Tautomerization reaction

Irradiation of the mixture of E and Z esters (118)/(119) with a 500-W high-pressure mercury lamp gave the two retro compounds (120) and (121), which were separated by chromatography (Ito et al., 1978). Using standard procedures. 

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Fast transient studies are largely focused on elementary kinetic processes in atoms and molecules, i.e., on unimolecular and bimolecular reactions with first and second order kinetics, respectively (although confonnational heterogeneity in macromolecules may lead to the observation of more complicated unimolecular kinetics). Examples of fast thennally activated unimolecular processes include dissociation reactions in molecules as simple as diatomics, and isomerization and tautomerization reactions in polyatomic molecules. A very rough estimate of the minimum time scale required for an elementary unimolecular reaction may be obtained from the Arrhenius expression for the reaction rate constant, k = A. The quantity /cg T//i from transition state theory provides... 

Additional sections of this chapter cover (1) tautomeric reactions of azoles and their derivatives in electronically excited states and (2) stabilization of certain tautomers in their metal complexes. 

The first condition is obviously met for the degenerate tautomeric reactions which were also termed as autotropic rearrangements [76AHC(S1), p. 268]. An illustrative and the most thoroughly studied example of such a tautomeric rearrangement is the interconversion of the degenerate isomers of pyrazole 1 (Scheme 1). 

The general scheme for this type of tautomeric reaction is 310a 310b. 

This retro-aldol-typc fragmentation is possible because the chlorin chromophore stabilizes the anion formed on loss of the a-oxo acid residue. A related reactivity is observed in the reduction of 3-vinylchlorins, e.g. 24, to 3-ethylporphyrins, e.g. 25, in the presence of hydrogen iodide in acetic acid. The mechanism of this reaction can be represented as a sequence of tautomeric reactions which lead to the completely conjugated porphyrin system.32c-40-54... 

In this case, the enol form is not more stable than the keto form (115). The enol form is less stable, and converts to the keto form upon prolonged heating. It can, however, be kept at room temperature for long periods of time because the tautomerization reaction (12-3) is very slow, owing to the electron-withdrawing power of the fluorines. 

Brouillard, R. and Delaporte, B., Chemistry of anthocyanin pigments. 2. Kinetic and thermodynamic study of proton-transfer, hydration, and tautomeric reactions of mal-vidin-3-glucoside, J. Am. Chem. Soc., 99, 8461, 1977. 

Salter LM, Chaban GM (2002) Theoretical study of gas phase tautomerization reaction for the ground and first excited electronic states of adenine. J Phys Chem A 106 4251... 

Another example of resonance stabilization of the conjugate base after proton removal is that of acety-lacetone (2,4-pentadione), which undergoes the tautomerization reaction... 

The 1,3-dipoles were generated by the addition of Et3N in 20% excess. Only imidazole was basic enough to generate a nitrile oxide in the absence of triethy-lamine. Due to prototropic tautomerism, reactions of triazoles and tetrazoles led to mixtures of two isomers. With unsubstituted pyrazole and imidazole only one hydroximoylazole was formed (117). 

This enzymic conversion involves two enzymes, a dehydrogenase and an isomerase. The dehydrogenase component oxidizes the hydroxyl group on pregnenolone to a ketone, and requires the oxidizing agent cofactor NAD+ (see Box 11.2). The isomerase then carries out two tautomerism reactions, enolization to a dienol followed by production of the more stable conjugated ketone. 

This is achieved by two keto-enol tautomerism reactions and a common enol (see Box 10.1). Mechanistically, it is identical to the isomerization of glucose 6-phosphate to fructose 6-phosphate seen earlier in the sequence, so we can move on to the next step of the pathway. 

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 ... 

Substitution of hydrogen with phenyl, as in trifluoro-substituted acetophenone derivative 14h, enhances the rate of Lhe isomerization reaction. In fact, small amounts of 15h have been formed in the preparation of 14h, presumably catalyzed by benzylamine.14 Use of the stronger base l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) accelerates the isomerization and is a very effective catalyst for the tautomerization reaction in the ketimine series.13... 

Four species participate in the tautomerization reaction, the ketone (K, e.g., acetone), the protonated ketone (K ), the enol (E), and its anion (Ee). These species are connected through two thermodynamic cycles. The Gibbs free energies for the individual elementary reactions r of any cycle must add up to naught, Equation (2). 

The addition of buffers is required to maintain constant pH during the reaction when experiments are to be carried out in the range 3 < PH <11. However, keto-enol tautomerization reactions usually exhibit so-called general acid and base catalysis. 26 The observed rate acceleration with increasing buffer concentration implies that the components of the buffer participate in some rate-determining step of the reaction. In most cases, the rate of reaction increases linearly with increasing buffer concentration at constant buffer ratio, chb/cb3 = const (Fig. 4a). 

The six parameters defining the kinetic and thermodynamic properties of tautomerization reactions that have been determined for a representative selection of carbon acids are collected in Table 1. The column headed by the symbol contains the observed pH-independent, uncatalyzed terms, k c=kf +, of the ketonization rate law [Equation (8)]. In general,... 

Scheme 5 Tautomerization reactions of 2-(2, 4 -dinitrobenzyl)pyridine a light-activated proton shuttle.32...

Kcto cnol tautomerization reactions usually exhibit general acid and general base catalysis (section General Acid and General Base Catalysis ). The rate coefficients for general acid catalysis, kHA, determined from a series of buffer dilution plots (Fig. 4) tend to obey a linear log-log relationship to the acidity... 

Using these relations, the rate coefficients for specific and general acid catalysis, Ah and Aha, of any keto-enol tautomeric reaction can be predicted from the appropriate free energy of reaction ArG°. The required... 

Flash photolysis has provided a wealth of kinetic and thermodynamic data for tautomerization reactions. Equilibrium constants of enolization, KE, spanning a range of 30 orders of magnitude, have thereby been determined accurately as the ratio of the rate constants of enolization, kE, and of ketonization, kK. Nowadays, tautomerization constants KE can be predicted with useful... 

These reactions are real tautomerization reactions, but the quite common water elimination reactions can also completely change the redox property of a radical. A case in point is the radical derived from ethylene glycol which is a reducing a-hydroxyalkyl radical which is transformed by water elimination into the fomylmethyl radical (see below) whose oxidizing property has been discussed above [reaction (20)]. Similarly, the phenol OH-adduct is a reason-... 

The mechanism presented here is somewhat at variance with that proposed by the authors (Yamamoto et al. 1995) who suggested that the /BuOI I-derived radical adds to the primarily formed electron-adduct radical. Since this has been shown above to have only a very short lifetime, it will not be capable of undergoing bimolecular recombination reactions. An isomerization of C(8)-H -adduct [reaction (183)] followed by an addition of the tert-butanol-derived radical and water elimination [reactions (184) and (185)] is not in conflict with the above pulse radiolysis results [note that the tautomerization reaction (183) cannot be excluded on the basis of the pulse radiolysis data]. 

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