Oxidation to Aldehydes and Ketones

Cyclization via attack at boron in 168 led to 169, which was opened by reaction with trifluoroacetate anion to give 170. Oxidation of the 170 gave the tertiary alcohol, 171. The ketone product is formed by oxidation of the intermediate 169, although oxidative cleavage of 170 is also possible. 25 

While this method is applicable to symmetrical alcohols or ketones, formation of unsymmetrical ketones is difficult unless selective transfer of the alkyl groups can be achieved. Thexylborane provides a solution since it can react sequentially with two different alkenes to generate an unsymmetrical trialkylborane. Reaction of thexylborane with cyclopentene and then arylalkene 172, for example, gave 173. Subsequent reaction with sodium cyanide gave the unsymmetrical ate complex ( 74) and this generated unsymmetrical ketone 175 upon treatment with trifluoroacetic anhydride followed by oxidation. 20b 

Unsymmetrical cyclic ketones can be formed by the hydroboration of dienes followed by the oxidation sequence just described. In a typical example, Bryson reacted 176 with thexylborane but sodium cyanide was present (in situ) and this mixture generated 177. When this product was treated with trifluoroacetic anhydride 

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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... 

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. 

The treatment of an alkene by 5yn-hydroxylation, followed by periodic acid (HIO4) cleavage, is an alternative to the ozonolysis, followed by reductive work-up. 5yn-diols are oxidized to aldehydes and ketones by periodic acid (HIO4). This oxidation reaction divides the reactant into two pieces, thus it is called an oxidative cleavage. 

Monosubstituted and 1,2-disubstituted olefins can be oxidized to aldehydes and ketones by palladium chloride and similar salts of noble metals.367 1,1-Disubstituted olefins generally give poor results. The reaction is used industrially to prepare acetaldehyde from ethylene... 

Collins reagent is used for the introduction of carbonyl groups at allylic positions." This transformation of alkenes into enones is much slower than the oxidation of alcohols, requiring a great excess of Cr03 2Py and prolonged reaction times. Consequently, alcohols can be oxidized to aldehydes and ketones by Collins reagent without interference from alkenes. 

It is important to note that the relative velocity of an uneventful oxidation of an alcohol with PCC versus a carbon-carbon bond breakage from a chromate ester, driven by the generation of a stable carbocation, is substantially substrate-dependent, and may change according to stereoelec-tronic factors, which may be difficult to predict. Thus, many alcohols are successfully oxidized to aldehydes and ketones, regardless of an apparently potential carbon-carbon bond breakage leading to stabilized carboca-tions.315 Consequently, failure to try an alcohol oxidation with PCC, because of fear of this side reaction is not recommended. 

In 1965, Albright and Goldman3 demonstrated that alcohols are oxidized to aldehydes and ketones by the action of a mixture of DMSO and acetic anhydride at room temperature. Two years later,56 they presented a full paper, in which optimized conditions for this oxidation were established using yohimbine (16) as a model substrate. Thus, it was found that treatment of yohimbine with a mixture of DMSO and AC2O produces the desired oxidation to yohim-binone (17), accompanied by formation of the methylthiomethyl ether 18. 

Newer methods of alcohol oxidation (Swem, Dess-Martin) are introduced as environmentally preferable to the older chromium methods, including a description of a general, unifying mechanism of alcohol oxidation to aldehydes and ketones. TEMPO is shown as an oxidation catalyst to enhance hypochlorite oxidation. 

Pyridinium fluorochromate is a stable solid, which can be stored for long periods." h is as reactive as PCC, but slightly less acidic (pH of a 0.01 M solution = 2.4S compared to 1.7S for PCQ, and thus substrates with acid labile groups can be oxidized without the need to add a buffer." With 1.S equiv. of oxidant in dichloromethane at room temperature, primary and secondary alcohols are oxidized to aldehydes and ketones in high yield (Table 9). Unfortunately, ( )/(Z)-isomerization has been observed during the oxidation of allylic alcohols." ... 

Different electrophiles have been used to effect this oxidation. For example, in Pfitzner-Moffatt oxidation, alcohols are oxidized to aldehydes and ketones by the DMSO and DCC (dicyclohexylcarbodiimide). The resulting alkoxysulfonium ylide C rearranges to generate aldehydes and ketones (Scheme 7.6). 

Hypervalent organoiodine compounds have found wide utility in organic synthesis. Primary and secondary alcohols can be oxidized to aldehydes and ketones with the 12-1-5 compound (28), according to equation (29). a-Hydroxy dimethyl acetals can be synthesized under basic conditions with the 10-1-3 reagent PhI(OAc)2 (equation 30). The topic... 

Oxidation of alcohols. Primary and secondary alcohols can be oxidized to aldehydes and ketones, respectively, by potassium chromate and sulfuric acid in a two-phase system (CHCI3-H2O) in the presence of tetra- -butylammonium hydrogen sulfate as catalyst. The method is particularly useful for oxidation of primary... 

PCC on alumina (7.5g, 6.1 mmol) is added to a flask containing a solution of citronellol (0.60 g, 3.8 mmol) in 10 mL of n-hexane. After stirring or shaking for up to 3 h (follow the course of the reaction by TLC), remove the solid by filtration, wash it with three 10-mL portions of ether, and remove the solvents from the filtrate by distillation or evaporation. The last trace of solvent can be removed under vacuum. (See Fig. 9 in Chapter 3.) The residue should be pure citronellal, bp 90°C at 14 mm. Check its purity by TLC and infrared spectroscopy. A large number of other primary and secondary alcohols can be oxidized to aldehydes and ketones using this same procedure. 

Primary and secondary alkyl chlorides and bromides were oxidized to aldehydes and ketones in high yields with 4-dimethylaminopyridine N-oxide (48) in the presence or absence of DBU in acetonitrile. In the second step, DBU proved to be the most useful base for the deprotonation of compounds 49(81BCJ2221). 

The ability to convert a protective group to another functional group directly without first performing a deprotection is a potentially valuable transformation. Silyl-pro-tected alcohols have been converted directly to aldehydes, ketones, bromides, acetates and ethers" without first liberating the alcohol in a prior deprotection step. The smaller sterically less demanding silyl ethers can often be oxidized to aldehydes and ketones with reagents such as pyridinium chlorochromate. 

Alcohols are oxidized (to aldehydes and ketones) by H2O2 with catal3ffic quantitites of RUCI3 and 3-iodobenzoic acid. ... 

We have seen that alkenes can be oxidized to 1,2-diols and that 1,2-diols can be further oxidized to aldehydes and ketones (Sections 20.6 and 20.7, respectively). Alternatively, alkenes can be directly oxidized to aldehydes and ketones by ozone (O3). When an alkene is treated with ozone at low temperatures, the double bond breaks and the carbons that were doubly bonded to each other find themselves doubly bonded to oxygens instead. This oxidation reaction is known as ozonolysis. 

Oxidation with sodium dichromate. Primary and secondary alcohols are oxidized to aldehydes and ketones in 80-90% yield by Na2Cr2 07 2H2O and sulfuric acid in DMSO at 70° (90 min.). DMSO is not involved and acts as a solvent. In its absence, considerable charring occurs with further oxidation of the carbonyl product. Cr03 can also be used as oxidant. ... 

The most characteristic reactions of alcohols are their oxidation to aldehydes and ketones, which may undergo further oxidation, producing carboxylic acids. While primary alcohols oxidize to aldehyde, secondary alcohols produce ketones ... 

Various primary and secondary alcohols can be oxidized to aldehydes and ketones with the ion-supported [bis(acyloxy)iodo]arene 99 in the ionic liquid [emim]+[BF4] (l-ethyl-3-methylimidazolium tetrafluorobo-rate) in the presence of bromide anion [94], or in water in the presence of ion-supported TEMPO [97]. Under similar conditions reagent 99 can be used for mild, efficient, highly selective and environmentally friendly oxidation of aliphatic and aromatic sulfides to sulfoxides in excellent yields [98]. This reaction is compatible with hydroxyl, nitrile, methoxy, carbon-carbon double bonds and ester functionalities. The analogous pyrrolidinium-derived ion-supported [bis(acyloxy)iodo]arenes are efficient oxidants of alcohols to carbonyl compounds in the presence of TEMPO [99]. 

Alcohols are oxidized to aldehydes and ketones, whereas thiols are oxidized to disulfides. 

Alcohols, particularly benzylic and allylic ones, are oxidized to aldehydes and ketones under PTC conditions. Both stoichiometric and catalytic processes have been performed As in the case of permanganate, the reaction is controlled by pH in... 

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