Photocatalytic Synthesis of Oxetane Derivatives by Selective CH Activation

Catalytic Asymmetric Hydrogen Atom Transfer Based on a Chiral Hydrogen Atom Donor Generated from TBADT and Chiral BINOL

Enantioselective radical reactions mediated by TBADT have seldom been seen due to the inherent challenges. Herein, we disclose a new chiral hydrogen atom transfer (HAT) reagent that was generated easily from 8H-BINOL, potassium carbonate, and TBADT under irradiation. The new complex 8H-BINOL/DTs could be used as a chiral H donor. A series of azaarenes could be converted into the corresponding chiral compounds via radical addition followed by enantioselective HAT.

Nickel/Photo-Cocatalyzed Three-Component Alkyl-Acylation of Aryl-Activated Alkenes

Herein, we disclose a nickel/photo-cocatalyzed three-component alkyl-acylation of aryl-substituted alkenes with aldehydes and electron-withdrawing-group-activated alkyl bromides, providing straightforward access to various ketones under mild and ligand-free conditions. The photocatalyst TBADT plays a dual role in activating the acyl C-H bond of aldehydes via hydrogen atom transfer and reducing the C-Br bond of alkyl bromides via single-electron transfer. While the terminal C-C bond is forged through polarity-matched radical-type addition, nickel is likely involved in the acylation step.

Asymmetric imino-acylation of alkenes enabled by HAT-photo/nickel cocatalysis

By merging nickel-mediated facially selective aza-Heck cyclization and radical acyl C-H activation promoted by tetrabutylammonium decatungstate (TBADT) as a hydrogen atom transfer (HAT) photocatalyst, we accomplish an asymmetric imino-acylation of oxime ester-tethered alkenes with readily available aldehydes as the acyl source, enabling the synthesis of highly enantioenriched pyrrolines bearing an acyl-substituted stereogenic center under mild conditions. Preliminary mechanistic studies support a Ni(i)/Ni(ii)/Ni(iii) catalytic sequence involving the intramolecular migratory insertion of a tethered olefinic unit into the Ni(iii)-N bond as the enantiodiscriminating step.

Visible Light Photoredox-Catalyzed Decarboxylative Alkylation of 3-Aryl-Oxetanes and Azetidines via Benzylic Tertiary Radicals and Implications of Benzylic Radical Stability

Four-membered heterocycles offer exciting potential as small polar motifs in medicinal chemistry but require further methods for incorporation. Photoredox catalysis is a powerful method for the mild generation of alkyl radicals for C-C bond formation. The effect of ring strain on radical reactivity is not well understood, with no studies that address this question systematically. Examples of reactions that involve benzylic radicals are rare, and their reactivity is challenging to harness. This work develops a radical functionalization of benzylic oxetanes and azetidines using visible light photoredox catalysis to prepare 3-aryl-3-alkyl substituted derivatives and assesses the influence of ring strain and heterosubstitution on the reactivity of small-ring radicals. 3-Aryl-3-carboxylic acid oxetanes and azetidines are suitable precursors to tertiary benzylic oxetane/azetidine radicals which undergo conjugate addition into activated alkenes. We compare the reactivity of oxetane radicals to other benzylic systems. Computational studies indicate that Giese additions of unstrained benzylic radicals into acrylates are reversible and result in low yields and radical dimerization. Benzylic radicals as part of a strained ring, however, are less stable and more π-delocalized, decreasing dimer and increasing Giese product formation. Oxetanes show high product yields due to ring strain and Bent's rule rendering the Giese addition irreversible.

Electrochemical C(sp3)-H functionalization of ethers via hydrogen-atom transfer by means of cathodic reduction

The chemo- and stereoselective electrochemical allylation/alkylation of ethers is presented via a C(sp³)-H activation event. The electrosynthetic protocol enables the realization of a large library of functionalized ethers (35 examples) in high yields (up to 84%) via cathodic activation of a new type of redox-active carbonate (RAC), capable of triggering HAT (Hydrogen-Atom-Transfer) events through the generation of electrophilic oxy radicals. The process displayed high functional group tolerance and mild reaction conditions. A mechanistic elucidation via voltammetric analysis completes the study.

Oxygen- and Sulphur-Containing Heterocyclic Compounds as Potential Anticancer Agents

Oxygen- and sulphur-based heterocycles form the core structure of many biologically active molecules as well as U.S. FDA-approved drugs. Moreover, they possess broad range of biological activities, viz. anticancer, antiinflammatory, antioxidant, antitumour, antibacterial, antiviral, antidiabetic, anticonvulsant, anti-tubercular, analgesic, anti-leishmanial, antimalarial, antifungal, and anti-histaminic, Hence, O- and S-based heterocycles are gaining more attention in recent years on the road to the discovery of innovative anticancer drugs after the extensive investigation of nitrogen-based heterocycles as anticancer agents. Several attempts have been made to synthesize fused oxygen- and sulphur-based heterocyclic derivatives as joining one heterocyclic moiety with another may lead to improvement in the biological profile of a molecule. Humans have been cursed with cancer since long time. Despite the development of several heterocyclic anticancer medications such as 5-fluorouracil, doxorubicin, methotrexate, and daunorubicin, cure of cancer is difficult. Hence, researchers are trying to synthesize new fused/spiro heterocyclic molecules to discover novel anticancer drugs which may show promising anticancer effects with fewer side effects. Furthermore, fused heterocycles behave as DNA intercalating agents which have the ability to interact with DNA, leading to cell death thereby exerting anticancer effect. This review article highlights the synthesis and anticancer potentiality of oxygen- and sulphur-containing heterocyclic compounds covering the period from 2011 to 2021.

CF3SO2Na-Mediated Visible-Light-Induced Cross-Dehydrogenative Coupling of Heteroarenes with Aliphatic C(sp3)-H Bonds

Minisci-type reaction is one of the important means to construct C(sp³)-H functionalization of heteroarenes. According to traditional methods, stoichiometric amounts of precious transition metal catalysts and chemical oxidants were required at high temperatures. Here, a green and gentle novel Minisci-type method was developed via visible-light-induced cross-dehydrogenative coupling of heteroarenes with aliphatic C(sp³)-H bonds under oxidant-free and transition-metal-catalyst-free conditions. Only the catalytic equivalent of CF₃SO₂Na and room temperature were required to maintain an efficient reaction.

Bioinspired Cobalt-Catalysis Enables Generation of Nucleophilic Radicals from Oxetanes

Oxetanes are valuable building blocks due to their well-explored propensity to undergo ring-opening reactions with nucleophiles. However, their application as precursors of radical species is still elusive. Herein, we present a bioinspired cobalt-catalysis-based strategy to access unprecedented modes of radical reactivity via oxetane ring-opening. This powerful approach gives access to nucleophilic radicals that engage in reactions with SOMOphiles and low-valent transition metals. Importantly, the regioselectivity of these processes complements known methodologies.

Visible Light-Induced Transition Metal Catalysis

In recent years, visible light-induced transition metal catalysis has emerged as a new paradigm in organic photocatalysis, which has led to the discovery of unprecedented transformations as well as the improvement of known reactions. In this subfield of photocatalysis, a transition metal complex serves a double duty by harvesting photon energy and then enabling bond forming/breaking events mostly via a single catalytic cycle, thus contrasting the established dual photocatalysis in which an exogenous photosensitizer is employed. In addition, this approach often synergistically combines catalyst-substrate interaction with photoinduced process, a feature that is uncommon in conventional photoredox chemistry. This Review describes the early development and recent advances of this emerging field.

Theoretical insight of decatungstate photocatalyzed alkylation of N-tosylimine via hydrogen atom transfer and proton-coupled electron transfer

Decatungstate as a photocatalyst can activate various C(sp3)−H bond to successfully construct C(sp3)−C(sp2) bond with N-tosylimines. Herein density functional theory (DFT) calculations reveal the unique radical mechanism triggered by the...

Direct Photocatalyzed Hydrogen Atom Transfer (HAT) for Aliphatic C-H Bonds Elaboration

Direct photocatalyzed hydrogen atom transfer (d-HAT) can be considered a method of choice for the elaboration of aliphatic C–H bonds. In this manifold, a photocatalyst (PC_HAT) exploits the energy of a photon to trigger the homolytic cleavage of such bonds in organic compounds. Selective C–H bond elaboration may be achieved by a judicious choice of the hydrogen abstractor (key parameters are the electronic character and the molecular structure), as well as reaction additives. Different are the classes of PCs_HAT available, including aromatic ketones, xanthene dyes (Eosin Y), polyoxometalates, uranyl salts, a metal-oxo porphyrin and a tris(amino)cyclopropenium radical dication. The processes (mainly C–C bond formation) are in most cases carried out under mild conditions with the help of visible light. The aim of this review is to offer a comprehensive survey of the synthetic applications of photocatalyzed d-HAT.

Dual Benzophenone/Copper-Photocatalyzed Giese-Type Alkylation of C(sp3 )-H Bonds

Photocatalyzed Giese-type alkylations of C(sp³ )-H bonds are very attractive reactions in the context of atom-economy in C-C bond formation. The main limitation of such reactions is that when using highly polymerizable olefin acceptors, such as unsubstituted acrylates, acrylonitrile, or methyl vinyl ketone, radical polymerization often becomes the dominant or exclusive reaction pathway. Herein, we report that the polymerization of such olefins is strongly limited or suppressed when combining the photocatalytic activity of benzophenone (BP) with a catalytic amount of Cu(OAc)₂ . Under mild and operationally simple conditions, the Giese adducts resulting from the C(sp³ )-H functionalization of amines, alcohols, ethers, and cycloalkanes could be synthesized. Preliminary mechanistic studies have revealed that the reaction does not proceed through a radical chain, but through a dual BP/Cu photocatalytic process, in which both Cu^II and low-valent Cu^I/0 species, generated in situ by reduction by the BP ketyl radical, may react with α-keto or α-cyano intermediate radicals, thus preventing polymerization.

Photocatalytic hydroacylation of trifluoromethyl alkenes

Hydrofunctionalization of trifluoromethyl alkenes is highly challenging, because the nucleophilic addition is oftentimes followed by β-F elimination in this case. By the use of tetrabutylammonium decatungstate (TBADT) as a hydrogen-atom-transfer (HAT) photocatalyst for acyl C-H activation, we successfully avoid the β-F elimination in the hydroacylation of trifluoromethyl alkenes with aldehydes. This light (390 nm) promoted reaction provides a facile and efficient access to various β-CF3 ketones in complete regiocontrol with high functionality tolerance and 100% atom economy.

Visible-light-initiated manganese-catalyzed Giese addition of unactivated alkyl iodides to electron-poor olefins

Visible-light-initiated Giese addition of unactivated alkyl iodides to electron-poor olefins with catalysis by decacarbonyl dimanganese, Mn₂(CO)₁₀ was reported.

Formal Giese addition of C(sp3)-H nucleophiles enabled by visible light mediated Ni catalysis of triplet enone diradicals

An unprecedented utilization of triplet excited enones in Ni-catalysis enabled a formal Giese addition of C(sp3)-H nucleophiles. This mechanism-based approach has greatly widened the reaction scope, allowing the synthesis of previously inaccessible structures. In this process, the enone diradical acted as two distinct reaction centers, participating in both metalation and hydrogen atom transfer, ultimately furnishing a range of formal Giese addition products in a highly general context. This reaction provides complementary access to traditional 1,4-addition reactions of enones, with a future perspective to develop triplet diradical-based transition metal catalysis.

Hydrogen Atom Transfer from Alkanols and Alkanediols to the Cumyloxyl Radical: Kinetic Evaluation of the Contribution of α-C-H Activation and β-C-H Deactivation

A kinetic study on the reactions of the cumyloxyl radical (CumO^•) with a series of alkanols and alkanediols has been carried out. Predominant hydrogen atom transfer (HAT) from the α-C-H bonds of these substrates, activated by the presence of the OH group, is observed. The comparable k_H values measured for ethanol and 1-propanol and the increase in k_H measured upon going from 1,2-diols to structurally related 1,3- and 1,4-diols is indicative of β-C-H deactivation toward HAT to the electrophilic CumO^•, determined by the electron-withdrawing character of the OH group. No analogous deactivation is observed for the corresponding diamines, in agreement with the weaker electron-withdrawing character of the NH₂ group. The significantly lower k_H values measured for reaction of CumO^• with densely oxygenated methyl pyranosides as compared to cyclohexanol derivatives highlights the role of β-C-H deactivation. The contribution of torsional effects on reactivity is evidenced by the ∼2-fold increase in k_H observed upon going from the trans isomers of 4- tert-butylcyclohexanol and 1,2- and 1,4-cyclohexanediol to the corresponding cis isomers. These results provide an evaluation of the role of electronic and torsional effects on HAT reactions from alcohols and diols to CumO^•, uncovering moreover β-C-H deactivation as a relevant contributor in defining site selectivity.

Versatile cross-dehydrogenative coupling of heteroaromatics and hydrogen donors via decatungstate photocatalysis

A facile sunlight-induced derivatization of heteroaromatics via photocatalyzed C-H functionalization in amides, ethers, alkanes and aldehydes is described. Tetrabutylammonium decatungstate (TBADT) was used as the photocatalyst and allowed to carry out the process under mild conditions.

Decatungstate Anion for Photocatalyzed "Window Ledge" Reactions

The majority of organic reactions are commonly carried out inside a lab, under a fume hood. A particular case is that of photochemical reactions, a field where the pioneering experiments by Giacomo Ciamician demonstrated more than one century ago that different processes can be carried out outdoors, for example, on the balcony of his own department, upon exposure of the reacting mixtures to sunlight. The main problem related to this chemistry of the "window ledge" is that most organic compounds are colorless and their absorption in the solar light region is in most cases negligible. Recently, the impressive development in the use of visible light absorbing photocatalysts (e.g., Ru^II or Ir^III complexes, as well as organic dyes) made light-induced processes convenient even for non-photochemistry practitioners. It is thus possible to easily perform the reactions by simply placing the reaction vessel in a sunny place outside the lab. However, most of these processes are based on single electron transfer (SET) reactions (photoredox catalysis). Other photocatalysts able to activate substrates via alternative paths, such as hydrogen atom transfer (HAT), are emerging. In the last years, we were deeply involved in the use of the decatungstate anion ([W₁₀O₃₂]^4-, a polyoxometalate) in synthesis. Indeed, such a versatile species is able to promote the photocatalytic C-H activation of organic compounds via either SET or HAT reactions. Interestingly, though the absorption spectrum of [W₁₀O₃₂]^4- does not extend into the visible region, it shows an overlap with solar light emission. In this Account, we provide an overview on the application of decatungstate salts as photocatalysts in window ledge chemistry. We initially discuss the nature of the photogenerated species involved in the mechanism of action of the anion, also supported by theoretical simulations. The first-formed excited state of the decatungstate anion decays rapidly to the active species, a dark state tagged wO, featuring the presence of electron-deficient oxygen centers. Next, we describe the main applications of decatungstate chemistry. A significant part of this Account is devoted to photocatalyzed synthesis (C-X bond formation, with X = C, N, O, and oxidations) carried out by adopting sunlight (or simulated solar light). This synthetic approach is versatile, and most of the reactions involved C-H activation in cycloalkanes, alkylaromatics, amides, ethers (1,4-dioxane, oxetane, benzodioxole, and THF), aldehydes, nitriles, and cyclopentanones, and the ensuing addition of the resulting radicals onto electron-deficient olefins. Finally, the increasing role of the decatungstate anion in water depollution and polymerization is briefly discussed.

Stereoselective synthesis of 2,3,4-highly substituted oxetanes by intramolecular C–C bond forming Michael addition

A wide variety of 2,3,3,4-tetrasubstituted oxetanes and 3-α,β-unsaturated ester substituted oxetanes were synthesized starting from vinylogous urethane derivatives.

Asymmetric chemoenzymatic synthesis of 1,3-diols and 2,4-disubstituted aryloxetanes by using whole cell biocatalysts

A simple and efficient chemoenzymatic approach for the asymmetric synthesis of 1,3-diols and 2,4-disubstituted aryloxetanes has been established.

Direct α-arylation of ethers through the combination of photoredox-mediated C-H functionalization and the Minisci reaction

The direct α-arylation of cyclic and acyclic ethers with heteroarenes has been accomplished through the design of a photoredox-mediated CH functionalization pathway. Transiently generated α-oxyalkyl radicals, produced from a variety of widely available ethers through hydrogen atom transfer (HAT), were coupled with a range of electron-deficient heteroarenes in a Minisci-type mechanism. This mild, visible-light-driven protocol allows direct access to medicinal pharmacophores of broad utility using feedstock substrates and a commercial photocatalyst.