Iron-Catalyzed C–H Activation/Aza-Annulation with 2-Butyne for Accessing Methylated Aza-Arenes

The incorporation of methyl groups into aza-arene frameworks not only often improves their biological activity as the “magic methyl” effect, but also notably affects their solid-state electronic properties by modulating π-π stackings, which makes them promising candidates for developing organic electronic materials. This study focuses on the iron-catalyzed C−H annulation of ketones with 2-butyne via their oxime ethers, offering an efficient pathway to synthesizing methylated π-extended aza-arenes. The reactions utilize isobutyl aluminum(III) catecholate as a base and Triphos (bis(2-diphenylphosphinoethyl)phenylphosphine) as a ligand. Reported C−H activation methods using rhodium catalysis proved less effective with π-conjugated substrates. Regioselective C−H aza-annulation with 1-(trimethylsilyl)propyne following desilylation provides also mono-methylated azaarenes. The reactions also generate tetra-methylated aza-arene products by two-fold C−H aza-annulation on chromeno[3,2-a]xanthene-13,14-dione and quinacridone. These methylated aza-arenes exhibit strong π-π stacking, a narrow emission band, and tunable photophysical properties, indicating their potential applications as electronic materials.