Cleavage reactions photolysis

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. 

The associative, thermochemical, and photochemical properties of PATE polymer will be described and evidence will be provided supporting the thesis of labile sulfur-sulfur bonds. Photolysis of a model compound and a polymer similar to PATE but lacking thiosulfate functional groups has been conducted to provide supportive evidence for the proposed sulfur-sulfur bond cleavage reactions. Finally, a preliminary investigation of the imaging characteristics of the PATE polymer is presented in order to place the photochemical investigation in perspective. 

The end result of these studies showed very clearly that two major processes were important, i.e. photolysis and photo-oxidation. Photolysis reactions were posited to be the result of the well-known Norrish Type 1 and Norrish Type 2 cleavage reactions. As we shall see, the Type 1 cleavage followed by several subsequent reactions can account for many of the observed degradation products. 

New photochemical cleavage reactions of ortho-substituted C=C double bonds were reported by introducing a 2-nitrophenyl group to the double bond104. Photolysis of 1-(2-nitrophenyl)-l-alkenes 174 in methylene chloride solution without oxygen affords aryl... 

A bond-cleavage reaction that has been light-induced. The term has also been used to describe irradiation of a sample by light however, such usage is discouraged. See Flash Photolysis Photoreactive Caged Compounds... 

Herz and Iyer have proposed an a-cleavage mechanism with the intermediacy of a cyclopropyl ketene for formation of the polycyclic acetal 42 from photolysis of the rigid cyclopentenone 4339 The cleavage reaction is thought to occur via an n,7r triplet state of unusually high energy for a cyclopentenone (75 kcal/mole). 

Reactions arising via photo-oxidation of diazo-compounds are thought to involve carbonyl oxide intermediates, and stilbene derivatives are formed via aryldiazomethane radical-cations on dicyanoanthracene-sensitized irradiation of aryldiazomethanes. " A 1,4-biradical is formed by stepwise cleavage on photolysis of l,12-bis(diazo)-l,12-dihydroindeno[2,3-a]fluorene. ... 

Photocleavage of benzyl-S bonds appears to proceed neither by an electron transfer pathway nor with participation of an exciplex. A meta effect seems to operate in these reactions as evidenced by the influence of 3-methoxy and of 3-cyano substituents on the efficiency of the cleavage process. Photolysis of 1,2-bis(phenoxymethyl)-, l,2-bis(phenylthiomethyl)-, and l,2-bis(phenylseleno-methyl)benzene induces a two-photon process to give o-quinonedimethane, which in the presence of dienophiles undergoes a cycloaddition reaction, and 4,8,10-trithiadibenzo[cd,ij]azulene 8-oxides gives the corresponding aldehydes and ketones together with 4,8,9-trithiacyclopenta[def]phenanthrene. The two... 

In addition to the difference in forms and the distribution of reaction products, the photocatalysis and photolysis processes exhibit different extents of mineralization. As shown in Figure 1C, the generation of carbon dioxide in direct photolysis of 4-chlorophenol is almost negligible. In contrast, pho-tocatalytic oxidation leads to total mineralization of 4-chlorophenol to carbon dioxide. There is, however, no evidence that further ring-cleavage reaction takes place in direct photolysis of 4-chlorophenol. 

Direct, or primary, photochemical reactions are the immediate chemical changes to CDOM such as isomerization, bond cleavage, and photolysis. Thus, we refer... 

Carbon-oxygen bond heterolysis is responsible for the observed photolyses of 9-aryl-9-xanthenols ° and llH-benzo[b]fluoren-11-ol. - - Evidence for the formation of the 9-fluorenol radical cation as well as the 9-fluorenyl cation has been obtained from a laser flash photolysis study of 9-fluorenol. 1, l-Di-2-thienylethanol undergoes light-induced dehydration to give i,i-di-2-thienylethylene.Single electron transfer pathways, however, are implicated in the ring cleavage reactions of a,j8-epoxy ketones in the presence of allyltributyltin - or alkylamines. 

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