Oppenauer oxidation hydrogen acceptors

When heated in the presence of a carboxyHc acid, cinnamyl alcohol is converted to the corresponding ester. Oxidation to cinnamaldehyde is readily accompHshed under Oppenauer conditions with furfural as a hydrogen acceptor in the presence of aluminum isopropoxide (44). Cinnamic acid is produced directly with strong oxidants such as chromic acid and nickel peroxide. The use of t-butyl hydroperoxide with vanadium pentoxide catalysis offers a selective method for epoxidation of the olefinic double bond of cinnamyl alcohol (45). 

The Oppenauer oxidation makes use of ketones (typically acetone) or alkenes as hydrogen acceptors and this absence of a strong oxidising agent allows to overcome some potential NHC oxidative instability. Reactions consist of an equilibrium between an alcohol and its oxidised form (Scheme 10.9). 

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. 

Acetone in conjunction with benzene as a solvent is widely employed. Alternatively cyclohexanone as the hydrogen acceptor, coupled with toluene or xylene as solvent, permits the use of higher reaction temperatures and consequently the reaction time is considerably reduced the excess of cyclohexanone can be easily separated from the reaction product by steam distillation. Usually at least 0.25 mol of aluminium alkoxide per mol of secondary alcohol is employed. However, since an excess of alkoxide has no detrimental effect, the use of 1 to 3 mol of alkoxide is desirable, particularly as water, either present in the reagents or formed during secondary reactions, will remove an equivalent quantity of the reagent. It is recommended that 50 to 200 mol of acetone or 10 to 20 mol of cyclohexanone be employed. Other oxidisable groups are usually unaffected in the Oppenauer oxidation and the reaction has found wide application in the steroid field. 

A zirconium complex, bis(cyclopenta(Uenyl)zirconium(IV) hydride will function as a catalyst for the chemoselective Oppenauer oxidation of primary alcohols in the presence of a hydrogen acceptor (cyclohexanone, benzaldehyde or benzophenone). This method appears to be of some value, since it also allows for the selective monooxidation of primary (and secondary) diols (Scheme 3). 1,2-Diols are not cleaved under these conditions and retro-aldol reactions appear not to be a problem. 

Meerwein-Ponndorf-Verley reduction was efficiently and selectively achieved by use of l-(4-dimethylaminophenyl)ethanol as the reducing alcohol (2-4 equiv.) and Zr(0-/-Bu)4 (0.2 equiv.) as the catalyst [32b]. Oppenauer oxidation was selectively achieved by using chloral (1.2-3 equiv.) as the hydrogen acceptor and Zr(0-t-Bu)4 (0.2 equiv.) as the catalyst [32c]. 

Acetone, cyclohexanone, benzophenone, cinnamaldehyde, and other carbonyl compounds are hydrogen acceptors in the Oppenauer oxidation of alcohols to carbonyl compounds. The reaction is catalyzed by Raney nickel [961], aluminum alkoxides [962], tris(isopropoxide), or tris(tert-bu-toxide) as bases soluble in organic solvents [963, 964]. These dehydrogenations of alcohols to aldehydes and ketones require refluxing or distillations and have given way to dimethyl sulfoxide oxidations, which take place at room temperature. 

Carbonyl compounds act as hydrogen acceptors in the Oppenauer oxidation of alcohols to aldehydes or ketones. The reaction is based on hydride transfer from the alkoxide ion of the starting alcohol prepared in situ from anhydrous bases, aluminum isopropoxide, or, better still, tert-butoxide (equation 256). 

Oppenauer oxidation The oxidation of cholesterol to cholestenone as described by Oppenauer is carried out by gentle refluxingfor 8 hrs. of a mixture of commercial chuleslerol, aluminum t-butoxide. acetone (hydrogen acceptor), and benzene. The... 

Another is use as hydrogen acceptor in the Oppenauer oxidation of a -3/9-hydroxysierold to produce a 4°. diene 3-one. In reporting discovery of this reaction. 

When PhCHO (or any other ArCHO) is added at the termination of the reaction as a hydrogen acceptor to the chromium(III) diarylmethanoates, an Oppenauer oxidation occurs and the products are diaryl ketones. 

The zirconocene complex [2ZrH2] catalyses an Oppenauer-type oxidation of alcohols in the presence of an appropriate hydrogen acceptor. On oxidation of diols containing two primary alcohols, and of diols containing two secondary alcohol groups, one of the alcohol groups is selectively oxidized to form hydroxy-aldehydes and hydroxy-ketones respectively. This system... 

Oppenauer applied this reaction to the oxidation of unsaturated steroidal alcohols using aluminum triiso-propoxide [Al(0/-Pr)3] in acetone. 37 The acetone acts as a hydrogen acceptor and the presence of excess acetone drives the reaction toward the oxidation product. Oppenauer used this method to oxidize the alcohol... 

Probably related mechanistically to the Oppenauer oxidations are several techniques for oxidation which involve transfer of hydrogen to trichloroacetal-dehyde. These reactions are mediated by alumina and the reaction is carried out by simply mixing the alcohol to be oxidized, the hydrogen acceptor, and alumina... 

The development of the CrOs-pyridine and DMSO-based methods has decreased the number of instances in which older oxidation techniques are used. One such method, the Oppenauer oxidation is the reverse of the Meerwein-Pondorff-Verley reduction (Chapter 5). It involves heating the alcohol to be oxidized with an aluminum alkoxide in the presence of a carbonyl compound, which acts as the hydrogen acceptor. The reaction is an equilibrium process and proceeds through a cyclic transition state. 

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