Manifestations of the Alder−Rickert Reaction in the Structures of Bicyclo[2.2.2]octadiene and Bicyclo[2.2.2]octene Derivatives

The Alder−Rickert ethylene extrusion reaction manifests in the ground state structures of compounds
9−12 which contain the bicyclo[2.2.2]octadiene moiety and compounds 13, 14, and 17−20 which contain
the bicyclo[2.2.2]octene moiety. A significant decrease of the 13C−13C one-bond coupling constants for
the C−C bonds, which break in this fragmentation reaction, suggests lengthening, and hence weakening
of these bonds. In the unsymmetrical systems these effects are also shown to be associated with
strengthening of the CH2−CH2 bond, which is ultimately lost from the molecule as ethylene. Low-temperature crystal structures of compounds 9−12 and 16 provide evidence for similar crystal packing
requirements of the CH2−CH2 and CHCH moieties.