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 norbornadien e, 1,3,5-cycloheptatriene, norcaradiene, and toluene. These include the [1, 5]hydrogen shift in cycloheptatriene, the [1,5]carbon shift (walk rearrange ment) in norcaradiene, the ring flip of cycloheptatriene, and the formation of norcaradiene from cycloheptatriene. Our best theoretical calculations f or the walk rearrangement predict that the Woodward-Hoffmann "forbidden" su prafacial-inversion transition state and the "allowed" suprafacial-retentio n pathway differ in energy by < 1 kcal/mol. Further, both transition states are effectively biradical-like. The transition state for formation of tolu ene from norcaradiene proceeds via a hydrogen transfer transition state tha t is formed directly from the "allowed" transition state for the walk rearr angement but not from the "forbidden" transition state. Further, the transi tion state for the transannular hydrogen shift from the "allowed" transitio n state to 3-methylene-1,4-cyclohexadiene was calculated to be too high in energy to participate in the formation of toluene.