Aldehydes overoxidation

In each case the initial product is the aldehyde or ketone. In (a) and (c) the aldehyde overoxidizes via addi tion of water to give a 1,1-diol, followed by a second oxidation ... 

Oxidation of primary alcohols to aldehydes (Section 15.10) Pyridinium dichromate (PDC) or pyridinium chloro-chromate (PCC) in anhydrous media such as dichloromethane oxidizes primary alcohols to aldehydes while avoiding overoxidation to carboxylic acids. 

Various experimental conditions have been used for oxidations of alcohols by Cr(VI) on a laboratory scale, and several examples are shown in Scheme 12.1. Entry 1 is an example of oxidation of a primary alcohol to an aldehyde. The propanal is distilled from the reaction mixture as oxidation proceeds, which minimizes overoxidation. For secondary alcohols, oxidation can be done by addition of an acidic aqueous solution containing chromic acid (known as Jones reagent) to an acetone solution of the alcohol. Oxidation normally occurs rapidly, and overoxidation is minimal. In acetone solution, the reduced chromium salts precipitate and the reaction solution can be decanted. Entries 2 to 4 in Scheme 12.1 are examples of this method. 

An alternative approach consisted of the use of an alcohol as aldehyde precursor since aldehydes are less stable (prone to overoxidation, polymerization or formation... 

To avoid overoxidation, primary amines (e.g. 128, equation 89) can be converted into Schiff bases with an aromatic aldehyde. Subsequent oxidation of the resultant imines 129 with an excess of peracids produces oxaziridines 130 and/or nitrones 131. Both of them produce hydroxylamines 132 (equation 89) upon hydrolysis in moderate to good overall yields. Yields of hydroxylamines are considerably better if anisaldehyde instead of benzaldehyde is used for the protection . ... 

Good selectivity for the oxidation of primary alcohols in the presence of secondary ones can be achieved. By appropriate choice of the reaction conditions, overoxidation of the aldehyde from a primary alcohol to carboxylic acid can be minimized. Kinetic isotope effects in the range of 2 to 3 testify about the relevance of the H+-elimination step upon the overall reactivity . In general, the efficiency of oxidation of alkanols is slightly lower... 

NaClO, or else in the two-phase system but with a quaternary ammonium (viz. AUquat) ion as a phase-transfer catalyst, overoxidation to the corresponding carboxylic acid is obtained (entry 4). Therefore, by proper choice of the experimental conditions, a synthetically useful distinction in products formation can be made for the oxidation of primary alcohols, even though we are far from a satisfactory understanding of the reason behind this different behaviour. In fact TEMPO, as a well-known inhibitor of free-radical processes is allegedly responsible for the lack of overoxidation of an aldehyde to carboxylic acid (entry 3) this notwithstanding, TEMPO is also present under those conditions where the overoxidation does occur (eutry 4). Moreover, a commou teuet is that the formation of the hydrated form of an aldehyde (in water solution) prevents further oxidation to the carboxylic acid however, both entries 3 and 4 refer to water-organic solutions, and their... 

The most profitable is certainly the use of laccase (Lc) with TEMPO. It enables the almost quantitative conversion of primary benzylic and allylic alcohols to aldehydes without overoxidation under mild conditions (Table 14, entries 1 and 2), that is, 25 °C and pH = 4.5 in the presence of atmospheric O2, for a reaction time of 24 h. The successful enzyme is the one obtained from the fungus Trametes villosa. 

Primary benzylic alcohols are oxidized to aldehydes in good yields without overoxidation (entry 1) lowering the pH from 5 to 3.5 increases the conversion, for reasons not fnUy understood yet (entry 2) . The aminoxyl radical is an electrophilic species" ... 

By suitable modification of reaction conditions, it was found possible to reduce 859 to keto alcohol 873 K The subsequent conversion of this intermediate to 874 proceeded without event. However, 874 could not be oxidized to aldehyde 875. Overoxidation to produce 876 or 877 (Jones conditions) invariably was observed due to the extreme sensitivity of874. This potentially expedient route to dodecahedrane therefore had to be abandoned and recourse made to blocking group methodology. 

Taylor and Flood could show that polystyrene-bound phenylselenic acid in the presence of TBHP can catalyze the oxidation of benzylic alcohols to ketones or aldehydes in a biphasic system (polymer-TBHP/alcohol in CCI4) in good yields (69-100%) (Scheme 117) °. No overoxidation of aldehydes to carboxylic acids was observed and unactivated allylic alcohols or aliphatic alcohols were unreactive under these conditions. In 1999, Berkessel and Sklorz presented a manganese-catalyzed method for the oxidation of primary and secondary alcohols to the corresponding carboxylic acids and ketones (Scheme 118). The authors employed the Mn-tmtacn complex (Mn/168a) in the presence of sodium ascorbate as very efficient cocatalyst and 30% H2O2 as oxidant to oxidize 1-butanol to butyric acid and 2-pentanol to 2-pentanone in yields of 90% and 97%, respectively. This catalytic system shows very good catalytic activity, as can be seen from the fact that for the oxidation of 2-pentanol as little as 0.03% of the catalyst is necessary to obtain the ketone in excellent yield. 

To prevent overoxidation of aldehydes, the very mild oxidant dimethyl sulfoxide or dmso, is used to react with T halides or sulfonates to give aldehydes. These reactants are in the same oxidation level as alcohols ... 

A few studies did address chemical selectivity in these oxidations. Pattenden and coworkers, for example, showed that primary alcohols could be selectively oxidized to the corresponding aldehydes, without appreciable overoxidation, when the platinized TiO2 photocatalyst was suspended in benzene, Eq. (11). Poor yields were obtained... 

When applied to N-acylglycofuranosylamines, the periodate oxidation showed an abnormal uptake of oxidant ( overoxidation ). For example, when oxidized with lead tetraacetate12 and with periodate,1065 N-acetyl-a-D-glucofuranosylamine (15) afforded formaldehyde (indicating a furanose structure), and it consumed more than 5 moles of oxidant per mole. This result can be attributed to subsequent oxidation of the formic acid produced,69 or to the formation,10 by hydrolysis, of the intermediate 2-hydroxypropanedial (tartron-aldehyde) (77) that would then be oxidized. This tendency to undergo overoxidation has been found common for the furanoid N-acyl-gly cos y lam ines.24,25... 

In Chapter 15 primary alcohols, RCH2OH, were shown to be readily oxidized to aldehydes, RCHO, and secondary alcohols, R2CHOH, to ketones, R2CO, by inorganic reagents such as Cr03 and KMn04. However, it is a problem to avoid overoxidation with primary alcohols because of the ease with which aldehydes... 

The oxidation of primary alcohols to aldehydes also suffers from the problem of overoxidation of the aldehyde to a carboxylic acid. Mild methods capable of stopping die oxidation at the aldehyde oxidation level are required if aldehydes are to be obtained. The most common and effective reagent for this purpose is pyridinium chlorochromate (PCC), produced by the reaction of pyridinium hydrochloride with chromium trioxide. This reagent is soluble in dichloromethane and smoothly oxidizes primary alcohols to aldehydes in high yields. Because of die mild, neutral reaction conditions and the use of stoichiomettic amounts of oxidant, the aldehyde product is not oxidized further. 

An eco-friendly oxidation of alcohols under an oxygen atmosphere using catalytic amounts of [bis(acetoxy)iodo]benzene-TEMPO-KN02 has been reported. The use of a catalytic amount of poly[4-(bis(acetoxy)iodo)]styrene allowed the successful recycling of this catalytic component. The protocol can be used to promote the oxidation of different kinds of alcohols in the presence of other functional groups and also in the oxidation of primary benzylic alcohols in the presence of secondary and aliphatic ones. Primary alcohols can be oxidized to the corresponding aldehydes without any noticeable overoxidation to the carboxylic acids.269... 

The quantitative stoichiometry of the D-glucose-chlorous acid reaction has been studied in detail the reagent used was sodium chlorite in a phosphoric acid-phosphate buffer at pH 2.4-3.4. The molar ratio of oxidant consumed to D-glucose consumed was 3 1 no overoxidation occurred over extended periods of time. The method is recommended for the determination of aldehyde groups in carbohydrates, especially alkali-sensitive carbohydrates. 

Although the a-hydrogen atoms in alcohols are very reactive, those of ethers and particularly esters are substantially more sluggish toward oxidation than those of alcohols. Thus, the overoxidation of primary and secondary alcohols may be prevented by protection of the newly formed hydroxy group by in-situ acylation with trifluoroacetic anhydride [23], Preparatively most significant, the C-H bonds adjacent to carbonyl (ketone, aldehyde, ester, and amide) and cyano groups are inert towards DMD and even TFD oxidation. 

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