Weiguo Zheng

Ionic liquid-functionalized graphene oxide as an efficient support for the chiral salen Mn(III) complex in asymmetric epoxidation of unfunctionalized olefins

Imidazolium-based ionic liquid (IL)-functionalized graphene oxide (GO) was prepared and used to support chiral salen Mn(III) complexes. Technologies of characterization suggested that the intact chiral complex was covalently appended on flat planes and edge of the GO sheet through an imidazolium-based IL linker. The flexible layer-structure, as well as the active role of the IL linker in overall reaction, makes the novel heterogeneous catalyst efficient and universal for the enantioselective epoxidation of unfunctionalized olefins using NaClO as an oxidant. Remarkable enhancement of the reaction rate with excellent conversion (99%) was observed over a wide range of alkenes (styrene, α-methylstyrene, indene, 1,2-dihydronaphthalene, 6-cyano-2,2-dimethylchromene, and 6-nitro-2,2-dimethylchromene). The enantiomeric excess (ee) for epoxides was also encouraging (in the range of 73–93%), except for styrene (ee, 40%) and α-methylstyrene (ee, 44%). More importantly, the catalyst was perfectly stable and could be reused several times without significant loss of activity and enantioselectivity.

Dendritic phosphotungstate hybrids efficiently catalyze the selective oxidation of alcohols with H2O2

Keggin-type phosphotungstic acid (HPW) was incorporated into the polyamidoamine (PAMAM) matrix by partly protonating the amino groups of the PAMAM dendrimer. Characterization results suggested that the PAMAM dendrimer accommodated trivalent PW anions through ionic interactions and hydrogen bonds, resulting in novel dendritic phosphotungstate hybrids (dendri-PW hybrids). The dendritic PAMAM matrix as a countercation not only fine-tunes the redox properties of the PW anions but also has a positive “dendrimer effect” on the enhancement of the catalytic activity of the hybrids. Higher conversion of alcohols was observed over the dendri-PW hybrids in the selective oxidation of alcohols with H2O2 than over their non-dendritic counterparts. Furthermore, the dendritic ammonium cations impart reaction-controlled phase-transfer function to the dendri-PW hybrids. The catalysts precipitated from the solvent upon consumption of H2O2 and could be reused several times without loss of activity and selectivity.

A highly efficient and recyclable catalyst—dendrimer supported chiral salen Mn(III) complexes for asymmetric epoxidation

A novel series of PAMAM dendrimer-supported chiral salen Mn(III) complexes has been synthesized and applied to the enantioselective epoxidation of unfunctionalized olefins. Technologies of characterization clearly suggested the axial grafting of the chiral salen Mn(III) complex on the surface of PAMAM dendrimers through a phenoxy group spacer. It was found that the peripherally modified dendrimers benefited from the cooperativity between neighboring sites confined to the surface of the dendrimers, thereby enhancing the asymmetric induction of the catalyst. Higher enantioselectivity for PAMAM dendrimer-supported chiral salen Mn(III) complexes over the homogeneous counterpart was observed in the enantioselective epoxidation of unfunctionalized olefins. Furthermore, the dendritic supports imparted selective solubility properties to the catalysts. These catalysts could be easily recovered from the reaction solution by using solvent precipitation, and could be reused for several times without loss of activity and enantioselectivity.

Molecularly Imprinted Polymer Containing Fe(III) Catalysts for Specific Substrate Recognition

A series of molecularly imprinted polymers (MIPs) containing equal amounts of iron(III) were prepared by the polymerization of acrylamide and ethylene dimethacrylate in the presence of the template of o‐, m‐, or p‐nitrobenzyl alcohol(NBA) and a FeCl3 complex. The samples were characterized by scanning electron microscopy, N2 adsorption, and Fourier transform infrared spectroscopy. The catalysts exhibited high catalytic activity and unique substrate recognition in the oxidation of benzyl alcohol derivatives in water using 30% H2O2 as the oxidant. The conversion of p‐NBA was 80% over the p‐Fe(III)‐MIP catalyst when the template molecule was p‐NBA, which had a good fit with the substrate. However, the conversion of p‐NBA was less than 58% over o‐Fe(III)‐MIP or m‐Fe(III)‐MIP due to the mismatch of the substrate with the cavities of the Fe(III)‐MIP. The results indicated that the Fe(III)‐MIP samples contained molecular recognizable shapes and sites in their cavities that match the corresponding substrate. The special recognizing cavities of the Fe(III)‐MIP catalyst exhibited unique substrate recognition, and therefore the selectivity for the substrate was improved.