Kinetic influences on enantioselectivity in asymmetric catalytic hydrogenation

Sun, Yuhang; Wang, Jian; LeBlond, Carl; Reamer, Robert A.; Laquidara, Joseph; Sowa, John R.; Blackmond, Donna G.

doi:10.1016/s0022-328x(97)00165-4

Cited by 32 publications

(36 citation statements)

References 13 publications

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“…However, work by Blackmond and others has highlighted situations in which applied hydrogen pressure and concentration of hydrogen in solution are not always related in a simple way for catalytic hydrogenation reactions 31. 32 Pfalz et al have also noted that H 2 diffusion effects might be important for catalysts formed from their phosphine–oxazolines for cases in which the substrate reacts relatively fast 9. 33 Further, there is evidence that, unlike some rhodium‐catalyzed hydrogenation reactions in which oxidative addition of hydrogen tends to be rate‐limiting, the corresponding reactions of cationic Ir + complexes tend to be fast even at −80 °C 34.…”

Section: Resultsmentioning

confidence: 99%

Mechanistic Insights into Iridium‐Catalyzed Asymmetric Hydrogenation of Dienes

Cui

Fan

Hall

et al. 2005

Chemistry A European J

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Hydrogenation of 2,3-diphenylbutadiene (1) with the chiral carbene-oxazoline-iridium complex C has been studied by means of a combined experimental and computational approach. A detailed kinetic profile of the reaction was obtained with respect to consumption of the substrate and formation of the intermediate half-reduction products, 2,3-diphenylbut-1-ene (2) and the final product, 2,3-diphenylbutane (3). The data generated from these analyses, and from NMR experiments, revealed several facets of the reaction. After a brief induction period (presumably involving reduction of the cyclooctadiene ligand on C), the diene concentration declines in a zero-order process primarily to give monoene intermediates. When all the diene is consumed, the reaction accelerates and compound 3 begins to accumulate. Interestingly, the prevalent enantiomer of the monoene intermediate 2 is converted mostly to meso-3 so the enantioselectivity of the reaction appears to reverse. The reaction seems to be first-order with respect to the catalyst when the catalyst concentration is less than 0.0075 M; diffusion of hydrogen across the gas-liquid interface complicates the analysis at higher catalyst concentrations. Similarly, these diffusion effects complicated measurements of reaction rate versus applied pressure of dihydrogen; other factors like stir speed and flask geometry come into play under some, but not all, the conditions examined. Density functional theory (DFT) calculations, using the PBE method, were used to probe the reaction. These studies indicate a transoid-eta(4)-diene-dihydride complex forms in the first stages of the catalytic cycle. Further reaction requires dissociation of one alkene ligand to give a eta(2)-diene-dihydride-dihydrogen intermediate. A catalytic cycle that features Ir(3+)/Ir(5+) seems to be involved thereafter.

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Section: Resultsmentioning

confidence: 99%

Mechanistic Insights into Iridium‐Catalyzed Asymmetric Hydrogenation of Dienes

Cui

Fan

Hall

et al. 2005

Chemistry A European J

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“…(gas solubility), and [C] = gas concentration in solution at time t. The analysis of gas absorption proceeding exponentially under experimental conditions provides the gas solubility C Ã and the value of k L · a. As a rule of thumb, this value should be approximately ten-fold larger than the rate of a subsequent chemical reaction in order to eliminate diffusion influences on the latter reaction [41].…”

Section: Monitoring Of Hydrogenations Via Hydrogen Consumptionmentioning

confidence: 99%

Kinetics of Homogeneous Hydrogenations: Measurement and Interpretation

Drexler¹,

Preetz²,

Schmidt³

et al. 2006

The Handbook of Homogeneous Hydrogenation

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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

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“…Ž . over binap-Ru II Sun et al, 1997 . Impact of mixing on autocatalytic and biological systems was reviewed by Epstein Ž .…”

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confidence: 99%