Bioinspired Manganese and Iron Complexes for Enantioselective Oxidation Reactions: Ligand Design, Catalytic Activity, and Beyond

Enabling highly (R)-enantioselective epoxidation of styrene by engineering unique non-natural P450 peroxygenases

Unlike the excellent (S)-enantioselective epoxidation of styrene performed by natural styrene monooxygenases (ee > 99%), the (R)-enantioselective epoxidation of styrene has not yet achieved a comparable efficiency using natural or engineered oxidative enzymes. This report describes the H₂O₂-dependent (R)-enantioselective epoxidation of unfunctionalized styrene and its derivatives by site-mutated variants of a unique non-natural P450BM3 peroxygenase, working in tandem with a dual-functional small molecule (DFSM). The observed (R)-enantiomeric excess (ee) of styrene epoxidation is up to 99% with a turnover number (TON) of 918 by the best enantioselective mutant F87A/T268I/L181Q, while the best active mutant F87A/T268I/V78A/A184L (with 98% ee) gave a catalytic TON of 4350, representing the best activity of a P450 peroxygenase towards styrene epoxidation to date. Following this approach, a set of styrene derivatives, such as o-, m-, p-chlorostyrenes and fluorostyrenes, could also be epoxidized with modest to very good TONs (362–3480) and high (R)-enantioselectivities (95–99% ee). The semi-preparative scale synthesis of (R)-styrene oxide performed at 0 °C with high conversion, maintaining enantioselectivity, and moderate isolated yields, further suggests the potential application of the current P450 enzymatic system in styrene epoxidation. This study indicates that the synergistic use of protein engineering and an exogenous DFSM constitutes an efficient strategy to control the enantioselectivity of styrene epoxidation, thus substantially expanding the chemical scope of P450 enzymes as useful bio-oxidative catalysts.

H₂O₂-dependent epoxidation of unfunctionalized styrenes is achieved with high (R)-enantioselectivity and moderate to excellent TONs by combining site-mutated variants of cytochrome P450BM3 monooxygenase and a dual-functional small molecule (DFSM).

Insight into the Selective Methylene Oxidation Catalyzed by Mn(CF3-PDP)(SbF6)2/H2O2/CH2ClCO2H) System: A DFT Mechanistic Study

DFT study was employed to gain insight into methylene oxidation catalyzed by Mn(CF₃-PDP)(NCMe)₂ (SbF₆)₂/H₂O₂/HOAc^Cl(OAC^Cl ═OC(O)CH₂Cl). The active catalyst was characterized to be [Mn](O)OAc^Cl ([Mn]═Mn(CF₃-PDP)²⁺) which is generated via a sequence from [Mn] to [Mn]OH to [Mn]OAc^Cl to [Mn]OOH. With the active catalyst, the methylene group is sequentially oxidized to an alcohol and then to a carbonyl group via rebound mechanism. The mechanism explains the observed site selectivity.

General Access to Modified α-Amino Acids by Bioinspired Stereoselective γ-C-H Bond Lactonization

α-Amino acids represent a valuable class of natural products employed as building blocks in biological and chemical synthesis. Because of the limited number of natural amino acids available, and of their widespread application in proteomics, diagnosis, drug delivery and catalysis, there is an increasing demand for the development of procedures for the preparation of modified analogues. Herein, we show that the use of bioinspired manganese catalysts and H₂ O₂ under mild conditions, provides access to modified α-amino acids via γ-C-H bond lactonization. The system can efficiently target 1°, 2° and 3° γ-C-H bonds of α-substituted and achiral α,α-disubstituted α-amino acids with outstanding site-selectivity, good to excellent diastereoselectivity and (where applicable) enantioselectivity. This methodology may be considered alternative to well-established organometallic procedures.

Photoredox-Catalyzed Decarboxylative Cross-Coupling of α-Amino Acids with Nitrones

A decarboxylative cross-coupling reaction of α-amino acids with nitrones via visible-light-induced photoredox catalysis has been established for easy access to β-amino hydroxylamines and vicinal diamines with structural diversity, which is featured with simple operation, mild conditions, readily available α-amino acids, and a broad scope of nitrone substrates. The application of this protocol can furnish efficient synthetic strategies for some valuable vicinal diamine-containing molecules.

A DFT study on the C-H oxidation reactivity of Fe(iv)-oxo species with N4/N5 ligands derived from l-proline

The hydroxylation of hexane by two FeIVO complexes bearing a pentadentate ligand (N5, Pro3Py) and a tetradentate ligand (N4, Pro2PyBn) derived from l-proline was studied by DFT calculations. Theoretical results predict that both FeIVO complexes hold triplet ground states. The hydrogen atom abstraction (HAA) processes by both FeIVO species proceed through a two-state reactivity, thus indicating that HAA occurs via a low-barrier quintet surface. Beyond the conventional rebound step, the dissociation path is also calculated and is found to potentially occur after HAA.

Exploration of highly electron-rich manganese complexes in enantioselective oxidation catalysis; a focus on enantioselective benzylic oxidation

The development of highly electron-rich manganese complexes for enantioselective benzylic oxidation (and asymmetric epoxidation) is described, to provide chiral benzylic alcohols and epoxides in good yields and enantioselectivites.

Biosynthesis of chiral cyclic and heterocyclic alcohols via CO/C–H/C–O asymmetric reactions

This review covers the recent progress in various biological approaches applied to the synthesis of enantiomerically pure cyclic and heterocyclic alcohols through CO/C–H/C–O asymmetric reactions.

Catalytic Enantioselective Methylene C(sp3)-H Hydroxylation Using a Chiral Manganese Complex/Carboxylic Acid System

Achieving direct C-H hydroxylation in a highly diastereo- and enantioselective manner is still a challenging goal. This reaction is mainly hindered by the potential for overoxidation of the generated alcohols as well as low stereoselectivity. Herein, we present an enantioselective benzylic C-H hydroxylation catalyzed by a manganese complex, H₂O₂, and a carboxylic acid in 2,2,2-trifluoroethanol. The benzylic alcohols were successfully furnished in excellent diastereoselectivities (up to >95:5) and enantioselectivities (up to 95% ee). As a highlight of this work, high diastereoselectivity of C-H hydroxylation could be achieved by tuning the amount of carboxylic acid additive.

Tumor Vessel Targeted Self-Assemble Nanoparticles for Amplification and Prediction of the Embolization Effect in Hepatocellular Carcinoma

Vessel embolization is recommended as the first line treatment for unresectable hepatocellular carcinoma (HCC). However, owing to the imprecise vessel embolization and heterogeneous response performance among patients, its survival benefits are often compromised. Herein, we reported an innovative strategy to extensively embolize the tumor by triggering the coagulation cascade, and predict the embolization effect with vessel density assessment. We synthesized manganese dioxide (MnO₂)/verteporfin (BPD) nanocomposites, in which BPD bound to the tumor vessel endothelial cells (TVECs) and MnO₂ nanosheets served as the carrier. MnO₂ was reduced to Mn²⁺ ions and self-assembled with BPD to produce nanoBPD, resulting in enhanced TVECs apoptosis and coagulation cascade compared to that with free BPD. Furthermore, multimodal imaging was used to visualize tumor vessel density, which can be used as a predictor to identify the patients who would benefit from embolization. Our findings describe a promising strategy for both tumor eradication and effect prediction to improve survival benefits in unresectable HCC patients.

Multifaceted aspects of charge transfer

Charge transfer and charge transport are by far among the most important processes for sustaining life on Earth and for making our modern ways of living possible. Involving multiple electron-transfer steps, photosynthesis and cellular respiration have been principally responsible for managing the energy flow in the biosphere of our planet since the Great Oxygen Event. It is impossible to imagine living organisms without charge transport mediated by ion channels, or electron and proton transfer mediated by redox enzymes. Concurrently, transfer and transport of electrons and holes drive the functionalities of electronic and photonic devices that are intricate for our lives. While fueling advances in engineering, charge-transfer science has established itself as an important independent field, originating from physical chemistry and chemical physics, focusing on paradigms from biology, and gaining momentum from solar-energy research. Here, we review the fundamental concepts of charge transfer, and outline its core role in a broad range of unrelated fields, such as medicine, environmental science, catalysis, electronics and photonics. The ubiquitous nature of dipoles, for example, sets demands on deepening the understanding of how localized electric fields affect charge transfer. Charge-transfer electrets, thus, prove important for advancing the field and for interfacing fundamental science with engineering. Synergy between the vastly different aspects of charge-transfer science sets the stage for the broad global impacts that the advances in this field have.

Ligand regulation for manganese-catalyzed enantioselective epoxidation of olefins without acid

A novel manganese catalyst bearing an l-proline-derived N4 ligand has been developed for enabling acid-free asymmetric epoxidation of olefins with tert-butyl hydroperoxide as the oxidant. A variety of olefins that are well-matched in size with the ligand pocket can be transformed to epoxides with excellent enantioselectivities. The smaller ligand pocket is also beneficial to the enantioselective epoxidation of simple olefins. Cryospray ionization mass spectrometry experiments reveal that a Mn^IV[double bond, length as m-dash]O species serves as an active epoxidizing species.

Concerted proton-electron transfer reactions of manganese-hydroxo and manganese-oxo complexes

The enzymes manganese superoxide dismutase and manganese lipoxygenase use MnIII-hydroxo centres to mediate proton-coupled electron transfer (PCET) reactions with substrate. As manganese is earth-abundant and inexpensive, manganese catalysts are of interest for synthetic applications. Recent years have seen exciting reports of enantioselective C-H bond oxidation by Mn catalysts supported by aminopyridyl ligands. Such catalysts offer economic and environmentally-friendly alternatives to conventional reagents and catalysts. Mechanistic studies of synthetic catalysts highlight the role of Mn-oxo motifs in attacking substrate C-H bonds, presumably by a concerted proton-electron transfer (CPET) step. (CPET is a sub-class of PCET, where the proton and electron are transferred in the same step.) Knowledge of geometric and electronic influences for CPET reactions of Mn-hydroxo and Mn-oxo adducts enhances our understanding of biological and synthetic manganese centers and informs the design of new catalysts. In this Feature article, we describe kinetic, spectroscopic, and computational studies of MnIII-hydroxo and MnIV-oxo complexes that provide insight into the basis for the CPET reactivity of these species. Systematic perturbations of the ligand environment around MnIII-hydroxo and MnIV-oxo motifs permit elucidation of structure-activity relationships. For MnIII-hydroxo centers, electron-deficient ligands enhance oxidative reactivity. However, ligand perturbations have competing consequences, as changes in the MnIII/II potential, which represents the electron-transfer component for CPET, is offset by compensating changes in the pKa of the MnII-aqua product, which represents the proton-transfer component for CPET. For MnIV-oxo systems, a multi-state reactivity model inspired the development of significantly more reactive complexes. Weakened equatorial donation to the MnIV-oxo unit results in large rate enhancements for C-H bond oxidation and oxygen-atom transfer reactions. These results demonstrate that the local coordination environment can be rationally changed to enhance reactivity of MnIII-hydroxo and MnIV-oxo adducts.

A New Tetradentate Mixed Aza-Thioether Macrocycle and Its Complexation Behavior towards Fe(II), Ni(II) and Cu(II) Ions

A new tetradentate mixed aza-thioether macrocyclic ligand 2,6-dithia[7](2,9)-1,10-phenanthrolinophane ([13]ane(phenN₂)S₂) was successfully synthesized. Reacting metal precursors [Fe(CH₃CN)₂(OTf)₂], Ni(ClO₄)₂·6H₂O, and Cu(ClO₄)₂·6H₂O with one equivalent of [13]ane(phenN₂)S₂ afforded [Fe([13]ane(phenN₂)S₂)(OTf)₂] (1), [Ni([13]ane(phenN₂)S₂)](ClO₄)₂ (2(ClO₄)₂), and [Cu([13]ane(phenN₂)S₂)(OH₂)](ClO₄)₂ (3(ClO₄)₂), respectively. The structures of [13]ane(phenN₂)S₂ and all of its metal complexes were investigated by X-ray crystallography. The [13]ane(phenN₂)S₂ was found to behave as a tetradentate ligand via its donor atoms N and S.

Biomimetic Aerobic Epoxidation of Alkenes Catalyzed by Cobalt Porphyrin under Ambient Conditions in the Presence of Sunflower Seeds Oil as a Co-Substrate

In this work, a mild and sustainable catalytic aerobic epoxidation of alkenes catalyzed by cobalt porphyrin was performed in the presence of sunflower seeds oil. Under ambient conditions, the conversion rate of trans-stilbene reached 99%, and selectivity toward epoxide formation was 88%. The kinetic studies showed that the aerobic epoxidation followed the Michaelis-Menten kinetics. Mass spectroscopy and in situ electron spin resonance indicated that linoleic acid was converted to fatty aldehydes via hydroperoxide intermediates. A plausible mechanism of epoxidation of alkenes was accordingly proposed.