Secododecahedradienes - Syntheses, reactivity, in-plane homoconjugated 3C/2e cations, 4C/3e radical cations, and sigma-bishomoaromatic 4C/2e dications?

Secodecahedradiene 2a, featuring very proximate, perfectly syn-periplanar a nd significantly pyramidalized C-C bonds, was synthesized as testing object for in-plane(c)-homoconjugational electron delocalization. starting from t he available pagodane 15b. The response of 2a (and in part its diester 2b) - in pi,pi -distance (average 3.08 Angstrom), olefinic pyramidalization (av erage 26.9 degrees), and pi,pi -split (PE. 1.15 eV) intermediate between di secododecahedradiene la and 1,16-dodecahedradiene 3a - to selected 4 pi -re agents, electrophiles, and radicals was explored experimentally and by calc ulations. Intriguing multistep reaction sequences attest to the ease of com peting stabilization pathways for the 3C/2e in-plane homoconjugated cationi c intermediates. PE, CV, and ESR measurements and calculations (DFT) charac terize the radical cation generated from 2a as in-plane homoconjugated 4C/3 e-species 2a(.-), persistent in a Freon matrix. but only very shortly exist ent in solution (CIDNP). Consequently. NMR control of the two-electron oxid ation in SbF5/SO2ClF did not disclose the sigma -bis-homoaromatic dication 4C/2e (see 2a(2+)). but a bis-allylic dication 75 as persistent species. In support of 2a(2+) as intermediate. evidence is presented for very limited kinetic protection offered by the secododecahedral framework to through H-c age sigma -homoconjugated cations.