Chemists Ponder Benzene Jailbirds

Chemists modeling how benzene might polymerize under high pressure have uncovered several as-yet-unsynthesized benzene dimers (J. Am. Chem. Soc., DOI: 10.1021/ja302597r). The Cornell University team of Andrey Yu. Rogachev, Xiao-Dong Wen, and Roald Hoffmann placed two benzene molecules in “uncomfortably close contact and then let loose the geometry optimization of a quantum chemical program.” Their computations led to a set of 12 benzene dimers, most known experimentally but four previously unknown (one shown). All of the dimers are higher in energy—that is, less stable—than two noninteracting benzene rings. The team then determined the possible dimer fragmentation “escape routes” back to two benzene rings or to reactions leading to many other molecules. These pathways include sigmatropic hydrogen shifts that relocate double bonds, retrocyclizations to form 12-membered rings, the ring opening of one benzene ring, and Diels-Alder dimerization of two dimers.