Molecular Iron-Based Oxidants and Their Stoichiometric Reactions

Abstract

Molecular iron-based oxidants can oxidize organic substrates under relatively mild conditions, sometimes even in water. If these reactions can be converted to catalytic protocols, they hold great promise for the development of greener methods for this particularly messy class of reaction. For application in organic synthesis, the biggest question is: How selective can the reactions be? The supporting ligands in these compounds are crucial for tuning the electronic structure of a catalytically competent iron-based oxidant and its selectivity in terms of the production of a single, even enantiopure, product. A thorough understanding of the stoichiometric generation of molecular iron-based oxidants and their subsequent reactivity is an important step for the development of new iron-based catalysts. Ideally, and in analogy to many of nature's iron-based enzymes, their regeneration under catalytic conditions could involve the activation of dioxygen from air. This chapter will focus on the stoichiometric reactions of iron compounds with potential oxidizing agents including O 2 , peroxides, oxygen-atom donor reagents, and even water, to form iron-based oxidizing species and the reactions of these usually very transient species with oxidizable substrates. This chapter might be especially inspiring for researchers in the field of iron catalyst design.

Iron Catalysis: Historic Overview and Current Trends

“…As further detailed in [64] by Christine McKenzie, the design of catalytically active iron complexes is complicated by the fact that under the oxidative reaction conditions, the ligand itself can get oxidized [69], and decomposition pathways have been investigated [70]. It is, thus, difficult to identify a catalytically active species in oxidation reactions.…”

“…These enzymes can roughly be divided into heme [3] and nonheme systems [63]. As further outlined by George Britovsek in [4] and Christine McKenzie in [64], synthetic efforts have been directed toward mimicking these efficient oxidation enzymes by artificial systems based on iron complexes [65]. The research activity in the field led to a number of iron-based catalytic systems that can oxidize a number of organic substrates, utilizing oxygen, peroxides, or other oxidizing agents as the oxidant.…”

“…A prototypical example is Davies' reagent, an ironbased chiral auxiliary, which can be employed in a number of enantioselective reactions, e.g., enolate alkylations [89]. Certain iron complexes and their stoichiometric reactions are currently intensely investigated, as outlined by Christine McKenzie in [64] with the example of iron-based oxidants. The understanding of the structure of oxygen-transferring species can help to develop catalytic systems for oxidation reactions.…”

“…These two concepts are only briefly described here, and readers are referred to [64,79] for an in-depth discussion. Many of the reactions described above have enantioselective counterparts, and iron has also increasingly been investigated in catalysts for asymmetric syntheses, as described by Thierry Ollevier in [29,86,87].…”

“…The latter species can also be found in natural oxidation processes and is more selective than oxidants based on free radicals [68]. The extensive investigations of iron oxo species Fe ¼ O, their impact on selectivities, and their potential role in catalytic cycles are outlined in detail in [4,64].…”