Stereochemistry of Hexakis(dimethylamino)benzene and Its Dication

The crystal structures of hexakis(dimethylamino)benzene (4) and its dication bis(triiodide) (8) have been determined. Crystals of 4 are monoclinic, space group C2/c, a = 18.440 (6) Å, b = 9.461 (3) Å, c = 12.017 (4) Å, β = 104.74 (3)°, Z = 4. Like hexaisopropylbenzene, hexakis(dimethylsilyl)benzene, and hexakis(dichloromethyl) benzene, molecules of 4 adopt approximate C_6h symmetry in the crystal, but, unlike these otherwise closely related compounds, 4 does not manifest an orientational disorder. Reaction of 4 with iodine yields 8, crystals of which belong to the monoclinic system, space group P2/c, a = 16.243 (5) Å, b = 11.051 (4) Å, c = 20.358 (7) Å, β = 112.02 (3)°, Z = 4. Molecules of 8 have crystallographic C₂ symmetry and approximate D₂ symmetry, with the benzene ring in a twist conformation. Variable-temperature NMR studies on l,3,5-tris(diethylamino)- 2,4,6-tris(dimethylamino)benzene (9) and hexakis(diethylamino)benzene, two molecules that are structurally related to 4, show that pyramidal inversion of the nitrogen atoms requires 8.2 and 10.0 kcal mol^-1, respectively. As shown by the interconversion of the diastereomers of 1,3,5-tris(ethylmethylamino)-2,4,6-tris(dimethylamino) benzene, which requires 16.0 kcal mol^-1, barriers to rotation about the C_ar-N bonds in this class of compounds are substantially higher than barriers to pyramidal inversion. According to AMI calculations, the homomerization (topomerization) of 4 is not concerted but involves stepwise inversion of the dimethylamino groups. The calculated activation energy,10.5 kcal mol^-1, is in reasonable agreement with the experimentally observed barrier of8.2 kcal mol^-1 for 9. Variable-temperature NMR studies on 8 reveal a site exchange of the methyl groups which is rationalized by a pseudorotational motion of the twisted benzene ring.