Heterogeneous Enantioselective Hydrogenation of Activated Ketones Catalyzed by Modified Pt‐Catalysts: A Systematic Structure‐Selectivity Study

Abstract

A systematic structure-selectivity study was carried out for the enantioselective hydrogenation of activated ketones with chirally modified Pt/Al 2 O 3 catalysts. For this, 18 modifiers containing an extended aromatic system able to form a strong adsorption complex with the Pt surface, and a suitable chiral group with an amino function capable to interact with the keto group of the substrate (HCd, Qd, HCn, Qn, and semi-synthetic derivatives, as well as synthetic analogues) were prepared and tested on 8 different activated ketones in AcOH and toluene under standard conditions. It was found that relatively small structural changes of the substrate and/or modifier structures strongly affected the enantioselectivity, and that no ™best∫ modifier exists for all substrates. The highest ees for all substrates were obtained with quinuclidine-derived modifiers in combination with naphthalene or quinoline rings, either in AcOH (substrates 1 ± 5 and 8, all carrying an sp 3 carbon next to the keto group) or toluene (6 and 7, with an sp 2 carbon next to the ketone). The presence and nature of the substituent R× at the quinuclidine significantly affected the ee (positive and negative effects). Certain combinations of an aromatic system and an amino function were preferred: For the quinuclidine moiety, quinoline and to a somewhat lesser extent naphthalene were a better match, while for the pyrrolidinylmethyl group anthracene was better suited. Methylation of the OH group often had a positive effect for hydrogenations in AcOH but not in toluene. With the exception of 8, higher ees were obtained for the Cd/ Qn series [leading to (R)-products] than for the Cn/ Qd series [leading to (S)-products]. In several cases, opposite structure-selectivity trends were detected when comparing reactions in toluene and AcOH, indicating a significant influence of the solvent.

Enantioselective hydrogenation of ethyl pyruvate and 1-phenyl-1,2-propanedione on catalysts prepared by impregnation of colloidal platinum on SiO2

“…[1][2][3][4] Thus, the synthesis of chiral compounds is of growing interest; among them, the hydrogenation of a-ketoesters and ketones is the most widely studied. [5][6][7][8][9][10][11][12][13][14][15] Since Orito et al reported the enantioselective hydrogenation of ethyl pyruvate to (R)-ethyl lactate over cinchonidine-modified Pt catalysts with ee's up to 95%, many other research teams have evaluated the performance of their catalytic systems in this reaction. [16][17][18] Indeed, this is nowadays a sort of test reaction in the field of asymmetric catalysis.…”

Effect of Microwave Irradiation on the Catalytic Activity of Palladium Supported Catalysts in the One-Step Isomerisation of Cinchona Alkaloids to Δ3,10-Isobases

“…This procedure was tested to find the optimal Literature data [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] Oxidative cleavage [12][13][14][15] time for conversion of 1a or 2a to the isomers (Z/E)-3a or (Z/E)-4a. We monitored the ongoing conversion progress of substrates by taking micro-samples of the reaction mixture regularly, at 10-min intervals, which after extraction with diethyl ether in the presence of aqueous ammonia were analyzed by TLC (silica gel plates, methoxyethanol).…”

“…This paper described our work on influence of controlled microwave heating on activity of palladium solid supported (Pd/SS) catalysts in the heterogeneous isomerisation of four natural Cinchona alkaloids. We studied also possibility of analysis and isolation of the geometric isomers Z and E in Δ 3,10 -isomers, as they will be used as chiral ligands [7,8], catalysts and modifiers in asymmetric syntheses [9][10][11][12].…”

“…The Cinchona Δ 3,10 -isomers 3a,b and 4a,b (Scheme 1) were investigated as effective chiral modifiers of solid-supported metallic catalysts for enantioselective hydrogenation of activated ketones [9][10][11]. Δ 3,10 -Isobases were applied also as valuable intermediates for modification of the native structure [12] including the oxidative cleavage of the double bond [13][14][15][16].…”

Selective Catalysis and Nanoscience: An Inseparable Pair

“…[115,116] Particularly striking is the progress made in the chiral modification of supported Pt and Pd catalysts by means of cinchona alkaloids, applied in enantioselective hydrogenation of functionalized ketones and enantioselective hydrogenation of C=C bonds, respectively. [102,[117][118][119][120][121] In these cases, an important role is not only played by the conformation and adsorption modes of cinchona alkaloid on the metallic surface (orange sphere in Figure 7), but also by the polarity of the solvent used during the reaction (yellow sphere in Figure 7). [122] Figure 7 provides a three-dimensional view of two surface conformations of cinchonidine adsorbed on Pt surface.…”