Kinetics of Homogeneous Hydrogenations: Measurement and Interpretation

Abstract

IntroductionRecently, the results of kinetic measurements have been summarized for homogeneous hydrogenations with transition metal complexes in a review [1]. Essential new results of kinetic investigations leading to the completion of hitherto existing ideas regarding the reaction mechanism of particular catalyses are represented in the respective chapters of this book, and shall not be repeated here. Rather, this chapter will introduce the kinetic treatment of reaction sequences with pre-equilibria typical for catalyses, together with the analysis and interpretation of Michaelis-Menten kinetics, the monitoring of hydrogenations, and a discussion of possible problems, with selected examples.Kinetic investigations deliver quantitative correlations regarding the concentration-time dependence of the participating reactants, and therefore serve as the major methodical approach in the elucidation of reaction mechanisms. A knowledge of funded mechanistic ideas opens the possibility of an aimed manipulation of activity and selectivity, respectively, which are important parameters of catalyses. As "operating values", pressure and temperature -as well as the concentration of particular reaction partners -are available. However, when scaling-up from a laboratory standard to an industrial application, kinetic results are indispensable. Moreover, kinetics provides essential indications about the nature of the actual catalyst and the distinction between homogeneous and heterogeneous catalysis. This objective has been investigated more intensively during the past few years, partly with surprising results, and naturally plays an important role when transition-metal complexes meet with hydrogen as reducing agent [2]. Thereby, the problem does not lie in the kinetics as the method. ("In the kinetic approach no frontiers exist today between homogeneous, enzymatic, and heterogeneous catalysis. There is a consistent science which permits the definition of useful and efficient rate laws describing sequences of elementary steps." [3])In spite of these capabilities of kinetics it is necessary to emphasize here that, in principle, it is not possible to prove that a reaction mechanism occurs only by 257The Handbook of Homogeneous Hydrogenation. Edited by J. G. de Vries and C. J. Elsevier

Extensive Re‐Investigations of Pressure Effects in Rhodium‐ Catalyzed Asymmetric Hydrogenations

“…To proceed further, we must accept here the assumptions of the steady-state approximation. The risk of drawing erroneous conclusions is, however, small since the steadystate assumption was the basis of Halperns work and others and is well verified for Rh-catalyzed asymmetric hydrogenations, [6,8] as long as the full derived rate laws are considered, without simplification.…”

“…Since these early works, many examples of hydrogen pressure effects have been reported. [6] The early work of Brown and Halpern has also lead to an abundant literature in the field of asymmetric hydrogenations, including mechanistic, [7] kinetic, [8] theoretical calculations, [9] and chemical engineering aspects. [10] Interestingly, while many of these researches have considerably improved our understanding of the mechanism at the molecular level, none have tried to systematically assess the generality of hydrogen pressure effect on ee.…”

Iridium‐Catalysed Asymmetric Hydrogenation of Enamides in the Presence of 3,3′‐Substituted H8‐Phosphoramidites

“…Here, we monitored closely the activity of the reduction with different catalyst loadings. [16] Among the highly enantioselective ligands, the pMeO-phenyl-substituted phosphoramidite 1c presented the highest TOF. Notably, the addition of salts like NaClO 4 or NaBF 4 improves the catalytic activity (average TOF estimated from the consumption curve: 22 h…”

Activation, Deactivation and Reversibility Phenomena in Homogeneous Catalysis: A Showcase Based on the Chemistry of Rhodium/Phosphine Catalysts

“…In principle, it is possible to track either the stoichiometric hydrogenation (by measuring the hydrogen uptake [99] using 31 P NMR [100] or UV-Vis spectroscopy [101]) or the catalytic hydrogenation with an automatic recording device [102,103] in the presence of an excess of the diolefin [104] under several pressure regimes [105]. It is likewise possible by means of 1 H NMR spectroscopy to assess the H 2 concentration in solution during diolefin hydrogenation under isochoric conditions [106,107].…”