Nitrosoaromatics

So far as the rearrangement is concerned, the scheme consistent with all the known facts, for the reactions of N-nitrosoaromatic amines in acid solution is set out below. It incorporates the rearrangement and the de-nitrosation, viz. 

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. 

When oxoketene acetals (12) are heated at 200°C in a scaled tube with nitrosoaromatics in acetic anhydride, imidazoles (13) are formed in moderate to good yields (Scheme 3.2.5) [28, 29]. Best results use excess of the nitrosoaromatic. Presumably the reaction is initiated by attack of the nitroso... 

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). 

Arylthioimidazoles can be made in high yields when thiophenates react with At-(l-cyanoalkyl) alkylidene A -oxides. Alkyl and aralkyl thiolates, however, react much less readily (see Section 2.1.1 and Scheme 2.1.10). Imidazole-4-thioethers can be made in a general reaction between nitriles and a range of isocyanides which are susceptible to o -metallation (see Section 4.2, Scheme 4.2.2 and Table 4.2.1). Oxoketene acetals bearing alkylthio substituents react with nitrosoaromatics to give 5-acyl-4-alkylthio-l-arylimidazoles in moderate to good yields (see Section 3.2 and Scheme 3.2.5). 

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>. 

Representatives of the 1,3,2-oxathiazine family are not expected to show aromatic or antiaromatic properties because of their structure. Only a few 2-aryl-4/7-3,l,2-benzoxathiazines, which are products of the thermal reaction between benzothiete and nitrosoaromatics, are known.43... 

P. Politzer and R. Bar-Adon,/. Phys. Chem., 91,3069(1987). Computational Analysis of the Reactive Properties of Some Nitrosoaromatic Molecules. 

Almost the same time, they showed that indoles could also be formed from [Cp Ru(CO)2]2 Catalyzed reductive annulation of nitrosoaromatics (ArNO) with alkynes (Eq. (7.20)) [26]. Although only moderate yields were observed for both methods, the transformations were conducted under neutral reaction conditions and exhibited high regioselectivity. 

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. 

An interesting process that leads to the formation of the indoles 226 is the reaction of nitrosoaromatics with alkynes catalyzed by (rj -C5Me5)Ru(CO)2 227 (Scheme 103) [141]. 

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