[2+2+2]-Cycloreversion reactions: a theoretical elucidation of thermodynamic and through-bond coupling effects on activation energies

[2+2+2]-Cycloreversion reactions of cyclohexane and ten fused cyclohexanes were studied computationally with B3LYP/6-31G* and CASSCF methods. Reaction s involving cleavage of bonds in three- and five-membered rings show distin ctly lower barriers to cycloreversion than cleavage of four-membered rings. The lower activation energies for the cleavage of odd-membered rings arise from interactions of the sigma framework of the odd-membered ring with the orbitals of the breaking bond. NICS values were calculated to determine th e aromaticity of the different rings involved in bond cleavage. In addition to concerted mechanisms, the stepwise diradical pathways for the [2 + 2 2]-cycloreversions of cyclohexane and cis-tris-cydopropacyclohexane were st udied.