Novel synthesis and structures of tris-annelated benzene donors for the electron-density elucidation of the classical Mills-Nixon effect

A versatile method fur 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 alpha,beta-dibromoalkenes and alpha'-alkenyl Grignard reagents-all 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 (pi-character) of the various aromatic C-C bonds-sufficient 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.