Recent advancements in palladium-catalyzed reactions involving molecular oxygen

Oxidation reactions have significant value in organic chemistry, having been in focus continuously due to the high efficiency in building up molecular complexity. In the past few decades, transition metal-catalyzed oxidation reactions have been significantly explored and have played important roles in organic synthesis. Compared to the widely-used oxidants, such as inorganic salts, peroxides, hypervalent iodine reagents and quinones, molecular oxygen (O₂), which is natural, inexpensive, and environmentally friendly, is a highly appealing oxidant in academic and industry area for green and sustainable chemistry. Recently, significant advances have been made in palladium-catalyzed reactions using O₂ as the oxidant. This critical review highlights some of the recent developments in molecular oxygen-involved Pd-catalyzed oxidation reactions with a focus on mechanistic strategies and new reaction developments.     

Catalytic Aerobic Oxidation of C(sp3)−H Bonds

Oxidation reactions are a key technology to transform hydrocarbons from petroleum feedstock into chemicals of a higher oxidation state, allowing further chemical transformations. These bulk‐scale oxidation processes usually employ molecular oxygen as the terminal oxidant as at this scale it is typically the only economically viable oxidant. The produced commodity chemicals possess limited functionality and usually show a high degree of symmetry thereby avoiding selectivity issues. In sharp contrast, in the production of fine chemicals preference is still given to classical oxidants. Considering the strive for greener production processes, the use of O₂, the most abundant and greenest oxidant, is a logical choice. Given the rich functionality and complexity of fine chemicals, achieving regio/chemoselectivity is a major challenge. This review presents an overview of the most important catalytic systems recently described for aerobic oxidation, and the current insight in their reaction mechanism.     

Ketone Catalyzed Oxidation Reactions

Dioxiranes are important oxidants for organic reactions such as epoxidation, heteroatom oxidation and oxygenation of C-H bonds. We have developed a mild and general method for epoxidation of olefins using dioxiranes generated in situ from ketones and Oxone. This method has not only extended the synthetic utility of dioxiranes, but also allowed us to discover a series of novel cyclic ketones for catalytic oxidation. In particular, we have demonstrated the potential of chiral ketones for catalytic asymmetric epoxidation of trans-olefins and trisubstituted olefins. We have also explored the potential of ketones in catalyzing oxidation of unactivated C-H bonds and decomposition of peroxynitrite.     

Demethylation and Nitration of Aryldimethilamines with Cerium (IV) Ammonium Nitrate

Studies about the oxidation of organic molecules with Ce (IV) have been performed by many groups and are summarised in two reviews¹. This oxidant has a particular behaviour when the salt cerium (IV) ammonium nitrate (CAN), soluble in organic solvents, is used. In previous papers² we explored the reactivity of some aromatic trinuclear compounds with CAN. The results reported in this paper indicate that the oxidation of aromatic amines with CAN differs from the behaviour of other metal oxidants as acetates with these substrates³.     

Recent Advances in Organic Electrochemical C—H Functionalization

Organic electrochemistry has a rich history in organic synthesis and has been considered as a promising alternative to traditional chemical oxidants and reductants because it obviates the use of stoichiometric amount of dangerous and toxic reagents. In particular, the electrochemical C—H bonds functionalization is one of the most desirable approaches for the construction of carbon–carbon (C—C) and carbon–heteroatom (C—X) bonds. This review summarizes the substantial progress made in the last few years in C—H functionalization via organic electrochemistry. It is divided into sections on C—C, C—N, C—O, C—S, C–Halogen and C—P bond formation.     

Catalytic oxidation of organic substrates by molecular oxygen and hydrogen peroxide by multistep electron transfer--a biomimetic approach

Oxidation reactions are of fundamental importance in nature, and are key transformations in organic synthesis. The development of new processes that employ transition metals as substrate-selective catalysts and stoichiometric environmentally friendly oxidants, such as molecular oxygen or hydrogen peroxide, is one of the most important goals in oxidation chemistry. Direct oxidation of the catalyst by molecular oxygen or hydrogen peroxide is often kinetically unfavored. The use of coupled catalytic systems with electron-transfer mediators (ETMs) usually facilitates the procedures by transporting the electrons from the catalyst to the oxidant along a low-energy pathway, thereby increasing the efficiency of the oxidation and thus complementing the direct oxidation reactions. As a result of the similarities with biological systems, this can be dubbed a biomimetic approach.     

Liquid-Phase Oxidation of Organic Reagents in Inorganic Analysis

It was shown that the sensitivity of analytical procedures using organic reagents can be significantly increased if a poorly soluble complex of an element with an analytical reagent is oxidized to the products of complete oxidation of organic compounds (CO₂, H₂O, and NH₃or N₂for nitrogen-containing reagents). Knowing the amount of the consumed oxidant, one can easily determine the amount of the organic constituent of the complex and, hence, the amount of the complex-forming element. Vanadium salts were proposed as oxidants. The effects of medium acidity, oxidant concentration, time of heating, and temperature on the completeness of oxidation were studied. Based on the results obtained, it was demonstrated for a number of organic compounds that liquid-phase oxidation is highly promising and that trace amounts of elements can be determined by conventional titrimetric methods. The determination of phosphorous with diantipyrylmethane is illustrated by an example (1 mL of a 0.1 M vanadate solution corresponds to 10 g phosphorous).     

Biologically Inspired C−H and C=C Oxidations with Hydrogen Peroxide Catalyzed by Iron Coordination Complexes

The development of catalysts for the selective oxidation of readily available hydrocarbons or organic precursors into oxygenated products is a long‐standing goal in organic synthesis. In the last decade, some iron coordination complexes have shown the potential to fit this role. These catalysts can mimic the O?O activation mode of far more sophisticated iron oxygenase enzymes, generating powerful yet selective oxidants. In this review, we report state‐of‐the‐art C?H and C=C oxidations catalyzed by non‐heme iron complexes and H₂O₂ as the oxidant. Finally, we briefly describe some novel oxidative reactivity and the perspectives of this chemistry.     

Metal and Non-metal Based Catalysts for Oxidation of Organic Compounds

The oxidation of organic compounds is of extreme importance in synthetic chemistry. The catalysts provide high selectivity towards the oxidation products employing with the oxidants. This review is not intended to give a complete survey of all oxidation catalysis reactions by different catalysts but rather to give a summary of some important catalytic oxidation reactions by different metal and non-metal based catalysts.     

The electrochemical advanced oxidation processes coupling of oxidants for organic pollutants degradation: A mini-review

In this mini-review, the homogeneous and heterogeneous EAOPs-oxidant processes were summarized. The reaction mechanisms of different EAOPs combined with different oxidants are elucidated in detail, as well as the synergistic effect for improving the treatment efficiency.     

Gold‐Catalyzed Ring Expansion of Alkynyl Heterocycles through 1,2‐Migration of an Endocyclic Carbon–Heteroatom Bond

A mild and efficient gold‐catalyzed oxidative ring‐expansion of a series of alkynyl heterocycles using pyridine‐N‐oxide as the oxidant has been developed, which affords highly valuable six‐ or seven‐membered heterocycles with wide functional group toleration. The reaction consists of a regioselective oxidation and a chemoselective migration of an endocyclic carbon–heteroatom bond (favored over C?H migration) with the order of migratory aptitude for carbon–heteroatom bonds being C?S>C?N>C?O. In the absence of an oxidant, polycyclic products are readily constructed through a ring‐expansion/Nazarov cyclization reaction sequence.     

Vinylbenziodoxol(on)es: Synthetic Methods and Applications

Cyclic hypervalent iodine reagents are now frequently used in synthetic organic chemistry, either as oxidants or group-transfer reagents. Vinylbenziodoxol(on)es (VBXs) bearing alkene substituents have been less investigated than the corresponding trifluoromethyl or alkynyl reagents. Nevertheless, since 2016 the development of new synthetic methods to access VBXs has awakened the interest of the synthetic community, leading to new transformations highlighting their unique reactivity as electrophilic alkene synthons. In this review, an overview of the synthesis and applications of VBX reagents will be presented. The review is organized according to the two main classes of VBX reagents reported so far – simple alkyl/aryl-substituted VBXs and heteroatom (S, O, N, X)-substituted VBXs – as they differ significantly from the point of view of synthetic access.     

An Overview of Selective Oxidation of Alcohols: Catalysts,Oxidants and Reaction Mechanisms

The oxidation of alcohols to carbonyl compounds is one of the most fundamental reactions in synthetic organic chemistry. In order to achieve the realization of dehydrogenation reactions with high atomic efficiency, suitable catalysts and oxidants are considered as the key factors to obtain the optimum activity and aldehydes/ketones selectivity. This review aims to make an overview on the reasonable reaction mechanism and promising catalytic system of alcohols dehydrogenation.     

Oxidation Kinetics of Simvastatin Using Cetyltrimethylammonium Dichromate

This article reports an attempt on the studies on resistance of oxidative stress by the prodrug, simvastatin (SV). Cetyltrimethylammonium dichromate has been used as a lipid compatible oxidant to study the oxidation kinetics of SV in organic media. The reaction undergoes via an ionic mechanism without any side product. The reaction is found to be acid catalyzed and sensitive to solvent polarity. The increase in the rate constant due to an increase in hydrophobicity (apolarity) of the solvent indicates the existence of a less polar transition state. Furthermore, the decrease in the rate constant due to an increase in [CTAB] suggests partitioning of the substrates and the oxidants into two different domains with different polar characteristics akin to a reversed micellar aggregates. Considering the above results and the thermodynamic parameters, a reaction mechanism has been proposed, wherein a complex formed at the interface of the two domains due to the reactant and the oxidant in a fast process decomposes to the products in a slow process in the nonpolar bulk.     

Amperometric biosensors employing an insoluble oxidant as an interference-removing agent

A strong oxidant membrane is introduced to amperometric biosensors in order to solve the problem associated with interference from readily oxidizable species. The proposed biosensors are in planar format, and are composed of four components, i.e. a base amperometric transducer, an enzyme layer, a protecting membrane, and an oxidant membrane. In this sensor format, interfering species are removed by an oxidation reaction during their diffusion through the oxidant membrane. The oxidant membrane is introduced by dispensing a mixture of an oxidant and a polymer matrix as dissolved in an organic solvent, and thus, could be easily adapted to mass fabrication of miniature biosensors. In this work, several different reagents are examined as oxidants: BaO₂, CeO₂, MnO₂ and PbO₂. Of these, PbO₂ is shown to yield biosensors with the best performance, in terms of reducing interfering signals. Two different matrix systems are devised for use in formulating oxidant membranes: hydrophilic polyurethane (HPU) and cellulose acetate incorporating poly(ethylene glycol) (CA/PEG). While the CA/PEG-type sensor displays better sensitivity and faster response behavior, the HPU-type is shown to exhibit more pronounced interference-removing ability. The analytical utility of the proposed oxidant membrane is demonstrated by fabricating amperometric glucose and creatinine sensors as the model biosensor systems, and by investigating their response characteristics.     

Olefin cis‐Dihydroxylation and Aliphatic C?H Bond Oxygenation by a Dioxygen‐Derived Electrophilic Iron–Oxygen Oxidant

Many iron‐containing enzymes involve metal–oxygen oxidants to carry out O₂‐dependent transformation reactions. However, the selective oxidation of C H and CC bonds by biomimetic complexes using O₂ remains a major challenge in bioinspired catalysis. The reactivity of iron–oxygen oxidants generated from an Fe^II–benzilate complex of a facial N₃ ligand were thus investigated. The complex reacted with O₂ to form a nucleophilic oxidant, whereas an electrophilic oxidant, intercepted by external substrates, was generated in the presence of a Lewis acid. Based on the mechanistic studies, a nucleophilic Fe^II–hydroperoxo species is proposed to form from the benzilate complex, which undergoes heterolytic O O bond cleavage in the presence of a Lewis acid to generate an Fe^IV–oxo–hydroxo oxidant. The electrophilic iron–oxygen oxidant selectively oxidizes sulfides to sulfoxides, alkenes to cis‐diols, and it hydroxylates the C H bonds of alkanes, including that of cyclohexane.     

Olefin cis‐Dihydroxylation and Aliphatic CH Bond Oxygenation by a Dioxygen‐Derived Electrophilic Iron–Oxygen Oxidant

Many iron‐containing enzymes involve metal–oxygen oxidants to carry out O₂‐dependent transformation reactions. However, the selective oxidation of C? H and C?C bonds by biomimetic complexes using O₂ remains a major challenge in bioinspired catalysis. The reactivity of iron–oxygen oxidants generated from an Fe^II–benzilate complex of a facial N₃ ligand were thus investigated. The complex reacted with O₂ to form a nucleophilic oxidant, whereas an electrophilic oxidant, intercepted by external substrates, was generated in the presence of a Lewis acid. Based on the mechanistic studies, a nucleophilic Fe^II–hydroperoxo species is proposed to form from the benzilate complex, which undergoes heterolytic O? O bond cleavage in the presence of a Lewis acid to generate an Fe^IV–oxo–hydroxo oxidant. The electrophilic iron–oxygen oxidant selectively oxidizes sulfides to sulfoxides, alkenes to cis‐diols, and it hydroxylates the C? H bonds of alkanes, including that of cyclohexane.     

Chemiluminescence (CL) emission generated during oxidation of pyrogallol and its application in analytical chemistry. I. Effect of oxidant compound

An investigation of chemiluminescence (CL)-emission generated by the oxidation of pyrogallol using various inorganic oxidant compounds is reported in this F.I.A.-merging zone application. The oxidant compounds that showed measurable CL-emission were permanganate, periodate, hypochlorite anions, cerium(IV) and hydrogen peroxide. The different oxidant compounds showed CL-emissions at different pH-ranges. The CL-emission was limited by the inner filter effect and this was more intense for oxidants of selective oxidation. Kinetic effects were also found in the case of oxidation by permanganate. Plots of CL-emission against pH give evidence of speciation and or deactivation mechanism effects. The analytical parameters for the determination of the oxidants are given. Sensitivities of 895 600, 19 500, 33 723, 10 680 and 56 703 mV M(-1) were found for the determination of permanganate, cerium(IV), periodate, hypochlorite and hydrogen peroxide, respectively. The calibration curves of the oxidant determination were generally S-shaped; the S-shaped calibration curve of periodate was closer to a straight line relationship while that of hypochlorite was almost a straight line; detection limits in the range of 10(-4) M oxidant concentration were found for nearly all oxidants. The analytical parameters for determination of pyrogallol by the CL-emission generated through oxidation by the different oxidants at optimum conditions were 1.16x10(6) mV M(-1) for permanganate; 0.086x10(6) mV M(-1) for cerium(IV); 0.91x10(6) mV M(-1) for periodate; 0.012x10(6) mV M(-1) for hypochlorite; and 0.25x10(6) mV M(-1) for hydrogen peroxide. The detection limit was 1.0x10(-4) M. The nearly straight-line relationship (initial part of the plot) for CL-emission with oxidant concentration gives an indication that the CL-reaction of pyrogallol oxidation by hypochlorite proceeds through a process that involves energy transfer while the pronounced S-shaped curve produced by permanganate gives the indication that the reaction proceeds through a process that does not involve energy transfer according to the mathematical model of CL-emission that controls the F.I.A.-merging zone technique of the flow apparatus used in this work. The sequence of completeness of the oxidation process by each oxidant was MnO(4)(-)>H(2)O(2)>IO(4)(-)>OCl(-); the stoichiometric quantity of the oxidant per pyrogallol molecule for the rapid part of the overall oxidation by each different oxidant was attempted; this is an index-value of the oxidation state of the fluorescent excited molecule. Finally, the impact of the above findings for further analytical applications is discussed.     

Studies on Oxidation of Active α-H of Aromatic Hydrocarbon by CAN/NaBrO3/H2O System

Cerium ammonium nitrate (CAN) is an expensive and powerful selective oxidant. It has played an important role in organic synthesis. Because it is a single electron oxidant,many organic reactions need large quantities of it. The application of CAN in organic synthesis in the oxidation of alkyl aromatics has so far been limited. In this paper, we wish to report a new method of oxidation of active α-H of aromatic hydrocarbon by CAN/NaBrO₃/H₂O system. Only catalytic amount of CAN was needed in this method and the selectivity was improved as well.     

A density functional study of O-O bond cleavage for a biomimetic non-heme iron complex demonstrating an Fe(v)-intermediate

Density functional theory using the B3LYP hybrid functional has been employed to investigate the reactivity of Fe(TPA) complexes (TPA = tris(2-pyridylmethyl)amine), which are known to catalyze stereospecific hydrocarbon oxidation when H(2)O(2) is used as oxidant. The reaction pathway leading to O-O bond heterolysis in the active catalytic species Fe(III)(TPA)-OOH has been explored, and it is shown that a high-valent iron-oxo intermediate is formed, where an Fe(V) oxidation state is attained, in agreement with previous suggestions based on experiments. In contrast to the analogous intermediate [(Por.)Fe(IV)=O](+1) in P450, the TPA ligand is not oxidized, and the electrons are extracted almost exclusively from the mononuclear iron center. The corresponding homolytic O-O bond cleavage, yielding the two oxidants Fe(IV)=O and the OH. radical, has also been considered, and it is shown that this pathway is inaccessible in the hydrocarbon oxidation reaction with Fe(TPA) and hydrogen peroxide. Investigations have also been performed for the O-O cleavage in the Fe(III)(TPA)-alkylperoxide species. In this case, the barrier for O-O homolysis is found to be slightly lower, leading to loss of stereospecificity and supporting the experimental conclusion that this is the preferred pathway for alkylperoxide oxidants. The difference between hydroperoxide and alkylperoxide as oxidant derives from the higher O-O bond strength for hydrogen peroxide (by 8.0 kcal/mol).