Oxidation of dienes by triphenylphosphine peroxopalladium

Thermal reaction of 1,3-diphenylisobenzofuran and tetramethylcyclopentadienone with PdLO₂ complex (L = PPh₃) gives compounds identical to those produced by singlet molecular oxygen. Photochemical reaction of 1,9-diphenylanthracene with PdLO₂ or PdL₃ in the presence of oxygen gives the 9,10-endoperoxide adduct.     

ab initio molecular orbital and density functional analysis of acetylene + O2 reactions with CHEMKIN evaluation

A number of researchers have indicated that a direct reaction of acetylene with oxygen needs to be included in detailed reaction mechanisms in order to model observed flame speeds and induction times. Four pathways for the initiation of acetylene oxidation to chain propagation are considered and the rate constants are compared with values used in the mechanisms:

• 1 ³O₂ + HCCH to triplet adduct and reaction on the triplet surface
• 2 ³O₂ + HCCH to triplet adduct, conversion of triplet adduct to singlet adduct via collision in the reaction environment, with further reaction of the singlet adduct
• 3 ¹O₂ + HCCH to singlet adduct
• 4 Isomerization of HCCH to vinylidene and then vinylidene insertion reaction with ³O₂

Elementary reaction pathways for oxidation of acetylene by addition reaction of O₂(³Σ) on the triplet surface are analyzed. ab initio molecular orbital and density functional calculations are employed to estimate the thermodynamic properties of the reactants, transition states, and products in this system. Acetylene oxidation reaction over the triplet surface is initiated by addition of molecular oxygen, O₂(³Σ), to a carbon atom, forming a triplet peroxy‐ethylene biradical. The reaction path to major products, either two formyl radicals or glyoxal radical plus hydrogen atom, involves reaction through three transition states: O₂(³Σ) addition to acetylene (TS1), peroxy radical addition at the ipso‐carbon to form a dioxirane (TS2), and cleavage of O O bond in a three‐member ring (TS3). Single‐point QCISD(T) and B3LYP calculations with large basis sets were performed to try to verify barrier heights on important transition states. A second pathway to product formation is through spin conversion of the triplet peroxy‐ethylene biradical to the singlet by collision with bath gas. Rapid ring closure of the singlet peroxy‐ethylene biradical to form a four‐member ring is followed by breaking of the peroxy bond to form glyoxal, which further dissociates to either two formyl radicals or a glyoxal radical plus hydrogen atom. The overall forward rate constant through this pathway is estimated to be k_f = 2.21 × 10⁷ T^1.46e^{−33.1(kcal/mol)/RT}. Two additional pathways from the literature, HCCH + O₂(¹Δ) and pressure‐dependent isomerization of acetylene to vinylidene and then vinylidene reaction with O₂(³Σ), are also evaluated for completeness. CHEMKIN modeling on each of the four proposed pathways is performed and concentration profiles from these reactions are evaluated at 0.013 atm and 1 atm over 35 milliseconds. Through reaction on the triplet surface is evaluated to be not important. Formation of the triplet adduct with conversion (via collision) to a singlet and the vinylidene paths show similar and lower rates than those used in mechanisms, respectively. Our implementation of the HCCH + O₂(¹Δ) pathway of Benson suggests the need to include: (i) reverse reaction, (ii) barriers to further reaction of the initial adduct plus (iii) further evaluation of the O₂(¹Δ) addition barrier. The pathways from triplet adduct with conversion to singlet and from vinylidene are both recommended for initiation of acetylene oxidation. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 623–641, 2000     

Reactions of urocanic acid (UCA) methyl esters with singlet oxygen and 4-methyl-1,2,4-triazoline-3,5-dione (MTAD)

Singlet oxygen adds to the imidazole ring of cis- and trans-methyl urocanate (MUC) to yield the corresponding 2,5-endoperoxides, which are modestly stable at low temperature but decompose upon warming to form complex reaction mixtures. MTAD, a singlet oxygen mimic, reacts with cis- and trans-MUC to yield stereospecific [4+2] reaction products involving the olefinic side chain and the C₄-C₅ double bond of the imidazole ring. trans-MUC forms a 1:2 MTAD adduct while the cis isomer yields only the 1:1 adduct at 25 °C. The stereospecificity and absence of MeOH trapping adducts indicate that these reactions may not involve open or trappable dipolar intermediates.     

The Reaction of Singlet Oxygen with Adamantylideneadamantane Mediated by Rose Bengal

The photo-oxygenation of adamantylideneadamantane ( 1 ) on siliceous supports using admixed granules of ion-exchange resin fixed to methylene blue (MB) and rose bengal (RB) gave exclusively the corresponding dioxetane derivative 2 for the former sensitizer, while the latter gave 2 and traces of the epoxide 3. RB and the charge-transfer complex produced from N-ethylcarbazole and 2,4,5,6-tetranitrofluoren-9-one both reacted with chemically generated singlet oxygen to give superoxide radical anion. Trapping of the latter with 5,5-dimethyl-1-pyrroline 1-oxide gave an adduct exhibiting a characteristic ESR spectrum. The treatment of 1 in MeOH with 30% aqueous H₂O₂ for 22 h at 60° gave 3 in 100% yield. Repetition of this experiment in the presence of 2,6-di(tert-butyl)-p-cresol caused no significant change. These results indicate that singlet oxygen reacts with 1 , in the presence of RB, by two different processes. The first leads to dioxetane formation. The second process involves conversion of singlet oxygen by RB to superoxide radical anion which subsequently gives H₂O₂ so producing epoxide 3 from 1 .     

Photo-oxidation of polymers without light: Oxidation of polybutadiene and an ABS polyblend with singlet oxygen

In recent years much evidence has been accumulated to implicate electronically excited oxygen (¹Δ_g) molecules as the agent responsible in photosensitized oxidations for the formation of allylic hydroperoxides from olefins and of endoperoxides from 1,3-dienes. Little regarding the mechanistic aspects of the photo-oxidative degradation of polybutadiene (PBD) is known, however. To determine if electronically excited oxygen (¹Δ_g) molecules can oxidize PBD, the ABS polyblend and standard samples of PBD's containing high trans, high cis, and high vinyl content were treated in homogeneous solution at low temperature with chemically produced singlet oxygen in situ. The source of the singlet oxygen was the triphenylphosphite-ozone adduct. Studies by spectroscopy, elemental analysis, viscosity determinations, and gel measurements showed only the cis- and the trans-PBD were susceptible to oxidation; no chain scission was involved in the attack of cis- and trans-PBD by singlet oxygen; the oxidation of the cis PBD involved the initial formation of hydroperoxides which on thermal decomposition yielded gel. The trans-PBD was found to oxidize but apparently by a mechanism different from that of cis-PBD. Initial singlet oxygen attack of ABS proceeds by oxidation of the PBD portion of the polyblend. It was also observed that when only a small amount of the double bonds in the cis-PBD polymer had been oxidized to hydroperoxides, subsequent thermal treatment of this sample resulted in gross structural changes in the whole polymer.     

Singlet Oxygen during Cycling of the Aprotic Sodium–O2 Battery

Aprotic sodium–O₂ batteries require the reversible formation/dissolution of sodium superoxide (NaO₂) on cycling. Poor cycle life has been associated with parasitic chemistry caused by the reactivity of electrolyte and electrode with NaO₂, a strong nucleophile and base. Its reactivity can, however, not consistently explain the side reactions and irreversibility. Herein we show that singlet oxygen (¹O₂) forms at all stages of cycling and that it is a main driver for parasitic chemistry. It was detected in‐ and ex‐situ via a ¹O₂ trap that selectively and rapidly forms a stable adduct with ¹O₂. The ¹O₂ formation mechanism involves proton‐mediated superoxide disproportionation on discharge, rest, and charge below ca. 3.3 V, and direct electrochemical ¹O₂ evolution above ca. 3.3 V. Trace water, which is needed for high capacities also drives parasitic chemistry. Controlling the highly reactive singlet oxygen is thus crucial for achieving highly reversible cell operation.     

A fullerene core to probe dendritic shielding effects

Dendrimers with a C₆₀ core have been obtained by cyclization of dendritic bis-malonate derivatives at the carbon sphere. The resulting bis-methanofullerene derivatives have been characterised by electrospray (ES) and/or MALDI-TOF mass spectrometries. UV-VIS absorption spectra, fluorescence spectra, and fullerene singlet excited state lifetimes have been determined in solvents of different polarity (toluene, dichloromethane, acetonitrile). These data suggest a tighter core/periphery contact upon increase of solvent polarity and dendrimer size. In all the investigated solvents, the fullerene triplet lifetimes are steadily increased with the dendrimer volume, reflecting lower diffusion rates of O₂ inside the dendrimers along the series. Measurements of quantum yields of singlet oxygen sensitization indicate that longer lived triplet states generate lower amounts of singlet oxygen (¹O₂^∗) in dichloromethane but not in apolar toluene suggesting a tighter contact between the dendritic branches and the fullerene core in CH₂Cl₂. In acetonitrile, the trend in singlet oxygen production is peculiar. Effectively, enhanced singlet oxygen production is monitored for the largest dendrimer. This reflects specific interactions of excited ¹O₂^∗ molecules with the dendritic wedges, as probed by singlet oxygen lifetime measurements, possibly as a consequence of trapping effects.     

A novel sensitized chemiluminescence: Infrared emissions from thiazine dyes sensitized by singlet oxygen

An unusual infrared chemiluminescence emission (8130Å) of methylene blue, and other thiazine dyes, sensitized by singlet molecular oxygen is reported. This chemiluminescence does not correspond to the ordinary fluorescence of the dye and cannot be explained by previously proposed mechanisms for singlet oxygen sensitized emissions of dyes. From energetic considerations singlet molecular oxygen in its ¹Σ_g⁺ state is postulated as the sensitizing agent for the thiazine dye chemiluminescences. Schemes in which ¹Σ_g⁺ oxygen transfers electronic excitation energy (a) to the lowest triplet state of the dye, (b) to a combined multiplicity state of the lowest triplet state of the dye, and triplet molecular oxygen, or (c) to a charge-transfer state between the dye and oxygen, are compared. The chemiluminescence of methylene blue in aqueous solution may be used as a luminescent probe for ¹Σ_g⁺ oxygen.     

Computational mechanistic study of methanol and molecular oxygen reaction on the triplet and singlet potential energy surfaces

The reaction of CH₃OH with the O₂ on the triplet and singlet potential energy surfaces (PES) was carried out using the B3LYP, MP2, and CCSD(T)//B3LYP theoretical approaches in connection with the 6-311++G(3df–3pd) basis set. Three pre-reactive complexes, ¹C1, ¹C2, and ³C1, on the singlet and triplet PES were formed between methanol and molecular oxygen. From a variety of the complexes, seven types of products are obtained, of which four types are found to be thermodynamically stable. Results reveal that there exists one intersystem crossing between triplet and singlet PES. For P4 adduct that is the main and kinetically the most favorable product, the rate constants are calculated in the temperature range of 200–1,000 K in the reliable pathway.     

A Targeting Singlet Oxygen Battery for Multidrug-Resistant Bacterial Deep-Tissue Infections

Traditional photodynamic therapy (PDT) is dependent on externally applied light and oxygen, and the depth of penetration of these factors can be insufficient for the treatment of deep infections. The short half-life and short diffusion distance of reactive oxygen species (ROS) also limit the antibacterial efficiency of PDT. Herein, we designed a targeting singlet oxygen delivery system, CARG-Py, for irradiation-free and oxygen-free PDT. This system was converted to the “singlet oxygen battery” CARG-¹O₂ and released singlet oxygen without external irradiation or oxygen. CARG-¹O₂ is composed of pyridones coupled to a targeting peptide that improves the utilization of singlet oxygen in deep multidrug-resistant bacterial infections. CARG-¹O₂ was shown to damage DNA, protein, and membranes by increasing the level of reactive oxygen inside bacteria; the attacking of multiple biomolecular sites caused the death of methicillin-resistant Staphylococcus aureus (MRSA). An in vivo study in a MRSA-infected mouse model of pneumonia demonstrated the potential of CARG-¹O₂ for the efficient treatment of deep infections. This work provides a new strategy to improve traditional PDT for irradiation- and oxygen-free treatment of deep infections while improving convenience of PDT.     

Singlet Sigma: The "Other" Singlet Oxygen in Solution

The lowest excited electronic state of molecular oxygen, O₂(a^1-DL_g), is often called simply singlet oxygen. This singlet delta state is an acknowledged and well-studied intermediate in many solution-phase photosystems. However, the second excited electronic state of oxygen, O₂(b¹δ_g+), is also a singlet. It has recently become possible to monitor this singlet sigma state in solution, which, in combination with studies of the singlet delta state, contributes to a better understanding of a variety of general problems in chemistry.     

INVESTIGATIONS ON THE LIGHT-EMITTING SPECIES IN THE REACTION OF METMYOGLOBIN AND METHEMOGLOBIN WITH HYDROGEN PEROXIDE

Abstract The formation of a compound I type ferryl complex in the reaction of methemoglobin (MetHb) and metmyoglobin (MetMyo) with hydrogen peroxide is accompanied by strong chemiluminescence. An approach to identify the nature of the light-emitting species was made by the use of quenchers and sensitizers reacting with singlet oxygen and compounds interfering in the formation and reactivity of other reactive oxygen species. Singlet oxygen is not the source of light emission. This could be concluded from the results obtained using the specific singlet oxygen trap 9,10-anthracenedipropionic acid (ADPA) in combination with high-performance liquid chromatography (HPLC) analysis. The singlet oxygen adduct of ADPA was not formed in the incubation systems (MetHb or MetMyo/H₂O₂). Instead, ADPA was oxidized by the ferryl ion to a different oxidation product, which was characterized by HPLC and IR spectroscopy. In the case of MetHb-related chemiluminescence, light emission does not result from a single source. Both, SH-groups and O₂^.¯ radicals are involved because blocking of thiol-groups with N-ethylmaleimide (NEM) and scavenging of O₂^.¯(by superoxide dismutase) suppressed chemiluminescence by 50% and 30%, respectively. Development of MetMyo-related chemiluminescence is not dependent on thiol groups (which are not present in the globin moiety) and also 0₂^.¯is not involved. Although generation of chemiluminescence in MetHb and MetMyo seems to follow different mechanisms, both types of light-emitting species are sensitive to antioxidants, such as uric acid and ascorbate. The detection of the respective free radicals by means of ESR demonstrates that both MetHb- and MetMyo-mediated chemiluminescence is associated with a strong one-electron oxidizing species, which seems to be identical with the light-emitting source itself. Also desferal, which was originally used to exclude the involvement of a Fenton-type reaction, was readily oxidized to the nitroxide free radical associated with a strong decrease of chemiluminescence. This quenching effect was not dependent on iron complexation because the addition of iron was ineffective. In summary, chemiluminescence is not restricted to a single chemical process but is related to different one-electron transfer reactions from globin residues to the oxo-heme center.     

Enhancement of singlet oxygen generation based on incorporation of oxoporphyrinogen (OxP) into microporous solids

Singlet oxygen, ¹O₂, generating compounds are highly useful for photodynamic therapy or organic oxidative transformations. In this work, the synthesis and photochemical performances for singlet oxygen generation of a range of oxoporphyrinogen-containing porous coordination polymers (OxP-PCPs) are reported. Oxoporphyrinogens, a previously unreported class of singlet oxygen generators derived from the oxidation of the antioxidant-substituted porphyrin tetrakis(3,5-di-tert-butyl-4-hydroxyphenyl)porphyrin, were converted to molecular tectons by the introduction of oligophenylene-carboxylate linkers and incorporated into porous coordination polymers using well-known oxo-Zr(IV)₆ cluster chemistry. Their structures and textural properties were analyzed revealing substantial surface areas up to 650 m² g^?1 for the optimum linker length (biphenylyl). The oxoporphyrinogen precursors exhibit good quantum yields of singlet oxygen generation (up to Φ = 0.37), and a high level of activity is maintained in the resulting coordination polymers, which appear to be superior for singlet oxygen generation to the precursors and to a reported reference material. These OxP-PCP materials were applied for the selective oxidation of sulfides to sulfoxides. This work demonstrates that the excellent singlet oxygen generator oxoporphyrinogens can be successfully incorporated as porous solids and conveniently applied in heterogeneous oxidative transformations.     

Singlet Oxygen Generation by Cyclometalated Complexes and Applications

While cyclometalated complexes have been extensively studied for optoelectronic applications, these compounds also represent a relatively new class of photosensitizers for the production of singlet oxygen. Thus far, singlet oxygen generation from cyclometalated Ir and Pt complexes has been studied in detail. In this review, photophysical data for singlet oxygen generation from these complexes are presented, and the mechanism of ¹O₂ generation is discussed, including evidence for singlet oxygen generation via an electron‐transfer mechanism for some of cyclometalated Ir complexes. The period from the first report of singlet oxygen generation by a cyclometalated Ir complex in 2002 through August 2013 is covered in this review. This new class of singlet oxygen photosensitizers may prove to be rather versatile due to the ease of substitution of ancillary ligands without loss of activity. Several cyclometalated complexes have been tethered to zeolites, polystyrene, or quantum dots. Applications for photooxygenation of organic molecules, including “traditional” singlet oxygen reactions (ene reaction, [4 + 2] and [2 + 2] cycloadditions) as well as oxidative coupling of amines are presented. Potential biomedical applications are also reviewed.     

PHOTOCHEMICAL DECOMPOSITION OF 1, 4-BENZODIAZEPINES. NITRAZEPAM*

Abstract— The photochemical activity of nitrazepam, a drug of which phototoxic effects are known, is investigated. Nitrazepam decomposes photochemically in an oxygen-poor medium, while it is relatively stable in the presence of oxygen. It appears that this quenching of excited nitrazepam by molecular oxygen leads to the formation of singlet oxygen. This is demonstrated in three different ways: 1. In the presence of 2-methyl-2-pentene as acceptor of singlet oxygen, two characteristic products are formed: 2-methyl-1-penten-3-ol and 2-methyl-3-penten-2-ol. 2. The photosensitized oxidation of L-dopa in the presence of nitrazepam is increased when D₂O, instead of H₂O, is added to the solvent system, due to the longer lifetime of singlet oxygen in D₂O. 3. The β-value, the ratio of the rate of decay of singlet oxygen to its rate of reaction, is determined with L-dopa as acceptor of singlet oxygen. The same value is obtained with both nitrazepam and methylene blue, a known singlet oxygen generator.     

Determination of the quantum yield of singlet oxygen sensitized by halogenated boron difluoride dipyrromethenes

The spectral–luminescent, photophysical, and photochemical properties of dichloro-, dibromo-, and diiodo-derivatives of boron dipyrromethenate (BODIPY) have been studied, as well as the feasibility of generating singlet oxygen (¹O₂) via its photosensitization by the dihalogenated derivatives of BF₂ dipyrromethene in solutions. Quantum yields of singlet oxygen have been determined using 1,3-diphenylisobenzofuran as the ¹O₂ trap. The lowest fluorescence quantum yields have been shown to correspond to the maximum yields of singlet oxygen. It has been found that the best ¹O₂ photosensitizer among the three test dihalotetraphenylaza- BODIPY is dibromotetraphenylaza-BODIPY, which in addition possesses the highest photostability. Diiodotetramethyl-BODIPY results in the singlet oxygen yield close to unity, but it has significantly lower photostability. The yield of singlet oxygen is affected by the solvent. Dibromtetraphenylaza-BODIPY and diiodotetramethyl-BODIPY may find use as a medium in photodynamic therapy and photocatalysis of oxidation reactions.     

Photooxygenolysis of 3,6-di-tert-butyl-o-benzoquinone

The photolysis of 3,6-and 3,5-di-tert-butyl-o-benzoquinones in benzene (λ > 380 nm, inert atmosphere) involves decarbonylation of the compounds to furnish respectively 2,5-and 2,4-di-tert-butylcyclopentadienones. The 2,5-isomer is stable, and the 2,4-di-tert-butylcyclopentadienone suffers a conversion into a Diels-Alder adduct. The participation of oxygen inhibited the decarbonylation and changed the direction of the photolysis: Here the products of the 3,5-di-tert-butyl-o-benzoquinones conversion were a di-tert-butylmuconic anhydride and dipivalylethylene. It was concluded that a singlet oxygen was involved in the process which formed by a triplet-triplet annihilation at the interaction of ³O₂ with a triplet-excited initial quinone.     

Taming of Singlet Oxygen: Towards Artificial Oxygen Carriers Based on 1,4-Dialkylnaphthalenes

Naphthalene endoperoxides are known as convenient sources of singlet oxygen (O₂, ¹Δ_g), which is the major product of endoperoxide cycloreversion reaction. However, their potential as carriers of ground-state molecular oxygen (O₂, ³Σ_g) similar to artificial oxygen carriers remains largely unexplored. This is due to the extreme reactivity and cytotoxic effects of the released singlet oxygen. We now report that a compound with a bimodular design, which incorporates an endoperoxide and an efficient physical quencher of singlet oxygen, 1,4-diazabicyclo[2.2.2]octane (DABCO), produces exclusively ground-state molecular oxygen. This result, coupled with the fact that oxygen release rates from endoperoxides are highly amenable to fine-tuning in a very broad range, and open to targeting by ligand attachment, raises the potential of these compounds far above any comparable chemical, or even biochemical sources. In cell culture experiments, we showed that the addition of the endoperoxide-quencher conjugate can enhance and sustain cell proliferation.     

Ein Kobalt(II)-Corrin-Komplex als reversibler Sauerstoffträger. Eine ESR.-spektroskopische Untersuchung

The diamagnetic crystalline Jodo-Co(II)-cobyrinicacid-heptamethylester dissolves in benzene or toluene forming solutions which contain the monomeric paramagnetic complex unit. From anisotropic ESR.-parameters, measured in frozen solutions, follows a ground state configuration (d_xz,d_yz)⁴(d_xy)²(d_z²)¹ for the Co(II). The unpaired electron is highly delocalized to the axially coordinated iodide ion, giving rise to a strong hyperfine coupling with the iodine nucleus. The complex forms in a very fast and completely reversible reaction a 1:1 adduct with molecular oxygen in toluene solutions below ca. 280° K. ESR.-parameters of the oxygen adduct are presented and discussed. Thermodynamic data for the formation reaction of the adduct are estimated.