Tautomer table

R = H R = Ph R = YC6H4) in favor of the cyclic tautomer (Table XI). A good linear correlation has been observed ... 

The ultraviolet absorption spectra of most 1-hydroxy-, 1-acyloxy-, and 1-alkoxyindoles are quite similar to those of the corresponding indoles representative examples are listed in Table 11. In general, they show a small shift towards the visible, and this is accentuated in the 1-hydroxyindoles on the addition of alkali when the proton can be removed. The UV spectrum reported (67BSF1296) for 1-hydroxyindole is certainly that of a polymer, for it shows no absorption at —280 nm, which is characteristic of these compounds and of 2,3,3-trimethyl-3//-indolenine 1-oxide, a model for the 3//-tautomer (Table II). 

The lower the polarity of the solvent, the higher the percentage of the enol tautomer. Table 1 lists the solvents for which data have been reported arranged in order of the... 

For 4-substituted-3(5)-monoamino derivatives, the existence of a 2-NH tautomer has been described (Section n,A,3,b), except for 4-cyano derivative 3c, which, based on C-NMR data, seems to exist in DMSO as a mixture of the 2-NH and 6-NH tautomers (Table II, Section II,A,3,b) (86MI1). 

The effect of substituents is profound. Isoindole itself appears to be essentially present as the isoindole tautomer. As Table VI shows, a 1-methyl substituent shifts the ratio remarkably far to the isoindolenine side however, the effect is not additive since the 1,3-dimethyl system has a smaller proportion of isoindole tautomer than the 1-methyl.16 Aryl substituents in the 1,3-positions reestablish the isoindole tautomer (Table VI), presumably because conjugation between the aromatic systems is energetically more... 

In marked contrast to the anion (40), which equilibrates with its (A) tautomer (Table 6), the p-triazole congener (41), once formed by cyclization of the respective azide (A), undergoes rapid... 

The last example is somewhat more complicated since four isomers (two tautomers and two conformations) are present at equilibrium (Figure 9) (78BSB189). The experimental value (3.73 D, Table 3) establishes the predominance of the 3-azido tautomer but does not allow the determination of the conformational equilibrium other methods (Section 4.04.2.3.4(v)) are necessary to establish definitely the Z conformation (43b). 

For )V-unsubstituted pyrazoles the tautomeric proton was generally located without ambiguity. 3-Substituted tautomers were favoured in the solid state (45), (46) and (48) (Table 5). For the pyrazolyltriazole (47) the authors (77JHC65) concluded that the X-ray analysis indicates that the proton on the pyrazole ring populates either nitrogen atom to... 

The H chemical shifts shown in Table 6 for 3-substituted pyrazoles are mean values of 3- and 5-substituted tautomers. Only in the case of the 3-azidopyrazole (56) have the... 

Acyl-, 4-alkoxycarbonyl- and 4-phenylazo-pyrazolin-5-ones present the possibility of a fourth tautomer with an exocyclic double bond and a chelated structure. The molecular structure of (138) has been determined by X-ray crystallography (Table 5). It was shown that the hydroxy group participates in an intramolecular hydrogen bond with the carbonyl oxygen atom of the ethoxycarbonyl group at position 4 (8OCSCII21). On the other hand, the fourth isomer is the most stable in 4-phenylazopyrazolones (139), a chelated phenyl-hydrazone structure. 

The aromaticity of the pyrimidine and purine ring systems and the electron-rich nature of their —OH and —NHg substituents endow them with the capacity to undergo keto-enol tautomeric shifts. That is, pyrimidines and purines exist as tautomeric pairs, as shown in Figure 11.6 for uracil. The keto tautomer is called a lactam, whereas the enol form is a lactim. The lactam form vastly predominates at neutral pH. In other words, pA) values for ring nitrogen atoms 1 and 3 in uracil are greater than 8 (the pAl, value for N-3 is 9.5) (Table 11.1). 

It has been proposed that protonation or complex formation at the 2-nitrogen atom of 14 would enhance the polarization of the r,6 -7i system and facilitate the rearrangement leading to new C-C bond formation. The equilibrium between the arylhydrazone and its ene-hydrazine tautomer is continuously promoted to the right by the irreversible rearomatization in stage II of the process. The indolization of arylhydrazones on heating in the presence of (or absence of) solvent under non-catalytic conditions can be rationalized by the formation of the transient intermediate 14 (R = H). Under these thermal conditions, the equilibrium is continuously pushed to the right in favor of indole formation. Some commonly used catalysts in this process are summarized in Table 3.4.1. 

The pKa method was first used by Tucker and Irvin to determine the proportion of the tautomers of quinol-4-one present at equilibrium and was subsequently applied to many other potentially tautomeric hydroxy compounds " these results are summarized in Table I. 

The ultraviolet spectrum of vitamin Be, or pyridoxine, measured in aqueous ethanol varies with the composition of the solvent indicating that this compound is in equilibrium with the zwitterion form 38. The equilibrium constant in pure water was obtained by extrapolation. Prior to this, equilibria which involved tautomers of type 39 had been suggested for vitamin Be, but see Section VI,A. In the case of pyridoxal, an additional equilibrium, 40 41, occurs (cf. Section VIII) other pyridoxal analogs have also been studied (Table II). 

Two independent molecular orbital calculations (HMO method) of delocalization energies for isoindole and isoindolenine tautomers agree that the isoindole form should possess the more resonance stabilization. The actual difference calculated for isoindole-isoindolenine is about 8 kcal/mole, but increases in favor of the isoindole with phenyl substitution at position 1 (Table VI).Since isoindole and isoindolenine tautomers have roughly comparable thermodynamic stabilities, the tautomeric proce.ss is readily obser-... 

The calculation of the proton affinities (PA) for a pair of tautomers and the comparison with experimental data [generally from ICR measurements (Section VII,F)] has been the subject of a series of publications with increasing sophistication (Table IV). Such calculations concerning the annular tautomerism of azoles and benzazoles have been reviewed [87AHC(41)187]. 

Examples of dynamic processes involving two, three, or four identical tautomers (degenerate or autotrope annular tautomerism) have been found in pyrazoles (type 2 of Table VII). Thus, 3,5-diphenyl-4-bromopyr-azole and 3,5-di-ferf-butylpyrazole (dimers), 3,5-dimethyl-pyrazole (trimer). 

Some papers (second column of Table XII) report the observation of individual tautomers on cooling. These results were used to determine the populations by integration and, therefore, of Kt and dG. 

Electron-releasing substituents stabilize the isoindolenine tautomer 7, whereas eleetron-withdrawing groups have the opposite effect. In Table IV some data are given. 

Hydroxythiophenes 56 have two possible keto tautomers (57 and 58) [for review see (86HC1)]. As has been pointed out earlier (76AHCS1, p. 229), the tautomerism of 5-substituted eompounds was extensively studied by Lawesson and eoworkers (63T1867) and by Hornfeldt (63MI1 68MI1). For 5-alkyl eompounds, only the keto forms were present, whereas with R = phenyl, thienyl and ethoxyearbonyl substantial amounts of the enol forms were deteeted. Computations for the parent system (R = H) showed that the 4 -thiobutenolide of type 57 is most stable (Table VII). 

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