Three-Dimensional Homoaromaticity

The structure and energy of a series of ions generated from penta-cyclo[3.3.1.13,7.01 3.05 7]decane (7) has been explored by using HF, MP2 and DFT methods to estimate enthalpy changes of isodesmic disproportionation reactions and by considering the reorganization of frontier orbitals as a consequence of addition or removal of electrons from the neutral molecule.8 The dication (72+), which is considered to be Three-dimensionally homoaromatic , is stable relative to a localized structure with similar features but is highly unstable compared to the radical cation (7+i)- hi contrast, the dianion (72 ) is unstable relative to the radical anion (T) and shows no evidence of electron delocalization. 

Schleyer, P. v. R., Bremer, M., Schotz, K., Kausch, M. Schindler, M. 1987 Four-center two-electron bonding in a tetrahedral topology. Experimental realization of three-dimensional homoaromaticity in the l,3-dehydro-5,7-adamantanediyl dication. Angew. Chem. Int. Ed. Engl. 26, 761 763. 

Aromaticity and homoaromaticity of a parent barbaralane and a tetraphospha-barbaralane of C2v-symmetry were visualized by means of three-dimensional nucleus-independent chemical shift maps [90]. In combination with CPMD simulations the fiuxional character of tetraphosphabarbaralane was revealed and it was shown that the ionic motion at room temperature leaves the aromaticity in this case unchanged [90]. 

Despite many controversial arguments regarding the definition and physical origin of aromaticity [1,15-17], the concept of aromaticity has crossed the boundary of benzenoid hydrocarbons [with (4n + 2)ji -electrons] to include heterosystems [50] like pyridine, thiophine, cations such as tropylium [12] and cyclopropenium [13], anions like cyclopentadienyl [51], organometallic systems, namely ferrocene [52], purely carbon-free systems [53,54], namely Pj, [(P5)2Ti]. The three-dimensional aromaticity of boron-based clusters [55] and of fullerenes [56], the homoaromaticity of cationic systems [57], aromaticity of triplet state annulenes [58], and pericyclic transition states [59] has enlarged the concept of aromaticity. Extension of aromaticity... 

Schleyer et al. prepared, and fully characterized as three-dimensionally homoaromatic (tetratrisho-moaromatic), the 4c/2e dehydroadamantyl dication 63. They also determined, from the isodesmic reaction 1, that 63 is stabilized by 47 kcal mol" (MINDO/ 3) relative to 64. Calculated and observed magnetic... 

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