ABSTRACT
The diazines – pyridazine, pyrimidine and pyrazine – contain azomethine nitrogen atoms, the lessons learnt with regard to pyridine are, in these heterocycles, exaggerated. A very characteristic feature of diazine chemistry, which is associated with their strongly electron-poor nature, is that they add nucleophilic reagents easily. Although there is always debate quantitative measures of aromaticity, it is agreed that the diazines are less resonance stabilised than pyridines – they are ‘less aromatic’. The most studied diazine derivatives are the oxy- and amino-pyrimidines since uracil, thymine and cytosine are found as bases in DNA and RNA. It is the enamide-like character of the double bonds in diazines with two oxygen substituents which allows electrophilic substitution – uracil, for example, can be brominated. Diazinones, like pyridones, react with phosphorus halides with overall conversion into halide. Anions produced by (N-) deprotonation of diazinones are ambident, with a phenolate-like resonance contributor, but they generally react with electrophilic alkylating agents at nitrogen, rather than oxygen.
