Nitration of aromatic rings

Nitration of aromatic rings is possible by use of Pd(N03)2[356], Pd(OAc)2-NaN02[357], Pd(0Ac)2-N02[358], and Pd(0)-NO2[359], The nitration can be carried out fully catalytically by Pd(0Ac)2-N02 and oxygen. This reaction offers a promising new method of nitration without using mixed acids of HNO3 and H2SO4. 

Nitration of aromatic rings by nitronium tetrafluoroborate is a general method. Fifty-seven arenes, haloarenes, nitroarenes,... 

Nitromum tetrafluoroborate, from nitric acid, boron tnfluonde, and hy drogen fluoride, 47, 56 m nitration of aromatic rings, 47,... 

Notable examples of general synthetic procedures in Volume 47 include the synthesis of aromatic aldehydes (from dichloro-methyl methyl ether), aliphatic aldehydes (from alkyl halides and trimethylamine oxide and by oxidation of alcohols using dimethyl sulfoxide, dicyclohexylcarbodiimide, and pyridinum trifluoro-acetate the latter method is particularly useful since the conditions are so mild), carbethoxycycloalkanones (from sodium hydride, diethyl carbonate, and the cycloalkanone), m-dialkylbenzenes (from the />-isomer by isomerization with hydrogen fluoride and boron trifluoride), and the deamination of amines (by conversion to the nitrosoamide and thermolysis to the ester). Other general methods are represented by the synthesis of 1 J-difluoroolefins (from sodium chlorodifluoroacetate, triphenyl phosphine, and an aldehyde or ketone), the nitration of aromatic rings (with ni-tronium tetrafluoroborate), the reductive methylation of aromatic nitro compounds (with formaldehyde and hydrogen), the synthesis of dialkyl ketones (from carboxylic acids and iron powder), and the preparation of 1-substituted cyclopropanols (from the condensation of a 1,3-dichloro-2-propanol derivative and ethyl-... 

Near the top of the list of reactivity towards hydrogenation lies the N02 group and in Chapter 22 we saw how the sequence of nitration of aromatic rings followed by reduction was a useful route to aromatic amines. The reduction can be carried out by Sn/HCl but catalytic hydrogenation is much simpler. The reaction is usually done in ethanol with a Pd or Pt catalyst and it may be necessary to add a weak acid to prevent the amine produced from poisoning the catalyst. 

Tlie diagram suggested by Fraudenberg (Vol. II, p. 435) with the nitration of aromatic ring has generally speaking been confirmed. 

An attractive idea for nitrate ions (NOs") is to convert them (with the aid of sulfuric acid) into nitronium ions (N02 ), which are the electrophilic agents in the nitration of aromatic rings. To facilitate the electrophilic attack, an activated aromatic ring, containing electron-donating substituents, should be employed. 

Nitration of aromatic rings is an important reaction because the nitro group can readily be reduced to an amino group, a common functional group required in many pharmaceutical compounds. Other substituents can subsequently replace the amino group (Section 13.8). 

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