An Unexpected Bispericyclic Transition Structure Leading to 4+2 and 2+4 Cycloadducts in the <i>E</i><i>ndo</i> Dimerization of Cyclopentadiene

Abstract

The stereospecific endo dimerization of cyclopentadiene takes place through an asynchronous and symmetrical bispericyclic transition structure, which shows a merging of the 4+2 and 2+4 cycloaddition paths. The shape of the transition structure testifies to the presence of attractive Salem/Houk secondary orbital interactions assisting the endo approach.