Double aromaticity in the 3,5-dehydrophenyl cation and in cyclo[6]carbon

Abstract

The presence of two (4n+2)-electron conjugated systems in perpendicular planes results in considerable aromatic stabilization. Despite having two fewer hydrogens, the 6 π e-2 σ e 3,5-dehydrophenyl cation (C₆H₃⁺, 1) is 32.7 (CCSD(T)/6-31G^∗∗) and 35.2 kcal/mol (RMP4sdtq/6-3iG^∗//RMP2(fu)/6-31G) more stable than the phenyl cation (evaluated by an isodesmic reaction involving benzene and m-dehydrobenzene (4)). Cation 1, the global C₆H₃⁺ minimum, is 11.7,24.2, 11.8, and 30.4 kcal/mol lower in energy than the 2,6- (11) and 3,4-dehydrophenyl (12) cations as well as the open-chain isomers 13 and 14 (RMP4sdtq/6-31G^∗//RMP2(fu)/6-31G^∗ + ZPE(RMP2(fu)/6-31G^∗)). The stability of 1 is increased hyperconjugatively by 2,4,6-trisilyl substitution. The double aromaticity of 1 is indicated by the computed magnetic susceptibility exaltations (IGLO/II//RMP2(fu)/6-31G^∗) of -5.2, -6.8, -15, and -23.2 relative to 11, 12, 13, and 14, respectively. Thus, 1 fulfills the geometric, energetic, and magnetic criteria of aromaticity. The double aromaticity of the D_6h cyclo[6]carbon is apparent from the same criteria.