[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% v/v dioxane/water using NMR spectrometry. The resulting s value of 0.06, consistent with a small electron-withdrawing effect of C60, is due to the fact that only inductive effects can be transmitted through the two tetracoordinated carbon atoms between the fullerene p-system and the para-position of the benzoic acid moiety in 1. The parameter p, which describes the hydrophobic character of the substituent C60, has been evaluated as the difference between that of 1 and model compound 2. The p ƒnvalue, larger than 3, indicates that the fullerene cage imparts high hydrophobicity to the molecule where it is inserted. Finally, we have evaluated how the fullerene spheroid influences the acid-base properties and nucleophilicity of the pyrrolidine nitrogen in a suitably functionalized fulleropyrrolidine. We have found that fulleropyrrolidine 4 (pKBD+ = 5.6) is six orders of magnitude less basic and 1000 times less reactive than its model 3 (pKBD+ = 11.6). This may be related to through-space interactions of the nitrogen lone pair and the fullerene p-system