Substrate-Dependent Two-State Reactivity in Iron-Catalyzed Alkene [2+2] Cycloaddition Reactions.

Iron-catalyzed alkene [2+2] cycloaddition reactions represent a promising stepwise pathway to effect the kinetically hindered concerted [2+2] cycloaddition. However, the fundamental reactivity paradigm of these reactions remains unclear. Based on high level combined CASPT2/DFT modelings, herein we reveal an unprecedented substrate-dependent two-state reactivity scenario for the key C-C coupling in this iron catalysis, in which the representative substrates of mono-olefins only and mono-olefin plus 1,3-diene exhibit different reactivity paradigms. The role of the redox-active ligand is found to generate a ferric oxidation state for the metallacyclic intermediate of C-C coupling, thereby rendering a thermodynamically more accessible Fe/Fe reductive elimination process compared with the otherwise Fe/Fe one. The enhancement of the spin state transition efficiency between the singlet and triplet states is predicted as an alternative way to increase the C-C coupling reactivity in the cross [2+2] cycloaddition reactions between mono-olefins and dienes. This work highlights the ab initio multi-reference method in describing very complicated open-shell iron catalysis.