Mechanism and Selectivity of Iron-Catalyzed [4+2] Cycloadditions of Unactivated Dienes: A Computational Study.

The mechanisms of iron-catalyzed [4 + 2] cycloadditions of unactivated dienes were investigated using density functional theory calculations. The calculation results show that the reaction involves sequential key steps of an initial ligand exchange followed by oxidative coupling, isomerization to form a seven-membered ferracycle intermediate, and C-C reductive elimination to form the cyclohexene product. The C-C reductive elimination step is shown to be the rate-determining step of the catalytic cycle. Moreover, energy profiles with three possible spin states (S_Fe = 0, 1, 2) have been considered. The results show that spin crossing occurs mainly through quintet intermediates and triplet transition states, which indicates that the reaction has a two-state reactivity. In addition, the origins of the chemical selectivities and enantioselectivities are analyzed in detail. It was found that the spatial effect between the catalyst ligand and the substrate leads to high [4 + 2] chemoselectivity, while the stabilizing attractive interaction between the ligand and the substrate leads to high enantioselectivity.