Katalytische, milde und selektive Oxyfunktionalisierung von linearen Alkanen: aktuelle Herausforderungen

Proton Switch in the Secondary Coordination Sphere to Control Catalytic Events at the Metal Center: Biomimetic Oxo Transfer Chemistry of Nickel Amidate Complex

High-valent metal-oxo species are key intermediates for the oxygen atom transfer step in the catalytic cycles of many metalloenzymes. While the redox-active metal centers of such enzymes are typically supported by anionic amino acid side chains or porphyrin rings, peptide backbones might function as strong electron-donating ligands to stabilize high oxidation states. To test the feasibility of this idea in synthetic settings, we have prepared a nickel(II) complex of new amido multidentate ligand. The mononuclear nickel complex of this N5 ligand catalyzes epoxidation reactions of a wide range of olefins by using mCPBA as a terminal oxidant. Notably, a remarkably high catalytic efficiency and selectivity were observed for terminal olefin substrates. We found that protonation of the secondary coordination sphere serves as the entry point to the catalytic cycle, in which high-valent nickel species is subsequently formed to carry out oxo-transfer reactions. A conceptually parallel process might allow metalloenzymes to control the catalytic cycle in the primary coordination sphere by using proton switch in the secondary coordination sphere.

Metal- and Additive-Free Oxidation of Sulfides into Sulfoxides by Fullerene-Modified Carbon Nitride with Visible-Light Illumination

Photocatalytic selective oxidation has attracted considerable attention as an environmentally friendly strategy for organic transformations. Some methods have been reported for the photocatalytic oxidation of sulfides into sulfoxides in recent years. However, the practical application of these processes is undermined by several challenges, such as low selectivity, sluggish reaction rates, the requirement of UV-light irradiation, the use of additives, and the instability of the photocatalyst. Herein, a metal-free C₆₀ /graphitic carbon nitride (g-C₃ N₄ ) composite photocatalyst was fabricated by a facile method, and well characterized by TEM, SEM, FTIR spectroscopy, XRD, X-ray photoelectron spectroscopy, diffuse reflectance spectroscopy, and photoluminescence spectroscopy. The C₆₀ /g-C₃ N₄ catalyst exhibited a high photocatalytic activity at room temperature for the selective oxidation of sulfides into the corresponding sulfoxides in the presence of other functional groups, due to the synergetic roles of C₆₀ and g-C₃ N₄ . Several important parameters have been screened, and this method afforded good to excellent yields of sulfoxides under optimal conditions. The superoxide radical (^. O₂^- ) and singlet oxygen (¹ O₂ ) were identified as the oxidative species for the oxidation of sulfides into sulfoxides by exploring EPR experiments, and hence, a plausible mechanism for this oxidation was proposed. Moreover, the C₆₀ /g-C₃ N₄ catalyst can be easily recovered by filtration and then reused at least four times without loss in activity.

Confined Carbon Mediating Dehydroaromatization of Methane over Mo/ZSM-5

Non-oxidative dehydroaromatization of methane (MDA) is a promising catalytic process for direct valorization of natural gas to liquid hydrocarbons. The application of this reaction in practical technology is hindered by a lack of understanding about the mechanism and nature of the active sites in benchmark zeolite-based Mo/ZSM-5 catalysts, which precludes the solution of problems such as rapid catalyst deactivation. By applying spectroscopy and microscopy, it is shown that the active centers in Mo/ZSM-5 are partially reduced single-atom Mo sites stabilized by the zeolite framework. By combining a pulse reaction technique with isotope labeling of methane, MDA is shown to be governed by a hydrocarbon pool mechanism in which benzene is derived from secondary reactions of confined polyaromatic carbon species with the initial products of methane activation.

Green synthesis of PbCrO4 nanostructures using gum of ferula assa-foetida for enhancement of visible-light photocatalytic activity

Photocatalytic selective oxidation has attracted considerable attention as an environmentally friendly strategy for organic transformations.

An Enzymatic Route to α-Tocopherol Synthons: Aromatic Hydroxylation of Pseudocumene and Mesitylene with P450 BM3

Aromatic hydroxylation of pseudocumene (1 a) and mesitylene (1 b) with P450 BM3 yields key phenolic building blocks for α-tocopherol synthesis. The P450 BM3 wild-type (WT) catalyzed selective aromatic hydroxylation of 1 b (94 %), whereas 1 a was hydroxylated to a large extent on benzylic positions (46-64 %). Site-saturation mutagenesis generated a new P450 BM3 mutant, herein named "variant M3" (R47S, Y51W, A330F, I401M), with significantly increased coupling efficiency (3- to 8-fold) and activity (75- to 230-fold) for the conversion of 1 a and 1 b. Additional π-π interactions introduced by mutation A330F improved not only productivity and coupling efficiency, but also selectivity toward aromatic hydroxylation of 1 a (61 to 75 %). Under continuous nicotinamide adenine dinucleotide phosphate recycling, the novel P450 BM3 variant M3 was able to produce the key tocopherol precursor trimethylhydroquinone (3 a; 35 % selectivity; 0.18 mg mL^-1 ) directly from 1 a. In the case of 1 b, overoxidation leads to dearomatization and the formation of a valuable p-quinol synthon that can directly serve as an educt for the synthesis of 3 a. Detailed product pattern analysis, substrate docking, and mechanistic considerations support the hypothesis that 1 a binds in an inverted orientation in the active site of P450 BM3 WT, relative to P450 BM3 variant M3, to allow this change in chemoselectivity. This study provides an enzymatic route to key phenolic synthons for α-tocopherols and the first catalytic and mechanistic insights into direct aromatic hydroxylation and dearomatization of trimethylbenzenes with O₂ .

Overcoming the Gas-Liquid Mass Transfer of Oxygen by Coupling Photosynthetic Water Oxidation with Biocatalytic Oxyfunctionalization

Gas–liquid mass transfer of gaseous reactants is a major limitation for high space–time yields, especially for O₂‐dependent (bio)catalytic reactions in aqueous solutions. Herein, oxygenic photosynthesis was used for homogeneous O₂ supply via in situ generation in the liquid phase to overcome this limitation. The phototrophic cyanobacterium Synechocystis sp. PCC6803 was engineered to synthesize the alkane monooxygenase AlkBGT from Pseudomonas putida GPo1. With light, but without external addition of O₂, the chemo‐ and regioselective hydroxylation of nonanoic acid methyl ester to ω‐hydroxynonanoic acid methyl ester was driven by O₂ generated through photosynthetic water oxidation. Photosynthesis also delivered the necessary reduction equivalents to regenerate the Fe²⁺ center in AlkB for oxygen transfer to the terminal methyl group. The in situ coupling of oxygenic photosynthesis to O₂‐transferring enzymes now enables the design of fast hydrocarbon oxyfunctionalization reactions.

Enzymatic C-H Oxidation-Amidation Cascade in the Production of Natural and Unnatural Thiotetronate Antibiotics with Potentiated Bioactivity

The selective activation of unreactive hydrocarbons by biosynthetic enzymes has inspired new synthetic methods in C-H bond activation. Herein, we report the unprecedented two-step biosynthetic conversion of thiotetromycin to thiotetroamide C involving the tandem oxidation and amidation of an unreactive ethyl group. We detail the genetic and biochemical basis for the terminal amidation in thiotetroamide C biosynthesis, which involves a uniquely adapted cytochrome P450-amidotransferase enzyme pair and highlights the first oxidation-amidation enzymatic cascade reaction leading to the selective formation of a primary amide group from a chemically inert alkyl group. Motivated by the ten-fold increase in antibiotic potency of thiotetroamide C ascribed to the acetamide group and the unusual enzymology involved, we enzymatically interrogated diverse thiolactomycin analogues and prepared an unnatural thiotetroamide C analogue with potentiated bioactivity compared to the parent molecule.

Semirational Protein Engineering of CYP153AM.aq. -CPRBM3 for Efficient Terminal Hydroxylation of Short- to Long-Chain Fatty Acids

The regioselective terminal hydroxylation of alkanes and fatty acids is of great interest in a variety of industrial applications, such as in cosmetics, in fine chemicals, and in the fragrance industry. The chemically challenging activation and oxidation of non-activated C-H bonds can be achieved with cytochrome P450 enzymes. CYP153AM.aq. -CPRBM3 is an artificial fusion construct consisting of the heme domain from Marinobacter aquaeolei and the reductase domain of CYP102A1 from Bacillus megaterium. It has the ability to hydroxylate medium- and long-chain fatty acids selectively at their terminal positions. However, the activity of this interesting P450 construct needs to be improved for applications in industrial processes. For this purpose, the design of mutant libraries including two consecutive steps of mutagenesis is demonstrated. Targeted positions and residues chosen for substitution were based on semi-rational protein design after creation of a homology model of the heme domain of CYP153AM.aq. , sequence alignments, and docking studies. Site-directed mutagenesis was the preferred method employed to address positions within the binding pocket, whereas diversity was created with the aid of a degenerate codon for amino acids located at the substrate entrance channel. Combining the successful variants led to the identification of a double variant-G307A/S233G-that showed alterations of one position within the binding pocket and one position located in the substrate access channel. This double variant showed twofold increased activity relative to the wild type for the terminal hydroxylation of medium-chain-length fatty acids. This variant furthermore showed improved activity towards short- and long-chain fatty acids and enhanced stability in the presence of higher concentrations of fatty acids.

Enhancing the efficiency and regioselectivity of P450 oxidation catalysts by unnatural amino acid mutagenesis

The development of effective strategies for modulating the reactivity and selectivity of cytochrome P450 enzymes represents a key step toward expediting the use of these biocatalysts for synthetic applications. We have investigated the potential of unnatural amino acid mutagenesis to aid efforts in this direction. Four unnatural amino acids with diverse aromatic side chains were incorporated at 11 active-site positions of a substrate-promiscuous CYP102A1 variant. The resulting "uP450s" were then tested for their catalytic activity and regioselectivity in the oxidation of two representative substrates: a small-molecule drug and a natural product. Large shifts in regioselectivity resulted from these single mutations, and in particular, for para-acetyl-Phe substitutions at positions close to the heme cofactor. Screening this mini library of uP450s enabled us to identify P450 catalysts for the selective hydroxylation of four aliphatic positions in the target substrates, including a C(sp(3))-H site not oxidized by the parent enzyme. Furthermore, we discovered a general activity-enhancing effect of active-site substitutions involving the unnatural amino acid para-amino-Phe, which resulted in P450 catalysts capable of supporting the highest total turnover number reported to date on a complex molecule (34,650). The functional changes induced by the unnatural amino acids could not be reproduced by any of the 20 natural amino acids. This study thus demonstrates that unnatural amino acid mutagenesis constitutes a promising new strategy for improving the catalytic activity and regioselectivity of P450 oxidation catalysts.

Molecular Cu(II)-O-Cu(II) complexes: still waters run deep

Research on O2 activation at ligated Cu(I) is fueled by its biological relevance and the quest for efficient oxidation catalysts. A rarely observed reaction is the formation of a Cu(II) -O-Cu(II) species, which is more special than it appears at first sight: a single oxo ligand between two Cu(II) centers experiences considerable electron density, and this makes the corresponding complexes reactive and difficult to access. Hence, only a small number of these compounds have been synthesized and characterized unequivocally to date, and as biological relevance was not apparent, they remained unappreciated. However, recently they moved into the spotlight, when Cu(II) -O-Cu(II) cores were proposed as the active species in the challenging oxidation of methane to methanol at the surface of a Cu-grafted zeolite and in the active center of the copper enzyme particulate methane monooxygenase. This Minireview provides an overview of these systems with a special focus on their reactivity and spectroscopic features.