Mixtures of chiral and achiral monodentate ligands in asymmetric Rh-catalyzed olefin hydrogenation: reversal of enantioselectivity

The recently described method of combinatorial asymmetric transition metal catalysis based on the use of mixtures of chiral monodentate P-ligands has been extended to include mixtures of chiral and achiral monodentate P-ligands, reversal of enantioselectivity in Rh-catalyzed olefin hydrogenation being possible in appropriate cases.     

Rh-catalyzed asymmetric hydrogenation of prochiral olefins with a dynamic library of chiral TROPOS phosphorus ligands

A library of 19 chiral tropos phosphorus ligands, based on a flexible (tropos) biphenol unit and a chiral P-bound alcohol (11 phosphites) or secondary amine (8 phosphoramidites), was synthesized. These ligands were screened, individually and as a combination of two, in the rhodium-catalyzed asymmetric hydrogenation of dehydro-alpha-amino acids, dehydro-beta-amino acids, enamides and dimethyl itaconate. ee values up to 98% were obtained for the dehydro-alpha-amino acids, by using the best combination of ligands, a phosphite [4-P(O)2O] and a phosphoramidite [13-P(O)2N]. Kinetic studies of the reactions with the single ligands and with the combination of phosphite [4-P(O)2O] and phosphoramidite [13-P(O)2N] have shown that the phosphite, despite being less enantioselective, promotes the hydrogenation of methyl 2-acetamidoacrylate and methyl 2-acetamidocinnamate faster than the mixture of the same phosphite with the phosphoramidite, while the phosphoramidite alone is much less active. In this way, the reaction was optimized by lowering the phosphite/phosphoramidite ratio (the best ratio is 0.25 equiv phosphite/1.75 equiv phosphoramidite) with a resulting improvement of the product enantiomeric excess. A simple mathematical model for a better understanding of the variation of the enantiomeric excess with the phosphite/phosphoramidite ratio is also presented.     

Carboranylphosphites as new ligands for Rh-catalyzed asymmetric hydrogenation

First representatives of chiral carborane-based phosphite ligands were synthesized. The enantiomeric excess in the Rh-catalyzed hydrogenation of dimethyl itaconate with the use of these ligands reaches 67% in CH₂Cl₂ and 62% in supercritical CO₂, the ee and conversion being strongly dependent on the hydrogen pressure. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 337–340, February, 2008.     

Cymantrene-derived monodentate phosphites: New ligands for Rh-catalyzed enantioselective hydrogenation

Novel chiral monodentate phosphite ligands bearing cymantrene fragment have been prepared. Phosphite 4 with PPh₃, instead of CO-ligand, in cymantrene moiety provided higher enantioselectivity than its unsubstituted analogue 3 in hydrogenation of (Z)-methyl-2-acetamido-3-phenylacrylate.     

Polymer-supported monodentate phosphite ligands for asymmetric hydrogenation

Several new polymer-supported monophosphite ligands have been developed and the rhodium complexes were shown to be highly efficient, highly enantioselective and easily separable catalysts for asymmetric hydrogenation of itaconates, enamides, α-dehydroamino acid derivatives and β-dehydroamino acid derivatives.     

P-Chiral monodentate diamidophosphites as ligands for Rh-catalyzed asymmetric reactions

A series of P-chiral monodentate diamidophosphite ligands of the 1,3-diaza-2-phosphabicyclo[3.3.0]octane family was tested in the Rh-catalyzed hydrogenation of dimethyl itaconate and addition of phenylboronic acid at the carbonyl group of trans-cinnamaldehyde. The enantioselectivities and conversions of these reactions are strongly dependent on the nature of the exo-cyclic substituent of the ligand. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2023–2025, October, 2007.     

First chiral phosphite with a quaternary ammonium fragment: Synthesis and use in Rh-catalyzed asymmetric hydrogenation

The first representative of chiral P-monodentate phosphite ligands containing quaternary ammonium substituents and its rhodium complex [Rh(COD)(L)₂]BF₄ (COD is 1,5-cycloctadiene) were obtained. The use of this ligand in Rh-catalyzed asymmetric hydrogenation of prochiral methyl esters of unsaturated acids allowed one to achieve optical yields up to 99%. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1395–1398, August, 2006.