Arenesulfonic acids

The iso)tazole ring is rather resistant to sulfonation. However, on prolonged heating with chlorosulfonic acid, 5-methyl-, 3-methyl- and 3,5-diraethyl-isoxazoles are converted into a mixture of the sulfonic acid and the corresponding sulfonyl chloride. The sulfonic acid group enters the 4-position even when other positions are available for substitution. The sulfonation of the parent isoxazole occurs only under more drastic conditions (20% oleum) than that of alkyl isoxazoles isoxazole-4-sulfonic acid is obtained in 17% yield. In the case of 5-phenylisoxazole (64), only the phenyl nucleus is sulfonated to yield a mixture of m-and p-arenesulfonic acid chlorides (65) and (66) in a 2 1 ratio (63AHC(2)365). 

Tetrabutylammonium iodide in trifluoroacetic anhydride is an effective reducing reagent [dS] This system can be used for direct reduction of arenesulfonic acids to the corresponding thiols or disulfides m moderate yields under mild conditions (equation 18) Alkanesulfonic acids are reduced by this system to disulfides with 30-57% yields [dfi]... 

Of these the most important are the sulfonic acids. In general, however, sulfonic acids are not prepared by oxidation of thiols. Arenesulfonic acids (ArS03H), for example, are prepared by sulfonation of arenes (Section 12.4). 

An important role must be attributed to intermediate mixed anhydrides of sulfonic acids and mineral acids sulfonic acid anhydrides are reported to need Friedel-Crafts conditions to generate sulfones327,476. Instead of arenesulfonic acids, their methyl esters may undergo insertion of sulfur trioxide477,478 yielding mixed anhydrides, which in turn furnish... 

Esters of arenesulfonic acids are prepared by reaction of the corresponding alcohol with the arenesulfonyl chloride in the presence of a basic reagent, which has the function of activating the alcohol and binding the hydrogen chloride. The reaction is often carried out with pyridine, which is used as solvent. In the procedure for tosylates of primary aliphatic alcohols, described in A.l. Vogel, A Textbook of Practical Organic Chemistry, an aqueous solution of sodium hydroxide is used. Esters of primary alcohols are formed more easily than secondary-alkyl esters, while tertiary alcohols cannot be esterified under the usual conditions. 

Reduction of arenesulfonic acids to arenethiols.1 The reagent reduces arenesulfononic acids to thiols in high yield. In actual practice, iodine can be used in catalytic amounts. The usual stoichiometry employed is ArS03H/I2/ P(C H,)3 = 2 1 10. 

Protection of aldehydes and ketones.1 Bis-o-nitrobenzyl acetals or ketals are removable in 85-95% yield on irradiation at 350 nm in benzene. The acetals or ketals are easily prepared from 1 by an exchange reaction using 2,2-dimethoxy-propane (1, 268-269) catalyzed by an arenesulfonic acid. In the case of hindered ketones (17-keto steroids), the glycol o-N02C6H4CH(OH)CH2OH (2) can be used. 

Esterification of sulfonic acids.1 Sulfonic acids are converted into methyl or ethyl esters on reaction with trimethyl or triethyl orthoformate. Yields are >80% in esterification of arenesulfonic acids, but are somewhat lower in the reaction of alkanesulfonic acids because of volatility of the products. 

Sulfur compounds, e.g. arenesulfonic acids, used in conjunction with the unsaturated compounds mentioned above can provide level, bright films. After several hours of operation of the bath, the odour of the aromatic hydrocarbon can be detected and it is well known that the bright deposits contain sulfur. Thus reduction of organic species at the developing nickel surface is a general phenomenon. 

Friedel-Crafts-type intermolecular sulfonylation of aromatics can also be conveniently carried out over Nafion-H by reacting aromatics with arenesulfonic acids at reflux temperature with azeotropic water removal465 [Eq. (5.168)]. Methanesulfonic acid also reacts with para-xylene to yield 2-methanesulfonyl-1,4-dimethylbenzene in much lower yield (30%). 

Sulfur trioxide, SO3 Reacts with arenes in sulfuric acid solution to yield arenesulfonic acids (Section 16.2). 

Roberts and co-workers prepared neopentyl (2,2-dimethylpropyl) esters of arenesulfonic acids which are compatible with a wide range of standard organic synthesis methodologies. These esters withstand several reagents such as terf-butyllithium, vinylmagnesium bromide, CrOs, NBS (AT-bromosuccinimide)-benzoyl peroxide, H2-Raney Ni, DIBAL-H, Nal,... 

FI. Kimoto, K. Saigo, Y. Ohashi, M. Flasegawa, Molecular recognition in the formation of conglomerate crystal. 2. The role of arenesulfonic acid in the conglomerate crystals of amino acid salts. Bull. Chem. Soc. Jap. 62 (1989) 2189-2195. 

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