Potassium ferricyanide radicals

Manganese(III)-promoted radical cyclization of arylthioformanilides and a-benzoylthio-formanilides is a recently described microwave-assisted example for the synthesis of 2-arylbenzothiazoles and 2-benzoylbenzothiazoles. In this study, manganese triacetate is introduced as a new reagent to replace potassium ferricyanide or bromide. The 2-substituted benzothiazoles are generated in 6 min at 110°C imder microwave irradiation (300 W) in a domestic oven with no real control of the temperature (reflux of acetic acid) (Scheme 15). Conventional heating (oil bath) of the reaction at 110 °C for 6 h gave similar yields [16]. 

As described, other nucleophilic reactions in the anthraquinone series also involve the production of anion-radicals. These reactions are as follows Hydroxylation of 9,10-anthraquinone-2-sulfonic acid (Fomin and Gurdzhiyan 1978) hydroxylation, alkoxylation, and cyanation in the homoaromatic ring of 9,10-anthraquinone condensed with 2,1,5-oxadiazole ring at positions 1 and 2 (Gorelik and Puchkova 1969). These studies suggest that one-electron reduction of quinone proceeds in parallel to the main nucleophilic reaction. The concentration of anthraquinone-2-sulfonate anion-radicals, for example, becomes independent of the duration time of the reaction with an alkali hydroxide, and the total yield of the anion-radicals does not exceed 10%. Inhibitors (oxygen, potassium ferricyanide) prevent formation of anion-radicals, and the yield of 2-hydroxyanthraquinone even increases somewhat. In this case, the anion-radical pathway is not the main one. The same conclusion is made in the case of oxadiazoloanthraquinone. 

The superoxide ion or its protonated form (hydroperoxy radical HO2 ) is produced in the system (Shugalei and Tselinskii 1993, 1994). Hydroquinone, which is known to interact effectively with superoxide ion (Afanas ev and Polozova 1978), exerts fairly strong inhibiting effect on the reaction. Addition of potassium ferricyanide to the system has, in contrast, an accelerating effect. The cause of the effect consists of transformation of the following type ... 

Dimethylcarbodihydrazide (280) reacts with aldehydes to afford 1,5-dimethyltetrahydro-l,2,4,5-tetrazin-6-ones (281), which can be oxidized by silver oxide, potassium ferricyanide or lead dioxide to yield radicals (282) related to the verdazyls these can be transformed into tetrahydro-l,2,4,5-tetrazines (283) by hydrogenation over palladium (80AG766). 

The pioneer in phenolate radical coupling was Pummerer. In 1925 he showed1 that one electron oxidation of />-cresol using potassium ferricyanide afforded a nicely crystalline ketonic dimer of the radical in up to 25 % yield. Pummerer s ketone, as it became known, was considered to result from the coupling of two p-cresol radicals to give the dienone 1. This then underwent spontaneous cyclization to furnish 2. As proof of the structure... 

Recently, aqueous potassium ferricyanide has been used to oxidize a series of tetrasubstituted pyrroles of the type (88) to their dimers (89), which may be dissociated on heating into two pyrryl radicals (90).103... 

Treatment of 2,2,4,4-tetramethyl-3-thietanone with diiron nonacarbonyl gives the binuclear iron complex 381. 2,2-Dimethyl-3-thietanone undergoes oxidative dimerization to 382 on treatment with potassium ferricyanide. Methylene-3-thietanones such as 359 add chlorine from thionyl chloride to the carbon-carbon double bond. 2,2,4,4-tetramethyl-3-thietanone is converted to the 3-thione in 14% yield by treatment with hydrogen sulfide-hydrogen chloride. Electrochemical reduction of the thione produces radical anions. 

Potassium ferricyanide [potassium hexacyanoferrate(lll)], K3Fe-(CN), in the presence of a base, dehydrogenates hydroaromatic compounds to aromatic compounds [919] and can cause dehydrogenative cy-clizations [920]. The reagent is used for the conversion of acid hydrazides into aldehydes [921], of sterically hindered phenols into phenoxy radicals [922, 923], and of primary amines into nitriles [924], Tertiary amines are demethylated to secondary amines [925, 926]. 

Potassium ferricyanide effects the one-electron oxidation of sterically hindered phenols to phenoxyl radicals (equation 308) [922, 923]. 

Lunarine (26), one of the typical neolignans, is biosynthesized by the ortho-para radical coupling between two molecules of p-hydroxycinnamic acid. In this connection, oxidative coupling reactions of 4-substituted phenols have been extensively stndied using thallium trifluoroacetate (TTFA), potassium ferricyanide (K3[Fe(CN)g]) and other reagents. p-Cresol (27) was also electrolyzed at a controlled potential (+0.25 V vi. SCE) in a basic medium to afford Pummerer s ketone 28 in 74% yield. The snggested mechanism is given in Scheme 4. 

Radicals Bromine azide. -Butyllithium. Cuprous acetate. N,N-Dichlorourethane. 4,4 -Dinilrodiphenylnitroxide. Iodine, oxidation. Iodobenzenedichloride. Manganic tris(acetyl-acetonate) Potassium /-butoxide. Potassium ferricyanide. Potassium mtrosodisulfonate. 

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