Acid-base interactions in some isoquinoline and quinazoline amino derivatives

Few groups of substituted 1-aminoisoquinoline, 4-aminoand 2,4-diaminoquinazoline derivatives have been examined in order to find general relationships between the basicity and a kind of substituent. pKa Values have been determined for these compounds and correlated with the original Hammett σm and σp constants. They clearly showed that the steric effects of the bulky substituents neighbouring the protonated ring nitrogen atom influence the electron transfer in such arrangements. Different tendencies in the electron transfer caused by the same groups in the position 1 of isoquinoline and position 4 of quinazoline have been also disscused. The ρ value, single crystal X-ray analysis, N-NMR spectra and MNDO calculations give evidence about the electronic effects and the preferential site of protonation in such arrangements.