Secododecahedradienes - Syntheses, Reactivity, in-Plane Homoconjugated 3C/2e Cations, 4C/3e Radical Cations, andσ-Bishomoaromatic 4C/2e Dications?

Abstract

Secodecahedradiene 2a, featuring very proximate, perfectly syn-periplanar and significantly pyramidalized C=C bonds, was synthesized as testing object for in-plane(σ)-homoconjugational electron delocalization, starting from the available pagodane 15b. The response of 2a (and in part its diester 2b) – in π,π-distance (average 3.08 Å), olefinic pyramidalization (average 26.9°), and π,π-split (PE, 1.15 eV) intermediate between disecododecahedradiene 1a and 1,16-dodecahedradiene 3a – to selected 4π-reagents, electrophiles, and radicals was explored experimentally and by calculations. Intriguing multistep reaction sequences attest to the ease of competing stabilization pathways for the 3C/2e in-plane homoconjugated cationic intermediates. PE, CV, and ESR measurements and calculations (DFT) characterize the radical cation generated from 2a as in-plane homoconjugated 4C/3e-species 2a.+, persistent in a Freon matrix, but only very shortly existent in solution (CIDNP). Consequently, NMR control of the two-electron oxidation in SbF5/SO2ClF did not disclose the σ-bis-homoaromatic dication 4C/2e (see 2a2+), but a bis-allylic dication 75 as persistent species. In support of 2a2+ as intermediate, evidence is presented for very limited kinetic protection offered by the secododecahedral framework to through H-cage σ-homoconjugated cations.