Saturated alcohols, manganese dioxide

Nickel peroxide is a solid, insoluble oxidant prepared by reaction of nickel (II) salts with hypochlorite or ozone in aqueous alkaline solution. This reagent when used in nonpolar medium is similar to, but more reactive than, activated manganese dioxide in selectively oxidizing allylic or acetylenic alcohols. It also reacts rapidly with amines, phenols, hydrazones and sulfides so that selective oxidation of allylic alcohols in the presence of these functionalities may not be possible. In basic media the oxidizing power of nickel peroxide is increased and saturated primary alcohols can be oxidized directly to carboxylic acids. In the presence of ammonia at —20°, primary allylic alcohols give amides while at elevated temperatures nitriles are formed. At elevated temperatures efficient cleavage of a-glycols, a-ketols... 

The selective oxidation of benzylic and allylic alcohols with active manganese dioxide in the presence of saturated alcohols is normally carried out by stirring or shaking a solution of the alcohol in an organic solvent in the presence of 5-20 equivalents of suspended active M11O2. 

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-... 

A useful addition to the roster of oxidants converting primary alcohols into aldehydes is manganese dioxide, which can be prepared by several methods [805, 806, 807, 808, 809, 810]. It is used as a suspension in petroleum ether [8< 5, 808], ether [806, 808, 811], hexane [806, 811], benzene [808, 813], chloroform [808, 811, 813], and carbon tetrachloride [808, 813]. The oxidations are carried out at room temperature and are especially suitable for allylic and benzylic alcohols, which are oxidized more readily than the saturated alcohols [808, 810], Yields vary widely depending on the substrate, on the ratio of the substrate to oxidant (which in turn depends on the particle size of the oxide [810]), on the solvent, and on the reaction time 811] (equation 214). 

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. 

Acetaldehyde—Acetic aldehyde—Acetyl hydrid——44 —is formed in all reactions in which alcohol is deprived of H without introduction of O. It is prepared by distilling from a capacious retort, connected with a well-cooled condenser, a mixture of HjSOj, 6 pts. HgO, 4 pts. alcohol, 4 pts. and pow dered manganese dioxid, G pts. The product is redistilled from calcium ehlorid below 50° (123° P.). The second distillate is mixed with two volumes of ether, cooled by a freezing mixture, and saturated with dry NHg there separate crystals of ammonium acetylid, CgHiO, NHi, -which are -washed with ether, dried, and decomposed in a distilling apparatus, over the water-bath, with tho proper quantity of dilute HjS04 the distillate is finally dried over calcium ehlorid and rectified below 35° (95° P.). 

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. 

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