VALENCE ISOMERISM AND REARRANGEMENTS IN METHANOFULLERENES

A PM3 computational and experimental study of the methanofullerenes 1- 5 demonstrates that the electronic basis for the experimentally prefer red formation of the [6,6]-closed and [6,5]-open over the [6,6]-open a nd [6,5]-closed isomers of methanofullerenes is the preservation of th e [5]radialene-type bonding pattern found in C-60, by these two struct ures. The [6,5]-open is the less stable of the two experimentally isol ated isomers as a result of the violation of Bredt's Rule. The PM3 met hod locates a [6,5]-closed structure for the methanofullerenes 2-5, bu t not for 1. This suggests that the thermal interconversion of the [6, 5]-open and [6,6]-closed isomers can only occur via a stepwise mechani sm-[6,5] -open to [6,5]-closed valence isomerisation followed by a 1,5 -shift-in cases where the corresponding [6,5]-closed structure is loca ted in a local energy minimum.