Phenothiazine-5-oxide, photooxidation

Photolysis reactions often are associated with oxidation because the latter category of reactions frequently can be initiated by light. The photooxidation of phenothiazines with the formation of N- and S-oxides is typical. But photolysis reactions are not restricted to oxidation. In the case of sodium nitroprusside, it is believed that degradation results from loss of the nitro-ligand from the molecule, followed by electronic rearrangement and hydration. Photo-induced reactions are common in steroids [36] an example is the formation of 2-benzoylcholestan-3-one following irradiation of cholest-2-en-3-ol benzoate. Photoadditions of water and of alcohols to the electronically excited state of steroids have also been observed [37],... 

Phenothiazine (PTZ) and ferrocene (Fc), both of which have a small reorganization energy associated to the electron transfer, are the first comediators that have been considered. Each has a potential that falls between 0.22 and 0.75 V versus SCE, respectively, the potential of the Co(DTB)33+/2+ and of the dye [Ru(H2DCB)-(dnbpy)(NCS)2]+/0. Because of the facile electron transfer, the photooxidized dye would be predominantly reduced by the comediator. Its oxidized form (PTZ+ and Fc +) can then be rapidly intercepted by Co(II), preventing the direct charge recombination between the oxidized comediator and the electrons in the 2. Nanosecond... 

A photochromic system based on the reversible photooxidation of the leuco forms of certain organic dyes has been described (B-76MI11401). Under appropriate conditions compounds such as the phenothiazine (150) can exist in different oxidation states (Scheme 17), each having significantly different absorption spectra. 

The identification of the major products of the photooxidation of phenothiazine upon its direct excitation does not support the idea of a N-hydroperoxy intermediate (Equation 5) (11). In fact, phenothia-zine-5-oxide (A) which has been first identified by comparing Rf values in thin layer chromatography of isolated and separately prepared samples (11) represents the best evidence of a singlet oxygen reaction (Equation 9) (12). 

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)... 

Our hypothesis is based on the observation that the photooxidation upon direct excitation of F yields G as well as a hydroxylated sulfone, too (28). Again, this reaction must involve oxidants other than singlet oxygen, for no reaction has been observed between the latter and ground state 10-methyl-phenothiazine-5-oxide (F). 

Photooxidation to 5-oxides was noticed when aqueous solutions of some phenothiazine drugs were exposed to light. ... 

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