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

Abstract

[2+2+2]-Cycloreversion reactions of cyclohexane and ten fused cyclohexanes were studied computationally with B3LYP/6-31G* and CASSCF methods. Reactions involving cleavage of bonds in three- and five-membered rings show distinctly 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 the 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-cyclopropacyclohexane were studied.