Manganese dioxide oxidation mechanism

Further support for structure CXXXVII for tazettine was derived from a study of tazettamide, the manganese dioxide oxidation product of tazettine (55, 140). Although the mechanism of the oxidation is obscure, tazettamide may be formulated as CXLII (R = CHO) from the spectral and analytical data and from numerous characterization reactions. Unlike most manganese dioxide oxidations, the double bond is not required for oxidation since dihydrotazettamide (CXLIII) was formed from dihydrotazettine in equally good yield and in the same reaction time. Alternatively, dihydrotazettamide was produced by catalytic reduction of tazettamide. Although mild basic hydrolysis... 

Polar solvents inhibit the reaction, presumably by interfering with the adsorption process as noted in the mechanism proposed for manganese dioxide oxidations. Oxidation of 1-heptanol to heptanal with Fetizon s reagent was quantitative when the solvent was 35% hexanes. When benzene was used as a solvent, the yield of heptanal dropped to 90% and was < 1% in ethyl acetate, methyl ethyl ketone, or acetonitrile. 69 Since the oxidation is a heterogeneous reaction, requiring adsorption of the alcohol substrate, as the surface area of the reagent increases (increased by precipitation on Celite), the rate of oxidation increases. An optimum ratio is reached beyond which increasing the silver carbonate/Celite ratio slows the oxidation. 69... 

Kubota and co-workers also prepared several 1,2-diols (61) and noted that these substances give the enediones (59) in much better yields than the diosphenols (58). This is consistent with a mechanism which requires, as the first step, hydroxylation of the -double bond of (58) by manganese dioxide followed by oxidation to the intermediate trione. 

Oxidation of toluene-o-sulphonamide to saccharin. In a 600-ml beaker, mounted on an electric hot plate and provided with a mechanical stirrer, place 12 g (0.07 mol) of toluene-o-sulphonamide, 200 ml of water and 3g of pure sodium hydroxide. Stir the mixture and warm to 34-40 °C until nearly all has passed into solution (about 30 minutes). Introduce 19g (0.32 mol) of finely powdered potassium permanganate in small portions at intervals of 10-15 minutes into the well-stirred liquid. At first the permanganate is rapidly reduced, but towards the end of the reaction complete reduction of the permanganate is not attained. The addition occupies 4 hours. Continue the stirring for a further 2-3 hours, and then allow the mixture to stand overnight. Filter off the precipitated manganese dioxide at the pump and decolourise the filtrate by the addition of a little sodium metabisulphite solution. Exactly neutralise the solution with dilute hydrochloric acid (use methyl orange or methyl red as external indicator). Filter off any o-sulphonamidobenzoic acid (and/or toluene-o-sulphonamide) which separates at this point. Treat the filtrate with concentrated hydrochloric acid until the precipitation of the saccharin is complete. Cool, filter at the pump and wash with a little cold water. Recrystallise from hot water. The yield of pure saccharin, m.p. 228 °C, is 7.5 g (58%). 

Allylic alcohols are more easily oxidised than saturated alcohols and in this case rates are greatest for the equatorial isomers [42]. Activated manganese dioxide is commonly used for selective oxidations of allylic alcohols, although it will also attack saturated alcohols slowly under special conditions [44]. Its mechanism of action is not understood, although some relevant observations have been reported [43]. 2,3-Dichloro-5,6 dicyanobenzoquinone (DDQ) is a very mild and selective oxidant for allylic alcohols [43]. Kinetic studies [46] on 3a- and 3 9 hydroxy-A -systems (8) revealed a higher rate of oxidation of the pseudo-equatorial 3 -alcohol K jK a = 6), and a large primary isotope effect Kj)IKb ca. 1/5) when the C(3) H was replaced by deuterium. These results indicate a rate-determining hydride abstraction from C(3>, with preferred removal of the pseudo-axial 3C1-H as a result of optimum a-7i overlap in the transition state (9). However, a detailed analysis of thermodynamic parameters shows that the cause of the rate difference appears mainly in the term (A5 ) rep-... 

The oxidation of a ketone by permanganate to the dicarboxylic acid takes place through the enol form of the ketone. The reaction can be followed as the bright purple permanganate solution reacts to give a brown precipitate of manganese dioxide. A possible mechanism for this reaction is the following ... 

In this case, the electron transfer takes place at the anode interface, probably through an electrocatalytic mechanism. Also using cobaltic, manganic ions, or insoluble oxides, such as lead or manganese dioxides. 

---