Immobilized Rose Bengal as Photocatalyst for Metal-Free Thiocyanation of Azaheterocycles under Continuous Flow Conditions

Organosulfur compounds play a crucial role as synthetic intermediates for biologically active target molecules, therefore the exploration of synthetic methodologies to incorporate sulfur-containing functional groups into organic scaffolds is a topic of great interest. Recently, various strategies to introduce the thiocyanate group (SCN) have been reported, due to its facile manipulation and conversion into a great variety of other functionalities. Notably, photochemical-induced thiocyanation reactions (which involve the use of a photocatalyst activated by visible light), received significant attention in the last few years, since this approach offers the advantage of reduced waste production and the use of traceless photons for synthetic transformations. In a novel advancement in this field, we investigated the use of a heterogeneous catalyst resulting from the immobilization of Rose Bengal organic dye onto a Merrifield resin (MR-RB) in the metal-free thiocyanation of azaheterocycles, under traditional batch conditions and under continuous flow conditions (Scheme 1). MR-RB proved to efficiently promote the reactions in both cases, providing a small library of thiocyanated azaheterocyclic rings, in up to 98% yield. When the resin-supported Rose Bengal was employed under continuous flow conditions, the desired products were obtained with high space-time-yields and reduced reaction times compared to the traditional batch approach. Furthermore, we explored the feasibility of recovering and recycling the supported dye for further applications.