Yolk‐Shell‐Mesostructured Silica‐Supported Dual Molecular Catalyst for Enantioselective Tandem Reactions

Abstract

Yolk‐shell‐mesostructured silica was used as a support in the development of an active‐site‐isolated bifunctional catalyst that can mediate a sequential organic transformation. Herein, through immobilization, the location of two catalytic species is controlled: a base functionality is anchored in the channels of the outer silica shell and a chiral ruthenium/diamine functionality is anchored on the inner silica yolk. The result is a yolk‐shell‐mesostructured silica‐supported active‐site‐isolated dual molecule catalyst. Structural analysis through solid‐state carbon 13C NMR spectroscopy reveals its well‐defined single‐site dual active centers. Electron microscopy investigations disclose its uniformly distributed mesoporous nanoparticles. As envisaged, this bifunctional catalyst enables a controllable aza‐Michael addition/asymmetric transfer hydrogenation catalytic sequence, where the base‐catalyzed aza‐Michael addition of enones and amines to aryl‐substitutedβ ‐secondary amino ketones is followed by a Ru‐catalyzed asymmetric transfer hydrogenation. Various aryl‐substituted γ‐secondary amino alcohols are obtained in high yields and enantioselectivities via this one‐pot enantioselective organic transformation. Furthermore, the heterogeneous catalyst can be applied in a continuous‐flow process, which was shown to be particularly attractive for the practical preparation of aryl‐substituted γ‐secondary amino alcohols in an environmentally friendly medium.

Functionalized Silicas for Metal‐Free and Metal‐Based Catalytic Applications: A Review in Perspective of Green Chemistry

“…In various reports, G. Liu's research group has been effectively utilized a sulfonamide that derived from the reaction of 2‐(4‐chlorosulfonylphenyl)ethyltrimethoxysilane with diphenylethylenediamine. Later, they have grafted the obtained sulphonamide on silica …”

“…Furthermore, OSs provide the facile mass transfer for the reactants as well as the products during the catalysis . Recently, a list of organosilanes is reported for the preparation of OSs which includes 1,4‐bis(triethoxysilyl)benzene, 4,4′‐bis(triisopropoxysilyl)‐2,2′‐bipyridine, 1,2‐bis(triethoxysilyl)ethane, tris[3‐(trimethoxysilyl)propyl] isocyanurate, 1,3‐bis(3‐trimethoxysilylpropyl)‐imidazolium iodide and 4,4′‐[4‐(trimethoxysilanyl)butyl]‐2,2′‐bipyridine . These organosilanes evidence that OSs can possess the organic groups such as ionic liquids and bipyridyl rather than just having phenyl and ethyl groups.…”

“…The OSs that prepared in presence of a surfactant under hydrothermal conditions are termed as the PMOs. Considering the surfactants used for the preparation of PMOs, P123 is very famous as compared to other surfactants such as CTAB and F127 . Choice of surfactant is very important during the preparation of PMOs as they influence the basic characteristics of OSs like porosity.…”

“…are able to anchor the entirely two different functional groups in the core and shell of yolk‐shell (FS@functionalized PMO) material in which the shell is made up of PMO prepared from 1,2‐bis(triethoxysilyl)ethane using CTAB. The authors have utilized this interesting strategy for the preparation of a dual functionalized yolk‐shell silica material as shown in Figure …”

“…Research group of G. Liu has made a remarkable contribution to the cascade reactions that catalyzed by dual FSs . In their contribution, a novel organosilane ((S,S)‐4‐((trimethoxysilyl)ethyl)phenylsulfonyl‐1,2‐diphenylethylene‐diamine), also a multidentate chelating ligand, has been frequently employed in the synthesis of dual FSs.…”