189518-59-0Relevant academic research and scientific papers
New low-temperature NMR studies establish the presence of a second equatorial-apical isomer of [(R, S)-binaphos](CO)2RhH
Castillo Molina, Dante A.,Casey, Charles P.,Mueller, Imke,Nozaki, Kyoko,Jaekel, Christoph
, p. 3362 - 3367 (2010)
The important enantioselective hydroformylation catalyst [(R,S)-Binaphos](CO)2RhH has been reexamained by low-temperature NMR spectroscopy. Both 1H and 31P NMR spectroscopy at -90 °C allow direct observation of a mixture of two apical-equatorial chelates. The major chelate, 1eq,ap, was shown to have an equatorial phosphine and an apical phosphite. Its structure was unambiguously assigned using low-temperature 31P NMR spectroscopy with selective decoupling of aromatic hydrogens but not of the rhodium hydride, which showed a 225 Hz trans phosphite to RhH coupling. The equilibrium constant for [1eq,ap]/ [1ap,eq] was determined over a wide temperature range.
High-pressure IR studies on the asymmetric hydroformylation of styrene catalyzed by Rh(I)-(R,S)-BINAPHOS
Nozaki, Kyoko,Matsuo, Takeshi,Shibahara, Fumitoshi,Hiyama, Tamejiro
, p. 594 - 600 (2008/10/08)
Rhodium-catalyzed asymmetric hydroformylation of styrene using (R,S)-BINAPHOS (1a) and its methoxy-substituted derivative 1b as chiral ligands was monitored by in situ high-pressure IR. The rate of aldehyde production is given by robs = kobs[Rh]1.0[styrene]0.6[1b]-0.1 PCO-0.9 (kobs is a constant under isothermal conditions). The higher catalytic activity of the Rh-1b catalyst compared with that of Rh-1a is attributed to a higher concentration of the active species, which is generated by dissociation of CO from the major resting state RhH(CO)2(1) (2). A rhodium carbonyl species 10, the structure of which is unknown, is suggested to be formed from the rhodium hydride RhH(CO)2(1) (2), especially when the rhodium concentration is high. Complex 10 is less active for hydroformylation and is probably responsible for the loss of %ee observed at high rhodium concentrations.
New chiral phosphine-phosphite ligands in the enantioselective rhodium-catalyzed hydroformylation of styrene
Deerenberg, Sirik,Kamer, Paul C. J.,Van Leeuwen, Piet W. N. M.
, p. 2065 - 2072 (2008/10/08)
A series of chiral phosphine-phosphite ligands 1a-g have been synthesized from monophosphines 2-4, enantiomerically pure propene oxide or styrene oxide, and 3,3′,5,5′-tetra(tert-butyl)-2,2′-bisphenol phosphorochloridite or enantiomerically pure 3,3′-bis(tri> methylsilyl)-2,2′-binaphthol phosphorochloridites. These phosphine-phosphites have been used in the rhodium-catalyzed asymmetric hydroformylation of styrene. The structures of the active catalysts, [HRh(L-L)(CO)2] complexes (L-L = ligands 1a-g), have been studied using high-pressure NMR and IR spectroscopy. The obtained spectroscopic data show that the ligands coordinate in an equatorial-apical fashion to the rhodium center with the phosphine in apical position. Systematic variation in configuration of the stereocenters at both the ligand bridge and the phosphine moiety revealed a remarkable cooperative effect on the selectivity of the hydroformylation reaction. Under mild reaction conditions ee's of 63% and regioselectivities up to 92% toward 2-phenylpropanal were obtained (25-60°C, 20 bar of syn gas CO:H2 [1:1]) for ligands 1. The absolute configuration of the product is governed by the stereogenic center of the backbone of the ligand. There is a large cooperative effect, however, from the phosphine moiety. Spectroscopic data, in combination with the obtained results in catalysis, suggest that phosphine-phosphite ligands (L-L) containing the conformationally flexible and axially chiral biphenyl moiety exist predominantly as single atropisomers in the HRh(L-L)(CO)2 complexes. Comparison of the bisphenol and binaphthol substituents suggests that the high enantiomeric excesses obtained with the former are caused by the preferential formation of the most selective diastereomer.
