[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.
Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37205, USA. hessba@ctrvax.vanderbilt.edu
B Andes
BA
Hess
John E
JE
Baldwin