Methanol photooxidation

Figure 4. Methanolic photooxidation of methoprene (reaction with singlet oxygen (40)...

Based on their experimental results concerning ethanol and methanol photooxidation by Au/Ti02 composites under visible-light illumination, Tatsuma et al. (Tian and Tatsuma 2005) proposed a plasmon-induced charge separation scheme. They observed a surprising phenomenon, in which the photoelectrons were excited from Au nanoparticles and transferred to the CB of Ti02 (Wood et al. 2001 Subramanian et al. 2004). Meanwhile, the oxidized Au species accepted electrons from the donor molecules present in the solution to recover the charge balance. The process is illustrated in Fig. 16.23. 

Similarly, photooxidation of dihydrocoralyne (108) in hot methanol at pH 8, subsequent addition of sodium methoxide and additional irradiation yielded 6,7-dimethoxyisoquinolone and 3-methyl-3,5,6-trimetho-xyphthalide via the betainic intermediate 109 (77H45) (Scheme 39). It was demonstrated earlier that dihydrocoralyne is oxidized to this betaine in quantitative yields under physiological conditions (76H153). The autoox-idative degradation of the mesomeric betaine was rationalized by the addition of singlet oxygen. 

Water, methanol, and n-hexane do not influence the photooxidation of PVC (43), but the photodegradation is accelerated by ferric chloride (70,71) and certain other compounds containing iron (70,71,72). Purification of the polymer might be expected to enhance its photostability by removing deleterious impurities such as iron compounds that are derived from metal equipment. This type of result was obtained in one recent study (58) but not in others (30,59). In contrast, the photo-oxidative degradation of PVC should be enhanced by admixture of the polymer with materials that are unusually susceptible to photooxidation themselves. Such behavior has been observed for impact-modified PVC containing polybutadiene-based polyblends (69,73). 

Behymer and Hites (1985) determined the effect of different substrates on the rate of photooxidation of anthracene (25 pg/g substrate) using a rotary photoreactor. The photolytic half-lives of anthracene using silica gel, alumina, and fly ash were 1.9, 0.5, and 48 h, respectively. Anthracene (5 mg/L) in a methanol-water solution (1 1 v/v) was subjected to a high pressure mercury lamp or sunlight. Based on a rate constant of 2.3 x lO /min, the corresponding half-life is 30 min (Wang et al., 1991). 

CASRN 20354-26-1 molecular formula CgHeCLNzOs FW 261.06 Photolytic. Ivie et al. (1973) studied the photooxidation of methazole in methanol, water, and surface droplets using UV light. In methanol, only small quantities of unreacted methazole... 

In a thorough study on photooxidation of 2,5-dimethyl-2,4-hexadiene (455) it was found that 1,2-dioxene 456, 1,2-dioxetane 457, hydroperoxy dienes 458 and 459 and, when methanol was used as solvent, also hydroperoxy(methoxy)octene 460 are formed (Scheme 124) . Product distribution was found to be highly solvent dependent. These results led investigators to postulate a mechanism involving the intermediacy of perepoxide 461 and zwitterion 462 (Scheme 124). Accordingly, the product of [4-1-21-cycloaddition 456, the product of [2 + 2]-cycloaddition 457, as well as the products 458 and 459 deriving from ene-addition would originate from polar intermediates 461 and... 

Completely different results from those obtained in the photooxidation of 2.4.6-tri-tert-butyl-X phosphorin 24 (p. 54) are obtained in the photooxidation of 1.1-dimethoxy-2.4.6-tri-tert-butyl-X -phosphorin 183, as Schaffer has found. In this case the 2-hydroxy-endoxy-phosphinic acid methyl ester 213 can be isolated in about 20% yield. Its formation can be explained by assuming normal 1.4-addition to 212 as the primary product which is transformed to 213 by hydrolytic ring cleavage of the peroxide bridge, followed by loss of methanol. 

Interesting photooxidation products of betaines 363 and 364 have also been reported. Irradiation of methanol solutions (> 0.1%) of compound 364 in the presence of oxygen gives a product (CjqH, 7NO7), mp 100.5-101.5°C, which has been formulated as adduct 373 (42%). In more dilute solutions the product is berberal (374 R = H) (56%). Products 375, 376, and 374 (R = OMe) have been obtained by photooxygenation of the 8-methoxy-betaine 363. ... 

Only one other photooxide of a benzo[c]furan has been described. Hydrocarbon 232 on treatment with oxygen in methanol gives 233 in 96% yield further treatment with oxygen in benzene or tetrahydrofuran leads to 234 probably the corresponding benzo[c]furan is involved. Whereas reduction with triphenylphosphine in benzene or KI in acetic acid gives 235, 2 hours reflux in benzene leads to 235, 236, and 237 in 7, 48, and 3% yield, respectively. 

Although the oxidation of indoles by air and light has been studied extensively, e.g., the photooxidation of tryptophan to kynurenine and 3-hydroxykynurenine in the presence of methylene blue as sensitizer,311 313 little is known about the course of pyrrole photooxidation.173180 184 Under very mild conditions, irradiation of 2,3,4,5-tetra-phenylpyrrole in methanol and in the presence of air and methylene blue effected its oxidation to 5-methoxy-3,4-epoxy-2,3,4,5-tetraphenyl-A -pyrroline (CXVI) and a-A-benzoylamino-a -benzoyl stilbene(CXVII).303 Photooxidation in the presence of potassium hydroxide gave the lactam (CXVIII). 

Benzo[c]furans undergo very rapid photooxidation. 1,3-Diphenylbenzo[c ]furan yields the oxide (368) on sensitized photooxygenation in ether at -50 °C. Reduction of the oxide with potassium iodide in acetic acid affords 1,2-dibenzoylbenzene and reaction with methanol supplies the hydroperoxide (369). 

Reports of the photosensitivity of polyoxometalates appeared as early as 1916 (396). Early systematic work on photocatalysis was done by Yamase et al. (397-402). For example, methanol is photooxidized to give formaldehyde in the presence of M07O24 according to Eq. (46). 

MPT reacts somewhat more slowly with singlet oxygen than PTH. The combined rate constants of physical quenching (e.g. Equation 32) and chemical reaction (e.g. Equation 27) have been calculated from photooxidation experiments in bromobenzene/methanol (2/1) to be 1.2x10 M s-1 for MPT and 4.2x10 for PTH (26). Hovey has calculated... 

In 1979, an interesting effect on the dioxetane/diendoperoxide product ratio was observed upon the addition of methanol to acetone. The addition of methanol increased the dioxetane/diendoperoxide ratio. The photooxidation of 2,3-dipheny-lindene 15 afforded its dioxetane 31 in methanol at —78 °C [26]. On the basis ofliquid chromatography (LC) analysis and NMR spectroscopy, dioxetane 31 formed in high yields in methanol or methanol-acetone (3 7) mixtures. However, the formation also occurred of a diendoperoxide with the solvents Freon-11 and acetone-d6 (the topic of bisperoxides will be discussed in Section 11.5.2.1). The synthesis of benzofuran dioxetanes has also been accomplished from a TPP-sensitized photooxidation of benzofurans [27, 28]. 

In a study of a nonchain photooxidation of acetone one is concerned with the oxidation of methyl radicals and acetyl radicals. Figure 4 shows that under simple conditions the yields of formaldehyde and methanol are equal and this is a feature of other systems (to be described later) that produce methyl radicals. 

The experimental facts are clear that equal quantities of methanol and formaldehyde are produced in the initial stages of the oxidation of methyl radicals under many conditions,81 46 100 but recently similar amounts of methyl hydroperoxide have been determined in the photooxidation of acetone81 and it has also been detected in the photooxidation of methyl iodide87 and azomethane.7 106 122 Under some conditions,67 e.g., low pressures, the oxidation of methyl radicals appears to give more formaldehyde than methanol in the early stages, while at high temperatures (500°C.)64 little methanol is produced. 

Studying the photooxidation of methyl iodide at room temperature they show that the addition of hydrogen iodide does not give the methanol yields which might be expected from the reaction... 

The first comprehensive study was made by Taylor and Blacet,20 who studied the nature of the products at 3130 A. at temperatures from 60 to 140°C. and in the presence of large pressures of oxygen (10-124 mm.) Table V. Conditioning of the cell walls by photooxidation or by boric acid helped to reduce the considerable scatter in their experimental results. Analyses, performed mass spectrometrically, gave the main products as carbon monoxide, carbon dioxide, formaldehyde, and water, while minor products were methanol, acetic acid, and a polymeric substance that gradually accumulated on the walls. No methane, ethane, or formic acid was deteoted. 

Calvert and Hanst88 using infrared analysis, have also re-investigated the photooxidation of acetaldehyde at 20°C. using 3130 A. radiation. Acetaldehyde pressures were chiefly about 42.5 nun., but the oxygen pressure was varied from 0 to 745 mm. Analyses were made for carbon monoxide, carbon dioxide, formic acid, methanol, acetic acid, peracetic acid, acetyl peroxide, methyl hydroperoxide, and unreacted acetaldehyde (Table X). Chains were short. Although they do not detect methyl hydroperoxide or diacetyl peroxide, the non-observance of a peroxide does not necessarily mean it is not formed. The decomposition of hydroperoxides on the smallest particle of catalyst is remarkably fast. 

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