Solubilization of potassium permanganate

Using dicyclohexyl-18-crown-6 it is possible to dissolve potassium hydroxide in benzene at a concentration which exceeds 0.15 mol dm-3 (Pedersen, 1967). The free OH- has been shown to be an excellent reagent for ester hydrolysis under such conditions. The related solubilization of potassium permanganate in benzene, to yield purple benzene , enables oxidations to be performed in this solvent (Hiraoka, 1982). Thus, it is possible to oxidize a range of alkenes, alcohols, aldehydes, and alkylbenzenes under mild conditions using this solubilized reagent. For example, purple benzene will oxidize many alkenes or alcohols virtually instantaneously at room temperature to yield the corresponding carboxylic acids in near-quantitative yields (Sam Simmons, 1972). 

The now classic example of this type, excluding nucleophilic substitution reactions, is the solubilization of potassium permanganate in benzene by [18]crown-6 derivatives. The resulting solution proved a mild and efficient oxidizing agent for many organic compounds. For example, a-pinene is oxidized to cw-pinonic acid in 90% yield (Scheme 8) in contrast to a 40-60% yield in aqueous solution. An additional advantage is that the products of these reactions (e.g. potassium salt of cw-pinonic acid in Scheme 8) are frequently insoluble in the hydrocarbon solvents and consequently are unable to react further. 

The physical properties of many macrocyclic polyethers and their salt complexes have been already described. - Dibenzo-18-crown-6 polyether is useful for the preparation of sharpmelting salt complexes. Dicyclohexyl-18-crown-6 polyether has the convenient property of solubilizing sodium and potassium salts in aprotic solvents, as exemplified by the formation of a toluene solution of the potassium hydroxide complex (Note 13). Crystals of potassium permanganate, potassium Lbutoxide, and potassium palladium(II) tetrachloride (PdClj + KCl) can be made to dissolve in liquid aromatic hydrocarbons merely by adding dicyclohexyl-18-crown-6 polyether. The solubilizing power of the saturated macrocyclic polyethers permits ionic reactions to occur in aprotic media. It is expected that this [)ropcrty will find practical use in catalysis, enhancement of... 

Potassium permanganate can be readily solubilized in nonpolar media by complex-ing the potassium ion with a crown ether. Sam and Simmons [7] found that dicyclo-hexyl-18-crown-6 could solubilize solid potassium permanganate in benzene to the extent of about 0.06 molar. The resulting purple benzene solution was used to oxidize a number of organic substrates in good to excellent yield. Some of their results are recorded in Table 11.3. Similar results have recently been reported by other workers [8, 9]. 

Apart from analytical applications, reports of the use of crowns in synthetic organic chemistry have been quite common. Typically, the solubilization of an inorganic reagent (such as potassium permanganate) or the production of a free counter ion (such as the fluoride ion) in an organic solvent such as benzene has formed the basis for many of these reports. 

Extension of the scope of the methodology to a full stereocontrol in the addition sequence A in order to obtain either, Z-anti- or l,2-ry -diol units was hampered by the inherent //-selectivity of the addition of 2-TST to a,/3-dialkoxy aldehydes. Control of the selectivity by the use of Lewis acids acting as chelating agents was also foiled by substantial decomposition of 2-TST. This limitation was overcome by conversion of the 7 //-adduct into the syn-isomer via an oxidation-reduction sequence <1989JOC702>. The secondary (Rj-alcohol 457 was oxidized to ketone 461 with potassium permanganate partly solubilized with TDA-1 (tris[2-(2-methoxyethoxy)ethyl]amine). It was proved that under these neutral nonaqueous oxidation conditions no appreciable racemization occurred via exchange... 

Supported reagent. One limitation to potassium permanganate is the limited solubility in organic solvents. Solubilization with a crown ether is one solution (4,143) another is impregnation on an inorganic solid support such as a molecular sieve, silica gel, and certain clays. Although the factors responsible for the enhanced reactivity of this form of KMn04 are not understood, such materials are useful for oxidation. ... 

The use of crown polyethers to solubilize potassium permanganate in benzene has been reported. In a study of the oxidation of alkyl toluene-/ -sulphonates with DMSO-sodium bicarbonate, the elements of carbon dioxide are retained in the product (86) as a cyclic carbonate (Scheme 170), inferring that, for this substrate, bicarbonate ion is a more effective nucleophile than DMSO this leads to a revision of the mechanism of such oxidations to that shown in Scheme 170. 

The macrocyclic polyethers were prepared by Pedersen a decade ago and shown to complex a variety of cationic substrates [39]. Among these substrates are alkali metal cations, alkaline earth cations and ammonium ions [40]. It has been shown that hindered esters could be saponified by KOH in toluene solution in the presence of dicyclo-hexyl-18-crown-6 [41] or cryptate [42]. Potassium hydroxide is both solubilized in toluene and activated by the crown ether [43]. Sam and Simmons, however, first clearly demonstrated the potential of crowns as phase transfer reagents by solubilizing potassium permanganate in benzene solution (see Eq. 1.15) and using the solubilized reagent in a variety of oxidation processes [44]. 

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