Tautomerism in hydroxy- and amino-diazines

We have seen that 2- and 4-hydroxypyridines exist primarily in their tautomeric amide-like pyridone forms (see Section 11.4.3). This preference over the phenolic tautomer was related to these compounds still retaining their aromatic character, with further stabilization from the carbonyl group. 3-Hydroxypyridine cannot benefit from this additional stabilization. In contrast, 2-aminopyridine and 4-aminopyridine exist almost entirely as the amino 

We also enconnter tautomerism in hydroxy- and amino-diazines, and the preference for one tautomeric form over the other follows what we have seen with the pyridine derivatives. Thus, with the exception of 5-hydroxypyrimidine, all the mono-oxygenated diazines exist predominantly in the carbonyl tan-tomeric form. We term these amide-like tautomers diazinones. 5-Hydroxypyrimidine is analogous to 3-hydroxypyridine, in that the hydroxyl is wrongly positioned for tautomerism. 

The diazinone tautomers are identified by using terminology such as 3(2//)-pyridazinone for the carbonyl tautomer of 3-hydroxypyrazine. The 3(2//) prefix signifies the position of the oxygen (3-pyridazinone) and specifies the NH is at position 2. Note that, in addition to the diazine-diazinone tau-tomerism, when the nitrogens have a 1,3-relationship there is further tautomerism possible, e.g. 4(1//)-pyrimidone 4(3//)-pyrimidone. 

Diazinones may be converted into chlorodiazines by the use of phosphorus oxychloride, just as 

Aminodiazines exist in the amino form. These compounds contain two ring nitrogens and a primary amino group. 

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