Inductive and Mesomeric Effects of the [60]Fulleropyrrolidine Fragment and [60]Fullerene Sphere: A Quantitative Evaluation Based on Theory and Experiments

Inductive and mesomeric effects of the [60]fulleropyrrolidine (Pyr=C60) and the [60]fullerene (C60) molecular frameworks have been investigated by linear free energy relationship analyses. The electronic effects of these moieties have been studied by expressly designing [60]fulleropyrrolidine derivative 1, which has a C–H bond α to both a carbonyl group (C–Hα) and a C60 cage. The extent of polarization of such a bond was then used to probe electron-withdrawing perturbation induced by the fullerene sphere. Thermodynamic measurements based on both theoretical and experimental approaches allowed the acidity of ketone 1 to be measured; this resulted in about a 1000000-fold increase in acidity with respect to that of the structurally correlated acetophenone. Experimental and theoretical kinetic determinations were used to determine the stereolability of ketone 1, which was, in fact, chiral due to the stereogenic carbon α to the carbonyl group. Also, these results confirmed the very strong tendency for the C60 sphere to promote breaking of the C–Hα bond. This means that the Pyr=C60 and C60 fragments are able to express electron-withdrawing effects stronger than those of halogen atoms, but less effective than those of carbonyl derivatives (e.g., ketones, carboxylic acids or acyl derivatives). Finally, the σp, σI and σR– descriptors of inductive and/or mesomeric effects were successfully estimated for both Pyr=C60 and C60 fragments thanks to the integrated use of an original procedure based on semi-empirical calculations and Hammett/Taft correlations involving mono- or dual parameter equations.