Herein, we report an eco-friendly photochemical oxidative Csp2-H thiocyanation and selenocyanation of activated arenes. The reaction proceeds under Violet LED irradiation in the presence of K2S2O8, which quickly oxidizes KSCN and KSeCN, finally producing arylthio/selenocyanates. Using this benign, atom-economic protocol, the desired chalcogenide products were obtained regioselectively, with isolated yields that range from very good to excellent. Although, mechanistic study indicates that it is difficult to distinguish between a radical to a SEAr reaction mechanism between the photo-induced formed •SCN, for the former, or NCSSCN, for the latter, to the aromatic heterocycles. The inhibition experiment together with the observed reactivity and regioselectivity, would be in agreement with the latter. The synthetic methodology designed could be successfully adapted to continuous-flow systems in a segmented-flow regime, employing the organic phase as the product reservoir. Using this setup, the advantage of the latter can be demonstrated by reducing the reaction time and improving the product yields. Similarly, the scaling up of the reaction to gram scale resulted in favorable outcomes by the flow setup, which installs the photo-flow chemistry as a powerful tool to be included into routine reaction procedures, which have great relevance for the pharmaceutical industry.

The employment of visible-light has emerged as a powerful, selective, and sustainable tool for the construction and functionalization of heterocyclic systems. This review highlights the recent progress in protocols to obtain benzothiazoles, triazoles, and selenoindoles mediated by light providing overviews of their synthetic scope. In addition, the mechanisms involved are discussed.

The photodecomposition of azides to generate nitrenes usually requires wavelengths in the <300 nm region. In this study, we show that this reaction can be readily performed in the UVA region (368 nm) when catalyzed by Pd-decorated TiO2. In aqueous medium the reaction leads to amines, with water acting as the H source; however, in non-protic and non-nucleophilic media, such as acetonitrile, nitrenes recombine to yield azo compounds, while azirine-mediated trapping occurs in the presence of nucleophiles. The heterogeneous process facilitates catalyst separation while showing great chemoselectivity and high yields.

The potential application of multistep continuous-flow systems has had a great impact on the syntheses of active pharmaceutical ingredients, natural products, and commodity chemicals. In this report, the highly efficient combination of a chemical reduction and a photochemical C-sp2-H activation reaction for selenylation of biologically relevant electron-rich arenes was achieved by means of a continuous-flow process. First, the reduction of alkyl and aryl selenocyanates by Rongalite was achieved giving the corresponding diselenides; second, the photoactivation of the Se-Se bond resulted in the selenylation of electron-rich arenes, both steps from good to excellent yields. In all cases, the reaction time was shortened, and isolated yields were improved when compared to batch reaction conditions. Furthermore, connecting both reactions in a multistep continuous-flow sequence was possible even when reductive and photooxidative transformations were coupled.

A novel and sustainable procedure was developed for the synthesis of 3-selenylindoles employing diorganyl diselenides and indoles or electron-rich arenes as starting materials. Visible blue light was used to promote the reaction without employing transition metal complexes or organic photocatalysts as sensitizers. Additives such as strong oxidants or bases were not required. Moreover, ethanol was employed as a benign solvent under mild reaction conditions. Through this easy and eco-friendly approach, several 3-selenylindoles and a number of asymmetric diarylselenides were obtained in good to excellent isolated yields.