Asymmetric Cascade Catalysis with Chiral Polyoxometalate‐Based Frameworks: Sequential Direct Aldol and Epoxidation Reactions

Abstract

Catalytic asymmetric cascade reactions in which the substrates are transferred through well‐choreographed consecutive independent steps by a single catalyst have received increasing interest. By incorporating a chiral organocatalyst pyrrolidine, an oxidation catalyst polyoxometalate and a Lewis acid catalyst copper(II) into a single porous coordination network (PCN), two enantiomorph CuW‐PYIs (PYI=pyrrolidine‐2‐yl‐imidazole) with novel Kagomé lattice were achieved and applied in the conversion of aldehyde with ketone into value‐added chiral epoxy ketones in a one‐pot procedure. The chiral pyrrolidine organocatalysts interact with the carbonyl group of ketones throughout the reaction, which is beneficial for achieving compatibility between the reaction intermediates and synergy of the multiple catalytic cycles, thus driving the direct aldol/epoxidation cascade reaction in an orderly and asymmetrical way. Moreover, the “two‐center catalysis” principle was well demonstrated in the direct aldol/epoxidation cascade reactions.

Chiral Phosphotungstate Functionalized with (S)-1-Phenylethylamine: Synthesis, Characterization, and Asymmetric Epoxidation of Styrene

“…In 2017, the Qiuxia group reported the use of synthesized of L-or D-pyrrolidine-2-ylimidazole-functionalized copper phosphotungstate (CuW−PYI1) for styrene oxidation using tert-butyl hydroperoxide (TBHP). 41 In 2008, Mirkhani et al reported Cu(salen)-silicotungstate for styrene oxidation using hydrogen peroxide (H 2 O 2 ) in acetonitrile (CH 3 CN). 47 O and its use for the said reaction using TBHP in CH 3 CN.…”

“…Reports are available on the synthesis and characterizations of copper-based inorganic–organic hybrid POMs having copper complexes as a countercations − consisting of phosphotungstates − and silicotungstates. ,− However, it should be highlighted that, out of the cited references, very few reports are available where the oxidation of styrene has been carried out. In 2017, the Qiuxia group reported the use of synthesized of l - or d -pyrrolidine-2-ylimidazole-functionalized copper phosphotungstate (CuW–PYI1) for styrene oxidation using tert -butyl hydroperoxide (TBHP) . In 2008, Mirkhani et al reported Cu­(salen)-silicotungstate for styrene oxidation using hydrogen peroxide (H 2 O 2 ) in acetonitrile (CH 3 CN) .…”

The synthesis and applications of chiral pyrrolidine functionalized metal–organic frameworks and covalent-organic frameworks

“…In contrast, a product with only a trace ee value was harvested when a racemic styrene oxide was employed as the starting material, which once again emphasizes that the enantioselectivity comes from the epoxidation rather than from the CO 2 cycloaddition reaction. 132 133 Metalloporphyrin is another kind of star molecule, and the metalloporphyrinic MOFs exhibit impressive catalytic performance in oxidation [134][135][136] and CO 2 fixation. 137 In 2019, Wu et al constructed a 3-periodic MOF with the reaction of a metalloporphyrinic ligand and Y(III).…”

Polyoxometalate-Supported Aminocatalyst for the Photocatalytic Direct Synthesis of Imines from Alkenes and Amines

“…In the structure, the [H 3 PW 11 O 39 Zn­(PYI)] 2– anion can be photoexcited by UV–vis light to form POM*, and then POM* accepts the electron, forming the reduced POM – , which is applicable to O 2 activation. , The direct assembly of [Zn­(PYI) 3 ] 2+ into the skeleton of POM makes electron transfer more efficient. PYIs are beneficial for interacting with the resulting aldehydes for the generation of iminium-ion intermediates, thus promoting a smooth tandem reaction …”

“…PYIs are beneficial for interacting with the resulting aldehydes for the generation of iminium-ion intermediates, thus promoting a smooth tandem reaction. 34 The diffuse-reflectance spectra for fresh ZnW−PYI exhibited two intense bands at approximately 301 and 372 nm, which are assigned to the transitions from n−π* and π−π* in PYI and the O b,c → W charge-transfer (CT) of [PW 11 ZnO 39 ] 5− , respectively (Figure 2a). After irradiation for 10 min, the absorption intensity increased slightly.…”