The Syntheses and Crystal-Structures of Decahydro-5,14:6,13:7,12-Trimethanopentacene and Decahydro-5,14-7,12-Dimethanopentacene: Two Useful Molecules for Investigating Laticyclic Hyperconjugation

Abstract

The syntheses of decahydro-5,14:6,13:7,12-trimethanopentacene and decahydro-5,14:7,12-dimethanopentacene are described (see Schemes 1 and 2 respectively). Diels -Alder reaction of diene (10) with two equivalents of tetrachlorothiophen 1,1-dioxide (11) gave the adduct (12), which was aromatized to (13) through treatment with base. Reductive dechlorination (Na/ PrⁱOH ) of (13) gave (6). A similar set of reactions performed on (17), itself prepared from cyclohexa-1,4-diene and two equivalents of hexachlorocyclopentadiene (15), gave (7). The crystal structures of (6) and (7) were determined. An interesting feature of the structure of (7) is the slight endo pyramidalization (by c. 3°) of the trigonal carbon atoms C 4a, C 7a, C 11a and C 14a. Model ab initio calculations (STO-3G basis set) on complexes (27) and (28), whose geometries were culled from the crystal structure of (6), indicate that significant laticyclic hyperconjugative interactions (c. 0.3 eV ) are mainly responsible for causing the observed splitting between the π_s+π_s and π_s-π_s levels in the photoelectron spectrum of (6). Orbital interactions through the pair of six connecting sigma bonds (OIT-6-B) in (6) have a negligible effect (c. 0.08 eV ) on these levels. In agreement with prediction, laticyclic hyperconjugation does not affect the π_a+π_a and π_a-π_a levels.