Generation of Carbon Radical from Iron-Hydride/Alkene: Exchange-Enhanced Reactivity Selects the Reactive Spin State

Iron-hydride and alkene constitute a promising pair of reactants to access alkyl carbon radicals. However, the fundamental mechanistic scenario remains unclear. High-level coupled cluster calculations reported in this work demonstrate that for iron(III)-hydrides, hydrogen atom transfer (HAT) is favored over hydrometalation. The oxidation state of iron makes a remarkable difference in the reactivity of the iron-hydride such that ferric FeIII–H is much more reactive in HAT than ferrous FeII–H. Akin to iron(IV)-oxo, exchange-enhanced reactivity (EER) dictates the intrinsic spin HAT reactivity of iron(III)-hydride. However, it is not two-state reactivity (TSR) but single-state reactivity (SSR) that operates in HAT with iron(III)-hydride. The reactivity insights gained herein for alkyl liberation from iron-alkyls may have profound mechanistic implications on the iron-dependent bioorganometallic radical chemistry in radical SAM enzymes.