A Greek cross dodecaphenylene: Sparteine-mediated asymmetric synthesis of chiral D₂-symmetric π-conjugated tetra-o-phenylenes

The asymmetric synthesis of a chiral, nonracemic π-conjugated system with D₂ point group of symmetry, dodecaphenylene 4 is described. In the key step, (-)-sparteine- and Cu(II)-mediated oxidation of 2,2′-dilithio-1,1′-biaryls in ether gives the corresponding dimers, tetra-o-phenylenes, in 80% isolated yields and 50-60% ee's. X-Ray crystallography confirms the structure of rac-4 and its molecular shape of a Greek cross. The torsional angles between the benzene rings in the tetra-o-phenylene core of rac-4 are in the 56.5-71.0° range. However, CD and UV spectra of 4 in CH₂Cl₂ are consistent with significant conjugation between the four terphenyl moieties.