Singlet-oxygen

Singlet oxygen is generated by excitation of polymathine dyes 

Deactivation of 2 by solvent molecules is a collisional E - V+R+T energy transfer process it occurs by coupling of the highest fundaumental vibrational mode X-Y of the acceptor molecule with an 02( Ag, v = 0) + v - m) transition. 

Oxidation of Aliphatic Compounds Alkfuies have been photoox id ised in aqueous solutions of Hg(II) salts and their use on a preparative scale described. An experimental study and theoretical modelling of methane CH bond photoactivation by M0O3 and Cu -doped M0O3 has appeared and cyclohexane and 1,4-dimethylcyclohex5me have been 

4-endoperoxides.The photocatalytic activity of silica gel-supported zirconium oxide catalysts in the oxidation of Isobutylene to acetone has been correlated with their luminescence intensity, and the binding related to the mechanism of active centre formation in the catalysts. The influence of the active sites and structure of inorganic supports on the oxidative cleavage of olefins with oxygen in dry media has been investigated and poisoning experiments have been used to show 

Singlet oxygen has been generated by theraal decoaposltion of the water-soluble endoperoxide (4), and light eiission due to the 

Seml-empiricai and ab initio calculations have been used to study the l,2 cycloaddition of 02( Ag) to aminoethylene and ethylene. Electron-rich olefins react via a zwitterionic intermediate and the total reaction paths were determined using Fakul s IRC theory.The same group has examined 

Singlet oxygenation of soae 6-substltuted cycloocta-l,4-dlenes has been shown to be a convenient selective route to cls-5.8-dlsubstituted and trans-5,6-dlsubstituted cycloocta-l,3-dlenes. The stable conformation of the starting material In the ground state appears to be responsible for the observed selectivity. 6-Substltuted cycloocta-l,4-dlenes are 

0- phellandrene produces a mixture of els- and trans-endooerox1des and all the hydroperoxides derivable by ene reaction of the substrate with 

A common first step for endoperoxide and hydroperoxide formation has been developed. An effective stereoselective synthesis of Z-2-(2-methyl-enecyclohexylidenelethanoi by the photosensitized oxygenation of 

Although singlet oxygen was postulated as the reactive agent in sensitized photooxidations over 60 years ago (Kautsky and de Bruin, 1931), its importance was not recognized until it was rediscovered by Foote and Wexler (1964) more than 30 years later. An explosion of interest in the species followed, which is still continuing today. 

In water, O2 is rapidly deactivated back to the ground state by collision with solvent molecules the lifetime has been calculated to be approximately 4 microseconds (Rodgers and Snowden, 1982). In organic solvents, particularly those with halogen atoms, the rate of quenching by solvent decreases greatly in CCI4, for example, its lifetime is 700 microseconds (Merkel and Kearns, 1972). In the vapor phase it can have lifetimes of several seconds. 

Singlet oxygen has been reported to be generated at soil surfaces (Gohre and Miller, 1983 Gohre et al., 1986), based on analyses of the products formed when 

Singlet oxygen interacts with organic molecules either by collisional (physical) 

The natural peroxide, ascaridole (10) could be biosynthesized by a O2 reaction from its diene precursor, a-terpinene (11). This reaction has been accomplished in vivo (Gollnick and Schenk, 1967). 

The photochemical generation of singlet oxygen has suffered from the insolubility of those anionic dyes which are commonly used as sensitizers (see Fig. 11.1). It has recently been found that the photosensitizing dyes eosin and rose bengal can be solu- 

On a first, very broad, approximation singlet oxygen behaves somehow like ethylene. Three types of reactions of 102 are usually observed and have been utilized in organic synthesis 606 608) a) the Diels-Alder like cycloaddition to dienes (6.1) b) the ene reaction with olefins (6.2) and c) cycloaddition to activated double bonds (6.3). 

Singlet oxygen adds to cyclic 1,3-dienes to afford 1,4-endoperoxides 609). This type of reaction has first been applied successfully to the synthesis of ascaridole from a-terpinene 610) (6.4) and of the vesicatory compound cantharidine 6U) (6.5) some 25 years ago. 

An interesting umpolung effect has been found in the alkylation of pyrroles with carbon nucleophiles via the endoperoxide (6.6)612). 

The corresponding endoperoxides of furans usually ring-open to afford 1,4-diketones. Such a sequence, followed by an intramolecular Diels Alder reaction was utilized in the synthesis of the tts-indacenyl dianion (6.7) 613 . 

Endoperoxides of carbocyclic 1,3-dienes are most often transformed into either 1,4-diols, 1,3-diepoxides or 1,4-hydroxyenones. The 1,4-diol formation is illustrated in the synthesis of the sesquiterpene ( )-cybullol (6.8)614). 

Molecular oxygen exists in the three lowest-lying states as a triplet state ( S ), singlet oxygen 02( Ag) and 02( Sg ) (Fig. 7.2) [1157]. Singlet oxygen has been reviewed in detail in several books [669, 1764, 1807, 1894, 2216], and papers [795, 1382, 1754, 1784, 1786, 1805, 1808, 1923]. 

Isolated molecules of 02( Ag) at low pressure undergo a spontaneous transition to the ground state ( 2 ), which has a half-life of 45 min [1505]  

Collisions with other molecules can shorten this lifetime by two methods  

They can induce an electric dipole transition at the same wavelength. 

They can induce a radiationless transition to the ground state. 

As usual, this Section serves to review only those papers concerning singlet molecular oxygen ( 2, XA0) which have general or organic significance. 

The possible participation of 1Oa in the mechanisms of photochemical air pollution has been reviewed by Pitts and Finlayson,97 and the photochemical reactions of 102 have been reviewed (in Japanese).98 Nilsson and Kearns99 have investigated the part played by singlet oxygen in some chemiluminescence and 

Sato and Y. Kanaska, Chem. and Pharm. Bull. Japan), 1974, 22, 799. 

New sources of singlet oxygen continue to be developed. Near i.r. radiation ( 900 nm), in the presence of a cyanine dye as sensitizer, has been used to generate 102.100 Thermal decomposition of the ozonide (52) produces 102 and the 

Bonneau, R. Pottier, O. Bagno, and J. Joussot-Dubien, Photochem. and Photobiol., 1975, 21, 159. 

Erythrocuprein inhibited the luminol-enhanced chemiluminescence caused by singlet oxygen produced in a xanthine-xanthine oxidase system as measured in a Packard scintillation counter (Weser and Paschen 1972). 

Electronically exited oxygen in the Ag state has been shown to be a product of a number of enzymatic reactions (Piatt et al. 1977, Piatt and O Brien 1979, Karnofsky 1983, 1984, 1985, 1988, 1991, Karnofsky et al. 1983, 1984, 1985, 1988, Khan et al. 1983). 

Lipid peroxidation in microsomal fraction initiated by hydroperoxides or iron/ascorbate, as well as lipid peroxidation of isolated hepatocytes under oxygenation, has been studied extensively by Cadenas et al. (1981) and Cadenas and Sies (1982). The available evidence points to 02 formation by the Russell mechanism (Russell 1957, Howard and Ingold 1968). Secondary lipid-peroxy radicals react via a transient tetroxide to yield an alcohol, a carbonyl and oxygen, the carbonyl or oxygen being in an electronically excited state. 

In phorbol myristate acetate-stimulated polymorphonuclear leucocytes it is important to note that the generation of Ag02 was dependent on the dose of phorbol myristate acetate added and that stimulation with high-dose phorbol myristate acetate resulted in lower amounts of detectable Ag02 (Karnofsky 1994). This finding was not entirely unexpected since higher concentration of phorbol myristate acetate lead to a greater production of 

Oj and little net acaunulation of AgOj, as Oj at high concentrations quenches Ag02 (Rosenthal 1975, Guiraud and Foote 1976, Khan 1977). 

Reviews have appeared on photo-oxidation and toxicity, and on the involvement of singlet oxygen in the photofading of dyes.  

Maruyama, S. Arakawa, A. Osuka, and H. Suzuki, Kokagaku Toronkai Koen Yoshishu, 1979, 24. 

The reactions of oxygen and oxygen (2 Pj) with halomethanes have been 

An equation has been derived for the quantum yield of photoperoxidation of unsaturated organic molecules, and has been applied to lipoic acid inhibition of the self-sensitized photoperoxidation of 1,3-diphenylisobenzofuran in acetonitrile. Evidence has been presented to suggest that in benzene solution photoperoxidation of 1,3-diphenylisobenzofuran proceeds with formation of 

A design has been described for an inexpensive apparatus for the measurement of the lifetimes of long-lived ( 10 s) exponential processes, and its use has been illustrated by an example in the study of photo-oxidation. 

A further important reaction involving the hypohalous acids is in the generation of singlet oxygen ( 02), which is characterised by the pairing of the two outer orbital electrons [12] as shown in Eq. 6  

The rate constants for reaction of singlet oxygen vary significantly between biomolecules with greatest reactivity towards amino acid side chains, thereby affording some specificity of response [ 15]. Nevertheless, it can be considered as a powerful short-lived oxidant. The most frequent type of reaction initiated by this species is addition across unsaturated bonds yielding a cyclised product. This has been extensively reviewed by Davies [16] and will not be discussed further here. 

C-Phenyl-N-tert-butylnitrone is reported to deactivate O C Ag) primarily by a reversible electron-tr msfer mechanism and queuititative aspects of the quenching of all-trans-retinol by singlet aund triplet oxygen have been described.Intermolecular perturbation and Cl calculations have been carried out to elucidate the attacking modes of on allyllc and 

The chemiluminescence intensity at x-1270 for the H2O2 + NaOCl reaction amd corresponding to the Cg trameition of has been measured as a function of the concentrations of H2O2 amd NaOCl. 

Quantum yields of formation of OaC Ag) obtained by irradiation of benzene solutions of various flavonoids, and by irradiation of oxodiperoxo-molybde-num(VI) complexes of the type (L-L)MoO(02 2 (L-L = 2-(l-alkyl-3-pyrazolyl)-pyridine, alkyl = Bu, n-octyl, n-octadecyl L-L = 2-(3-n-octyl-2- 

Photo-oxidation of olefins in pyridine solution in the presence of FeCIs has been found to take one of three courses, depending on the substitution pattern of the substrate olefin. These routes lead to formation of a-chloroketones, gem-dichloroketones, and a, t -dichloroketones, and have all been illustrated by their use in actual syntheses. a-Chloroketones have also been produced by irradiation of pyridine solutions of vinylsUanes or vinyl sulphides under similar conditions. DCA-sensitized photo-oxidation of l,2-diphenyl-3,3,4,4-tetramethylcyclobutene in MeCN leads to the corresponding ozonide in a process which occurs by initial formation of an oxirane. Jasmine lactone (12  

Kuiild, X Sakai, and A. Takeda, Chem. Lett., 1983, 911. 

Zelnik, E. Rabenhorst, A. Haider, J. Lauterwein, and H. Wyler, Hetv. Chim. Acta, 1983, 66, 780. 

Oestrone has been photo-oxygenated to a 1 1 mixture of the oestradienes (13 R = HOO, HO) and this transformation is a new application of photooxygenation to the synthesis of 19-norsteroids.  

An examination has been made of substituent effects on the product distribution of tropylidene- and norcaradiene-derived endoperoxides arising on singlet oxygenation of 7-aryl- and 7-alkylcyclohepta-l,3,5-trienes. The ratio 

20-Tetrakis(heptafluoropropyl)porphyrin (TPFPP) in a fluorous biphasic medium is effective for 02( Ag) generation and possesses good stability for 

4- diazabicyclo[2.2.2]octane, 2,6-di-tert-butyl-4-methylphenol, and lauric acid is reported to occur by 02( Ag)-quenching only. SOSDF is emerging as a highly sensitive method for the measurement of rate constants for 02( Ag) quenching. 

The quenching kinetics of 02( Ag) by the colourless trap cyclohexa-l,3-dien- 

We see in nature not words, but rather only the first letters of words, and if we then wish to read, we discover that the new so-called words are again merely first letters of others. 

Explanation of the Development of Photoluminescence in Fluorescing Systems as a Result of Oxygen The Formation of Active, Diffusible Oxygen Molecules through Sensitization. 

This experiment should only be carried out in a well-ventilated hood. Safety glasses and protective gloves must be worn. Chlorine gas is highly toxic  

Sodium hydroxide solution and H2O2 are very corrosive Skin contact must be avoided at all costs. 

CI2 gas cylinder or Kipp s gas generation apparatus, 500-mL wash bottle with glass frit, 250-mL and 150-mL beakers, large plastic basin for use as an ice bath, two measuring cylinders, funnel, stand, clamp and boss, PVC tubing, protective gloves, safety glasses. 

Photodissociation of O3 using 335 -352 nm radiation leads to the formation of 02(b Sg ), and studies indicate that absorption of only a single photon is involved.  

The absolute phosphorescence quantum yield (Op) of 02( Ag) sensitized by phenalenone and its lifetime have been determined in a range of different solvents, and from measurements of the radiative rate constant and lifetime of OiC Ag) in various solvents, it has emerged that charge transfer interaction is the main factor responsible for removing the prohibition on the radiative spin-forbidden transition Ag Low temperature photolysis 

The importance of the reaction environment is a theme which is common to a number of reports relating to the generation of 02. Microcrystalline cellulose 

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Recently the solvent effect on the [4+2] cycloaddition of singlet oxygen to cyclic dienes has been subjected to a multiparameter analysis. A pre-equilibrium with charge-transfer character is involved, which is affected by the solvent through dipolarity-polarisability (n ) and solvophobic interactions ( Sjf and Another multiparameter analysis has been published by Gajewski, demonstrating the... 

Hindered amines, such as 4-(2,2,6,6-tetramethylpiperidinyl) decanedioate, serve as radical scavengers and will protect thin Aims under conditions in which ultraviolet absorbers are ineffective. Metal salts of nickel, such as dibutyldithiocarbamate, are used in polyolefins to quench singlet oxygen or elecbonically excited states of other species in the polymer. Zinc salts function as peroxide decomposers. 

Meta.1 Complexes. The importance of Ni complexes is based on their effectiveness as quenchers for singlet oxygen. Of disadvantage is their low colorfastness and their lower ir-reflectance compared to cyanine dyes (qv) therefore they are used in combination with suitable dyes. Numerous complexes are described in the Hterature, primarily tetrathiolate complexes of Pt or Ni, eg, dithiolatonickel complexes (3). Well known is the practical use of a combination of ben2othia2ole dyes with nickel thiol complexes in WORM disks (Ricoh, TDK) (17). 

The mechanism of chemiluminescence is still being studied and most mechanistic interpretations should be regarded as tentative. Nevertheless, most chemiluminescent reactions can be classified into (/) peroxide decomposition, including biolurninescence and peroxyoxalate chemiluminescence (2) singlet oxygen chemiluminescence and (J) ion radical or electron-transfer chemiluminescence, which includes electrochemiluminescence. 

Dioxetanes are obtained from an a-halohydroperoxide by treatment with base (41), or reaction of singlet oxygen with an electron-rich olefin such as tetraethoxyethylene or 10,10 -dimethyl-9,9 -biacridan [23663-77-6] (16) (25,42). 

Classical chemiluminescence from lucigenin (20) is obtained from its reaction with hydrogen peroxide in water at a pH of about 10 Qc is reported to be about 0.5% based on lucigenin, but 1.6% based on the product A/-methylacridone which is formed in low yield (46). Lucigenin dioxetane (17) has been prepared by singlet oxygen addition to an electron-rich olefin (16) at low temperature (47). Thermal decomposition of (17) gives of 1.6% (47). 

Dioxetane decomposition has also been proposed to account for chemiluminescence from other reactions (43), including gas-phase reactions of singlet oxygen with ethylene and vinyl ethers (53). 

Most likely singlet oxygen is also responsible for the red chemiluminescence observed in the reaction of pyrogaHol with formaldehyde and hydrogen peroxide in aqueous alkaU (152). It is also involved in chemiluminescence from the decomposition of secondary dialkyl peroxides and hydroperoxides (153), although triplet carbonyl products appear to be the emitting species (132). 

Decomposition of adducts such as triphenyl phosphite—ozone provides a convenient method for accomplishing chemical oxidations involving singlet oxygen and making it a useful oxygenating agent for synthetic and mechanistic appHcations. 

Many hydroperoxides have been prepared by autoxidation of suitable substrates with molecular oxygen (45,52,55). These reactions can be free-radical chain or nonchain processes, depending on whether triplet or singlet oxygen is involved. The free-radical process consists of three stages ... 

Singlet oxygen reacts with olefins presumably by the "ene" reaction to form allyflc hydroperoxides (45,57), eg, l-methyl-2-propenyl hydroperoxide [20733-08-8] is produced from 2-butene (eq. 19). The regioselectivity of this reaction has been investigated (58). 

The endoperoxides of polynuclear aromatic compounds are crystalline soHds that extmde singlet oxygen when heated, thus forming the patent aromatic hydrocarbon (44,66,80,81). Thus 9,10-diphenyl-9,10-epidioxyanthrancene [15257-17-7] yields singlet oxygen and 9,10-diphenylanthracene. 

Hydrolysis is accelerated in the presence of strong acids. However, in the presence of aqueous bases such as sodium hydroxide, the rate of decomposition increases with increasing pH and teaches a maximum at the of the petoxycatboxyhc acid (ca 8.25), then decreases at higher pH (169,170). The basic decomposition products include the parent catboxyhc acid and singlet oxygen (171,172). Because the maximum rate of decomposition occurs at the p-K, the petoxycatboxyhc acid and its anion ate involved in the transition state (169). 

Phenols with bulky ortho- and para-substituents, eg, phenoHc antioxidants, do not undergo this reaction however, they scavenge radicals generated by thermolysis of diacyl peroxides and other peroxides. Diacyl peroxides react with potassium superoxide, KO2, forming singlet oxygen (207). 

The role of rose bengal and other sensitizer dyes in the photodimerization of 2-acet5i-l,4-benzoquinone [1125-55-9] involves electron transfer but not singlet oxygen (42) (see Dyes, SENSITIZING). 

Reaction between oxygen and butadiene in the Hquid phase produces polymeric peroxides that can be explosive and shock-sensitive when concentrated. Ir(I) and Rh(I) complexes have been shown to cataly2e this polymerisation at 55°C (92). These peroxides, which are formed via 1,2- and 1,4-addition, can be hydrogenated to produce the corresponding 1,2- or 1,4-butanediol [110-63-4] (93). Butadiene can also react with singlet oxygen in a Diels-Alder type reaction to produce a cycHc peroxide that can be hydrogenated to 1,4-butanediol. 

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