Sulfites aromatic nucleophilic substitution

If the double salt 158 is heated with sodium sulfite (Na2S03) in water, aromatic nucleophilic substitution 160 leads to the sodium salt of pyridine-4-sulfonic acid 162 or to the sulfonic acid itself 163 after acidification. These are nucleophilic substitutions following electrophilic attack at nitrogen. 

The reaction of benzenesulfonic acid with sodium hydroxide (first entry in Table 24.3) proceeds by the addition-elimination mechanism of nucleophilic aromatic substitution (Section 23.6). Hydroxide replaces sulfite ion (S03 ) at the carbon atom that bear s the leaving group. Thus, p-toluenesulfonic acid is converted exclusively to p-cresol by an analogous reaction ... 

Sulfite is an extremely good nucleophile for activated aromatic systems and reaction with l-substituted-2,4,6-trinitrobenzenes (1) may result in cr-adduct formation or in displacement of the 1-substiment as shown in Scheme 1. When X = OEt or SEt, adducts (2) and (3) formed by reaction at unsubstituted positions are long-lived. 

Among nucleophiles used are enols " (including mono- and bis-TMS ethers, which are particularly favorable ) derivatives of P N and S in hydrogen sulfite dithionate and sulfide Si, Ge, Sn and alkyl or aryl in derivatives of Cd, Zn or Li . The Li reagents in particular require special experimental conditions, including dichloro methane as solvent and radical scavengers. Many nucleophiles cause electron addition to the cations, with formation of radicals. Aromatic substitutions displace not only H but SnMej or SiMe3 . Allylsilanes and trialkyl alkynylborates provide synthetically useful products. 

Nitropyridine itself, and some of its substituted derivatives, can now be prepared efficiently by reaction with dinitrogen pentoxide as shown below. The initially formed iV-nitropyridinium nitrate suffers addition of a nucleophile - sulfur dioxide when this is used as solvent or co-solvent, or sulfite, added subsequently - forming a 1,2-dihydropyridine. Transfer of the nitro group to a 3- or 5-position, via a [1,5]-sigmatropic migration, is then followed by elimination of the nucleophile regenerating the aromatic system. ... 

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