Asymmetric Hydrogenation Using Rhodium Complexes Generated from Mixtures of Monodentate Neutral and Anionic Phosphorus Ligands

Abstract

A series of monodentate neutral and anionic phosphorus ligands was synthesized and evaluated in the asymmetric rhodium-catalyzed hydrogenation of functionalized olefins by using either catalysts containing identical ligands or catalysts generated from mixtures of two different ligands. We expected that the combination of an anionic ligand with a neutral ligand would favor the formation of hetero over homo bis-ligand complexes due to charge repulsion. NMR spectroscopic studies confirmed that charge effects can indeed shift the equilibrium toward the hetero bis-ligand complexes. In several cases, the combination of a neutral phosphane with an anionic phosphane, one chiral and the other achiral, furnished significantly higher enantioselectivities than analogous mixtures of two neutral ligands. The best results were obtained with a mixture of an anionic phosphoramidite and a neutral phosphoric acid diester. It is supposed that in this case a hydrogen bond between the two ligands additionally stabilizes the hetero ligand combination.

Rhodium(I)‐Catalyzed Enantioselective Hydrogenation of Substituted Acrylic Acids with Sterically Similar β,β‐Diaryls

“…[9] The salient features of SPO ligands, including ready accessibility, good air and moisture stability, and the potentially H-bonding OH group in the ligand, [9,10] prompted us to explore the catalytic potential of Rh I /SPO systems in the asymmetric hydrogenation of challenging b,b-diarylacrylic acid substrates. Inspired by the fact that mixtures of chiral monodentate ligands (or chiral and achiral) can often improve the enantioselectivity and reactivity in many transition-metalcatalyzed reactions, [11] as evidenced by the elegant studies of several groups, [12][13][14][15][16][17] we moved to employ this mixed-ligand strategy in the present reaction. However, an initial screening of the catalysts generated in situ by mixing [Rh(cod) 2 ]BF 4 (cod = 1,5-cyclooctadiene) and L1-L7 in a 1:2 molar ratio resulted in only poor conversions under the conditions tested (entries 1-9), which indicates that the Rh complexes of L1-L7 alone are not active enough for catalysis.…”

“…[11][12][13][14][15][16][17] The synergistic effect of ligand mixtures has also been observed in a variety of catalytic asymmetric reactions. [11][12][13][14][15][16][17] The synergistic effect of ligand mixtures has also been observed in a variety of catalytic asymmetric reactions.…”

Combinatorial Strategies to find New Catalysts for Asymmetric Hydrogenation Based on the Versatile Coordination Chemistry of METAMORPhos Ligands

“…More recently, Pfaltz et al. made a small library of phosphites and phosphoramidites equipped covalently with noninteracting anion moieties that were used in combination with neutral ligands and a cationic Rh precursor for asymmetric hydrogenation . They showed that the formation of the neutral heterocomplexes was favored over that of the anionic or the cationic homocomplexes and that in some cases, the neutral complexes were more stereoselective.…”

Catalytic Asymmetric Hydrogenation of α‐CF3‐ or β‐CF3‐Substituted Acrylic Acids using Rhodium(I) Complexes with a Combination of Chiral and Achiral Ligands

“…[8] The feasibility of using Rh I /SPO-type catalysts in AHs of CF 3substituted acrylic acid derivatives was first investigated by screening several SPO ligands (L1-L6) with a-CF 3 -substituted cinnamic acid ((Z)-1 a) as a model substrate ( Table 1). Gratifyingly, when we investigated the reaction using Rh I complexes with SPO/PPh 3 [9] monodentate ligand mixtures, [10][11][12][13][14][15][16] the addition of achiral PPh 3 had a profound influence on the catalysis, as evidenced by the substantial improvement in the catalytic performance (30-95 % conversions, 11-92 % ee, Table 1, entries 1-6). Gratifyingly, when we investigated the reaction using Rh I complexes with SPO/PPh 3 [9] monodentate ligand mixtures, [10][11][12][13][14][15][16] the addition of achiral PPh 3 had a profound influence on the catalysis, as evidenced by the substantial improvement in the catalytic performance (30-95 % conversions, 11-92 % ee, Table 1, entries 1-6).…”