Stereochemistry of the [4+2] cycloadditions of trans,trans- and cis,trans-2,4-hexadiene to C60

The [4 + 2] cycloaddition of trans, trans-2,4-hexadiene with C-60 proceeds via a concerted mechanism with retention of stereochemistry in the cycloadduct la. However, when cis, trans-2,4-hexadiene reacts with C-60d, isomerization of the cis,trans to the thermodynamically more stable trans,trans isomer occurs. Subsequently, the cis,trans diene isomerized to the trans,trans isomer and cycloadds to C-60, to form adduct la. When the reaction is carried out at higher temperatures, the formation of cycloadduct 1b is also obtained. This result is consistent with a concerted cycloaddition of cis, trans-2,4-hexadiene with C-60, which is more reactive at elevated temperatures and leads to the formation of the Diels-Alder adduct 1b.