Oxidation reactions Lead tetraacetate-Manganese

Oxidation of 1-aminobenzimidazoles with manganese dioxide or lead tetraacetate can give either 1,1 -azobenzimidazoles (735) or 3-substituted benzo-l,2,4-triazines (736). Electrochemical measurements have shown that the first step in this reaction is removal of an electron from a ir-orbital of benzimidazole rather than from the A-amino group. Because the cation radical which is formed must be stabilized by loss of a proton, for (736) to form the 2-substituent must contain an NH or OH group. This is unnecessary for the formation of the azo product (735) which may form via a nitrene intermediate. 

Another oxidation reactions have been reported that led to C-C bond formation. They deal with treatment of A-allylsilane-substituted SMAs with CAN, lead tetraacetate or manganese triacetate to form methylene piperidines and azepines. Yields are in the same range as those obtained using SET-photolytic or electrolytic processes.359... 

The formation of benzimidazoles by cyclization of aryl amidines can be accomplished in a number of ways, but is usually carried out under anhydrous conditions in the presence of a base. Thus cyclizations of (45 R = OH) induced by benzenesulfonyl chloride in pyridine or triethylamine generally give yields of greater than 60%. The more direct process of oxidizing the parent compounds (45 R = H) with sodium hypochlorite under basic conditions gives even higher yields (70-98%). Intermediates in these reactions are the )V-haloamidines (45 R = C1) a nitrene intermediate may be involved or merely dehydrochlorination with concomitant cyclization (Scheme 25). Alternative cyclization procedures include oxidation of the parent amidines (45 R = H) by manganese dioxide or lead tetraacetate and thermal... 

Pyrazolines can also be prepared by the oxidation of pyrazolidines. 2-Pyrazolines, which are readily obtained in the reaction of a,j -unsaturated ketones with hydrazine, also undergo nitrogen extrusion at elevated temperature usually in the presence of a basic catalyst. The reaction is believed to proceed via 1-pyrazolines. Treatment of 3,3,5-trialkyl-2-pyrazolines with lead tetraacetate followed by thermolysis affords cyclopropyl acetates ". Oxidation of certain 2-pyrazolines with manganese dioxide gives 3H-pyrazoles, which in turn produce cyclopropenes in the photolysis (equation 7). ... 

Red mercuric oxide is less reactive [387], Less often, oxidations are carried out with silver oxide [370, mercurous tri-fluoroacetate [405], mercuric trifluoroacetate [406], lead tetraacetate [445], manganese dioxide [370], and nickel peroxide [955]. The most common solvents are pentane, petroleum ether, benzene, toluene, ether, and ben-zonitrile. Basic catalysts such as alcoholic potassium hydroxide are sometimes used. Anhydrous calcium oxide, sodium sulfate, and magnesium sulfate are frequently added to remove the water of reaction. The reaction temperatures range from room temperature to the reflux temperature of the solvents. 

Although the oxidation pathway is determined mainly by the nature of the azole nucleus, the conditions and the oxidants also have great significance. In the first experiments, the oxidant most used was lead tetraacetate. It still dominates, but many other oxidants have appeared, and they frequently give better results, for instance, nickel peroxide, manganese dioxide, potassium periodide, bromine water, and N-bromosuccinimide. Reactions with LTA are usually carried out in absolute methylene chloride at about 0-20°C, and for neutralization of the released... 

Alkoxy radicals for ring expansion can be generated from alcohols by oxidative methods such as hypohalite thermolysis/photolysis [19a] and lead tetraacetate oxidation [19b], or peroxide reduction [19c]. The recent development of the hyper-valent organoiodine reagent (diacetoxyiodo)benzene (DIB) provides another way for efficient generation of alkoxy radicals (Scheme 11) [19d]. Additional oxidative methods to prepare cyclopropyloxy radicals include reaction of tertiary cyclopropanols or their silyl ether derivatives with various reagents such as manganese(III) tris(pyridine-2-carboxylate) [Mn(pic)3] [20a], Fe(III) salts [20b], and vanadyl ace-tylacetate [20c] (Scheme 12). 

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