Autoxidation of aldehydes

Another method for producing petoxycatboxyhc acids is by autoxidation of aldehydes (168). The reaction is a free-radical chain process, initiated by organic peroxides, uv irradiation, o2one, and various metal salts. It is terrninated by free-radical inhibitors (181,183). In certain cases, the petoxycatboxyhc acid forms an adduct with the aldehyde from which the petoxycatboxyhc acid can be hberated by heating or by acid hydrolysis. If the petoxycatboxyhc acid remains in contact with excess aldehyde, a redox disproportionation reaction occurs that forms a catboxyhc acid ... 

The peroxycarboxyhc acid can be generated m situ by autoxidation of aldehydes, either in the presence of anhydrides or an acyl chloride and a base, eg, sodium carbonate, or basic ion-exchange resins (44,187,188,210) ... 

The autoxidation of aldehydes, and of other organic compounds, may be lessened considerably by very careful purification—removal of existing peroxides, trace metal ions, etc.—but much more readily and effectively by the addition of suitable radical inhibitors, referred to in this context as anti-oxidants. The best of these are phenols and aromatic amines which have a readily abstractable H atom, the resultant radical is of relatively low reactivity, being able to act as a good chain terminator (by reaction with another radical) but only as a poor initiator (by reaction with a new substrate molecule). 

For reviews of (he autoxidation of aldehydes, see Vardanyan Nalbandyan Russ. Chem. Rev. 1985, 54. 532-543 (gas phase) Sajus S6r6c de Roch, in Bamford Tipper, Ref. 37. vol. 16. 1980. pp. 89-124 (liquid phase) Maslov Blyumberg Russ. Chem. Rev. 1976, 45, 155-167 (liquid phase). For a review of photochemical oxidation of aldehydes by O . see Niclause Lcmaire Letort Adv. Pholochem. 1966, 4, 25-48. For a discussion of the mechanism of catalyzed atmospheric oxidation of aldehydes, see Larkin J. Org. Chem. 1990, 55, 1563. 

The stereoselective nature of the reaction supports the suggestion that epoxidation in this case does not occur by acylperoxy radicals but rather by peracids generated from autoxidation of aldehydes. 

Other methods for preparing peroxycarboxylic acids include (/) autoxidation of aldehydes. (2) reaction of acid chlorides, anhydrides, or boric-carboxylic anhydrides with hydrogen or sodium peroxide, and (3) basic hydrolysis or perhydrolysis of dtacyl peroxides. 

Free-radical autoxidation of aldehydes with 02 is facile and affords the corresponding peradds, which are used as oxidants for carbonyl compounds. The peracid can transfer an oxygen atom to a substrate such as an olefin or ketone, resulting in the formation of one equivalent of epoxide or ester and add as a co-produd in the absence of metal catalysts [59]. Kaneda and coworkers have developed several HT materials that are active for heterogeneous Baeyer-Villiger reactions with 02/aldehyde [60]. Combination with Lewis addic metals improved the reaction by allowing coordination of the peracid and the intermediate. 

Autoxidation of aldehydes is analogous to that of hydrocarbons. Acylperoxy radicals are involved as principal chain carriers and peracids are the primary products in the following manner ... 

Oxidation of Co(II) or Mn(II) complexes by peracids [reaction (106) or (107)] is a facile reaction.124 Reaction (108) proceeds much more slowly, if at all. Thus, in contrast to the reaction with alkyl hydroperoxides (see preceding section), during the decomposition of peracetic acid the cobalt catalyst is present largely as Co(III) in both chlorobenzene and acetic acid solutions. Peracids are intermediates in the autoxidation of aldehydes, but the direct oxidation of aldehydes is a more favorable pathway for the regeneration of the reduced form of the catalyst (see Section II.B.3.e). The predominance of Co(III) supports the reaction (109) as the slower step in both solvents. 

The propagation step 8.14 is of special importance. Its rate depends on the R-H bond strength the weaker the bond, the faster the reaction. This explains why autoxidation of aldehydes is much easier than that of saturated hydrocarbons. The C-H bond energies of -CHO- and -CH2- groups are approx-... 

In a similar manner, autoxidation of aldehydes gives the corresponding carboxylic acids. 

Table I presents numerical values for the autoxidation of aldehydes and sodium bisulfite in a current of ozonized oxygen.

It has long been known that autoxidation of aldehydes leads to carboxylic acids via a radical mechanism which involves the formation and oxidation of acyl radicals, leading to acyl cations via one-electron oxidation processes [68]. However, the recent topic in this field relates to the fact that many new synthetic methods for the synthesis of carboxylic acids derivatives have been developed which rely on the power of one-carbon homologative radical reactions. 

The initial step of both the above reactions, the formation of an acyl radical, is catalysed by light and metal ions that are capable of a one-electron reduction transition (e.g. Fe3+ — Fe2+). Thus, the autoxidation of aldehydes can be greatly slowed down by keeping the compounds in the dark and by very careful purification. However, the most efficient method is the addition of antioxidants, such as phenols and aromatic amines that react preferentially with any radicals that may be present. 

The liquid phase autoxidation of aldehydes by molecular oxygen is almost always a homogeneous reaction which is brought about by the intervention of active radical species. The initial kinetics for the different cases studied may be deduced from the general scheme of long kinetic chain radical oxidation already worked out for the oxidation of hydro-... 

The autoxidation of aldehyde to form benzoic acid is also favoured by the presence of vanadium. The reaction is usually represented by ... 

Primary alcohols afforded the corresjjonding carboxylic adds via further oxidation of the aldehyde intermediate for example, 1-hexanol afforded 1-hexanoic acid in 95% yield. It is important to note, however, thatthis was achieved without the requirement of one equivalent of base to neutralize the carboxylic acid product (which is the case with supported noble metal catalysts [5]). In contrast, when lmol% TEMPO (4 equivalents per Pd) was added the aldehyde was obtained in high yield for example, 1 -hexanol afforded 1 -hexanal in 97% yield. Some representative examples of primary alcohol oxidations using this system are shown in Table 5.9. The TEMPO was previously shown to suppress the autoxidation of aldehydes to the carboxyhc adds (see earlier). 

Functional groups that stabilize radicals would be expected to increase susceptibility to autoxidation. This is illustrated by two cases that are relatively well studied. Aldehydes, in which abstraction of the aldehydic hydrogen is facile, are easily autoxidized. The autoxidation of aldehydes can lead to peroxycarboxylic acids, but usually carboxylic acids are isolated because the peroxyacid is capable of oxidizing unreacted aldehyde ... 

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