Nonheme iron-containing enzymes can carry out hydroxylation of hydrocarbons owing to their ability to generate high-valent oxoiron(IV) oxidants to perform hydrogen atom transfer (HAT) on substrate C–H bonds. The transfer of the hydroxo group to the product is typically accomplished through a so-called “rebound” mechanism, where the nascent carbon-centered radical generated through HAT collapses onto the same iron center, forming the hydroxylated product and reducing the iron center from FeIII(OH) to FeII. In synthetic models, however, it has been proposed that rebound does not occur, and instead the substrate radical is long-lived enough to escape the solvent cage, leading to its interception by another FeIV(O) species or radical scavengers. This scenario not only gives lower product yields, but also results in the formation of ferric byproducts. We have investigated the cause of this apparent discrepancy in reaction pathways between enzymatic and synthetic systems through the use of a series of substrates with varying C–H bond strengths and steric bulks. Our study of synthetic FeIV(O) complexes finds evidence for rebound under certain conditions and reveals interactions that may be responsible for directing the substrate radical to undergo rebound. Our findings may be useful designing ligand frameworks/substrates that facilitate directed rebound, which could enable more selective and efficient functionalization of hydrocarbons.

To rebound or not in hydrogen atom transfer reactions by synthetic nonheme oxoiron(IV) complexes

2019 Fall National Meeting

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