Journal of the Chemical Society. Perkin Transactions 2 (2001) p. 2263 - 2269 (1996)
Update date:2022-08-10
Topics:
Bruneau, Pierre
Taylor, Peter J.
Wilkinson, Anthony J.
Correction factors derived in another study have been applied to the basic pKa 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 pKas values; as with the basic pKas, the resultant of these agrees very closely with the measured, macroscopic pKa, 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 C=O not C=N is the π-acceptor; that its origin lies in a-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'.
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