A reusable citric acid modified V/AC catalyst prepared by dielectric barrier discharge for hydroxylation of benzene to phenol

Direct hydroxylation of benzene with high selectivity and durability toward phenol production is still a challenge. Herein, a reusable VxOy/AC(cit)-DBD catalyst was synthesized by a facile and environmentally benign method...

Advanced Methods for Hydroxylation of Vegetable Oils, Unsaturated Fatty Acids and Their Alkyl Esters

Vegetable oils and their derivatives have great potential as renewable and sustainable raw materials for the production of polyurethanes and bio-based polyols. For industry an important process is their modification. Chemical reactions that are carried out on vegetable oils and their derivatives are: transesterification, auto-oxidation, hydrogenation, epoxidation, hydroxylation, acrylation, isocyanation and others. One of the modifications are reactions performed on double bonds and/or carbonyl moieties of plants oils and their derivatives. These reactions result in products that are actively used as binders in coating materials due to their unique structural properties. In this manuscript, we describe important technological methods for the introduction of hydroxyl groups: opening of oxirane rings by nucleophilic reagents such as: water, alcohols, glycols, amino alcohols, carboxylic acids; direct hydroxylation of unsaturated bonds with carboxylic peracids in combination with hydrolysis of carboxyl groups and hydration; hydroformylation of unsaturated bonds with subsequent hydrogenation and alkoxylation; and ozonolysis of unsaturated bonds in combination with subsequent hydrogenation and alkoxylation.

Corticosteroid Biosynthesis Revisited: No Direct Hydroxylation of Pregnenolone by Steroid 21-Hydroxylase

Cytochrome P450s (CYPs) are an essential family of enzymes in the human body. They play a crucial role in metabolism, especially in human steroid biosynthesis. Reactions catalyzed by these enzymes are highly stereo- and regio-specific. Lack or severe malfunctions of CYPs can cause severe diseases and even shorten life. Hence, investigations on metabolic reactions and structural requirements of substrates are crucial to gain further knowledge on the relevance of different enzymes in the human body functions and the origin of diseases. One key enzyme in the biosynthesis of gluco- and mineralocorticoids is CYP21A2, also known as steroid 21-hydroxylase. To investigate the steric and regional requirements of substrates for this enzyme, we performed whole-cell biotransformation assays using a strain of fission yeast Schizosaccharomyces pombe recombinantly expressing CYP21A2. The progestogens progesterone, pregnenolone, and their 17α-hydroxy-derivatives were used as substrates. After incubation, samples were analyzed using gas chromatography coupled to mass spectrometry. For progesterone and 17α-hydroxyprogesterone, their corresponding 21-hydroxylated metabolites 11-deoxycorticosterone and 11-deoxycortisol were detected, while after incubation of pregnenolone and 17α-hydroxypregnenolone, no hydroxylated product was observed. Findings were confirmed with authentic reference material. Molecular docking experiments agree with these results and suggest that interaction between the 3-oxo group and arginine-234 of the enzyme is a strict requirement. The presented results demonstrate once more that the presence of an oxo-group in position 3 of the steroid is indispensable, while a 3-hydroxy group prevents hydroxylation in position C-21 by CYP21A2. This knowledge may be transferred to other CYP21A2 substrates and hence help to gain essential insights into steroid metabolism.

A [6+4]-cycloaddition adduct is the biosynthetic intermediate in streptoseomycin biosynthesis

Streptoseomycin (STM, 1) is a bacterial macrolactone that has a unique 5/14/10/6/6-pentacyclic ring with an ether bridge. We have previously identified the biosynthetic gene cluster for 1 and characterized StmD as [6 + 4]- and [4 + 2]-bispericyclase that catalyze a reaction leading to both 6/10/6- and 10/6/6-tricyclic adducts (6 and 7). The remaining steps, especially how to install and stabilize the required 10/6/6-tricyclic core for downstream modifications, remain unknown. In this work, we have identified three oxidoreductases that fix the required 10/6/6-tryciclic core. A pair of flavin-dependent oxidoreductases, StmO1 and StmO2, catalyze the direct hydroxylation at [6 + 4]-adduct (6). Subsequently, a spontaneous [3,3]-Cope rearrangement and an enol-ketone tautomerization result in the formation of 10/6/6-tricyclic intermediate 12b, which can be further converted to a stable 10/6/6-tricyclic alcohol 11 through a ketoreduction by StmK. Crystal structure of the heterodimeric complex NtfO1-NtfO2, homologues of StmO1-StmO2 with equivalent function, reveals protein-protein interactions. Our results demonstrate that the [6 + 4]-adduct instead of [4 + 2]-adduct is the bona fide biosynthetic intermediate.

Effect of Natural Organic Matter on the Ozonation Mechanism of Trimethoprim in Water

Trimethoprim (TMP) is often used for the treatment of various bacterial infections. It can be detected in water, and it is difficult to be biodegraded. In this study, the degradation mechanism of TMP through ozonation and the effect of humic acids (HA) were investigated. Excessive ozone (pH 6, 0 °C) could reduce the content of TMP to less than 1% in 30 s. However, when ozone (O3) was not excessive (pH 6, 20 °C), the removal efficiency of TMP increased with the increase of O3 concentration. Four possible degradation pathways of TMP in the process of ozonation were speculated: hydroxylation, demethylation, carbonylation, and cleavage. The presence of HA in water inhibit the generation of ozonation products of TMP. The excitation-emission matrices (EEM) analysis showed that with the extension of ozonation time, the fluorescence value in the solution decreased and the fluorescence peak blue shifted. These results indicated that the structure of HA changed in the reaction and was competitively degraded with TMP. According to the free radical quenching test, the products of pyrolysis, direct hydroxylation and demethylation were mainly produced by indirect oxidation.

On the Ability of Nickel Complexes Derived from Tripodal Aminopyridine Ligands to Catalyze Arene Hydroxylations

The development of catalysts for the selective hydroxylation of aromatic C–H bonds is an essential challenge in current chemical research. The accomplishment of this goal requires the discovery of powerful metal-based oxidizing species capable of hydroxylating inert aromatic bonds in a selective manner, avoiding the generation of non-selective oxygen-centered radicals. Herein we show an investigation on the ability of nickel(ii) complexes supported by tripodal tetradentate aminopyridine ligands to catalyze the direct hydroxylation of benzene to phenol with H2O2 as oxidant. We have found that modifications on the ligand structure of the nickel complex do not translate into different reactivity, which differs from previous findings for nickel-based arene hydroxylations. Besides, several nickel(ii) salts have been found to be effective in the oxidation of aromatic C–H bonds. The use of fluorinated alcohols as solvent has been found to result in an increase in phenol yield; however, showing no more than two turn-overs per nickel. These findings raise questions on the nature of the oxidizing species responsible for the arene hydroxylation reaction.