Targeting of High-Valent Iron-TAML Activators at Hydrocarbons and Beyond

TAML activators of peroxides are iron(III) complexes. The ligation by four deprotonated amide nitrogens in macrocydic motifs is the signature of TAMLs where the macrocydic structures vary considerably. TAML activators are exceptional functional replicas of the peroxidases and cytochrome P450 oxidizing enzymes. In water, they catalyze peroxide oxidation of a broad spectrum of compounds, many of which are micropollutants, compounds that produce undesired effects at low concentrations as with the enzymes, peroxide is typically activated with near-quantitative efficiency. In nonaqueous solvents such as organic nitriles, the prototype TAML activator gave the structurally authenticated reactive iron(V)oxo units ((FeO)-O-V), wherein the iron atom is two oxidation equivalents above the Fe-III resting state. The iron(V) state can be achieved through the intermediacy of iron(IV) species, which are usually mu-oxo-bridged dimers ((FeFeIV)-Fe-IV), and this allows for the reactivity of this potent reactive intermediate to be studied in stoichiometric processes. The present review is primarily focused at the mechanistic features of the oxidation by (FeO)-O-V of hydrocarbons including cyclohexane. The main topic is preceded by a description of mechanisms of oxidation of thioanisoles by (FeO)-O-V, because the associated studies provide valuable insight into the ability of (FeO)-O-V to oxidize organic molecules. The review is opened by a summary of the interconversions between Fe-III, (FeFeIV)-Fe-IV, and (FeO)-O-V species, since this information is crucial for interpreting the kinetic data. The highest reactivity in both reaction classes described belongs to (FeO)-O-V. The resting state Fe-III is unreactive oxidatively. Intermediate reactivity is typically found for (FeFeIV)-Fe-IV; therefore, kinetic features for these species in interchange and oxidation processes are also reviewed. Examples of using TAML activators for C-H bond cleavage applied to fine organic synthesis conclude the review.