Sulfonic-acid functionalized hypercrosslinked porous organic polymer as a highly efficient heterogeneous catalyst for synthesis of 2H-chromene derivatives

Abstract

A sulfonic acid-functionalized hypercrosslinked porous organic polymer (HCP-H-SO₃H) with high surface area (S_BET = 604 m² g⁻¹) was successfully synthesized via a Friedel–Crafts alkylation of benzene and dimethoxymethane, followed by sulfonation of aromatic rings using sulfuric acid. This material was used as an efficient heterogeneous catalyst in a cyclization between various ortho-hydroxy aromatic aldehydes and acetylenic diesters, affording the corresponding 2H-chromenes in moderate to excellent yields. The material could be reused for at least five reaction cycles, suggesting the sustainable application potential of this sulfonic-acid functionalized materials as a heterogeneous acid catalyst. A mechanistic investigation using ¹³C NMR kinetic isotope effect at natural abundance experiments provided insights into a similar nature of the rate-limiting steps in the catalytic processes of the homogeneous catalyst (p-toluenesulfonic acid monohydrate; PTSA·H₂O) and the heterogeneous catalyst (HCP-H-SO₃H).