Epoxidation and 1,2-Dihydroxylation of Alkenes by a Nonheme Iron Model System − DFT Supports the Mechanism Proposed by Experiment

Abstract

The FeII complexes of two isomeric pentadentate bispidine ligands in the presence of H2O2 are catalytically active for the epoxidation and 1,2-dihydroxylation of cyclooctene (bispidine = 3,7-diazabicyclo[3.3.1]nonane; the two isomeric pentadentate bispidine ligands discussed here have two tertiary amine and three pyridine donors). The published spectroscopic and mechanistic data, which include an extensive set of 18O labeling experiments, suggest that the FeIV=O complex is the catalytically active species, which produces epoxide as well as cis- and trans-1,2-dihydroxylated products. Several observations from the published experimental study are addressed with hybrid density functional methods and, in general, the calculations support the proposed, for nonheme iron model systems novel mechanism, where the formation of a radical intermediate emerges from the reaction of the FeIV=O oxidant and cyclooctene. The calculations suggest that the S = 1 ground state of the FeIV=O complex reacts with cyclooctene in a stepwise reaction, leading to the formation of a carbon-based radical intermediate. This radical is captured by O2 from air to produce the majority of the epoxide products in an aerobic atmosphere. Under anaerobic conditions, the produced epoxide product is due to the cyclization of the radical intermediate. Several possible spin states (ST = 3, 2, 1, 0) of the radical intermediate are close in energy. As a result of the substantial energy barrier, calculated for the ST = 3 spin ground state, a spin-crossover during the cyclization step is assumed, and a possible two-state scenario is found, where the S = 2 state of the FeIV=O complex participates in the catalytic mechanism. The 1,2-dihydroxylation proceeds, as suggested by experiment, via an unprecedented pathway, where the radical intermediate is captured by a hydroxyl radical, the source of which is FeIII-OOH, and this reaction is barrierless. The calculations suggest that dihydroxylation can also occur by a direct oxidation pathway from FeIII-OOH. The strikingly different reactivities observed with the two isomeric bispidine FeII complexes are rationalized on the basis of structural and electronic differences.

Biomimetic High‐Valent Non‐Heme Iron Oxidants for the cis‐Dihydroxylation and Epoxidation of Olefins

“…From our previous DFT calculations with the iron complexes of the pentadentate ligand L 2 it emerges that the reaction of O 2 with the Fe III radical intermediate has a lower barrier than the ring closure, and this is required for the formation of epoxide by an autoxidation process. [16] A similar behavior is expected in this case.…”

“…[10] For [14] The experimentally determined mechanism of the L 2 -based system [10] was confirmed by DFT calculations, [16] and a similar set of calculations was therefore done with the L 1 -based system to more thoroughly understand the new reaction mechanism, and specifically to validate the above hypotheses.…”

Strengths, Weaknesses, Opportunities and Threats: Computational Studies of Mn- and Fe-Catalyzed Epoxidations

“…Despite this, a limited number of sources has explored other routes, such as the formation of an hydroperoxo-Fe(porphyrin) complexes in P45models [55][56][57][58][59], in non-porphyrinic Fe catalysts [60][61][62][63] and also on Mn catalysts [64][65][66][67]. Nevertheless, the activity of such hydroperoxo species towards epoxidation reactions has been questioned by Shaik and co-workers [68], at least in models of cytochrome P450.…”

Three‐Membered Heterocycles. Structure and Reactivity

“…Thus, the binaphthyl strapped iron-porphyrin catalyst 424 promotes the enantioselective epoxidation of styrene (10) with iodosylbenzene to give (R)-styrene oxide (421) in excellent yield and enantiomeric excess [457]. Effective non-heme iron catalysts using pentadentate bispidine ligands have also been studied [458].…”