1,4-Selective Hydrovinylation of Diene Catalyzed by an Iron Diimine Catalyst: A Computational Case Study on Two-State Reactivity

First-row transition-metal catalysis has been attracting great attention in recent years, partly due to its low toxicity and low cost, as well as a wide variety in reactivities. However, the theoretical understanding behind this important class of reactions is still quite limited, and how the presence of low-lying high-spin states benefits their reactivities is not well known. In this work, we have performed a detailed density functional theory (DFT) study on a previously reported iron diimine-catalyzed hydrovinylation, which we have found to exhibit an interesting two-state reactivity. Specifically, we found that despite the fact that the resting state of the reaction was experimentally determined to be a triplet state previously, the rate-determining and product-determining steps are found to preferably proceed at a singlet state. A triplet state is better at stabilizing the intermediates and imposes fewer geometric constraints for the substrate to adjust its conformations. A singlet state allows an extra available metal d orbital for interaction with ligand, which facilitates oxidative coupling of diene with an incoming alkene, beta-hydride transfer, and ligand substitution. Through in-depth analysis of the electronic structures, we found that the two-state reactivity phenomenon is due to the interplay between orbital interactions, exchange interactions, and coordination geometry, a conclusion that would also serve as an important step in the pursuit of understanding in the first-row transition-metal catalysis and benefit the future design of catalysis with earth-abundant metals.