Heterocycles, phenyl, nitration

The nitration of phenylpyridines and related compounds has attracted attention for a long time, and measurements of isomer proportions have been made for several compounds of this type. Nitration occurs in the phenyl ring. For 2-phenylpyridine and 2-phenylpyridine i-oxide measurements of the dependence of rate of nitration upon acidity in 75-81 % sulphuric acid at 25 °C show that both compounds are nitrated as their cations (table 8.1). The isomer distribution did not depend significantly upon the acidity, and by comparison with the kinetic data for quinolinium ( 10.4.2) the partial rate factors illustrated below were obtained.They should be compared with those for the nitration of 2-nitrobiphenyl ( 10.1). The protonated heterocyclic groups are much... 

The features of the electronic structure of aryl-substituted pyrazolines influence their chemical properties. For example, in the case of 3-substituted 7V-phenyl-pyrazolines 100 reactions of formylation, acylation, nitration, sulfonation, azocoupling and other electrophilic processes involve the para position of the 7V-phenyl ring, with formation of compounds 101 [103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113]. On the other hand, some electrophilic reactions, including nitration, bromination, chlorination, formylation and azocoupling, for 3-unsubstituted pyrazolines 102 occur at position 3, yielding heterocycles 103 and in some cases as a mixture with 104 [108, 114, 115] (Scheme 2.26). This fact provides evidence for orbital control of these reactions. 

This review was originally intended to cover electrophilic substitution in phenyl-substituted heterocycles in general. However, it was decided to concentrate on nitration since this provides the substantial majority of studies in this area. It is the only reaction that seems to have been systematically investigated to any extent and is the only reaction for which there are data for a wide range of phenyl-substituted heterocycles. The literature has been searched to 1991. However, few references have been found in the last 5 years. 

Schofield (80MI1) has provided a comprehensive review of aromatic nitration but with slight emphasis on phenyl heterocycles. There have also been extensive reviews of the methods and mechanisms of nitration... 

Although nitration has been studied for many years, little systematic work seems to have been carried out in the area of phenyl-substituted heterocycles. There has been a report, Nitration of Phenyl Substituents of Heterocyclic Nuclei (30JCS397) but this only describes afew examples... 

Sec. II.B] NITRATION OF PHENYL-SUBSTITUTED HETEROCYCLES 239 5. Oxazoles and Isoxazoles... 

The nitration of 5-phenyl-1,3,4-oxathiazolin-2-one (47) has been reported to give a 50% isolated yield of the 4-nitrophenyl derivative. The residue was analyzed after pyrolysis and found to contain 62% ortho and 38% meta and para nitro products. The nitration was thus indicated to give >54% para, <18% meta, and 28% ortho products (69TL5131 73ACS2161). The heterocyclic ring was deactivating and ortho para directing. 

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