(pi+sigma)-double aromatic and pi,sigma-mixed aromatic boron compounds with two electrons delocalized over three centers

作者：Hofmann M^*; Berndt A

来源：Heteroatom Chemistry, 2006, 17(3): 224-237.

DOI：10.1002/hc.20225

Using Schleyers measure for aromaticity, computed nuclear independent chemical shifts (NICS), we review molecules with two electrons delocalized cyclically over three centers: one class with two pi- as well as two sigma-electrons, i.e. (pi + sigma)-double aromatics and a second class with two electrons intermediate between pi and sigma, i.e. pi, sigma-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 pi,sigma-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 pi,sigma-mixed aromatics. New representatives with large aromatic stabilization energies (tip to 60 kcal mol(-1)) and very strong pyramidalizations at tricoordinate B, C, and Si centers (up to 63 degrees) are designed by computations.

• 出版日期2006