Photocatalyst-Free Regioselective C–H Thiocyanation of 4-Anilinocoumarins under Visible Light

Abstract

In the present study, we describe the successful development of a novel and efficient C­(sp2)–H thiocyanation reaction of electron-rich heteroarenes and arenes with readily available ammonium thiocyanate under visible light exposure. Most importantly, the reaction proceeded smoothly without addition of any photocatalyst or strong oxidant, ultimately minimizing the production of chemical waste. The advantages of this reaction met the requirements of green and sustainable synthetic chemistry and may be widely used in organic synthesis and pharmaceutical chemistry.

Catalyst-Free Organic Transformations under Visible-Light

“…Taking into account that the fact coumarin derivatives are very significant motifs in various pharmacological and biological activities and the importance of thiocyanation reaction, − in 2019, visible-light was employed for the synthesis of SCN-containing coumarin compounds using NH 4 SCN as thiocyanate source (Scheme ). Although initial experiments for a model reaction were successfully carried out in the presence of various organic photocatalysts, such as rose bengal, acridine red, eosin Y, methylene blue, and rhodamine B, the photocatalyst-free condition gave the corresponding product in higher yield. It was found that the addition of TFA (2 equiv) could increase the yield, and using green LED lamp resulted in a significantly lower yield than blue LED.…”

The catalytic and radical mechanism for ethanol oxidation to acetic acid

“…22 From this study, the nature of the active oxygen, such as singlet oxygen, oxygen radicals, and hydrogen peroxide, cannot be determined. 23,24 Hence, the ethanol oxidation in the liquid-phase involves two consecutive steps, shown in Scheme 1: (1) the oxidative dehydrogenation of ethanol to acetaldehyde on the catalytic surface (reaction (1)) and (2) the non-catalytic, likely radicalinvolved, autoxidation of acetaldehyde to acetic acid in the liquid phase (reaction (2)). A catalytic pathway of acetaldehyde oxidation is possible depending on the catalyst and conditions.…”

Recent advances in photochemical and electrochemically induced thiocyanation: a greener approach for SCN-containing compound formation

“…Using visible light and the widely available ammonium thiocyanate, Yang et al found a novel photocatalyst-free oxidative C(sp 2 )–H thiocyanation process for electron-rich heteroarenes and arenes 36. 21 According to mechanistic research, singlet oxygen is created when light is irradiated and can be a vital stimulant for the reaction to continue. The most appealing aspect of the reaction is that it allows simple bench-top processes because it only requires simple and widely affordable compounds 37 and does not necessitate the use of a transition metal catalysts, photocatalysts, bases, or strong oxidants.…”