Hollow Hyper-Cross-Linked Nanospheres with Acid and Base Sites as Efficient and Water-Stable Catalysts for One-Pot Tandem Reactions

A task-specific functionalized hyper-cross-linked polymer (HCP) with hollow spherical structure was synthesized by an easily accessible Friedel–Crafts reaction-based approach. A harmonious coexistence of acid (sulfonic acid) and base (amine) sites on a microporous organic material was achieved. The acid–base bifunctional HCP catalyst (HCP–A–B) structure was fully characterized by many physicochemical methods. In the subsequent tandem reactions (hydrolysis/Henry and hydrolysis/Knoevenagel reactions), the HCP–A–B catalyst displayed high catalytic efficiency and chemical stability toward water or organic solvent. These HCP–A–B catalyst characteristics led to the development of a previously unreported transformation of 2-ethoxy-3,4-dihydropyran derivative to a 2-cyclohexen-1-one derivative through a tandem reaction involving water-assisted ring-opening hydrolysis of the dihydropyran, an intramolecular aldol reaction, and a dehydration reaction. In all of these reactions, the bifunctional HCP–A–B catalyst can be recovered and reused more than 10 times without significant loss of activity.