Benzophenone-sensitized photooxidation

Type I mechanism, which does not involve the singlet oxygen reaction, is the another mechanism for the photosensitized oxidation of flavor compounds. Benzophenone-sensitized photooxidation of tetrahydrofuran leads to formation of the a-hydroperoxide (Scheme 27) (Schench et al, 1963), which is the same product formed in the unsensitized photooxidation of tetrahydrofuran as demonstrated in Scheme 11. 

It has been shown that the benzophenone sensitized decomposition of benzoyl peroxide is due in part to formation of the benzophenone ketyl radical, which induces decomposition.98,99 Hydrocarbon sensitized peroxide decomposition is discussed in Section IV.A.4. The formation of benzonitrile from the benzophenone sensitized irradiation of benzalazine, which was originally attributed to hydrogen abstraction by benzophenone,100 actually results from a photooxidation.101... 

Benzophenone (BP) sensitized photooxidation of phenothiazine leads mainly to the formation of products A and B (17). In view of the many data available from the literature and from our own experiments the published interpretation needs, however, some revision provided 3H-phenothiazine-3-ones are not products of a singlet oxygen reaction with ground state phenothiazine (c.f. methylphenothiazine), formation of products A and B implies two different oxidation mechanisms. Since singlet oxygen sensitization by benzophenone (Equations 10 to 12) is known to be very inefficient, reaction sequence (10), (13), (14), and (9)... 

Not all photooxidation pathways can be attributed to radical cations as the only intermediate. For example, in the benzophenone-sensitized reaction, some and P—and a—>p isomerization (in the case of a-dGuo) is observed (Vialas et al. 1999). while no such isomerization occurs upon the Mn-TMPyP-mediated oxidation by KHS05 (Vialas et al. 1999). Here, Iz is the only detected product [90% yield Vialas et al. 1998] which points to different intermediates in these two systems. Ascorbate or cysteine, may intercept G% and in their presence the oxazolone yield is strongly reduced (Douki et al. 1999). 

Sensitized photooxidation of sulfur-containing amino acids in neutral aqueous solution occurs via electron transfer from the sulfur to the triplet state of the sensitizer (the mechanism can be adapted to other dye-sensitized photoinitiated polymerization). Electron transfer quenching in the system is followed mainly by diffusion of the sulfur-centered radical cation and benzophenone radical anion. Laser... 

However, while 313 and 366 nm direct irradiation yields a cis- trans quantum yield of-0.005, the benzophenone triplet sensitized and the low energy direct irradiation yield is only —0.001. These results support the notion that two reactive excited states are involved one associated with the styrylferrocene which leads to the more efficient isomerization and a ferrocene-type excited state at lower energy which has a smaller tendency to undergo cis -trans isomerization. It is interesting to note that the 100% trans photostationary state is likely very close to the amount expected at thermal equilibrium. Such a result would be consistent with an intramolecular CT transition to yield a radical-anion ferricinium complex as in reaction (47). The radical anion could yield cis trans isomerization just as in the Ru(II)-styrylpyridine complexes. The wavelength dependence of the isomerization of the styrylferrocene is consistent with (47) in that upper excited states are required for the photooxidation of ferrocene to ferricinium in CC14.135 ... 

Certain features of alcohol photo-oxidation are connected with the presence of sensitizers. As found by Backstrom [60], the rate of photooxidation of 2-propanol (with benzophenone as sensitizer) is inversely proportional the oxygen pressure. Dependence of the 2-propanol oxidation rate on alcohol concentration and on light intensity, /, (with anthraquinone as sensitizer) is expressed as... 

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