Selective oxidations of allylic alcohols

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... 

Selective oxidation of allylic alcohols.1 This zircononcene complex when used in catalytic amount can effect an Oppenauer-type oxidation of alcohols, including allylic ones, in the presence of a hydrogen acceptor, usually benzaldehyde or cyclohexanone. This system oxidizes primary alcohols selectively in the presence of secondary ones. Thus primary allylic alcohols are oxidized to the enals with retention of the configuration of the double bond in 75-95% yield. The method is not useful for oxidation of propargylic alcohols. 

Selective oxidation of allylic alcohols.2 PCC in CH2C12 containing pyridine (2%) selectively oxidizes steroidal equatorial allylic hydroxyl groups. PCC buffered with... 

XI. Selective Oxidation of Allylic Alcohols / 244 Manganese dioxide / 244 Dichlorodicyanobenzoquinone / 247 Nickel peroxide / 248... 

Activated manganese dioxide, which is generally used for the selective oxidation of allylic alcohols, permitted the synthesis of an unsaturated (3-keto-arabino-hexopyranosyl)pyridine.13 New oxidative systems for alcohols, involving molecular sieve-assisted... 

Choi K-M, Akita T, Mizugaki T et al (2003) Highly selective oxidation of allylic alcohols catalysed by monodispersed 8-shell Pd nanoclusters in the presence of molecular oxygen. New J Chem 27(2) ... 

Selective oxidation of allylic alcohols. The complex of PCC with pyrazole or 3,5-dimethylpyrazole- is recommended for selective oxidation of allylic alcohols in the presence of nonallylic hydroxyl groups. The complex of chromic anhydride and dimethyl-pyrazole (8, 110 9. 116) has been used for this oxidation. 

The chemoselective oxidation of a primary alcohol in the presence of a secondary alcohol is a somewhat more difficult task. Not only is the inherent difference in reactivity less than in the case of the selective oxidation of allylic alcohols discussed above, but most reagents will oxidize secondary alcohols somewhat more rapidly than primary alcohols. Nevertheless there are reagents which will carry out the selective oxidation of a primary alcohol to an aldehyde without oxidizing a secondary alcohol, some of which will be considered here. 

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-... 

Figure 2.9 Evidence that surface PdO catalyzes the aerobic selective oxidation of allylic alcohols over Pd/Al20j (a) Strong dependence of TOF on surface oxide concentration and (b) in situ reduction of active PdO phase accompanying onstream deactivation. (Adapted from Refs [96] and [144] by permission of the Royal Society of Chemistry.)...

Method for selective oxidation of allylic alcohols) Helv (1961) 44 179... 

Oxidation of 101 to ester 2 and epoxidation of the sterically most accessible olefin, completed the synthesis of CJH. The conversion of 101 to 2 is notable, and relies on the selective oxidation of allylic alcohols to aldehydes by manganese dioxide. The sequence of events is (1) oxidation of 101 to the corresponding aldehyde, (2) cyanohydrin formation, (3) oxidation of the cyanohydrin to an acyl cyanide and (4) methanolysis of the acyl cyanide to provide the ester. 

Note selective oxidation of allylic alcohol in presence of a benzyiic alcohol. 

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