Iron-containing enzymes are one of Nature’s main means of effecting key biological transformations. The mononuclear non-heme iron oxygenases and oxidases have received the most attention recently, primarily because of the recent availability of crystal structures of many different enzymes and the stunningly diverse oxidative transformations that these enzymes catalyze. The wealth of available structural data has furthermore established the so-called 2-His-1-carboxylate facial triad as a new common structural motif for the activation of dioxygen. This superfamily of mononuclear iron(II) enzymes catalyzes a wide range of oxidative transformations, ranging from the cis-dihydroxylation of arenes to the biosynthesis of antibiotics such as isopenicillin and fosfomycin. The remarkable scope of oxidative transformations seems to be even broader than that associated with oxidative heme enzymes. Not only are many of these oxidative transformations of key biological importance, many of these selective oxidations are also unprecedented in synthetic organic chemistry. In this critical review, we wish to provide a concise background on the chemistry of the mononuclear non-heme iron enzymes characterized by the 2-His-1-carboxylate facial triad and to discuss the many recent developments in the field. New examples of enzymes with unique reactivities belonging to the superfamily have been reported. Furthermore, key insights into the intricate mechanistic details and reactive intermediates have been obtained from both enzyme and modeling studies. Sections of this review are devoted to each of these subjects, i.e. the enzymes, biomimetic models, and reactive intermediates (225 references).

The oxidative cleavage of the CC double bond in unsaturated fatty acids into aldehydes or carboxylic acids is a reaction of current interest in biomass valorization. The products of this reaction, which is currently being performed on an industrial scale by means of ozonolysis, can be applied for the production of commodity chemicals such as plasticizers and polymers. Alternative, catalytic methods for this conversion that are not based on the hazardous ozone oxidant are of high interest. In this respect, the use of transition metal catalysts would allow the application of milder reaction conditions and more benign oxidants. This review covers the various transition metal-based catalysts that have been reported for the oxidative cleavage of alkenes in general and the cleavage of unsaturated fatty acids in particular. Catalytic systems based on second- and third-row metals such as Ru, Os, and W have been widely studied and are well-known to catalyze the cleavage of various alkenes and unsaturated fatty acids. In addition to simple metal salts, metal oxides or peroxides, various metal coordination complexes have been investigated for chemo- and regioselective olefin cleavage. Heterogeneous catalyst systems based on these metals have, in addition, been studied with a focus on catalyst reuse. To circumvent the use of expensive, toxic or less abundant second- and third-row metal catalysts, the use of first-row transition metal catalysts for the cleavage reaction might be preferred. However, examples of such catalysts for oxidative olefin cleavage are much less documented. The application of Fe- and Mn-based catalysts currently is largely limited to activated alkenes, e.g. styrene derivatives. The many possibilities that are offered by careful ligand design together with the recent advances in the field of first-row transition metal oxidation catalysis may allow for the development of catalyst systems that substitute ozone or second- and third-row metal catalysts for the oxidative cleavage of simple alkenes and unsaturated fatty acids.

An iron complex with a C₁-symmetric tetradentate N-based ligand catalyzes the asymmetric epoxidation of cyclic enones and cyclohexene ketones with aqueous hydrogen peroxide, providing the corresponding epoxides in good to excellent yields and enantioselectivities (up to 99% yield, and 95% ee), under mild conditions and in short reaction times. Evidence is provided that reactions involve an electrophilic oxidant, and this element is employed in performing site selective epoxidation of enones containing two alkene sites.