Benzophenone as sensitizer

Photodecomposition of A -l,2,3-triazolines gives aziridines. In cyclohexane the cis derivative (304) gives the cis product (305), whereas photolysis in benzene in the presence of benzophenone as sensitizer gives the same ratio of cis- and trans-aziridines from both triazolines and is accounted for in terms of a triplet excited state (70AHC(ll)i). A -Tetrazo-lines are photolyzed to diaziridines. 

Irradiation in polar solvents (acetonitrile, dimethylformamide, or methanol) leads to mixtures of (78) and (79) with the former predominating.<90 91) Nonpolar solvents (dioxane, benzene, or ethyl acetate) or the presence of benzophenone as sensitizer lead to a predominance of the anti isomer... 

Bis-3,4-dichlorobenzoyl peroxide, 49, 4S decomposition in benzene, 49, 44 4,4 -Bis(dimethylamino)benzophenone as sensitizer for butadiene photolysis, 47, 65... 

A dry, weighed polyethylene film (length 50 mm, breadth 25 mm, thickness 0.1-0.2 mm) is placed in a tube (about 70 ml capacity) fitted with a ground glass joint and stopcock. After addition of some crystals of benzophenone (as sensitizer) the sample is heated for 1 h on a water bath at 60 °C.The outside of the tube is dried, the stopcock closed, and the tube exposed to 15 min irradiation from a mercury lamp (maximum of emission in the region of A = 253-254 nm). 

Benzophenone as sensitizer, 19 94 1,4-Benzoquinone, benzoin catalysis, 33 62 Benzoselenadiazole complexes, osmium, 37 290-291... 

The photodimerization of coumarin has been studied in several solvents and their nature has an effect on this rather complex reaction. In a polar medium such as methanol, the only product is the cis head-to-head isomer (244) but in acetonitrile, this is accompanied by the trans head-to-head dimer (245), which becomes the main product of the reaction in non-polar solvents like benzene or dioxan. Small amounts of the head-to-tail isomers (246) are also formed in non-polar solvents (66JA5415). In some solvents, the presence of benzophenone as sensitizer is essential and it also alters the relative proportion of dimeric isomers (64JA3103, 66CB625). Photolysis of osthole (247) in the presence of acetophenone gives a mixture of two quite different dimers (248) and (249) (80MI22302), representing addition at the exocyclic and endocyclic double bond respectively. 

Exercise 28-14 Suppose the rates of the processes marked kc and kt in Figure 28-4 were the same. Which isomer would be favored at photoequilibrium if the rate of the reaction represented by k were greater than the rate represented by k On the basis of steric hindrance, would you expect the rate of the k process or the k c process to be greater, using benzophenone as sensitizer Explain. 

Boese and Goldman investigated the photocarbonylation of cyclohexane 105 cocatalyzed by d8 metal complexes, such as 106, and aromatic ketones, like acetophenone 107 or benzophenone, as sensitizers (Fig. 27) [170]. The transformation furnished cyclohexanecarbaldehyde 108. The formation of 108 was... 

A triplet route using benzophenone as sensitizer is also possible, but with reduced efficiency.42 The kinetic scheme for the photoprocess is shown in Figure 6.3 (see also Refs. 3-7). 

First attempts to induce chirality in the photoinduced addition of ketyl radicals (e.g., U) involved a,p-usaturated carbonyl compounds such as 208 derived from carbohydrates (Scheme 56) [126]. With benzophenone as sensitizer, these radicals could be added stereoselectively, and similar reactions were carried out with dioxolane and a,p-usaturated nitropyranones [127]. 

Similar fragmentation occurs on irradiation of /1-phenylthioalcohols such as 84a with benzophenone as sensitizer, resulting in the formation of products such as 85a in 40-93% yield [201]. The mechanism for this reaction involves the formation of the corresponding sulfur radical cation by electron transfer from the sulfide to excited state benzophenone (a process which will be discussed in more detail in a subsequent section). The benzophenone anion radical formed in this process then deprotonates the alcohol moiety concomitantly with C=0 bond formation and C-C bond cleavage as indicated in 86. 

Certain features of alcohol photo-oxidation are connected with the presence of sensitizers. As found by Backstrom [60], the rate of photooxidation of 2-propanol (with benzophenone as sensitizer) is inversely proportional the oxygen pressure. Dependence of the 2-propanol oxidation rate on alcohol concentration and on light intensity, /, (with anthraquinone as sensitizer) is expressed as... 

Benzo[ ]thiophenes undergo photochemical [2+2] cycloaddition, for instance with 1,2-dichloro-ethene in the presence of benzophenone as sensitizer ... 

An alkynyliodonium salt, namely, phenyl(phenylethynyl)iodonium hexafluorophosphate, has been tested for application as cationic photoinitiator [38]. The high activity of phenyl(phenylethynyl)iodonium salt as a photoinitiator was verified by photo differential scanning calorimetry (photo-DSC) experiments in direct irradiation and in photosensitized initiation using 9,10-dibutylanthracene, 2-isopropylthioxanthone and benzophenone as sensitizers [38]. 

A soln. of methanesulfonyl azide in isopropanol-water irradiated 80 min. in a slow Ng-stream with a high-pressure Hg-arc lamp until gas-evolution ceases -> methane-sulfonamide. Y 99.8%. F. e., also in the presence of benzophenone as sensitizer, s. M. T. Reagan and A. Nickon, Am. Soc. 90, 4096 (1968). 

The deamination of cyclohexylamine has been brought about photochemically using benzophenone as sensitizer . Cyclohexanone was produced in 80-90% yields (equation 173). The reaction is also successful with secondary amines. 

A soln. of (n-butylthio)acetic acid irradiated 2.67 hrs. with Pyrex-filtered light of a 100 w. medium-pressure Hg-lamp in benzene containing benzophenone as sensitizer and thiophenol -> n-butyl methyl sulfide. Y ca. 100% based on startg. m. consumed. F. e. and sensitizers s. R. S. Davidson and P. R. Steiner, Soc. (C) 1971, 1682, 3480 without sensitizer cf. R. S. Goudie and P. N. Preston, Soc. (C) 1971, 3081. 

Startg. m. in benzene irradiated with the Duran-50-filtered light of a high-pressure Hg-lamp in the presence of benzophenone as sensitizer product. Y ca. 100%. F. e., also thermolysis and isolation of intermediates, s. W. Welter and M. Regitz, Tetrah. Let. 1976, 1473. 

Ghosh et aP have recently demonstrated that the CuOTf-catalyzed [2 -H 2]-photocycloaddition can be successfully carried out when 1,6-heptadienes are incorporated in a carbohydrate derivative, thus widening the scope of the Cu(I)-catalyzed photocycloaddition. The diene 67a, on irradiation in the presence of CuOTf, afforded the thermodynamically less stable cis-syn-cis adduct 68a (Scheme 20). Initially, this unexpected stereochemical outcome was thought to arise through the tri-coordinated Cu(I) complex 69. However, photocycloaddition of the analogous diene 67b (with benzophenone as sensitizer) in the absence of a Cu(I) catalyst also produced the cis-syn-cis adduct 68b, excluding the possibihty of the involvement of the Cu(I)-complex 69. Possibly, the observed stereoselectivity arises through the Cu(I)-... 

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