Profoundly Hindered Aryl-Aryl Single Bond

Polyaromatic hydrocarbons (PAHs) come in a range of shapes that depend on how their six-membered rings are connected. Chemists use the terms "bay," "cove," and "fjord" to describe indentations that resemble topographical shoreline features in the PHA periphery. Benjamin T. King and coworkers of the University of Nevada, Reno, have now coined the term "gulf" to describe a large PAH indentation and report the preparation of the first compound in which an aryl-aryl single bond joins two PAHs across their gulfs (J. Am. Chem. Soc., DOI: 10.1021/ja803396n). Key to the synthesis is a fourfold biphenylation of a brominated biphenyl compound using a new zirconium-based biphenylating reagent. X-ray diffraction analysis revealed that the two halves of the molecule are twisted in the same chiral sense, akin to a handshake, and that the close quarters makes the bridging bond one of the most rotationally hindered aryl-aryl single bonds in chemistry. Although the hindered bond is exceptionally weak, it has a "surprisingly ordinary" 1.49-Å length, King says.