Basicities and tautomerism of 4,4,6-trimethyl-2-aryl-amino-4H-1, 3-thiazines

Abstract

The relative basicities of 4,4,6-trimethyl-2-arylamino-4H-1,3-thiazines, which are capable of amine-imine tautomerism, and of model compounds with fixed amino and imino structures were determined by potentiometric titration in methanol. Good correlation was observed between the pKa values and the Hammett σ constants in the investigated reaction series. The anomalous pKa values of the model amines and imines are explained by the different character of the electronic interactions in them, which is due to the change in the three-dimensional orientation of the aromatic ring and the amidine fragment.