[60]Fullerene as a substituent.

The substituent effect of the dihydro[60]fullerenyl group and its hydrophobic parameters have been evaluated quantitatively. The substituent constant has been determined from the pK value of a fullerene-based, para-substituted benzoic acid 1 in 80% dioxane/water (v/v) by NMR spectroscopy. The resulting Hammett sigma value of 0.06, consistent with a small electron-withdrawing effect of C(60), is a consequence of the fact that only inductive effects can be transmitted through the two tetracoordinate carbon atoms between the fullerene pi system and the para-position of the benzoic acid moiety in 1. The parameter pi, which describes the hydrophobic character of the substituent C(60), has been evaluated as the difference between that of 1 and model compound 2. The pi value, which is larger than 3, indicates that the fullerene cage imparts high hydrophobicity to the molecule to which it is attached. Finally, we have evaluated how the fullerene spheroid influences the acid-base properties and nucleophilicity of the pyrrolidine nitrogen in a suitably functionalized fulleropyrrolidine. The fulleropyrrolidine 4 (pK(BD)(+)=5.6) is six orders of magnitude less basic and 1000 times less reactive than its model 3 (pK(BD)(+)=11.6). This may be related to through-space interactions of the nitrogen lone pair and the fullerene pi system.