Stereoelectronic effects on the retro-Diels-Alder fragmentation of ionized bicyclo[4.3.0]nona-3,7-dienes

The electron impact and collision-induced dissociation mass spectra of cis- and trans-annulated bicyclo[4.3.0]nona-3,7-dienes differ in their relative abundances of [C₅H₆]⁺˙ fragments formed by the retro-Diels-Alder decomposition. The formation of [C₅H₆]⁺˙ is not preceded by hydrogen migration in the short-lived and long-lived molecular ions. The appearance energy of [C₅H₆]⁺˙ from both annulation isomers is identical within experimental error: AE_cis([C₅H₆]⁺˙)=10.56±0.10 eV and AE_trans([C₅H₆]⁺˙)=10.54±0.15 eV. The barrier to the retro-Diels-Alder fragmentation lies 68–76 kJ mol^?1 above the thermo-chemical threshold corresponding to [C₅H₆]⁺˙ + C₄H₆. Investigation of the two-dimensional reaction coordinate by the Topological Molecular Orbital treatment shows that the lowest energy path for the retro-Diels-Alder reaction involves a two-step dissociation of the C(5)? C(6) and C(1)? C(2) bonds in the molecular ion, the latter step overcoming a barrier, calculated as 80 kJ mol^?1 above the thermochemical threshold. The stereochemical difference between the geometric isomers is due to stereoelectronic assistance of the π orbitals of the cis-annulated isomer in the cleavage of the C(5)? C(6) bond. Other mechanisms of the retro-Diels–Alder reaction are discussed.