Applications Alcohol oxidation

Another reagent that finds application of oxidations of alcohols to ketones is ruthenium tetroxide. The oxidations are typically carried out using a catalytic amount of the ruthenium source, e.g., RuC13, with NaI04 or NaOCl as the stoichiometric oxidant.16 Acetonitrile is a favorable solvent because of its ability to stabilize the ruthenium species that are present.17 For example, the oxidation of 1 to 2 was successfully achieved with this reagent after a number of other methods failed. 

Although the ability of microwaves (MW) to heat water and other polar materials has been known for half a century or more, it was not until 1986 that two groups of researchers independently reported the application of MW heating to organic synthesis. Gedye et al. [1] found that several organic reactions in polar solvents could be performed rapidly and conveniently in closed Teflon vessels in a domestic MW oven. These reactions included the hydrolysis of amides and esters to carboxylic acids, esterification of carboxylic acids with alcohols, oxidation of alkyl benzenes to aromatic carboxylic acids and the conversion of alkyl halides to ethers. 

The attractive (80) features of MOFs and similar materials noted above for catalytic applications have led to a few reports of catalysis by these systems (81-89), but to date the great majority of MOF applications have addressed selective sorption and separation of gases (54-57,59,80,90-94). Most of the MOF catalytic applications have involved hydrolytic processes and several have involved enantioselec-tive processes. Prior to our work, there were only two or three reports of selective oxidation processes catalyzed by MOFs. Nguyen and Hupp reported an MOF with chiral covalently incorporated (salen)Mn units that catalyzes asymmetric epoxidation by iodosylarenes (95), and in a very recent study, Corma and co-workers reported aerobic alcohol oxidation, but no mechanistic studies or discussion was provided (89). 

Oxidation of ethyl alcohol was one of the two important commercial routes to acetaldehyde until the 1950s, The other, much older route was the hydration of acetylene. The chemical industry was always after a replacement of acetylene chemistry, not just for acetaldehyde production, but all its many applications. Acetylene was expensive to produce, and with its reactive, explosive nature, it was difficult to handle. In the 1950s, acetylene chemistry and the ethyl alcohol oxidation route were largely phased out by the introduction of the liquid phase direct oxidation of ethylene. Almost all the acetaldehyde produced uses the newer process. 

Oxidation of Alcohols in a Direct Alcohol Fuel Cell The electrocatalytic oxidation of an alcohol (methanol, ethanol, etc.) in a direct alcohol fuel cell (DAFC) will avoid the presence of a heavy and bulky reformer, which is particularly convenient for applications to transportation and portable electronics. However, the reaction mechanism of alcohol oxidation is much more complicated, involving multi-electron transfer with many steps and reaction intermediates. As an example, the complete oxidation of methanol to carbon dioxide ... 

Another reagent that finds application in oxidations of alcohols to ketones is ruthenium tetroxide. For example, the oxidation of 1 to 2 was successfully achieved with this reagent after a number of other methods failed. 

The RuHAP-y-Fe203 catalyst was also found to be applicable to oxidation of sterically bulky alcohols 3,5-dibenzyloxybenzyl alcohol and cholestanol were successfully converted into the corresponding carbonyl compounds with excellent yields, (5.2) and (5.3), respectively. In particular, the oxidation of 7-hexadecyn-l-ol proceeded quantitatively without any effect on the alkynyl group (5.4) ... 

The oxidation of primary or secondary alcohols to aldehydes or ketones respectively with dimethyl sulphoxide activated by oxalyl chloride has wide applicability (Swern oxidation).243b The initial reaction between the acid chloride and dimethyl sulphoxide in dichloromethane solvent is vigorous and exothermic at — 60 °C and results in the formation of the complex (7) this complex spontaneously decomposes, even at this low temperature, releasing carbon dioxide and carbon monoxide to form the complex (8). The alcohol is added within 5 minutes, followed after 15 minutes by triethylamine. After a further 5 minutes at low temperature the reaction mixture is allowed to warm to room temperature and work-up follows standard procedures. The ratio of reactants is dimethyl sulphoxide oxalyl chloride alcohol triethylamine 2.2 1.1 1.0 5. 

It should be noted that the related imine-oxaziridine couple E-F finds application in asymmetric sulfoxidation, which is discussed in Section 10.3. Similarly, chiral oxoammonium ions G enable catalytic stereoselective oxidation of alcohols and thus, e.g., kinetic resolution of racemates. Processes of this type are discussed in Section 10.4. Whereas perhydrates, e.g. of fluorinated ketones, have several applications in oxidation catalysis [5], e.g. for the preparation of epoxides from olefins, it seems that no application of chiral perhydrates in asymmetric synthesis has yet been found. Metal-free oxidation catalysis - achiral or chiral - has, nevertheless, become a very potent method in organic synthesis, and the field is developing rapidly [6]. 

Chloroperoxidase (CPO) is a very versatile enzyme capable of carrying out a number of reactions including epoxidation (1,2), sulfoxidation, alcohol oxidation, N-dealkylation, (3) and hydroxylation in the presence of a suitable reductant (4-7). Most of these hydrophobic molecules require the use of an organic solvent in the reaction medium to enhance solubility. However, the enzyme has very low activity in organic solvents (8), reducing its potential for industrial application. 

In an alternative application of asymmetric alcohol oxidation, Rychnovsky has reported the use of the chiral nitroxyl radical 34 (Fig. 12.14) along with bleach, allowing kinetic resolution of secondary alcohols [89]. The best substrates were simple benzylic alcohols, for which S factors (= ks/kR) were in the range 3.9 to 7.1 (Scheme 12.22). Other chiral C2-symmetric nitroxyl radicals reported recently give lower selectivities [90]. 

Modern variations include the in situ, and thus catalytic, use of this high-valent selective reagent, not only for alcohols but also for ethers (see later). Ru(VII) (perruthenate) in the compounds tetra-n-butylammonium perruthenate (TBAP) and tetra-n-propylammonium perruthenate (TPAP) has found wide application in alcohol oxidation. Ru-oxo complexes with valence states of IV to VI are key intermediates in, for example, the selective oxygen transfer to alkenes, leading to epoxides. On the other hand 16-electron Ru(II) complexes can be used to catalyse hydrogen transfer thus these are excellent catalysts for oxidative dehydrogenation of alcohols. A separate section is included to describe the different mechanisms in more detail. 

This system found another application—the oxidation of secondary alcohols into ketones in excellent yield. It is worthy of note that the oxidation procedure tolerates other functional groups including iodide, ester, terminal alkyne, aromatic ether, and 1,3-dioxolane. Certain secondary allylic alcohol such as 2-cyclododecen-l-ol (31) produced the corresponding epoxy ketone 32 in one pot, as exemplified in Sch. 17. 

Redox silicalites with elements other than titanium have found catalytic applications in various organic oxidations. V-ZSM-11 is reported to catalyze alkane oxidation and is found to oxidize even primary C-H bonds [98]. Detailed studies indicate that V silicalites are better than Ti silicalites for alkane oxidation, as it is found that molecular oxygen can act as the oxidant in the former case [99]. V-ZSM-5 shows selectivity toward alcohol oxidation in allyl and methallyl alcohols... 

The main applications of oxidation with chromium trioxide are transformations of primary alcohols into aldehydes [184, 537, 538, 543, 570, 571, 572, 573] or, rarely, into carboxylic acids [184, 574], and of secondary alcohols into ketones [406, 536, 542, 543, 575, 576, 577, 578, 579, 580, 581, 582, 583, 584]. Jones reagent is especially successful for such oxidations. It is prepared by diluting with water a solution of 267 g of chromium trioxide in a mixture of 230 mL of concentrated sulfuric acid and 400 mL of water to 1 L to form an 8 N CrOj solution [565, 572, 579, 581, 585, 556]. Other oxidations with chromic oxide include the cleavage of carbon-carbon bonds to give carbonyl compounds or carboxylic acids [482, 566, 567, 569, 580, 587, 555], the conversion of sulfides into sulfoxides [541] and sulfones [559], and the transformation of alkyl silyl ethers into ketones or carboxylic acids [590]. 

Zinc dichromate tiihydrate, ZnCr207<3H20, is obtained as an orange-red solid by adding zinc carbonate to a cold solution of chromium trioxide in dilute sulfuric acid [660]. The applications are oxidations of acetylenes lo a-diketones, of aromatic hydrocarbons to quinones, of alcohols to aldehydes, and of ethers to esters and the oxidative regeneration of carbonyl compounds from their oximes [660]. 

A chromium(VI) oxidant that is applicable to oxidations of acid-sensitive substrates is the complex of chromium trioxide with two molecules of pyridine (Collins reagent). As described on pages 22 and 274, its preparation requires the portionwise addition of chromium trioxide to dry pyridine at 15-20 C (addition of pyridine to chromium oxide could cause ignition) [592, 595, 599]. Up to 6 mol of the complex is used to oxidize alcohols in dichloromethane solutions at 25 °C, and the reaction is finished in 5-15 min [595]. Alternatively, the oxidation can be carried out in pyridine cooled with an ice bath and is finished at room temperature within 15-22 h [592, 599]. 

Doorslaer CV, ScheUekens Y, Mertens P, Binnemans K, De Vos D (2010) Spontaneous product segregation from reactions in ionic hquids application in Pd-catalyzed aliphatic alcohol oxidation. Phys Chem Chem Phys 12 1741-1749... 

Nitroxide-mediated oxidation based on oxoammonium salts is a very common application of nitroxides in organic synthesis. In addition to a variety of alcohol oxidations, applications using oxoammonium species as one-electron oxidants have been utilized with a number of different substrates [30]. 

Oxidation of sulfides. This oxide in combination with bromine has been used for selective oxidation of a secondary alcohol in the presence of a primary alcohol (7, 26 27). This mild procedure is also applicable to oxidation of sulfides (o sulfoxiilcs without further oxidation to sulfones (equation I). The method can also be used lo oxidi/c hydroxy sulfides to keto sulfoxides in one operation. The reageni also can be used lor oxidation of sidfenamides to sulfinamidcs without loi II union of sn I Iona in ides (eipial ion II). ... 

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