(π+σ)-double aromatic and π,σ-mixed aromatic boron compounds with two electrons delocalized over three centers
✍ Scribed by Matthias Hofmann; Armin Berndt
- Publisher
- John Wiley and Sons
- Year
- 2006
- Tongue
- English
- Weight
- 613 KB
- Volume
- 17
- Category
- Article
- ISSN
- 1042-7163
- DOI
- 10.1002/hc.20225
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✦ Synopsis
Abstract
Using Schleyer's measure for aromaticity, computed nuclear independent chemical shifts (NICS), we review molecules with two electrons delocalized cyclically over three centers: one class with two π‐ as well as two σ‐electrons, i.e. (π + σ)‐double aromatics and a second class with two electrons intermediate between π and σ, i.e. π,σ‐mixed aromatics. These classes show distinctly different patterns of NICS values. On this basis, sila‐, germa‐, and stannaboriranes, recently presented by Jemmis et al., are shown to be the first π,σ‐mixed aromatics which are not homoaromatics. “Improvement of overlap by pyramidalization” a building principle introduced by Haddon in 1988, is the key to understanding the structure of π,σ‐mixed aromatics. New representatives with large aromatic stabilization energies (up to 60 kcal mol^−1^) and very strong pyramidalizations at tricoordinate B, C, and Si centers (up to 63°) are designed by computations. © 2006 Wiley Periodicals, Inc. Heteroatom Chem 17:224–237, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20225