Efficient Aliphatic C–H Oxidation and C═C Epoxidation Catalyzed by Porous Organic Polymer-Supported Single-Site Manganese Catalysts

Bioinspired Non-Heme Mn Catalysts for Regio- and Stereoselective Oxyfunctionalizations with H2 O2

In recent years, metalloenzymes-mediated highly selective oxidations of organic substrates under mild conditions have been inspiration for developing synthetic bioinspired catalyst systems, capable of conducting such processes in the laboratory (and, in the future, in industry), relying on easy-to-handle and environmentally benign oxidants such as H2 O2 . To date, non-heme manganese complexes with chiral bis-amino-bis-pyridylmethyl and structurally related ligands are considered as possessing the highest synthetic potential, having demonstrated the ability to mediate a variety of chemo- and stereoselective oxidative transformations, such as epoxidations, C(sp3 )-H hydroxylations and ketonizations, oxidative desymmetrizations, kinetic resolutions, etc. Furthermore, in the past few years non-heme Mn based catalysts have become the major platform for studies focused on getting insight into the molecular mechanisms of oxidant activation and (stereo)selective oxygen transfer, testing non-traditional hydroperoxide oxidants, engineering catalytic sites with enzyme-like substrate recognition-based selectivity, exploration of catalytic regioselectivity trends in the oxidation of biologically active substrates of natural origin. This contribution summarizes the progress in manganese catalyzed C-H oxygenative transformations of organic substrates, achieved essentially in the past 5 years (late 2018-2023).

The elusive reaction mechanism of Mn(II)-mediated benzylic oxidation of alkylarene by H2O2: a gem-diol mechanism or a dual hydrogen abstraction mechanism?

The direct oxygenation of alkylarenes at the benzylic position employing bioinspired nonheme catalysts has emerged as a promising strategy for the production of bioactive arene ketone scaffolds in drugs. However, the structure-activity relationship of the active species and the mechanism of these reactions remain elusive. Herein, the reaction mechanism of the Mn(II)-mediated benzylic oxygenation of phenylbutanoic acid (PBA) to 4-oxo-4-phenylbutyric acid (4-oxo-PBA) by H2O2 was investigated using density functional theory calculations. The calculated results demonstrated that the MnIII-OOH species (1) is a sluggish oxidant and needs to be converted to a high-valent manganese-oxo species (2). The conversion of PBA to 4-oxo-PBA by 2 occurs via the consecutive hydroxylation of PBA to 4-hydroxyl-4-phenylbutyric acid (4-OH-PBA) and the alcohol oxidation of 4-OH-PBA to 4-oxo-PBA. The hydroxylation of PBA proceeds via a novel hydride transfer/hydroxyl-rebound mechanism and the alcohol oxidation of 4-OH-PBA occurs via three pathways (gem-diol, dual hydrogen abstraction (DHA), and reversed-DHA pathways). The regio-selectivity of benzylic oxidations was caused by a strong π-π stacking interaction between the pyridine ring of the nonheme ligand and the phenyl ring of the substrate. These mechanistic findings enrich the knowledge of biomimetic alcohol oxidations and play a positive role in the rational design of new non-heme catalysts.

Trading Symmetry for Stereoinduction in Tetradentate, non-C2 -Symmetric Fe(II)-Complexes for Asymmetric Catalysis

A series of stereogenic-at-metal iron complexes comprising a non-C₂ -symmetric chiral topology is introduced and applied to asymmetric 3d-transition metal catalysis. The chiral iron(II) complexes are built from chiral tetradentate N4-ligands containing a proline-derived amino pyrrolidinyl backbone which controls the relative (cis-α coordination) and absolute metal-centered configuration (Λ vs. Δ). Two chloride ligands complement the octahedral coordination sphere. The modular composition of the tetradentate ligands facilitates the straightforward incorporation of different terminal coordinating heteroaromatic groups into the scaffold. The influence of various combinations was evaluated in an asymmetric ring contraction of isoxazoles to 2H-azirines revealing that a decrease of symmetry is beneficial for the stereoinduction to obtain chiral products in up to 99 % yield and with up to 92 % ee. Conveniently, iron catalysis is feasible under open flask conditions with the bench-stable dichloro complexes exhibiting high robustness towards oxidative or hydrolytic decomposition. The versatility of non-racemic 2H-azirines was subsequently showcased with the conversion into a variety of quaternary α-amino acid derivatives.

Photoelectrochemical Epoxidation of Cyclohexene on an α-Fe2O3 Photoanode Using Water as the Oxygen Source

This study developed a safe and sustainable route for the epoxidation of cyclohexene using water as the source of oxygen at room temperature and ambient pressure. Here, we optimized the cyclohexene concentration, volume of solvent/water (CH₃CN, H₂O), time, and potential on the photoelectrochemical (PEC) cyclohexene oxidation reaction of the α-Fe₂O₃ photoanode. The α-Fe₂O₃ photoanode epoxidized cyclohexene to cyclohexene oxide with a 72.4 ± 3.6% yield and a 35.2 ± 1.6% Faradaic efficiency of 0.37 V vs Fc/Fc⁺ (0.8 V_Ag/AgCl) under 100 mW cm^-2. Furthermore, the irradiation of light (PEC) decreased the applied voltage of the electrochemical cell oxidation process by 0.47 V. This work supplies an energy-saving and environment-benign approach for producing value-added chemicals coupled with solar fuel generation. Epoxidation with green solvents via PEC methods has a high potential for different oxidation reactions of value-added and fine chemicals.

Chemoselective Oxyfunctionalization of Functionalized Benzylic Compounds with a Manganese Catalyst

Whilst allowing for easy access to synthetically versatile motifs and for modification of bioactive molecules, the chemoselective benzylic oxidation reactions of functionalized alkyl arenes remain challenging. Reported in this study is a new non‐heme Mn catalyst stabilized by a bipiperidine‐based tetradentate ligand, which enables methylene oxidation of benzylic compounds by H₂O₂, showing high activity and excellent chemoselectivity under mild conditions. The protocol tolerates an unprecedentedly wide range of functional groups, including carboxylic acid and derivatives, ketone, cyano, azide, acetate, sulfonate, alkyne, amino acid, and amine units, thus providing a low‐cost, more sustainable and robust pathway for the facile synthesis of ketones, increase of complexity of organic molecules, and late‐stage modification of drugs.