Phase transfer catalysed oxidation

Phase-transfer catalysed oxidation of sulphoxides to sulphones using copper(II) permanganate or a mixture of potassium permanganate and copper(II) sulphate is also possible156. In this case hexane is used as the solvent for the organic phase and the reaction is carried out under reflux for 24 hours. Sulphones are prepared by this method in quantitative yields and the mechanism proposed is given in equation (51). 

An early study of the attempted phase-transfer catalysed oxidation of dibenzyl sulphoxide to the sulphone using a variety of conditions and oxygen, periodate, hypochlorate and permanganate as oxidants showed that only potassium permanganate gave any of the required product, albeit in low yields. A further study, using Adogen 464 as... 

Although alcohols are oxidized by tetra-n-butylammonium persulphate when the reaction is conducted in dichloromethane, tetrahydropyranyl ethers have been produced (>90%) when attempts to oxidize the alcohol are conducted in tetrahydro-pyran (see Chapter 10) [ 19], Tetrahydrofuranyl ethers have been prepared by an analogous method [20,21 ]. Base-mediated elimination of halo acids from P-halo alcohols under phase-transfer catalysed conditions produce oxiranes in high yield (70-85%). The reaction has particular use in the synthesis of epihalohydrins from p,y-dihalo alcohols [22],... 

Although there are other convenient procedures for the conversion of sulphides into sulphoxides and sulphones, the phase-transfer catalysed reaction using Oxone has the advantage that the oxidation can be conducted in the presence of other readily oxidized groups, such as amines, alkenes, and hydroxyl groups, and acid-labile groups, such as esters and carbamates [6, 7], Hydrolysis of very acid-labile groups, such as ketals, can result in production of the keto sulphone. 

The phase-transfer catalysed oxidative iodination of a,p-unsaturated carbonyl compounds (Table 10.23) using iodine/persulphate to produce the a-iodo derivatives has been described [16]. Activated methylene groups are also iodinated under similar conditions. 

Phase-transfer catalysed oxidation of ketones with dioxygen under basic conditions in the presence of triethyl phosphite and a cinchonium salt produces a-hydroxy-ketones (Schemes 12.14 and 12.15, Table 12.9) in good overall yield (-95%) and with a high enantiomeric excess [>70% ee using N-(4-trifluoromethyIbenzyl)cincho-nium bromide] [29], Lower asymmetric induction is observed with ephedrinium salts, polymer-supported salts and, surprisingly, by cinchonidinium salts. 

An exciting addition to the armoury of asymmetric phase transfer catalysed reactions has been the oxidative cyclisation of 1,5-dienes (Scheme 13) [21]. This tandem reaction process leads to the formation of tetrahydrofurans such as 35 in a single step from the open chain dienes 34. The step which determines the sense of asymmetry is the initial attack of permanganate anion, and this chiral information is efficiently relayed in the cyclisation to give products with three new stereogenic centres. For example, oxidation of the di-enone 34 with potassium permanganate, catalysed by the salt 36, gave the tetrahydrofuran 35 in 72% ee. 

An Sjuyl-type (S l ) mechanism has been proposed in the synthesis of poly(2,6-dimethyl-l,4-phenylene ether) through the anion-radical polymerization of 4-bromo-2,6-dimethylphenoxide ions (204) under phase-transfer catalysed conditions269. Ions 204 are oxidized to give an oxygen radical 205. The propagation consists of the radical nucleophilic substitution by 205 at the ipso position of the bromine in 204 (equation 144). The anion-radical 206 thus formed eliminates a bromide ion to form a dimer phenoxy radical 207 (equation 145). A polymeric phenoxy radical results by continuation of this radical nucleophilic substitution. 

Diazines. - Like phenylglyoxal, 2-furylglyoxal condenses with amino-acetamide to give a 5-arylpyrazin-2-one (127) (28%) chlorination of this material, followed by phase-transfer-catalysed oxidation of the furyl group, affords the carboxylic acid (128) (Scheme 66). ... 

Ketones are reduced to alcohols in the phase-transfer catalysed H-transfer reduction with isopropanol, or better PhCH2CH20H, in the presence of Fe3(C0)i2 Fe(C0)5 is far less active. Mono-, di- and trinuclear iron hydride carbonyl anions are generated in situ. a-Trimethylsilylketones can be prepared via Rh catalysed oxidations with butenones (eqn.9). Azobenzene is isomerised and reduced to o-phenylenediamines by a RuCl3/PPh3/C0/Li0Ac system in secondary alcohols. By contrast, n-butanol leads to formation of benzimidazoles (Scheme 3). 

Ylides of Sulphur, Selenium, Tellurium, and Related Structures 341 halide ion present and the NaOH concentration." Under phase-transfer-catalysed conditions, involving methylene chloride and water solvents, sodium hydroxide as the base, and tetrabutylammonium iodide as a transfer agent, trimethylsulphonium iodide and benzaldehyde afforded a 90% yield of styrene oxide." An equally good yield was obtained with cinnamal-dehyde, but low yields were obtained with acetophenone and benzo-phenone. 

As indicated above, the traditional base-catalysed hydrolysis of 0,5-dialkyl thio-carbonates for the synthesis of thiols is generally unsatisfactory, as oxidation leads to the formation of disulphides. Under phase-transfer conditions, the procedure produces thioethers to the virtual exclusion of the thiols, as a result of the slow release of the thiolate anions in the presence of the electrophilic ester. However, a simple modification of the reaction conditions provides an efficient one-pot reaction [50] from haloalkanes (Table 4.15) via the intermediate formation of the thermally labile (9-/ert-butyl-5-alkyl dithiocarbonates (Scheme 4.8). 

Table 3.5 Oxidation of alcohols to the corresponding carbonyl compounds with hydrogen peroxide catalysed by Mo(VI) or W(VI) complexes under phase-transfer conditions at 70°C ...

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