Visible-light heterogeneous photocatalysis with an ionic porous organic polymer (IPOP): synthesis of β-keto sulfoxides and benzothiophene diesters by tandem radical addition–oxidation reactions

Abstract

An ionic porous organic polymer, CyNTzTz-IPOP, constructed by the nucleophilic substitution reaction of cyanuric chloride with readily accessible 2,5-di(pyridin-4-yl)thiazolo[5,4-d]thiazole (BpyTzTz), is shown to exhibit excellent visible light absorption and robust redox properties to serve as a metal-free heterogeneous photocatalyst. An expedient synthesis of diverse β-keto sulfoxides and benzothiophene-2,3-dicarboxylate esters by tandem addition–oxidation and addition–annulation–oxidation, respectively, is demonstrated by employing CyNTzTz-IPOP as a photocatalyst. Besides the fact that the reactions proceed under mild reaction conditions, leading to the products in very good to excellent isolated yields, the catalyst can be recycled at least ten times without any significant change in the catalytic activity. Mechanistically, both reactions occur by an initial single electron transfer from aryl thiols to the photocatalyst in its excited state, leading to arylthiyl radicals, which subsequently add to olefin/acetylene dicarboxylate; the resultant species undergo oxidation to yield the final products. Given that visible light-mediated heterogeneous photocatalysis is an environmentally benign and sustainable alternative to the traditional metal-based homogeneous catalysis, the advantages of the development of POPs in a bottom-up fashion for photocatalysis are compellingly borne out. To the best of our knowledge, the results constitute the first report of the syntheses of β-keto sulfoxides and benzothiophene diesters by heterogeneous photocatalysis.