Oxidations to aldehydes

In some cases where there is a neighboring group participation effect, aldehydes are formed. The a-vinyl group in the / -lactam 29 is mainly oxidized to aldehyde 30[83],... 

The use of silver (II) salts, particularly argentic picolinate, as reagents for hydroxyl oxidation has also been disclosed recently. The reaction may be run in acid, neutral or basic media in aqueous or polar organic solvents at room or slightly elevated temperatures. Primary alcohols may be oxidized to aldehydes or acids depending on the conditions used. Amines and trivalent phosphorous compounds are more sensitive to oxidation with this reagent than are hydroxyl groups. 

Dipyridiue-chromium(VI) oxide2 was introduced as an oxidant for the conversion of acid-sensitive alcohols to carbonyl compounds by Poos, Arth, Beyler, and Sarett.3 The complex, dispersed in pyridine, smoothly converts secondary alcohols to ketones, but oxidations of primary alcohols to aldehydes are capricious.4 In 1968, Collins, Hess, and Frank found that anhydrous dipyridine-chromium(VI) oxide is moderately soluble in chlorinated hydrocarbons and chose dichloro-methane as the solvent.5 By this modification, primary and secondary alcohols were oxidized to aldehydes and ketones in yields of 87-98%. Subsequently Dauben, Lorber, and Fullerton showed that dichloro-methane solutions of the complex are also useful for accomplishing allylic oxidations.6... 

In another type of oxidative decarboxylation, arylacetic acids can be oxidized to aldehydes with one less carbon (ArCH2COOH ArCHO) by tetrabutylammonium... 

Another reagent that convert benzylic halides to aldehydes is pyridine followed by /7-nitrosodimethylaniline and then water, called the Krohnke reaction. Primary halides and tosylates have been oxidized to aldehydes by trimethylamine N-oxide, and by pyridine N-oxide with microwave irradiation. ... 

Primary aliphatic amines can be oxidized to aldehydes or ketones. Other reagents... 

Primary and secondary aliphatic nitro compounds have been oxidized to aldehydes and ketones, respectively (RR CHN02 RR C=0) with sodium chlorite under phase-transfer conditions, TPAP, Oxone , as well as with other reagents. 

Reactions of partial electrochemical oxidation are of considerable interest in the electrosynthesis of various organic compounds. Thus, at gold electrodes in acidic solutions, olefins can be oxidized to aldehydes, acids, oxides, and other compounds. A good deal of work was invested in the oxidation of aromatic compounds (benzene, anthracene, etc.) to the corresponding quinones. To this end, various mediating redox systems (e.g., the Ce /Ce system) are employed (see Section 13.6). 

Ten Brink et al. (2000) have shown how biphasic systems, sometimes with the sparingly soluble alcohols as one phase and an aqueous phase as the other phase, benefit from the strategy for air oxidation to aldehydes/ketones by using water soluble Pd complex of bathophenanthroline disulphonate. This is a nice example of green technology. 

Hydroxymethylation (formaldehyde) of nitro-imidazole 76 affords 77, which is oxidized to aldehyde 78. To prepare the other fragment for this convergent synthesis, reaction of epichlorohydrin with morpholine leads to the aminoepoxide 79, which is reacted with hydrazine to afford 80. Reaction of this substituted hydrazine with dimethyl carbonate affords oxazolinone 81 by sequential ester interchange reactions. Condensation of 81 with aldehyde 78 affords the antitricho-... 

Carbon-Carbon Bond Cleavage with Oxidation to Aldehyde Residues... 

Carbon-Carbon Bond Breakage with Oxidation to Aldehydes... 

You learned earlier that primary alcohols are oxidized to aldehydes, and secondary alcohols are oxidized to ketones. You can think of the reduction of aldehydes and ketones as the reverse of these reactions. Aldehydes can be reduced to produce primary alcohols. Ketones can be reduced to produce secondary alcohols. 

Aromatic methyl group hydroxylation and oxidation to aldehyde and carboxylic acid Aspergillus sclerotiorum, Aspergillus sp.)... 

Varieties of primary and secondary alcohols are selectively oxidized to aldehyde or carbonyl compounds in moderate to excellent yields as summarized in Table 3. As can be seen, /(-substituted benzyl alcohols (e.g., -Cl, -CH3, -OCH3, and -NO2) yielded > 90% of product conversion in 3-4 h of reaction time with TOP in the range of 84-155 h (entries 2-5, Table 3), Heterocyclic alcohols with sulfur- and nitrogen-containing compoimds are found to show the best catalytic yield with TOP of 1517 and 902 h for (pyrindin-2-yl)methanol and (thiophene-2-yl) methanol, respectively (entries 9 and 10, Table 3). Some of aliphatic primary alcohols (long chain alcohols) and secondary alcohols (cyclohexanol, its methyl substituted derivatives and norboman-2-ol) are also selectively oxidized by the membrane catalyst (entries 11-14 and 15-17, Table 3) with TOP values in the window of 8-... 

Alcohols are oxidized to aldehydes by the liver enzyme alcohol dehydrogenase, and aldehydes to carboxylic acids by aldehyde dehydrogenase. In mammals, monooxygenases can be induced by plant secondary metabolites such as a-pinene, caffeine, or isobornyl acetate. Reduction is less common and plays a role with ketones that cannot be further oxidized. Hydrolysis, the degradation of a compound with addition of water, is also less common than oxidation. 

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. 

With the fully functionalized heterocyclic core completed, synthetic attention next focused on introduction of the 3,5-dihydroxyheptanoic acid side-chain. This required initial conversion of the ethyl ester of 35 to the corresponding aldehyde through a two-step reduction/oxidation sequence. In that event, a low-temperature DIBAL reduction of 35 provided primary alcohol 36, which was then oxidized to aldehyde 37 with TRAP. Subsequent installation of the carbon backbone of the side-chain was accomplished using a Wittig olefination reaction with stabilized phosphonium ylide 38 resulting in exclusive formation of the desired -olefin 39. The synthesis of phosphonium ylide 38 will be examined in Scheme 12.5 (Konoike and Araki, 1994). 

Allylic and benzylic alcohols were oxidized to aldehydes or ketones with BnPhsPHSOs in refluxing CHsCN. The yield increased in the presence of bismuth chloride in a catalytic amount. Selective oxidation of various alcohols under solvent free conditions was also reported Interestingly, benzyl alcohols were oxidized selectively to benzaldehydes in very high yield (95-100%) when reacted with BnPhsPHSOs (1.2 eq.) and AICI3 (1 eq.) in the presence of an equimolar amount of 2-phenethyl alcohol, diphenyl carbinol or methyl phenyl sulfide (equation 72). 

More recently, the Noyori group described an organic solvent- and haUde-free oxidation of alcohols with aqueous H202 . The catalyst system typically consists of Na2W04 and methyltrioctylammonium hydrogen sulfate, with a substrate-to-catalyst ratio of 50-500. Secondary alcohols are converted to ketones, whereas primary alcohols, in particular substituted benzyUc ones, are oxidized to aldehydes or carboxylic acid by selecting appropriate reaction conditions This system also catalyzed the chemoselective oxidation of unsaturated alcohols, the transformation exemplified in equation 65, with a marked prevalence for the hydroxy function. 

---