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

Abstract

The asymmetric synthesis of a chiral, nonracemic π-conjugated system with D 2 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 CH 2Cl 2 are consistent with significant conjugation between the four terphenyl moieties.