Mobius annulenes

The fact that there is at least one antibonding overlap in the cyclic array of orbitals does not reduce aromaticity. Mobius transition states are as aromatic as normal Htickel transition states. In contrast to stable Mobius annulenes, Mobius transition states in pericyclic reactions are quite frequent. They also include c aromatic... 

Figure 1.5. View of the Mobius stabilized annulene. Crystallographic data from Ajami, 2003. The tetradehydrodianthracene and twisted structures are shown left and right of the figure, respectively. C and H are represented by black and light grey balls, respectively.

The overlap of this twisted radical with a monoatomic radical to give a twisted polyene is positive. It is zero for an annulene (Figure 3.5). A Mobius An + 2 annulene is... 

Figure 3.5 Formal unions giving a Mobius in + 2 polyene and a Mobius in + 2 annulene.

E. Heilbronner, Tetrahedron Lett. 5, 1923 (1964). Hiickel Molecular Orbitals of Mobius-Type Conformations of Annulenes. 

Even though the stability of Mobius systems was predicted over 40 years ago [103], for a long time no such systems were synthesized. One possible reason for this is the expected strain in the twisted structure. Based on quantum chemical calculations, different groups suggested the stability of [4 ]annulenes [(CH) , with n = 3-5], but it was also shown that there are many possible isomers of this system that are close in energy and thus the Mobius system might easily flip back to the less strained Hiickel structures [104],... 

The first real Mobius systems ([16]annulenes) were only synthesized a few years ago [105], In these systems the authors achieved enough rigidity for the molecules so that they would not flip back to a Hiickel system. It was also determined that these Mobius-twisted annulenes are more aromatic in their character than the Hiickel-systems [106],... 

Figure 3.11 B3LYP/6-31G optimized structures of the Mobius annulenes 49-51. ...

It seems clear that definitive evidence of a synthesized aromatic Mobius annu-lene has not been produced. We note in passing that a few examples of aromatic Mobius hexaporphyrins have been reported. Castro and Kamey have suggested that an aromatic Mobius annulene has been prepared, but as a transition state. The optimized transition state 54 for the twist-coupled bond shifting process then connects the tri-frani-[12]annulene 53 with the di-fra i-[12]annulene... 

The common Mobius strip has a single half twist. Ribbons can be even more twisted and so higher order annulenes might be possible, expressing potential aromatic or antiaromatic properties. Rzepa et al. have found examples of these higher order twisted annulenes, such as Ci4Hx4 (63) with one full twist, Ci Hj ... 

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. 

Heilbronner, E. Hiickel molecular orbitals of Mobius-type conformations of annulenes, Tetrahedron Lett. 1964, 5,1923-1928. 

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

Okoronkwo, T. Nguyen, P. T Castro, C. Karney, W. L. [14]Annulene cis/trans isomerization via two-twist and nondegenerate planar bond shifting and Mobius conformational minima, Org. Lett. 2010,12, 972-975. 

Edgar Heilbronner in 1964 worked out that such a twisted system (Mobius system) would be aromatic if it contained 4n conjugated tr-electrons. Curiously, no actual examples were identified until relatively recently, when it was proposed that [12], [16] and [20] annulenes have several higher energy conformations which adopt this mode. In 2003, the first crystal structure of a true stable Mobius [16] annulene was completed, and various heteroannulenes were identified as existing in Mobius form. 

The antiaromatic geometry found along the concerted path of ground-state-forbidden pericyclic reactions, which is topologically equivalent to an antiaromatic Hiickel [4n]annulene or MObius [An + 2]annulene, is a particularly interesting type of biradicaloid geometry. (Cf. Section 4.4.) Other biradicaloid geometries and combinations of those mentioned are equally possible. 

Fig. 8.7. Most stable (top) and most aromatic (bottom) Mobius structures for [12], [16], and [20]annulene. Reproduced from Org. Lett., 4, 3431 (2002), by permission of the American Chemical Society.

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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