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

Abstract

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, β-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.

The role of the intermediate triplet state in iron-catalyzed multi-state C–H activation

“…The spin-orbit coupling (SOC) is often the main contributor to this interstate coupling. In recent years, the two-state reaction mechanisms [5][6][7] received significant attention due to their important roles in C-H bond activation, [8][9][10][11] hydrogen atom transfer and oxidation reactions, [12][13][14][15] catalyzed hydroxylation, [16][17][18] isomerization, 19 catalytic and electron-transfer properties of metalloproteins, [20][21][22] excited-state relaxation, 23 and interstellar chemistry. 24 To explain the efficient low-temperature iron-catalyzed C-H bond activation, 3 Sun et al proposed a two-state mechanism involving a transition between the singlet and quintet spin states of Fe(II).…”

Mechanism and selectivity of photocatalyzed CO2reduction by a function-integrated Ru catalyst

“…The principal interaction orbital (PIO) analysis can be used to recover a chemically intuitive description for the interaction between the metal and ligand. 83–85 For 1 3-CO 2 , through the PISO analysis, we see two important bonding interactions between the catalyst and the CO 2 − ligand. The first PIO pair corresponds to the σ-donation from carbon to the empty d z 2 orbital of Ru with a PIO-based bond index (PBI) value of 0.985.…”

Crossed Regio‐ and Enantioselective Iron‐Catalyzed [4+2]‐Cycloadditions of Unactivated Dienes

“…In contrast, cyclooctadienes 4 ai and 4 mi were exclusively formed using Fe10 as the catalyst. [28] To rationalize the effects of the catalyst on the chemoselectivity of the transformation, a working model for the mechanism could be formulated by combining previous findings from Chirik [26,27,36] and Lin [37] To gauge the influence of the (E/Z)-geometry of the linear diene 2 on the cycloaddition outcome, we independently reacted (E)-2 f and (Z)-2 f with 1 a in the presence of Fe5 (Scheme 5 A). Irrespectively of the starting double bond configuration, cyclohexene 3 af bearing an (E)-exocyclic double bond was obtained in both cases as the sole geometrical isomer in excellent yield.…”

“…To rationalize the effects of the catalyst on the chemoselectivity of the transformation, a working model for the mechanism could be formulated by combining previous findings from Chirik[ 26 , 27 , 36 ] and Lin [37] and our experimental observations (Scheme 4 ). An oxidative cyclization of in situ formed intermediate Fe‐I and linear diene 2 connects both diene substrates.…”