Terminal Oxidant Research Articles

A Low-Cost Ecofriendly Oxidation Process to Manufacture High-Performance Polymeric Biosurfactants Derived from Municipal Biowaste.

Biosurfactants account for about 12% of the global value of the surfactant market, which is currently dominated by synthetic surfactants obtained from fossil sources. Yet, the production of biosurfactants from renewable feedstock is bound to increase, driven by the increasing pressure from both society and governments for chemistry-based industries to become more ecofriendly and economically sustainable. A photo-chemical oxidation process is reported here, yielding new biosurfactants from urban biowaste in water that perform as a solvent and terminal oxidant reagent at room temperature without the addition of conventional oxidants and catalysts. Products with 200-500 kDa molecular weight are obtained. They lower the surface tension of water down to 34 mN/m at 0.5-2 g/L concentration. The estimated cost is rather low (0.1-1.5 EUR/kg), which is competitive with the cost of synthetic surfactants but much lower than the cost of the best-performing bacterial surfactants. For the implementation of the photo-chemical oxidation process at the industrial level, the results suggest that the new biosurfactants obtained in the present work may not reach the performance level of the best-performing bacterial surfactants capable of lowering the surface tension of water down to 28 mN/m. Yet, the biosurfactants produced by the photo-chemical process have a greater chance of being marketed on large scales.

Bifunctional Squaramide-Catalyzed Oxidative Kinetic Resolution: Simultaneous Access to Axially Chiral Thioether and Sulfoxide.

Axially chiral thioethers and sulfoxides emerge as two pivotal classes of ligands and organocatalysts, which have remarkable features in the stereoinduction of various asymmetric transformations. However, the lack of easy methods to access such molecules with diverse structures has hampered their broader utilization. Herein, an oxidative kinetic resolution for sulfides using a chiral bifunctional squaramide as the catalyst with cumene hydroperoxide as the terminal oxidant is established. This asymmetric approach provides a variety of axially chiral thioethers as well as sulfoxides bearing both axial and central chirality, with excellent diastereo- and enantioselectivities. This catalytic system also successfully extends to the kinetic resolution of benzothiophene-based sulfides. Preliminary mechanism investigation indicates that the multiple hydrogen bonding interactions between the bifunctional squaramide catalyst and substrates play a crucial role in determining the enantioselectivity and reactivity.

Visible‐Light‐Mediated Oxidative Desulfurization‐Oxygenation in Cyclic Thio‐Amide

AbstractVisible‐light assisted a very simple and mild protocol for photocatalytic oxidative transformation of thioamide to amide group has been realized in cyclic system. For this purpose 5‐arylalkylidene‐rhodanine derivatives were chosen as model substrates to afford thiazolidine‐2,4‐diones. Light mediated reactions are intrinsically beneficial, since activation occurs by cleanest energy source light. Here K2S2O8 is used as terminal oxidant which does not leave any hazardous and corrosive waste to discard after the reaction fostering the practices of green chemistry. This is the first report of conversion of thioxo group in rhodanine derivatives into the corresponding oxygen analogue by visible light photo‐catalytic method. The reaction was very general since various 5‐arylalkylidene‐rhodanines underwent the reaction smoothly affording high product yields. Moreover, the broadness of this protocol was further established by applying this reaction on other systems like 3,4‐dihydropyrimidine‐2(1H)‐(thio)ones.

Oxidovanadium(V) complexes with tridentate hydrazone ligands as oxygen atom transfer catalysts

Four isostructural oxovanadium(V) complexes with hydrazone ligands have been synthesised, characterised, and evaluated as epoxidation and sulfoxidation catalysts. The reactions between [VO(acac)2] (acac– = acetylacetonate) and H2Ln (n = 1–4), precursors for monoanionic tridentate hydrazone ligands, afford complexes formulated as [VO(Ln)(bzh)·MeOH] (1–4) when bidentate benzohydroxamic acid (Hbzh) is included as a co-ligand. Single crystal X-ray structure analyses showed that complexes 1–3 have a distorted octahedral coordination geometry with an O5N coordination environment. Cyclic voltammetry showed that all complexes undergo two quasi-irreversible reduction peaks and a single irreversible oxidation peak. The bonding in 1 has been investigated by electronic structure calculations, and these data are discussed with respect to the electrochemical results. Complexes 1–4 were tested as catalysts for the epoxidation of cis-cyclooctene at 50 °C and sulfoxidation of methyl-p-tolylsulfide at room temperature using tert-butyl hydroperoxide (tBuOOH) and aqueous H2O2 as the terminal oxidants.

Synthesis, Characterization and Reactivity Studies of Cobalt(III) Porphyrin‐Iodosylarene Adduct and Cobalt(III) Porphyrin π‐Cation Radical Species

AbstractBiomimetic metalloporphyrin complexes have been employed in a number of catalytic oxidation reactions by utilizing terminal oxidants such as iodosylarenes (ArIO). Although high‐valent metal‐oxo species have been considered as the reactive intermediates, their precursors, metal‐iodosylarene adduct species, also exhibit intriguing oxidation capability under certain conditions. However, late transition metal porphyrin‐oxidant adduct species have not been explored in oxidation reactions yet. Herein, we report the synthesis, characterization and reactivity studies of cobalt(III) porphyrin‐ArIO adduct complexes. These adduct species exhibit moderate oxidation capability in electron transfer reactions. More interestingly, addition of Brønsted acid or Lewis acid facilitated the O−I bond cleavage, resulted in the formation of cobalt(III) porphyrin π‐cation radical species, which is much more reactive than the corresponding adduct species in electron transfer reactions. Kinetic studies and theoretical calculations demonstrate that the O−I bond cleavage is triggered in the presence of acid, affording the porphyrin ligand oxidation while the formation of high‐valent cobalt‐oxo species is prohibited due to the “oxo‐wall” for late transition metals. This study provides a novel model of a late transition metal‐iodosylarene adduct species as an active oxidant in oxidation reactions, while in the cases of iron and manganese complexes, high‐valent metal‐oxo species are generated.

En Route to a Molecular Terminal Tin Oxide.

In the pursuit of terminal tin chalcogenides, heteroleptic stannylenes bearing terphenyl- and hexamethyldisilazide ligands were reacted with carbodiimides to yield the respective guanidinato complexes. Further supported by quantum chemical calculations, this revealed that the iso-propyl-substituted derivative provides the maximum steric protection achievable. Oxidation with elemental selenium produced monomeric terminal tin selenides with four-coordinate tin centers. In reactions with N2O as oxygen transfer reagent, silyl migration toward putative terminal tin oxide intermediates gave rise to tin complexes with terminal ─OSiMe3 functionality. To prevent silyl migration, the silyl groups were substituted with cyclohexyl moieties. This analogue exhibited distinctively different reactivities toward selenium and N2O, yielding a 1,2,3,4,5-tetraselenastannolane and chalcogenide-bridged dimeric compounds, respectively.

RETRACTED ARTICLE: Metal free cross-dehydrogenative N-N coupling of primary amides with Lewis basic amines

Hydrazides, N-N containing structural motifs, are important due to their presence in a wide variety of biologically significant compounds. While the homo N-N coupling of two NH moieties to form the hydrazide N-N bond is well developed, the cross-dehydrogenative hetero N-N coupling remains very unevolved. Here we present an efficient intermolecular N-N cross-coupling of a series of primary benzamides with broad range of Lewis basic primary and secondary amines using PhI(OAc)2 as both a terminal oxidant and a cross-coupling mediator, without the need for metal catalysts, high temperatures, and inert atmospheres, and with substantial potential for use in the late-stage functionalization of drugs.

Racemic total synthesis of oxomaritidine by iodobenzene-catalyzed oxidative phenolic coupling under mild reaction conditions

A hypervalent iodine-catalyzed phenolic coupling reaction of norbelladine derivatives was examined with a catalytic amount of iodobenzene and meta-chloroperbenzoic acid as the terminal oxidant. The addition of acetic acid dramatically accelerated the reaction rate even at room temperature, and the cyclized products were successfully obtained under mild reaction conditions. The methods were applicable to the racemic total synthesis of oxomaritidine.

Tuning Catalytic Activity of Palladium in Direct C−H Arylation of Biologically Active Xanthines using Silver Additive.

AbstractWe describe an efficient and versatile method for the direct C−H arylation of xanthines with aryl halides using Pd/P(o‐tolyl)3 as catalyst and Ag2O as an additive. Silver salts have been frequently used as an additive in the Pd‐catalyzed C−H bond activation reactions albeit in excessive amounts (2 to 4 eq.). Herein, we use of a stoichiometric amount (0.1 to 1 eq.) of Ag2O to obtain the optimum yields of C‐8 arylated products. This method exhibits broad substrate scope and enables synthesis of a wide range of xanthine derivatives by variation of aryl component at the C‐8 position. A key advantage of silver additive is that it reduces the catalyst loading and time required for the completion of reactions. Although kinetic experiments show that the inclusion of silver salt accelerates the rate of reaction, catalytic role of silver salt could not be conceived since the C−H bond cleavage is not a rate determining step. Silver salt presumably act as a terminal oxidant in the catalytic cycle and assist the regeneration of Pd catalyst. Moreover, preliminary assessment of biological activities of xanthines demonstrates that the C‐8 arylated xanthines derivatives efficiently inhibit the α‐Synuclein fibrillation, a key factor behind Parkinson's disease.

Preparation of a Key Intermediate En Route to the Anti-HIV Drug Lenacapavir.

A very efficient four-step synthesis of the main fragment of Gilead's anti-HIV drug lenacapavir is described. The route showcases a 1,2-addition to an intermediate aldehyde using an organozinc halide derived from a commercially available difluorobenzyl Grignard reagent. This sets the stage for the oxidation of the resulting secondary alcohol to the desired ketone, which relies solely on catalytic amounts of TEMPO together with NaClO as the terminal oxidant, affording the targeted ketone in 67% overall yield.

Nitrogen loss in coastal sediments driven by anaerobic ammonium oxidation coupled to microbial reduction of Mn(IV)-oxide

Coastal sediments play a central role in regulating the amount of land-derived reactive nitrogen (Nr) entering the ocean, and their importance becomes crucial in vulnerable ecosystems threatened by anthropogenic activities. Sedimentary denitrification has been identified as the main sink of Nr in marine environments, while anaerobic ammonium oxidation with nitrite (anammox) has also been pointed out as a key player in controlling the nitrogen pool in these locations. Collected evidence in the present work indicates that the microbial biota in coastal sediments from Baja California (northwestern Mexico) has the potential to drive anaerobic ammonium oxidation linked to Mn(IV) reduction (manganammox). Unamended sediment showed ammonification, but addition of vernadite (δMnO2 with nano-crystal size ∼15 Å) as terminal electron acceptor fueled simultaneous ammonium oxidation (up to ∼400 μM of ammonium removed) and production of Mn(II) with a ratio ∆[Mn(II)]/∆[NH4+] of 1.8, which is very close to the stoichiometric value of manganammox (1.5). Additional incubations spiked with external ammonium also showed concomitant ammonium oxidation and Mn(II) production, accounting for ∼30 % of the oxidized ammonium. Tracer analysis revealed that the nitrogen loss associated with manganammox was 4.2 ± 0.4 μg 30N2/g-day, which is 17-fold higher than that related to the feammox process (anaerobic ammonium oxidation linked to Fe(III) reduction, 0.24 ± 0.02 μg 30N2/g-day). Taxonomic characterization based on 16S rRNA gene sequencing revealed the existence of several clades belonging to Desulfobacterota as potential microorganisms catalyzing the manganammox process. These findings suggest that manganammox has the potential to be an additional Nr sink in coastal environments, whose contribution to total Nr losses remains to be evaluated.

Oxidovanadium(V) complexes with chiral tetradentate Schiff bases: Synthesis, spectroscopic characterization, catalytic and biological activity

New mononuclear oxidovanadium(V) complexes, [(−)VOL1–10] and [(+)VOL1–4], with chiral tetradentate Schiff bases have been obtained by monocondensation of salicylaldehyde derivatives and optically active amino alcohols, i.e. 1R,2R-(−)-2-amino-1-phenyl-1,3-propanediol or 1S,2S-(+)-2-amino-1-phenyl-1,3-propanediol. All complexes have been characterized by elemental analysis, circular dichroism, UV–vis, IR, 1D (1H, 51V) and 2D (COSY, NOESY) NMR spectroscopy. The catalytic activity studies have also been performed for all complexes in the epoxidation of styrene, cyclohexene and two monoterpenes, i.e. S(−)-limonene and (−)-α-pinene, using aqueous 30 % H2O2 or tert-butyl hydroperoxide (TBHP) as the terminal oxidant. In addition, biological properties of these chiral oxidovanadium(V) compounds, as well as cis-dioxidomolybdenum(VI) complexes with the same chiral Schiff bases, were studied in relation to their cytoprotective activity against the oxidative damage generated exogenously by 30 % hydrogen peroxide in the HT-22 hippocampal neuronal cells. The latter compounds have revealed distinct cytoprotective properties causing the highest increase in the viability of HT-22 cells. Concentration dependent effect in the range of 10-100 μM concentration of the investigated compounds has also been observed.

Visible-Light-Driven Oxidative Chlorination of Alkyl sp3 C–H Bonds with HCl/Air at Room Temperature

AbstractAn aerobic chlorination of alkyl sp3 C–H bonds of functionalized alkanes and simple alkanes has been established successfully under visible light at room temperature. The reaction is performed in the presence of catalytic NaNO2 using HCl or NaCl/acid as a chlorine source and air as a terminal oxidant under atmospheric pressure. It is suggested that NOCl from NaNO2/HCl is a crucial species in the catalytic cycle involving an aerobic oxidation and a photochemical radical chlorination.

Mechanistic Insights into Ru‐catalyzed Alkene Epoxidation with Nitrous Oxide as a Terminal Oxidant

AbstractNitrous oxide (N2O) is a greenhouse gas produced in the manufacture of 6,6‐nylon and nitric acid. While an attractive oxidant that releases only N2 as a by‐product, the kinetic stability of N2O typically requires high temperatures and pressures for activation. This work describes initial kinetics of oxygen transfer in the epoxidation of cholesteryl acetate with N2O catalysed by D4‐Ru(VI)(por)(O)2 complexes in efforts to provide a better mechanistic understanding of this chemistry. Insights include a need for low concentrations of the alkene to avoid competitive binding to the metal, possible saturation behavior at high N2O pressures, transfer of only one oxygen of RuVI(O)2 to substrate and a possible catalyst turnover involving disproportionation of RuIV(O) and RuIV(O)(N2O) to active RuVI(O)2, RuIV(O) and N2. These insights will be used in future designs of improved catalysts and reaction protocols that may operate efficiently at low pressures of N2O and ambident temperature.

Nitrosonium Ion Catalyzed Oxidative Bromination of Arenes

AbstractA nitrosonium catalyzed bromination of electron‐rich arenes has been developed, using bromide salts as bromine sources, oxygen gas as a terminal oxidant and Brønsted acids (sulfuric acid, TFA or HBr). This arene bromination process is free of transition metals, proceeding in acetic acid and under mild temperatures (25–50 °C) to give brominated arenes with good to excellent yields. Our mechanistic studies indicated that this oxidative bromination of arenes reaction is catalyzed by nitrosnium ion (NO+), without the involvement of Br2 or Br+. The kinetic isotope effect of this reaction was measured using pentadeuteriophenyl phenyl ether (kH/kD=1.1).

Cu-Catalyzed Coupling of Aliphatic Amines with Alkylboronic Esters.

We report a Cu-catalyzed oxidative coupling of aliphatic amines with benzylic and aliphatic boronic esters to give high value alkyl amines, products found widely in applications from medicinal chemistry to materials science. This operationally simple reaction, which can be performed on gram scale, runs under mild conditions and exhibits broad functional group tolerance. The terminal oxidant of the reaction is O2 from the air, avoiding the need for additional chemical oxidants. Investigation into the reaction mechanism suggests that the boronic ester is activated by an aminyl radical, formed through oxidation of the amine by the Cu catalyst, to give a key alkyl radical intermediate. To demonstrate its utility and potential for late-stage functionalization, we showcase the method as the final step in the total synthesis of a TRPV1 antagonist.

Simultaneous removal of aliphatic and aromatic crude oil hydrocarbons by Pantoea agglomerans isolated from petroleum-contaminated soil in the west of Iran.

Hydrocarbons are considered as one of the most common and harmful environmental pollutants affecting human health and the environment. Bioremediation as an environmentally friendly, highly efficient, and cost-effective method in remediating oil-contaminated environments has been interesting in recent decades. In this study, hydrocarbon degrader bacterial strains were isolated from the highly petroleum-contaminated soils in the Dehloran oil field in the west of Iran. Out of 37 isolates, 15 can grow on M9 agar medium that contains 1.5 gL-1 of crude oil as the sole carbon source. The morphological, biochemical, and 16SrRNA sequencing analyses were performed for the isolates. The choosing of the isolates as the hydrocarbon degrader was examined by evaluating the efficacy of their crude oil removal at a concentration of 10gL-1 in an aqueous medium. The results showed that five isolates belonging to Pseudomonas sp., Pseudomonas oryzihabitans, Roseomonas aestuarii, Pantoea agglomerans, and Arthrobacter sp. had a hyper hydrocarbon-degrading activity and they could remove more than 85% of the total petroleum hydrocarbon (TPH) after 96h. The highest TPH removal of about 95.75% and biodegradation rate of 0.0997gL-1h-1 was observed for P. agglomerans. The gas chromatography-mass spectroscopy (GC-MS) analysis was performed during the biodegradation process by P. agglomerans to detect the degradation intermediates and final products. The results confirmed the presence of intermediates such as alcohols and fatty acids in the terminal oxidation pathway of alkanes in this biodegradation process. A promising P. agglomerans NB391 strain can remove aliphatic and aromatic hydrocarbons simultaneously.

New Isoxazole‐Substituted Aryl Iodides: Metal‐Free Synthesis, Characterization and Catalytic Activity

AbstractA series of new isoxazole‐substituted aryl iodides 1 a–1 d have been synthesized by DIB‐mediated [3+2] cycloaddition reaction of 2‐iodo‐1,3‐bis(prop‐2‐yn‐1‐yloxy) benzene (4) with corresponding benzaldehyde oximes 5 a–5 d. Structure of the synthesized aryl iodides 1 were characterized by IR, 1H NMR, 13C NMR and HRMS. The structure of 1 a was also confirmed by single‐crystal X‐ray crystallography. Further, catalytic activity of iodoarenes 1 a–1 d was screened for the oxidation of hydroquinones and sulfides. On oxidation using aryl iodides 1 with m‐CPBA as terminal oxidant, hydroquinones afforded benzoquinones while sulfides gave corresponding sulfoxides in good to excellent yields. Iodoarene 1 b showed the best catalytic activity for the oxidation of sulfides and hydroquinones. Moreover, iodoarene 1 b, was also utilized for α‐oxytosylation of acetophenones.

Pd-Catalyzed Oxidative Functionalization of Alkenes, Arenes, and 1,3-Dienes Using Molecular Oxygen as the Terminal Oxidant

AbstractThis Account presents palladium-complex-catalyzed oxidative couplings mainly developed by the author’s group, including oxidative amination and silylation of terminal alkenes, direct oxidative arylation of aromatic compounds, and oxidative difunctionalization of 1,3-dienes. The catalytic cycles in these representative reactions feature re-oxidation of the palladium species with molecular oxygen as the terminal oxidant. Varying the combination of palladium catalyst and re-oxidant enables the formation of a variety of bonds through dehydrogenative cross-coupling reactions.1 Introduction2 Oxidative Amination of Terminal Alkenes3 Direct Oxidative Arylation of Aromatic Compounds4 Oxidative Silylation of Terminal Alkenes5 Oxidative Difunctionalization of 1,3-Dienes6 Conclusions and Perspectives

Cu(II) salts as terminal oxidants in visible-light photochemical oxidation reactions.

Photochemistry provides an important platform for the discovery of synthetically useful transformations. The development of new oxidative photoreactions, however, has proven to be relatively challenging. The importance of the identity of the terminal oxidant has been an underappreciated consideration in the design of these reactions. Many of the most common terminal oxidants used in ground-state catalytic methods are poorly compatible with the one-electron oxidation state changes characteristic of photoredox reactions and result in hard-to-control deleterious side reactions. As an alternative, Cu(II) salts have emerged as versatile terminal oxidants in photochemical oxidation reactions that are terrestrially abundant, cost-effective, and readily compatible with one-electron oxidation state changes. This review highlights recent reaction methods that leverage Cu(II) oxidation in combination with the photochemical activation of substrates or that use Cu(II) salts as both the active chromophore and terminal oxidant.