Aldehydes, reaction with peroxy acids

Methoxyalkenes (45), derived from IV-Boc or N-Cbz a-amino acids, undergo stereoselective addition of PhSeCl in the presence of (Pr 0)4Ti and LiCl to give the corresponding phenylselenenyl aldehydes (46) that can be easily transformed into enantiomerically pure aziridines (47) on reaction with peroxy acids. 

The initiation step was proposed to proceed via the formation of the oxygenated adduct, like Co -0-0, which reacts with aldehyde in the rate-determining step [26]. However, the fact that the induction time is more for CoPc as compared to C0W12 (the latter is unable to form any adducts), shows that species different from oxygenated cobalt adducts may promote the chain initiation. We believe that for most cobalt catalysts studied these species are Co(III) forms of the catalyst produced by one-electron oxidation of Co(II) initial forms with peroxy acid, which in turn is produced in the course of aldehyde autoxidation. Usually, the end of the induction period coincides with the change of the reaction mixture colour expected for Co(III) appearance. 

Oxidation of aldehydes with peroxy-acids is not so synthetically useful as oxidation of ketones and generally gives either carboxylic acids or formate esters. However, reaction of ortho- and para-hydroxy-benzaldehydes or -acetophenones with alkaline hydrogen peroxide (the Dakin reaction) is a useful method for making catechols and quinols. With benzaldehyde itself, only henzoic acid is formed, but orr/jo-hydroxy-benzaldehyde (salicylaldehyde) gives catechol almost quantitatively (6.65) and 3,4-dimethylcatechol was obtained by oxidation of 2-hydroxy-3,4-dimethylacetophenone. 

Mechanism of Baeyer-Villiger Oxidation Both aldehydes and ketones are oxidized by peroxy acids. This reaction, called the Baeyer- Villiger oxidation, is especially useful with ketones. 

The mechanism is the same as that of the Dakin reaction of aromatic aldehydes (equation 359). The reaction is acid-catalyzed. The peroxy acid transfers oxygen onto the carbon of the carbonyl group and generates an unstable intermediate. A rearrangement of the groups bonded to the original carbonyl carbon results in the formation of esters or, with cyclic ketones, lactones [262, 303] (equation 379). 

An important addition to the arsenal of oxidants for the Baeyer-Villiger reaction is peroxytrifluoroacetic add [282, 283, 284]. Although this reagent is less easily accessible than peroxyacetic acid and aromatic peroxy acids, it is more reactive. The yields of esters obtained by oxidation of ketones with peroxytrifluoroacetic acid are high enough to justify the use of this oxidant for the quantitative determination of aldehydes and... 

Reaction (4.7) leads to a proliferation of hydroxy radicals, which non-selectively abstract hydrogen atoms, see Reactions (4.8) and (4.9). Acids are formed by the following two reactions, which start from a hydroperoxy-peroxy radical, see Reaction (4.4), and an aldehyde [9, 10]. Carboxylic acids (RCOOH), formed according to Reaction sequences (4.18) and (4.19), represent one of the principal products under these oxidation conditions. In a subsequent step they can react with alcohols R OH, produced by Reactions (4.10) and (4.14), to form esters, RCOOR. In addition, when the rate of oxidation becomes limited by diffusion, ethers are formed. Reaction sequence (4.20) ... 

Oxidation of an aldehyde group to a carboxyl group has often been used for identification of the compounds. As far as can be judged from the results, such oxidations, using chromium trioxide/acetic acid, bromine-water, - peroxy acids, - or chlorite, have not been accompanied by any important side-reactions. Hypoiodite titration, using the iodine in sodium bicarbonate-sodium carbonate procedure, has sometimes been used, giving almost stoichiometric aldehyde determinations. - - Reduction by the Meerwein-Ponndorf reaction, with borohydride - - ... 

Dienolic ethers (379), in contrast with the acetates, are cleaved by peroxy-acid at the 3,4-unsaturated link under anhydrous conditions, giving carbo-methoxy-aldehydes (380). The reaction is probably mechanistically similar to the oxidative cleavage of dihydropyrans e.g. 381 — 382), although the... 

Epoxidation in the absence of a metal catalyst is possible. Hydrogen peroxide, in the presence of a nitrile, aldehyde or ketone, or a relatively acidic alcohol (e.g. phenol), can effect epoxidation of an alkene. Peroxy-imidic acids RC(=NH)OOH, formed in situ by reaction of nitriles (RC=N) with hydrogen peroxide, react under mildly alkaline or neutral conditions. For example, 2-aUyl-cyclohexanone was readily converted into the corresponding epoxide with the alkaline reagent, whereas with peroxy-acetic acid Baeyer-Villiger ring-expansion intervenes. The perhydrate 39,... 

In remote atmospheres, where NOx concentrations are very low, reactions of peroxy radicals with HO2, or with other peroxy radicals, compete with the reaction with NOx and result in the formation of various oxidised compounds such as aldehydes, alcohols, organic acids and hydroperoxides. Several studies performed as part of LACTOZ have provided important data for modelling these processes in the atmosphere. 

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