Hydrazones from hydroxy naphthaldehydes and N-aminoheterocycles: structure and stereodynamics

Schiff bases derived from 2-hydroxy-1-naphthaldehyde, or 1-hydroxy-2-naphthaldehyde, and different saturated N-aminoheterocycles have been prepared. Their structures have been elucidated in both solution and the solid state, including unequivocal X-ray diffraction analyses. Experimental data evidence the presence of imine (or hydrazone) structures as the most stable tautomers, while all attempts to switch to enamine (or enhydrazine) structures based on electronic and steric considerations were unsuccessful. A complete conformational analysis assisted by DFT calculations at B3LYP/6-31G^∗ and M06-2X/6-311++G^∗∗ levels has been performed on each series of representative structures.