Density Functional Study of the Retrocyclization of Norbornadiene and Norbornene Catalyzed by Fe

In the presence of Fe+ catalyst, the retro Diels−Alder reaction of norbornadiene (NBD) is predicted to be stepwise with an activation barrier of 18.8 kcal/mol, which is 3.1 kcal/mol lower than the concerted retro reaction. For norbornene (NBN), the Fe+-catalyzed retro reaction is also calculated to be stepwise with an activation barrier of 24.9 kcal/mol, which is 8.5 kcal/mol lower than the uncatalyzed stepwise reaction but 3.8 kcal/mol higher than the concerted reaction. The intermediates from the NBD and NBN retro Diels−Alder reactions, C5H6FeC2H2 + and C5H6FeC2H4 +, are predicted to have low activation barriers for ligand-to-ligand hydrogen transfers (through an iron−hydrido intermediate) to form CpFeC2H3 + and CpFeC2H5 + and, ultimately, vinyl- and ethyl-substituted cyclopentadiene−iron complexes, respectively. In contrast to FeC2H2 + and FeC2H4 +, the lowest-energy pathways on the C5H6FeC2H2 + and C5H6FeC2H4 + potential energy surfaces involve only one multiplicity (quartet). The C2H2 and C2H4 complexes of CpFe+ and C5H6Fe+ are compared.