Neutral Mobius Aromatics: Derivatives of the Pyrrole Congener Aza[11]annulene as Promising Synthetic Targets

Amino- and methyl-disubstituted derivatives of an aromatic 12 pi electron pyrrole homologue Mobius 1H-aza[11]annulene are predicted computationally at the B3LYP/6-31G* level of theory to be distinctly aromatic and a neutral ground state on its potential surface. The parent annulene has a nonaromatic conformation, which is 6.0 kcal mol(-1) lower in energy at the CCSD(T)/D95*//B3LYP/6-31G* level of theory than a structurally very similar Mobius conformation. After derivatization, however, geometry optimizations of the two lowest-energy - Mobius and non-Mobius - heteroannulene conformations both converged two a single Mobius minimum, whereas two (Mobius and non-Mobius) distinct difluoro-substituted species could be located 1.8 kcal mol(-1) apart in energy at the B3LYP level. The Mobius minimum is 7.5 kcal mol(-1) lower in energy at the coupled cluster level than a Huckel topology form derived from a known bridged 1H-[4,9]methano[11]annulene. Several possible reactive modes are investigated in order to assess the relative stability of the Mobius parent annulene. In addition, we report formal novel. examples for charged and neutral Mobius triplet aromatic annulenic and heteroannulenic systems, some of which also exhibit distinct aromatic properties. Further fundamental insights into the phenomenon of Mobius aromaticity are suggested. The aromaticity of Mobius annulenes is for the first: time linked to wavefunction boundary conditions. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)