[1,5] Sigmatropic Hydrogen Shifts in Cyclic 1,3-Dienes

Density functional calculations have been carried out for [1,5] hydrogen shifts in 1,3-cycloalkadienes
(cyclohexadiene, cycloheptadiene, and cyclooctadiene). The complexity of the potential surfaces of
these reactions was found to increase with ring size. For 1,3-cyclohexadiene a single transition
structure for the [1,5] hydrogen shift was located, which connects the two enantiomeric conformers.
For 1,3-cycloheptadiene two enantiomeric transition structures for the [1,5] hydrogen shift were
located, which interconnect three conformers of the diene, a pair of enantiomeric conformers and
a third achiral conformer. Finally for 1,3-cyclooctadiene two diastereomeric transition structures
were found in addition to six conformers (three pairs of enantiomeric conformers) of the diene.
Calculated activation energies for the [1,5] hydrogen shifts were found to be in qualitative agreement
with experiment. Variation in these energies are attributed to strain energies present in either
the diene or the transition structure.