From Dynamics to Kinetics: Investigation of Interconverting Stereoisomers and Catalyzed Reactions

Abstract

The integration of chemical reactions and chromatographic separations into a single process offers many advantages. In particular, the reactants and products of the reactions can be separated and, in the case of reversible reactions, there is the possibility to shift the chemical equilibrium to the favored side. This integrated experimental set-up not only allows the simultaneous study of the reaction kinetics, but also yields valuable data on adsorption as well as parameters of diffusion. The contributions of diffusion processes to the overall reaction rate constant can be determined by the measurement of van Deemter data [1][2][3][4][5][6]. The model of retention increments [7] allows selectivity differences to be revealed, which is of interest in the optimization of enantioselective processes.By using this method, reaction rate constants and the kinetic activation parameters of reversible first-order reactions have been determined for stereoisomers with enantioselective dynamic chromatography [8][9][10]. The rate constants of the interconversion processes were obtained by an iterative comparison of experimental and simulated chromatograms [11][12][13][14], though this was computationally expensive and a major drawback of the technique. As a result, only a limited number of research groups have utilized dynamic chromatography, despite its overwhelming advantages that include the use of standard experimental set-ups, the minute analyte consumption, the use of arbitrary mixtures of interconverting stereoisomers, the lack of previous analyte purification, and the precise determination of kinetic and thermodynamic parameters.The term dynamic gas chromatography, in analogy to the term dynamic NMR (DNMR) [15], stresses the dynamic [16] behavior of analytes to interconvert between two stereoisomeric forms, or even between two constitutional isomers, which typically are in equilibrium. To achieve a quantitative separation of enantiomers, the prerequisite is the chiral recognition between the racemic selectand and the Modeling of Molecular Properties, First Edition. Edited by Peter Comba.