Concise Cycloparaphenylene Synthesis

Significance

A three-step synthesis of [8]para­cyclophenylene is reported in remarkable 25% overall yield. The assembly of hoop-shaped π-conjugated molecules is particularly demanding as significant ring strain must be imposed on each of the aromatic subunits in order to form a cyclic structure. As such the first examples of para­cyclophenylene were described only recently by Bertozzi and co-workers (J. Am. Chem. Soc. 2008, 130, 17646). Here, Yamago and co-workers describe how they facilitated the challenging macrocycle-forming step by preorganizing four biphenylene units (1) into a square-shaped platinum complex (2a) via transmetalation. Following ligand exchange to form 2b, bromine-induced reductive elimination resulted in the formation of macrocycle 3, the smallest paracyclophenylene described to date.