De Novo Access to SO₃H-Grafted Porous Organic Polymers (POPs@H): Synthesis of Diarylbenzopyrans/Naphthopyrans and Triazoles by Heterogeneous Catalytic Cyclocondensation and Cycloaddition Reactions

Covalent polymerization of contrived molecular building blocks leads to porous organic polymers (POPs) with intrinsic microporosity. Leveraging the results of our extensive investigations with molecular systems based on twisted biaryl scaffold as applied to metal−organic frameworks (MOFs) and organic light-emitting diodes (OLEDs), we envisioned that the Fridel-Crafts polyalkylation of the 2,2′,6,6′-tetramethylbiphenyl core with α,α′-dichloro-p-xylene should lead to a hypercrosslinked polymer with assured porosity. Furthermore, sulfonation of the latter in a postsynthetic fashion was surmised to yield a POP grafted with sulfonic acid groups for catalysis. Indeed, the polyalkylation of tetramethylbiphenyl led to a highly porous POP (MeBP) with a BET surface area of 1277 m2 g–1. Also, its sulfonated derivative (MeBP@H) exhibited a surface area of 766 m2 g–1, as determined by N2 sorption analysis. Polymer MeBP@H with palpable porosity and endowed with Bronsted acid sites installed through sulfonation was explored for the heterogeneous catalysis of cyclocondensation and cycloaddition reactions in a recyclable fashion. We show that MeBP@H exhibits exceptionally high catalytic activity in the synthesis of a wide variety of photochromic diarylbenzopyrans/napththopyrans and pharmacologically relevant triazoles at room temperature. Diverse functional groups are shown to be tolerated in both condensation and cycloaddition reactions. The catalyst is found to be efficient for the synthesis of triazoles having both electron-rich and electron-poor functionalities. Although a variety of methods are known in the literature for the synthesis of both diarylbenzopyrans/naphthopyrans and triazoles, MeBP@H allows the reactions to be conducted in shorter reaction times and very cleanly, leading to the isolation of products with high operational simplicity. The catalyst can be isolated back by filtration and recycled; indeed, MeBP@H is shown to be recycled for at least 10 catalytic cycles for both reactions without any loss of efficiency. Additionally, MeBP and MeBP@H are found to exhibit higher uptake capacities for CO2 and C2H2 over N2 at 298 K and inverse uptake capacity for C2H2 over CO2.