Synthetic and mechanistic aspects of 1,3-diene photooxidation

Singlet oxygen: Properties, generation, detection, and environmental applications

Singlet oxygen (1O2) is molecular oxygen in the excited state with high energy and electrophilic properties. It is widely found in nature, and its important role is gradually extending from chemical syntheses and medical techniques to environmental remediation. However, there exist ambiguities and controversies regarding detection methods, generation pathways, and reaction mechanisms which have hindered the understanding and applications of 1O2. For example, the inaccurate detection of 1O2 has led to an overestimation of its role in pollutant degradation. The difficulty in detecting multiple intermediate species obscures the mechanism of 1O2 production. The applications of 1O2 in environmental remediation have also not been comprehensively commented on. To fill these knowledge gaps, this paper systematically discussed the properties and generation of 1O2, reviewed the state-of-the-art detection methods for 1O2 and long-standing controversies in the catalytic systems. Future opportunities and challenges were also discussed regarding the applications of 1O2 in the degradation of pollutants dissolved in water and volatilized in the atmosphere, the disinfection of drinking water, the gas/solid sterilization, and the self-cleaning of filter membranes. This review is expected to provide a better understanding of 1O2-based advanced oxidation processes and practical applications in the environmental protection of 1O2.

Desymmetrization and Kinetic Resolution of Endoperoxides Using a Bifunctional Organocatalyst

Bifunctional thiourea/amine organocatalysts have been used for the desymmetrization of meso-endoperoxides using the Kornblum-DeLaMare reaction, giving 4-hydroxyketones in 78-98% yields with ≤98:2 enantioselectivity. The influence of the catalyst structure, solvent, and temperature was examined. The most promising catalyst was applied to the kinetic resolution of racemic endoperoxides to give enantioenriched materials (≤99:1 er).

Silyl Tether-Assisted Photooxygenation of Electron-Deficient Enaminoesters: Direct Access to Oxamate Formation

Photooxygenation reactions of electron-deficient enaminoesters bearing an oxophilic silyl tether at the α-position of the nitrogen atom using methylene blue (MB) were explored to develop a mild and efficient photochemical strategy for oxidative C-C double bond cleavage reactions via singlet oxygen (¹O₂). Photochemically generated ¹O₂, through energy transfer from the triplet excited state of MB (³MB*) to molecular oxygen (³O₂), was added across a C-C double bond moiety of enaminoesters to form perepoxides, which rearranged to form dioxetane intermediates. The cycloreversion of the formed dioxetane via both C-C and O-O bond cleavage processes led to the formation of oxamates. Importantly, contrary to alkyl group tether-substituted electron-deficient enaminoesters that typically disfavor photooxygenation, the silyl tether-substituted analogues undergo this photochemical transformation efficiently with the assistance of a silyl tether, which facilitates formation of the perepoxide. The observations in this study provide useful information about photosensitized oxygenation reactions of unsaturated C-C bonds, and, moreover, this photochemical strategy can be utilized as a mild and feasible method for the preparation of diversely functionalized carbonyl compounds including oxamates.

Aromatic Endoperoxides

The fundamental aspects of aromatic endoperoxide chemistry are reviewed including their synthesis and reactions. The discussion will focus on factors that will both enhance and prevent the formation of aromatic endoperoxides, and on structural features that will provide control over their ability to release singlet oxygen. This approach recognizes the dual use of aromatic hydrocarbons as both precursors of endoperoxides and as valuable materials for incorporation in electronic and photonic devices. Improvement of the existing methods and development of new methods for the synthesis of endoperoxides is necessary as result of the demand to improve existing and to create new applications for these valuable materials. On the other hand, prevention of endoperoxide formation is crucial to inhibit irreversible oxidative degradation of aromatic hydrocarbons and to extend their lifetimes as useful organic semiconductors.

Synthesis of a novel naphthalenone endoperoxide and structural elucidation by nuclear magnetic resonance spectroscopy and theoretical calculation

Sesquiterpene lactones are found in plants of Asteraceae family, and endoperoxides are known for their antimalarial activity. Structural elucidation is a relevant aspect; however, it is not uncommon to find incorrect or incomplete structural assignments in the literature. Calculations based in quantum mechanics are frequently used to compute ¹ H and ¹³ C NMR chemical shifts, and after comparing with the experimental data, the correct structure is established from diverse candidates. Targeting the synthesis of bioactive compounds, we envisaged the synthesis of a novel endoperoxide from the natural sesquiterpene lactone α-santonin (2). Photochemical transformation of α-santonin (2) to mazdasantonin (4) followed by photooxidation catalyzed by rose bengal afforded the novel endoperoxide 5. This new endoperoxide contains five stereogenic centers and is analogous to the antimalarial agent artemisinin (1). The relative configuration of the stereogenic centers of the endoperoxide were established by nuclear magnetic resonance (NMR) analyses and confirmed by theoretical calculations. All approaches were in complete agreement, and the structure of mazdasantonin endoperoxide was established as (3S,3aS,5aS,8R,9bS)-3,6,6-trimethyl-3,3a,4,5,8,9b-hexahydro-2H-5a,8-epidioxynaphtho[1,2-b]furan-2,7(6H)-dione.

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.

Influence of Functional Groups on the Ene Reaction of Singlet Oxygen with 1,4-Cyclohexadienes†

The photooxygenation of 1,4-cyclohexadienes has been studied with a special focus on regio- and stereoselectivities. In all examples, only the methyl-substituted double bond undergoes an ene reaction with singlet oxygen, to afford hydroperoxides in moderate to good yields. We explain the high regioselectivities by a "large-group effect" of the adjacent quaternary stereocenter. Nitriles decrease the reactivity of singlet oxygen, presumably by quenching, but can stabilize proposed per-epoxide intermediates by polar interactions resulting in different stereoselectivities. Spiro lactams and lactones show an interesting effect on regio- and stereoselectivities of the ene reactions. Thus, singlet oxygen attacks the double bond preferentially anti to the carbonyl group, affording only one regioisomeric hydroperoxide. If the reaction occurs from the opposite face, the other regioisomer is exclusively formed by severe electrostatic repulsion in a perepoxide intermediate. We explain this unusual behavior by the fixed geometry of spiro compounds and call it a "spiro effect" in singlet oxygen ene reactions.

Catalytic synthesis of benzannelated macrocyclic di- and triperoxides based on phenols

An efficient method for the synthesis of benzannelated macrocyclic di- and triperoxides by cyclocondensation of aromatic compounds with bis-hydroperoxides and formaldehyde in the presence of lanthanide catalysts has been developed.

Synthetic feasibility of oxygen-driven photoisomerizations of alkenes and polyenes

This review describes O₂-dependent photoreactions for possible routes to double-bond isomerizations. E,Z-isomerizations triggered by O₂ and visible light are a new area of potential synthetic interest. The reaction involves the reversible addition of O₂ to form a peroxy intermediate with oxygen evolution and partial regeneration of the compound as its isomer. Targeting of O₂-dependent photoisomerizations also relates to a practical use of visible light, for example the improved light penetration depth for visible as opposed to UV photons in batch sensitized reactions. This review is intended to draw a link between visible-light formation of a peroxy intermediate and its dark degradation with O₂ release for unsaturated compound isomerization. This review should be of interest both to photochemists and synthetic organic chemists, as it ties together mechanistic and synthetic work, drawing attention to an overlooked subject.

Fundamental Studies of the Singlet Oxygen Reactions with the Potent Marine Toxin Domoic Acid

Domoic acid (DA), a potent marine toxin, is readily oxidized upon reaction with singlet oxygen (¹O₂). Detailed product studies revealed that the major singlet oxygenation reaction pathways were the [2 + 2] cycloaddition (60.2%) and ene reactions (39.8%) occurring at the Z double bond. Diene isomerization and [4 + 2] cycloaddition, common for conjugated diene systems, were not observed during the singlet oxygenation of DA. The bimolecular rate constant for the DA reaction with ¹O₂ determined by competition kinetics was 5.1 × 10⁵ M^-1 s^-1. Based on the rate constant and steady-state concentrations of ¹O₂ in surface waters, the environmental half-life of DA due to singlet oxygen-induced transformations is between 5 and 63 days. The ¹O₂ reaction product mixture of DA did not exhibit significant biological activity based on ELISA studies, indicating that singlet oxygenation could be an important natural detoxification process. The characteristic oxidation products can provide valuable markers for the risk assessment of DA-contaminated natural waters.

Probing the Reactivity of Singlet Oxygen with Cyclic Monoterpenes

Monoterpenes represent a class of hydrocarbons consisting of two isoprene units. Like many other terpenes, monoterpenes emerge mainly from vegetation, indicating their significance in both atmospheric chemistry and pharmaceutical and food industries. The atmospheric recycling of monoterpenes constitutes a major source of secondary organic aerosols. Therefore, this contribution focuses on the mechanism and kinetics of atmospheric oxidation of five dominant monoterpenes (i.e., limonene, α-pinene, β-pinene, sabinene, and camphene) by singlet oxygen. The reactions are initiated via the ene-type addition of singlet oxygen (O2 1Δg) to the electron-rich double bond, progressing favorably through the concerted reaction mechanisms. The physical analyses of the frontier molecular orbitals agree well with the thermodynamic properties of the selected reagents, and the computed reaction rate parameters. The reactivity of monoterpenes with O2 1Δg follows the order of α-pinene > sabinene > limonene > β-pinene > camphene, i.e., α-pinene and camphene retain the highest and lowest reactivity toward singlet oxygen, with rate expressions of k(T) (M-1 s-1) = 1.13 × 108 exp(-48(kJ)/RT(K)) and 6.93 × 108 exp(-139(kJ)/RT(K)), respectively. The effect of solvent on the primary reaction pathways triggers a slight reduction in energy, ranging between 12 and 34 kJ/mol.

Release of Singlet Oxygen from Aromatic Endoperoxides by Chemical Triggers

The generation of reactive singlet oxygen under mild conditions is of current interest in chemistry, biology, and medicine. We were able to release oxygen from dipyridylanthracene endoperoxides (EPOs) by using a simple chemical trigger at low temperature. Protonation and methylation of such EPOs strongly accelerated these reactions. Furthermore, the methyl pyridinium derivatives are water soluble and therefore serve as oxygen carriers in aqueous media. Methylation of the EPO of the ortho isomer affords the parent form directly without increasing the temperature under very mild conditions. This exceptional behavior is ascribed to the close contact between the nitrogen atom and the peroxo group. Singlet oxygen is released upon this reaction, and can be used to oxygenate an acceptor such as tetramethylethylene in the dark with no heating. Thus, a new chemical source of singlet oxygen has been found, which is triggered by a simple stimulus.

A biosynthetically inspired route to substituted furans using the Appel reaction: total synthesis of the furan fatty acid F5

Appel reaction conditions have been harnessed to affect a mild biosynthetically inspired dehydration of endoperoxides to deliver multi-substituted electron rich furans. Unlike traditional dehydrative procedures, this method is metal and acid free, and can be achieved under redox neutral conditions. It is general for a range of aryl and alkyl substituted endoperoxides, and is functional group tolerant. Furthermore, this procedure has been used to deliver an effective total synthesis of the furan fatty acid (FFA) F₅.

Reaction of Aniline with Singlet Oxygen (O21Δg)

Dissolved organic matter (DOM) acts as an effective photochemical sensitizer that produces the singlet delta state of molecular oxygen (O₂¹Δ_g), a powerful oxidizer that removes aniline from aqueous solutions. However, the exact mode of this reaction, the p- to o-iminobenzoquinone ratio, and the selectivity of one over the other remain largely speculative. This contribution resolves these uncertainties. We report, for the first time, a comprehensive mechanistic and kinetic account of the oxidation of aniline with the singlet delta oxygen using B3LYP and M06 functionals in both gas and aqueous phases. Reaction mechanisms have been mapped out at E, H, and G scales. The 1,4-cycloaddition of O₂¹Δ_g to aniline forms a 1,4-peroxide intermediate (M1), which isomerizes via a closed-shell mechanism to generate a p-iminobenzoquinone molecule. On the other hand, the O₂¹Δ_g ene-type reaction forms an o-iminobenzoquinone product when the hydroperoxyl bond breaks, splitting hydroxyl from the 1,2-hydroperoxide (M3) moiety. The gas-phase model predicts the formation of both p- and o-iminobenzoquinones. In the latter model, the M1 adduct displays the selectivity of up to 96%. A water-solvation model predicts that M1 decomposes further, forming only p-iminobenzoquinone with a rate constant of k = 1.85 × 10⁹ (L/(mol s)) at T = 313 K. These results corroborate the recent experimental findings of product concentration profile in which p-iminobenzoquinonine represents the only detected product.

Preparation of Antimalarial Endoperoxides by a Formal [2 + 2 + 2] Cycloaddition

A formal [2 + 2 + 2] cycloaddition reaction between a 1,3-dione, an olefin, and molecular oxygen mediated by light is reported, which delivers endoperoxides in good yield through the formation of two C-O and one C-C bond in one step. The resulting 1,2-dioxanes are stable compounds and can be further derivatized at the hemiacetal position via alkylation or acetylation. All compounds have been evaluated against Plasmodium falciparum, and the best compound displayed an IC50-value of 180 nM. A potential mechanistic rationale for the formation of these compounds is presented.

Synthesis of five- and six-membered cyclic organic peroxides: Key transformations into peroxide ring-retaining products

The present review describes the current status of synthetic five and six-membered cyclic peroxides such as 1,2-dioxolanes, 1,2,4-trioxolanes (ozonides), 1,2-dioxanes, 1,2-dioxenes, 1,2,4-trioxanes, and 1,2,4,5-tetraoxanes. The literature from 2000 onwards is surveyed to provide an update on synthesis of cyclic peroxides. The indicated period of time is, on the whole, characterized by the development of new efficient and scale-up methods for the preparation of these cyclic compounds. It was shown that cyclic peroxides remain unchanged throughout the course of a wide range of fundamental organic reactions. Due to these properties, the molecular structures can be greatly modified to give peroxide ring-retaining products. The chemistry of cyclic peroxides has attracted considerable attention, because these compounds are used in medicine for the design of antimalarial, antihelminthic, and antitumor agents.

Intermediates in the Formation and Thermolysis of Peroxides from Oxidations with Singlet Oxygen

Herein we describe the recent mechanistic understandings of the singlet oxygen ene reaction to give hydroperoxides and the [4+2] cycloaddition affording endoperoxides. Both experimental findings and theoretical work conclude in the formation of intermediates structurally similar to perepoxides during the ene reaction. Such intermediates mainly control the regio- and stereoselectivities of this reaction class. For the [4+2] cycloaddition, both a synchronous concerted reaction (benzene, naphthalenes) and a stepwise reaction with a non-symmetric zwitterionic intermediate (larger acenes) have been found. The thermolysis of endoperoxides derived from acenes proceeds stepwise for anthracenes, but in a concerted manner for less stable adducts such as naphthalene.

Julia-Kocienski reaction-based 1,3-diene synthesis: aldehyde-dependent (E,E/E,Z)-selectivity

A new modification of Julia-Kocienski olefination reaction based on the use of cation-specific chelating agents that yields 1,3-dienes with predictable (E/Z)-selectivity on newly created double bond was developed. The influence of the aldehyde structure on reaction (E/Z) selectivity is discussed and rationalized.

Endoperoxide synthesis by photocatalytic aerobic [2 + 2 + 2] cycloadditions

Structurally novel endoperoxides can be sythesized by the photocatalytic cyclotrimerization of bis(styrene) substrates with molecular oxygen. The optimal catalyst for this process is Ru(bpz)(3)(2+), which is a markedly more efficient catalyst for these photooxygention reactions than conventional organic photosensitizers. The 1,2-dioxolane products are amenable to synthetic manipulation and can be easily processed to 1,4-diols and γ-hydroxyketones. An initial screen of the biological activity of these compounds reveals promising inhibition of cancer cell growth.

Synthesis and Photooxygenation of the Methyl Ester of 11-Homodrim-6,8(9)-Diene-12-Oic Acid

Starting with the methyl 11-homodrim-8-ene-7-oxo-12-oate a two steps synthesis of methyl-11-homodrim-6,8(9)-diene-12-oate was accomplished in 87% overall yield, which on photooxygenation in the presence of tetraphenylporphyrin gave a mixture of methyl esters of 11-homodrim-7-ene-6α,9α-peroxy-12-oic and 11-homodrim-5,8-diene-7-oxo-12-oic acids 21% and 54% yields, respectively.

Lewis acid-promoted addition of 1,3-bis(silyl)propenes to aldehydes: a route to 1,3-dienes

The Lewis acid-promoted addition of 1,3-bis(silyl)propenes to aldehydes to provide the corresponding (E)-1,3-dienes in excellent stereoselectivity and good to excellent yields is reported. The procedure is mild, base-free, and operationally straightforward.

Photooxidation of a platinum-anthracene pincer complex: formation and structures of Pt(II)-anthrone and -ketal complexes

Irradiating the cyclometalated pincer complex Pt(II)(DPA)Cl (1, DPA = 1,8-bis(diphenylphosphino)anthracene) in the presence of O(2) led to three sequential oxidations of the anthracenyl ring. The first photoproduct, a Pt(II)-9,10-endoperoxide complex, was converted photochemically to a Pt(II)-9-hydroxyanthrone complex A which was further oxygenated to a Pt(II)-hemiketal (B). The oxidation of A, which could be accelerated by light irradiation, probably involved a Pt(II)-anthraquinone intermediate. B underwent acid-catalyzed ketalization to form a binuclear Pt(II)(2)-diketal (B1). The photolysis was followed by UV-vis absorption and NMR spectroscopy, and the structures of A and B1 were characterized by single crystal X-ray diffraction.

Inside vs "Outside" photooxygenation reactions: singlet-oxygen-mediated surface passivation of polymer films

Films of poly(acrylonitrile- co-2,3-dimethyl-1,3-butadiene) were exposed to singlet oxygen. The extent of polymer oxygenation was monitored for singlet oxygen generated (1) within the polymer film and (2) at the polymer surface in an aqueous medium. When singlet oxygen is generated within the film, oxygenation of the polymer is pronounced and extensive. When singlet oxygen is generated at the polymer surface, oxygenation reactions are limited to the surface. The data suggest that the initial oxygenation reactions at the film surface passivate the polymer against further reaction with singlet oxygen and, hence, also minimize the progressively detrimental effects of secondary reactions. These results indicate that one should exercise restraint when implicating singlet oxygen as a reactive species in some processes of polymer oxygenation.

Remote substituent effects on the photooxygenation of 9,10-diarylanthracenes: strong evidence for polar intermediates

Two different reaction pathways in the photooxygenation of 9,10-diarylanthracenes are identified, with strong evidence for polar (forward, singlet oxygen addition) and radical (backward, thermolysis) intermediates.