Nitrosoaromatic compounds

There is also some debate over whether nitrations like the one in the last example may alternatively proceed via a nitrosyl cation (NO ) instead of a HN03 molecule as the electrophile. Small amounts of the nitrosyl cation occur in diluted nitric acid and would—via a Wheland complex intermediate—initially lead to a nitrosoaromatic compound (Ar-N=0) as the Ar-SE product. (Remember Figure 5.23 presented a different reaction mode between nitrosyl cations and—less electron-rich—aromatic compounds.) This nitrosoaromatic compound would subsequently undergo rapid oxidation by the diluted nitric acid to finally yield the nitroaromatic compound. 

Heating oxoketene acetals with nitrosoaromatic compounds in acetic anhydride at 200 °C gives imidazoles. With nitrosyl chloride in pyridine or acetonitrile the A(-unsubstituted imidazole can be made (Equation (64)) <84CC430,85IJC(B)463>. 

Yoshioka, T. and T. Uematsu. 1993. Formation of N-hydroxy-N-arylacetamides from nitrosoaromatic compounds by the mammalian pyruvate dehydrogenase complex. Biochem. J. 290 783-790. 

Acylainino-4-acylimidazoles have been made from 3-amino-l,2,4-oxadiazoles and 1,3-dicarbonyl reagents (see Section 2.2.1 and Scheme 2.2.5). 4(5)-Acylimidazoles can be derived from 4-acylaininoisoxazoles (see Section 6.1.2 and Scheme 6.1.3). (See also the discussion in Section 2.2.1 on 4-acylimidazole synthesis.) 5-Acyl-l-arylimidazoles can be made from or-oxoketene-SJV-acetals and nitrosoaromatics (see Section 3.2 and Scheme 3.2.5), and 4-acyl-imidazoles by nitration of 1,3-dicarbonyl compounds in their enolic forms, reduction to iV-alkenylformamides and subsequent cyclization (see Section 3.2 and Scheme 3.2.4). Examples have also been isolated from reactions of 2-oximino-l,2,3-tricarbonyls and amines (see Section 4.1 and Scheme 4.1.7), from compounds such as 3-chloro-4,4-dimethoxy-2-butanone and 3,4-disubstituted 3-buten-2-ones (see Section 4.3 and Scheme 4.3.5), and by ultraviolet irradiation of 1-alkenyltetrazoles which bear an acyl group conjugated with the exocyclic double bond (see Section 6.1.2.3). 

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