An artificial metalloenzyme that can oxidize water photocatalytically: design, synthesis, and characterization

In nature, light-driven water oxidation (WO) catalysis is performed by photosystem II via the delicate interplay of different cofactors positioned in its protein scaffold. Artificial systems for homogeneous photocatalytic WO are based on small molecules that often have limited solubility in aqueous solutions. In this work, we alleviated this issue and present a cobalt-based WO-catalyst containing artificial metalloenzyme (ArM) that is active in light-driven, homogeneous WO catalysis in neutral-pH aqueous solutions. A haem-containing electron transfer protein, cytochrome B5 (CB5), served to host a first-row transition-metal-based WO catalyst, CoSalen (CoIISalen, where H2Salen = N,N'-bis(salicylidene)ethylenediamine), thus producing an ArM capable of driving photocatalytic WO. The CoSalen ArM formed a water-soluble pre-catalyst in the presence of [Ru(bpy)3](ClO4)2 as photosensitizer and Na2S2O8 as the sacrificial electron acceptor, with photocatalytic activity similar to that of free CoSalen. During photocatalysis, the CoSalen-protein interactions were destabilized, and the protein partially unfolded. Rather than forming tens of nanometer sized CoOx nanoparticles as free CoSalen does under photocatalytic WO conditions, the CB5 : CoSalen ArM showed limited protein cross-linking and remained soluble. We conclude that a weak, dynamic interaction between a soluble cobalt species and apoCB5 was formed, which generated a catalytically active adduct during photocatalysis. A detailed analysis was performed on protein stability and decomposition processes during the harsh oxidizing reaction conditions of WO, which will serve for the future design of WO ArMs with improved activity and stability.

Reacting Molecular Oxygen with Butanone under Visible Light Irradiation: A General Aerobic Oxidation of Alkenes, Sulfides, Phosphines, and Silanes

Here, we present a novel oxidation technique by reacting molecular oxygen with butanone under visible light irradiation. This method enables the mild oxidation of various functionalized compounds, including olefins, sulfides, phosphines, and silanes. Preliminary mechanistic experiments and theoretical calculations suggest that visible light triggers molecular oxygen to produce singlet oxygen in butanone. This singlet oxygen then reacts with butanone, producing an active oxidizing species.

The Synthetic Approaches to 1,2-Chlorohydrins

This short review highlights the hitherto realised synthetic approaches towards organic 1,2-chlorohydrins by functionalisation of alkenes (i.e., 1,2-chlorohydroxylation), which is the most prominent access route to this class of compounds. Also, some other synthetic approaches involving the reduction of α-chloroketones, the epoxide opening ring by chloride anions and the utilisation of Grignard reagents for the synthesis of these compounds and chlorination of allylic alcohols are highlighted. Finally, enzymatic reactions for the formation of chlorohydrins are briefly summarised followed by a short view on natural products containing this moiety.

1 Introduction

2 Applications for the Synthesis of 1,2-Chlorohydrins

2.1 Chlorohydroxylation of Alkenes

2.2 Reduction of Chloroketones

2.3 Metalorganic Reagents

2.4 Epoxide Ring Opening

2.5 Chlorination of Allylic Alcohols

2.6 Biochemical Methods

2.7 Selected Applications in Natural Product Total Synthesis

3 Conclusion

Bis(4-carboxylpyrazol-1-yl)acetic acid: a scorpionate ligand for complexes with improved water solubility

Bis(4-carboxylpyrazol-1-yl)acetic acid (H₃bcpza) (2), obtained in a one-pot synthesis, contains carboxyl groups that differ in their pK_a values. The ligand exhibits heteroscorpionate κ³-N,N,O-coordination whereupon the peripheral carboxyl groups are not involved in metal binding. The corresponding carbonyl complexes [Mn(H₂bcpza)(CO)₃] (4), [Re(H₂bcpza)(CO)₃] (5) and [Ru(H₂bcpza)Cl(CO)₂] (6a/6b) are partially soluble in water but readily soluble in PBS buffer.

Electrophilic chloro(ω-alkoxy)lation of alkenes employing 1-chloro-1,2-benziodoxol-3-one: facile synthesis of β-chloroethers

A four-component reaction for electrophilic chloro(ω-alkoxy)lation of alkenes has been described. The stable chloro-iodine(iii) reagent and SOCl2 were used as electrophilic and nucleophilic chlorine sources, respectively. This approach provides a straightforward way to synthesize various useful β-chloro ω-chloroalkyl ethers from a wide range of alkenes, including electron-deficient, aromatic and unactivated alkenes. The synthetic applications of this approach were also explored in some useful transformations.

Halofunctionalization of alkenes by vanadium chloroperoxidase from Curvularia inaequalis

The vanadium-dependent chloroperoxidase from Curvularia inaequalis is a stable and efficient biocatalyst for the hydroxyhalogenation of a broad range of alkenes into halohydrins. Up to 1 200 000 TON with 69 s^-1 TOF were observed for the biocatalyst. A bienzymatic cascade to yield epoxides as reaction products is presented.

Cross-Linked Artificial Enzyme Crystals as Heterogeneous Catalysts for Oxidation Reactions

Designing systems that merge the advantages of heterogeneous catalysis, enzymology, and molecular catalysis represents the next major goal for sustainable chemistry. Cross-linked enzyme crystals display most of these essential assets (well-designed mesoporous support, protein selectivity, and molecular recognition of substrates). Nevertheless, a lack of reaction diversity, particularly in the field of oxidation, remains a constraint for their increased use in the field. Here, thanks to the design of cross-linked artificial nonheme iron oxygenase crystals, we filled this gap by developing biobased heterogeneous catalysts capable of oxidizing carbon-carbon double bonds. First, reductive O₂ activation induces selective oxidative cleavage, revealing the indestructible character of the solid catalyst (at least 30 000 turnover numbers without any loss of activity). Second, the use of 2-electron oxidants allows selective and high-efficiency hydroxychlorination with thousands of turnover numbers. This new technology by far outperforms catalysis using the inorganic complexes alone, or even the artificial enzymes in solution. The combination of easy catalyst synthesis, the improvement of "omic" technologies, and automation of protein crystallization makes this strategy a real opportunity for the future of (bio)catalysis.