Novel Synthesis and Structures of Tris-Annelated Benzene Donors for the Electron-Density Elucidation of the Classical Mills−Nixon Effect
American Chemical Society
Journal of the American Chemical Society
A versatile method for the high-yield synthesis of various tris-, bis-, and mono-annelated benzenes (as well as cyclooctatetraene) is based on the Pd-catalyzed coupling of three (or four) ethylenic units comprised of α,β-dibromoalkenes and α‘-alkenyl Grignard reagentsall carried out in a single pot. The particular application to tris(bicyclopentyl)-annelated benzene yields the syn isomer 1s in high purity; X-ray diffraction analysis confirms the aromatic bond alternation relevant to the Mills−Nixon effect. Most importantly, the efficient synthesis of 1s crystals of extraordinary quality allows us (for the first time) to make precise electron-density measurements of the “banana-type” distortion and the ellipticity (π-character) of the various aromatic C−C bondssufficient to identify the electronic origin of the classical Mills−Nixon effect. The unique electron-donor properties of tris-annelated benzenes also relate to their highly reversible one-electron oxidation potentials even in nonpolar solvents.