Kinetische Untersuchungen zur Ligandenhydrierung in Katalysatorvorstufen für die asymmetrische Reduktion prochiraler Olefine
TL;DR: In this paper, a comparison of chiral rhodium(I) complex catalysts of the type [Rh(L)PP*]A (L = cyclic diolefin; PP* = chiral bis(phosphane) and A = anion like BF) regarding their hydrogenating activity against prochiral substrates was made.
Abstract: Kinetic Investigations of the Hydrogenation of Ligands in Catalyst Precursors for Asymmetric Reduction of Prochiral Olefines1
The comparison of chiral rhodium(I) complex catalysts of the type [Rh(L)PP*]A (L = cyclic diolefin; PP* = chiral bis(phosphane) and A = anion like BF) regarding their hydrogenating activity against prochiral substrates is hampered by the induction period in the hydrogen consumption which can be attributed to the formation of the catalytically active species from the diolefin precatalyst. The competing hydrogenation of diolefin [e.g. cis,cis-cycloocta-1,5-diene (COD), norborna-2,5-diene (NBD)] and prochiral substrate may cause a maximum of the hydrogenation rate, which is characteristic for different catalyst/substrate/solvent systems. Michaelis-Menten rate constants for the hydrogenation of COD and NBD were determined for different chiral catalysts by stoichiometric and catalytic hydrogenations. The rate constants differ for one selected diolefin up to a factor of 40. The hydrogenation of NBD is generally 5–8 times faster than the COD hydrogenation. In the special case of precatalysts based on he-xopyranoside bis(phosphinites) the rate constants for the NBD hydrogenation in comparison with those for COD are higher up to a factor of 48. In some cases the hydrogenation of the mono-ene is faster than that of the diolefin (e.g. COD, NBD). The high hydrogenation rate reported in the literature for some precatalysts in the asymmetric hydrogenation of prochiral olefins is caused in part by the relatively fast diolefin hydrogenation which facilitates the complete formation of the catalyst. The influence of substrate, solvent, and some experimental conditions on the induction period will be discussed.
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TL;DR: In this paper, it was shown that a relation between the sense of rotation of the diolefin in the precatalyst (clockwise or anticlockwise twist) is not very likely.
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TL;DR: The aim of this work is to quantify the hydrogenation of the diolefins cyclooctadiene (cod) and norborna-2,5-diene (nbd) for cationic complexes of the type [Rh(ligand)(diolefin)]BF(4) for the ligands Binap (1,1'-binaphthalene-1,2'-diylbis(phenylphosphine), Me-Duphos
Abstract: The use of diolefin-containing rhodium precatalysts leads to induction periods in asymmetric hydrogenation of prochiral olefins. Consequently, the reaction rate increases in the beginning. The induction period is caused by the fact that some of the catalyst is blocked by the diolefin and thus not available for hydrogenation of the prochiral olefin. Therefore, the maximum reaction rate cannot be reached initially. Due to the relatively slow hydrogenation of cyclooctadiene (cod) the share of active catalysts increases at first, and this leads to typical induction periods. The aim of this work is to quantify the hydrogenation of the diolefins cyclooctadiene (cod) and norborna-2,5-diene (nbd) for cationic complexes of the type [Rh(ligand)(diolefin)]BF(4) for the ligands Binap (1,1'-binaphthalene-2,2'-diylbis(phenylphosphine)), Me-Duphos (1,2-bis(2,5-dimethylphospholano)benzene, and Catasium in the solvents methanol, THF, and propylene carbonate. Furthermore, an approach is presented to determine the desired rate constant and the resulting respective pre-hydrogenation time from stoichiometric hydrogenations of the diolefin complexes via UV/Vis spectroscopy. This method is especially useful for very slow diolefin hydrogenations (e.g., cod hydrogenation with the ligands Me-Duphos, Et-Duphos (1,2-bis(2,5-diethylphospholano)benzene), and dppe (1,2-bis(diphenylphosphino)ethane).
57 citations
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TL;DR: In this paper, the steady-state concentration of the COD complex under hydrogen is higher than that of the NBD complex under argon, and the complete reversion of the thermodynamically determined ratios of COD to NBD under hydrogenation conditions was proven by means of UV/Vis spectroscopy.
Abstract: Asymmetric hydrogenations of prochiral olefines by means of chiral rhodium(I) complexes of the type [Rh(L)(PP*)]A (L = COD, [(Z,Z)-cycloocta-1,5-diene], or NBD (norborna-2;5-diene), PP* = chiral bisphosphane forming seven-membered chelate rings, A = anion like BF4−) are often associated with induction periods caused by partial blocking of the catalyst. NBD complexes are hydrogenated faster than the corresponding COD complexes. Catalytic hydrogenation of COD/NBD mixtures and the determination of the ratio of the Michaelis constants showed that the steady-state concentration of the COD complex under hydrogen is higher than that of the NBD complex. However, under argon the NBD complex predominates owing to its higher thermodynamic stability compared with that of the COD complex as determined by 31P-NMR spectoscopy. This complete reversion of the ther-modynamically determined ratios of COD to NBD complex concentration under hydrogenation conditions was proven by means of UV/Vis spectroscopy.
39 citations
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TL;DR: In this paper, an overview of possible activation and deactivation phenomena in homogeneous catalytic processes promoted by different types of rhodium complexes containing diphosphine ligands is provided.
33 citations
References
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