Computational Study of Iron-Catalyzed Intramolecular [2 + 2] Cycloaddition and Cycloisomerization of Enyne Acetates: Mechanism and Selectivity.

The mechanism of iron-catalyzed intramolecular [2 + 2] cycloaddition and cycloisomerization of enyne acetates has been investigated with DFT computations. Both mechanisms start the catalytic cycle from the stepwise 1,2-acyloxy migration to afford the iron carbene. The [2 + 2] cycloaddition mechanism involves subsequent key steps of [2 + 2] cycloaddition, 1,2-acyloxy migration, and reductive elimination to generate the azabicyclo [3.2.0] heptane product, with the reductive elimination being the rate-determining step. The cycloisomerization mechanism involves subsequent key steps of [2 + 2] cycloaddition, stepwise 1,4-acyloxy migration to produce the allenylpyrrolidine product, with the 1,4-acyloxy migration being the rate-determining step. Reaction potential energy surfaces for two model substrates that have or do not have alkene-terminal substituents have been investigated and the origins of the selectivities have been disclosed. Moreover, energy profiles with three possible spin states (S Fe = 0, 1, 2) have been considered. The reaction is suggested to occur mainly on the singlet potential energy surface with a few spin crossovers between singlet and triplet states involved, which indicates that this reaction should have two-state reactivity (TSR).