NH tautomerism

A multiply bonded nitrogen atom deactivates carbon atoms a or y to it toward electrophilic attack thus initial substitution in 1,2- and 1,3-dihetero compounds should be as shown in structures (110) and (111). Pyrazoles (110 Z = NH), isoxazoles (110 Z = 0), isothiazoles (110 Z = S), imidazoles (111 Z = NH, tautomerism can make the 4- and 5-positions equivalent) and thiazoles (111 Z = S) do indeed undergo electrophilic substitution as expected. Little is known of the electrophilic substitution reactions of oxazoles (111 Z = O) and compounds containing three or more heteroatoms in one ring. Deactivation of the 4-position in 1,3-dihetero compounds (111) is less effective because of considerable double bond fixation (cf. Sections 4.01.3.2.1 and 4.02.3.1.7), and if the 5-position of imidazoles or thiazoles is blocked, substitution can occur in the 4-position (112). 

Recently, zero kinetic energy (ZEKE) photoelectron spectroscopy has been used to study the OH/NH tautomerism of 2-pyridone in the gas phase (95JPC8608). This work, which is expected to develop considerably, provides a wealth of information about that equilibrium for the states So, Sj, and Do (cation ground state). 

Smith et al determined the activation barrier for H2OETNP by NMR in CD2CI2 at 300 MHz (ZIGI73 = 55.2 kJ mor )(94JA3261) and found it to be similar to that measured for NH tautomerism in other free-base dodeca-substituted porphyrins (90JA8851, 92JA9859, 93IC1716). Finnish authors have reported a detailed study of the tautomerism of a natural chlorin. 

All the known porphyrin isomers are typical benzoid aromatic compounds which show distinctly porphyrin-like characteristic electronic absorption spectra.13 Also the complexation properties for metal ions, NH tautomerism and the NMR spectra are quite similar to the parent porphyrin structure. 

In symmetrical porphyrins NH-tautomers do not distinguish in structure and have identical electronic spectra. Correspondingly, in most cases the spectral separation between the individual tautomers (a E) does not exceed 100 cm l and is due to crystalline matrix effect (6,7). Therefore, NH-tautomerism can be effectively detected at 77 K and 4.2 K in the Shpol skii matrix only at selective monochromatic excitation (5-7). In chlorins, the... 

Up to now the four-orbital model-based theory of porphyrin absorption electronic spectra ( ) has not taken into account the influence of NH-tautomerism in non-symmetrical porphyrins on the positions and intensities of electronic transitions in the visible region. The theoretical consideration of this problem involves solving the fundamental question of the absolute orientation of electronic transition oscillators for each tautomer. First of all. 

Near IR spectra—See Electronic absorption spectra New IR spectra, R sphaeroides reaction centers, 207,208f NH-tautomer structure, porphyrins with nonsymmetrical substitutions, 84 NH-tautomerism, porphyrins with nonsymmetrical substitution, 74-93,89-91 Ni(II)-reconstituted hemoglobin—See Nickel-reconstituted heme proteins Nickel isobacteriochlorin... 

Four NH-tautomeric forms are possible for pyrazolo[3,4-d]pyrimidine (3-6). In the solid state as well as in neutral media this compound was shown to exist as 3. In basic media, the mesomeric anion 300 is the predominant species. In acidic solutions, 301 predominates as the monocation (58JCS2973). 

A multiply bonded nitrogen atom deactivates carbon atoms a or t to it toward electrophilic attack thus initial substitution in 1,2- and 1,3-dihetero compounds should be as shown in structures (136) and (137). Pyrazoles (136 Z=NH), isoxazoles (136 Z = 0), isothiazoles (136 Z=S), imidazoles (137 Z=NH, tautomerism can make the 4- and 5-positions equivalent) and thiazoles (137 Z=S) do indeed... 

Apart from the obvious use of proton NMR spectroscopy as an aid in structure elucidation, the technique has also been used to probe the phenomena of intermolecular aggregation, apical ligand binding, NH tautomerism, rates of protonation, and a host of theoretical aspects. An extensive review of porphyrin and chlorin NMR spectroscopy has appeared... 

Purine can form four NH tautomeric forms depending on the site of attachment of the proton at the ring nitrogens (A-D). The CH tautomers are of minor importance. Different tautomeric forms can be observed in the gas phase, in solution, or in the solid slate. According to quantum mechanical calculations, the H- and the 3/T-taulomers A and B are much less stable than the IH- and the 9//-tautomers C and D, respectively. Only the latter are detected in solution. 

AMI semiempirical calculations have shown that, as far as tautomerism is concerned, there is a structural relationship between jS-dicarbonyl compounds and NH-pyrazoles, and in a wide variety of NH-pyrazoles studied " the most stable tautomer was found to be that having the largest single-bond character between the C(3)—C(4) bond. The problem of proton transfer in NH-pyrazole crystals has been subjected to a detailed theoretical study, while a study of the tautomerism of 2-aryl and 2-heteroaryl derivatives of benzimidazole has indicated that tautomerism takes place by the intermolecular relay of protons between stacked molecules. The first report of the stable co-existence of two different histidine tautomers in one peptide crystal structure has appeared. Ab initio calculations have been used to study the tautomerism of both histamine and pyrazolo[3,4-i/]pyridazine in the gas phase and in aqueous solution, and a theoretical study of the NH tautomerism in free-base porphyrin has been undertaken. ... 

Figure 4.3.5 The three NH-tautomeric forms observed for macrocycle 4.143a...

Ab initio and density functional theory (DET) methods have been exploited to determine the structures and the interaction energies of 2/7-isoxazol-5-one B, and its dimer and trimer structures in the gas phase. For the cyclic trimer, the computed structural parameters resulted in excellent agreement with the X-ray determination of the supramolecular aggregate of 4-(2-methoxybenzyl)-3-phenyl-4//-isoxazol-5-one, involving very strong intermolecular H-bonds of the NH tautomeric form, interpreted in terms of the RAHB (resonance-assisted hydrogen bond) model (see Section 4.03.3.1) <2002HCA2364>. 

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