Manganese dioxide secondary alcohols

The selective oxidation of a secondary alcohol in the presence of a tertiary amine function has been carried out with manganese dioxide (equations 43 and 44) in the context of an alkaloid synthesis. ... 

Activated manganese dioxide (MnO2) reliably oxidizes acetylenic, allylic, and benzylic alcohols to aldehydes and ketones. Saturated primary and secondary alcohols are also oxidized, albeit more slowly. The two main concerns are the activity of the manganese dioxide and the slow filtration of salts after the reaction. Activated MnOi is available commercially or may be prepared. 

In broad terms it is only in special cases that the oxidation of hydrocarbons to primary or secondary alcohols is of practical importance. One such case is oxidation of toluene and its derivatives to the corresponding benzyl alcohols, by e.g., manganese dioxide, lead dioxide, lead tetraacetate, or Caro s acid. It is advantageous to use lead tetraacetate in glacial acetic acid solution since then the alcohol formed is protected by esterification against further oxidation. 

Manganese dioxide is capable of oxidizing alcohols to ketones or aldehydes. The reaction proceeds via a radical intermediate (see below), producing MnO (which is Mn +) as the byproduct. Manganese dioxide is an important reagent in organic synthesis since it oxidizes primary and secondary alcohols to the aldehyde or ketone, respectively, in neutral media.This reaction was discovered by Ballet al when they precipitated manganese dioxide and used it to convert vitamin A (94) to retinal (95) in 80% yield. 49... 

Manganese dioxide oxidizes allylic and benzylic alcohols faster than primary saturated alcohols, but primary and secondary allylic alcohols react at about the same rate. This use of manganese dioxide is particularly important.i Oxidation of benzylic alcohols is also facile and a secondary benzylic alcohol is oxidized faster than a primary saturated alcohol. The secondary benzylic alcohol group in 102 was oxidized to give aryl ketone 103 (94% yield) in preference to reaction at the primary aliphatic hydroxyl. 

Peroxymonosulfuric acid is highly unstable, decomposes dangerously on heating, and evolves oxygen at room temperamre. It may react violently with organic matter and readily oxidizable compounds. Violent explosions have been reported with acetone, due to the formation of acetone peroxide (Toennis 1937). It may explode when mixed with many primary and secondary alcohols, manganese dioxide, cotton, many metals in finely divided form, and aromatics such as benzene, phenol, and aniline. 

Enhancing the utility of the allq lation-rearrangenient sequence in synthesis, the Evans group addressed the problems of a-versus-y alkylation as well as low anion reactivity by employing heterocyclic sulfides as the alleviation substrates tScheme IR.IfiE For instance, allylic imidazolyl sulfide 56 could be allqvlated efficiently, reaction at the a-position being favored by a chelated but reactive allyl lithium intermediate. Oxidation of 57 to the allylic sulfoxide and treatment with a secondary amine thiophile provided allylic alcohol 58 in high yield and with excellent stereoselectivity at the trisubstituted alkene. Allylic oxidation with manganese dioxide completed a synthesis of the sesquiterpene nuciferal (59). ... 

Several references have appeared on the use of solid-phase oxidants. Solid potassium permanganate-copper sulphate mixtures oxidize secondary alcohols to ketones in high yield, and pyridinium chromate or chromic acid on silica gel are described as convenient off-the-shelf reagents for oxidation of both primary and secondary alcohols. Anhydrous chromium trioxide-celite effects similar transformations only when ether is present as co-solvent. An excellent review, with over 400 references, on supported oxidants covers the use of silver carbonate-celite, chromium trioxide-pyridine-celite, ozone-silica, chromyl chloride-silica, chromium trioxide-graphite, manganese dioxide-carbon, and potassium permanganate-molecular sieve. 

Allylic and benzylic primary and secondary alcohols are more easily oxidized, and a number of reagents selective for these are in use, including freshly precipitated manganese dioxide, silver carbonate, dichlorodicyanoquinone, and potassium ferrate. 4-(Dimethylamino) pyridinium chlorochromate is mild and selective as demonstrated in Equation 6.26 [44]. 

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