Tautomerism of Heterocycles: Five-Membered Rings with One Heteroatom

Chemistry of heterocyclic compounds at the A. E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch of the Russian Academy of Sciences, over 50 years (Review)

“…Essentially it involves the use of heterocumulenes (isothiocyanates, isocyanates) and carbanions readily generated in situ from available alkynes or 1,2-and 1,3-dienes, as key building blocks. The results of these investigations were summarized and analyzed in dissertations [98,148,149] and in recently published reviews and monographs [99,101,[150][151][152][153][154][155][156][157][158][159][160][161][162][163][164][165].…”

The first example of tautomerism in 2-aminopyrroles: effect of structure and solvent

“…During the variable temperature NMR study, H-5 in the 2-amino tautomer 4b, coalesced at 218 K. The lifetime (s was calculated using the equation s = p 22pDm, with Dm = 4.97 Yz and s = 0.045 s. 14 Heterocyclic tautomeric equilibria are known to be sensitive to solvent effects. 1 Amino/imino tautomerism was therefore studied in seven solvents and the results are summarized in Table 1. Values for K T (amino/imino ratio) varied from 0.8 in CD 3 1.0 CD 3 OD 1.3 DMSO-d 6 1.4 DMF-d 6 1.4 (CD 3 ) 2 CO 1.8 Figure 1.…”

“…Analogous results have been reported for 3-hydroxypyrroles (hydroxy/keto). 1,15 In 2-aminothiophenes on increasing solvent polarity (CDCl 3 vs CCl 4 ) shifted the equilibrium further to the imino form; whereas, in (CD 3 ) 2 CO, the amino form was favored. 16 An attempt was made to correlate K T with the solvent parameter E T 30 (7 solvents), 17 but no correlation was found indicating that hydrogen bonding, not solvent polarity, was likely the determining factor in K T .…”

“…1 Investigators have searched for evidence of amino/imino tautomerism, in 2-aminopyrroles, without success. 2 An early report 3 for an imino tautomer was not substantiated 4 by later 1 H and 13 C NMR data.…”

“…1 Investigators have searched for evidence of amino/imino tautomerism, in 2-aminopyrroles, without success. 2 An early report 3 for an imino tautomer was not substantiated 4 by later 1 H and 13 C NMR data. The first examples of 2-aminopyrroles, without ring substituents, have been reported 5 and isolated 6 as stable tetraphenylborate salts.…”