Autooxidation of ketones

According to Quinkert, photoexcited cyclic ketones may be transformed to open-chain unsaturated carboxylic acids in the presence of molecular oxygen. This reaction may compete efficiently with a-cleavage and secondary transformations thereof. Thus, both stereo iso meric 17-ketones (109) and (110) yield as much as 20% of the unsaturated acid (111) when irradiated in benzene under a stream of oxygen. This photolytic autoxidation has been used notably for partial syntheses of naturally occurring unsaturated 3,4-seco-acids from 3-oxo triterpenes (for references, see ref. 72). 

It has been proposed that oxygen adds to the excited keto group [- (112)]. The rearrangement of the resulting hydroxyhydroperoxy diradical (112) could then proceed by intramolecular hydrogen abstraction involving a six-membered cyclic transition state, followed by fission of the former C —CO bond to form the unsaturated peracid (113) as the precursor of the final product. Such a reaction sequence demands a hydrogen atom in the J -position sterically accessible to the intermediate hydroperoxy radical. 

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Photochemical autooxidation of ketones, 316 Photochemical a-cleavage, 296 Photochemical cleavage of bonds attaching atoms to the a-carbon of ketones, 307 Photochemical cleavage of exocyclic. S-bonds, 323... 

Chemiluminescence is also obtained by anionic autooxidation of (41) with oxygen ia alkaline dimethyl sulfoxide (DMSO) (216). Qc has been reported to be 10% and ketone (43) and CO2 are obtained. Several analogues of luciferin have been prepared that are also chemiluminescent when they react with oxygen ia alkaline DMSO (62). 

Formation of an enamine radical cation 45 was proposed as the chain initiation step in the autooxidation of enamines and SchifFs bases of a,/ -unsaturated ketones to give unsaturated 1,4-diones37. Pyrrolidine enamine of 10-methyl-A1(9)-octal-2-one (44) reacts with oxygen at room temperature to produce, after acid hydrolysis, 10-methyl-A1 (9)-octalin-2,8-dione (47) in 20% yield. Addition of a catalytic amount of FeCl3, Cu(OAc)2 or CuCl2 causes a pronounced enhancement in the oxidation rate and increases the yield to 80-85% after 1 h. 

The proposed free radical chain mechanism for this reaction is given in Scheme 3. The striking catalytic effect of the metal ions such as Cu2+ and Fe3+ is attributed to their ability to accept an electron from the enamine in the chain initiation step. The autooxidation of the SchifFs bases of a,/ -unsaturated ketones is thought to proceed similarly via the enamine form of the SchifFs bases. 

Autooxidation of 2-phenyloxetane followed a somewhat different oouree, since the most easily ahstraoted hydrogen must be tertiary, rather than secondary. The product was a lower polymer with carbonyl absorption characteristic of a phenyl ketone. ... 

Deodorization Deodorization, which removes the volatile eompounds along with residual FFA, is a eritieal step in ensuring the purity of any vegetable oil and improves flavor, odor, eolor, and oxidative stability. Many of the volatile eompounds removed are formed by the autooxidation of fat, whieh produees aldehydes, ketones, aleohols, and hydrocarbons that are associated with undesirable flavors and odors. The proeess is also effective in removing any remaining pestieide residues or metabolites that may be in the oil. 

A -dien-3-ol ethers gives rise to 6-substituted A" -3-ketones. 6-Hydroxy-A" -3-ketones can be obtained also by autooxidation.Structural changes in the steroid molecule may strongly affect the stability of 3-alkyl-A -ethers. Thus 11 j5-hydroxyl and 9a-fluorine substituents greatly increase the lability of the enol ether/ while halogens at C-6 stabilize this system to autooxidation and acid hydrolysis. 

Radical hydroxylation of hydrocarbons by autooxidation yields alcohols (major products), ketones, and acids (minor products). Cyclohexanol, for example, is formed in 90% yield from cyclohexane and peroxyacetic acid (275). The high -ol/-one ratio at low conversions can sometimes be used as a partial diagnostic tool to distinguish between the radical and electrophilic pathways. The predominant reaction of electrophilic radicals, such as HO, ROO, and CH 3 is H-atom abstraction from reactants (S-H) or peracids, as exemplified by the following ... 

Hydrolysis of the acetonide followed by oxidation and bromination provided the ketone 13, which is itself a natural product, hamigeran A. Hydrolysis under aerobic conditions led first to decarboxylation, then to autooxidation, to give(-)-hamigeran 3. 

In a brief survey of other simple binary carbonyls we find that the compounds M(CO)6(M = Cr, Mo, W) and Ru3(CO)12 have only minimal catalytic activity for autooxidizing alcohols or ketones. The compounds Fe(CO)5 and Fe3(CO)12 are decomposed completely when we try to use them as catalysts. When the compound Mn2(CO)i0 is used, there is a considerable enhancement in acid formation. During this reaction there is extensive decomposition to manganese dioxide, and we believe that this compound is the one primarily involved in the catalytic oxidation. 

Molecular oxygen can also oxidize a variety of organic compounds, including hydrocarbons, aldehydes, amines, ethers and ketones. These autooxidation reactions can be used to make a variety of small molecules and a number of industrial processes rely on the controlled oxidation of organics using molecular oxygen (often with a metal catalyst). Examples include the formation of phenol and acetone from cumene (isopropylbenzene) and cyclohexanone from cyclohexane. Phenol is a popular starting material for a number... 

On the other hand, carbonyl oxygen occurring as aldehyde functions is very susceptible to oxidation whereas ketone functions are much less susceptible. The oxidation of aldehydes in air (autooxidation) occurs readily by way of a free-radical mechanism ... 

In systems where such radicals appear (alcohols, amines, some unsaturated compounds), variable-valence metal ions manifests themselves as catalysts for chain termination (see Chapter 11). The reaction of the ions with peroxyl radicals appears also in the composition of the oxidation products, especially at the early stages of oxidation. For example, the only primary oxidation product of cyclohexane autooxidation is hydroperoxide the other products, in particular, alcohol and ketone, appear later as the decomposition products of hydroperoxide. In the presence of stearates of such metals as cobalt, iron, and manganese, all three products (ROOH, ROH, and ketone) appear immediately with the beginning of oxidation and in the initial period (when ROOH decomposition is insignificant), they are formed in parallel with a constant rate. The ratio of rates of their formation is determined by the catalyst. The reason for this behavior is evidently related to the fast reaction of R02 with the catalyst. Thus, the reaction of peroxyl radicals widi variable-valence ions manifests itself in the kinetics as well (the induction period appears imder certain conditions), and alcohol and ketone are formed in parallel with ROOH from R02 among the oxidation products. 

Oxidative rancidity Chemieal reaetion in which the double bonds of fatty acids are oxidized to form peroxides, which eventually decompose into aldehydes, ketones, and alcohols. These decomposition products cause off-flavors, odors, and rancidity. Two types of rancidities are known enzyme-mediated and autooxidation. 

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