Electrophilic sulfonation

Figure 16.5 The mechanism of electrophilic sulfonation of an aromatic ring. An electrostatic potential map of the reactive electrophile H0S02 shows that sulfur and hydrogen are the most positive atoms (blue).

Direct electrophilic sulfonation of imidazole and 2-substituted imidazoles has been traditionally carried out using oleum at around 160°C. Yields of the 4-sulfonic acid can be above 80%, but if a three-fold excess of sulfur trioxidc in refluxing 1,2-dichloroethane is used, the acids arc obtained under much milder conditions. Care has to be taken, for if only a 1 1 or 1 2 ratio of S03 imidazole is used, a charge transfer complex is formed instead [6]. 

In Scheme 57, the nucleophilic enamine (135) adds to the strained cyclic vinyl sulfone (136) to give the bicydic sulfone (137) a similar [2+2] cydoaddition occurs between (138) and the ynamine (139) to form the cyclic phenyl sulfone (140). The regiospecific reactions probably involve stepwise attack of the nucleophilic enamine (or ynamine) on the electrophilic sulfone. The four-membered ring products (137) and (140) are obtained in good yields and can be readily hydrolysed by dilute acid to form the corresponding cyclobutanones. 

Because sulfonation is reversible, a sulfonic acid group is sometimes introduced to occupy a position on the ring, temporarily protecting that position from attack by another electrophile. Sulfonation is also a useful way of directing further substitution to a specific position. 

Electrophilic sulfonation of isoxazole is of no preparative value the substitution of only the phenyl-substituent of 5-phenylisoxazole with chlorosulfonic acid makes the same point. Both isothiazole and pyrazole can be satisfactorily sulfonated at C-4. 

The S-PEEK polymers can be synthesized by means of a direct electrophilic sulfonation via concentrated sulphuric acid or via chlorosulphuric acid. 

Besides electrophilic sulfonation, sulfonated phosphines can be produced by nucleophilic substitution reactions using PH3, primary or secondary phosphines, and fluoroarenes in superbasic media. This method has been applied for the synthesis of pura-isomer of TPPTS (p-TPPTS) (Scheme 10) as well as dozens of other interesting ligands including chelating, chiral, and amphiphilic phosphines (a selection is in Scheme 11). ... 

In Summary Nitration of benzene requires the generation of the nitronium ion, N02, which functions as the active electrophile. The nitronium ion is formed by the loss of water from protonated nitric acid. Sulfonation is achieved with fuming sulfuric acid, in which sulfur trioxide, SO3, is the electrophile. Sulfonation is reversed by hot aqueous acid. Benzenesulfonic acids are used in the preparation of detergents, dyes, compounds containing leaving groups, and sulfa drugs. 

Sidfonated hydrocarbon polymers are widely used as a host matrix to prepare organic-inorganic nanocomposite for high-temperature PEM applications due to their excellent stability and high susceptibility to electrophilic sulfonation reactions for acid fimctionalization. Sulfonation of thermally resistant polymers such as polysulfones (PSF), fluoropolymers, and polyetheretherketone (PEEK) was widely studied. The sulfonation of certain polymers such as polystyrenes... 

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