Heteroatoms, nitric acid

These compounds are less common than indole (benzo[ ]pyrrole). In the case of benzo[i>]furan the aromaticity of the heterocycle is weaker than in indole, and this ring is easily cleaved by reduction or oxidation. Electrophilic reagents tend to react with benzo[Z ]furan at C-2 in preference to C-3 (Scheme 7.21), reflecting the reduced ability of the heteroatom to stabilize the intermediate for 3-substitution. Attack in the heterocycle is often accompanied by substitution in the benzenoid ring. Nitration with nitric acid in acetic acid gives mainly 2-nitrobenzo[Z ]furan, plus the 4-, 6- and 7-isomers. When the reagent is in benzene maintained at 10 °C, both 3- and 2-nitro[ ]furans are formed in the ratio 4 1. Under Vilsmeier reaction conditions (see Section 6.1.2), benzo[Z ]furan gives 2-formylbenzo[6]furan in ca. 40% yield. 

Nitration of Heteroatoms. Secondary amines and primary and secondary amides can be converted to A -nitro compounds by direct nitration with nitric acid. 46 Although most primary amines cannot be nitrated directly, it is possible to obtain primary nitramines by nitration of the corresponding dichloroamines (eq 9). Treatment of pyrrolidone with nitric acid and acetic acid in the presence of copper provides lV-nitroso-2-pyrrolidone in 70% yield note that this transformation represents A -nitrosation. 

Oxidation of Heteroatoms. The nitroso group is oxidized efficiently to the nitro group by nitric acid. For example, 2,4-dinitrosoresorcinol is converted efficiently to 2,4,6-trinitroresorcinol with concentrated nitric acid (eq 19). ... 

Oxidation of Phosphorus. In addition to the previously mentioned heteroatom oxidations, the oxidation of trialkyl phosphines to the corresponding trialkylphosphine oxides has been accomplished with nitric acid (eq 25). ... 

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