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 CC 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 CC coupling, thereby
rendering a thermodynamically more accessible Fe<sup>III</sup>/Fe<sup>I</sup> reductive elimination process compared with the otherwise
Fe<sup>II</sup>/Fe<sup>0</sup> one. The enhancement of the spin state
transition efficiency between the singlet and triplet states is predicted
as an alternative way to increase the CC 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
