Über die Azo‐Hydrazon‐Tautomerie

Since the azo‐hydrazone tautomerism was discovered, it has been the subject of much research and much heated debate. Only modern spectroscopic methods led to clear experimental evidences. Therefore, the first part of the review describes the spectroscopic methods (UV/VIS, IR, mass, NMR and X‐ray photoelectron spectra and crystal structure analysis) associated with a more detailed evaluation of the certainty of the results so obtained. It is shown that nuclear magnetic resonance (δH, δ13C, δ15N, δ14N, J15NH) is a generally applicable tool and it leads to the most reliable results. In the second part, a survey of the tautomerism of p‐hydroxyazo‐benzenes, phenylazonaphthols and the azocoupling products of pyrazolones and ß‐diketones is given. The effects of substituents, anellation and H‐bonding are summarized. The various assumptions, indications and evidences of tautomeric structures and solvent effects are discussed critically.