B3LYP calculations on bishomoaromaticity in substituted semibullvalenes*

✍ Scribed by David A. Hrovat; Richard Vaughan Williams; Alan C. Goren; Weston Thatcher Borden

• Publisher: John Wiley and Sons
• Year: 2001
• Tongue: English
• Weight: 199 KB
• Volume: 22
• Category: Article
• ISSN: 0192-8651
• DOI: 10.1002/jcc.1110

✦ Synopsis

Abstract

B3LYP/6‐31G* calculations on the degenerate rearrangements of substituted semibullvalenes spuriously predict the relative enthalpies of the bishomoaromatic TSs to be lower than the experimental values. However, the calculations do make the useful and experimentally testable prediction that the two cyano and two phenyl substituents in 2,6‐dicyano‐4,8‐diphenylsemibullvalene (9d) are more likely than four cyano substituents in 2,4,6,8‐tetracyanosemibullvalene (9f) or the four phenyl substituents in 2,4,6,8‐tetraphenylsemibullvalene (9g) to produce a semibullvalene that has a bishomoaromatic equilibrium geometry in the gas phase. The major reason for the surprising finding that 9d is more likely to be bishomoaromatic than 9g is shown to be steric interactions between the phenyl groups at C‐2 and C‐8 and at C‐4 and C‐6 in bishomoaromatic structure 10g. These interactions inhibit the conjugative stabilization of 10g; but they are absent in bishomoaromatic structure 10d, where cyano groups replace the phenyl groups at C‐2 and C‐6 in 10g. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1565–1573, 2001