Powerful annulation fuses polyaromatics

More than a century ago, chemists discovered the homocoupling of aryl halides, in which a copper catalyst convinces twin halogen atoms to give up their positions to bring together two aryl rings. This mighty dimerization reaction has long dominated biarene synthesis. Now, scientists are offering an upgrade. A research team at Nagoya University, led by Kei Murakami and Kenichiro Itami, has developed an annulative dimerization reaction of chlorophenylenes that connects the rings with two new bonds instead of one (Science 2018, DOI: 10.1126/science.aap9801). In the reaction, a palladium catalyst with bulky adamantane ligands promotes dual carbon-hydrogen bond activation events to produce the partially fused triphenylene core structure. This type of π-conjugated architecture holds precious properties for modern applications such as organic light-emitting diodes and photovoltaics. Itami notes that the partially fused arenes can be successfully “zipped up” to the fully fused polyaromatic structure using the iron-catalyzed Scholl reaction. Previous attempts to use this route starting from nonfused arenes have been plagued with undesirable side reactions. This additional step provides access to graphene nanoribbon materials, which are coveted for their unique optoelectronic properties.