Tautomerism kinetic investigations

Kinetic investigation of the reaction of cotarnine and a few aromatic aldehydes (iV-methylcotarnine, m-nitrobenzaldehyde) with hydrogen eyanide in anhydrous tetrahydrofuran showed such differences in the kinetic and thermodynamic parameters for cotarnine compared to those for the aldehydes, and also in the effect of catalysts, so that the possibility that cotarnine was reacting in the hypothetical amino-aldehyde form could be completely eliminated. Even if the amino-aldehyde form is present in concentrations under the limit of spectroscopic detection, then it still certainly plays no pfi,rt in the chemical reactions. This is also expected by Kabachnik s conclusions for the reactions of tautomeric systems where the equilibrium is very predominantly on one side. 

The 21//,22//-tautomer 2 with hydrogens at adjacent pyrrole rings is less stable because of penetration of each hydrogen into the van der Waals sphere of the other. However, NMR studies with unsymmetrically substituted porphyrins at low temperature have allowed the observation of both tautomers 1 and 2. The kinetic parameters of tautomerism investigated by NMR measurements at different temperatures are consistent with a two-step process forming 3 from 1 via 2 rather than a concerted two-hydrogen shift which could form 3 from 1 directly. 

An important feature of saturated oxazolo[3,4- ]pyridines is their easy epimerization at the aminal C-l stereocenter. A quite explicit example has been reported by Moloney et al. and is depicted in Scheme 46. The reaction between lactam 157 and benzaldehyde produces a mixture of hexahydro-oxazolo[3,4- ]pyridines, the kinetic product 158 being the major one. Equilibration of the mixture with boric acid allows the ratio of diasteroisomers to be reversed since /rarcr-oxazolidine 159 is now the major product <1998TL1025> the equilibration of epimeric oxazolidines via ring-chain tautomerism has been investigated in detail and explains the epimerization observed for some hexahydro-oxazolo[3,4-4]pyridines <1993JOC1967>. 

The kinetics and mechanism of pyrrole pyrolysis were investigated by ab initio quantum-chemical calculations. It was revealed that pyrrole undergoes tautomerization to form 2H- and 37/-pyrroles prior to any thermal decomposition. It has been shown that the major product, HCN, arises from a hydrogen migration in pyrrole to form a cyclic carbene with the NH bond intact. Ring scission of the carbene leads to an allenic imine of HCN and propyne which is the lowest energy pathway. The 277-pyrrole... 

Despite the fact that the concept of pseudobase formation has entered the folklore of heterocyclic chemistry, 15 the example presented by most heterocyclic chemistry texts of a reaction involving this phenomenon has recently been shown to be incorrect (Section V,C). It is only recently that systematic quantitative studies on pseudobase stability have been attempted or that the kinetics of pseudobase-cation equilibration have been investigated in detail. Thus, the current review covers a period in which extensive quantitative studies of pseudobase formation have become available. In addition, work in this area since Beke s earlier review4 has been characterized by the detailed application of spectroscopic techniques to the determination of pseudobase structure and tautomerism. 

Second, careful analysis of anisotropy data is useful not only with respect to investigation of structural and kinetic aspects of tautomerization, but also as a means to obtain detailed spectroscopic information about transition moment directions. Both procedures will be described below in more detail. 

Nanosecond and picosecond photochemical kinetics of quinoid fluorescence produced by excitation of the enol form salicylideaniline have been investigated in various environments.An excited-state tautomeric proton transfer occurs within 5 ps at temperatures above 4 K in both protic and aprotic solvents. 

The differentiation of anomeric acyclonucleosides by FAB tandem mass spectrometry was reported (90MI1 91MI10). Regioisomeric compounds could be differentiated by kinetic energy release measurements [87-AQ(C)271 91RCM72]. Electronic structure, spectral properties, and acid-base and tautomeric equilibria of some adenosine acyclo derivatives were investigated (89MI5). 

The next stage of oxidation for organic compounds is to aldehydes and ketones. In acid solutions, aldehydes and ketones are often hydrolyzed to produce ketohydrates (two OH groups attached to the same carbon atom). They also undergo keto-enol tautomerism to convert into more acidic species which are then more capable of forming ionic bonds with metal ions. The principal kinetic question is whether the keto, enol, or ketohydrate form is the one involved in the oxidation reaction. The results of the most complete investigations of ceric oxidation of ketones and aldehydes are contained in table 4. 

The commonest use of n.m.r. in chemistry is to decide whether certain groups are present or absent. Hence it is much used in finding the constitution of newly synthesized or isolated substances. It is also valuable in investigating potentially tautomeric molecules. When the tautomeric equilibrium is reached slowly (as usually occurs when the mobile proton is attached to a carbon atom in one tautomeric form), signals from both tautomeric forms are recorded independently, and a kinetic study may be made. The equilibrium is reached almost instantaneously when the mobile proton is attached to a nitrogen or oxygen atom. 

The thermodynamic and kinetic parameters of the stepwise hydration of 1-methylcytosine and its imino-oxo tautomer in the presence of the Na" " cation have been investigated [139]. Hydrationof 1-methylcytosineby one water molecule leads to an increase of the concentration of its imino-oxo tautomer in the equilibrium mixture and decrease of the barrier of the tatutomer formation (to 15.6 kcal/mol). If the sodium cation is present the tautomeric form is much less favored and tautomerization barrier increases to 25.2 kcal/mol. The computationally predicted values of the rate constants suggest that the tautomerization of 1-methylcytosine to its imino-oxo form proceeds mainly due to a presence of the hydrated (MeCW) species. Based on the kinetic analysis of the tautomerization process in hydrated MeC in the presence of sodium ions it was concluded that complexes of hydrated MeC with Na+ are unlikely to contribute to the frequency of DNA point mutations caused by the tautomers. This is due to the fact that the interactions with Na" " lead to a decrease of both the rate and the equilibrium constants of the tautomerization reactions in hydrated 1-methylcytosine. 

The tautomeric equilibria of aldopentoses, aldohexoses and hexuloses in DMSO have been examined by C-n.m.r. spectroscopy the mutarotation kinetics of aqueous D-glucose have been investigated by i.r. methods, and Raman spectroscopic data on the tautomeric transformations of glucose in aqueous solution over a wide pH range confirmed that the ring-chain tautomerism depends on pH." The ring shapes of the two fiiranose and two pyranose forms of psicose in aqueous solution, riiown by n.m.r. spectroscopy to be present in almost equal proportions, have been analyzed by MM3."... 

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