomolybdic acid leaching showed that its incorporation in metal-organic framework and bead structure could suppress its leaching, which is considered a drawback for this compound. Other merits of this bi-functional catalyst were its simplicity, use of bio-based compounds and true catalysis, which was proved via hot filtration.

A catalytic alkene insertion approach to bicyclo[2.1.1]hexane bioisosteres

C(sp³)-rich bicyclic hydrocarbon scaffolds, as exemplified by bicyclo[1.1.1]pentanes, play an increasingly high-profile role as saturated bioisosteres of benzenoids in medicinal chemistry and crop science. Substituted bicyclo[2.1.1]hexanes (BCHs) are emerging bicyclic hydrocarbon bioisosteres for ortho- and meta-substituted benzenes, but are difficult to access. Therefore, a general synthetic route to BCHs is needed if their potential as bioisosteres is to be realized. Here we describe a broadly applicable catalytic approach that delivers substituted BCHs by intermolecular coupling between olefins and bicyclo[1.1.0]butyl (BCB) ketones. The SmI₂-catalysed process works for a wide range of electron-deficient alkenes and substituted BCB ketones, operates with SmI₂ loadings as low as 5 mol% and is underpinned by a radical relay mechanism that is supported by density functional theory calculations. The product BCH ketones have been shown to be versatile synthetic intermediates through selective downstream manipulation and the expedient synthesis of a saturated hydrocarbon analogue of the broad-spectrum antimicrobial, phthalylsulfathiazole.

Formal Cycloadditions Driven by the Homolytic Opening of Strained, Saturated Ring Systems

The field of strain-driven, radical formal cycloadditions is experiencing a surge in activity motivated by a renaissance in free radical chemistry and growing demand for sp³ -rich ring systems. The former has been driven in large part by the rise of photoredox catalysis, and the latter by adoption of the "Escape from Flatland" concept in medicinal chemistry. In the years since these broader trends emerged, dozens of formal cycloadditions, including catalytic, asymmetric variants, have been developed that operate via radical mechanisms. While cyclopropanes have been studied most extensively, a variety of strained ring systems are amenable to the design of analogous reactions. Many of these processes generate lucrative, functionally decorated sp³ -rich ring systems that are difficult to access by other means. Herein, we summarize recent efforts in this area and analyze the state of the field.

Nitrosoarenes Implement Cascade Cyclization of 1-Allenyl-2-alkynylbenzenes through Diradical Mechanism

This work reports cascade cyclization between 1-allenyl-2-alkynylbenzenes and nitrosoarenes. When these two components reacted alone under N₂, N,O-functionalized indane-fused isoxazolidines 3 were obtained selectively. DFT calculations verify that this reaction sequence involves unprecedented nitrone/alkyne cycloadditions, followed by diradical rearrangement.

A Diastereoselective Cascade Annulation Approach to Bridged Polycyclic Heterocycles Involving an Unexpected Rearrangement

A diastereoselective cascade annulation between allenoates and in-situ generated isoquinoline N-oxides generating sp³-rich bridged polycyclic heterocycles is disclosed. The reaction proceeds through an unprecedented non-rearomatized rearrangement and allows access to a broad range of bridged heterocycles in 38-93% yields with excellent functional group tolerance and high diastereoselectivity. Density functional theory calculations provided key insights into the possible reaction pathway and the stereoselectivity of this procedure.

Photochemical Intermolecular Cyclopropanation Reactions of Allylic Alcohols for the Synthesis of [3.1.0]-Bicyclohexanes

Cyclopropane-fused lactones are highly desirable in drug and natural products synthesis. Herein, we report on a photochemical, chemoselective reaction of aryldiazoacetates with allylic alcohols that furnishes cyclopropane-fused lactone skeletons efficiently in one step. The diastereoselectivity of the protocol was precisely controlled, and chemoselective cyclopropanation of allylic alcohols via free carbene intermediate followed by transesterification constitutes a series of bicyclic lactones in high yield without the formation of ether byproducts via typical O-H insertion reactions.