Tautomeric mixture composition

Tautomeric equilibria, elucidation using basicity measurements, 327 Tautomeric mixture composition of, 318 equilibrium position of, 318 influence of external factors on composition of, 318... 

In a neutral azole, the apparent rate of formation of an A-substituted derivative depends on the rate of reaction of a given tautomer and on the tautomeric equilibrium constant. For example, with a 3(5)-substituted pyrazole such as (199), which exists as a mixture of two tautomers (199a) and (199b) in equilibrium, the product composition [(200)]/[(201)] is a function of the rate constants Ha and fcs, as well as of the composition of the tautomeric mixture (Scheme 16) <76AHC(Si)l). 

Our decision also obviates consideration of the additional structural ambiguity arising from the temperature, phase and solvent dependence of the keto vs enol composition of the tautomeric mixture. 

Spectroscopic techniques which do not influence the position of the equilibrium can more certainly be used to ascertain the composition of the equilibrium tautomeric mixture. The presence of significant amounts of... 

In this book, we have tried to put together a number of approaches to these topics, which, on one hand, highlight these problems, and, on the other, try to offer solutions to at least a few of them. In a number of chapters, tools are presented for the experimental and theoretical study of tautomerism absorption in combination with chemometrics to unravel the composition of a tautomeric mixture (Chapter 2) steady-state and time-resolved optical techniques to investigate transfer dynamics (Chapters 3 and 4) the use of NMR to elucidate equilibrium properties... 

If the refractivity of the pure tautomeric constituents is known, the composition of the equilibrium mixture can be determined. This method has been used to study, for example, the keto and enol tautomers of ethyl acetoacetate. So far it has not been applied to heterocyclic compounds in this series the isolation of the pure... 

After succeeding in the asymmetric reductive acylation of ketones, we ventured to see if enol acetates can be used as acyl donors and precursors of ketones at the same time through deacylation and keto-enol tautomerization (Scheme 8). The overall reaction thus corresponds to the asymmetric reduction of enol acetate. For example, 1-phenylvinyl acetate was transformed to (f )-l-phenylethyl acetate by CALB and diruthenium complex 1 in the presence of 2,6-dimethyl-4-heptanol with 89% yield and 98% ee. Molecular hydrogen (1 atm) was almost equally effective for the transformation. A broad range of enol acetates were prepared from ketones and were successfully transformed into their corresponding (7 )-acetates under 1 atm H2 (Table 19). From unsymmetrical aliphatic ketones, enol acetates were obtained as the mixtures of regio- and geometrical isomers. Notably, however, the efficiency of the process was little affected by the isomeric composition of the enol acetates. 

Azo-hydrazone tautomerism Azo-hydrazone tautomerism (83) (Section 58.2.3.2) is a well-established phenomenon in o,o -dihydroxydiarylazo compounds, the various species existing in solution as an equilibrium mixture the composition of which is determined by a number of factors including substituents in the azo compound, the nature of the solvent, etc. It has been invoked to account for the formation of isomeric 2 1 cobalt(III) complexes by symmetrical o,o -dihydroxydi-... 

The mono-thio, -seleno and -telluro analogues of phosphonic and phosphinic acids are tautomeric (equations 1, 2 and 4). In the case of the sulphur-containing compounds, an attempt has been made to estimate the equilibrium compositions of a variety of acids by means of physicochemical measurements. The alkali metal salts of phosphonothioate and phosphinothioate acids possess anions which are mesomeric and, theoretically, it is possible for such ions to act as sources of both O-and S-substituted products. The same products can be obtained through reactions between the free acids and diazoalkanes, and because these reactions are relatively fast, it might be expected that the composition of the product mixtures might be a reflection of the tautomer composition of the acids. 

These spectra can be processed as three-component mixtures, following the philosophy of Eqs. (2.25) and (2.26) [30], which yields the individual spectra of the neutral, mono, and biprotonated forms 6-8, as shown in Figure 2.7a. The spectrum of the neutral form is identical to that of the solution at pH 3.7, which is evidence for the reliability of the computational procedure. In addition, the availability of the molar fractions of the components allows corresponding pfC values to be estimated [47]. However, at this stage nothing can be concluded about the tautomeric ratios in 7 and 8, because they do not depend on the acidity. As a consequence, the individual spectra of the mono and biprotonated forms are constant in the given solvent composition, because no shift in the tautomeric equilibrium can be achieved with acid addition. But if the solvent is changed, the... 

---