Oxidation of aliphatic aldehydes

Investigation of the oxidation of aliphatic aldehydes has been confined to ew-diols which behave as secondary alcohols, being most easily oxidised in the anionic forms, e.g. 

Oxidation of aliphatic aldehydes in methanol with potassium iodide as mediator follows a reaction path like that of the saccharides. The corresponding carboxylic... 

The oxidation of a-hydroxy acids by benzyltrimethylammonium tribromide (BTMAB) to the corresponding carbonyl compounds shows a substantial solvent isotope effect, A (H20)/A (D20) = 3.57, but no KIE for a-deuteromandelic acid.133 The oxidation of glucose by hypobromous acid is first order in glucose and the acid.134 [l,l-2H2]Ethanol shows a substantial kinetic isotope effect when oxidized by hexamethylenetetramine-bromine (HABR) in acetic acid to aldehyde.135 Kinetics of the oxidation of aliphatic aldehydes by hexamethylenetetramine-bromine have been studied by the same group.136 Dioxoane dibromide oxidizes y-tocopherol to 5-bromomethyl-y-tocopherylquinone, which spontaneously cyclizes to 5-formyl-y-tocopherol.137... 

Anodic oxidation of aliphatic aldehydes and ketones is generally difficult because their oxidation potentials are very high (>2.5 V). However, silylation at the carbonyl carbon causes a marked decrease in the oxidation potential as shown in Table 933,40. This silicon effect is much smaller in the case of aromatic carbonyl analogues40. The silicon effect is attributed to the rise of the HOMO level by the interaction between the C—Si a orbital and the nonbonding p orbital of the carbonyl oxygen, which in turn favors the electron transfer. 

Oxidation of aliphatic aldehydes by quinolinium dichromate in aqueous acetic acid shows first-order kinetics in substrate and oxidant, and second-order with respect to H+.319 Hydrated aldehyde and protonated oxidant are suggested to be the reactive species, with Zucker-Hammett plots supporting proton abstraction by water in the slow step. 

A kinetic study of a reaction can be simplified by running the reaction with one or more of the components in large excess, so that the concentration remains effectively constant. Me Tigue and Sime [2] consider the oxidation of aliphatic aldehydes such that ethanal with bromine in aqueous solution follows second-order kinetics ... 

On the other hand, Cordova et al. reported that 7a gave better stereochemical results than 1 in the direct singlet-ojgrgen photochemical a-oxidation of aliphatic aldehydes, albeit being less reactive (Scheme 11.4). ... 

Scheme 11.4 Direct organocatal34ic a mmetric a-oxidations of aliphatic aldehydes with singlet oxygen catdysed by (S)-proline (1) and (S)-a-2-methyl-proline (7a). TPP = tetraphenylporphine.

Anodic oxidation of aliphatic aldehydes and ketones is generally difficult because their oxidation potentials are very high (>2.5 V). However, silylation at the carbonyl carbon causes a marked decrease in the oxidation potential as shown in Table This silicon... 

Correlation analysis of reactivity in the oxidation of benzyl alcohol by benzyltrimethyl-ammonium tribromide has revealed that the reaction involves an almost synchronous cleavage of the a-C—H and O-H bonds. An intermediate complex between the aldehyde hydrate and tribromide ion is decomposed in the rate-limiting step of the oxidation of aliphatic aldehydes to the corresponding carboxylic acids by benzyltrimethylammon-ium tribromide. ... 

Oxidation of aliphatic aldehydes by benzyltrimethylammonium chlorobromate to the corresponding carboxylic acid proceeds via the transfer of a hydride ion from the aldehyde hydrate to the oxidant. The oxidation of aUyl alcohol with potassium bromate in the presence of osmium(Vin) catalyst in aqueous acidic medium is first order in bromate, Os(Vni) and substrate, but inverse fractional order in H+ the stoichiometry of the reaction is 2 3 (oxidantsubstrate). The active species of oxidant and catalyst in the reaction were understood to be BrOs and H2OSO5, respectively, which form a complex. Autocatalysis by Br, one of the products, was observed, and attributed to complex formation between Br and osmium(VIII). First-order kinetics each in BrOs, Ru(VI), and substrate were observed for the ruthenium(VI)-catalyzed oxidation of cyclopentanol by alkaline KBrOs containing Hg(OAc)2. A zero-order dependence on HO concentration was observed and a suitable mechanism was postulated. The oxidation reaction of aniUne blue (AB+) with bromate at low pH exhibits interesting non-linear phenomena. The depletion of AB+ in the presence of excess of bromate and acid occurs at a distinctly slow rate, followed by a very rapid reaction. A 12-step reaction mechanism, consistent with the reaction dynamics, has been proposed. The novel cyclohexane-l,4-dione-bromate-acid system has been shown to exhibit a rapid oscillatory redox reaction superimposed on a slower... 

Catalytic oxidation of H2O by peroxodiphosphate has been studied under acid conditions where competing hydrolysis to peroxomonophos-phate is avoided. The stoichiometry (36) has been established and the rate law found to be first order in [P20g ] and [Ag ] from initial rate measurements, H2P20g and HP20g being the most important species. Oxidations of aliphatic aldehydes with peroxomonophosphate have been studied mechanistically. ... 

The effect of cetylpyridinium chloride surfactant on the rate of oxidation of propi-onaldehyde by bromate has been studied. Solvent effects and Michaelis-Menten kinetics found for acidic oxidation of aliphatic aldehydes by benzimidazolium dichromate (BIDC) suggest that in the rate-determining step an aldehyde-BIDC complex reacts via a cyclic transition to give a carbocationic species by hydride transfer to oxidant = 6.36 for MeCHO). ... 

Fehling s solution. Aqueous solutions of aliphatic aldehydes are almost invariably acidic owing to atmospheric oxidation, and therefore... 

In the aerobic oxidation of the non-activated aliphatic primary and secondary alcohols to the corresponding aldehydes and ketones, co-catalysts or other additives are normally required 223-226). The catalytic aerobic oxidation of aromatic aldehydes to the corresponding carboxylic acids with Ni(acac)2 in ionic liquids was the first example of an aerobic oxidation in ionic liquids 227). 

The oxidation of 2-ethylhexan-l-ol to 2-ethyl-hexanal by the Oppenauer oxidation with aliphatic aldehydes such as acetaldehyde, propionaldehyde, and isobutyr-aldehyde has been investigated with gas-phase reactants and MgO as the catalyst (196). Reaction with propionaldehyde was found to be an effective synthetic route for 2-ethylhexanal preparation, whereas with acetaldehyde and isobutyraldehyde a gradual catalyst deactivation in a flow reactor was observed. 

Recently, Borhan and coworkers reported the facile oxidation of aliphatic and aromatic aldehydes to acids and esters in DMF or methanol with Oxone (equation 53) . These reactions are considered to be valuable alternatives to traditional metal-mediated oxidations. 

Pincer-ligated iridium complexes have been used as homogeneous catalysts for the dehydrogenation of aliphatic polyalkenes to give partially unsaturated polymers. The catalyst appears to be selective for dehydrogenation in branches as compared with the backbone of the polymer.56 The mechanism shown in Scheme 1 has been suggested for an [IrCl(cod)]2-catalysed oxidative esterification reaction of aliphatic aldehydes and olefinic alcohols.57... 

Fig. 17.17. Sodium chlorite oxidation of aliphatic or aromatic aldehydes to form a carboxylic acid. The extra additive destroys the reduction product of the oxidant, i.e., sodium hypochlorite or hypochloric acid.

The oxidation of aliphatic and aromatic aldehydes to their corresponding acids allows the acids to be excreted or conjugated in phase II reactions ... 

Nanoparticlulated gold supported by nanocrystalline or mesostruc-tured nanocrystalline ceria catalysts represents an alternative to catalysts for selective aerobic oxidation of aliphatic and aromatic aldehydes which is much better than the gold supported by the precipitated ceria (Corma and Domine, 2005). The ceria or yttria supported Au are also active and extremely selective for the homocoupling of arylboronic acids, and the activity is directly correlated with Au(lll) (Carrettin et al., 2005). 

The carbonyl group is a reactive function and, although aromatic aldehydes are somewhat less reactive than their aliphatic counterparts, benzaldehydes have an extensive chemistry. Many reactions replicate those of aliphatic aldehydes, but are mentioned here for completeness. Thus, oxidation of the carbonyl group leads to carboxylic acids and reduction gives alcohols. The aldehyde group reacts with a range of N-nucleophiles (Scheme 6.9). Imines (Schiff bases) are formed with amines and hydrazones with hydrazines. Semicarbazide gives semicarbazones and hydroxylamine forms oximes. 

Pyridinium dichromate in dichloromethane solution converts primary alcohols into aldehydes. In dimethylformamide at 25 °C, carboxylic acids are formed. Cyclohexylmethanol thus gives cyclohexanecarboxylic acid in 84% yield [603]. Oxidations of aliphatic alcohols with ten-butyl chromate yield mixtures of acids with aldehydes and esters [677]. 

The oxidation of aliphatic alcohols in benzene or petroleum ether with /ert-butyl chromate at 1-2 °C for 6 h leads to mixtures of aldehydes, acids, and their esters. Butanol gives 30% of butanal, 27% of butyric acid, and 36% of butyl butyrate [677], Also, electrolysis of aliphatic alcohols on platinum or carbon electrodes in aqueous potassium iodide at room temperature results in 80-83% yields of the corresponding esters [121]. 

Failure of aliphatic aldehydes to be produced in high yields by application of this steam distillation method to estragole and 1-dodecene may be attributed to the increased stability of the intermediate hydroperoxides toward hydrolysis. In other words, the conversion rates of the corresponding intermediates, V to VI to VII, are so slow that the hydroperoxides are either steam distilled and/or undergo the relatively more rapid oxidative rearrangement to VIII and subsequent conversion to IX. 

Due to the wide availability of aliphatic aldehydes from hydroformylation (cf. Section 2.1.1), the principal method for the production of C3-C]o carboxylic acids is the catalytic oxidation of the corresponding aldehyde (eq. 1). 

---