Scheme 1.

Relevant tautomeric forms of o-hydroxyaryl Schiff bases. The keto-amine tautomer exists in the Z and E isomers as shown in the left and right bottom images, respectively. However, the geometric isomers of this tautomer do not have a direct correspondence to the geometric isomers of the enol-imine tautomer. To avoid confusion, in this article the keto-amine conformers will be grouped in two sets, one related to the E-enol-imine tautomer and other to the Z-enol-imine tautomer, which will be designated as “keto-amine forms of the E-enol-imine tautomer” and “keto-amine forms of the Z-enol-imine tautomer”, respectively. In both sets, there are Z-keto-amine and E-keto-amine conformers.