Iron-catalysed C(sp²)–H activation for aza-annulation with alkynes on extended π-conjugated systems

Sustainable C–H activation methods for expanding large π-systems, crucial for electronic materials, continue to pose a challenge due to the metal–π interaction involving bonding and back-bonding. Here we present an iron-catalysed method for aza-annulation π-extension reactions of conjugated carbonyl compounds via oxime ether, employing a sterically bulky trisphosphine ligand to mitigate Fe–π interaction. Inexpensive isobutyl aluminium(III) catecholate serves as a base. These reactions convert readily available substrates, such as quinacridone, pentacenedione and pentacenetetraone, into conjugated skeletons, yielding several narrow-band-emissive molecules, including an actively pursued deep-blue emitter with peak emission at 450 nm and a narrow emission band of 13 nm. The tetrafold C–H activation of pentacenetetraone, with each step achieving an average yield of 95%, underscores the efficacy of iron catalysis in selectively activating C–H bonds on π-extended systems, offering promise for expanding organic electronic materials. Metal–π interactions typically hinder metal-catalysed C–H functionalization reactions involving large π-conjugated systems. Now, an iron-catalysed aza-annulation method, employing a bulky trisphosphine ligand and an aluminium base, proves efficient for large π-extended substrates, holding promise for electronic materials discovery.