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Theoretical chemistry Theoretical chemistry Z 0350 Some New Views on the Tautomerization Mechanism -[107 refs.]. -(KERESELIDZE, J. A.; ZARQUA, T. S.; KIKALISHVILI, T. J.; CHURGULIA, E. J.; MAKARIDZE, M. C.; Russ. Chem. Rev. (Engl. Transl.) 71 (2002) 12, 993-1003; Dep. Chem., Javakhishvili State Univ., Tbilisi 380028, Georgia; Eng.) -Lindner 22-292

A new proton transfer mechanism is proposed for hydrazone-enehydrazine tautomerism through the cyclic dimer of the phenylhydrazones that take part in the E. Fischer Hydrazone--enehydrazine tautomerism plays a special role in the chemistry of nitrogen-containing heterocyclic compounds. In particular, this process is considered to be the controlling stage in the E. Fischer indolization of arylhydrazones [1]. Although we do not share this opinion [2], there is no doubt about the importance of the hydrazone--enehydrazine tautomeric transformation for the planned synthesis of indole derivatives. The results from investigations devoted to this problem have mostly been qualitative in nature [3]. With the exception of a few isolated cases [4, 5] experimental difficulties and, in particular, the isolation and description of the enehydrazine tautomer have not made it possible to analyze the tautomeric process quantitatively. This is due either to the instability of the enehydrazine tautomers or to the inadequacy of the experimental technique.In this connection it was extremely interesting to investigate theoretically the possibility of obtaining a quantitative description of the hydrazone-enehydrazine tautomeric transformation. As in the case of keto--enol tautomerism [6, 7], proton transfer in this process must take place by means of an inter-and intramolecular hydrogen bond. The mobility of the proton, as leaving particle, is actuated under the influence of the neighboring functional groups, temperature, catalyst, solvent, and the concentration of the solution [8]. In the absence of external promoting factors, however, the description of the mechanism of the tautomeric process requires a different approach. In this case it is tacitly assumed that the proton transfer results from a redistribution of electron I R ~ = R 2 = H, R 3 = C6H5; II R ~ = CH3, m 2 = H, R 3 = C6H5; III R l = C2Hs, R 2 = H, R 3 = C6H5; IV R I = COOC2H5, R 2 = H, R 3 = C6H5; V R I = C6H5, R 2 = H, R 3 = C6H5; VI R 1 =p-CH3C6H4, R 2 = H, R 3 = C6H5; VII R 1 =p-O2NC6H4, R 2 = H, R 3 = C6H5; VIII R 1 = C6H5, R 2 = H, R 3 =p-CH3C6I-I4; IX R I = C6Hs, R 2 = H, R 3 = p-O2NC6I-L; X RI,R 2 = cyclo C4Hs (cyclohexanone residue), R 3 = C6H5