Trapping a Highly Reactive Nonheme Iron Intermediate That Oxygenates Strong CH Bonds with Stereoretention

An unprecedentedly
reactive iron species (2) has been
generated by reaction of excess peracetic acid with a mononuclear
iron complex [FeII(CF3SO3)2(PyNMe3)] (1) at cryogenic temperatures,
and characterized spectroscopically. Compound 2 is kinetically
competent for breaking strong CH bonds of alkanes (BDE ≈
100 kcal·mol–1) through a hydrogen-atom transfer
mechanism, and the transformations
proceed with stereoretention and regioselectively, responding to bond
strength, as well as to steric and polar effects. Bimolecular reaction
rates are at least an order of magnitude faster than those of the
most reactive synthetic high-valent nonheme oxoiron species described
to date. EPR studies in tandem with kinetic analysis show that the
490 nm chromophore of 2 is associated with two S = 1/2 species in rapid equilibrium. The minor component 2a (∼5% iron) has g-values at 2.20,
2.19, and 1.99 characteristic of a low-spin iron­(III) center, and
it is assigned as [FeIII(OOAc)­(PyNMe3)]2+, also by comparison with the EPR parameters
of the structurally characterized hydroxamate analogue [FeIII(tBuCON­(H)­O)­(PyNMe3)]2+ (4). The major component 2b (∼40% iron, g-values = 2.07, 2.01, 1.95) has unusual EPR parameters,
and it is proposed to be [FeV(O)­(OAc)­(PyNMe3)]2+, where the OO bond in 2a has
been broken. Consistent with this assignment, 2b undergoes
exchange of its acetate ligand with CD3CO2D
and very rapidly reacts with olefins to produce the corresponding cis-1,2-hydroxoacetate product. Therefore, this work constitutes
the first example where a synthetic nonheme iron species responsible
for stereospecific and site selective CH hydroxylation is
spectroscopically trapped, and its catalytic reactivity against CH
bonds can be directly interrogated by kinetic methods. The accumulated
evidence indicates that 2 consists mainly of an extraordinarily
reactive [FeV(O)­(OAc)­(PyNMe3)]2+ (2b) species capable of hydroxylating unactivated alkyl CH
bonds with stereoretention in a rapid and site-selective manner, and
that exists in fast equilibrium with its [FeIII(OOAc)­(PyNMe3)]2+ precursor.