The tautomerism of indazolinone in aqueous solution. A note on the 'principle of vinylogy'

Correction factors derived in another study have been applied to the basic pK(a) values of tired model tautomers in order to elucidate the tautomeric balance for indazolinone 1 in aqueous solution. The ore-form B dominates to the extent of ca. 95%; this is consistent with past studies in other solvents. Of the two possible hydroxy tautomers, the benzenoid form A accounts for almost all of the remainder, the proportion of the quinonoid form C being estimated as ca. 10(-4.7). It has also proved possible to estimate all six of the microscopic acid pK(a) values; as with the basic pK(a)s, the resultant of these agrees very closely with the measured, macroscopic pK(a) of 1 itself. 1-Substitution engenders a switch to tautomer A; AM1 calculations suggest that the reason may be enforced planarity, leading to a Severe(R)N-NH lone pair clash in B which destabilises this form. Comparison with N-unsubstituted pyrazolones shows that benzene ring annelation has the expected effect of stabilising B and destabilising C. However, it is noted that A is more stable than C even when no quinonoid form is possible, and that this reflects a greater basicity for heterocycles in general when substituted with pi-donors gamma- rather than alpha- to aza-nitrogen. It is suggested that this effect applies equally in other contexts, as when C=O not C=N is the pi-acceptor; that its origin lies in sigma-bond-no-bond resonance which acts specifically to limit conjugation when pi-donor and pi-acceptor are contiguous; and that this phenomenon throws much light on the (principle of vinylogy'.