Azoles, tautomers, stabilization

Additional sections of this chapter cover (1) tautomeric reactions of azoles and their derivatives in electronically excited states and (2) stabilization of certain tautomers in their metal complexes. 

Trends in Relative Stability of Annular Tautomers of Azoles... 

The tautomers of azole and azine derivatives, stabilized as a result of complex formation, are widely found [11,15,16,18,261-269]. Thus, the metal-cyclic structures with anionic ligands are formed in chelates of 2-hydroxy- 436, 438) and 2-mercapto-substituted (484, 485) derivatives of these nitrogen-containing heterocycles described above. At the same time, the ketone (thion) isomers are stabilized in molecular adducts. In this respect, the syntheses (4.64) [266], (4.65) [267], and (4.66) [269] are quite representative for this group ... 

Table 42 gives an overview of annular tautomerism data for azoles in the gas phase and in solution or crystals. In the gas phase the stability of alternative tautomers largely depends on their relative aromaticities. In Section 2 A.4.2.2 it was noted that 1,2-relationships between pyrrole- and pyridine-type nitrogen atoms favor aromaticity (Figure 21) and this is consistent with the relative stabilities of triazole and tetrazole tautomers in the gas phase (Table 42) <2010T2695>. In solution (and crystals) other factors such as solvent polarity, hydrogen bonding, and temperature become important and the relative stabilities can be reversed. Polar solvents tend to stabilize the tautomer with the largest dipole moment and this probably accounts for the observation of both 2H-1,2,3-triazole (p = 0.12D) and H-1,2,3-triazole (p = 4.55D) in...

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