Phenothiazine oxidative dimerization

Preparative scale electrochemical oxidation of phenothiazine in aqueous acetonitrile, with no added acid, leads to the radical formed by proton loss from the radical-cation. The radical dimerizes and further oxidation leads to the green qui-nonoid cation 67 [230]. 

Various phenothiazines (176) have been oxidized in CH3CN-Et4NC104 to a stable radical-cation (R = OCH3). The free radical resulting from deprotonation of the radical-cation gives in those cases where R = H, SCN, or t-Bu a C—N bond dimer, possibly 177272,2-73 [Eq. (107)]. 

Methyl-5-(trimethylammoniomethyl)pyrazine hydroxide (183) (made in situ from the corresponding chloride and silver oxide) gave among other products two separable dimeric isomers of the general formula (184) (PhMe, trace of phenothiazine, reflux with water removal, 8 h low yields for details and related products, see originals).550 1481... 

Similarly, phenothiazine may be oxidized to the cation radical species which then dimerizes forming the 3,10 -diphenothiazinyl species (Tsujino, 1969). The product of the electron-transfer step may react, via a second-order process, with a species in solution to form a new product. An example of this type of mechanism involves the reduction of anthraquinone and its derivatives in the presence of oxygen (Jeziorek etal., 1997). To understand quantitatively an EC and EC2 process, the concentration and scan-rate dependence of the associated cyclic voltammograms is matched with theory deriving from the mass transport/kinetic equations for each species. 

There are also several reports of formation of dimers upon oxidation of various phenothiazine derivatives, without more detailed structural investigations. ... 

In acetonitrile phenothiazine is oxidized in two, one-electron waves the radical cation obtained in the first oxidation loses a proton and is oxidized to the phenazothio-nium ion and isolated as the perchlorate [291] or dimerizes at nitrogen or at C-3 [292]. 

Phenothiazine (178) when oxidized with bromine forms a radical cation bromide (179) which reacts with phenoselenazine to give a complex cation radical salt involving both ring systems <75BCJ3747>. Phenoselenazines initially form cation radicals on oxidation which then undergo dimerization reactions <75BCJ1043>. Similarly, oxidation of phenoselenazine with dimethyl sulfoxide... 

The bromide, picrate, perchlorate and disulfate of phenothiazine are well known (Sato et al., 1969 Kehrmann and Diserens, 1915 Iida, 1971). Oxidations of phenothiazines are usually direct, in the presence of the appropriate anion. An acidic solution is advisable so as to avoid dimerization of neutral phenothiazinyl radicals. Oxidation of 3,7-dimethoxyphenothiazine by Fremy s salt in a biphthalate-buffered solution led to the stable perchlorate salt (Medzhikov et al.,... 

Tetrathiafulvalenes can be produced by an oxidative electrodimeri2ation of 1,3-dithiole derivatives (Scheme 125) [235], Unsymmetrical dimers may also be elec-trochemically produced as illustrated in Schemes 126, 127, and 128 respectively, from phenothiazine derivatives [236], tetra-hydrocarbazole [146] and dihydrobenzaze-pine derivatives [140]. 

Additional evidence has been presented in support of hydro-aromatic species, e.g. (82), as intermediates in the thermolysis of aryl 2-azidophenyl sulphides, leading to phenothiazines (Vol. 2, p. 780). A successful synthesis of 117-phenothiazin-l-one (83), a member of a novel class of heterocyclic o-quinone-imines, by oxidation of 1-hydroxyphenothiazine in an inert solvent under carefully controlled conditions has been reported. A dominant feature of the chemical behaviour of (83) is its tendency to undergo dimerization to give a rather intractable product, identified tentatively as (84). A multi-step synthesis of l-methyl-2-aminophenothiazin-3-one has also been described. ... 

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