Mobius aromatics

Interestingly, the next most stable form of diethyl ether and diethyl ketone may correspond to an 8 pi electron Mobius aromatic system ... 

M. Mauksch, V. Gogonea, H. Jiao, and Paul, von R. Schleyer, The Remarkable Nature of the Monocyclic (CH)g Cation. A Heilbronner Mobius Aromatic System Revealed, Angew. Chem. Int. Ed. Engl. 1998, 37, 2395. B3LYP calculations provide an interpretation of puzzling experimental results, obtained nearly three decades earlier. 

The Hiickel anti-aromaticity versus Mobius aromaticity effects for the seven-membered systems 1 and 2 have been studied computationally using Gaussian98 at the closed shell B3LYP/6-31G(d) level <20030BC182>. It was shown that Mobius aromaticity was preferred for the respective perfluorinated derivatives 3 and 4. 

D. Ajami, O. Oeckler, A. Simon, R. Herges, Synthesis of a Mobius Aromatic Hydrocarbon. Nature, 2003, 426, 819-821. 

Martln-Santamarfa, S. Lavan, B. Rzepa, H. S. Hiickel and Mobius aromaticity and trimerous transition state behaviour in the pericyclic reactions of [10], [14], [16] and [18]annulenes, J. Chem. Soc., Perkin Trans. 2000,2,1415-1417. 

Rzepa, H. S. A double-twist Mobius-aromatic conformation of [14]annulene, Org. Lett. 2005, 7, 4637-4639. 

The example of Mobius aromatic is (perfluorocyclo-octatetraene dianion, a... 

Mobius aromaticity A monocyclic array of orbitals in which a single out-of-phase overlap (or, more generally, an odd number of out-of-phase overlaps) reveals the opposite pattern of aromatic character to Hiickel systems with 4n electrons it is stabilized (aromatic), whereas with 4n + 2 it is destabilized (antiaromatic). In the excited state 4n + 2, Mobius pi-electron systems are stabilized, and 4n systems are destabilized. No examples of ground-state Mobius pi systems are known, but the concept has been applied to transition states of PERI-CYCLIC REACTIONS (see AROMATIC [3]). 

Whereas methylenecyclopropanes only react with highly electron-deficient dienophiles in a [ 2n + 2fT) + 2n] fashion, alkenylidenecyclopropanes 1 readily undergo this cycloaddition type. A number of comprehensive and elaborate investigations with various alkenylidenecyclopropanes and 4-phenyl-l,2,4-triazoline-3,5-dione indicate that these reactions are concerted and proceed via [( 2j+,25+ 2 J -I-, 2 J transition states, involving the terminal double bond in an eight-electron Mobius aromatic transition structure 4. 

While all cyclic, conjugated six-membered phosphorus heterocycles with AV -phosphorus are clearly Hiickel-aromatic systems, the bonding situation in the cyclic conjugated six-membered systems containing AV -phosphorus has been discussed rather controversially. Schweig and coworkers, for example, discussed a mixture of Huckel and Mobius aromaticity and ylide character... 

The [nlannulenes may be defined as cyclic polyaUcenes possessing a closed circuit of n Ti-conjugated pz-orbitals. The first three small annulenes are shown in Fig. 1 and include [4]annulene (cyclobutadiene), [6]annulene (benzene), and [8]annulene (cyclooctatetraene). The bracketed number, n, can be classified as either a 4n+2 (Hiickel aromatic/Mobous antiasomatic) or 4n (Huckel antiaromatic/ Mobius aromatic) delocalized n-electron species. That said, it is not surprising that the concept of aromaticity is closely associated with the annulenes. Indeed, the first three annulenes listed cover the concepts of Hiickel aromatic, antiaromatic, and nonaromatic properties (see Fig. 1). 

Mobius Aromaticity An aromatic annulene composed of a continuous array of p -orbitals containing a 4n... 

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