Photolytic decomposition pathways

The synthetic utility of the photolytic decomposition pathway of pyrrolo[3,4-J]-1,2,3-triazole derivatives has been realised in the asymmetric synthesis of (—)-quinocarcin. Thus, the triazine (62) was irradiated using a mercury light source to give aziridine (63) in 90% yield via loss of nitrogen <93JA10742>. 

Wakahara, T., Niino, Y, Kato, T, Maeda, Y, Akasaka, T., Liu, M.T.H., Kobayashi, K., and Nagase, S., A Nonspectroscopic Method to Determine the Photolytic Decomposition Pathways of 3-Chloro-3alkyldiazirine Carbene, Diazo and Rearrangement in the Excited State, /. Am. Chem. Soc., 124, 9465, 2002. 

Figure 7. Photolytic and thermal decomposition pathways in crystalline ABP. Initial photolysis at 300-400 nm gives methyl-benzoyloxyl (MB) radical pairs, which can either collapse to give methyl benzoate, or decarboxylate thermally or photochemi-cally to give methyl-phenyl radical pairs.

Nitrate and nitrite, N03 and NCV respectively, absorb light over the wavelength range of 200 to 400 nm and undergo photolytic decomposition to yield a variety of products [173-176]. The k > 300 nm photolysis of nitrate in aerated aqueous solutions at pH < 6 proceeds via two predominant pathways ... 

Another type of reaction is the addition of OH with the possible photolytic decomposition of the product back to OH but in competition with other pathways of HO-C ... 

The primary photodissociation probably occurs from high vibrational levels of the electronic ground state reached by rapid intersystem crossing directly from the originally excited Bg state. No evidence was found for collisional deexcitation, and the dissociation via reaction (4) occurred with a quantum yield close to unity. Some molecular dissociation Into N2 and H2 can not be excluded, because this process Is symmetry-allowed for C/S-N2H2 [1, 12]. The successive fission of the NH bonds is predicted from ab Initio SCF Cl studies to be the preferred pathway in the photolysis of C/S-N2H2 [13, 14]. The photolytic decomposition into N2 and H2 was already observed earlier in a few qualitative experiments at very low pressures (< 0.1 Torr) [9]. 

N-Nitrosamides are much less stable than the parent N-nitros-amines and they can decompose by either thermal, photolytic or acid and base (nucleophilic) catalysed pathways Thermal decomposition has attracted much attention as a clean method of deamin-... 

Despite patient and exhaustive effort by many researchers, all attempts to isolate or trap a benzazirine intermediate (214) have so far failed, and unequivocal evidence for their participation in either the photolytic or thermal decomposition of aryl azides is still awaited. Evidence in favor of the proposed reaction pathway (Scheme 22) comes from the work of Sundberg and coworkers, who succeeded in identifying 3-alkyl-2-diethylamino-lff-azepines as oxygen-sensitive, metastable intermediates in the photolysis of o-alkylphenyl azides in diethylamine (72JA513). Later studies on the flash photolysis of aryl azides in dialkylamine solution provided kinetic data which not only confirmed the Iff- to 3/f-azepine tautomer-ism, but also strongly supported the involvement of a benzazirine intermediate (74JA7491). 

FIGURE 6 Potential interactive pathways and processes of humic substances emanating from decomposition products of higher plant tissues with extracellular and surface-bound enzymes and photolytic reactions, particularly with UV irradiance. Humic acid-enzyme complexes can be stable for long periods (weeks and months) and subsequently reactivated upon exposure to weak UV light. Further photolysis can cleave simple compounds from the macromolecules for subsequent utilization by microbes. 

The photolytic degradations of drugs are complex, and include reactions such as oxidations, reductions, cis-trans isomerizations, structural rearrangements, hydrolyses, dechlorinations, etc. Oxidation is one of the major pathways of photochemical reactions. Table 1 lists the pathways of photochemical decomposition of some therapeutic and nontherapeutic agents. 

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