B3LYP calculations on bishomoaromaticity in substituted semibullvalenes*

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

Homoaromaticity in aza‐ and phosphasemibullvalenes. A computational study

“…Over the last few decades, researchers have discovered and explored a wealth of structures based on 1 ,3, 4 the intriguing fundamental goal being the experimental identification of a semibullvalene having a delocalized bishomoaromatic ground state. Strategies towards this goal have mainly been twofold: (a) introduction of substituents5–15 or coordinating entities16, 17 aiming at electronic stabilization of 1b relative to 1a and (b) annelation of small rings7, 18–24 which by strain destabilizes 1a relative to 1b . Both strategies have been pursued by experimental and computational means, the latter proving to be a valuable tool in the search for homoaromatic structures 5–8, 11–17, 19–21, 23.…”

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“…[20] In addition, several studies could show the excellent agreement of the B3LYP with the CCSD(T) method for the Cope rearrangement of many different molecules, including bullvalenes; see ref. [21] and references therein. The CCSD(T) calculations are needed to support the other methods, as such calculations lack exact exchange (DFT/BP86), nondynamical correlation (DFT/BP86, DFT/B3LYP), and dynamical correlation (CASSCF).…”

A C2v-Symmetric Barbaralane

“…In addition, several studies could show the excellent agreement of the B3LYP with the CCSD(T) method for the Cope rearrangement of many different molecules, including bullvalenes; see ref. 21 and references therein. The CCSD(T) calculations are needed to support the other methods, as such calculations lack exact exchange (DFT/BP86), nondynamical correlation (DFT/BP86, DFT/B3LYP), and dynamical correlation (CASSCF).…”