Photochemical Functionalization of Heterocycles with EBX Reagents: C-H Alkynylation versus Deconstructive Ring Cleavage

The development of novel methodologies for the functionalization of saturated heterocycles is highly desirable. Herein, we report a cheap and efficient photochemical method for the C-H functionalization of saturated O-heterocycles, as well as the deconstructive ring-cleavage of S-heterocycles, employing hypervalent iodine alkynylation reagents (ethynylbenziodoxolones, EBX). This photochemical alkynylation is performed utilizing phenylglyoxylic acid as the photoinitiator, leading to the corresponding products in good to high yields, under household fluorescent light bulb irradiation. When O-heterocycles were employed, the expected alpha-C-H alkynylation took place. In contrast, oxidative ring-opening to form a thioalkyne and an aldehyde was observed with S-heterocycles. Preliminary mechanistic experiments are presented to give first insights into this puzzling divergent reactivity.