Photocatalytic aerobic thiocyanation/sulfonylation of alkynes by D-A type COFs: Enhanced generation of superoxide and sulfur-based radicals via building units adjusted band structures

Abstract

Photocatalytic radical-mediated aerobic thiocyanation/sulfonylation of alkynes represents the most straightforward and atom-economic method to synthesize α-thiocyanate/sulfonyl ketones. The core issue is the generation of desired thiocyanate/sulfonyl radicals and their subsequent conversion into target products. In this work, three polyimide-based donor–acceptor COFs, named TAPB-PMDA, TTA-PMDA, and TTA-NDA, respectively, have been prepared for the photosynthesis of α-thiocyanate/sulfonyl ketones. Based on efficient adjusting electron donor-accept capacity of building units, TTA-NDA possesses broader visible absorption, richer redox properties, faster carrier separation efficiency and shows higher photocatalytic activity in thiocyanation/sulfonylation of alkynes under blue LED light illumination. Control experiments and EPR tests provide insight into the tandem radical addition/cross-coupling mechanism, in which the thiocyanate/sulfonyl radicals generation via h+ oxidation and the reactive oxygen species (ROS, e.g. •OH, H2O2, 1O2, O2•−) production by e− reduction carry out simultaneously, avoiding the employing of electron sacrificial agents or extra additives as precursors of singe electron transfer reagents. This operable strategy opens up a door for the difunctionalization of alkynes in photosynthetic manner.