Are 1,5-disubstituted semibullvalenes that have C2v equilibrium geometries necessarily bishomoaromatic?

Eric C Brown; Daven K Henze; Weston Thatcher Borden

doi:10.1021/ja027392p

Are 1,5-disubstituted semibullvalenes that have C2v equilibrium geometries necessarily bishomoaromatic?

J Am Chem Soc. 2002 Dec 18;124(50):14977-82. doi: 10.1021/ja027392p.

Authors

Eric C Brown¹, Daven K Henze, Weston Thatcher Borden

Affiliation

¹ Department of Chemistry, Box 351700, University of Washington, Seattle, WA 98195-1700, USA. ecbrown@u.washington.edu

PMID: 12475340
DOI: 10.1021/ja027392p

Abstract

Unrestricted density functional theory (UB3LYP), CASSCF, and CASPT2 calculations have been employed to compute the relative energies of the C(s) and C(2v) geometries of several 1,5-disubstituted semibullvalenes. Substitution at these positions with R = F, -CH(2)-, or -O- affords semibullvalenes that are predicted to have C(2v) equilibrium geometries. Calculated singlet-triplet energy splittings and the energies of isodesmic reactions are used to assess the amount of bishomoaromatic character at these geometries. The results of these calculations show that employing strain to destabilize the C(s) geometries of semibullvalenes can lead to a significant decrease in the amount of bishomoaromatic stabilization of the C(2v) geometries, due to reduced through-space interaction between the two allyl groups. However, the C(2v) equilibrium geometries of the 1,5-disubstituted semibullvalenes with R = F and -RR- = -O- do benefit from stabilizing through-bond interactions between the two allyl groups. These interactions involve mixing of the bisallyl HOMO with the low-lying C-F or C-O sigma orbital combinations of the same symmetry. In contrast, for -RR- = -CH(2)-, through-bond interactions destabilize the bisallyl HOMO and are predicted to make the ground state of this semibullvalene a triplet.