Heterogeneous Enantioselective Catalyzed Carbonyl‐ and Imino‐Ene Reactions using Copper Bis(Oxazoline) Zeolite Y

Abstract

The current high demand in the pharmaceuticals industry for efficient and economical routes to enantiomerically pure ahydroxy and a-amino carbonyl compounds has focused interest on carbonyl-ene and imino-ene reactions as a potential source of these compounds.[1] Homogeneously catalyzed carbonyl-ene reactions have been reported using a chiral methyl-aluminum-binaphthol complex, [2] but the scope of this catalyst was very limited. Subsequently, the scope of the reaction was widened to the use of 1,1-disubstituted alkenes with titanium-binol (binol = 2,2'-dihydroxy-1,1'-binaphthyl) homogeneous catalysts. [3] Recently, interest has focused on the use of soluble copper(ii) bis(oxazoline) complexes as catalysts for the reaction of a range of glyoxylate and pyruvate esters and substituted alkenes to generate ahydroxy carbonyl compounds in high yields and with high ee values.[4a] In contrast, there is only a single example of a homogeneously catalyzed imino-ene reaction involving copper binap (binap = [1,1'-binaphthalene]-2,2'-diylbis(diphenylphospane)) complexes.[5] These catalysts and processes remain, however, limited in their economic efficiency because of the inability to recover the catalysts from the reaction medium. Previously, bis(oxazoline) bound to soluble poly-(ethylene glycol) has been used to generate a recyclable catalyst which is recovered by precipitation at the end of each reaction cycle, but the enantioselection decreased with each reuse. [6] Herein, we present the first heterogeneously catalyzed, efficient enantioselective carbonyl-and imino-ene reactions using a readily recyclable catalyst. Our approach to designing highly enantioselective heterogeneous catalysts uses electrostatic immobilization of cations within zeolites and mesoporous materials. Previously, we have shown that heterogeneous catalysts for the enantioselective aziridination of alkenes, with ee ! 95 %, can be designed on this basis. [7] In addition, we have shown that the copper bis(oxazoline) complex, when constrained within the zeolite structure, [8] gives substantially higher enantioselection than the homogeneous catalysts. [7,9] Similar effects have been noted for other immobilized catalysts.[10] We have now found that bis(oxazoline)-modified CuH zeolite Y catalysts are extremely effective catalysts for asymmetric carbonyl-and imino-ene reactions and, furthermore, these catalysts can be readily recovered and reused without loss of catalyst performance.The reaction of ethyl glyoxylate with a range of alkenes has been investigated using the immobilized copper-zeolite Y (Cu-HY) bis(oxazoline) catalyst and the results are shown in Table 1. In the absence of the bis(oxazoline), negligible reaction was observed. The results show that the reactions proceed with good yields and, in many cases, high enantioselection. Interestingly, the use of (S,S)-2 as the ligand with amethylstyrene leads to significantly higher ee value (93 %) than the use of (S,S)-1 (77 %). Even removal of the methyl groups on the bridging carbon atom, that is, using (S...

Zeolite Framework Stabilized Copper Complex Inspired by the 2-His-1-carboxylate Facial Triad Motif Yielding Oxidation Catalysts

“…31,[36][37][38][39] CuCl 2 , Cu(NO 3 ) 2 , or Cu(OTf) 2 was used as the copper source. For each Cu(II) source, copper ions were ion-exchanged into the zeolite Y with concentrations corresponding to 0.5, 1, 2, or 4 copper atoms per unit cell (UC) of zeolite Y.…”

“…[19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] In most cases, the aim is to improve the separation of the catalyst from the products or the selectivity of a catalytic reaction. A zeolite supercage can also be seen as a protective structure around the active center of the catalyst, similar to the protein mantle surrounding the active site of enzymes.…”

Improved methodology for non-covalent immobilization of tert-butyl-azabis(oxazoline)–copper complex on Al-MCM41

“…The subsequent thermal treatment would then convert the copper counter anion to a stable form that would bind the copper sites firmly to the support. A similar idea had in fact already been described by Hutchings and coworkers [22,[32][33][34] for Cu-exchanged zeolites and mesoporous materials, using the thermal treatment of copper(II) acetate at 550ºC, followed by treatment with bis(oxazoline) ligands. Nonetheless, in our hands it was not possible to prepare stable catalysts following the same process with our Al-MCM41 supports.…”

Studies on the Synthesis of a Chiral Salen Mn (III) Complex Immobilised onto Zirconium Aminophosphonates and Catalytic Asymmetric Epoxidation of α–methylstyrene

“…[31][32][33] Complex 1 was also examined for comparison purposes. [34][35][36] The results, given in Table 2, showed that the ee values increased significantly (from 57.5 to 79.5%) with linkage length. Heterogeneous catalysts 4a-e gave much higher ee values than complex 1 under the same conditions.…”