Cleavage reactions photolytic

Photolytic cleavage reactions (e.g., of o-nitrobenzyl, phenacyl, nitrophenylsul-fenyl derivatives) take place in high yield on irradiation of the protected compound for a few hours at 254-350 nm. For example, the o-nitrobenzyl group, used to protect alcohols, amines, and carboxylic acids,has been removed by irradiation. Protective groups that have been removed by photolysis are described at the appropriate places in this book in addition, the reader may wish to consult five review articles. 

A single photon of visible light does not have enough energy to break the bonds in water four photons are required in this photolytic cleavage reaction. 

Morrison [78] has also investigated photolytic cleavage of C- Cl bonds in a- [82] and P-chloroketones [78]. These potential PET bond cleavage reactions are rationalized on the basis of a LUMO which is a combination of it and ct molecular orbitals. We have classified the intramolecular ET from a it HOMO to a it or ct LUMO as a PET process. A similar process occurs in the photolytic bond cleavage reactions of onium salts where the LUMO level is clearly ct in character. This topic will be reviewed in the following section. 

Two general routes to formyl-esters using ring cleavage reactions have been described one employs an electrolysis of lactams whereas a second involves a photolytic cleavage of cyclic /3-keto-esters to give w-formyl-a,/3-unsaturated esters. ... 

The most intriguing hydrocarbon of this molecular formula is named buUvalene, which is found in the mixture of products of the reaction given above. G. SchrOder (1963, 1964, 1967) synthesized it by a thermal dimerization presumably via diradicais of cyciooctatetraene and the photolytical cleavage of a benzene molecule from this dimer. The carbon-carbon bonds of buUvalene fluctuate extremely fast by thermal Cope rearrangements. 101/3 = 1,209,6(X) different combinations of the carbon atoms are possible. 

According to Quinkert, photoexcited cyclic ketones may be transformed to open-chain unsaturated carboxylic acids in the presence of molecular oxygen. This reaction may compete efficiently with a-cleavage and secondary transformations thereof. Thus, both stereo iso meric 17-ketones (109) and (110) yield as much as 20% of the unsaturated acid (111) when irradiated in benzene under a stream of oxygen. This photolytic autoxidation has been used notably for partial syntheses of naturally occurring unsaturated 3,4-seco-acids from 3-oxo triterpenes (for references, see ref. 72). 

Photolysis, 350 nm, CH3CN, H2O. The by-product from the reaction is 2-phenyl-benzo[ ]furan. Cleavage with TBAF and PhCH2SH has been demonstrated (70-94% yield). The related 3,5-dimethoxybenzoin analogue is cleaved with a rate constant of >10 sec Photolytic cleavage occurs by heterolytic bond dissociation. ... 

Acyl azides can undergo photolytic cleavage and rearrangement upon irradiation at room temperature or below. In that case acyl nitrenes 8 have been identified by trapping reactions and might be reactive intermediates in the photo Curtius rearrangement. However there is also evidence that the formation of isocyanates upon irradiation proceeds by a concerted reaction as in the case of the thermal procedure, and that the acyl nitrenes are formed by an alternative and competing pathway " ... 

The photolytic cleavage of cyclic ketones 14 leads to formation of a diradical species, that can undergo analogously the various reactions outlined above. The decarbonylation followed by intramolecular recombination yields a ring-contracted cycloalkane 15 ... 

Another free-radical arylation method consists of the photolysis of aryl iodides in an aromatic solvent. Yields are generally higher than in 14-17 or 14-21. The aryl iodide may contain OH or COOH groups. The mechanism is similar to that of 14-17. The aryl radicals are generated by the photolytic cleavage ArI AR + T. The reaction has been applied to intramolecular arylation (analogous to the Pschorr reaction). A similar reaction is photolysis of an arylthallium bis(trifluoroacetate) (12-21) in an aromatic solvent. Here too, an unsymmetrical biaryl is produced in good yields. ... 

The photolytic activation of 5m was also shown to lead to DNA cleavage [33,35-38]. This reaction appeared to be faster and more efficient than the Cu+-catalyzed cleavage conditions. The mechanism(s) of DNA cleavage should be different because aryl cations (not aryl radicals) are believed to be produced under photolytic conditions (Fig. 12) [7]. Such electrophiles should target the nucleic acid bases and/or the positively charged phosphodiester backbone, and both of these could lead to DNA cleavage. 

Photolytic. Photooxidation products reported include 2,2 -dihydroxy-4,4 -dimethylbiphenyl, 2-hydroxy-3,4 -dimethylbiphenyl ether, and 4-methylcatechol (Smith et al., 1978). Anticipated products from the reaction of 4-methylphenol with ozone or OH radicals in the atmosphere are hydroxynitrotoluene and ring cleavage compounds (Cupitt, 1980). Absorbs UV light at a maximum wavelength of 278 nm (Dohnal and Fenclov4, 1995). 

Interestingly, it should not be assumed that a polymer will be useless above its ceiling temperature. A dead polymer that has been removed from the reaction media will be stable and will not depolymerize unless an active end is produced by bond cleavage of an end group or at some point along the polymer chain. When such an active site is produced by thermal, chemical, photolytic, or other means, depolymerization will follow until the monomer concentration becomes equal to [M]c for the particular temperature. The thermal behavior of many polymers, however, is much more complex. Degradative reactions other than depolymerization will often occur at temperatures below the ceiling temperature. 

The formation of the oxepanes (160) and (161) by treatment of 2,2,5,5-tetramethyl-tetrahydrofuran-3-one with diazomethane is included in the present category although it is probable that the reaction proceeds by two separate ring expansion steps (cf. equation 47) as shown in Scheme 32 (72HC(26)l). Photolytic C—O bond cleavage of a dihydrofuran... 

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