Vollhardt's group synthesizes longer, helical phenylenes

Chemistry professor K. Peter Vollhardt

From C&E News Science Concentrates: Sept 2, 2002

[N]Phenylenes are extended aromatic compounds made up of N benzene rings fused together by N-1 cyclobutadiene rings. When this fusion occurs angularly, the terminal benzene rings start to overlap one another beginning with N = 6, forming helical structures. Previously, the N = 5 analog was the longest that could be synthesized.

But now chemistry professor K. Peter C. Vollhardt and coworkers report a novel triple cyclization strategy to prepare the N = 6, 7, 8, and 9 analogs [Angew. Chem. Int. Ed., 41, 3223 and 3227 (2002)]. These "heliphenes" are made by stitching together ethynyl-substituted benzenes using palladium-catalyzed reactions, followed by a cobalt-catalyzed step in which ethynyl substituents and bridges join to form additional benzene rings and the cyclobutadiene linkers.

The crystal structure of the N = 7 analog (shown) reveals an interplanar angle about the helix of 30º, with the overlapping benzene rings separated by 4.07 Å. Vollhardt and his group are looking toward possibly preparing longer analogs and eventually the hypothetical polyheliphene.