Heteroatom Oxidation Research Articles

Asymmetric N-oxidation catalyzed by bisguanidinium dinuclear oxodiperoxomolybdosulfate

N-oxides play a pivotal role in natural products and emerging drug design, while also serving as valuable ligand scaffolds in organometallic chemistry. Among heteroatom oxidations, the conversion of amines to N-oxides is a critical and challenging facet. We present here a highly enantioselective N-oxidation methodology for both cyclic and acyclic amines. The method employs an ion-pair catalyst comprising a chiral bisguanidinium [BG]2+ cation and an achiral oxodiperoxomolybdosulfate anion [(µ-SO4)2Mo2O2(µ-O2)2(O2)2]2-. Notably, the bisguanidinium cation undergoes modification through silyl group incorporation and is elucidated by X-ray crystallography. Our findings underscore the crucial role of the side chain in the determination of the chiral pocket size, allowing for the oxidation of diverse tertiary amines with enantioselectivities. Comprehensive mechanistic investigations are conducted to explain the catalytic system’s efficacy in achieving dynamic kinetic resolution (DKR) with high efficiency.

Biocatalytic role of cytochrome P450s to produce antibiotics: A review.

Cytochrome P450s belong to a family of heme-binding monooxygenases, which catalyze regio- and stereospecific functionalisation of C-H, C-C, and C-N bonds, including heteroatom oxidation, oxidative C-C bond cleavages, and nitrene transfer. P450s are considered useful biocatalysts for the production of pharmaceutical products, fine chemicals, and bioremediating agents. Despite having tremendous biotechnological potential, being heme-monooxygenases, P450s require either autologous or heterologous redox partner(s) to perform chemical transformations. Randomly distributed P450s throughout a bacterial genome and devoid of particular redox partners in natural products biosynthetic gene clusters (BGCs) showed an extra challenge to reveal their pharmaceutical potential. However, continuous efforts have been made to understand their involvement in antibiotic biosynthesis and their modification, and this review focused on such BGCs. Here, particularly, we have discussed the role of P450s involved in the production of macrolides and aminocoumarin antibiotics, nonribosomal peptide(NRPSs) antibiotics, ribosomally synthesized and post-translationally modified peptide(RiPPs) antibiotics, and others. Several reactions catalyzed by P450s, as well as the role of their redox partners involved in the BGCs of various antibiotics and their derivatives, have been primarily addressed in this review, which would be useful in further exploration of P450s for the biosynthesis of new therapeutics.

Methods for Biophysical Characterization of SznF, a Member of the Heme-Oxygenase-Like Diiron Oxidase/Oxygenase Superfamily.

Nonheme diiron enzymes harness the chemical potential of oxygen to catalyze challenging reactions in biology. In their resting state, these enzymes have a diferrous cofactor that is coordinated by histidine and carboxylate ligands. Upon exposure to oxygen, the cofactor oxidizes to its diferric state forming a peroxo- adduct, capable of catalyzing a wide range of oxidative chemistries such as desaturation and heteroatom oxidation. Despite their versatility and prowess, an emerging subset of nonheme diiron enzymes has inherent cofactor instability making them resistant to structural characterization. This feature is widespread among members of the heme-oxygenase-like diiron oxidase/oxygenase (HDO) superfamily. HDOs have a flexible core structure that remodels upon metal binding. Although ~9600 HDOs have been unearthed, few have undergone functional characterization to date. In this chapter, we describe the methods that have been used to characterize the HDO N-oxygenase, SznF. We demonstrate the overexpression and purification of apo-SznF and methodology specifically designed to aid in obtaining an X-ray structure of holo-SznF. We also describe the characterization of the transient SznF-peroxo-Fe(III)2 complex by stopped-flow absorption and Mössbauer spectroscopies. These studies provide the framework for the characterization of new members of the HDO superfamily.

Heteroatom oxidation controls singlet-triplet energy splitting in singlet fission building blocks.

Singlet fission (SF) is a promising multiexciton-generating process. Its demanding energy splitting criterion - that the S1 energy must be at least twice that of T1 - has limited the range of materials capable of SF. We propose heteroatom oxidation as a robust strategy to achieve sufficient S1/T1 splitting, and demonstrate the potential of this approach for intramolecular SF.

Singlet Oxygen Generation from a Water-Soluble Hypervalent Iodine(V) Reagent AIBX and H2O2: An Access to Artemisinin

Herein, we report an efficient method for the chemical generation of 1O2 by treatment of H2O2 with AIBX, a highly water-soluble, bench-stable, recyclable hypervalent iodine(V) reagent developed by our group. The generation of 1O2 was confirmed by the following results: (1) capture of 1O2 with the sodium salt of anthracene-9,10-bis(ethanesulfonate) produced the corresponding endoperoxide and (2) TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) produced by the oxidation of 2,2,6,6-tetramethylpiperidine with 1O2 generated using the AIBX/H2O2 system was detected by electron spin resonance spectroscopy. To illustrate the potential utility of this method for organic synthesis, we used the AIBX/H2O2 system to perform typical reactions of 1O2: [2 + 2]/[4 + 2] cycloadditions, Schenck ene reactions, and heteroatom oxidation reactions, which afforded the corresponding products in high yields. Moreover, we used the method to synthesize the antimalarial drug artemisinin. Finally, we demonstrated that AIBX could be regenerated after the reaction by means of a workup involving extraction and removal of water to obtain a precursor of AIBX, which could then be re-oxidized.

Natural Variation in the ‘Control Loop’ of BVMOAFL210 and Its Influence on Regioselectivity and Sulfoxidation

Baeyer-Villiger monooxygenases (BVMOs) are flavin-dependent enzymes that primarily convert ketones to esters, but can also catalyze heteroatom oxidation. Several structural studies have highlighted the importance of the ‘control loop’ in BVMOs, which adopts different conformations during catalysis. Central to the ‘control loop’ is a conserved tryptophan that has been implicated in NADP(H) binding. BVMOAFL210 from Aspergillus flavus, however, contains a threonine in the equivalent position. Here, we report the structure of BVMOAFL210 in complex with NADP+ in both the ‘open’ and ‘closed’ conformations. In neither conformation does Thr513 contact the NADP+. Although mutagenesis of Thr513 did not significantly alter the substrate scope, changes in peroxyflavin stability and reaction rates were observed. Mutation of this position also brought about changes in the regio- and enantioselectivity of the enzyme. Moreover, lower rates of overoxidation during sulfoxidation of thioanisole were also observed.

Radiation induced oxidation of [18F]fluorothia fatty acids under cGMP manufacturing conditions

ObjectiveThe objectives of the present work were to optimize and validate the synthesis and stability of 14(R,S)-[18F]fluoro-6-thia-heptadecanoic acid ([18F]FTHA) and 16-[18F]fluoro-4-thia-palmitic acid ([18F]FTP) under cGMP conditions for clinical applications. MethodsBenzyl-14-(R,S)-tosyloxy-6-thiaheptadecanoate and methyl 16-bromo-4-thia-palmitate were used as precursors for the synthesis of [18F]FTHA and [18F]FTP, respectively. For comparison, a fatty acid analog lacking a thia-substitution, 16-[18F]fluoro-palmitic acid ([18F]FP), was synthesized from the precursor methyl 16-bromo-palmitate. A standard nucleophilic reaction using cryptand (Kryptofix/K222, 8.1 mg), potassium carbonate (K2CO3, 4.0 mg) and 18F-fluoride were employed for the 18F-labeling and potassium hydroxide (0.8 M) was used for the post-labeling ester hydrolysis. The final products were purified via reverse phase semi-preparative HPLC and concentrated via trap and release on a C-18 plus solid phase extraction cartridge. The radiochemical purities of the [18F]fluorothia fatty acids and [18F]FP were examined over a period of 4 h post-synthesis using an analytical HPLC. All the syntheses were optimized in an automated TRACERlab FX-N Pro synthesizer. Liquid chromatography mass spectrometry (LCMS) and high resolution mass spectrometry (HRMS) was employed to study the identity and nature of side products formed during radiosynthesis and as a consequence of post-synthesis radiation induced oxidation. ResultsRadiosyntheses of [18F]FTHA, [18F]FTP and [18F]FP were achieved in moderate (8–20% uncorrected) yields. However, it was observed that the HPLC-purified [18F]fluorothia fatty acids, [18F]FTHA and [18F]FTP at higher radioactivity concentrations (>1.11 GBq/mL, 30 mCi/mL) underwent formation of 18F-labeled side products over time but [18F]FP (lacking a sulfur heteroatom) remained stable up to 4 h post-synthesis. Various radiation protectors like ethanol and ascorbic acid were examined to minimize the formation of side products formed during [18F]FTHA and [18F]FTP synthesis but showed only limited to no effect. Analysis of the side products by LCMS showed formation of sulfoxides of both [18F]FTHA and [18F]FTP. The identity of the sulfoxide side product was further confirmed by synthesizing a non-radioactive reference standard of the sulfoxide analog of FTP and matching retention times on HPLC and molecular ion peaks on LC/HRMS. Radiation-induced oxidation of the sulfur heteroatom was mitigated by dilution of product with isotonic saline to reduce the radioactivity concentration to <0.518 GBq/mL (14 mCi/mL). ConclusionsSuccessful automated synthesis of [18F]fluorothia fatty acids were carried out in cGMP facility for their routine production and clinical applications. Instability of [18F]fluorothia fatty acids were observed at radioactivity concentrations exceeding 1.11 GBq/mL (30 mCi/mL) but mitigated through dilution of the product to <0.518 GBq/mL (14 mCi/mL). The identities of the side products formed were established as the sulfoxides of the respective thia fatty acids caused by radiation-induced oxidation of the sulfur heteroatom.

Electrochemical characterization of cefadroxil β-lactam antibiotic and Cu(II) complex formation

The electrochemical behaviour of cefadroxil, a first-generation β-lactam antibiotic, was studied at glassy carbon electrodes in aqueous media over a wide range of pH. The first oxidation process is of the phenol moiety and follows an ECE mechanism, generating catechol and resorcinol derivatives as sub-products, which are then reduced and oxidized in subsequent cycles. The sulphur heteroatom present in the cyclic structure close to the β-lactam moiety is oxidized in two steps generating sulphoxide and sulphone. This process was identified from direct comparison with amoxicillin, which has a similar molecular structure, although they belong to different classes of β-lactam antibiotics. For amoxicillin, oxidation of the sulphur heteroatom occurred at more positive potentials, most likely due to structural difficulties in stabilizing the charged oxidized species. Formation of a complex between copper (II) and each of the antibiotics was studied by cyclic voltammetry. Finally, determination of cefadroxil in commercial samples was successfully carried out.

Chemoselective oxidation of unsaturated organosulfur, selenium and phosphorus compounds by molybdenum oxodiperoxo complexes: A computational investigation

Oxidation is fundamental for many chemical processes and the search for chemoselective oxidants is relevant because most substrates have different functional groups. For unsaturated organo-heteroatom (E) substrates R1E(CH2)nCHCR2H, the olefin and/or the heteroatom can be oxidized. It is known that molybdenum oxodiperoxo [MoO(O2)2L] complexes can selectively oxidize sulfide groups in the presence of alkenes. Thus, the effects of the substituents (R1 and R2) and of the separation n between the functional groups on the chemoselectivity were investigated by methods based on the density functional theory (DFT) for sulfides (E=S). The chemoselectivity was quantified by the difference between the Gibbs energy of activation at the double bond (TS@CC) and at the heteroatom (TS@E) oxidation pathways, ΔΔ‡G=Δ‡G(TS@CC)−Δ‡G(TS@E). Consistent with experimental observations, this oxidation is chemoselective towards the heteroatom (E=S, Se, and PCH3) for any unsaturated substrate or ancillary ligand L in the complex. For unsaturated organosulfur compounds, it is shown that an increase in the electronegativity of R1 leads to a decrease of the chemoselectivity, which can be correlated with the atomic charge at the sulfur center. The separation n affects the chemoselectivity similarly to the electronegativity of R1, namely, differences between sp2 and sp3 carbon centers. The ancillary ligand L=OPH3, pyrazole, pyridine N-oxide, and Si(OH)4, affects the reactivity of the complex and its chemoselectivity, where L=Si(OH)4 shows the highest reactivity and the least selectivity; however, ΔΔ‡G is still large enough (4.3kcal/mol) to provide high chemoselectivity. This agrees with the experimental observations related to the oxidations by [MoO(O2)2L] complexes supported on silica. These quantitative results and the qualitative trends and correlations can be helpful in the design of more efficient and greener Mo-based oxidants.

Catalytic Allylic Oxidation of Cyclic Enamides and 3,4-Dihydro-2H-Pyrans by TBHP

Allylic oxidation of heteroatom substituted cyclic alkenes by tert-butyl hydroperoxide (70% TBHP in water) using catalytic dirhodium caprolactamate [Rh2(cap)4] forms enone products with a variety of 2-substituted cyclic enamides and 3,4-dihyro-2H-pyrans. These reactions occur under mild reaction conditions, are operationally convenient to execute, and are effective for product formation with as low as 0.25 mol% catalyst loading. With heteroatom stabilization of the intermediate allylic free radical two sites for oxidative product formation are possible, and the selectivity of the oxidative process varies with the heteroatom when R = H. Cyclic enamides produce 4-piperidones in good yields when R = alkyl or aryl, but oxidation of 2H-pyrans also gives alkyl cleavage products. Alternative catalysts for TBHP oxidations show comparable selectivities but give lower product yields.

High throughput photo-oxidations in a packed bed reactor system

The efficiency gains produced by continuous-flow systems in conducting photochemical transformations have been extensively demonstrated. Recently, these systems have been used in developing safe and efficient methods for photo-oxidations using singlet oxygen generated by photosensitizers. Much of the previous work has focused on the use of homogeneous photocatalysts. The development of a unique, packed-bed photoreactor system using immobilized rose bengal expands these capabilities as this robust photocatalyst allows access to and elaboration from these highly useful building blocks without the need for further purification. With this platform we were able to demonstrate a wide scope of singlet oxygen ene, [4+2] cycloadditions and heteroatom oxidations. Furthermore, we applied this method as a strategic element in the synthesis of the high-volume antimalarial artemisinin.

Triol-Ligand Modification and Structural Transformation of Anderson-Evans Oxomolybdates via Modulating Oxidation State of Co-Heteroatom.

Various covalent decorations and structural transformations of a series of R-C(CH2OH)3 triol ligands on CoII/CoIII-centered Anderson-Evans polyoxomolybdates were studied in aqueous and/or organic solution. Single-crystal X-ray structural analysis demonstrated that four manners, including (1) double-sided χ/χ-isomer and (2) double-sided δ/χ-isomer with CoII central heteroatom, and (3) single-sided δ-isomer and (4) double-sided δ/δ-isomer with CoIII central heteroatom, appeared on the decoration of the mother polyanion [CoII/III(OH)6Mo6O18]4-/3-. Through careful manipulation of the reaction conditions, a gradual oxidation process from CoII to CoIII was disclosed to happen in air, and accompanying that process, the CoII-centered double-sided δ/χ-isomer that formed in the initial stage transformed into a CoIII-centered double-sided δ/δ-isomer. When the electron-withdrawing group -NO2 substituted triol ligand was used to replace CH3C(CH2OH)3, the oxidation process accelerated spontaneously. A further investigation showed that this oxidation process can be blocked by acid. While the triol ligands' decoration on Anderson-Evans polyoxometalates (POMs) took place along with the oxidation of the central heteroatom, that codecoration of other components was also accomplished. In the presence of excess acetic acid, an unprecedented single-sided δ-isomer with an acetate ligand covalently attaching on the other side by replacing one μ3-O atom was obtained in aqueous solution. The reaction of the mother cluster [(n-C4H9)4N]3[Co(OH)6Mo6O18] and triol ligands in methanol resulted in a codecoration of methanol with the double-sided δ/δ-isomer, which has never been reported in B-type Anderson-Evans POMs. The obtained results revealed that the oxidation state of the heteroatom has a significant impact on the triol ligand decoration styles, such as the χ/χ- or δ/χ-isomer for the divalent heteroatom and the δ- or δ/δ-isomer for the trivalent heteroatom.

Characterization of a new Baeyer-Villiger monooxygenase and conversion to a solely N-or S-oxidizing enzyme by a single R292 mutation

BackgroundAr-BVMO is a recently discovered Baeyer-Villiger monooxygenase from the genome of Acinetobacter radioresistens S13 closely related to medically relevant ethionamide monooxygenase EtaA (prodrug activator) and capable of inactivating the imipenem antibiotic. MethodsThe co-substrate preference as well as steady-state and rapid kinetics studies of the recombinant purified protein were carried out using stopped-flow spectroscopy under anaerobic and aerobic conditions. Kd values were measured by isothermal calorimetry. Enzymatic activity was determined by measuring the amount of product formed using high pressure liquid chromatography or gas chromatography. Site-directed mutagenesis experiments were performed to decipher the role of the active site arginine-292. ResultsAr-BVMO was found to oxidize ethionamide as well as linear ketones. Mechanistic studies on the wild type enzyme using stopped-flow spectroscopy allowed for the detection of the characteristic oxygenating C4a-(hydro)peroxyflavin intermediate, which decayed rapidly in the presence of the substrate. Replacement of arginine 292 in Ar-BVMO by glycine or alanine resulted in greatly reduced or no Baeyer-Villiger activity, respectively, demonstrating the crucial role of this residue in catalysis of ketone substrates. However, both the R292A and R292G mutants are capable of carrying out N- and S-oxidation reactions. ConclusionsSubstrate profiling of Ar-BVMO confirms its close relationship to EtaA; ethionamide is one of its substrates. The active site Arginine 292 is required for its Baeyer-Villiger activity but not for heteroatom oxidation. General significanceA single mutation converts Ar-BVMO to a unique S- or N-monooxygenase, a useful biocatalyst for the production of oxidized metabolites of human drug metabolizing enzymes.

Oxidizing intermediates in P450 catalysis: a case for multiple oxidants.

Cytochrome P450 (P450 or CYP) catalysis involves the oxygenation of organic compounds via a series of catalytic intermediates, namely, the ferric-peroxo, ferric-hydroperoxo, Compound I (Cpd I) and FeIII-(H2O2) intermediates. Now that the structures of P450 enzymes have been well established, a major focus of current research in the P450 area has been unraveling the intimate details and activities of these reactive intermediates. The general consensus is that the Cpd I intermediate is the most reactive species in the reaction cycle, especially when the reaction involves hydrocarbon hydroxylation. Cpd I has recently been characterized experimentally. Other than Cpd I, there is a multitude of evidence, both experimental as well as theoretical, supporting the involvement of other intermediates in various types of oxidation reactions. The involvement of these multiple oxidants has been experimentally demonstrated using P450 active-site mutants in epoxidation, heteroatom oxidation and dealkylation reactions. In this chapter, we will review the P450 reaction cycle and each of the reactive intermediates to discuss their role in oxidation reactions.

Combined Experimental and Theoretical Study on the Reactivity of Compounds I and II in Horseradish Peroxidase Biomimetics

For the exploration of the intrinsic reactivity of two key active species in the catalytic cycle of horseradish peroxidase (HRP), Compound I (HRP-I) and Compound II (HRP-II), we generated in situ [Fe(IV) O(TMP(+.) )(2-MeIm)](+) and [Fe(IV) O(TMP)(2-MeIm)](0) (TMP=5,10,15,20-tetramesitylporphyrin; 2-MeIm=2-methylimidazole) as biomimetics for HRP-I and HRP-II, respectively. Their catalytic activities in epoxidation, hydrogen abstraction, and heteroatom oxidation reactions were studied in acetonitrile at -15 °C by utilizing rapid-scan UV/Vis spectroscopy. Comparison of the second-order rate constants measured for the direct reactions of the HRP-I and HRP-II mimics with the selected substrates clearly confirmed the outstanding oxidizing capability of the HRP-I mimic, which is significantly higher than that of HRP-II. The experimental study was supported by computational modeling (DFT calculations) of the oxidation mechanism of the selected substrates with the involvement of quartet and doublet HRP-I mimics ((2,4) Cpd I) and the closed-shell triplet spin HRP-II model ((3) Cpd II) as oxidizing species. The significantly lower activation barriers calculated for the oxidation systems involving (2,4) Cpd I than those found for (3) Cpd II are in line with the much higher oxidizing efficiency of the HRP-I mimic proven in the experimental part of the study. In addition, the DFT calculations show that all three reaction types catalyzed by HRP-I occur on the doublet spin surface in an effectively concerted manner, whereas these reactions may proceed in a stepwise mechanism with the HRP-II mimic as oxidant. However, the high desaturation or oxygen rebound barriers during CH bond activation processes by the HRP-II mimic predict a sufficient lifetime for the substrate radical formed through hydrogen abstraction. Thus, the theoretical calculations suggest that the dissociation of the substrate radical may be a more favorable pathway than desaturation or oxygen rebound processes. Importantly, depending on the electronic nature of the oxidizing species, that is, (2,4) Cpd I or (3) Cpd II, an interesting region-selective conversion phenomenon between sulfoxidation and H-atom abstraction was revealed in the course of the oxidation reaction of dimethylsulfide. The combined experimental and theoretical study on the elucidation of the intrinsic reactivity patterns of the HRP-I and HRP-II mimics provides a valuable tool for evaluating the particular role of the HRP active species in biological systems.

Electrochemical degradation of tricyclazole in aqueous solution using Ti/SnO2–Sb/PbO2 anode

The electrochemical degradation of the fungicide tricyclazole(TC) in aqueous solution on a Ti/SnO2–Sb/PbO2 anode was investigated for the first time. Effect of key operating parameters such as the current density, the initial pH, the initial concentration of supporting electrolyte and TC was studied on the degradation efficiency. The current density and concentration of Na2SO4 exerted a prominent effect on degradation rate, and no significant effect of initial pH on TC mineralization and COD removal was observed. The efficiency of TC and COD removal decreased with increasing initial TC concentration, from 0.53 to 1.06mM. Kinetic analyses indicated that the electrochemical degradation of TC was a pseudo-first-order reaction. The apparent rate constant (k1) value of TC oxidation was determined as 1.50×10−4s−1 at 25mAcm−2 in the presence of 7.0gL−1 Na2SO4 in acidic media (pH: 5.0). The mineralization intermediates were analyzed by chromatographic techniques, several aromatic intermediates and short-chain carboxylic acids generated along the reaction time had been quantified. Heteroatom oxidation products like NO2-,NO3- had been quantified and explanations of their release were proposed. Finally, considering the identified reaction intermediates, a pathway for electrocatalytic degradation of TC in aqueous solution was proposed.

Synthesis of 2′‐Deoxy‐1′‐homo‐N‐nucleosides with Anti‐Influenza Activity by Catalytic Methyltrioxorhenium (MTO)/H2O2 Oxyfunctionalization

This paper describes a new route for the synthesis of 1'-homo-N-nucleoside derivatives by means of either methyltrioxorhenium (MTO) or supported MTO catalysts, with H(2)O(2) as the primary oxidant. Under these selective conditions, the oxyfunctionalization of the heterocyclic ring and the N heteroatom oxidation were operative processes, regardless of the type of substrate used, that is, purine or pyrimidine derivatives. In addition, the oxidation of 1'-homo-N-thionucleosides, showed the occurrence of site-specific oxidative nucleophilic substitutions of the heterocyclic ring. The MTO/H(2)O(2) system showed, in general, high reactivity under both homogeneous and heterogeneous conditions, affording the final products with high conversion values of substrates and from medium to high yields. Many of the novel 1'-homo-N-nucleoside analogues were active against the influenza A virus, without any cytotoxic effects, retaining their activity in both protected and unprotected forms.

A Nano‐Fibrillated Mesoporous Carbon as an Effective Support for Palladium Nanoparticles in the Aerobic Oxidation of Alcohols “on Pure Water”

A novel nano-fibrillated mesoporous carbon (IFMC) was successfully prepared via carbonization of the ionic liquid 1-methyl-3-phenethyl-1H-imidazolium hydrogen sulfate (1) in the presence of SBA-15. The material was shown to be an efficient and unique support for the palladium nanoparticle (PdNP) catalyst Pd@IFMC (2) in aerobic oxidation of heterocyclic, benzylic, and heteroatom containing alcohols on pure water at temperatures as low as 40 °C for the first time and giving almost consistent activities and selectivities within more than six reaction runs. The catalyst has also been employed as an effective catalyst for the selective oxidation of aliphatic and allylic alcohols at 70-80 °C. The materials were characterized by X-ray photoelectron spectroscopy (XPS), N(2) adsorption-desorption analysis, transmission electron microscopy (TEM), and electron tomography (ET). Our compelling XPS and ET studies showed that higher activity of 2 compared to Pd@CMK-3 and Pd/C in the aerobic oxidation of alcohols on water might be due to the presence of nitrogen functionalities inside the carbon structure and also the fibrous nature of our materials. The presence of a nitrogen heteroatom in the carboneous framework might also be responsible for the relatively uniform and nearly atomic-scale distribution of PdNPs throughout the mesoporous structure and the inhibition of Pd agglomeration during the reaction, resulting in high durability, high stability, and recycling characteristics of 2. This effect was clearly confirmed by comparing the TEM images of the recovered 2 and Pd@CMK-3.

Degradation pathway of the naphthalene azo dye intermediate 1-diazo-2- naphthol-4-sulfonic acid using Fenton's reagent

Degradation of naphthalene dye intermediate 1-diazo-2- naphthol-4-sulfonic acid (1,2,4-Acid) by Fenton process has been studied in depth for the purpose of learning more about the reactions involved in the oxidation of 1,2,4-Acid. During 1,2,4-Acid oxidation, the solution color initially takes on a dark red, then to dark black associated with the formation of quinodial-type structures, and then goes to dark brown and gradually disappears, indicating a fast degradation of azo group. The observed color changes of the solution are a result of main reaction intermediates, which can be an indicator of the level of oxidization reached. Nevertheless, complete TOC removal is not accomplished, in accordance with the presence of resistant carboxylic acids at the end of the reaction. The intermediates generated along the reaction time have been identified and quantified. UPLC–(ESI)–TOF–HRMS analysis allows the detection of 19 aromatic compounds of different size and complexity. Some of them share the same accurate mass but appear at different retention time, evidencing their different molecular structures. Heteroatom oxidation products like SO42- have also been quantified and explanations of their release are proposed. Short-chain carboxylic acids are detected at long reaction time, as a previous step to complete the process of dye mineralization. Finally, considering all the findings of the present study and previous related works, the evolution from the original 1,2,4-Acid to the final products is proposed in a general reaction scheme.

Bacterial dioxygenase- and monooxygenase-catalysed sulfoxidation of benzo[b]thiophenes

Asymmetric heteroatom oxidation of benzo[b]thiophenes to yield the corresponding sulfoxides was catalysed by toluene dioxygenase (TDO), naphthalene dioxygenase (NDO) and styrene monooxygenase (SMO) enzymes present in P. putida mutant and E. coli recombinant whole cells. TDO-catalysed oxidation yielded the relatively unstable benzo[b]thiophene sulfoxide; its dimerization, followed by dehydrogenation, resulted in the isolation of stable tetracyclic sulfoxides as minor products with cis-dihydrodiols being the dominant metabolites. SMO mainly catalysed the formation of enantioenriched benzo[b]thiophene sulfoxide and 2-methyl benzo[b]thiophene sulfoxides which racemized at ambient temperature. The barriers to pyramidal sulfur inversion of 2- and 3-methyl benzo[b]thiophene sulfoxide metabolites, obtained using TDO and NDO as biocatalysts, were found to be ca.: 25-27 kcal mol(-1). The absolute configurations of the benzo[b]thiophene sulfoxides were determined by ECD spectroscopy, X-ray crystallography and stereochemical correlation. A site-directed mutant E. coli strain containing an engineered form of NDO, was found to change the regioselectivity toward preferential oxidation of the thiophene ring rather than the benzene ring.