Theoretical Study on the Reactivity and Regioselectivity of the Ene Reaction of1Δg O2 with α,β-Unsaturated Carbonyl Compounds

Hydroxylamine mediated Fenton-like interfacial reaction dynamics on sea urchin-like catalyst derived from spent LiFePO4 battery

Herein, we converted spent LiFePO₄ battery to the sea urchin-like material (SULM) with a highly efficient and environment-friendly method, which can contribute to building a zero-waste city. With SULM as a Fenton-like catalyst, a highly-efficient degradation process was realized for organic pollutants with interface and solution synergistic effect. In our SULM+NH₂OH+H₂O₂ Fenton-like system, NH₂OH can effectively promote the interface iron (Fe(Ⅲ)/Fe(Ⅱ)) and solution iron (Fe(Ⅲ)/Fe(Ⅱ)) redox cycle, thus promoting the generation of reactive oxygen species (ROS). However, the ROS generation process and organic pollutants degradation pathway with the presence of NH₂OH remains a puzzle. Here the detailed ROS generation mechanism and pollutants degradation pathway have been illustrated carefully based on experimental exploration and characterization. Therein, hydroxyl radicals (·OH) and singlet oxygen (¹O₂) are the main ROS for oxidizing and degrading organic pollutants. Notably, ¹O₂ can be converted from superoxide radicals (·O₂) in SULM+NH₂OH+H₂O₂ system. This study not only demonstrates the strategy of "trash-to-treasure" and "waste-to-control-waste" to simultaneously reduce the hazardous release from industrial solid waste and organic wastewater, it also provides new mechanistic insights for NH₂OH mediated Fenton-like redox system.

Mechanism study of CoS2/Fe(III)/peroxymonosulfate catalysis system: The vital role of sulfur vacancies

Peroxymonosulfate (PMS) activation methods have attractive advantages in advanced oxidation process (AOPs) due to their powerful ability of directly or indirectly generating various reactive oxygen species (ROS). Herein, trace amount of Fe(III) ions were added into the commercial-CoS₂/PMS system to improve the CoS₂/PMS decomposition for organics removal. The organics removal efficiency could reach >90% towards methylene blue (MB), diclofenac sodium (DCF), sulfamethoxazole (SMX) and bisphenol A (BPA) in the CoS₂/Fe(III)/PMS system, with the kinetic apparent rate constant k_obs of 0.141, 0.206, 0.247 and 0.091 min^-1, respectively. The synergistic effect between Fe(III) ions and sulfur-vacancies on CoS₂ for PMS degradation were revealed for the first time in cobalt sulfides/PMS system. Quenching experiments and ESR analysis proved that ¹O₂ was the major ROS and was produced mainly by the hydrolysis of SO₅^•-. Besides, the high degradation efficiency was obtained by the contribution of SO₄^•- and ^•OH. Electron spin-resonance spectroscopy (ESR), cyclic voltammetry (CV) and Raman spectrum data revealed that the addition of Fe(III) ions could optimize the intensity of sulfur vacancies on the CoS₂ surface, which hindered the PMS reduction ability of Co(II), but accelerated the PMS oxidation to form ¹O₂. The degradation path of MB was analyzed by liquid chromatograph-mass spectrometer (LC-MS). The mechanism studies speculated that the sulfur vacancies of CoS₂ provided the binding sites for Fe(III) ions with Co(II), which facilitated the PMS activation by Co(III).

Singlet Oxygen: Discovery, Chemistry, C60 -Sensitization†

This review article refers to the discovery of excited molecular oxygen, in particular on its lower singlet excited state (¹ Δ_g , ¹ O₂ ). After a short report on singlet oxygen generation, the review is focused on the chemistry of this reactive species. Specifically, the three major reactions of ¹ O₂ with unsaturated organic substrates, namely the [4 + 2] and [2 +2] cycloadditions as well as the ene reaction, are reviewed. The proposed mechanisms of these reactions, through the years, based on experimental and computational work, have been presented. Selected examples of singlet oxygen-synthetic applications are also mentioned. The [60]fullerene and fullereno-materials photosensitized oxidations in homogeneous, as well as in heterogeneous conditions, are also comprehensively discussed. Finally, the self-sensitized photooxidation of open cage fullerenes as well as fullerenes bearing oxidizable groups is reported.

Singlet Oxygen Triggered by Superoxide Radicals in a Molybdenum Cocatalytic Fenton Reaction with Enhanced REDOX Activity in the Environment

As an important reactive oxygen species (ROS) with selective oxidation, singlet oxygen (¹O₂) has wide application prospects in biology and the environment. However, the mechanism of ¹O₂ formation, especially the conversion of superoxide radicals (·O₂^-) to ¹O₂, has been a great controversy. This process is often disturbed by hydroxyl radicals (·OH). Here, we develop a molybdenum cocatalytic Fenton system, which can realize the transformation from ·O₂^- to ¹O₂ on the premise of minimizing ·OH. The Mo⁰ exposed on the surface of molybdenum powder can significantly improve the Fe³⁺/Fe²⁺ cycling efficiency and weaken the production of ·OH, leading to the generation of ·O₂^-. Meanwhile, the exposed Mo⁶⁺ can realize the transformation of ·O₂^- to ¹O₂. The molybdenum cocatalytic effect makes the conventional Fenton reaction have high oxidation activity for the remediation of organic pollutants and prompts the inactivation of Staphylococcus aureus, as well as the adsorption and reduction of heavy metal ions (Cu²⁺, Ni²⁺, and Cr⁶⁺). Compared with iron powder, molybdenum powder is more likely to promote the conversion from Fe³⁺ to Fe²⁺ during the Fenton reaction, resulting in a higher Fe²⁺/Fe³⁺ ratio and better activity regarding the remediation of organics. Our findings clarify the transformation mechanism from ·O₂^- to ¹O₂ during the Fenton-like reaction and provide a promising REDOX Fenton-like system for water treatment.

Reaction of phenol with singlet oxygen

Photo-degradation of organic pollutants plays an important role in their removal from the environment. This study provides an experimental and theoretical account of the reaction of singlet oxygen O2(1Δg) with the biodegradable-resistant species of phenol in an aqueous medium. The experiments combine customised LED-photoreactors, high-performance liquid chromatography (HPLC), and electron paramagnetic resonance (EPR) imaging, employing rose bengal as a sensitiser. Guided by density functional theory (DFT) calculations at the M062X level, we report the mechanism of the reaction and its kinetic model. Addition of O2(1Δg) to the phenol molecule branches into two competitive 1,4-cycloaddition and ortho ene-type routes, yielding 2,3-dioxabicyclo[2.2.2]octa-5,7-dien-1-ol (i.e., 1,4-endoperoxide 1-hydroxy-2,5-cyclohexadiene) and 2-hydroperoxycyclohexa-3,5-dien-1-one, respectively. Unimolecular rearrangements of the 1,4-endoperoxide proceed in a facile exothermic reaction to form the only experimentally detected product, para-benzoquinone. EPR revealed the nature of the oxidation intermediates and corroborated the appearance of O2(1Δg) as the only active radical participating in the photosensitised reaction. Additional experiments excluded the formation of hydroxyl (HO˙), hydroperoxyl (HO2˙), and phenoxy intermediates. We detected for the first time the para-semibenzoquinone anion (PSBQ), supporting the reaction pathway leading to the formation of para-benzoquinone. Our experiments and the water-solvation model result in the overall reaction rates of kr-solvation = 1.21 × 104 M-1 s-1 and kr = 1.14 × 104 M-1 s-1, respectively. These results have practical application to quantify the degradation of phenol in wastewater treatment.

Visible Light-Induced Aerobic Epoxidation of α,β-Unsaturated Ketones Mediated by Amidines

An aerobic photoepoxidation of α,β-unsaturated ketones driven by visible light in the presence of tetramethylguanidine (3b), tetraphenylporphine (H₂TPP), and molecular oxygen under mild conditions was revealed. The corresponding α,β-epoxy ketones were obtained in yields of up to 94% in 96 h. The reaction time was shortened to 4.6 h by flow synthesis. The mechanism related to singlet oxygen was supported by experiments and density functional theory (DFT) calculations.

Theoretical study on the photooxygenation and photorearrangement reactions of 3-hydroxyflavone

The mechanisms of three photodegradation reactions of 3-hydroxyflavone – its photosensitized oxygenation, photooxygenation with ³O₂ and photorearrangement into an indanedione derivative – have been investigated by computing the free energy profiles.

Singlet oxygen and natural substrates: functional polyunsaturated models for the photooxidative degradation of carotenoids

The primary chemical reactions of singlet molecular oxygen with polyunsaturated carotenoids are the focus of this research report. Model compounds that exhibit electronic properties and substituent pattern similar to natural carotenes, xanthophylls or apocarotenoids, respectively, were investigated with regard to photooxygenation reactivity. For dienes and trienes as substrates, high tandem reactivity was observed and hydroperoxy-endoperoxides were isolated as the secondary products of singlet oxygen reaction. The electronic gem-effect on the regioselectivity of the ene reaction is conserved also in vinylogous positions and thus appears to originate from a radical-stabilizing effect. In an attempt to combine different peroxide groups derived from natural products as a tool for new pharmaceutically active products, a dyade synthesis of an artemisinine-safranol with subsequent singlet oxygen addition was realized.

Ene-diene transmissive cycloaddition reactions with singlet oxygen: the vinylogous gem effect and its use for polyoxyfunctionalization of dienes

The singlet oxygen reactivities and regioselectivities of the model compounds 1b-d were compared with those of the geminal (gem) selectivity model ethyl tiglate (1a). The kinetic cis effect is k(E)/k(Z) = 5.2 for the tiglate/angelate system 1a/1a' without a change in the high gem regioselectivity. Further conjugation to vinyl groups enabled mode-selective processes, namely, [4 + 2] cycloadditions versus ene reactions. The site-specific effects of methylation on the mode selectivity and the regioselectivity of the ene reaction were studied for dienes 1e-g. A vinylogous gem effect was observed for the γ,δ-dimethylated and α,γ,δ-trimethylated substrates 1h and 1i, respectively. The corresponding phenylated substrates 1j-l showed similar mode selectivity, as monomethylated 1j exhibited exclusively [4 + 2] reactivity while the tandem products 12 and 14 were isolated from the di- and trimethylated substrates 1k and 1l, respectively. The vinylogous gem effect favors the formation of 1,3-dienes from the substrates, and thus, secondary singlet oxygen addition was observed to give hydroperoxy-1,2-dioxenes 19 and 20 in an ene-diene transmissive cycloaddition sequence. These products were reduced to give alcohols (16, 17, and 18) or furans (24 and 25), respectively, or treated with titanium(IV) alkoxides to give the epoxy alcohols 26 and 27. The vinylogous gem effect is rationalized by DFT calculations showing that biradicals are the low-energy intermediates and that no reaction path bifurcations compete.

On the origin of regio- and stereoselectivity in singlet oxygen addition to enecarbamates

The reactions of excited state singlet molecular oxygen ((1)Δ(g),(1)O(2)) continue to witness interesting new developments. In the most recent manifestation, (1)O(2) is tamed to react with enecarbamates in a stereoselective manner, which is remarkable, in view of its inherently high reactivity (Acc. Chem. Res. 2008, 41, 387). Herein, we employed the CAS-MP2(8,7)/6-31G* as well as the CAS-MP2(10,8)/6-31G* computations to unravel the origin of (i) diastereoselectivities in dioxetane or hydroperoxide formation and (ii) regioselectivity leading to a [2 + 2] cycloadduct or an ene product when (1)O(2) reacts with an oxazolidinone tethered 2-phenyl-1-propenyl system. The computed Gibbs free energy profiles for E- and Z-isomers when (1)O(2) approaches through the hindered and nonhindered diastereotopic faces (by virtue of chiral oxazolidinone) of the enecarbamates exhibit distinct differences. In the case of E-isomer, the relative energies of the transition structures responsible for hydroperoxide (ene product) are lower than that for dioxetane formation. On the other hand, the ene pathway is predicted to involve higher barriers as compared to the corresponding dioxetane pathway for Z-isomer. The energy difference between the rate-determining diastereomeric transition structures involved in the most favored ene reaction for E-enecarbamate suggests high diastereoselectivity. In contrast, the corresponding energy difference for Z-enecarbamate in the ene pathway is found to be diminishingly close, implying low diastereoselectivity. However, the dioxetane formation from Z-enecarbamate is predicted to exhibit high diastereoselectivity. The application of activation strain model as well as the differences in stereoelectronic effects in the stereocontrolling transition structures is found to be effective toward rationalizing the origin of selectivities reported herein. These predictions are found to be in excellent agreement with the experimental observations.

Unraveling the mechanism of the singlet oxygen ene reaction: recent computational and experimental approaches

The mechanism of the singlet oxygen ene reaction has been a subject of renewed interest within the last few years. The main question being whether this reaction proceeds through a concerted mechanism or if it involves discrete intermediates. In general, the majority of experimental and computational studies support a traditional stepwise mechanism involving a perepoxide-like intermediate. In this minireview we highlight the most prominent and recent theoretical, as well as experimental results relating to the challenging mechanism of the singlet oxygen ene oxyfunctionalization.

Stereoselectivity in ene reactions with 1O2: matrix effects in polymer supports, photo-oxygenation of organic salts and asymmetric synthesis

The ene reaction of chiral allylic alcohols is applied as a tool for the investigation of intrapolymer effects by means of the stereoselectivity of the singlet-oxygen addition. The diastereo selectivity strongly depends on the structure of the polymer, the substrate loading degree and also on the degree of conversion demonstrating additional supramolecular effects evolving during the reaction. The efficiency and the stability of polymer-bound sensitizers were evaluated by the ene reaction of singlet oxygen with citronellol. The ene reaction with chiral ammonium salts of tiglic acid was conducted under solution phase conditions or in polystyrene beads under chiral contact ion-pair conditions. The products thus obtained precipitate during the photoreaction as ammonium salts. Moderate asymmetric induction was observed for this procedure for the first time.