The perchlorates of 7-tropylionorbornane (3), 2-tropyliobicyclo[2.2.2]octane (4), and 2-tropylioadamantane (5) were found to undergo stereomutation in acetonitrile at 25-85-degrees-C, as evidenced by following the change in the C-13 NMR spectra of their deuterium-labeled compounds in acetonitrile-d3. Addition of tetracyanoethylene to their solutions in acetonitrile gave the [8 + 2] cycloaddition products of the corresponding heptafulvenes, indicating the intermediacy of the heptafulvenes. This stereomutation was too slow to detect in dichloromethane at the same temperature. Therefore, it is concluded that the stereomutation proceeds through the abstraction of the alpha-hydrogen by acetonitrile as a base followed by the protonation of the heptafulvene intermediate from the rear side. The entropies of activation which are significantly negative and large indicate abstraction of the alpha-hydrogen is the rate-determining step. The enthalpy of activation for the norbornane system (22.1 +/- 1.1 kcal/mol) is about 8 kcal/mol greater than that for the bicyclo[2.2.2]octane system (15.4 +/- 1.1 kcal/mol) and that for the adamantane system (13.1 +/- 1.3 kcal/mol). Molecular mechanics calculations (MMPI, MM2') for the three heptafulvenes and the corresponding phenyl derivatives (instead of the tropylium ions) indicated that the angle strain at the 8-position of the heptafulvenes controls the rate of the steremutation.
IKAI, KEIZO;TAKEUCHI, KENICHI;KINOSHITA, TOMOMI;HAGA, KENICHI;KOMATSU, KO+, J. ORG. CHEM., 56,(1991) N, C. 1052-1058