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 b asic pK(a) values of tired model tautomers in order to elucidate the t automeric balance for indazolinone 1 in aqueous solution. The ore-form B dominates to the extent of ca. 95%; this is consistent with past st udies in other solvents. Of the two possible hydroxy tautomers, the be nzenoid form A accounts for almost all of the remainder, the proportio n 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) val ues; as with the basic pK(a)s, the resultant of these agrees very clos ely with the measured, macroscopic pK(a) of 1 itself. 1-Substitution e ngenders a switch to tautomer A; AM1 calculations suggest that the rea son may be enforced planarity, leading to a Severe(R)N-NH lone pair cl ash in B which destabilises this form. Comparison with N-unsubstituted pyrazolones shows that benzene ring annelation has the expected effec t 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 subs tituted with pi-donors gamma- rather than alpha- to aza-nitrogen. It i s suggested that this effect applies equally in other contexts, as whe n C=O not C=N is the pi-acceptor; that its origin lies in sigma-bond-n o-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'.