Amphiphilic Hyperbranched Polyethoxysiloxane: A Self-Templating Assembled Platform to Fabricate Functionalized Mesostructured Silicas for Aqueous Enantioselective Reactions

Fabrication of amphiphilic mesostructured silica as a heterogeneous catalyst is beneficial to facilitate an aqueous reaction due to its highly dispersed nature in water. In this work, by taking advantage of a self-templating assembled strategy, we construct two types of mesostructured silicas as heterogeneous catalysts, chiral ruthenium/diamine-functionalized and chiral cinchonine-based squaramide-functionalized heterogeneous catalysts, through the use of amphiphilic poly(ethylene glycol) monomethyl ether-modified hyper branched polyethoxysiloxane as a silica precursor. As presented in the study, the chiral ruthenium/diamine-functionalized catalyst performs an asymmetric transfer hydrogenation of acyclic alpha-trifluoromethylimines to chiral alpha-trifluoromethylamines in water, whereas the chiral squaramide-functionalized catalyst enables efficiently asymmetric Michael addition of acetylacetone to nitroalkenes in brine. Both highly catalytic performances are attributed to the combined multifunctionalities of well-defined single-site chiral active species, highly dispersed catalytic centers, and practical phase-transferred function. Furthermore, both catalysts can also be recovered easily and reused repeatedly for at least seven times without loss of catalytic activity. Such a feature makes this self-templating assembly attractive for construction of various heterogeneous catalysts.