Asymmetric Transfer Hydrogenation of Aromatic Ketones in Water using a Polymer‐Supported Chiral Catalyst Containing a Hydrophilic Pendant Group

Abstract

Hydrophilic polymers having pendant groups of carboxylates or sulfonates have been used as a polymer support for chiral 1,2-diamine monosulfonamides. The polymeric chiral complex prepared from the polymer-supported chiral ligand with ruthenium dichloride·p-cymene was used in the asymmetric transfer hydrogenation of prochiral ketones in water. The balance between hydrophilicity and hydrophobicity of the polymer support influenced both the reactivity and the enantioselectivity of the reaction in water. The chiral polymeric complex having a quaternary ammonium salt structure as the pendant group worked well in water. In most cases the polymer-supported catalyst having a quaternary ammonium sulfonate pendant group showed superior enantioselectivity compared to the corresponding nonsupported model catalyst in the solution system. The polymeric catalysts can be reused without loss of catalytic activity.

Immobilization of a Modified Tethered Rhodium(III)‐p‐Toluenesulfonyl‐1,2‐diphenylethylenediamine Catalyst on Soluble and Solid Polymeric Supports and Successful Application to Asymmetric Transfer Hydrogenation of Ketones

“…Soluble poly(ethylene glycol)-supported Ru-TsDPEN catalysts were used by Xiao et al [33] achieving more than ten runs without observable loss of activity and an ee of 93%, and by Li, Chan and coworkers [48] achieving eight runs with ees of 95-97%. High enantioselectivity (up to 98% ee) was achieved by Itsuno et al [25] in five consecutive runs using hydrophilized polystyrene-supported catalysts, and silica materials were shown to be readily recoverable supports, but lower ees were observed. [29,49] The herein presented approach provides a promising example of a repeatedly reusable immobilized ATH catalyst keeping the enantioselectivity constantly high at 98% ee for at least seven runs (PC-supported version).…”

“…[19] For Ru catalysts in aqueous systems with HCOONa, similar enantioselectivities and varying activities are found. [25,46,47] It is remarkable that due to the high catalyst robustness all experiments could be performed under non-inert conditions. Thus, no protective gas or degassed solvents were necessary.…”

“…This could be ascribed to the amphiphilic properties of the functional matrix of the support material which affords a high local concentration of substrate and hydrogen donor in the microenvironment of the catalyst. [25] Catalyst recycling was performed either with or without washing the supports with methanol after each run. A high decrease of activity was observed within the first three cycles when the supports were repeatedly used in the standard water/HCOONa system [model substrate acetophenone (16a)] without washing or further treatment (see Table 5).…”

“…[23] Although first recycling results were poor, this approach was successfully followed by other groups. [24][25][26] Solid organic and inorganic supports, to which TsDPEN or derivatives were covalently bound, have also widely been used, [27][28][29][30][31][32] and some attempts have been made to use soluble supported [33,34] as well as micellar-embedded TsDPEN-based catalysts. [35,36] Here we report the modification and covalent binding of a tethered RhA C H T U N G T R E N N U N G (III)-TsDPEN complex to both soluble hyperbranched polymers and solid polymeric chips as supports.…”

Catalytic asymmetric transfer hydrogenation of ketones: recent advances

“…141 Specially active in aqueous medium is the bifunctionalized catalyst 176 formed from flower-like mesoporous silica and the rhodium complex generated from [RhCl 2 Cp*] 2 and a TsDPEN fragment, and used in the ATH of different phenones with sodium formate as the reducing agent. 142 The two polymeric ionic ligands 177 143 and 178 144 with CRu as metal complex were successfully used in the ATH of aromatic ketones in water and with sodium formate as the hydrogen source. The polymeric complex 179 was very efficient in the ATH of aromatic ketones in pure water and with sodium formate.…”