Kinetics of tautomerization

Computer Modeling of the Kinetics of Tautomerization (Mutarotation) of Aldoses Implications for the Mechanism of the Process... 

Magnesium bis(hexamethyldisilazide), Mg(HMDS)2, catalyses the enolization of ketones.287 On addition to propiophenone in toluene at ambient temperature, a ca 3 1 E Z mixture of enolates (103, R=SiMe3) is formed. These enolates, and an initial ketone complex, have been characterized by NMR, X-ray, IR, and UV-visible spectroscopy and computational studies. Kinetics of tautomerization have been measured, with proton transfer confirmed as rate determining ( hAd = 18.9 at 295 K). The significant temperature dependence of the primary isotope effect is indicative of tunnelling. 

As ivill be demonstrated below, these differences in the cavity shape and distances have a huge impact on both the thermodynamics and kinetics of tautomerization. Moreover, it has been shown that the porphycene cavity dimensions are very sensitive to peripheral substituents, even such mild ones as alkyl groups. For variously alkylated porphycenes, the N-N distances can be varied from an extremely small value of 2.53 A, resembling that in proton sponges [35] to 2.80 A, close to the value for porphyrin. This provides a unique opportunity to study distance dependences in a class of very similar molecules. The material presented in this chapter will often be based on such comparisons and on the relationship with... 

The study of prototropic tautomerization is intimately related to the study of proton transfer reactions. The study of the dynamics of proton transfer is as old as the study of reaction kinetics itself Indeed, the first reactions studied, that is, the inversion of sugar by Wilhehny in 1850 [65], involves a proton transfer as the elementary step in the reaction. In the first studies on the dynamics of tautomerization, primarily keto-enol tautomerization in acetone-like compounds were studied, which is a slow process involving a number of reaction steps of which the acid catalyzed keto-enol conversion was taken as the rate determining one [66]. In the past century, since 1910, nearly 2000 papers have been published on the kinetics of tautomerization, and in the first 60 years most of those were devoted to the ground-state reactions of the keto-enol type involving a C atom. Until the mid-1950s, only a handful of papers can be found this was obviously due to experimental hmitations. Two things are needed a method to start the reaction, and a method to follow it. In Dawson s experiments [66], the rate could be influenced by the amount of acid present, and the reaction could be followed because the enol produced... 

Fluorescence techniques are especially suitable for studying NH tautomerism since they offer a variety of experimental approaches in which the emitting reactants and/or products can be monitored, either in steady state or in a time-resolved maimer. These methods carry information about both the photophysical properties of a fluorescing molecule and the kinetics of tautomerization processes. Fluorescence studies reveal uUrahigh sensitivity to reporting molecules, high speed of response, and high spatial resolution. 

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