Study of Cyclohexane and Methylcyclohexane Functionalization Promoted by Manganese(III) Compounds

Alkane functionalization using safe and low-energy processes is of great interest to industry and academia. Aiming to contribute to the process of saturated hydrocarbon functionalization, we have studied a set of three manganese(III) complexes as catalysts for promoting the oxidation of saturated hydrocarbons (cyclohexane and methylcyclohexane) in the presence of hydrogen peroxide or trichloroisocyanuric acid (TCCA). The mononuclear manganese(III) compounds were prepared using the ligands H2LMet4 (6,6’-((1,4-diazepane-1,4-diyl)bis(methylene))bis(2,4-dimethylphenol), H2salen (2,2’-((1E,1’E)-(ethane-1,2-diylbis(azaneylylidene))bis(methaneylylidene))diphenol) and H2salan (2,2’-((ethane-1,2-diylbis(azanediyl))bis(methylene))diphenol). The catalytic processes were carried out in acetonitrile at 25 and 50 °C for 24 h. The increase in the temperature was important to get a better conversion. The compounds did not promote cyclohexane oxidation in the presence of H2O2. However, they were active in the presence of TCCA, employing a ratio of 1000:333:1 equivalents of the substrate:TCCA:catalyst. The best catalytic activity was shown by the compound [Mn(salen)Cl], reaching conversions of 14.5 ± 0.3% (25 °C) and 26.3 ± 1.1% (50 °C) (yield for chlorocyclohexane) and up to 12.1 ± 0.5% (25 °C) and 29.8 ± 2.2% (50 °C) (total yield for the mixture of the products 1-chloro-4-methylcyclohexane, 3-methylcyclohexene and 1-methylcyclohexene). The interaction of the catalysts with TCCA was studied using electron paramagnetic resonance (EPR), suggesting that the catalysts [Mn(LMet4)Cl] and [Mn(salan)Cl] act via a different mechanism from that observed for [Mn(salen)Cl].

Insights into the Binding Interaction of Catechol 1,2-Dioxygenase with Catechol in Achromobacter xylosoxidans DN002

Microbial remediation has become one of the promising ways to eliminate polycyclic aromatic hydrocarbons (PAHs) pollution due to its efficient enzyme metabolism system. Catechol 1,2-dioxygenase (C12O) is a crucial rate-limiting enzyme in the degradation pathway of PAHs in Achromobacter xylosoxidans DN002 that opens the benzene ring through the ortho-cleavage pathway. However, little attention has been given to explore the interaction mechanism of relevant enzyme-substrate. This study aims to investigate the binding interaction between C12O of strain DN002 and catechol by means of a molecular biological approach combined with homology modeling, molecular docking, and multiple spectroscopies. The removal rate of catechol in the mutant strain of cat A deletion was only 12.03%, compared to the wild-type strain (54.21%). A Ramachandran plot of active site regions of the primary amino acid sequences in the native enzyme showed that 93.5% sequences were in the most favored regions on account of the results of homology modeling, while an additional 6.2% amino acid sequences were found in conditionally allowed regions, and 0.4% in generously allowed regions. The binding pocket of C12O with catechol was analyzed to obtain that the catalytic trimeric group of Tyr164-His224-His226 was proven to be great vital for the ring-opening reaction of catechol by molecular docking. In the native enzyme, binding complexes were spontaneously formed by hydrophobic interactions. Binding constants and thermodynamic potentials from fluorescence spectra indicated that catechol effectively quenched the intrinsic fluorescence of C12O in the C12O/catechol complex via conventional static and dynamic quenching mechanisms of C12O. The results of ultraviolet and visible (UV) spectra, synchronous fluorescence, and circular dichroism (CD) spectra revealed conspicuous changes in the local conformation, and site-directed mutagenesis confirmed the role of predicted key residues during catalysis, wherein His₂₂₆ had a significant effect on catechol utilization by C12O. This is the first report to reveal interactions of C12O with substrate from the molecular docking results, providing the mechanistic understanding of representative dioxygenases involved in aromatic compound degradation, and a solid foundation for further site modifications as well as strategies for the directed evolution of this enzyme.

Mononuclear copper(ii) Schiff base complexes as effective models for phenoxazinone synthase

Copper(ii) complexes of tridentate (N2O) Schiff base ligands as efficient catalysts for 2-aminophenol oxidation to 2-aminophenoxazin-3-one with excellent reaction rates.

Degradation of 2,4,6-Trinitrotoluene (TNT): Involvement of Protocatechuate 3,4-Dioxygenase (P34O) in Buttiauxella sp. S19-1

Extensive use and disposal of 2,4,6-trinitrotoluene (TNT), a primary constituent of explosives, pollutes the environment and causes severe damage to human health. Complete mineralization of TNT via bacterial degradation has recently gained research interest as an effective method for the restoration of contaminated sites. Here, screening for TNT degradation by six selected bacteria revealed that Buttiauxella sp. S19-1, possesses the strongest degrading ability. Moreover, BuP34O (a gene encoding for protocatechuate 3,4-dioxygenase—P34O, a key enzyme in the β-ketoadipate pathway) was upregulated during TNT degradation. A knockout of BuP34O in S19-1 to generate S-M1 mutant strain caused a marked reduction in TNT degradation efficiency compared to S19-1. Additionally, the EM1 mutant strain (Escherichia coli DH5α transfected with BuP34O) showed higher degradation efficiency than DH5α. Gas chromatography mass spectrometry (GC-MS) analysis of TNT degradation by S19-1 revealed 4-amino-2,6-dinitrotolune (ADNT) as the intermediate metabolite of TNT. Furthermore, the recombinant protein P34O (rP34O) expressed the activity of 2.46 µmol/min·mg. Our findings present the first report on the involvement of P34O in bacterial degradation of TNT and its metabolites, suggesting that P34O could catalyze downstream reactions in the TNT degradation pathway. In addition, the TNT-degrading ability of S19-1, a Gram-negative marine-derived bacterium, presents enormous potential for restoration of TNT-contaminated seas.

1,2-Disubstituted Benzimidazoles by the Iron Catalyzed Cross-Dehydrogenative Coupling of Isomeric o-Phenylenediamine Substrates

Benzimidazoles are common in nature, medicines, and materials. Numerous strategies for preparing 2-arylbenzimidazoles exist. In this work, 1,2-disubstituted benzimidazoles were prepared from various mono- and disubstituted ortho-phenylenediamines (OPD) by iron-catalyzed oxidative coupling. Specifically, O₂ and FeCl₃·6H₂O catalyzed the cross-dehydrogenative coupling and aromatization of diarylmethyl and dialkyl benzimidazole precursors. N,N'-Disubstituted-OPD substrates were significantly more reactive than their N,N-disubstituted isomers, which appears to be relative to their propensity for complexation and charge transfer with Fe³⁺. The reaction also converted N-monosubstituted OPD substrates to 2-substituted benzimidazoles; however, electron-poor substrates produce 1,2-disubstituted benzimidazoles by intermolecular imino-transfer. Kinetic, reagent, and spectroscopic (UV-vis and EPR) studies suggest a mechanism involving metal-substrate complexation, charge transfer, and aerobic turnover, involving high-valent Fe(IV) intermediates. Overall, comparative strategies for the relatively sustainable and efficient synthesis of 1,2-disubstituted benzimidazoles are demonstrated.

Iron amino-bis(phenolate) complexes for the formation of organic carbonates from CO2 and oxiranes

Air-stable iron complexes display good activity for CO₂-epoxide cycloadditions and reactivity trends across family are reported.

Redox and acid-base properties of asymmetric non-heme (hydr)oxo-bridged diiron complexes

The diiron unit is commonly found as the active site in enzymes that catalyze important biological transformations. Two μ-(hydr)oxo-diiron(iii) complexes with the ligands 2,2'-(2-methyl-2-(pyridine-2-yl)propane-1,3-diyl)bis(azanediyl)bis(methylene)diphenol (H2L) and 2,2'-(2-methyl-2(pyridine-2-yl)propane-1,3-diyl)bis(azanediyl)bis(methylene)bis(4-nitrophenol) (H2L(NO2)), namely [(FeL)2(μ-O)] () and [(FeL(NO2))2(μ-OH)]ClO4 () were synthesized and characterized. In the solid state, both structures are asymmetric, with unsupported (hydr)oxo bridges. Intramolecular hydrogen bonding of the ligand NH groups to the phenolate O atoms hold the diiron cores in a bent configuration (Fe-O-Fe angle of 143.7° for and 140.1° for ). A new phenolate bridged diferrous complex, [(FeL)2] (), was synthesized and characterized. Upon exposure to air the diferrous complex is oxidized to the diferric . Cyclic voltammetry at different scan rates and chemical reduction of [(FeL)2(μ-OH)]BPh4 () with cobaltocene revealed disproportionation followed by proton transfer, and a mixed-valence species could not be trapped. Subsequent exposure to molecular oxygen results in the formation of . Electrochemical studies of indicate easier reduction of the diiron(iii/iii) to the mixed-valence state than for . The protonation of by benzoic acid to form [(FeL)2(μ-OH)](+) only changes the Fe-O-Fe angle by 5° (from 143.7° to 138.6°), and the pKa of the hydroxo bridge is estimated to be about 20.4. We attribute this high pKa partly to stabilization of the benzoate by hydrogen bonding to the ligand's amine proton. Magnetic susceptibility studies on solid samples of and yielded values of the antiferromagnetic exchange coupling constants, J, for these S = 5/2 dimers of -13.1 cm(-1) and -87.5 cm(-1), respectively, typical of such unsupported hydroxo- and oxo-bridges.

A dual colorimetric-ratiometric fluorescent probe NAP-3 for selective detection and imaging of endogenous labile iron(iii) pools in C. elegans

The first dual colorimetric and ratiometric fluorescent probe NAP-3 for selective visualization of labile iron(iii) pools in Caenorhabditis elegans is reported.

Transition metal complexes and the activation of dioxygen

In order to address how diverse metalloprotein active sites, in particular those containing iron and copper, guide O₂binding and activation processes to perform diverse functions, studies of synthetic models of the active sites have been performed. These studies have led to deep, fundamental chemical insights into how O₂coordinates to mono- and multinuclear Fe and Cu centers and is reduced to superoxo, peroxo, hydroperoxo, and, after O-O bond scission, oxo species relevant to proposed intermediates in catalysis. Recent advances in understanding the various factors that influence the course of O₂activation by Fe and Cu complexes are surveyed, with an emphasis on evaluating the structure, bonding, and reactivity of intermediates involved. The discussion is guided by an overarching mechanistic paradigm, with differences in detail due to the involvement of disparate metal ions, nuclearities, geometries, and supporting ligands providing a rich tapestry of reaction pathways by which O₂is activated at Fe and Cu sites.

Ring-opening polymerization of rac-lactide mediated by tetrametallic lithium and sodium diamino-bis(phenolate) complexes

Sodium complex contains interesting intramolecular η⁶-arene interaction and shows excellent catalytic behaviour for polymerization of lactide.

Protocatechuate 3,4-dioxygenase: a wide substrate specificity enzyme isolated from Stenotrophomonas maltophilia KB2 as a useful tool in aromatic acid biodegradation

Protocatechuate 3,4-dioxygenases (P34Os) catalyze the reaction of the ring cleavage of aromatic acid derivatives. It is a key reaction in many xenobiotic metabolic pathways. P34Os characterize narrow substrate specificity. This property is an unfavorable feature in the biodegradation process because one type of pollution is rarely present in the environment. Thus, the following study aimed at the characterization of a P34O from Stenotrophomonas maltophilia KB2, being able to utilize a wide spectrum of aromatic carboxylic acids. A total of 3 mM vanillic acid and 4-hydroxybenzoate were completely degraded during 8 and 4.5 h, respectively. When cells of strain KB2 were grown on 9 mM 4-hydroxybenzoate, P34O was induced. Biochemical analysis revealed that the examined enzyme was similar to other known P34Os, but showed untypical wide substrate specificity. A high activity of P34O against 2,4- and 3,5-dihydroxybenzoate was observed. As these substrates do not possess ortho configuration hydroxyl groups, it is postulated that their cleavage could be connected with their monodentate binding of substrate to the active site. Since this enzyme characterizes untypical wide substrate specificity it makes it a useful tool in applications for environmental clean-up purposes.

Monomeric Ti(IV) homopiperazine complexes and their exploitation for the ring opening polymerisation of rac-lactide

BACKGROUND

The area of biodegradable/sustainable polymers is one of increasing importance in the 21st Century due to their positive environmental characteristics. Lewis acidic metal centres are currently one of the most popular choices for the initiator for the polymerisation. Thus, in this paper we report the synthesis and characterisation of a series of monometallic homopiperazine Ti(IV) complexes where we have systematically varied the sterics of the phenol moieties.

RESULTS

When the ortho substituent of the ligand is either a Me, tBu or amyl then the β-cis isomer is isolated exclusively in the solid-state. Nevertheless, in solution multiple isomers are clearly observed from analysis of the NMR spectra. However, when the ortho substituent is an H-atom then the trans-isomer is formed in the solid-state and solely in solution. The complexes have been screened for the polymerisation of rac-lactide in solution and under the industrially preferred melt conditions. Narrow molecular weight material (PDI 1.07 - 1.23) is formed under melt conditions with controlled molecular weights.

CONCLUSIONS

Six new Ti(IV) complexes are presented which are highly active for the polymerisation. In all cases atactic polymer is prepared with predictable molecular weight control. This shows the potential applicability of Ti(IV) to initiate the polymerisations.

Manganese(II) semiquinonato and manganese(III) catecholato complexes with tridentate ligand: modeling the substrate-binding state of manganese-dependent catechol dioxygenase and reactivity with molecular oxygen

Catecholate catwalk: Monomeric manganese(III) catecholato and manganese(II) semiquinonato complexes as the substrate-binding model of catechol dioxygenase have been synthesized and structurally characterized. The semiquinonato complex reacted with molecular oxygen to give ring-cleaved products and benzoquinone in the catalytic condition.

Influence of metal ions on bioremediation activity of protocatechuate 3,4-dioxygenase from Stenotrophomonas maltophilia KB2

The aim of this paper was to describe the effect of various metal ions on the activity of protocatechuate 3,4-dioxygenase from Stenotrophomonas maltophilia KB2. We also compared activity of different dioxygenases isolated from this strain, in the presence of metal ions, after induction by various aromatic compounds. S. maltophilia KB2 degraded 13 mM 3,4-dihydroxybenzoate, 10 mM benzoic acid and 12 mM phenol within 24 h of incubation. In the presence of dihydroxybenzoate and benzoate, the activity of protocatechuate 3,4-dioxygenase and catechol 1,2-dioxygenase was observed. Although Fe(3+), Cu(2+), Zn(2+), Co(2+), Al(3+), Cd(2+), Ni(2+) and Mn(2+) ions caused 20-80 % inhibition of protocatechuate 3,4-dioxygenase activity, the above-mentioned metal ions (with the exception of Ni(2+)) inhibited catechol 1,2-dioxygenase to a lesser extent or even activate the enzyme. Retaining activity of at least one of three dioxygenases from strain KB2 in the presence of metal ions makes it an ideal bacterium for bioremediation of contaminated areas.

Iron(III) complexes with meridional ligands as functional models of intradiol-cleaving catechol dioxygenases

Six dichloroiron(III) complexes of 1,3-bis(2'-arylimino)isoindoline (BAIH) with various N-donor aryl groups have been characterized by spectroscopy (infrared, UV-vis), electrochemistry (cyclic voltammetry), microanalysis, and in two cases X-ray crystallography. The structurally characterized Fe(III)Cl(2)(L(n)) complexes (n = 3, L(3) = 1,3-bis(2'-thiazolylimino)isoindoline and n = 5, L(5) = 1,3-bis(4-methyl-2'-piridylimino)isoindoline) are five-coordinate, trigonal bipyramidal with the isoindoline ligands occupying the two axial and one equatorial positions meridionally. These compounds served as precursors for catechol dioxygenase models that were formed in solution upon addition of 3,5-di-tert-butylcatechol (H(2)DBC) and excess triethylamine. These adducts react with dioxygen in N,N-dimethylformamide, and the analysis of the products by chromatography and mass spectrometry showed high intradiol over extradiol selectivity (the intradiol/extradiol product ratios varied between 46.5 and 6.5). Kinetic measurements were performed by following the change in the intensity of the catecholate to iron ligand-to-metal charge transfer (LMCT) band, the energy of which is influenced by the isoindolinate-ligand (827-960 nm). In combination with electrochemical investigations the kinetic studies revealed an inverse trend between reaction rates and oxidation potentials associated with the coordinated DBC(2-). On the basis of these results, a substrate activation mechanism is suggested for this system in which the geometry of the peroxide-bridged intermediate may be of key importance in regioselectivity.