Type I and Type II Photosensitized Oxidation Reactions: Guidelines and Mechanistic Pathways

Here, 10 guidelines are presented for a standardized definition of type I and type II photosensitized oxidation reactions. Because of varied notions of reactions mediated by photosensitizers, a checklist of recommendations is provided for their definitions. Type I and type II photoreactions are oxygen‐dependent and involve unstable species such as the initial formation of radical cation or neutral radicals from the substrates and/or singlet oxygen (¹O₂ ¹?_g) by energy transfer to molecular oxygen. In addition, superoxide anion radical () can be generated by a charge‐transfer reaction involving O₂ or more likely indirectly as the result of O₂‐mediated oxidation of the radical anion of type I photosensitizers. In subsequent reactions, may add and/or reduce a few highly oxidizing radicals that arise from the deprotonation of the radical cations of key biological targets. can also undergo dismutation into H₂O₂, the precursor of the highly reactive hydroxyl radical () that may induce delayed oxidation reactions in cells. In the second part, several examples of type I and type II photosensitized oxidation reactions are provided to illustrate the complexity and the diversity of the degradation pathways of mostly relevant biomolecules upon one‐electron oxidation and singlet oxygen reactions.     

Photosensitized Oxidation of Tetrabromobisphenol A by Humic Acid in Aqueous Solution†

The brominated flame retardant 3,3′,5,5′‐tetrabromobisphenol A (TBBPA) may accumulate in the environment, including surface waters, and degrade there to potentially toxic products. We have previously shown that singlet oxygen (¹O₂), produced by irradiation of rose bengal with visible light, oxidizes Triton X‐100‐solubilized TBBPA to yield the 2,6‐dibromo‐p‐benzosemiquinone anion radical while consuming oxygen (Environ. Sci. Technol. 42 , 166, 2008). Here, we report that a similar ¹ O ₂ ‐induced oxidation can be initiated in aqueous solutions by the irradiation of TBBPA dissolved in a humic acid (HA) solution. HA is a known weak ¹ O ₂ photosensitizer and we indeed detected the infrared ¹ O ₂ phosphorescence from HA preparations in D ₂ O. When an aqueous preparation of HA was irradiated (λ > 400 nm) in the presence of TBBPA, oxygen was consumed, and the 2,6‐dibromo‐ p ‐benzosemiquinone anion radical was generated and detected using electron paramagnetic resonance. Radical formation and oxygen consumption were inhibited by sodium azide, a singlet oxygen quencher. Our results suggest that solar radiation, in the presence of HA, may play an important role in the photodegradation of TBBPA in the aquatic environment.     

Photosensitized DNA Damage: The Case of Fluoroquinolones†

This review focuses on DNA damage photosensitized by the fluoroquinolone (FQ) antibacterial drugs. The in vivo evidence for photocarcinogenesis mediated by FQs is presented in the introduction. The different methods employed for detection of DNA‐photodamage mediated by FQs are then summarized, including gel electrophoresis (with whole cells, with isolated DNA and with oligonucleotides) and chromatographic analysis (especially HPLC with electrochemical and MS/MS detection). The chemical mechanisms involved in the formation of the reported lesions are discussed on the basis of product studies and transient spectroscopic evidence. In general, the literature coverage is limited to the last decade, although some earlier citations are also included.     

卟啉类化合物对生物分子的光敏化氧化

本文就光敏化氧化反应、光敏剂、生物小分子的光敏化氧化机制以及生物大分子的光敏氧化与切割进行了系统的介绍与评述。同时, 介绍了国内外有关生命分子的光敏化氧化反应的最新研究成果以及它们潜在的应用前景。     

Kinetics and Mechanism of the Photosensitized Oxidation of Furosemide*

Detection of O2(1δg) phosphorescence emission, γmax = 1270 nm, following laser excitation and steady-state competitive methods was employed to measure total rate constants, kT, for the reactions of the diuretic furosemide, 2-methylfurane and furfurylamine with singlet oxygen in several solvents. Correlation of kT values with solvent parameters and product identification shows that the reaction mechanism is strongly solvent dependent. In aliphatic alcohols, the dependence of kT on solvent parameters is similar to the one observed for triethylamine, suggesting a reaction mechanism involving partial charge transfer from the amino group to the singlet oxygen. In nonprotic solvents, the dependence of kT on solvent parameters resembles the behavior found for 2-methylfur-ane and furfurylamine, implying that mostly a 2 + 4 cy-cloaddition mechanism of singlet oxygen to furane ring of furosemide occurs in these solvents. These mechanistic differences are explained in terms of hydrogen-bonding interactions between the carboxylic group in the aromatic ring and the amino group of furosemide. Furthermore, direct generation of C2(1δg) by furosemide was detected. Quantum yields of 0.047 ± 0.003 and 0.078 ± 0.004 were determined in acetonitrile and benzene, respectively. This last result may be related, at least partially, to the photodynamic effects of this diuretic drug.     

Electrocatalytic Hydrogenation and Oxidation in Aqueous Conditions†

Water molecule contains one oxygen and two hydrogen atoms, making it a potential oxygen and hydrogen source. Electrocatalytic organic reduction and oxidation using water as oxygen and/or hydrogen donors provide an environmentally friendly and sustainable strategy to replace traditional chemical‐driven stoichiometric reactions that use sacrificial reagents. Furthermore, the development of electrochemical synthesis provides a potential application for low tension photoelectricity, which is not cost‐effective during boosted voltage and application. In the last decade, electrocatalytic redox reactions of organic molecules in aqueous media had shown progress owing to the development of electrode materials and water‐splitting technology. This paper highlights several electrocatalytic systems and corresponding mechanisms for both hydrogenation and oxidative transformation of representative compounds. The activation process of protons and water on the working electrode surface has received special focus. Furthermore, paired electrolysis using water as the oxygen and hydrogen source has been demonstrated. This paired system combines hydrogenation and oxidation half‐reactions in one cell using water as the hydrogen and oxygen source, resulting in high atomic and electron utilization rates.     

Photosensitized decompositions of oxasilacyclopropane ; unusual carbene formation

TCNE photo-sensitized decompositions of oxasilacyclopropane (1) generated carbene and silanone, while the DCA sensitized or direct photolysis of (1) gave silylene and indanone.     

All‐Trans Retinal Mediates Light‐Induced Oxidation in Single Living Rod Photoreceptors†

All‐trans retinal is a potent photosensitizer that is released in photoreceptor outer segments by the photoactivated visual pigment following the detection of light. Photoreceptor outer segments also contain high concentrations of polyunsaturated fatty acids, and are thus particularly susceptible to oxidative damage such as that initiated by light via a photosensitizer. Upon its release, all‐trans retinal is reduced within the outer segment to all‐trans retinol, through a reaction requiring metabolic input in the form of NADPH. The phototoxic potential of physiologically generated all‐trans retinal was examined in single living rod photoreceptors obtained from frog (Rana pipiens) retinas. Light‐induced oxidation was measured with fluorescence imaging using an oxidation‐sensitive indicator dye from the shift in fluorescence between the intact and oxidized forms. Light‐induced oxidation was highest in metabolically compromised rod outer segments following photoactivation of the visual pigment rhodopsin, and after a time interval, sufficiently long to ensure the release of all‐trans retinal. Furthermore, light‐induced oxidation increased with the concentration of exogenously added all‐trans retinal. The results show that the all‐trans retinal generated during the detection of light can mediate light‐induced oxidation. Its removal through reduction to all‐trans retinol protects photoreceptor outer segments against light‐induced oxidative damage.     

Significance of Photosensitized Oxidation of Alkanes During the Photochemical Degradation of Petroleum Hydrocarbon Fractions in Seawater

Abstract

A crude oil hydrocarbon fraction was photooxidized as a surface film on seawater under simulated environmental conditions. After irradiation, gas chromatography and gas chromatography/mass spectrometry analyses allowed to demonstrate the presence of relatively high quantities of compounds deriving from the photosensitized oxidation of n-alkanes, pristane and phytane, being among the non-acidic photoproducts of this fraction. The results suggest that the photochemical degradation of alkanes should be considered in studies of the fate of petroleum crudes in the marine environment.     

Photosensitized degradation of polyethylene glycols in aqueous solutions. I. Viscosimetric measurements

Investigations were performed on acetone and benzene-sensitized degradation of triethylene glycol (TEG), polyethylene glycol 400 (PEG 400), and polyethylene glycol 4000 (PEG 4000) in aqueous solutions exposed to irradiation at λ = 254 nm and λ ～ 313 nm. The systems investigated were exposed at 20°C (±1°C) in both deaerated and undeaerated solutions. The course of the photosensitized degradation was examined viscosimetrically. The determined quantum yield (?) of chain scission decreases distinctly as the molecular weight of the polymer rises, irrespective of the wavelength of the light absorbed and deaeration of system exposed.     

Photosensitized oxidation of acetylenes

The sensitized (electron transfer) photo-oxidation of acetylenes to yield benzils is described and the mechanism is discussed.     

Oxidation of <Emphasis Type="Italic">p</Emphasis>-Xylene

The review discusses the main procedures for producing terephthalic acid from p-xylene (primarily the Amoco process with the participation of Co, Mn, and Br salts), the oxidation mechanisms, alternative processes, and perspectives.     

Unique Thia‐Baeyer–Villiger‐Type Oxidation of Dibenzothiophene Sulfoxides Derivatives

The present research has demonstrated that selective C?S bond cleavages of dibenzothiophene and its derivatives are feasible by thia‐Baeyer–Villiger type oxidation, i. e. the oxygen insertion process within a sulfoxide‐carbon linkage, in the presence of porphyrin iron (III) and by ultraviolet irradiation originating from sunlight, high pressure Hg‐lamp or residentially germicidal ultraviolet lamp under very mild conditions. This reaction with tert‐butylhydroperoxide at 30.0 °C leads to dibenzo[1,2]oxathiin‐6‐oxide ( PBS ) in 83.2 % isolated yield or its hydrated products, 2‐(2‐hydroxyphenyl)‐benzenesulfinic derivatives ( HPBS ) in near 100 % yield based HPLC data. PBS and HPBS are a type of biological products detected on the C?S bond cleavage step through various oxidative biodesulfurization ( OBDS ) pathways, and are useful synthetic intermediates and fine chemicals. These observations may contribute on understanding delicately molecular aspect of OBDS in the photosynthesis system, expanding the C‐S cleavage chemistry of S‐heterocyclic compounds and approaching toward biomemic desulfurization with respect to converting sulfur contaminants to chemically beneficial blocks as needed and performing under the ambient conditions.     

Photosensitized oxygenation of 3-diazocamphor

Photosensitized oxygenation of 3-diazocamphor gave camphorquinone and camphoric anhydride in benzene. The yield of camphoric anhydride did not depend on the concentration of added camphorquinone or diazocamphor. When the reaction was carried out in acetonitrile or benzonitrile, amide $\text{7}$ or $\text{8}$ was produced. The formation of amide strongly supports the intermediacy of a carbonyl oxide.     

Photosensitized generation of singlet oxygen

This work gives an overview of what is currently known about the mechanisms of the photosensitized production of singlet oxygen. Quenching of pi pi* excited triplet states by O2 proceeds via internal conversion of excited encounter complexes and exciplexes of sensitizer and O2. Both deactivation channels lead with different efficiencies to singlet oxygen generation. The balance between the deactivation channels depends on the triplet-state energy and oxidation potential of the sensitizer, and on the solvent polarity. A model has been developed that reproduces rate constants and efficiencies of the competing processes quantitatively. Sensitization by excited singlet states is much more complex and hence only qualitative rules could be elaborated, despite serious efforts of many groups. However, the most important deactivation paths of fluorescence quenching by O2 are again directed by excess energies and charge-transfer interactions similar to triplet-state quenching by O2. Finally, two recent developments in photosensitization of singlet oxygen are reviewed: Two-photon sensitizers with particular application potential for photodynamic therapy and fluorescence imaging of biological samples and singlet oxygen sensitization by nanocrystalline porous silicon, a material with very different photophysics compared to molecular sensitizers.     

NCP‐Type Pincer Iridium Complexes Catalyzed Transfer‐Dehydrogenation of Alkanes and Heterocycles†

A series of NCP‐type pincer iridium complexes, (^RNC^CP)IrHCl ( 2a — 2c ) and (^BQ‐NC^OP)IrHCl 3 , have been studied for catalytic transfer alkane dehydrogenation. Complex 3 containing a rigid benzoquinoline backbone exhibits high activity and robustness in dehydrogenation of alkanes to form alkenes. Even more importantly, this catalyst system was also highly effective in the dehydrogenation of a wide range of heterocycles to furnish heteroarenes.     

Visible‐Light‐Photosensitized Aryl and Alkyl Decarboxylative Functionalization Reactions

Despite significant progress in aliphatic decarboxylation, an efficient and general protocol for radical aromatic decarboxylation has lagged far behind. Herein, we describe a general strategy for rapid access to both aryl and alkyl radicals by photosensitized decarboxylation of the corresponding carboxylic acids esters followed by their successive use in divergent carbon–heteroatom and carbon–carbon bond‐forming reactions. Identification of a suitable activator for carboxylic acids is the key to bypass a competing single‐electron‐transfer mechanism and “switch on” an energy‐transfer‐mediated homolysis of unsymmetrical σ‐bonds for a concerted fragmentation/decarboxylation process.