Probing the Mechanisms of Enantioselective Hydrogenation of Simple Olefins with Chiral Rhodium Catalysts in the Presence of Anions

Abstract

The strong influence of various anions upon the hydrogenation of 2-phenyl-1-butene, catalyzed by chiral rhodium catalysts was investigated. Both sulfonates and halides exert large increases in the enantioselectivity when [Rh[(-)-bdpp](NBD)]ClO4 (bdpp = 2,4-bis(diphenylphosphino)pentane, NBD = 2,5-norbornadiene) is used as the catalyst precursor at high pressures (70 atm) of dihydrogen in nonpolar solvents. A dihydride mechanism similar to that for Wilkinson's catalyst [RhCl(PPh3)3] was shown to be operating at both high- and low-pressure conditions through a combination of catalytic studies, 31P, 1H and parahydrogen-induced polarization (PHIP) NMR experiments. With sulfonate and in neat methanol, however, a mechanistic switch takes place from a dihydride route (dihydrogen addition before olefin binding) at high pressure to an unsaturate route (olefin binding before dihydrogen addition) at low pressures (<30 atm). Olefin isomerization is inhibited by halide addition, but occurs with sulfonate and in neat methanol through what is most likely a pi-allyl mechanism. A detailed understanding of the effects of addition of these anions is crucial for development of new classes of catalysts capable of efficient enantioselective reduction of prochiral olefins lacking a secondary polar binding group.

Mechanism of nuclear spin initiated para-H2 to ortho-H2 conversion

“…The mechanisms of the ortho/para conversion of hydrogen have been the topic of various experimental and theoretical studies. This interest in the conversion was triggered by three main reasons, namely (i) the application of combined studies of the ortho/para spin conversion of dihydrogen and of the hydrogen/deuterium scrambling reactions as tools for the elucidation of the mechanisms of catalytic hydrogenation reactions; 3,4 (ii) the application of para-hydrogen as an agent for the sensitivity enhancement of liquid state NMR, [5][6][7] in particular for studies of catalysis; and (iii) the quantum mechanical nature of the ortho-and para-states, which is interesting by itself; (iv) the usage of ortho/para-water ratios as remote sensors for the thermal history of the ice core of comets; 8-11 (v) to understand the mechanisms of the conversion of para-formaldehyde to ortho-formaldehyde by surface adsorption. 12 The studies of dihydrogen metal complexes started when Wilkinson found the first catalysts which are able to take up and easily activate molecular dihydrogen during the hydrogenation of double bonds.…”

Immobilization of Optically Active Rhodium-Diphosphine Complexes on Porous Silica via Hydrogen Bonding

“…Although the use of solvents with different polarity rules out a reliable comparison between the tethered and homogeneous catalysts, one may draw out some general conclusions: i) The hydrogen-bonding immobilization of the [(R)-(R)-BDPBzPSO 3 ] À and BINAP complexes on silica does not reduce the enantioselectivity (this latter is intrinsically low for the catalyst/substrate systems employed in this work). [12,13,14] ii) The supported DIOP complex is less enantioselective than the homogeneous derivative, which may tentatively be related to the presence of ether oxygen atoms in the ligand backbone and hence to possible H-bonding interactions with the support leading to less efficient enantio-discrimination. However, a different hydrogenation mechanism in the two-phase systems cannot be ruled out.…”

Places and chemistry: Strasbourg—a chemical crucible seen through historical personalities

“…553 He also worked on the enantioselective hydrogenation of double bonds (e.g. of olefins with chiral rhodium catalysts, 554 of imines with iridium catalysts 555 ), on the isomerization of allyl alcohols, 556 as well as on other subjects related to organometallic catalysis.…”