N-heteroaromatic bases

This reaction is based on the proposition that the sensitivity to polar effects in free-radical chemistry is the result of polarity and polarizability of both the radical and the substrate. This means that the polarity of the heteroaromatic base plays a key role in the process. Actually, the nucleophilic character of an alkyl radical, for example, is not so marked as to justify the addition to the N-heteroaromatic base, and in fact either no substitution occurs or low yields and selectivity are observed. 

Scheme 14.3 Mechanism for the carbamoylation of N-heteroaromatic bases with NHPI/Ce(IV) system.

In the last few years we have developed new methods for the photochemical carbamoylation of electron-poor N-heteroaromatic bases. The introduction of Ti02 photocatalysis has supplied new and environmentally friendly synthetic methods that operate under mild conditions (Scheme 14.4) [19, 20]. 

The perfluoro alkylation of protonated N-heteroaromatic bases (Scheme 14.5a) gives high conversion, because of the high enthalpic effect, which can be ascribed to the stronger C—C bond formed when Rr rather than R radicals add to the substrate [26]. However, Rf-being an electrophilic radical, low selectivity is observed. 

Scheme 14.6 Perfluoroalkylation of N-heteroaromatic bases and 1,4-quinones in the presence of electron rich olefins.

Rates and Activation Energies for the JV-Oxida-tion of N-Heteroaromatic Bases with Perbenzoic Acid... 

Proton sponges derived from N-heteroaromatic bases, geometry of hydrogen bonds 99AG(E)865. 

Six-membered N-heteroaromatic bases are known to be deactivated systems toward electrophilic substitution. Thus, reactions following the Friedel-Crafts protocol generally give on them products substituted onto the less electron-poor positions. Otherwise, their reactions with nucleophilic carbon-centered radicals reproduce most of the Friedel-Crafts aromatic substitutions, but with opposite reactivity and selectivity. The general reaction mechanism is shown in Scheme 6. 

In 1968, a pioneering paper by Minisci et al. described the alkylation of N-heteroaromatic bases, olefins, and ben-zoquinones by means of alkyl radicals [1]. 

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