Pyrazole, tautomerism complexes

Complex tautomerism for azoles with heteroatoms in the 1,2-positions occurs for pyrazoles which are not substituted on nitrogen. Scheme 10 shows the four important tautomeric structures (148)-(151) for 3-methylpyrazolin-5-one, and (152) and (153) as examples of other possible structures. A detailed investigation of this system disclosed that in aqueous solution (polar medium) the importance of the tautomers is (149) > (151) (150) or (148), whereas in cyclohexane solution (non-polar medium) (151) > (148) (149) or (150). 

The tautomerism of these compounds has already been discussed (Section 4.04.1.5.2). Owing to the extra functionality they have a complex reactivity, and to the reactions of the substituent must be added those of the pyrazole ring itself (Section 4.04.2.3), which are different for each tautomer. In addition to the three neutral tautomers, one has to bear in mind that the NH and OH tautomers of 5-pyrazolones can be written as betaines as in formula (158) which is important in understanding their reactivity. 

The tautomerism of crystalline pyrazoles which is discussed in the following, is particularly interesting because of the variety of hydrogen bonded complexes... 

The number of structures of tautomers determined by X-ray crystallography (and some by neutron diffraction) is considerable [21]. An aspect of tautomerism, usually but not always studied by crystallography (silver complex of chloroquine [22]), is the effect of the coordination with metals on tautomerism. Many examples have been reported, such as (i) 2-(l//-pyrazol-5-yl)phenol to 2-(l//-pyrazol-3-yl)phenol by coordination to a ruthenium complex [23] (ii) IH- to 2//-indazole when forming an osmium(IV) complex... 

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