Cyclopentacorannulene: pi-Facial stereoselective deuterogenation and determination of the bowl-to-bowl inversion barrier for a constrained buckybowl

Attachment of an ethane or ethylene unit to the rim of corannulene produced relatively rigid bowls of dihydrocyclopenta- and cyclopentacorannulene, respectively. In contrast to the parent corannulenes, their inversion barriers are too high to be determined by the NMR coalescence methods. Due to the significant curvature of cyclopentacorannulene, deuterogenation is pi-facial specific; both heterogeneous and homogeneous catalysis lead exclusively to exo-dideuteriocyclopentacorannulene 2a. Equilibration of the endo- and exo-isotopomers allowed the determination of Delta G(double dagger) at 27.61-27.67 kcal/mol over the temperature range 52.1-99.3 degrees C and the estimation of Delta H-double dagger (27.3 +/- 0.7 kcal/mol) and Delta S-double dagger (-1.1 +/- 0.2 eu) for the bowl-to-bowl inversion. The inversion barrier calculated at the HF/6-31G*//3-21G level (25.9 kcal/mol) compares well with the experimental result.