Other Oxidants

A very useful group of procedures for oxidation of alcohols to ketones have been developed that involve dimethyl sulfoxide (DMSO) and any one of a number of electrophilic molecules, particularly dicyclohexylcarbodiimide, acetic anhydride, and sulfur trioxide. The initial work involved the DMSO-dicyclo-hexylcarbodiimide system. The utility of the method has been greatest in the oxidation of molecules that are highly sensitive to more powerful oxidants and therefore cannot tolerate alternative methods. The mechanism of the oxidation involves formation of intermediate A by nucleophilic attack of DMSO on the carbodiimide, followed by reaction of this species with the alcohol. A major portion 

The role of activating DMSO toward the nucleophilic addition step can be accomplished by other electrophilic species. A method that appears to have certain advantages of convenience is use of the pyridine complex of A mechanism 

Other reagents that have been found capable of activating DMSO toward nucleophilic attack include acetic anhydride and phosphorus pentoxide.  

Oxidation of alcohols under extremely mild conditions can be effected using a procedure that is mechanistically related to the DMSO method. Dimethyl sulfide is converted to a sulfonium derivative by reaction with N-chlorosuccinimide. This sulfur species reacts readily with alcohols, generating the same kind of alkoxysul-fonium salts that are involved in the DMSO procedures. In the presence of mild base, elimination of dimethyl sulfide completes the oxidation.  

Similarly, reaction of chlorine and DMSO at low temperature gives an adduct that reacts with alcohols, presumably by displacement of chloride from sulfur, to give the ketone and DMSO t 

The activation of DMSO toward the addition step can be accomplished by other electrophiles. All of these reagents are believed to form a sulfoxonium species by electrophilic attack at the sulfoxide oxygen. The addition of the alcohol and the departure of the sulfoxide oxygen as part of a leaving group generates an intermediate comparable to C in the above mechanism. 

SECTION 12.1. OXIDATION OF ALCOHOLS TO ALDEHYDES, KETONES, OR CARBOXYLIC ACIDS 

The development of the CrOs-pyridine complex and the DMSO-based systems has decreased the number of instances in which older oxidation techniques are used. One such method, the Oppenauer oxidation, is the reverse of the Meerwein-PondorfT-Verley reduction (Chapter 5). It involves heating the alcohol to be 

The high solubility of the MTO catalyst in almost any solvent opens up a broad spectrum of reaction media from vhich to choose when performing epoxidations. The most commonly used solvent, however, is still dichloromethane. From an environmental point of view this is certainly not the most appropriate solvent in large scale epoxidations. Interesting solvent effects for the MTO-catalyzed epoxida-tion were reported by Sheldon and coworkers, who performed the reaction in trifluoroethanol [86]. The change from dichoromethane to the fluorinated alcohol allowed for a further reduction of the catalyst loading down to 0.1 mol%, even for terminal alkene substrates. It should be pointed out that this protocol does require 60% aqueous hydrogen peroxide for efficient epoxidations. 

A very useful group of procedures for oxidation of alcohols to ketones have been developed which involve DMSO and any one of several electrophilic reagents, such as dicyclohexylcarbodiimide, acetic anhydride, trifluoroacetic anhydride, oxalyl chloride, or 

5= (CH3)2CHCH=CHCH=CHCH20H (CH3 2CHCH=CHCH=CHCH=o 

5e (CH3)2CHCH=CHCH=CHCH2OH c1( ()(.()ci - (CH3)2CHCH=CHCH=CHCH=0 93% 

Oxidations for oxidation of alcohols to ketones employs dimethyl sulfoxide (DMSO) and any 

Preparatively useful procedures based on acetic anhydride,25 trifluoroacetic anhydride,26 and oxalyl chloride27 have been developed. The last method, known as the Swern oxidation, is currently the most popular. 

Oxidation of Alcohols to Aldehydes, Ketones, or Carboxylic Acids 

Oxidation by the Dess-Martin Reagent. Another reagent that has become important for laboratory synthesis is known as the Dess-Martin reagent,28 which is a hypervalent iodine(V) compound.29 The reagent is used in inert solvents such as chloroform or acetonitrile and gives rapid oxidation of primary and secondary alcohols. The by-product, o-iodosobenzoic acid, can be extracted with base and recycled. 

Olefin UHP[bl SPC[cl BTSP[dl UHP[el ionic liquid H202[fl CF3CH2OH H202[s (CF3)2CHOH 

Begue and coworkers recently achieved an improvement in this method by performing the epoxidation reaction in hexafluoro-2-propanol [120]. They found that the activity of hydrogen peroxide was significantly increased in this fluorous alcohol, in relation to trifluoroethanol, which allowed for the use of 30% aqueous H202. Interestingly, the nature of the substrate and the choice of additive turned out to have important consequences for the lifetime of the catalyst. Cyclic dis-ubstituted olefins were efficiently epoxidized with 0.1 mol% of MTO and 10 mol% 

Oxidation with permanganate588 592 636 637 usually requires the use of a suitable solvent mixture, such as aqueous THF,638 which dissolves both permanganate and the alkene. Better results may be obtained by phase-transfer agents.440 Reaction with tetraalkylammonium permanganate may allow selective cleavage of a phenyl-substituted double bond in dienes.639 

One-electron reduction by hydrogen abstraction from the solvent leads to 82, which decomposes to diol monoanion 83. The latter is oxidatively cleaved to carbonyl compounds with the concomitant reduction of manganese. 

Oxidation with hexavalent chromium usually leads to complex product mixtures.641 However, certain Cr(VI) reagents [Cr02(00CCCl3)2,642 Cr03 supported on silica,643 bistriphenylsilyl chromate644], and a Cr(V) complex,645 may be used to form carbonyl compounds. 

Alkali periodates (in aqueous solution)646 and ammonium periodate (in anhydrous aprotic solvent),647 when applied in the presence of a catalytic amount of 0s04, are selective oxidizing agents used to form carbonyl compounds. Os04 apparently transforms the alkene to an 1,2-diol that is subsequently cleaved by the periodate to the carbonyl compound end products. The periodate also serves to regenerate Os04. 

According to the patent literature, Ru02 can be used as a stoichiometric oxidant in aldehyde formation.648 RuC13649 and a ruthenium-substituted heteropoly anion [SiRu(H20)Wn0395-]650 651 have been found to serve as a selective catalyst in this oxidation. H202649 and NaI04650 651 as oxidants, or electrochemical oxidation651 have been used. 

Barluenga reported that the intramolecular formal Friedel-Crafts acylation of arenes with aldehydes can be promoted by IPyBF4 and HBF4 

CH2CI2 (9mL)/MeS03H (1 rtiL) CH2CI2 (9 mL)/CF3C02H (1 rtiL) 

EWG = C02Me, C02Et, C02f-Bu, CN, allyl, prenyl, geranyl 

Selectfluor works as the oxidant in the Mannich-type reaction of a piperidine derivative (104) (Seheme 8.53). The piperidine derivative can be oxidized by Selectfluor to form an iminium ion intermediate that can be attacked by nucleophiles such as furan or iV-methylpyrrole to form the formal CDC products 105. 

10 mol% pyrazole, 30% H2O2. Additional 26% ofthe diol was formed. 1-dodecene was used as 

The use of non-volatile ionic liquids as environmentally benign solvents has received significant attention in recent years. Abu-Omar and coworkers developed an 

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Since scanning tunneling microscopy requires flat conducting surfaces, it is not surprising that most of its early application was to study inorganic materials [17, 19, 20, 29-34]. These studies include investigations of catalytic metal surfaces [24, 35-37], silicon and other oxides [21], superconductors [38], gold... 

Fluorine is known to form three other oxides, OjFj, O3F2 and O4F2 but all these decompose below 200 K. 

Acetoxybenzene is prepared by the reaction of benzene with Pd(OAc)2[325,342-345], This reaction is regarded as a potentially useful method for phenol production from benzene, if carried out with only a catalytic amount of Pd(OAc)2. Extensive studies have been carried out on this reaction in order to achieve a high catalytic turnover. In addition to oxygen and Cu(II) salts, other oxidants, such as HNOi, nitrate[346,347], potassium peroxodisulfate[348], and heteropoly acids[349,3S0], are used. HNO is said to... 

Several Pd(0) complexes are effective catalysts of a variety of reactions, and these catalytic reactions are particularly useful because they are catalytic without adding other oxidants and proceed with catalytic amounts of expensive Pd compounds. These reactions are treated in this chapter. Among many substrates used for the catalytic reactions, organic halides and allylic esters are two of the most widely used, and they undergo facile oxidative additions to Pd(0) to form complexes which have o-Pd—C bonds. These intermediate complexes undergo several different transformations. Regeneration of Pd(0) species in the final step makes the reaction catalytic. These reactions of organic halides except allylic halides are treated in Section 1 and the reactions of various allylic compounds are surveyed in Section 2. Catalytic reactions of dienes, alkynes. and alkenes are treated in other sections. These reactions offer unique methods for carbon-carbon bond formation, which are impossible by other means. 

Because of their use in the rubber industry various sulfenamido thiazoles (131) have been prepared. They are obtained in good yields through the oxidation of A-4-thiazoline-2-thiones (130) in aqueous alkaline solution in the presence of an amine or ammonia (Scheme 66) <123, 166, 255, 286, 308, 309). Other oxidizing agents have been proposed (54, 148. 310-313) such as iodine (152), chlorine, or hydrogen peroxide. Disulfides can also be used as starting materials (3141. 

The oxidation of 2- and 5-sulfides is usually performed in acetic acid and 30% hydrogen peroxide (213, 229, 263, 345-350) Or with m-chloroperbenzoic acid (341). Ary] (8, 272. 349, 351-353) and alkyl sulfones (129, 203, 214, 270, 274, 275) are thus obtained in good yields. Other oxidative reagents such as KMn04 (7, 273) or CrO (7) in acetic add have also been used. 

Thiazolecarboxaldehydes are very easily oxidized to carboxylic acids by most oxidizing agents, the most common being KMn04 in cold pyridine or boiling acetone. Thiazolecarboxylic acids are obtained in 50% yield (29). Other oxidizing agents such as Ag 0 in dioxane and water (29, 103), chromic acid, and so forth are also used. 

In view of the widespread use of nitrogen and argon in surface area and porosity studies, data for the construction of the standard a,-curves for these adsorbates on hydroxylated silica, are given in Table 2.14 (p. 93) for nitrogen and in Table 2.15 for argon. From the arguments of Section 2.12, these should be adequate for other oxides such as alumina, if high accuracy is not called for. 

Most metals will precipitate as the hydroxide in the presence of concentrated NaOH. Metals forming amphoteric hydroxides, however, remain soluble in concentrated NaOH due to the formation of higher-order hydroxo-complexes. For example, Zn and AP will not precipitate in concentrated NaOH due to the formation of Zn(OH)3 and Al(OH)4. The solubility of AP in concentrated NaOH is used to isolate aluminum from impure bauxite, an ore of AI2O3. The ore is powdered and placed in a solution of concentrated NaOH where the AI2O3 dissolves to form A1(0H)4T Other oxides that may be present in the ore, such as Fe203 and Si02, remain insoluble. After filtering, the filtrate is acidified to recover the aluminum as a precipitate of Al(OH)3. 

In this manner, a current efficiency of 100% is maintained. Furthermore, since the concentration of Ce + remains at its initial level, the potential of the working electrode remains constant as long as any Fe + is present. This prevents other oxidation reactions, such as that for liiO, from interfering with the analysis. A species, such as Ce +, which is used to maintain 100% current efficiency, is called a mediator. 

Activated carbons contain chemisorbed oxygen in varying amounts unless special cate is taken to eliminate it. Desired adsorption properties often depend upon the amount and type of chemisorbed oxygen species on the surface. Therefore, the adsorption properties of an activated carbon adsorbent depend on its prior temperature and oxygen-exposure history. In contrast, molecular sieve 2eohtes and other oxide adsorbents are not affected by oxidi2ing or reducing conditions. 

The lower molecular weight PCTFE oils, waxes, and greases are used as inert sealants and lubricants for equipment handling oxygen and other oxidative or corrosive media. Other uses include gyroscope flotation fluids and plasticizers for thermoplastics. 

Si02, AI2O2—Si02, and many other oxides. 

The heavy mineral sand concentrates are scmbbed to remove any surface coatings, dried, and separated into magnetic and nonmagnetic fractions (see Separation, magnetic). Each of these fractions is further spHt into conducting and nonconducting fractions in an electrostatic separator to yield individual concentrates of ilmenite, leucoxene, monazite, mtile, xenotime, and zircon. Commercially pure zircon sand typically contains 64% zirconium oxide, 34% siUcon oxide, 1.2% hafnium oxide, and 0.8% other oxides including aluminum, iron, titanium, yttrium, lanthanides, uranium, thorium, phosphoms, scandium, and calcium. 

Chain lengths of some oxidations can be quite long (>100), especially for substrates with easily abstractable hydrogens when they are oxidized under mild conditions at low conversions. Aldehydes are good examples of such substrates (26). Many other oxidations have chain lengths estimated from 3 to 10. At limiting rates, the chain length is near 1 (25). 

The quantitative conversion of thiosulfate to tetrathionate is unique with iodine. Other oxidant agents tend to carry the oxidation further to sulfate ion or to a mixture of tetrathionate and sulfate ions. Thiosulfate titration of iodine is best performed in neutral or slightly acidic solutions. If strongly acidic solutions must be titrated, air oxidation of the excess of iodide must be prevented by blanketing the solution with an inert gas, such as carbon dioxide or... 

Fluxes are usually added in the form of either limestone or dolomite. The fluxes provide the basic constituents (CaO and MgO) needed to balance the acid constituents (Si02 and AI2O2) from the coke and ore. These are the four primary oxides which form the slag, although minor amounts of other oxides such as MnO, Na20, K2O, P2 S Ti02, and sulfur are also present. Proper adjustment of the slag chemistry is necessary to obtain the desired... 

Lignosulfonate Uses. Large-volume uses iaclude productioa of vanillin (qv) and DMSO (76). Commercially, softwood spent sulfite Hquors or lignosulfonates can be oxidized ia alkaline media by oxygea or air to produce vanillin [121 -33-5]. Other oxidizing ageats, such as copper(Il) hydroxide, nitrobenzene, and ozone, can also be used. 

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