Visible Light-Induced and Singlet Oxygen-Mediated Photochemical Conversion of 4-Hydroxy-α-benzopyrones to 2-Hydroxy-3-oxo-2,3-dihydrobenzofuran-2-carboxamides/carboxylates Using Rose Bengal as a Photosensitizer

De Novo Synthesis of Acridone-Based Zn-Metal-Organic Framework (Zn-MOF) as a Photocatalyst: Application for Visible Light-Mediated Oxidation of Sulfides and Enaminones

Acridone, a cyclic analogue of benzophenone that undergoes efficient intersystem crossing (ISC) to the triplet-excited state with near-unity quantum yield, was elaborated as a 3-connecting triacid linker, i.e., H3AcTA, to develop a photocatalytic metal-organic framework (MOF) for energy transfer applications; the triacid linker inherently features concave shapes, an attribute that is important for the construction of MOFs with significant porosity. Metal ion (Zn2+)-assisted self-assembly of the triacid yielded a Zn-MOF, i.e., Zn-AcTA, with a solvent-accessible volume of ca. 31%. The protection of the acridone chromophore in the MOF in conjunction with a wider cross-section of its absorption in the visible region renders the MOF an excellent heterogeneous photosensitizer for singlet oxygen (1O2) generation by energy transfer to the ground-state triplet oxygen (3O2). It is shown that the Zn-MOF can be applied as a photosensitizing catalyst for visible light-mediated oxidation of various sulfides to sulfoxides and enaminones to amino-esters via 1,2-acyl migration. It is further demonstrated that the photocatalyst can be easily recycled without any loss of catalytic activity and structural integrity. Based on mechanistic investigations, 1O2 is established as the reactive oxygen species in photocatalytic oxidation reactions. The results constitute the first demonstration of rational development of a photocatalytic MOF based on acridone for heterogeneous oxidations mediated by 1O2.

Mechanochemical Solvent-Free One-Pot Synthesis of Poly-Functionalized 5-(Arylselanyl)-1H-1,2,3-triazoles Through a Copper(I)-Catalyzed Click Reaction

A mechanochemistry-driven practical and efficient synthetic protocol for accessing diverse series of biologically relevant poly-functionalized 5-(arylselanyl)-1H-1,2,3-triazoles through copper(I)-catalyzed click reaction between aryl/heteroaryl acetylenes, diaryl diselenides, benzyl bromides, and sodium azide has been accomplished under high-speed ball-milling. Advantages of this method include operational simplicity, avoidance of using solvent and external heating, one-pot synthesis, short reaction time in minutes, good to excellent yields, broad substrate scope, and gram-scale applications. Furthermore, synthesized organoselenium compounds were synthetically diversified to biologically promising selenones.

C(sp)-C(sp3) Bond Formation through Ligand- and Additive-Free CuO-Mediated Decarboxylative Direct Cross-Coupling of Coumarin-/Chromone-3-carboxylic Acids and Terminal Alkynes

A practical and efficient method for the synthesis of functionalized 4-(aryl-/heteroaryl-ethynyl)chroman-2-ones and 2-(aryl-/heteroaryl-ethynyl)chroman-4-ones through copper-catalyzed decarboxylative direct cross-coupling of coumarin-/chromone-3-carboxylic acids with terminal alkynes, leading to the formation of C(sp)-C(sp3) bonds, has been unearthed. Advantages of this protocol include avoidance of any ligands and bases, a broad substrate scope, tolerance of diverse functional groups, and good to excellent yields.

Electrochemical Regioselective C(sp2)-H Selenylation and Sulfenylation of Substituted 2-Amino-1,4-naphthoquinones

A straightforward and efficient electrochemical method for regioselective C(sp²)-H selenylation and sulfenylation of substituted 2-amino-1,4-naphthoquinones has been unearthed. This oxidative cross-coupling reaction avoids using transition metal catalysts, oxidants, and high temperatures. The other notable advantages of this protocol are the tolerance of diverse functional groups, mild reaction conditions at ambient temperature, energy efficiency, good to excellent yields, short reaction times (in minutes), gram-scale applicability, and eco-friendliness.

Organobase-Catalyzed Umpolung of Amides: The Generation and Transfer of Carbamoyl Anion

A novel organobase-catalyzed umpolung reaction of amides was disclosed. This method provides an efficient method to generate and transfer carbamoyl anions. In this transformation, some of the inherent disadvantages of carbamoyl metal were avoided. The mechanistic analysis revealed that the reaction proceeds through polarity inversion of amide, and various carbamoyl anions were applied in the reaction. Moreover, a wide range of substrates was achieved with moderate to excellent yield.

Catalyst-Controlled Selectivity Switch in Three-Component Reaction: An NHC-Catalyzed Strategy for the Synthesis of δ-Lactone-Fused Spirobenzofuran-3-ones

An efficient, three-component reaction of aldehydes and benzofuran-3-ones was developed. This process provides a new approach for the preparation of synthetically and biologically important spirobenzofuran-3-one derivatives with moderate-to-good yields under mild conditions. A switch of intramolecular to intermolecular domino Michael–aldol–lactonization leading to differential product formation was achieved by different NHCs catalysis.

Sensitized Photooxidation of Ortho-prenyl Phenol: Biomimetic Dihydrobenzofuran Synthesis and Total 1 O2 Quenching

The sensitized photooxidation of ortho-prenyl phenol is described with evidence that solvent aproticity favors the formation of a dihydrobenzofuran [2-(prop-1-en-2-yl)-2,3-dihydrobenzofuran], a moiety commonly found in natural products. Benzene solvent increased the total quenching rate constant (k_T ) of singlet oxygen with prenyl phenol by ~10-fold compared to methanol. A mechanism is proposed with preferential addition of singlet oxygen addition to prenyl site due to hydrogen bonding with phenol OH group, which causes a divergence away from the singlet oxygen 'ene' reaction toward the dihydrobenzofuran as the major product. The reaction is a mixed photooxidized system since an epoxide arises by a type I sensitized photooxidation.

The complex photochemistry of coumarin-3-carboxylic acid in acetonitrile and methanol

Irradiation of coumarin-3-carboxylic acid in acetonitrile and methanol solutions at 355 nm results in complex multistep photochemical transformations, strongly dependent on the solvent properties and oxygen content. A number of reaction intermediates, which themselves undergo further (photo)chemical reactions, were identified by steady-state and transient absorption spectroscopy, mass spectrometry, and NMR and product analyses. The triplet excited compound in acetonitrile undergoes decarboxylation to give a 3-coumarinyl radical that traps molecular oxygen to form 3-hydroxycoumarin as the major but chemically reactive intermediate. This compound is oxygenated by singlet oxygen, produced by coumarin-3-carboxylic acid sensitization, followed by a pyrone ring-opening reaction to give an oxalic acid derivative. The subsequent steps lead to the production of salicylaldehyde, carbon monoxide, and carbon dioxide as the final products. When 3-coumarinyl radical is not trapped by oxygen in degassed acetonitrile, it abstracts hydrogen from the solvent and undergoes triplet-sensitized [2 + 2] cycloaddition. The reaction of 3-coumarinyl radical with oxygen is largely suppressed in aerated methanol as a better H-atom donor, and coumarin is obtained as the primary product in good yields. Because coumarin derivatives are used in many photophysical and photochemical applications, this work provides detailed and sometimes surprising insights into their complex phototransformations.

Visible-Light-Promoted Intramolecular C-O Bond Formation via Csp3-H Functionalization: A Straightforward Synthetic Route to Biorelevant Dihydrofuro[3,2-c]chromenone Derivatives

A photochemical method for the synthesis of functionalized dihydrofuro[3,2-c]chromenones via intramolecular C_sp³-H cross-dehydrogenative oxygenation within a warfarin framework has been unearthed. Advantages of this protocol include abundant sunlight or low-energy visible light as the energy source, mild reaction conditions, and avoidance of metal catalysts.

Photochemical and electrochemical regioselective cross-dehydrogenative C(sp2)–H sulfenylation and selenylation of substituted benzo[a]phenazin-5-ols

The essence of photo- and electrochemistry: sulfenylation and selenylation of substituted benzo[a]phenazin-5-ols through cross-dehydrogenative C(sp2)–H functionalization.

Visible Light-Driven and Singlet Oxygen-Mediated Photochemical Cross-Dehydrogenative C3-H Sulfenylation of 4-Hydroxycoumarins with Thiols Using Rose Bengal as a Photosensitizer

A visible light (white light-emitting diode/direct sunlight)-driven photochemical synthesis of a new series of biologically interesting 3-(alkyl/benzylthio)-4-hydroxy-2H-chromen-2-ones has been achieved through a cross-dehydrogenative C₃-H sulfenylation of 4-hydroxycoumarins with thiols at ambient temperature in the presence of rose bengal in acetonitrile under an oxygen atmosphere. The notable features of this newly developed method are mild reaction conditions, energy efficiency, metal-free synthesis, good to excellent yields, use of low-cost materials, and eco-friendliness.

Light-Triggered Antibacterial Hydrogels Containing Recombinant Growth Factor for Treatment of Bacterial Infections and Improved Wound Healing

Microbial infection and the limitation of tissue regeneration are main obstacles to chronic wound healing. Herein, a biofunctional hydrogel is prepared to simultaneously kill bacteria efficiently and promote would healing. First, a rose bengal/polypyrrole hybrid poly(vinyl alcohol) hydrogel (RB/PPy PVA HD) is synthesized and its antibacterial property is investigated under coirradiation of 550 nm visible light and 808 near-infrared light. The hydrogel exhibits excellent antibacterial activity within 10 min below 45 °C in vitro due to the synergistic effect of photothermal and photodynamic antibacterial therapy. Next, the recombined human epidermal growth factor (rhEGF) is physically absorbed on the surface of the porous hydrogel to form a RB/PPy/rhEGF hybrid PVA HD (rhEGF/RB/PPy PVA HD). The introduction of rhEGF enables the hydrogel to promote fibroblast proliferation and collagen secretion. Furthermore, the in vivo results indicate that the rhEGF/RB/PPy PVA HD can control infection effectively and promote wound healing significantly.

Synthesis of Pyrrolo[2,1,5-cd]indolizine Rings via Visible-Light-Induced Intermolecular [3+2] Cycloaddition of Indolizines and Alkynes

A range of indolizine smoothly underwent visible-light-induced intermolecular [3+2] annulations with internal alkynes to afford pyrrolo[2,1,5-cd]indolizine in good to excellent yields with high regioselectivity. Through this cascade reaction, a series of fluoroactive fused indolizines with a large π-system were conveniently synthesized. The usage of visible light as energy source with air as a stoichiometric oxidant under simple conditions makes this process attractive and practical.