Fragmentation photoinduced

Photochemical elimination reactions include all those photoinduced reactions resulting in the loss of one or more fragments from the excited molecule. Loss of carbon monoxide from type I or a-cleavage of carbonyl compounds has been previously considered in Chapter 3. Other types of photoeliminations, to be discussed here, include loss of molecular nitrogen from azo, diazo, and azido compounds, loss of nitric oxide from organic nitrites, and loss of sulfur dioxide and other miscellaneous species. 

Maslak, P. Fragmentations by Photoinduced Electron Transfer. Fundamentals and Practical Aspects. 168, 1-46 (1993). 

The P-aryl phosphanorbomenes (85 and 104) prepared by us served as excellent model compounds in photoinduced fragmentation-related phosphorylations. On the one hand, new //-phosphinates (105) were synthesized (Scheme 27), while on the other hand, our experiments indicating a high sensitivity towards the steric effects substantiated the AE mechanism involving a pentacoordinated pentavalent intermediate (106) (Scheme 28) [74, 75],... 

An interesting application of photoinduced electron transfer involving fragmentation and subsequent cyclization reactions is reported by Kirschberg and... 

An useful alternative to the already known retropinacol reactions is presented by Liu and co-workers [7], This works demonstrates that pinacols bearing (dimethylamino)phenyl substiments can be subjected to fast oxidative fragmentation via photoinduced electron transfer with chloroform as the electron acceptor in yields up to 80%. The extremely fast dechlorination of the chloroform radical anion inhibits back-electron transfer and thus leads to effective fragmentation of the pinacol radical cation (Scheme 8). 

Photoinduced single-electron transfer followed by fragmentation of the radical cation is an efficient method for generating carbon-centered radicals under exceptionally mild conditions. The fate of the thus formed radicals depends primarily on their interaction with the acceptor radical anions. Typically observed reactions are either back-electron transfer or radical coupling, but from the synthetic point of view, another most intriguing possibility is the trapping of the radical with suitable substrates such as olefins (Scheme 16). 

Stationary spectroscopy on the C and D states of Na3 already indicated the onset of photoinduced fragmentation. Fragmentation becomes more important as the cluster size increases. As a result, nondissociative electronic excitation processes have not yet been observed for free metal clusters larger than trimers [20]. An alternative to conventional spectroscopy of such bound-free transitions was provided by depletion spectroscopy [2]. A deep insight into the dynamics of such photoinduced cluster fragmentation, however, is obtained with ultrafast observation schemes. The principle of such an... 

Isomeric C Hn radical ions fragment not very differently by the different mass spectro-metric methods. The metastable decays are nearly identical, but the collisionally activated spectra of 14 isomeric hexenes, measured by Nishishita and McLafferty240, exhibit some quantitative differences. Bensimon, Rapin and Gaumann251 compared the metastable decay and the photoinduced fragmentation by infrared photons of long-lived parent ions of six hexene isomers and cyclohexane. If the linear isomers are practically identical, some notable differences are observed for branched isomers. Cyclohexane behaves similar to n-hexenes. The metastable fragmentation of H/D-labeled 4-Me-2-pentene, 2-Me-2-pentene... 

One important discrepancy should be noted between photochemical and chemical ion radical reactions. In the photochemical mode, an oxidized donor and a reduced acceptor remain in the same cage of a solvent and can interact instantly. In the chemical mode, these initial products of electron transfer can come apart and react separately in the bulk solvent. For example, one-electron oxidation of phenylbenzyl sulfide results in formation of the cation radical both in the photoinduced reaction with nitromethane and during treatment with ammoniumyl species. Sulfide cation radicals undergo fragmentation in the chemical process, but they form phenylbenzyl sulfoxide molecules in the photochemical reaction. The sulfoxide is formed at the expense of the oxygen atom donor. The latter comes from the nitromethane anion radical and is directly present in the solvent cage. As for the am-... 

Fries-type rearrangements (Sch. 15) (36) also occur photochemically this is generally a homolytic process, in contrast to the ionic mechanism of the thermal process. Azapropazone undergoes photoinduced 1,3-acyl shift in the solid state and fragmentation in solution (Sch. 16) (37). 

For the spironaphthoxazines conjugated with aza-15(18)-crown-5(6)-ether moieties at 6 -position of naphthalene fragment (13a,b) it was found that the addition of Li+ and alkaline earth (Mg2+, Ca2+, Sr2 and Ba2+) metal cations to 13a,b solutions results in a hypsochromic shift of the UV absorption band of the spiro form and a bathochromic shift of the absorption band of the merocyanine form in the visible region [36], In addition, the equilibrium shifts to the merocyanine form, and the lifetime of the photoinduced merocyanine form increases (Scheme 15). The isomerization of crown-containing compound 13a,b to the colored merocyanine form was promoted most strongly by the presence of metal ions, which are expected to be the best recognized by the crown ether ring (Scheme 15). 

Products of oxidation of sulfides by chemical electron transfer (CET) with cerium (IV) ammonium nitrate and photoinduced electron transfer (PET) with C(N02)4 depend on the nature of the sulfide. CET led to fragmentation and a-deprotonation products, whereas PET gave mainly sulfoxides, although in some cases fragmentation was also observed.256... 

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