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

Abstract

Correction factors derived in another study have been applied to the basic pK_a values of fixed model tautomers in order to elucidate the tautomeric balance for indazolinone 1 in aqueous solution. The oxo-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_as, 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 π-donors γ- rather than α- to aza-nitrogen. It is suggested that this effect applies equally in other contexts, as when CO not CN is the π-acceptor; that its origin lies in σ-bond-no-bond resonance which acts specifically to limit conjugation when π-donor and π-acceptor are contiguous; and that this phenomenon throws much light on the ‘principle of vinylogy’.