Thermal Rearrangements of Norcaradiene

Multireference ab initio methods and density functional theory with a 6-31G* basis set have been applied to study the interconversions of norbornadiene, 1,3,5-cycloheptatriene, norcaradiene, and toluene. These include the [1,5]hydrogen shift in cycloheptatriene, the [1,5]carbon shift (walk rearrangement) in norcaradiene, the ring flip of cycloheptatriene, and the formation of norcaradiene from cycloheptatriene. Our best theoretical calculations for the walk rearrangement predict that the Woodward−Hoffmann “forbidden” suprafacial-inversion transition state and the “allowed” suprafacial-retention pathway differ in energy by <1 kcal/mol. Further, both transition states are effectively biradical-like. The transition state for formation of toluene from norcaradiene proceeds via a hydrogen transfer transition state that is formed directly from the “allowed” transition state for the walk rearrangement but not from the “forbidden” transition state. Further, the transition state for the transannular hydrogen shift from the “allowed” transition state to 3-methylene-1,4-cyclohexadiene was calculated to be too high in energy to participate in the formation of toluene.