Photoeliminator

The effects of uv radiation on V/-nitroso compounds depend on the pH and the medium. Under neutral conditions and ia the absence of radical scavengers, these compounds often appear chemically stable, although the E—Z equiUbrium, with respect to rotation around the N—N bond, can be affected (70). This apparent stabiUty is due to rapid recombination of aminyl radicals and nitric oxide [10102-43-9] formed duting photolysis. In the presence of radical scavengers nitrosamines decay rapidly (71). At lower pH, a variety of photoproducts are formed, including compounds attributed to photoelimination, photoreduction, and photo-oxidation (69). Low concentrations of most nitrosamines, even at neutral pH, can be eliminated by prolonged kradiation at 366 nm. This technique is used ki the identification of /V-nitrosamines that are present ki low concentrations ki complex mixtures (72). 

QUINONE METHIDES FROM BENZYLIC PHOTOELIMINATION 1.2.1 Photoelimination of Fluoride... 

Application of this technique to a study of the photoelimination of azo compounds has been reported by Porter, Landis, and Marrett.aw Photolysis of the unsymmetrically substituted azo compound (1) in solvents of varying viscosity revealed a dependence of on solvent viscosity as shown in Table 11.2. Photolysis of optically active (1) (40% completion) and examination of the remaining azo compound indicated that 26% of the original optical activity had been lost. This is explained by the following mechanism involving stepwise homolysis ... 

Photoelimination of nitrogen from diazoketones is complicated by Wolff rearrangement of the intermediate carbene, as shown below for diazoaceto-phenone<35) ... 

Other sulfur compounds are known to undergo photoelimination and fragmentation reactions(85 6S) ... 

In contrast to aromatic hydrocarbons, heavy-atom substitution onto carbonyl and heterocyclic molecules appears to have little effect on radiative and nonradiative intercombinational transitions. Wagner(138) has shown that as determined by the type II photoelimination, aliphatic ketones (n -> it excited states) are not sensitive to external heavy-atom perturbation. As seen previously in our discussion of type II photoelimination, aliphatic ketones undergo this cleavage from both the excited singlet and triplet states (in... 

This chapter contains discussions of photoelimination, photoaddition, and photosubstitution. Although there may appear to be some degree of overlapping between the first two topics in that the species produced by photo-elimination may undergo addition to another substrate, our approach will be to concentrate on the reactions brought about by light absorption rather than subsequent dark reactions. 

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. 

Compounds containing nitrogen-nitrogen double bonds exhibit extreme photoreactivity due to the stability of the photoelimination product, nitrogen gas. In fact, the photodecomposition of azo compounds occurs so readily that... 

Irradiation of acetone or methanol solutions of this compound at — 50°C, however, yields the cis photoisomer, which upon warming (0°C), decomposes to yield photoelimination products. This is taken as evidence that the major path at room temperature involves photoisomerization of the trans to cis isomer and rapid thermolysis of the latter. Similar behavior was observed for azobis(isobutyronitrile), azobis(cyanocyclohexane), and azobis(2-methyl-propyl acetate).(4>... 

---