Abstract
Ambident reactivities of alkoxycarbonyl isothiocyanates (2) with heterocyclic amines were discussed from the concept of charge and frontier controlled interactions which is a qualitative interpretation of the hard and soft acids and bases (HSAB) principle. The calculated charge densities indicate hard nucleophiles have the tendency to attack on the carbonyl carbon atom and soft nucleophiles on the isothiocyanato moiety of 2. To clarify the difference in basic behavior of heterocyclic amines, correlations existing in the ionization data of pyridinium and thiazolium ions were analyzed using the dual substituent parameters of F (field) and R (resonance). It seems likely that the F effect results mostly from charge controlled interactions and the R effect from electron transfer interactions between the nearly degenerate overlapping orbitals. It was suggested that the magnitude of the F and R factors of the regression equations can be an indication of the charge and frontier controlled interactions between the two interacting molecules and the F effect may have an important role in determining the pathway of approach in the initial stage of the reactions.
