Heterocyclic Tautomers

Two conformations of the hydroxy tautomer (7, not indicated) were considered in HOCC syn and anti arrangements, and two oxo tautomers (8, 9) were investigated when the hydroxy proton migrates to the nitrogen or the atom ofthe ring, 

The largest deviations were observed, however, when the relative solvation free energy was calculated by the FEP/MC method in comparison with the continuum solvent approximation. The - AG . values scattered between -2 to 

Nucleobases, such as cytosine, thymine, and uracil are the most important pyrimidine derivatives. In a recent gas-phase study [64], three low-energy tautomers were identified on the basis of CCSD(T)/cc-pVQZ energy calculations andMP2/6-311++G(2d,2p) estimation ofthe vibrational frequencies. The authors emphasize the importance of the high-level estimation of the frequencies and the related thermal correction in order to obtain rehable AG within a 0.7 kcal mol free energy range of the tautomers at T = 450 K. The lowest-free-energy structure is the canonical hydroxy-amine form of cytosine. 

Relative energies of 13 tautomers/conformers for each of thymine and uracil were determined at a high theoretical level in the gas-phase by Rejnek et al. [66]. 

The dioxo form for both molecules is more stable by at least about 10 kcal mol than any of the corresponding 12 other species. Optimized geometries for monohydrates and dehydrates, as well as their relative energies help explore the tau-tomerization pathways if they emerge in aqueous solution. Schneider and Berman [67] determined, for example, ordered hydration sites for the nucleotide bases in B-type conformations using crystal structure data on 14 B-DNA decamers. The number of the water molecules, W, within 3.4 A of the atoms of the nucleotide bases were found as 101W/42G, 84W/43C, 92W/43A, and 95W/45T (G, C, A, T the standard code for nucleobases). The two to three water molecules that were identified per base on average in their first hydration shell confirm the importance of such studies. 

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The mechanism of recognition of most supramolecular entities (such as abiotic receptors) is the formation of several hydrogen bonds. Since heterocyclic tautomers possess both strong HBA and HBD properties (see Sections III,G, V,D,2, and VI,G), they are often used for this purpose. For instance, the hydrogen bond network formed by 5,5 -linked bis(2-pyridones) has been used by Dickert to obtain sensors (96BBG1312). 

Nitrosation of 2-methylaminobenzyl alcohol (14) produces the A-nitroso compound (15), a ring-chain tautomer of 2-hydroxy-1-methyl-l,2-dihydro-4//-3,l,2-benzoxadiazine (16) (Scheme 1) <88IJC(B)653>. However, no chemical or spectroscopic evidence for the heterocyclic tautomer (16) was found. 

In the early days of spectroscopy, Dobbie et al. [6] introduced the so-called model compounds whose spectra might be used in Eq. (2.2) instead of the missing spectra of the individual tautomers. In these compounds, the tautomeric proton is replaced by a Me group, whose weak electronic effect leads to only a minor spectral shift in planar tautomeric systems. For this reason, model compounds have been extensively used for quantitative analysis in heterocyclic tautomers, assuming that (i) molar absorptivity of the model tautomer and of the real tautomer are equal at their absorption maxima, and (ii) neglecting that these absorption maxima could be shifted [7]. From Table 2.1, where the predicted spectral characteristics of some real and fixed tautomers are collected, it is obvious that these assumptions are reasonable in the case of 1. 

IR spectroscopy has been particularly helpful in detecting the presence of keto tautomers of the hydroxy heterocycles discussed in Section 3.01.6. Some typical frequencies for such compounds are indicated in Figure 4. Here again the doublets observed for some of the carbonyl stretching frequencies have been ascribed to Fermi resonance. 

The tautomeric equilibria of these heterocycles always involve one or more non-aromatic tautomers. An important factor in determining the extent to which such non-aromatic tautomers are involved is the magnitude of the potential loss of resonance energy. 

Examples of the remaining potential 3,4-dihydroxy heterocycles are presently restricted to furan and thiophene. Although the parent 3,4-dihydroxyfuran apparently exists as the dioxo tautomer (86), derivatives bearing 2-alkyl or 2,5-dialkyl substituents prefer the keto-enol structure (87) (71T3839, 73HCA1882). The thiophene analogues also prefer the tautomeric structure (87), except in the case of the 2,5-diethoxycarbonyl derivative which has the fully aromatic structure (88) (71T3839). 

Concerted cycloadditions are observed with heterocyclics of all ring sizes. The heterocycles can react directly, or via a valence tautomer, and they can utilize all or just a part of unsaturated moieties in their rings. With three-membered rings, ylides are common reactive valence tautomers. Open chain 47T-systems are observed as intermediates with four-membered rings, and bicyclic valence tautomers are commonly reactive species in additions by large rings. Very often these reactive valence tautomers are formed under orbital symmetry control, both by thermal and by photochemical routes. 

Many heterocyclic compounds exist as mixtures of tautomers. For example, 2-hydroxypyridine exists in equilibrium with 2-pyridone. 

Protons bound to heteroatoms in heterocyclic compounds are likely to be very mobile in solution and, where two or more heteroatoms are present in a structure, different isomers (tautomers) may be in equilibrium. As a case in point, consider the nucleotide bases (indicates the point of attachment to the sugar-phosphate backbone). 

Identification of the product(s) resulting from the reaction of heterocyclic compounds with diazomethane has been used in attempts to elucidate their tautomeric composition (for summaries, see references 7 and 41). This work was based on the assumption that if a compound which is capable of existing in both an —NH and an —OH form produced only the =NMe derivative when it w as treated with diazomethane, it existed entirely in the =NH form. On the other hand, formation of the —OMe derivative was interpreted to mean that a finite amount of the compound existed in the —OH form. In some cases the tautomer present in the solid state w as concluded to be different from that present in solution for example, 41 42 gave a higher proportion of the 3,4-dimethoxy derivative when ethereal diaz-... 

The determination of pi a values is probably the most generally useful method for the investigation of tautomerism. This method was first employed in the heterocyclic field in the early 1950 s by Tucker and Irvin and by Angyal and Angyal. There are two empirical dissociation constants, and K2, for the conjugate acid (HXH+) of a tautomeric compound. Constants Kt and K2 are, in effect, a summation of the true dissociation constants Ka, Kb, Kc, and Kd of the individual tautomeric forms (see scheme 43, where XH and HX are tautomers) and the tautomeric constant, Kt] these constants are related by the following equations ... 

If the refractivity of the pure tautomeric constituents is known, the composition of the equilibrium mixture can be determined. This method has been used to study, for example, the keto and enol tautomers of ethyl acetoacetate. So far it has not been applied to heterocyclic compounds in this series the isolation of the pure... 

The imbalance between and NMR studies in the solid state (Section VI,F) partly reflects the fact that it is easier to introduce N than into heterocyclic compounds, particularly azoles (DNMR in the solid state usually requires isotopic enrichment). Compared to solution studies, solid-state intermolecular proton transfer between tautomers has the enormous advantage that the structure of the species involved is precisely defined. 

Our previous treatment (76AHCS1, p. 12) contained a section called Chemical Methods to Study Tautomerism where the relationship between tautomerism and reactivity was discussed. Today, nobody uses chemical methods to study tautomerism. However, a great many reactions are carried out on tautomeric heterocycles, although few papers contain new insights on that topic. Authors desiring to explain reactivity results based on tautomerism must take great care to verify that the substrate is in the neutral form AH and not as a conjugated anion A or cation HAH, which are usually devoid of tautomerism. They must also realize that most frequently the reaction path from tautomers to products in-... 

Quantum chemical predictions of tautomeric equilibria Ab initio multireference Cl studies of tautomers UPS spectra Combined approach to the tautomerism in azaaromatic heterocycles by N, C, and H NMR Fluoro-azoles Experimental data and MNDO studies Tautomerism A review... 

Since Woodward s work on the synthesis of chlorophyll a (60JA3800) it is known that the intrinsic unstable thioformyl moiety can be stabilized by the delocalization effect of heterocyclic systems. Recently the synthesis of 2-amino- and 3-aminothioformylthiophenes (and furans) and the corresponding benzo derivatives (Scheme 19) has been reported (96S1185). These compounds exist as amino tautomers (91S609 96S1185). 

It is known that unsaturated three-membered nitrogen heterocycles display tautomerism involving nonaromatic and antiaromatic (i.e., Air systems) forms. In all cases, the nonantiaromatic tautomer is the most stable 1-azirine la and 1-diazirine 2a. Nonetheless, antiaromatic tautomers are known, for instance, triazirines 3. 

Tautomeric studies of equilibria between enamino and methylene imino form have been reported for several heterocycles. Tautomerism of 1,2,4-tri-azino[4,3- ]quinoxalin-5-ones was deduced to be solvent dependent the enamino form 170b is predominant in DMSO-dg ( H NMR), whereas 170a is the major tautomer in the solid state (IR in nujol) (90JHC691 95H2057) (cf. Section III,B). 

The application of spectroscopic methods to the study of tautom-erism proved especially fruitful. The tautomerism of hydroxy and amino derivatives of isoxazole is of great interest to the chemistry of isoxazole this subject, as well as the tautomerism of functional derivatives of other five-membered heterocycles, has been reviewed by Katritzky and Lagowski. We shall therefore only... 

As is common in heterocyclic chemistry, many studies concern tautomeric equilibria. While quantum chemical calculations are straightforward for the question of the most stable isomer, experiments are sometimes very demanding. Therefore, quantum chemistry can easily provide answers that may require substantial experimental effort. Comparatively few studies concern the investigation of entire reaction paths. This is much more demanding than computing a limited number of tautomers, of course, but usually provides a very detailed picture of the reaction mechanism. In certain cases, it was only possible to judge the nature of a chemical reaction on the basis of quantum chemical calculations. 

There are also some couplings in which hydrazones are formed but for which the azo tautomer is not detectable and probably does not exist. This is the case in some coupling reactions involving methyl groups of aromatic heterocycles (see, for example, 12.48 and 12.49 in Sec. 12.5). Replacement of a methyl proton by an arylazo group (Scheme 12-3) would result in an azo compound containing an sp3-hybridized — CH2 — group (12.1). The latter is less stable than the tautomeric hydrazone (12.2), in which there is a n-n orbital overlap from the heteroaromatic to the aromatic system. 

For most simple phenols this equilibrium lies well to the side of the phenol, since only on that side is there aromaticity. For phenol itself, there is no evidence for the existence of the keto form. However, the keto form becomes important and may predominate (1) where certain groups, such as a second OH group or an N=0 group, are present (2) in systems of fused aromatic rings and (3) in heterocyclic systems. In many heterocyclic compounds in the liquid phase or in solution, the keto form is more stable, although in the vapor phase the positions of many of these equilibria are reversed. For example, in the equilibrium between 4-pyridone (118) and 4-hydroxypyridine (119), 118 is the only form detectable in ethanolic solution, while 119 predominates in the vapor phase. " In other heterocycles, the hydroxy-form predominates. 2-Hydroxypyridone (120) and pyridone-2-thiol (122) are in equilibrium with their tautomers, 121 and 123, respectively. In both cases, the most stable form is the hydroxy tautomer, 120 and 122. ... 

The recently reported (757) conversion of 5-pyrazolones directly to a,j8-acetylenic esters by treatment with TTN in methanol appears to be an example of thallation of a heterocyclic enamine the suggested mechanism involves initial electrophilic thallation of the 3-pyrazolin-5-one tautomer of the 5-pyrazolone to give an intermediate organothallium compound which undergoes a subsequent oxidation by a second equivalent of TTN to give a diazacyclopentadienone. Solvolysis by methanol, with concomitant elimination of nitrogen and thallium(I), yields the a,)S-acetylenic ester in excellent (78-95%) yield (Scheme 35). Since 5-pyrazolones may be prepared in quantitative yield by the reaction of /3-keto esters with hydrazine (168), this conversion represents in a formal sense the dehydration of /3-keto esters. In fact, the direct conversion of /3-keto esters to a,jS-acetylenic esters without isolation of the intermediate 5-pyrazolones can be achieved by treatment in methanol solution first with hydrazine and then with TTN. 

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