Novel Synthesis and Structures of Tris-Annelated Benzene Donors for the Electron-Density Elucidation of the Classical Mills−Nixon Effect

Abstract

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 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 (π-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.

Molecular Recognition of NO/NO+ via Multicenter (Charge-Transfer) Binding to Bridged Diarene Donors. Effect of Structure on the Optical Transitions and Complexation Thermodynamics

“…The bridged (cofacial) diarenes used in this and related studies 11 were conveniently synthesized in high yields utilizing the palladium-catalyzed cross coupling of readily available 1,2 -dibromoalkenes with the desired Grignard reagent, e.g. which is based on a versatile process originally described by Kochi and co-workers 12 (for additional details see the Experimental Section).…”

Aromaticity Analysis of Lithium Cation/ π Complexes of Aromatic Systems

“…Several examples of NICS values that are seriously affected by ring currents in adjacent 5-or 6-MRs have been recently reported. [30,31] The Mills-Nixon effect [79][80][81][82][83] is associated with distortions in aromatic rings fused to saturated or small strained rings. This effect reduces the aromaticity of the 6-MRs of annelated benzene derivatives such as benzo [3,4]cyclobuta[1,2-b]biphenylene or [3]phenylene (M9) and benzo [3,4]cyclobuta[1,2-a]biphenylene or angular [3]phenylene (M10), as compared to that of benzene, according to all indicators of aromaticity (see Table 1).…”

A tetrabromo-1,4-ethanonaphthalene and related dibromo-1,4-ethenonaphthalene

“…The latter two differ, respectively, in the endo versus exo orientation of the Br atom at position 3. The geometry of the parent hydrocarbon framework without substituents on the bicyclic cage is available in 5,8-diacetoxy-1,2,3,4-tetrahydro-1,4ethanonaphthalene, (VIII) (refcode EDAGUM; Goh et al, 2007), and in the structure of a cocrystal of 5,6,7,8-tetramethyl-1,2,3,4-tetrahydro-1,4-ethanonaphthalene, (IX) (refcode PAWDIA; Rathore et al, 1998). In (IV)-(IX), the two single bonds attached to benzene [equivalent to C1-C2 and C7-C8 in (I)] average 1.505 (9) Å , a value not statistically different from the average value in (I) of 1.517 (6) Å .…”