Iridium-Catalyzed Enantioselective Hydrogenation of Imines in Supercritical Carbon Dioxide

Abstract

Supercritical carbon dioxide (scCO 2 ) was shown to be a reaction medium with unique properties for highly efficient iridium-catalyzed enantioselective hydrogenation of prochiral imines. Cationic iridium(I) complexes with chiral phosphinodihydrooxazoles, modified with perfluoroalkyl groups in the ligand or in the anion, were synthesized and tested in the hydrogenation of N-(1-phenylethylidene)aniline. Both the side chains and the lipophilic anions increased the solubility, but the choice of the anion also had a dramatic effect on the enantioselectivity with tetrakis-3,5-bis(trifluoromethyl)phenylborate (BARF) leading to the highest asymmetric induction. (R)-N-phenyl-1-phenylethylamine was formed quantitativley within 1 h in scCO 2 [d(CO 2 ) ) 0.75 g mL -1 ] at 40°C and a H 2 pressure of 30 bar with enantiomeric excesses of up to 81% using 0.078 mol % catalyst. The use of scCO 2 instead of conventional solvents such as CH 2 Cl 2 allowed the catalyst loading to be lowered significantly owing to a change in the rate profile of the reaction. The homogeneous nature of the catalytically active species under the reaction conditions was demonstrated and was found to depend strongly on the composition of the reaction mixture and especially on the presence of the substrate. Utilizing the selective extractive properties of scCO 2 , the product could be readily separated from the catalyst, which could be recycled several times without significant loss of activity and enantioselectivity. High-pressure FT-IR and NMR investigations revealed that the reactivity of the products to form the corresponding carbamic acids plays an important role for the application of this new methodology.

Tail‐Tied Ligands: An Immobilized Analogue of (R)‐2‐Piperidino‐1,1,2‐triphenylethanol with Intact High Catalytic Activity and Enantioselectivity

“…asc.wiley-vch.de functionalized polymers, their catalytic activity was monitored using online FTIR analysis. [14] The addition of diethylzinc to benzaldehyde could be easily monitored following the absorbance values due to the characteristic stretching vibration of the benzaldehyde carbonyl at 1710 cm…”

The Effects of Solvent and Added Bases on the Protection of Benzylamines with Carbon Dioxide

“…The reactions of benzylamines under four distinct reaction conditions are presented in Figure 1. Neat 1-(4-bromophenyl)ethan-1-amine (1) and solutions of (1) dissolved in an aprotic solvent such as acetonitrile, tetrahydrofuran, ethyl acetate, acetone, dichloromethane, or 1,4-dioxane, were reacted with CO2 to produce the corresponding ammonium carbamate as an insoluble white solid [21][22][23]. Attempts to dissolve the carbamate salt in these solvents by raising the temperature to 50 °C resulted in significant reversal back to the starting amine.…”

Enantioselective Synthesis of Alcohols and Amines by Iridium‐Catalyzed Hydrogenation, Transfer Hydrogenation, and Related Processes

“…With the similar ligand 64, catalyst loadings could be further decreased to 0.078 mol % when supercritical carbon dioxide was used as the solvent and the catalyst system could be recycled. [67] The use of BAr F suppressed catalyst deactivation through formation of hydride-bridged trimers, and also resulted in better moisture and air stability, and higher turnover numbers than those previously obtained with 63.…”