Pyrazolidine-3,5-diones with heterocyclic substituents. IV. Ionization constants

Abstract

Spectrophotometric pK_a determinations are reported for N¹-3-phenyl-1 H- 1,2,4-triazol-5-ylpyrazol-idine-3,5-diones and N-phenyl analogues; 4- monoalkyl derivatives are too unstable for this purpose unless both nitrogen centres of the pyrazolidinedione are substituted. Substitution of the pyrazolidinedione by triazolyl instead of phenyl increases the acidity by 0.54-1.74 pK units; a comparable strengthening of the acidity of the triazole moiety by the pyrazolidinedione is observed. The nature of alkyl substituents has little effect on the value (+0.10 to +0.15) of the Hammett σ_m calculated for anions of the investigated pyrazolidinediones. Keto and en01 tautomers of 4-benzyl-1-phenyl-2-(3- phenyl-1H-1,2,4-triazol-5-yl)pyrazolidine-3,5-dione have been isolated.