Ketones with peroxyacids

Oxidation of ketones with hydrogen peroxide or with a peroxyacid, RC020H (cf. p. 330) results ia their conversion into esters ... 

Peroxyacids are powerful oxidants and so side reactions are to be expected. Ketones are known to undergo Baeyer-Villiger " oxidation to the carboxylic acid ester on treatment with peroxyacids and so these by-products can be observed if oxime hydrolysis occurs during the oxidation. Paquette and co-workers " " observed such a by-product (61) during the oxidation of the dioxime (59) to the dinitro compound (60) with m-CPBA in hot acetonitrile. 

Another route to the preparation of a-hydroxy derivatives consists of the transformation of enolates into silyl enolates and their subsequent oxidation. Oxidation of triaLkylsUyl enolates with peroxyacids, most frequently cpba, has been applied for preparation of a-hydroxy- and a-acetoxy aldehydes or ketones. Reactions require mild conditions and generally give good yields of the expected compound . Mechanistic investigations suggest the intermediate formation of epoxides which evolve to the final products via 1,4-sUyl group migration. 

Conjugated dienes can be epoxidized (1,2-addition), although the reaction is slower than for corresponding alkenes, but a,p-unsaturated ketones do not generally give epoxides when treated with peroxyacids.The epoxidation of a,p-unsaturated ketones with hydrogen peroxide under basic conditions is known as the Waits-Schejfer epoxidation, discovered in 1921. This fundamental reaction has been extended to a,()-unsaturated ketones (including quinones), aldehydes, and... 

Baeyer-Villiger rearrangement The rearrangement of a ketone with a peroxyacid, e.g. /ra-CPBA, to give an ester. 

Esters can also be formed upon oxidation of ketones with a peroxyacid oxidizing agent (such as mCPBA or CF3CO3H). This Baeyer-Villiger oxidation is especially useful in the preparation of cyclic esters (called lactones) from cyclic ketones. 

Also due to the high barrier of inversion, optically active oxaziridines are stable and were prepared repeatedly. To avoid additional centres of asymmetry in the molecule, symmetrical ketones were used as starting materials and converted to oxaziridines by optically active peroxyacids via their ketimines (69CC1086, 69JCS(C)2648). In optically active oxaziridines, made from benzophenone, cyclohexanone and adamantanone, the order of magnitude of the inversion barriers was determined by racemization experiments and was found to be identical with former results of NMR study. Inversion barriers of 128-132 kJ moF were found in the A-isopropyl compounds of the ketones mentioned inversion barriers of the A-t-butyl compounds lie markedly lower (104-110 kJ moF ). Thus, the A-t-butyloxaziridine derived from adamantanone loses half of its chirality within 2.3 days at 20 C (73JCS(P2)1575). 

The Baeyer-Villiger process for conversion of open-chain ketones to esters, or cyclic ketones to lactones by a peracid involves an intermediate a-hydroxyperester (235) step, as shown in equation 83. Determination of 235 in the reaction mixture involves selective reduction of the peroxyacid with diphenyl sulfide and reduction of 235 with excess iodide, followed by titration of the liberated iodine. The presence of various transition metal ions may affect the determination by accelerating the final step of the synthetic process, thus... 

Carboxylic Acids. The peroxyacids formed may be unstable and explosive.13 Ketones. Hazardous reaction with acetone and nitric acid other ketones yield shock-and heat-sensitive explosive peroxides.14... 

The two pieces were joined together by making the enolate of 5 and reacting it with 7 X = OTs. The unsaturated ketone 9 was then oxidised with a peroxyacid to give the epoxide 10 and... 

The perhydroxyl anion, HOf, is a powerful nucleophile (see Chapter 3) and will attack substrates such as electron-deficient olefins (e.g. a,/ -unsaturated ketones) and aldehydes. The perhydroxyl anion is also of value in bleaching and product purification, particularly of natural products. In addition, it can be used to generate more powerful oxidants by mixing with electron-deficient acyl compounds (giving peroxyacids) or with nitriles (Figure 2.5). 

This is called an initiation step. It may happen spontaneously or may be induced by heat or light (see the discussion on p. 279), depending on the type of bond. Peroxides, including hydrogen peroxide, dialkyl, diacyl, and alkyl acyl peroxides, and peroxyacids are the most common source of free radicals induced spontaneously or by heat, but other organic compounds with low-energy bonds, such as azo compounds, are also used. Molecules that are cleaved by light are most often chlorine, bromine, and various ketones (see Chapter 7). Radicals can also be formed... 

When a ketone is treated with a peracid (or peroxyacid), RCO3H, in the presence of an acid catalyst,... 

In the Baeyer-Villiger rearrangement, a ketone is treated with a peracid (or peroxyacid), in the presence of an acid catalyst, and the related ester of the ketone is produced by the equivalent of an insertion of an oxygen atom. If a cyclic ketone is treated in this manner, a lactone with a ring size one larger than the original ketone results. 

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