Chiral Aromaticities. AIM and ELF Critical Point and NICS Magnetic Analyses of Mobius-Type Aromaticity and Homoaromaticity in Lemniscular Annulenes and Hexaphyrins

• Published in: Journal of organic chemistry 2008-09, Vol.73 (17), p.6615-6622
• Main Authors: ALLAN, Charlotte S. M, RZEPA, Henry S
• Format: Article
• Language: English
• Subjects: Alicyclic compounds; Alicyclic compounds, terpenoids, prostaglandins, steroids; Chemical reactivity; Chemistry; Exact sciences and technology; Heterocyclic compounds; Heterocyclic compounds with several n hetero atoms in the same ring, in separated rings or in fused rings; Organic chemistry; Preparations and properties; Reactivity and mechanisms
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo801022b

An atoms-in-molecules (AIM) and electron localization function (ELF) critical point analysis is reported for two types of lemniscular system, each of which exhibits double-half-twist Mobius topology. This reveals that this type of conformation for [14]annulene 1 has, in addition to the obvious bond critical points (BCPs), two weaker transannular points in the central cross-over region. These can be interpreted in terms of local rings showing single-half-twist Mobius homoaromaticity in addition to the double-half-twist aromaticity revealed by the annulene as a whole. Another example of a single-half-twist Mobius homoaromatic 9 is suggested here to show aromatic properties as strong as its nonhomoaromatic analogue 8. The AIM critical points in 1 are relatively insensitive to the ring size (varied from 12 to 16), and only small changes are seen in the critical point properties when the pi-electron count is incremented from 4n+2 to 4n by dianion formation. These results are discussed in terms of the reported transformation of the 14-pi-electron octalene 10 by reduction/alkylation into 12, an isomer of 1. Another class of molecule that exhibits lemniscular topology is the phyrins. A transannular BCP in the central cross-over region for the double-half-twist aromatic [26]hexaphyrin 3 is revealed, which is not present for the double-half-twist antiaromatic [28]hexaphyrin 2. The NICS(rcp) for the former indicates strong Mobius homoaromaticity.