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 preferred formation of the [6,6]-closed and [6,5]-open over the [6,6]-open and [6,5]-closed isomers of methanofullerenes is the preservation of the [5]radialene-type bonding pattern found in C₆₀ by these two structures. The [6,5]-open is the less stable of the two experimentally isolated isomers as a result of the violation of Bredt's Rule. The PM3 method locates a [6,5]-closed structure for the methanofullerenes 2-5, but 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 mechanism - [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 located in a local energy minimum.