Photocatalytic Direct C3-Amidation of 2<i>H</i>-Indazole

The direct functionalization of privileged heterocycles could efficiently and site-selectively functionalize pharmacophores to expedite the construction of a focused library for mapping druglike chemical space. A variety of biologically active 2<i>H</i>-indazoles have been documented, but the direct amidation of 2<i>H</i>-indazoles remains underdeveloped. Herein, we report the first transition metal-free 3-amidation of 2<i>H</i>-indazoles under photoredox conditions. Employing 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) as an inexpensive photocatalyst and <i>N</i>-aminopyridinium salts as the amidyl radical sources, various 3-amidated 2<i>H</i>-indazoles were generated (22 examples, up to 90% yield). Furthermore, we also achieved photocatalytic late-stage modification of drug molecules. This protocol may represent a feasible molecular editing strategy on privileged drug scaffolds to efficiently generate a focused library.