Nonaqueous oxidizing reagents

Hajipour and coworkers prepared benzyltriphenylphosphonium peroxymonosulfate (BnPhsPHSOs) in a very high yield (95%) and purity (99%). This new oxidizing reagent was applied successfully in various deprotection reactions such as the conversion of oximes, phenylhydrazones, 2,4-dinitrophenylhydrazones and semicarbazones to the corresponding carbonyl compounds in the presence of bismuth chloride under nonaqueous conditions . Oxidative deprotection of trimethylsilyl ethers, tetrahydropyranyl ethers and ethylene acetals with BnPh3PHS05 under microwave irradiation affords the corresponding carbonyl compounds in very high yields (equation 71). The same reaction also proceeds under nonaqueous conditions ". 

Aldehydes are more easily oxidized than alcohols which is why special reagents such as PCC and PDC (Section 15 10) have been developed for oxidizing primary alco hols to aldehydes and no further PCC and PDC are effective because they are sources of Cr(VI) but are used m nonaqueous media (dichloromethane) By keeping water out of the reaction mixture the aldehyde is not converted to its hydrate which is the nec essary intermediate that leads to the carboxylic acid... 

Another important example of a redox titration for inorganic analytes, which is important in industrial labs, is the determination of water in nonaqueous solvents. The titrant for this analysis is known as the Karl Fischer reagent and consists of a mixture of iodine, sulfur dioxide, pyridine, and methanol. The concentration of pyridine is sufficiently large so that b and SO2 are complexed with the pyridine (py) as py b and py SO2. When added to a sample containing water, b is reduced to U, and SO2 is oxidized to SO3. 

A recently discovered (2) oxidizing system promises to become very important for the oxidation of acid-sensitive compounds. The reagent is chromium trioxide-pyridine complex, which may be isolated after preparation and employed in nonaqueous solvents (usually methylene chloride). A remarkable feature of the reagent is that good yields of aldehydes are obtained by direct oxidation of primary alcohols. The preparation of the reagent and its use are given. 

Some of the common factors that control the accuracy of these direct methods are incomplete removal of water, thermal decomposition, volatility of nonaqueous components, and certain side reactions, such as oxidation and nonspecificity of the Fischer reagent. 

Scheme 1. Preparation of oxo complexes. Reagents (i) NH3, (ii) RNH2 (R = tertiary alkyl group), (iii) RN=PPh3, (iv) RNH—SiMe3, (v) NR3, (vi) reductant, nonaqueous solvent (vii) excess L, (viii) excess HX, (ix) OH, (x) nonreducing acid, (xi) reductant, (xii) excess HX or X-, (xiii) L, (xiv) hv plus reductant or electrochemical reduction, (xv) R C=CR5, (xvi) L = py, etc., (xvii) oxidation of Os(II), Os(III), or Os(IV) complexes, and (xviii) A...

Transition metal oxidants such as permanganate, ruthenium tetroxide and diromium(VI) oxide are convenient and efficient reagents for routine cleavage reactions. The use of phase transfer catalysts (quaternary ammonium and phosphonium ions, primarily) has made it possible to solubilize transition metal oxides such as permanganate and chromatt in nonaqueous solvents, and to therdry increase the scope of these reactions substantially. ... 

Oxidations with dichromates can be accomplished in nonaqueous media or in a two-phase system by using phase-transfer reagents. The addition of 1-phenylethanol to a slurry of potassium dichromate and Adogen 464 (methyltrialkylammonium chloride) in benzene heated at 55 °C for 15 h results in an 80% yield of acetophenone. However, 2-octanol gives only a 33% yield of the ketone after being heated at 55 °C for 24 h [651]. Other solvents suitable for the reaction are dichloromethane, chloroform, and carbon tetrachloride, but not hexane [651], Better results are obtained when the alcohol is shaken for 1 min at room temperature with a mixture of a catalytic amount (10%) of tetrabutylammonium bisulfate and sodium dichromate in dichloromethane and aqueous 3 M sulfuric acid [630. ... 

An oxidizing agent prepared from CrOj in water containing NH Cl at 40° followed by addition of alumina (cf. reagent on silica gel ) retains its power when kept in the air for several months. Deoximation under nonaqueous conditions is accomplished with dimethylammonium chlorochromate adsorbed on alumina. ... 

Corrigan it was not exploited synthetically until Corey prepared the reagent by addition of pyridine to neutral chromium trioxide solutions and used it for the oxidation of alcohols to aldehydes, ketones, and acids.xhe reagent is not acidic and the neutral conditions required for the oxidation are superior for the oxidation of allylic alcohols. In dichloromethane (nonaqueous workup) the oxidation is similar to that of PCC. Addition of catalytic amounts of pyridinium trifluoroacetate in dichloromethane significantly increases the rate of oxidation. Allylic alcohols are oxidized faster than aliphatic alcohols, making PDC the reagent of choice for this transformation. Cyclohexenol, for example, is oxidized 10 times faster than cyclohexanol with... 

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