Heteroaromatic radicals definitions

Carbamoyl radicals, like acyl radicals, show a net nucleophilic character which permits the amidation of protonated heteroaromatic bases. Quantitative studies concerning the polar character of the carbamoyl radicals have not yet been published, but the complete selectivity of attack at the a- and /-positions of protonated heteroaromatic bases indicates a definite nucleophilic character and synthetic value. 

The definition also includes species in which a common radical function such as a ketyl, nitro anion-radical, or semiquinone is conjugated with a heteroaromatic system, e.g., 6-9. Certain major radical series are, however,... 

Russell and co-workers have obtained cation-radicals in the 1,2-dithiete system. Such radicals are cyclic, conjugated within the heterocycle, and possess (4 -P 1) electrons, with = 1 they are consequently heteroaromatic witliin the definition given in the introduction to Part I. 3,4-Dimethyl-l,2-dithiete cation radical 89 (R = R = Me) was obtained by treatment of acetoin in sulfuric acid with sulfide ion. The radical is persistent at ambient temperature and unaffected by oxygen. This evident stability, and its formation from open-chain precursors to the exclusion of acyclic possibilities such as 90, whose oxygen equivalent exists, implies that aromatic stabilization of 89 and similar radicals is a matter of fact and not merely definition. 

A detailed study was carried out on the radical anion derived from (10c) (Table 2) in connection with the conformational analysis of heteroaromatic carbonyl compounds. Notably different ESR signals could be observed originating from distinct rotational isomers. Unfortunately no information on the interconversion of the rotamers of (10c) could be definitely derived since these radicals were highly unstable at the temperature necessary for interconversion <74JCS(P2)562>. 

---