C–H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C–H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C–H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C–H activation until summer 2018.

3d Transition Metals for C-H Activation.

Recent efforts to design synthetic iron catalysts for the selective and efficient oxidation of C-H and C═C bonds have been inspired by a versatile family of nonheme iron oxygenases. These bioinspired nonheme (N4)Fe(II) catalysts use H2O2 to oxidize substrates with high regio- and stereoselectivity, unlike in Fenton chemistry where highly reactive but unselective hydroxyl radicals are produced. In this Account, we highlight our efforts to shed light on the nature of metastable peroxo intermediates, which we have trapped at -40 °C, in the reactions of the iron catalyst with H2O2 under various conditions and the high-valent species derived therefrom. Under the reaction conditions that originally led to the discovery of this family of catalysts, we have characterized spectroscopically an Fe(III)-OOH intermediate (EPR g(max) = 2.19) that leads to the hydroxylation of substrate C-H bonds or the epoxidation and cis-dihydroxylation of C═C bonds. Surprisingly, these organic products show incorporation of (18)O from H2(18)O, thereby excluding the possibility of a direct attack of the Fe(III)-OOH intermediate on the substrate. Instead, a water-assisted mechanism is implicated in which water binding to the iron(III) center at a site adjacent to the hydroperoxo ligand promotes heterolytic cleavage of the O-O bond to generate an Fe(V)(O)(OH) oxidant. This mechanism is supported by recent kinetic studies showing that the Fe(III)-OOH intermediate undergoes exponential decay at a rate enhanced by the addition of water and retarded by replacement of H2O with D2O, as well as mass spectral evidence for the Fe(V)(O)(OH) species obtained by the Costas group. The nature of the peroxo intermediate changes significantly when the reactions are carried out in the presence of carboxylic acids. Under these conditions, spectroscopic studies support the formation of a (κ(2)-acylperoxo)iron(III) species (EPR g(max) = 2.58) that decays at -40 °C in the absence of substrate to form an oxoiron(IV) byproduct, along with a carboxyl radical that readily loses CO2. The alkyl radical thus formed either reacts with O2 to form benzaldehyde (as in the case of PhCH2COOH) or rebounds with the incipient Fe(IV)(O) moiety to form phenol (as in the case of C6F5COOH). Substrate addition leads to its 2-e(-) oxidation and inhibits these side reactions. The emerging mechanistic picture, supported by DFT calculations of Wang and Shaik, describes a rather flat reaction landscape in which the (κ(2)-acylperoxo)iron(III) intermediate undergoes O-O bond homolysis reversibly to form an Fe(IV)(O)((•)OC(O)R) species that decays to Fe(IV)(O) and RCO2(•) or isomerizes to its Fe(V)(O)(O2CR) electromer, which effects substrate oxidation. Another short-lived S = 1/2 species just discovered by Talsi that has much less g-anisotropy (EPR g(max) = 2.07) may represent either of these postulated high-valent intermediates.

Bioinspired Nonheme Iron Catalysts for C-H and C═C Bond Oxidation: Insights into the Nature of the Metal-Based Oxidants.

The discovery of simple and efficient catalyst systems for the asymmetric oxofunctionalization of hydrocarbons is a challenging task of catalytic chemistry. In this paper, we give an overview of catalyst systems capable of conducting asymmetric oxygenative transformations of organic molecules and, in line with the major trend to sustainability, relying on green oxidants H2O2 and O2 as the ultimate oxygen source. The full historical period of asymmetric oxidation catalysis (1970 to the present day) is covered; both transition-metal-based and organocatalytic systems are considered. The focus of this review is the catalytic properties of the existing catalyst systems, in particular stereoselectivity, activity, efficiency, and synthetic outlook. At the same time, mechanistic peculiarities of stereoselective oxygen transfer are given attention.

Catalytic Asymmetric Oxygenations with the Environmentally Benign Oxidants H2O2 and O2.

Utilization of O2 as an abundant and environmentally benign oxidant is of great interest in the design of bioinspired synthetic catalytic oxidation systems. Metalloenzymes activate O2 by employing earth-abundant metals and exhibit diverse reactivities in oxidation reactions, including epoxidation of olefins, functionalization of alkane C–H bonds, arene hydroxylation, and syn-dihydroxylation of arenes. Metal–oxo species are proposed as reactive intermediates in these reactions. A number of biomimetic metal–oxo complexes have been synthesized in recent years by activating O2 or using artificial oxidants at iron and manganese centers supported on heme or nonheme-type ligand environments. Detailed reactivity studies together with spectroscopy and theory have helped us understand how the reactivities of these metal–oxygen intermediates are controlled by the electronic and steric properties of the metal centers. These studies have provided important insights into biological reactions, which have contributed to ...

Heme and Nonheme High-Valent Iron and Manganese Oxo Cores in Biological and Abiological Oxidation Reactions

This review describes new reactions catalyzed by recently discovered types of metal complexes and catalytic systems (catalyst + co-catalyst). Works of recent years (mainly 2010–2016) devoted to the oxygenations of saturated, aromatic hydrocarbons and other carbon–hydrogen compounds are surveyed. Both soluble metal complexes and solid metal compounds catalyze such transformations. Molecular oxygen, hydrogen peroxide, alkyl peroxides, and peroxy acids were used in these reactions as oxidants.

New Trends in Oxidative Functionalization of Carbon–Hydrogen Bonds: A Review

Herein, we report the EPR spectroscopic study of the bioinspired catalyst systems for selective hydrocarbon oxofunctionalizations based on dinuclear ferric complexes with TPA* and PDP* aminopyridine ligands, hydrogen peroxide, and acetic acid (TPA* = tris(3,5-dimethyl-4-methoxypyridyl-2-methyl)amine, PDP* = bis(3,5-dimethyl-4-methoxypyridyl-2-methyl)-(S,S)-2,2′-bipyrrolidine). Using very low temperatures, −75 to −85 °C, the extremely unstable and reactive iron–oxygen intermediates, directly reacting with olefins even at −85 °C, have been detected for the first time. Their EPR parameters (g1 = 2.070–2.071, g2 = 2.005–2.008, g3 = 1.956–1.960) were very similar to those of the known oxoiron(V) complex [(TMC)FeV═O(NC(O)CH3)]+ (g1 = 2.053, g2 = 2.010, g3 = 1.971, TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). On the basis of EPR and reactivity data, the detected intermediates were assigned to the FeV═O active oxidizing species of the catalyst systems studied.

EPR Spectroscopic Detection of the Elusive FeV═O Intermediates in Selective Catalytic Oxofunctionalizations of Hydrocarbons Mediated by Biomimetic Ferric Complexes

Iron complexes with chiral bipyrrolidine-derived aminopyridine (PDP) ligands are among the most efficient Fe-based bioinspired catalysts for regio- and stereoselective oxidation of C–H and C═C moieties with hydrogen peroxide. Besides hydrogen peroxide, other oxidants (peroxycarboxylic acids and organic hydroperoxides) can be effectively used. In this work, we have examined the mechanistic landscape of the Fe(PDP) catalyst family with various oxidants: H2O2, organic hydroperoxides, and peracids. The combined EPR spectroscopic, enantioselectivity, Hammett, Z-stilbene epoxidation stereoselectivity, and 18O-labeling data witness that the same oxoiron complexes [(L)FeV═O(OC(O)R)]2+ are the actual epoxidizing species in both the catalyst systems (L)Fe/H2O2/carboxylic acid and (L)Fe/AlkylOOH/carboxylic acid. On the contrary, in the systems (L)Fe/R2C(O)OOH (R2 = CH3 or 3–Cl-C6H4), in the presence or in the absence of carboxylic acid, the epoxidation is predominantly conducted by the acylperoxo-iron(III) intermedi...

https://pubs.acs.org/doi/pdf/10.1021/acscatal.6b02851

Iron-Catalyzed Enantioselective Epoxidations with Various Oxidants: Evidence for Different Active Species and Epoxidation Mechanisms

The electronic structure of the iron–oxygen intermediates responsible for catalytic transformations in the biomimetic catalyst systems [((S,S)-PDP)FeII(OTf)2]/H2O2/RCOOH has been found to be strongly dependent on the structure of the carboxylic acid RCOOH. For carboxylic acids with primary and secondary α-carbon atoms (acetic acid, butyric acid, caproic acid), the active species exhibit electron paramagnetic resonance (EPR) spectra with large g-factor anisotropy (g1 = 2.7, g2 = 2.4, g3 = 1.7), whereas for those with tertiary α-carbon atoms (2-ethylhexanoic acid, valproic acid, 2-ethylbutyric acid), the active species display EPR spectra with small g-factor anisotropy (g1 = 2.07, g2 = 2.01, g3 = 1.96). The EPR spectra of the latter intermediates are very similar to those of the intermediates previously assigned to oxoiron(V) species. The systems featuring intermediates of the second type ensure higher enantioselection in the epoxidation of electron-deficient olefins.

Dramatic Effect of Carboxylic Acid on the Electronic Structure of the Active Species in Fe(PDP)-Catalyzed Asymmetric Epoxidation

The reactivity of nonheme iron(V)–oxo intermediates toward aromatic C–H oxidation at −70 °C has been directly evaluated. The intermediates were generated upon the interaction of the ferric complex [(PDP*)FeIII(μ-OH)2FeIII(PDP*)](OTf)4 (4◊; PDP* = N,N′-bis(3,5-dimethyl-4-methoxypyridyl-2-methyl)-(S,S)-2,2′-bipyrrolidine) with peracetic acid in the presence of acetic or 2-ethylhexanoic acid. The second-order rate constants (k2) for the reaction of substituted benzenes with iron–oxo intermediates [(PDP*)FeV═O(OC(O)R)]2+ at −70 °C were determined (R = CH3, 3-heptyl). For more electron rich arenes, much higher k2 values were observed, increasing in the order nitrobenzene < acetophenone < chlorobenzene < benzene < toluene, in accordance with the electrophilic aromatic substitution mechanism. The catalytic oxidation of mono- and dialkylbenzenes with H2O2 proceeded with good efficiency (up to 36.5 TN per Fe atom) and high selectivity toward aromatic oxidation products (up to 91%).

Direct Evaluation of the Reactivity of Nonheme Iron(V)–Oxo Intermediates toward Arenes

The catalytic activity of a series of iron complexes of the PDP family (PDP=N,N′‐bis(2‐pyridylmethyl)‐2,2′‐bipyrrolidine) in the oxidation of aromatic substrates with H2O2 has been studied. In the presence of acetic acid, these complexes efficiently catalyze the oxidation of benzene and alkylbenzenes with high selectivity for oxygen incorporation into the aromatic ring (up to 93 %), performing up to 84 catalytic turnovers. The parent complex, [(PDP)(OTf)2], has demonstrated the highest catalytic efficiency and aromatic oxidation selectivity. The yield of products of oxidation of different substrates increases in line with increasing number of electron‐donating alkyl groups of the substrates: halogenbenzenes

Alexandra M. Zima

Papers

EPR Spectroscopic Detection of the Elusive FeV═O Intermediates in Selective Catalytic Oxofunctionalizations of Hydrocarbons Mediated by Biomimetic Ferric Complexes

Iron-Catalyzed Enantioselective Epoxidations with Various Oxidants: Evidence for Different Active Species and Epoxidation Mechanisms

Dramatic Effect of Carboxylic Acid on the Electronic Structure of the Active Species in Fe(PDP)-Catalyzed Asymmetric Epoxidation

Direct Evaluation of the Reactivity of Nonheme Iron(V)–Oxo Intermediates toward Arenes

Highly Efficient Aromatic C−H Oxidation with H2O2 in the Presence of Iron Complexes of the PDP Family