Phenylglyoxylic Acid: An Efficient Initiator for the Photochemical Hydrogen Atom Transfer C−H Functionalization of Heterocycles

C−H functionalization at the α‐position of heterocycles has become a rapidly growing area of research. Herein, a cheap and efficient photochemical method was developed for the C−H functionalization of heterocycles. Phenylglyoxylic acid (PhCOCOOH) could behave as an alternative to metal‐based catalysts and organic dyes and provided a very general and wide array of photochemical C−H alkylation, alkenylation, and alkynylation, as well as C−N bond forming reaction methodologies. This novel, mild, and metal‐free protocol was successfully employed in the functionalization of a wide range of C−H bonds, utilizing not only O‐ or N‐heterocycles, but also the less studied S‐heterocycles.