Proton exchange tautomerism

Photochromic LB materials offer much potential for applications in optical data storage. The molecular processes involved include proton exchange, tautomerism, charge transfer and cis-trans isomerization. 

Tile tautomeric equilibrium of pyrimido[4,5-h][4, 5 -e]thiazine 157 was studied in DMSO-dg by NMR spectroscopy (92CHE1219). Based on and NMR chemical shifts, fast proton exchange was concluded to occur between 157b and 157c. Monoprotonation of 157 has been assumed to form... 

Tautomeric equilibrium in the symmetrical phenoxy-substituted derivative 136 (R = Ph, r = R = OPh) is fast at ambient temperature on the NMR time scale however, at —84°C the proton exchange becomes frozen and both annular tautomers 136a and 136b can be observed (Scheme 40). The similar exchange was also found for P-aryl-substituted 136 (R = Me, Ft, Ph R = R = Ph). In these cases, the equilibrium is very slow, even at ambient temperature, which was attributed to increased steric demands of four phenyl substituents. Unsymmetrically substituted azaphosphorinanes (R R ) provide even more interesting examples. These compounds (R = Ph R = Me, -Pr R = MeO, -PrO) were found to... 

Tautomerism of the proposed pyrrolenin-2-one nucleus of V-2, resulting in proton exchange at C3 and C5 of the five membered ring. Ri, R2, and R3 as in figure 9. 

It is important to emphasize that the presence of two or three basic centres of comparable basicity in a conjugated molecule may lead to protonation processes at all of them. These processes may occur at various rates, but if all the rates are fast, nmr spectra do not enable us to say what are the relative amounts of variously protonated species present at equilibrium. Increasing the acidity of the medium and lowering the temperature in order to observe the resonances of the captured proton may have the effect of shifting the tautomeric equilibria in favour of one or other of the protonation sites. Information on the position of tautomeric equihbria in protonation processes is thus not obt iinable from nmr spectra under the conditions of rapid proton exchange. 

The generally accepted pathway for the hydration of alkynes are the generation and subsequent tautomerization of an intermediate enol. The use of fairly concentrated acids, usually H2S04, is necessary to achieve suitable reaction rates. Addition of catalytic amounts of metal salts, however, greatly accelerates product formation. In most cases mercury(II) salts are used. Mercury-impregnated Nafion-H [with 25% of the protons exchanged for Hg(II)] is a very convenient reagent for hydration 35... 

C NMR studies, especially in the solid state (83H(20)1713), are of value in studies of tautomerism (83H(20)1713, 86ZC378). Solid state studies on imidazole (and pyrazole) show there are three distinct signals for the annular carbon atoms (imidazole C-2, 136.3 C-4, 126.8 C-5, 115.3 ppm). Proton exchange does not occur in the solid, hence the compounds resemble their crystal structures. Comparison with the corresponding chemical shifts for 1-methylimidazole (137.6, 129.3, 119.7 ppm) implies that the tautomerism has been frozen in the solid state (81CC1207). Solid-state examination of 2,2 -bis-lH-imidazole also reveals frozen tautomerism. 

Crystalline 2-methylimidazole exhibits different 13C (CPMAS) chemical shifts for C-4 and C-5 (125.0, 115.7 ppm). The average (120.3 ppm) is close to that reported for imidazole in deuterated DMSO (121.2 ppm). These results imply that solid state chemical shifts can be used instead of N-methyl models in tautomerism studies (87H(26)333). For imidazole the solid state l3C shifts are 137.6 (C-2), 129.3 (C-4), and 119.7 (C-5) (81JA6011). No proton exchange occurs in the solid, and the data support a structure resembling the crystal structure. Cooling imidazole solutions has not yet allowed the detection of individual tautomers, but by symmetry the compound exists in equal tautomeric forms, as does pyrazole (81CC1207). 

Imidazoles and benzimidazoles unsubstituted on nitrogen exhibit fast prototropic tautomerism which leads to equilibrium mixtures of unsymmetrically substituted compounds, for example, 4(5)-substituted imidazoles, 4(7)- or 5(6)-substituted benzimidazoles. The proton exchange is an intermolecular process (which may involve a protic solvent), and is so fast on the NMR time scale that only one species is usually apparent in the spectrum. iV-Substituted azoles cannot exhibit such tautomerism, and strongly acidic media also inhibit proton exchange. Much work since the mid-1980s has focussed on determination of the proportions of isomers in tautomeric mixtures, with theoretical studies applied to the gas phase (and solution), and solid state studies are also receiving considerable attention. Extrapolation of the results of gas phase and solid state studies to solution, though, can only be less than perfect. 

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