Azaindoles, tautomerism

Pyrrolo[2,3-c]pyridines — see also 6-Azaindoles reaction with O-mesitylhydroxylamine, 4, 508 synthesis, 4, 516, 522 tautomerism, 4, 502 UV spectra, 4, 501... 

At its best, the study of solvent kies by the formalism given can be used to learn about proton content and activation in the transition state. For this reason it is known as the proton inventory technique. The kinetics of decay of the lowest-energy electronic excited state of 7-azaindole illustrates the technique.25 Laser flash photolysis techniques (Section 11.6) were used to evaluate the rate constant for this very fast reaction. From the results it was suggested that, in alcohol, a double-proton tautomerism was mediated by a single molecule of solvent such that only two protons are involved in the transition state. In water, on the other hand, the excited state tautomerism is frustrated such that two water molecules may play separate roles. Diagrams for possible transition states that can be suggested from the data are shown, where of course any of the H s might be D s. 

Mentus S, Maroncelli M (1998) Solvation and the excited-state tautomerization of 7-azaindole and 1-azacarbazole computer simulations in water and alcohol solvents. J Phys Chem A 102 3860-3876... 

Recently, we reported observations of the femtosecond dynamics of tautomerization in model base pairs (7-azaindole dimers) containing two hydrogen bonds. Because of the femtosecond resolution of proton motions, we were able to examine the cooperativity of formation of the tautomer (in... 

B. Kohler 1 would like to ask two questions to Prof. Zewail. First, in your investigation of the electron transfer reaction in a benzene- complex, the sample trajectory calculations you showed appear to suggest that the charge transfer step may induce vibrationally coherent motion in h-. Have you tested this possibility experimentally My second question concerns your intriguing results on a tautomerization reaction in a model base-pair system. In many of the barrierless chemical reactions you have studied, you have been able to show that an initial coherence created in the reactant molecules is often observable in the products. In the case of the 7-azaindole dimer system your measurements indicate that reaction proceeds quite slowly on the time scale of vibrational motions (such as the N—H stretch) that are coupled to the reaction coordinate. What role do you think coherent motion might play in reactions such as this one that have a barrier ... 

The pyrrolopyridines contain one nitrogen heteroatom in the five-membered pyrrole ring and one nitrogen atom in the six-membered pyridine ring. A more common name is azaindoles, with 7-azaindole (l//-pyrrolo[2,3-6]pyridine (1)) being the most widely studied isomer of the series. Twelve isomers of pyrrolopyridines are listed in the Chemical Abstracts Ring Formula Index. Table 1 lists the pyrrolopyridines with their Chemical Abstracts Registry Numbers. The 1//-pyrrolopyridines (l)-(6) are the most frequently prepared, but some work has also been carried out on the isomers (7), (8), and (10). Isomers (8), (9), (11), and (12) are tautomeric structures which are considered later (see Section 7.06.4.4). 

The structures of the azaindoles were discussed in terms of tt-electron density calculations in Section IV. Tautomeric forms of the azaindoles will be discussed under the various physical methods below. 

The tautomeric 6-hydroxy-7-azaindole shows a shift to longer wavelength, which is characteristic of 2-hydroxypyridine as the neutral molecule or of l-methyl-2-pyridone. The degree of tauto-merization is solvent-dependent, and Yakhontov et have determined the lactam/lactim R = Me) ratio from the position of... 

Some experimental evidence for a DPT process in the excited state leading to tautomerization not observed in the ground state has been reported (Chang et al., 1980 Sepiol and Wild, 1982) for solutions of 1-azacarbazole in 3-methylpentane and mixed l-azacarbazole/7-azaindol solutions in the same solvent. For these systems the tautomeric forms that are unstable in the ground state are formed adiabatically from the excited state hydrogen-bonded dimers via a DPT and detected in the fluorescence spectra. These results are especially relevant to excited state DPT processes possible for nucleic acid base pairs. Clearly, further experimental and theoretical studies on heterocyclic dimer systems are needed. 

To illustrate the relative efficiency of long and short proton conduits, we return to the 7-azaindole, molecule, whose dimer was briefly discussed in Section 29.7. As pointed out in Section 29.7, excitation of this molecule to the fluorescent state redistributes the charge between the two nitrogens. This redistribution renders the tautomeric form produced by the excitation unstable relative to the form in... 

Figure 29.18 Calculated structures of the stationary points along the reaction coordinate for the excited state tautomerization in 7-azaindole complexes with one, two and five water molecules [17].

Mishra and coworkers have reported spectral oscillation in the absorption spectra of adenine solution in water when air was passed through the solution and exposed to UV-radiation intermittently over a long period of time. The main absorption peak near 260 nm (4.77 eV) was shifted to near 300 nm (4.13 eV) region which was explained in terms of the formation of the N3H tautomer of adenine,the process of which is closely related to the tautomerization of 7-azaindole. This assertian (the formation of the N3H tautomer in the excited state) is also supported fixim a recent theoretical study of adenine tautomers at the MCQDPT2/CASSCF levels of theory. ... 

Pronm tautomerization 7-Azaindole, 3 iydroxyfiavoae AiN+ir AfNir AcridhK 5.1 10.6 P... 

Fig. 52 Optimized geometries of the 7-azaindole homodimer in So, Si (normal species) and Si (tautomer) states The double proton exchange path leading to tautomeric conversion is denoted by arrows,Reprinted from M, Mukherjee, B. Bandyopadhyay and T. Chakraborty Ultraviolet and infrared spectroscopy of matrix-isolated 7-azaindole dimer Matrix effect on excited state tautomerization, Chem. Phys. Lett., 2012. 546. 74-79. Copyright (2012), with permission from Elsevier,...

Keywords On-the-lly dynamics simulation Excited-state proton transfer Excited-state tautomerization Water-assisted proton transfer Hydrogen bond rearrangement 7-Azaindole ADC(2)... 

Figure 20-11. Hydrogen-bonded complex of 7-azaindole with ethanol and its tautomeric form,...

Figure 3.9 Doubly hydrogen-bonded dimers capable of double proton NH---N tautomerization 7-azaindole (5) [2, 38, 39], 2-aminopyridine (6) [44], 1-azacarbazole (13) [40-42], lH-pyrazolo[3,4-b]quinoline (14) [43], and H-propyl-6H-indolo-[2,3-b]quinoline (15) [44].

Figure 3.10 Role of conjugation of extra phenyl rings in the excited-state tautomerization of 7-azaindole dimers (16 and 17) [48],...

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