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

Abstract

The [4 + 2] cycloaddition of trans,trans-2,4-hexadiene with C60 proceeds via a concerted mechanism with retention of stereochemistry in the cycloadduct 1a. However, when cis,trans-2,4-hexadiene reacts with C60, 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 C60, to form adduct 1a. 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 C60, which is more reactive at elevated temperatures and leads to the formation of the Diels-Alder adduct 1b.