Mobius homoaromaticity

While the initial formulation of homoaromaticity pre-dated the introduction of orbital symmetry by some eight years33, the two concepts are inextricably linked34. This is most evident when pericyclic reactions are considered from the perspective of aromatic or antiaromatic transitions states35 and the Huckel/Mobius concept31. The inter-relationship can be demonstrated by the electrocyclic reaction shown in Scheme 136. 

The closed and open forms, 4 and 5, respectively, represent the formal starting and end points of an electrocyclic reaction. In terms of this pericyclic reaction, the transition state 6 can be analysed with respect to its configurational and electronic properties as either a stabilized or destabilized Huckel or Mobius transition state. Where 4 and 5 are linked by a thermally allowed disrotatory process, then 6 will have a Hiickel-type configuration. Where the process involves (4q + 2) electrons, the electrocyclic reaction is thermally allowed and 6 can be considered to be homoaromatic. In those instances where the 4/5 interconversion is a 4q process, then 6 is formally an homoantiaromatic molecule or ion. 

A potential example of Mobius n-conjugation is the cychc cation C9H/ (212). On the basis of computational evidence provided by Schleyer et al.," it is considered a chiral Mobius 4n-7i-electron aromatic species of a helical C2-symmetrical structure with eight delocalized electrons. Allan and Rzepa" have recently explored topological homoaromaticity by analyzing a series of... 

For further discussion of homoaromatic systems, see (a) McEwen, A. B., Schleyer, P. v. R. (1986). In-plane aromaticity and trishomoaromaticity a computational evaluation. Journal of Organic Chemistry, 51,4357-4368. (b) Allan, C. S. M., Rzepa, H. S. (2008). Chiral Aromaticities. A Topological Exploration of Mobius Homoaromaticity. Journal of Chemical Theory and Computation, 4, 1841-1848. (c) Allan, C. S. M., Rzepa, H. S. (2008). Chiral Aromaticities. AIM and ELF Critical Point and NICS Magnetic Analyses of Mobius-Type Aromaticity and Homoaromaticity in Lemniscular Annulenes and Hexaphyrins. Journal of Organic Chemistry, 73, 6615-6622. 

Barzaghi M, Gatti C (1987) Homoaromaticity versus Mobius aromaticity. J Chimie Physique... 

This has been very well extended to Mobius systems and hetero [66-69] and homoaromatic [70] systems. The concept of Topological resonance energy as a mea-snre of aromaticity has been extended to the lowest excited state of annnlenes. Here the excited state in the lowest n-n triplet state and the reference strnctnre is the most stable biradical. 

The term aromaticity was introduced 150 years ago to indicate pleasant aroma in compounds like benzene and later on it was associated with extra stability of certain organic compounds. Since then the scope of the term is ever expanding and various types of aromaticity such as antiaromaticity, pseudoaromaticity, homoaromaticity, heteroaromaticity, Mobius aromaticity, and metal aromaticity have been defined. The measures for assessment of the aromaticity are plentiful and ranges from purely qualitative to virtually quantitative. Although the concept has a profound impact on chanistry and chanical thinking over several decades, it remains conceptual, as it is not an experimentally measurable or verifiable property. 

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