Restricted rotation in (phenylpyrrolidino)fullerene derivatives

A complete series of (phenylpyrrolidino)fullerene derivatives has been prepared. A detailed conformational analysis of these compounds has been carried out by variable-temperature H-1 NMR experiments and computational studies. In the case of (phenylpyrrolidino)fullerene derivatives without ortho substituents, dynamic phenomena arising from restricted rotation around the phenyl-pyrrolidine bond are observed. In contrast, as soon as one of the ortho positions of the phenyl ring is substituted, the rotational energy barrier is high enough to prevent observation under our experimental conditions (room temperature to 120 degrees C) of any dynamic exchange resulting from rotation of the phenyl substituent on the pyrrolidine ring. Whereas, in principle, two diastereoisomeric conformers can exist for the ortho-substituted (phenyl-pyrrolidino)fullerenes, only the atropisomers in which the unsubstituted ortho position is located atop the fullerene sphere are obtained. We conclude that the reaction of the ortho-substituted benzaldehyde derivatives with C-60 is diastereoselective, affording only one of the two possible atropisomers. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).