Kinetic substituent and solvent effects in 1,3-dipolar cycloaddition of diphenyldiazomethanes with fullerenes C60 and C70: a comparison with the addition to TCNE, DDQ, and chloranil

Kinetics of 1,3-dipolar cycloaddition of a series of meta- and para-substituted diphenyldiazomethanes (DDMs) with fullerenes C60 and C70 as dipolarophiles have been investigated in toluene at 30 degrees C. Fullerene C60 was ca. 1.5 times more reactive than C70. The rate constants (k) for the primary [3 + 2] additions increased with the increase of the electron-releasing ability of the meta and para substituent. The log k/k0 values were well correlated by the Yukawa-Tsuno (Y-T) equations with the smaller negative rho values (-1.6 and -1.7 for C60 and C70) and the reduced resonance reaction constants r (0.22 and 0.17) compared to similar reactions of common acceptors, TCNE, DDQ, and chloranil (CA). The plots of log k (acceptor) versus log k (C60) as reference gave good regression equations and the slopes became larger in the order of TCNE > DDQ > CA > C70 > or = C60. The rates were also found to decrease with the increase of solvent polarity due to the ground-state solvation of fullerenes. However, the relative reactivity of these acceptors toward the unsubstituted DDM increased in the order of DDQ > C60 > or = C70 > TCNE > CA. The unexpected higher reactivity of fullerenes was interpreted in terms of the inherent steric strain by the pyramidalization of the sp2 C-atoms as well as the shorter [6,6] bonds with larger pi-electron densities.