Synthetic utility of metal catalyzed hydrogen peroxide oxidation of C-H, C-C and C=C bonds in alkanes, arenes and alkenes: Recent advances

This review presents the current state of the art of homogenic and heterogenic oxygen transfer processes using hydrogen peroxide as oxygen source and compounds and complexes of tranisition metals, Fe, V, Cu, Mn, Rh, Mo, Ti, W, Pd, Os, Zr, Cr, Nb, Co, as the catalysts. Based on the reactions of C-H, C-C and C=C bonds in alkanes, arenes and alkenes various mechanisms of the oxygen-transfer processes are discussed and new methodologies for synthesis of alcohols, phenols, aldehydes, ketones, epoxides and carboxylic acids are indicated.


Introduction
Oxidation reactions are of fundamental importance in nature, and are key transformations in practical organic synthesis, in both laboratory and industry.The development of processes that employ hydrogen peroxide as an environmentally friendly oxidant is one of the most important goals in oxidation chemistry.Hydrogen peroxide is supplied mainly as a 30% aqueous solution.It is relatively stable, easy to store, commercially available and relatively cheap reagent of low molecular weight which can be used on any scale.It contains a large amount (40% by weight) of active oxygen (next after O2), much more than other oxidants.The standard reduction potential E o =1.776 V is higher than that of dichromates in acidic medium.][3][4][5][6] The most common oxidation catalysts contain a transition metal that activates hydrogen peroxide for the oxyfunctionalization of the organic substrate.The broad spectrum of applications of metal catalysts for hydrogen peroxide oxidation have been reviewed elsewhere e.g. in refs 7-18.Alternatively, one may perceive the use of organocatalysts and enzymes, which mediate catalytic homogenous oxidations without the need for metal-containing catalysts.These include α-hydroxyperoxides, hydroperoxyflavines, imine-derived hydroperoxysulfonyl amides (e.g.][21][22][23][24] Following the discovery of a broad spectrum of new metal catalysts for hydrogen peroxide oxidation in recent years, and the huge progress in their use in organic synthesis, this review covers the scientific literature from 2011 to early 2016, but includes a few significant earlier references where necessary to the discussion.Since, during this period, hundreds of articles covering this topic have been published, this presentation is limited to those catalysts which meet the conditions definied by Gladysz, 25 make the greatest impact on modern practical synthesis, and allow an understanding of the reaction mechanisms.
Among oxyfunctionalizations of various groups of organic compounds, oxidation of hydrocarbon C-H, C-C and C=C bonds is a fundamental problem in the synthetic laboratory and industrial chemistry because this group of compounds is derived from natural sources on a large scale and their transformations lead to many products used in modern technologies and medicine.This review focuses on the use of metal-containing catalysts for oxidation of hydrocarbons, but some oxidations of hydrocarbon moieties in more complex compounds are also mentioned.Procedures for the preparation of the catalysts and analysis of their structures can be found in the cited literature

Oxometal vs. peroxometal pathways
Metal-catalyzed oxidations with hydrogen peroxide can be conveniently divided into two categories, involving peroxometal and oxometal species, respectively, as the active oxidant.An illustrative example is given for alcohol oxidations in Scheme 1.In the peroxometal pathway, the metal ion does not undergo any change in oxidation state during the catalytic cycle, and no stoichiometric oxidation is observed in the absence of H2O2.In contrast, oxometal pathways involve a two electron change in oxidation state of the metal ion and a stoichiometric oxidation is observed, with the oxidized state of the catalyst, in the absence of H2O2.Indeed, this is a test for distinguishing between the two pathways.Peroxometal pathways are typically observed with early transition metal ions with a d 0 configuration, e.g.Mo(VI), W(VI), Ti(IV), Re(VII), that are relatively weak oxidants.Oxometal pathways are characteristic of late transition elements and first row transition elements, e.g., Cr(VI), Mn(V), Os(VIII), Ru(VI) and Ru(VIII), that are strong oxidants in their highest oxidation states. 1,26,27t is also worth noting that an oxometal intermediate can be formed via heterolysis of the O-O bond of an initially formed peroxometal species.The metal ion increases the oxidizing power (electrophilicity) of the peroxo moiety by withdrawing electrons, i.e. by acting as a Lewis acid.Hence, the peroxometal pathway is not limited to variable-valence elements.In the oxometal pathway, in contrast, the metal center undergoes a two electron reduction and is subsequently reoxidized by the oxygen donor.Some metals can operate via both pathways, depending on the substrate, e.g.vanadium(V) operates via a peroxometal pathway in olefin epoxidations and via an oxometal pathway in alcohol oxidations.It was shown that simple monovanadate VO3 -efficiently catalyzes the oxidations of alkanes, benzene, and styrene with H2O2 in acetonitrile, if pyrazine-2-carboxylic acid (PCA) is added as a cocatalyst in a low concentration.Kinetic studies and DFT calculations have demonstrated that the mechanism of this process involves the formation of HO • and HOO • radicals.The former highly reactive radical abstracts the hydrogen from alkane RH, to give radical R • , which upon reaction with O2 affords the corresponding alkyl hydroperoxide, ROO • .This not a very stable species and can easily be transformed into the industrially useful alcohol or ketone (Scheme 3). 28Bu The vanadium-catalyzed oxidations of alkanes are very sensitive to the presence of H + in the reaction mixture.Simple monovanadate VO3 -does not exhibit any catalytic activity toward alkanes, but the addition of an acid in low concentration dramatically accelerates this reaction.The hypothesis of the formation of oligovanadates in acidic medium, which are more active toward the oxidation of alkanes than the simple monovanadate, was confirmed by both experimental and theoretical methods.51 V NMR and kinetic studies indicate, that in the systems VO3 -/H2O2/TFA and [V10O26] 4-/H2O2 in acetonitrile, oligovanadates are responsible for the efficient generation of hydroxyl radicals (key species in the alkane oxidation) (Scheme 4).29,30  On the basis of the experimental and theoretical data on copper-catalyzed oxidative alkane dehydrogenation, the mechanistic proposal shown in Scheme 5 was formulated.The trispyrazolylborate (Tp x Cu) core reacts with hydrogen peroxide via an oxometal pathway, to give a copper-oxo intermediate with strong oxyl character.Interaction with cyclohexane induces a hydrogen abstraction process, yielding the cyclohexyl radical and Tp x Cu-OH.From here, two competitive pathways may occur.Through the hydroxylation pathway, the cyclohexyl radical collapses with Tp x Cu-OH leading to cyclohexanol formation, whereas through the dehydrogenation pathway, a second hydrogen abstraction from the α C-H of the cyclohexyl radical, leads to cyclohexene formation.In the latter case, the oxidant itself acts as hydrogen acceptor, with the net reaction providing two molecules of water from one molecule of H2O2 and cyclohexane.These pathways involve spin crossover through MECPs of similar energy, which leads to product mixtures.The lowest-energy MECP yields cyclohexanol as major reaction product.None of these pathways involve the formation of free hydroxyl radicals.30 Studies on catalytic oxidation of benzene to phenol over CuO-impregnated mesoporous silica provided the evidence that the reaction path involves a first stage, where the interaction of Cu/MCM-41 catalysts with H2O2 yields HO· radical and HOO· radical species via a redox mechanism.Next, the HO· radical attacks the benzene ring and yields phenol as the major product, with hydroquinone and benzoquinone as the side products.Oxygen and water are formed as side products by the decomposition of the hydroperoxy radical and H2O2, respectively.So the process mainly follows a free radical mechanism.31,32 Scheme 5.The mechanism of copper-catalyzed oxidative alkane dehydrogenation.
Mechanistic consideration of enantioselective vinylarene (indene, chromene, styrene) epoxidation, under mild conditions by the Mn(III)-containing Jacobsen catalyst, led to the conclusion that activation of H2O2 by this complex is similar to activation of H2O2 by other metal complexes, namely metalloporphyrins and by peroxidase.Upon addition of hydrogen peroxide to the catalyst 1, the peroxyl group is activated through formation of a [Mn(III)salen-H2O2] adduct 2 (Scheme 5), which by deprotonation leads to a hydroperoxy intermediate [Mn(III)salen-OOH] 3. Next, the hydroperoxy intermediate 3 can undergo protonation and dehydration, generating the oxo-species 4. The hydroperoxy species is able to epoxidize olefins through a concerted mechanism, as observed for the epoxidation by peracids, namely m-CPBA, since the electrophilicity of the distal peroxidic oxygen is enhanced due to metal bond activation.On the other hand, the activity of the oxo-species 4 was found to be high.Independent of the oxidant used it was considered to be the intermediate mainly responsible for the high enantioselectivity of the Jacobsen catalyst, relative to other systems.It is a consequence of the close proximity of the active oxygen to the non-planar and hindered salen ligand, schematically represented in Scheme 6. 33 © ARKAT USA, Inc Scheme 6. Hydrogen peroxide enantioselective epoxidation of vinylarenes under mild conditions by the Jacobsen catalyst.

Solid supported catalysts
A broad range of solid supported catalysts including peroxometalates, framework-substituted microporous and mesoporous molecular sieves, layer-type materials, inorganic oxides, porous materials, encapsulated metal complexes, supported porphyrin and nanomaterials, have been reported during last decades as catalysts suitable for liquid phase oxidation with H2O2.Among these, catalyst systems with W, Ti and Mo have much better prospects of industrial applications.][36][37][38] Due to the exclusive properties of titanosilicates, especially TS-1, on oxidation of different groups of organic compounds, the mechanisms of their action have been extensively studied.They vary depending on the microstructure of the catalyst and the substrate used.In the TS-1 catalyst, titanium is tetrahedrally coordinated, which coordinates a protic solvent such as alcohol and is transformed into pentahedral and/or octahedral coordination.Increasing the size of the R group in the alcohol facilitates the formation of hydroperoxide intermediate 5 and favors the approach of the alkene.The oxygen atom is transferred to the alkene C=C bond by intermediate complex 6 and an epoxide is formed, as shown in Scheme 7. 39,40  Normally, titanium-silicates produce products typical of both two-electron oxidation mechanisms (cyclohexene epoxide and trans-cyclohexane-1,2-diol), and one-electron oxidation mechanisms (cyclohex-2en-1-ol and cyclohex-2-en-1-one).A Ti-hydroperoxide with a Ti-OOH bond is formed by the chemisorption of hydrogen peroxide on Ti-sites.This can be the active species in a heterolytic mechanism as in the oxidation of olefins and possibly of alcohols or the precursor of a Ti-centred radical species, responsible for homolytic oxidations, as in the hydroxylation of alkanes and in the decomposition of hydrogen peroxide.The adsorption of a protic component of the solution, affects the activity of active species.The direction of the oxidation path is determined by a combination of different factors: the nature of catalyst surface, the polarity of the solvent, the reactivity of the substrate and the temperature.On these grounds, a general oxidation mechanism was proposed for TS-1 and other Ti-zeolites. 41,42he metal hydroperoxide was also postulated as an active intermediate formed during the epoxidation, catalyzed by layered hydrotalcite in the presence of isobutyramide (Scheme 8).Hydrogen peroxide attack a basis hydroxyl function of the aluminium atoms on the surface of hydrotalcite to form hydroperoxy species, which react with an amide, to generate a peracid together with NH 2 in the aqueous phase.Further, oxygen transfer from the peracid to an alkene occurs at the surface boundary between aqueous and organic phases, hence addition of an anionic surfactant is preferred.When the Mg/Al reaction was carried in the presence of acetonitrile the iminoperacetic acid, formed in situ, was an active oxygen donor. 43,44he reaction mechanism of the Al2O3/H2O2-catalyzed alkene epoxidation probably involves an Al-OOH species, which react with alkene to give the resulting peroxide. 45,46In the rhenium-catalyzed epoxidation of alkenes on the surface of alumina, the tetrahedral ReO4 ̶ species are attached to alumina surfaces by covalent Al-O-Re bonds.Reaction on surface-Al-O-ReO3 7 gives the peroxo species 8, which reacts further with H2O2 to form complex 9. Oxygen transfer from 9 to the alkene C=C bond, yields the epoxide via intermediate complex 10 (Scheme 9).Scheme 9. Proposed mechanism for the Re-catalyzed epoxidation of alkenes on the surface of alumina.

The Fenton reaction
The well-known Fenton reaction is the reaction between ferrous ion and hydrogen peroxide, resulting in the formation of hydroxyl and perhydroxyl radicals and ferric ion Equations (1-3).The ferric ion with the hydrogen peroxide generate a complex, which is the ferrous ion coordinated with perhydroxy radical, which decomposes to ferrous ion and perhydoxy radical as seen in Equations ( 2) and ( 3).This regenerative nature is characteristic of the Fenton reagent.
On the other hand, the perhydroxyl radicals can be formed from hydrogen peroxide and hydroxyl radical, directly generated in the reaction between the ferrous ion and H2O2 Equation (4).
The hydroxyl radicals are more selective than perhydroxyl radicals with organic compounds and it was assumed that it is the hydroxyl radicals that take part in the oxidation of organic compounds Eq. ( 6), while the perhydroxyl radicals decompose to molecular oxygen Eq. ( 5) and it was assumed that an addition complex is formed between the substrate (e.g.phenol) and the hydroxyl radical. 48

Oxidation of alkanes
The selective oxidation of alkanes and arenes remains a fundamental problem in organic synthesis, because their components, carbon and hydrogen, do not have lone electron pairs and the molecules do not have orbitals of sufficient energy, that are easily accessible.When very reactive reagents are used and the reaction is carried out under harsh conditions, the initial products are generally more reactive that the starting compounds and undesired side reactions occur.Despite extensive development, efforts to find new catalysts for alkane and arene oxidation, current commercial processes still suffer from low conversion, poor selectivity and excessive production of waste.Thus, high-efficiency and high-selectivity catalytic oxidation of C-H groups under mild conditions is a major objective of current catalysis and remains of one of the greatest challenges in organic synthesis.
In recent years, significant progress was been made in the oxidation of hydrocarbons with hydrogen peroxide in the presence vanadium, copper, iron, osmium, tungsten, manganese, molybdenum, cobalt compounds and complexes.Vanadium compounds have attracted much attention, because they have been used widely for homogeneous, heterogeneous, industrial, and biological oxidation processes with H2O2, alkyl hydroperoxides, and O2.Reactions of vanadium complexes and vanadium-containing polyoxometalates (POMs) with H2O2 have been extensively studied and several mono-, di-, and poly-nuclear peroxovanadium complexes have been isolated and characterized.The isolated and/or in situ-generated peroxovanadium complexes catalyze H2O2-based oxidation of alkanes, as well as alcohols, aromatic compounds, sulfides, and halides.The new hexanuclear mixed-valence vanadium complex [V3O3(OEt)(ashz)2(μ-OEt)]2 (ashzH3 = N-acetyl-salicylhydrazide), with an N,O-donor ligand, acts as a highly efficient catalyst toward alkane oxidations by aqueous H2O2.Remarkably, high turnover numbers up to 25,000 with product yields of up to 27% (based on alkane), being one of the most active systems for such reactions. 49The efficient oxidation of alkanes (cyclohexane, n-heptane, methylcyclohexane, isooctane, cis-and trans-1,2-dimethylcyclohexane) to the corresponding alkyl hydroperoxides with the system (n-Bu4N)[VO3]/H2O2/trifluoroacetic acid/MeCN-H2O was investigated and it has been established on the basis of the cyclohexane oxidations, that the reaction mechanism includes the formation of HO• radicals. 50he oxovanadium(IV) complexes [VO(acac)2(Hpz)]•HC(pz)3 I (acac = acetylacetonate, Hpz = pyrazole, pz = pyrazolyl) and [VOCl2{HOCH2C(pz)3}] II under either heterogeneous (immobilized on a PDMS-based membrane) or homogeneous conditions, exhibit appreciable catalytic activity for the oxidation of cyclohexane to cyclohexanol/cyclohexanone mixtures.The stoichiometric oxidant was H2O2, or better more active benzoylperoxide or m-chloroperbenzoic acid in acetonitrile in acidic medium, at room temperature, in moles products/moles catalyst ratios (TONs) up to 1100. 51The same authors reported the catalytic activity of transition-metal complexes of the first-row (V(IV), Mn(III), Fe(III) Co(III) and Cu(II)) with biomimetic Schiff base ligands with N 2 O 2 coordination sphere in the room-temperature oxidation of cyclohexane using hydrogen peroxide 30% H2O2 as the oxygen source, acetonitrile as the solvent and nitric acid as promoter of the oxidation reaction. 52A new binuclear oxovanadium(V) complex [{VO(OEt)(EtOH)}2L], where H4L was bis(2hydroxybenzylidene)terephthalohydrazide, was synthesized and used in combination with pyrazine-2carboxylic acid (PCA; a cocatalyst) for the efficient oxidation of saturated hydrocarbons RH, with H2O2 and air in acetonitrile solution at 50 °C, to produce alkyl hydroperoxides ROOH, as the main primary products. 53The catalytic performances of three new peroxo-and oxovanadium(IV) complexes: [VO(O2)(bpz*eaT)•VO(C4H4O6)]•H2O, [VOSO4(bpz*eaT)]•C6H8O7 (bpz*eaT=2,4-bis-(3,5-dimethyl-1H-pyrazol-1yl)-6-diethylamino-1,3,5-triazine) and [VO(C12H8N2)(C9H7NO3)]•MeOH•0.5H2O, were studied in the reaction of cyclohexane oxidation.The first of them exhibited the highest catalytic activity. 54wo novel oxomolybdenum(V)-trispyrazolylborate complexes [(TpMoO)(μ2-O)2(MoOTp)](Tp = tris-(pyrazolyl)borate) and [(Tp4IMoOCl)(μ-O)(MoOClTp4I)](Tp4I = tris(4-iodinpyrazolyl)borate) were synthesized and characterized.Their catalytic performances in cyclohexane oxidation were significantly better than the molybdenum(VI) complexes with the similar structure. 55 promising approach for mild oxidative functionalization of alkanes, concerns the development of bioinspired metal complex catalysts which, with an appropriate oxidizing agent and under tuned reaction conditions, are capable of converting alkanes into valuable functionalized products.In view of the well recognized biological function of copper, which is present in the active sites of many oxidation enzymes, several complexes were prepared and used as catalyst while the oxide source was hydrogen peroxide. 56The dehydrogenation of n-hexane and cycloalkanes, giving n-hexene and cycloalkenes, was observed in the reaction of such hydrocarbons with hydrogen peroxide in the presence of copper complexes bearing trispyrazolylborate ligands.Experimental data exclude the participation of hydroxyl radicals derived from Fenton-like reaction mechanisms. 30he photocatalytic conversion of methane into methanol from an aqueous suspension containing mesoporous WO 3 was studied, as well as the effect of the addition of electron scavengers (Fe 3+, Cu 2+ , Ag + ) and H2O2 species.In the presence of Fe 3+ ions the production of methanol was about two and a half times higher than that of pure mesoporous WO3. 57he tricopper complex [CuICuICuI(7-N-Etppz)] 1+ , where 7-N-Etppz denotes the ligand 3,3'-(1,4-diazepane-1,4-diyl)bis[1-(4-Et-piperazine-1-yl)propan-2-ol], is capable of mediating facile conversion of methane into methanol, upon activation of the tricopper cluster by dioxygen and/or H2O2 at room temperature.This is the first catalyst that can catalyze selective oxidation of methane to methanol, without over-oxidation, under ambient conditions.When this tricopper complex is activated by dioxygen or H2O2, the tricopper cluster harnesses a "singlet oxene", the strongest oxidant that could be used to accomplish facile O-atom insertion across a C-H bond.To elucidate the properties of this novel catalytic system, methane oxidation over a wide range of conditions was investigated and the study was extended to other small alkanes, including components of natural gas. 58Continuous selective oxidation of methane to methanol and production of C2 oxygenated products from ethane using H2O2 as an oxidant over Cu-and Fe-modified ZSM-5 catalysts, was reported.A trickle bed continuous flow reactor was utilized to control the contact time effectively between reactants and catalyst, to minimise C-C scission and over oxidation to COx. 59,60 Titania-supported gold palladium copper AuPdCu/TiO2 catalysts are able to catalyze the oxidation of methane to methanol with H2O2 under mild aqueous reaction conditions, with methanol selectivity of up to 83%. 61he Cu(I) compound [Cu(μ4-4-ptz)]n [1,4-ptz = 5-(4-pyridyl)tetrazolate] revealed high catalytic activity toward oxidation of cyclic alkanes (cyclopentane, cyclohexane and cyclooctane) with aqueous hydrogen peroxide, under very mild conditions at room temperature. 62The catalytic peroxidative oxidation (with H 2 O 2 ) of cyclohexane in an ionic liquid (IL) using the tetracopper(II) complex [(CuL)2(μ4-O,O,O,O-CDC)]2•2H2O [HL = 2-(2-pyridylmethyleneamino)benzenesulfonic acid, CDC = cyclohexane-1,4-dicarboxylate] as a catalyst was reported.Significant improvements on the catalytic performance, in terms of product yield (up to 36%), TON (up to 529), reaction time, selectivity towards cyclohexanone and easy recycling were observed using 1-butyl-3-methylimidazolium hexafluorophosphate as the chosen IL instead of a molecular organic solvent including the commonly used acetonitrile. 63The easily prepared Cu(II), Co(II) and Ni(II) complexes, immobilized on imine-functionalized silica gel through a 3-aminopropyltriethoxysilane linker, were reported as efficient catalysts for cyclohexane oxidation to cyclohexanol and cyclohexanone under ambient conditions. 64The mononuclear water-soluble copper(II) complexes of pyridine 2-carboxamide, were reported as suitable catalysts for the oxidation of toluene, ethylbenzene and cyclohexane. 65The novel water-soluble 2D copper(II) coordination polymers were found to be highly active precatalysts for the mild water-promoted oxidation of cycloalkanes, from cyclopentane to cyclooctane, with 50% aqueous hydrogen peroxide to mixtures of alcohols and ketones in 16.5-35.8%yields. 66 family of imine-based nonheme iron(II) complexes (LX)2Fe(OTf)2 has been prepared, characterized and employed as C-H oxidation catalysts.Ligands LX (X = 1, 2, 3, and 4) stand for tridentate imine groups, resulting from spontaneous condensation of 2-picolylamine and 4-substituted-2-picolyl aldehydes.Reactions occur with stereoretention at the hydroxylated carbon and selectively at tertiary over secondary C-H bonds. 67

Mononuclear non-heme iron coordination complexes
] catalyze the fast, efficient, and selective oxidation of nonactivated alkyl C-H groups.Several branched alkanes and cycloalkanes were oxidized and regioselectivities determined with these catalysts.The oxidation of menthyl acetate 11 to alcohol 12 and ketone 13 is an example (Scheme 10).Scheme 10.Oxidation of menthyl acetate catalyzed by iron complex Δ-1P.
The efficient oxidation of alkanes to the corresponding alkyl hydroperoxides by H2O2 in the presence of catalytic amounts of ferrocene, proceeded in MeCN at 40-50 °C.Benzene was oxidized in the same system to phenol.An obligatory component of the catalytic system for both reactions is pyrazine-2-carboxylic acid (PCA) or trifluoroacetic acid (TFA). 69Both alkanes (cyclohexane, adamantane, alkylarenes) and alcohols (isopropanol, cyclohexanol, benzyl alcohols), were selectively oxidized to their corresponding aldehydes and ketones in excellent yields with 30% H2O2, in the presence of eco-friendly, cheap and reusable polymer-anchored iron(III)ferrocene Schiff base complex. 70Supporting the iron-salen complexes on montmorillonite, affords active, selective and recyclable catalysts for the liquid phase oxidation of n-octane with hydrogen peroxide under mild conditions, with ketones as the major products, although conversions of the substrate were low (3.6-7.4%). 71The hydrotris(pyrazol-1-yl)methane iron(II) complex immobilized on commmercial zeolite (MOR) or desilicated (MOR-D), catalyzes the oxidation of cyclohexane with hydrogen peroxide to cyclohexanol and cyclohexanone, under mild conditions.MOR-D/Fe (desilicated zeolite supported [FeCl2{η 3 -HC(pz)3}] complex) provides an outstanding catalytic activity (TON up to 2.90 × 103) with a concomitant overall yield of 38%, and can be easy recovered and reused. 72When hexadentate binding 8-quinolinolato iron(III) complexes were used for the oxygenation of cyclohexane to cyclohexanol and cyclohexanone by aqueous H2O2 in acetonitrile, the reactions were accelerated to some extent by heating or under visible light irradiation.Based on UV-visible spectroscopic characterization, a free radical mechanism for this catalytic oxygenation system was proposed. 73t was disclosed that two heteroleptic 8-quinolinolato iron(III) complexes (Qa1Qb2Fe III and Qa2Qb1Fe III (synthesized conveniently via coordination of FeCl2•6H2O with suitable equivalents of 5,7-dichloro-8-hydroxyquinoline (Qb) or 5-chloro-8-hydroxyquinoline (Qa), showed catalytic activity in the oxygenation of cyclohexane to a mixture of cyclohexanol (0.3-10.% yield) and cyclohexanone (0.2-12.8-yield) as well as in the hydroxylation of benzene to phenol, oxygidation of toluene to benzaldehyde and ethylbenzene to acetophenone in yields up to 17.2%. 74t was demonstrated that simple soluble Bi(III) salts -systems unexplored earlier in the homogeneous oxidation of alkanes -exhibit a pronounced catalytic activity in the homogenous oxidation of these hydrocarbons with hydrogen peroxide.The yield of the oxygenates in the oxidation of cyclooctane, catalyzed by the aqueous system Bi(NO3)3/H2O2/HNO3/CH3CN, attains 32% and the TON reaches 112.The reaction has a free-radical character and proceeds via hydroperoxides, finally hydrolyzed to ketones the main final products.(Scheme 11). 75© ARKAT USA, Inc Scheme 11.Radical mechanism of the Bi(III)salts-catalyzed alkane oxidation with H2O2 (the rate limiting step of the whole process is boxed).
An efficient and selective oxidation of secondary carbon-hydrogen sites (C-H methylene group) of alkanes, was described and the synthesis of the target compounds was achieved by using a low catalyst loading of an inexpensive, readily available iron catalyst [Fe(II)(CF3SO3)2(mcp) mcp = N,N'-dimethyl-N,N'-bis(2pyridylmethyl)cyclohexane-trans-1,2-diamine] and H2O2 as oxidant by a simple reaction protocol.Natural products were selectively oxidized and isolated in synthetically amenable yield.It is a particularly convenient tool to carry out alkane C-H oxidation on a preparative scale in a short reaction time.Reactants included 2,2dimethylpropanoic acid cis-4-methylcyclohexyl ester, 3,7-dimethyl-1-octanol 1-acetate, (1R,2S,5R)-5-methyl-2-(1-methylethyl)cyclohexanol acetate [(-)-menthyl acetate], (+)-neomenthyl acetate, linear heptane, (3aR,5aS,9aS,9bR)-dodecahydro-3a,6,6,9atetramethyl[naphtho [2,1-b]furan] [(-)-ambroxide], (+)-sclareolide (sesquiterpene lactone), cis-decalin, (3β,5α)-3-cetyloxyandrostan-17-one (steroid, androstane). 76Catalytic activity of the new iron(II) α-iminopyridine complexes was demonstrated in the oxidation of cyclohexane, activated methylene groups and secondary alcohols to the corresponding ketones using H2O2 and t-BuOOH as the oxidants. 77ow temperature selective transformation of light alkanes of natural gas into useful products continues to be an important challenge in chemistry and industry.For this purpose, nitrido diiron phthalocyanines were recently identified as powerful oxidation catalysts, capable of oxidizing methane, benzene and transforming poly-and perfluorinated aromatic compounds with hydrogen peroxide.Acetic acid was obtained from ethane with selectivity up to 71%.Turnover number up to 58 and 50-65 % product yields can be achieved.The reactivities of C-H bonds in methane, ethane and propane were very similar, when the reaction was performed in water.In contrast, in dilute acidic solution the nitrido diiron phthalocyanineH2O2 system, exhibits 5 times higher activity in the oxidation of a methane C-H bond compared to an ethane C-H bond. 78A new μ-oxo diiron(III) complex of the lithium salt of the pyridine-based unsymmetrical ligand 3- was synthesized and the ability of the complex to catalyze oxidation of several alkanes and alkenes has been investigated by using CH 3 COOH/H 2 O 2 (1:1) as an oxidative system.Moderate activity in cyclohexane oxidation to cyclohexanol (18% yield) and cyclohexanone (15% yield), and in cyclohexene oxidation to epoxyclohexane (13% yield), 2-cyclohexenol (15% yield) and 2hydroxycyclohexanone (8% yield) was detected.The formation of a transient peroxo species, [Fe2(O)(O2)(LiDPCPMPP)2] 2+ , which might be an intermediate in the metal-based component of the oxidation process, was suggested. 79atalytic and mechanistic insights of the low-temperature selective oxidation of methane over Cupromoted Fe-ZSM-5 demonstrated, that this system is able to catalytically convert methane to methanol with turnover frequencies (TOFs) of over 14 000 h −1 by using H2O2 as terminal oxidant. 80The same authors reported that the oxidation with hydrogen peroxide of methane to methanol is effectively catalyzed by supported Au-Pd catalysts.The primary product is Me hydroperoxide formed from the reaction of H2O2 with radical •CH3. 81he Au nanoparticles/CQDs (CQD = carbon quantum dots) composite photocatalyst yields 64% conversion efficiency and 99.9% selectivity for the green oxidation of cyclohexane to cyclohexanone, using H2O2 under visible light at room temp. 82enzylic, aliphatic C-H as well as tertiary C-H oxidation with hydrogen peroxide was efficiently catalyzed by a benzimidazole-based nonheme manganese complex in the presence of acetic acid as additive.For example, oxidation of ethylbenzene gave benzophenone in 70% yield. 83Catalytic properties of a series of manganese(III) complexes, with tetradentate phenol-and pyridine-based ligands, on the oxidation of the C-H bond of diphenylmethane in the presence of hydrogen peroxide as a terminal oxidant at ambient conditions, were investigated.In this process, diphenylmethane was oxidized to the corresponding ketone and alcohol. 84n in situ prepared manganese catalyst, based on manganese(II) salts, pyridine-2-carboxylic acid, and butanedione, was employed for hydrogen peroxide oxidation of C-H bonds at ambient temperatures.It provides good-to-excellent conversions of benzyl and alkyl groups as well as secondary alcohols with high turnover numbers (up to 10 000). 85he triosmium dodecacarbonyl cluster Os3(CO)12 catalyzes oxidation of linear (n-heptane) and cyclic alkanes (cyclohexane, cyclooctane, methylcyclohexane, cis-and trans-1,2-dimethylcyclohexanes) to the corresponding cycloalkyl hydroperoxides by hydrogen peroxide in acetonitrile.Addition of pyridine leads to an acceleration of the process.Turnover numbers in the case of cyclooctane attain 6•10 4 and turnover frequencies are up to 24,000 h -1 .The alkyl hydroperoxide partly decomposes in the course of the reaction to afford cyclooctanone and cyclooctanol. 86ther efficient catalysts for the oxidation of alkanes (cyclohexane, n-heptane, methylcyclohexane, isooctane, cis-and trans-1,2-dimethycyclohexane) to corresponding hydroperoxides, are osmium complexes containing p-cymene ligands.The yield of cyclohexyl hydroperoxide, based on cyclohexane, is up to 30%, which is also amongst the highest reported.The oxidation of cyclohexane at 60 o C after 24 h, gave a turnover number of 2•10 5 and turnover frequencies of 10 3 h -1 .These parameters are the highest ones, observed for catalytic oxidation of alkanes, and are even comparable to enzymatic oxidation.Although osmium compounds are rather expensive, they can be successfully employed in oxidation of hydrocarbons and alcohols by peroxides, because the reactions require only very low concentrations of the catalysts. 87,88he osmium(VI) nitrido complex, [Os VI (N)(L)(CH3OH)] + (L = N,N'-bis(salicylidene)-o-cyclohexyldiamine dianion) is an efficient catalyst for the oxidation of alkanes under ambient conditions using H2O2 as the oxidant.Alkanes are oxidized to the corresponding alcohols and ketones, with yields up to 75% and turnover numbers up to 2230.Experimental and computational studies are consistent with a mechanism that involves O-atom transfer from H2O2 to [Os VI (N)(L)] + to generate an [Os VIII (N)(O)(L)] + active intermediate. 89someric cage-like tetracopper(II) silsesquioxane complexes [(PhSiO1.5)12(CuO)4(NaO0.5)4](a), [(PhSiO1.5)6(CuO)4(NaO0.5))4(PhSiO1.5)6](b) and binuclear complex [(PhSiO1.5)10(CuO)2(NaO0.5)2](c), can be considered as models of some multinuclear copper-containing enzymes.Compounds (a) and (c) are good precatalysts for alkane oxygenation with hydrogen peroxide, in air in acetonitrile solution.Thus, the (a)-catalyzed reaction with cyclohexane at 60 °C, gave mainly cyclohexyl hydroperoxide in 17% yield. 90The hybrid [DPyAM(H2)]1.25PMoV2was tested as a heterogeneous catalyst for the oxidation of cyclohexane by aqueous H2O2 (30 wt%) into a mixture of cyclohexanol as a major product and cyclohexanone as a minor one (total yield up to 31.9%), with acetonitrile as the solvent.The oxygenation of cyclohexane gives rise to the formation of alkyl hydroperoxide as the intermediate product, that is further decomposed to yield stable products, cyclohexanol and cyclohexanone. 91nO-supported Ag nanoparticles efficiently catalyzed the oxidation reactions of of benzylic C-H bonds in alkyl-substituted arenes with H2O2. 92Selective oxidation of ethylbenzene using tert-butyl hydroperoxide and H2O2 in the presence of mixed oxide spinels, derived from layered double hydroxides, gave acetophenone.The activity of the catalyst increased in the order MgCuMnAl > MgCoMnAl >> MgMnAl > MgCrAl > CoMnAl oxides. 93The catalytic activity of SBA-15 supported Co3O4, was evaluated in liquid phase benzylic oxidation with hydrogen peroxide under mild conditions.The benzylic oxidation was ascribed to hydrogen abstraction of the C-H bond.The 5 wt% of catalyst was found to exhibit high conversion (75%), with product selectivity of 87%. 94The SiO2-included cesium polyoxotungstate and titanosilicate were also reported as efficient catalysts for oxidation of cyclohexane to cyclohexanol and cyclohexanone under mild conditions in liquid phase. 95itanium silicalite with hollow structure (HTS), is a new type of titanosilicate used in oxidation process.Its catalytic property on the mild oxidation of cyclohexane with hydrogen peroxide (H2O2) in liquid phase was investigated, and the difference of catalytic performance between HTS and compared sample (TS-1) discussed.Cyclohexanone and cyclohexanol were the major products. 96

Oxidation of arenes
Most recently published works on the synthetic application of arenes with hydrogen peroxide concern the reactions catalyzed by iron, copper and vanadium compounds.The FeCl3 and mesoporous carbon nitride hybrid (FeCl3/mpg-C3N4) was reported as an active and selective photocatalyst to activate H2O2 for the oxidation of benzene to phenol under visible-light illumination.By fine-optimizing of the FeCl3 loading amount in catalyst and reaction conditions, the one-step process achieved a 38% benzene conversion with high 97% selectivity for phenol.The excellent catalytic performance of FeCl3/mpg-C3N4 should be attributed to the fast reduction of Fe 3+ to Fe 2+ by photo-irradiated electrons from mpg-C3N4.A plausible reaction path for the oxidation of benzene by the FeCl3/mpg-C3N4 system is illustrated in Scheme 12.The visible-light-induced catalytic cycle is assumed to involve two coupled steps: (1) the light-irradiated electrons from the CB of FeCl3/mpg-C3N4 reduce Fe 3+ to Fe 2+ , which could smoothly decompose H2O2 with the formation of active ·OH radical; (2) the hydroxylation of benzene by ·OH radical, produces a cyclohexadienyl radical intermediate A and then the positive hole of mpg-C3N4 (h + ) with moderate oxidation potential oxidizes A to yield phenol as the product.This environmental-friendly and efficient method could be a new avenue for one-step phenol preparation. 97ther iron-containing hybrid materials, such as Fe-mpg-C3N4 (denoted as Fe-CN) and titanium silicate zeolite (TS-1), could be used as heterogeneous catalysts for the photocatalytic selective oxidation of benzene to phenol using H2O2, as benign oxidant under ambient conditions.It was found, that Fe-CN/TS-1 shows superior photocatalytic performance for phenol production from benzene to Fe-CN and TS-1 catalysts alone.Under the optimal conditions, up to 10% phenol yield (based on benzene) is achieved over the hybrid materials, with 18.4% phenol selectivity (based on H2O2). 98Highly selective benzene hydroxylation to phenol can be achieved over two Fe-based metal-organic frameworks [MIL-100(Fe) and MIL-68(Fe)] under visible light irradiations using H2O2 as an oxidant.An optimal benzene conversion of 30.6% was achieved with a H2O2:benzene ratio of 3:4 over MIL-100(Fe) after irradiation for 24 h. 99In another heterogenous photo-Fenton system, with ferrocene-modified carbon nitride as the synergistic donor-acceptor, interaction between the carbon nitride matrix and ferrocene group, also improved exciton splitting, and coupled photocatalytic performance allowed the direct synthesis of phenol from benzene in the presence of H2O2 under visible light irradiation. 100he promotional effect of nanosized Fe ( also Ru, Au, and Mn) particles on VOHPO4•0.5H2O(VHP) catalytic properties, was investigated in benzene hydroxylation with H2O2.Catalytic results indicated a profound effect of the nanoparticle dopants on VHP catalyst activity and product distribution.Among the promoted VHP catalysts, Au/VHP exhibited a high catalytic effect with benzene conversion of 76% at a combined 85.5% selectivity, toward the formation of phenol and hydroquinone achieved in six hours, under optimized reaction conditions. 101Among different metal-doped graphitic carbon nitride catalysts (Cu-, Fe-, V-, Co-, and Ni-mpg-C3N4), the vanadium-doped graphitic carbon nitride catalyst V-mpgg-C3N4 was found to be the most efficient. 102A bio-inspired oxo-bridged diiron(III) complex ([(bpmen)2Fe2O(μ-O)(μ-OH)](ClO4)3 , bpmen = N,N'dimethyl-N,N'-bis(2-pyridylmethyl)-1,2-diaminoethane) also showed promising activity towards aromatic hydroxylation, with H2O2 and acetic acid. 103yridine(Py)-modified vanadium substituted heteropoly acid (Py3PMo11V) with Keggin structure was applied for direct oxidation of benzene to phenol.Conventional heating gave a phenol yield of 7.8%, whereas a higher phenol yield of 24.7% and selectivity of 100% was obtained when irradiated with microwave energy. 104Direct hydroxylation of benzene to phenol, using H2O2 as an oxidant over vanadium-containing mesoporous carbon catalysts, or molybdovanadophosphoric anion ionic liquid as a reusable catalysts for benzene oxidation to phenol by H2O2, has also been reported. 105,106he catalytic performance of a γ-Keggin divanadium-substituted phosphotungstate, (Bu4N)4-[γ-PW10O38V2(μ-O)(μ-OH)], has been evaluated in a selective oxidation of 1,2,4-trimethylbenzene (pseudocumene) 14 and 2-methylnaphthalene 15 with the 35% aq.hydrogen peroxide (Scheme 13).Under conditions of H2O2 deficiency (14/[H2O2] = 17-22), oxidation of pseudocumene proceeded, with unusually high chemoselectivity and regioselectivity, producing exclusively 2,4,5-trimethylphenol 16 and 2,3,5trimethylphenol 17 in a molar ratio of 7.3/1 and a yield of 73%, based on the oxidant.Isomeric 2,3,6trimethylphenol was found in trace amounts.Under conditions of H2O2 excess ([H2O2]/14 = 8), 2,3,5-trimethyl-1,4-benzoquinone (vitamin E key intermediate) formed with 41% selectivity at 41% substrate conversion.A typical regioselectivity was also found in the oxidation of 2-methylnaphthalene 15, which gave predominantly 6-methyl-1,4-naphthoquinone 17 rather, than isomeric 18.The ratio between the isomers could be altered by varying the catalyst and oxidant amounts.The alternative method for synthesis of 2-methyl-1,4-naphthoquinone, is liquid phase oxidation of 2methylnaphthalene with H2O2 carried out under mild reaction conditions, over lanthanum doped MCM-41 in acetic acid as solvent.The catalyst exhibited very high substrate conversion (95.8%) and reasonable product (2-methyl-1,4-naphthoquinone) selectivity (69.3%).The catalyst can be reused twice, without losing its activity to a great extent. 108Using aqueous hydrogen peroxide and palladium(II) chloride for oxidation of differently substituted benzene rings of 2-phenylpyridines, lead to phenols in up to 74% yield. 109he direct oxidations of benzene to phenol and anthracenes to anthraquinones with H2O2 were achieved with Tp*Cu(NCMe)[Tp* = hydrotris(3,5-dimethylpyrazolylborate)] complexes as the catalysts. 110The copper(II) complexes with the bidentate ligand bis(1,4,5-trimethyl-2-imidazolyl)methane (Me6bim), [Cu(Me6bim)X2] (X = Cl, Br), exhibit catalytic activity in oxidation of benzene to phenol and 1,4-benzoquinone the presence of excess H2O2 at room temperature in MeCN/H2O solution. 111Catalytic activity of copper(II) 1,3,5benzenetricarboxylate in the catalytic oxidation of benzene to phenol and hydroquinone, with hydrogen peroxide as an oxidant was reported.The corresponding oxygenates had a yield of 36.5%, and the selectivity to phenol and hydroquinone was 53.2% and 35.5%, respectively.The turnover frequency (TOF) was 35.1 h -1 .Moreover, water treatment protected the catalyst from decomposition due to formation of a new oxidation mode. 112Selective hydroxylation of benzene to phenol has been achieved in 21% yield using H2O2 in the presence of a catalytic amamount of the nickel complex [Ni II (tepa)] 2+ (tepa = tris[2-(pyridin-2-yl)ethyl]amine) at 60 °C. 113The selective hydroxylation of benzene to phenol was also achieved by using of [(tpa)Mn II ] 2+ (tpa = tris(2-pyridylmethyl)amine) incorporated in mesoporous silica-alumina as the catalyst support, which prohibits further oxidation of phenol due to the strongly acidic sites. 114

Oxidation of alkenes
Derivatives of alkenes are one of the most important chemical products and they can be obtained by using oxidation or hydrolysis methods.Catalytic oxidation has the advantage over non-catalytic oxidation methods, in order to improve yields and selectivity and decrease environmental pollution.Oxidation of alkenes 20, depending on the substrate and the reaction conditions, may result in the various products illustrated in Scheme 14, such as epoxides 21, αhydroxyalkenes 22 and diols 23, which may be subsequently oxidized to α-hydroxyketones 24, diketones 25 or can be cleaved into aldehydes 26, ketones 27 or carboxylic acids 28.During recent years, significant progress was been achieved in the oxidation of alkenes with hydrogen peroxide in the presence of titanium, manganese, copper, rhenium, iron, silver, cobalt, molybdenum, palladium and niobium compounds and complexes, carried out in liquid phase.][118][119] Asymmetric epoxidation of alkenes using hydrogen peroxide as oxidant, has been a very active research field and has been investigated by many research groups.The most recent progress has been reviewed. 1203.1 Epoxidation.The titanium(salalen) complex has been known as an effective catalyst for asymmetric epoxidation of enol esters.Although (E)-enol esters were reluctant to proceed, (Z)-enol esters underwent asymmetric epoxidation with H202 to give the epoxides in high yields, with high enantioselectivity ranging from 86 to 99% ee.Complete enantioselectivity was observed in the reaction of (Z)-3,3-dimethylbut-1-en-1-yl-4-methoxybenzoate.The obtained epoxide was readily transformed into the corresponding 1,2-diol, by reduction with lithium borohydride, without erosion of the high enantiomeric excess.121,122 Bryliakov and co-workers systematically studied the influence of the ligand structure on the catalytic activity and selectivity, by surveying the performance of a series of chiral salan ligands, with varying steric and electronic properties in the asymmetric epoxidation of aromatic epoxides.The results revealed, that the electronic effect controls the catalytic activity, whereas the steric effect accounts for the facial selectivity of the epoxidation.123 Recently, it was reported that Ti-salan complex is able to catalyze distal selective epoxidation of conjugated dienes, even in the presence of other olefins and adjacent stereocenters.In this context, a variety of wellestablished methods for asymmetric epoxidation were surveyed for the regioselective epoxidation of conjugated dienes.The reaction, using Ti-salan complex or its enantiomer as catalyst, furnished the products, with complete regioselectivities in favor of the Z-or trisubstituted olefins over E-olefins.Notably, these reactions also proceeded with high diastereo-or enantio-selectivities.124 The situation becomes more complicated when the oxidized molecule contains more active centers.An example is catalyzed oxidation of limonene 29 by titanosilicates microporous TS-1 and mesoporous Ti-SBA-15, since the substrate has both an exocyclic and an endocyclic bond.Scheme 15 showa the different possible ways of transformation, not only in the direction of 1,2-epoxylimonene and its corresponding diol, but also in the direction of other products.When TS-1 was a catalyst in this process, as a result of the electrophilic attack on the double bond (position 1-2), 1,2epoxylimonene is formed, almost exclusively at 0 o C. 1,2-Epoxylimonene can undergo further hydration of the epoxide ring, and 1,2-dihydroxylimonene is formed.The next product of this process is carveol, formed as a result of the cyclic allylic hydrogen abstraction (allylic oxidation-hydroxylation at the 6-position).As a result of the acyclic allylic hydrogen abstraction (allylic oxidation-hydroxylation at the 7-position), perillyl alcohol is formed.Carvone is also observed among the products of this process,.This compound is formed as a result of oxidative dehydrogenation of carveol.Although, at 0 o C 1,2-epoxylimonene is a main product, at higher temperatures (40 °C, 80 °C and 120 °C) allylic oxidation products predominate over other products, mainly carveol resulteing from cyclic allylic hydrogen abstraction.The other product is perillyl alcohol, which is obtained as a result of acyclic allylic hydrogen abstraction.This direction of the reaction over the TS-1 catalyst to carveol and perillyl alcohol, can be explained on the basis of the mechanism of allylic oxidation.The evidence shows that the microporous TS-1 is an active catalyst of limonene epoxidation but higher functions describing the process were obtained for the mesoporous Ti-SBA-15.Taking into account the size and the structure of the transition states for the formation of perillyl alcohol, it is proposed that its formation probably proceeds inside the narrow pores of the TS-1 catalyst.This is possible because the cyclohexene ring is flexible and it can adopt a very narrow configuration and can enter the pores.The narrow size of the pores ensures that further transformation of perillyl alcohol into the corresponding aldehyde does not occur. For theaction of Ti-SBA-15 it was assumed that the reaction of allylic oxidation and oxidative dehydrogenation happen inside the wide pores, as well as on the outside surface of the catalyst.125 The oxidation of limonene 29 to 1,2-epoxy-1,2-dihydrolimonene and 1,2-dihydrolimonene-1,2-diol, carveol, carvone and perillyl alcohol with hydrogen peroxide and tert-Bu hydroperoxide (TBHP) in the presence of :TS-1, TS-2, Ti-Beta, Ti-MCM-41 Ti-MWW, Fe/EuroPh catalysts were also studied.126 For epoxidation of alkenes and their derivatives, other titanium catalysts, such as Ti-YNU-1, that contains 12-membered ring interlayer pores, 129 titanosilicate MFI (a three-letter structural code among ∼200 zeolite framework codes) nanosheets of single-unit-cell thickness, 130 Ti substituted phosphotungstate [DMIM]5PTiW11O40, 131 Ti-SBA-15, 132 and layered and pillared TS-1 catalysts, were also applied.133 Methyltrioxorhenium (CH3ReO3, MTO) in the presence of H2O2 has proven itself as an efficient and versatile oxidation catalyst, with interesting selectivity towards natural compounds, which can be oxidized under quite mild conditions.The active species, involved in the oxygen transfer to the olefinic double bond, are probably a monoperoxo complex [MeRe(O)2(O2)] and a diperoxo complex [MeRe(O)(O2)2], obtained respectively by the addition of one or two H2O2 molecules to MTO.However, depending on the nature of the epoxide, a ring opening catalyzed by the Re(VII) metal center, can also occur to give 1,2-diols.Moreover, the epoxide ring opening, can be minimized by employing pyridine as a basic ligand.Epoxidation reactions of some natural compounds by MTO/H2O2/pyridine, lead to products of practical interest as synthons in the synthesis of fine chemicals, with the aim of assessing the parameters controlling the process.Examples are epoxidation of some natural products, having C=C bonds, of the same or different reactivity (Scheme 16).134 Using 1-octene as a model system, under optimized conditions, a high yield and good chemoselectivity (>90 %) of epoxidation were achieved.In applications to other aromatic and aliphatic olefins, similar chemoselectivities and up to 19% enantioselectivity were obtained.Schiff bases coordinate smoothly with methyltrioxorhenium (MTO), to yield the corresponding MTO-Schiff base adducts, which were used as catalysts for cyclooctene epoxidation in ionic liquids.While the urea hydrogen peroxide adduct (UHP) was used as an oxidant at room temperature, no diol byproduct formation was observed.136 MTO, (2,6-dimethylphenyl)trioxorhenium (XTO) and benzimidazole complexes of methyltrioxorhenium were successfully employed for the epoxidation of various alkenes with H2O2, UHP or t-butyl hydroperoxide.[137][138][139] The ionic liquids were used as additives in the MTO catalyzed epoxidation and it was found that they performed well in suppressing the epoxide ring-opening reaction.140 A green solvent such as diethyl carbonate was used in epoxidation reactions catalysed by oxorhenium(V) complexes [ReOCl2(PPh3)(L)] and [ReOCl(L)2], with L being monoanionic bidentate phenolate-pyrazole ligands, albeit resulting in lower yields (up to 30%).141 A new manganese(III) complex [(N-O)2Mn(OAc)] was synthesized using 2-(2-hydroxyphenyl)-5,6-dihydro-1,3-oxazine (N-O) as a bidentate O,N-donor.Epoxidation of alkenes to epoxides, as well as oxidation of sulfides to sulfoxides, using UHP as oxidant, resulted in high yields and selectivities.142 Manganese(II) sulfate in the presence of polyamine was reported as an efficient catalyst for epoxidation of styrene with hydrogen peroxide in 78% yield. 143Organic carbonates, e.g.dimethyl carbonate and propylene carbonate, were used as reaction media in enantioselective epoxidation of non-functionalized alkenes, by using a series of chiral macrocyclic Mn(III) salen complexes as catalysts, with pyridine N-oxide as an axial base.This protocol worked effectively with UHP as oxidant, to give epoxides in high yields and ee (up to >91%).144 The complex, [Mn(saldien)(N3)], anchored on mesoporous SBA-15 produced a stable, active, and selective heterogeneous catalyst SBA15-[Mn(saldien)(N 3 )] (saldien = N,N'-bis(salicylidene)diethylenetriamine), used for oxidation of cyclohexene (as well as cycloalkanes) in acetonitrile, with H2O2 as terminal oxidant.Cyclohexene was converted in 38% yield to epoxide (90%), accompanied by small amounts of enol (7%) and diol (3%).145 Very recently, first example of efficient catalytic oxidation of alkenes and alcohols by a manganese(V) nitrido complex (PPh4)2[Mn(N)(CN)4] using H2O2 as an oxidant, was reported.Epoxidation of various alkenes was investigated in the presence and absence of acetic acid.In the absence of acetic acid, 100% conversion could be achieved for electron-rich alkenes, with excellent yields of epoxide products.On the other hand, for terminal alkenes such as 1-octene, the reaction rate and yield were lower.The epoxidation of cis-2-heptene, cis-β-methylstyrene, and trans-β-methylstyrene, were highly stereospecific; 100% yields of the corresponding epoxides could be obtained and no epimerized products could be detected.The rate of alkene epoxidation is greatly enhanced by acetic acid.When di-substituted and tri-substituted alkenes were used as substrates, the reactions were complete within five minutes.Longer reaction times, up to one hour, were required for the epoxidation of terminal alkenes.146 Polyoxomolybdates, generated in situ by treating a carboxylic acid-functionalized ionic liquid with aqueous sodium molybdate, showed a good performance in the epoxidation reaction (e.g.cyclooctene) and can be reused several times without significant loss of activity.147 An efficient and highly selective heterogenous catalyst was developed by immobilization of a manganese complex on an inorganic support.Mn(II) has been anchored on the surface of functionalized silica by means of N,O-coordination to the covalently Si-O bound, modified salicylaldiminato Schiff base ligand.Cycloalkenes and linear alkenes were oxidized efficiently to their corresponding epoxides with 83-98% conversion and 87-100% selectivity, in the presence of this catalyst.Furthermore, this material was shown to be very active catalyst in clean epoxidation reactions using combined oxidant of aqueous hydrogen peroxide and acetonitrile in the presence of aqueous NaHCO3.The obtained results showed that this catalytic system was a robust and stable heterogenous catalyst which can be recovered quantitatively by simple filtration and reused multiple times without loss of its activity.148 These same authors introduced a new catalytic system, based on an immobilized imidazole-manganese-porphyrin (Mn(Porph)) for the oxidation of olefins.Merrifield resin (MR) and functionalized silica gel (SG) were chosen as supports. Theesults indicate, that the Mn(Porph)-MR system shows high reaction rates, high efficiency with hydrogen peroxide as oxidant and good recyclability up to four times, without a dramatic loss in the catalytic efficiency.149 In another article, they also presented a manganese-substituted polyoxomolybdate tetrabutylammonium (TBA) salt TBA4[PMo11Mn(H2O)O39]•2H2O, which was used as the catalyst in the homogeneous oxidation of cis-cyclooctene, cyclohexene, styrene and geraniol with H2O2 in acetonitrile.Oxidation of cis-cyclooctene and geraniol gave only epoxides; cyclohexane-1,2-diol was the main product for cyclohexene; styrene was preferentially oxidized to benzaldehyde.150 The potassium salt of PW11Zn and the new composite PW11Zn-APTES@SiO2, were shown to be versatile catalysts for the oxidation of alkenes using H2O2 as the oxidant and acetonitrile as the solvent.Cyclohexene was oxidized to epoxide, geraniol to 2,3-epoxygeraniol while styrene gave benzaldehyde accompanied with 2hydroxy-1-phenylethanone and benzoic acid as minor products.151 Iron catalysts, bearing tridentate non-heme ligands for use in the epoxidation of alkenes with hydrogen peroxide as the oxidant, were also developed.152 Nb-SiO2 catalysts displayed an excellent performance in the epoxidation of limonene.153 The synthesis and characterization of new V2O5/Al2O3-MgO catalysts and their application in oxidative dehydrogenation and epoxidation reactions, were reported.154 Oxovanadium(IV) complex (VOL-Y) of a Schiff base ligand, derived from 2,4-dihydroxyacetophenone and 2,2'-dimethylpropanediamine, was encapsulated in the nanopores of zeolite-Y.The encapsulated complex catalyzed the oxidation of various olefins, using hydrogen peroxide in good yield.Under the optimized reaction conditions, in the presence of VOL-Y, 86% conversion of cyclooctene with 100% selectivity for epoxide was observed.155 New mononuclear oxovanadium(IV) complexes [VO(acac)(R-BIAN)]Cl (BIAN = 1,2-bis{(R-phenyl)imino}acenaphthene) were successfully used for the catalytic oxidation of alkenes such as cyclohexene, cis-cyclooctene, and styrene, with both tert-butylhydroperoxide or H2O2 as the oxidant.156 Alkylaminophosphazenes (RPN) are efficient and tuneable phase-transfer agents for environmentally benign biphasic oxidation with hydrogen peroxide, catalyzed by polyoxometalates (POMs).Substituent groups on the phosphazene ring, can be varied to modify the phase-transfer efficiency of RPN for use in water-organic two-phase systems.Active phosphazene-polyoxometalate aggregates that self-assemble in situ from RPN and Keggin POMs, such as PMo12O40 3-and PW12O40 3-, were found to be efficient and recyclable catalysts for the epoxidation.157 The Keggin and Wells-Dawson sandwich-type POMs, with various transition metals in the central belts, proved to be active catalysts for the oxidation of cyclooctene and cyclohexene with H2O2.Under the reaction conditions, Wells-Dawson-type POMs were less active than Keggin ones.Zn-and Fe-substituted POMs, of both types, gave higher conversions.With tetra-n-butylammonium salts of [Zn4(PW9O34)2] 10− , [Fe4(PW9O34)2] 6− , [Zn4(P2W15O56)2] 16− and [Fe4(P2W15O56)2] 16− , high to excellent conversions of cyclooctene and cyclohexene were obtained.The main products were epoxides and the minor ones α-alcohols or α-ketones.158 Over the past few decades the use of peroxotungstates as catalysts for epoxidation with H2O2 has attracted much attention as a result of their high capability for oxygen transfer and low activity for disproportion of H2O2.Recently, the first tungsten-catalyzed asymmetric epoxidation of allylic and homoallylic alcohols was developed (Scheme 17).With catalysis by the W-bishydroxamic acid (BHA) system, primary, secondary, and tertiary allylic, as well as homoallylic alcohols, were successfully employed as precursors in asymmetric epoxidation reactions to furnish chiral products, generally with excellent enantioselectivities. Notably, the reactions were performed under air, in most cases at room temperature, and requiring no anhydrous solvent or preparation of the metal-complex catalyst prior to the catalytic process.Moreover, this method demonstrates good chemoselectivity for primary alcohols over secondary and tertiary alcohols.For instance, two farnesol derivatives bearing three olefin and two alcohol moieties were reacted as precursors of the epoxidation reaction; they provided the corresponding products with almost complete regioselectivity in favor of oxidation of the C=C double bond which is closer to the primary alcohol.This method seems very promising for use in the late stages of synthesis of complex molecules.159
Four new molybdenum and tungsten complexes bearing tetradentate tripodal amino bisphenolate ligands with either hydroxyethylene or hydroxyglycolene substituents have been described and tested as catalysts in the epoxidation of cyclooctene eco-friendly conditions, using an aqueous solution of H2O2 as the oxidant and dimethyl carbonate (DMC) as solvent or neat conditions as substitute for chlorinated solvents.Molybdenum complexes showed good catalytic activity using H2O2 without added solvent, and tungsten complexes showed very high activity. 160Composites of [γ-H2PV2W10O40] 3− and [α-SiW12O40] 4− supported on Fe2O3 acted as effective and reusable heterogeneous catalysts for selective oxidation with aqueous hydrogen peroxide.In the presence of PV2-SiW12/Fe2O3, various kinds of organic substrates such as alkenes, sulfides, arenes, and alkanes were selectively converted into the corresponding oxygenated products in moderate to high yields.For example 1-octene gave epoxide in 85% preparative yield and cyclooctene was oxidized to epoxide in 89% yield while selectivity was above 99% in both reactions.The catalysts could be reused at least three times without appreciable loss of their catalytic performance. 161The catalytic behavior in olefin epoxidation of a one-dimensional tungsten oxide/2,2'-bipyridine hybrid material was also recently reported. 162ron-catalyzed asymmetric epoxidation is attractive owing to the availability, low cost, and low toxicity of iron salts.Generally, the iron catalysts for asymmetric epoxidation can be classified into two categories: porphyrin-and non-porphyrin iron complexes.The oxidant used in these catalytic systems has generally been iodosyl benzene.In 2012, Simonneux and his co-workers developed a series of chiral iron-porphyrins, which were able to promote asymmetric epoxidation using hydrogen peroxide as oxidant.Notably, the sulfonated Halterman iron-porphyrin is water soluble and thus under its catalysis the epoxidation can be conducted under aqueous conditions.However, the substrate scope of these reactions is narrow and includes only styrene derivatives and 1,2-dihydronaphthalene.Furthermore, the products are afforded mostly with only moderately good enantioselectivities. 163,164 Tetradentate nitrogen-containing ligands, which have electronic properties that resemble those of porphyrin, also proved to be efficient catalysts for the Fe-catalyzed asymmetric epoxidation of olefins.For instance, a chiral Fe-aminopyridine catalyst was developed, which was capable of promoting highly enantioselective epoxidation of diverse olefins with aqueous hydrogen peroxide as oxidant.Furthermore, the authors also discovered that the enantioselectivity of this process increased with growing steric demand of acidic additives.A plausible mechanism was also proposed. 165The catalytic behaviour of the Fe-aminopyridine complex was significantly improved by introducing a dimethylamino moiety to the pyridine rings.The addition of a catalytic amount of a carboxylic acid as cocatalyst, improved both yields and enantioselectivities of the epoxidation reaction through synergistical cooperation with the iron complex leading to more efficient O−O bond cleavage and the formation of epoxidizing species.After careful screening of a variety of carboxylic acids, Ibuprofen turned out to be the best coligand.This new epoxidation method demonstrates a broad substrate scope including various enones and cis-β-substituted styrenes. 166ecently, amino acids were employed as synergistic coligands for the Fe-aminopyridine-complex-catalyzed epoxidation of α-substituted styrenes, which are challenging substrates for asymmetric epoxidation.Generally, under optimum conditions, the epoxide products were obtained in high enantiomeric excess.The authors' new observations revealed that the substrate spectrum of a catalytic system can be simply expanded through variation of coligands instead of onerous development of novel ligands. 167Sun et al. developed a Feaminobenzimidazole complex, which proved to be an efficient catalyst for H2O2-mediated enantioselective epoxidation of chalcones, furnishing the products in most cases with high enantioselectivities. 168 Mn-and Fecatalyzed asymmetric epoxidations are similar in terms of the ligands employed, the substrate scope, and the reaction mechanism.The use of hydrogen peroxide as oxidant in Mn-catalyzed epoxidation was reviewed recently by Krishnan et al. 169 Being similar to iron, the manganese-porphyrin complexes were found to be able to catalyze the asymmetric epoxidation of unfunctionalized olefins with aqueous hydrogen peroxide as oxidant but the substrate scope was relatively narrow and the level of the asymmetric induction was only moderate. 170In 2011, Bryliakov et al. discovered that a Mn complex with a tetradentate aminopyridine ligand featuring a cyclohexanediamine scaffold was an efficient catalyst for the enantioselective epoxidation of olefins using aqueous hydrogen peroxide as the terminal oxidant and a large excess amount of acetic acid as additive, although the products were obtained in lower enantioselectivities.The same research group also developed a chiral tetradentate aminopyridine ligand based on a bis-pyrrolidine structure, which turned out to be a good ligand for the Mn-catalyzed H2O2-mediated asymmetric epoxidation of electron-deficient olefins, thereby yielding the products in moderate-to-good enantioselectivities. Furthermore, the authors also observed that the use of a sterically demanding carboxylic acid instead of acetic acid as additive improved the enantioselectivities of this Mn-catalyzed epoxidation reaction. 171Later, Costas et al. also reported a chiral bispyrrolidine-based ligand bearing modified pyridine rings as ligand arms for the Mn-catalyzed asymmetric epoxidation.In this case, the products were provided in high yields albeit with low to moderate enantioselectivities. 172 In 2012, Sun et al. described a Mn-catalyzed asymmetric epoxidation by employing a tetradentate N-ligand containing chiral bis-pyrrolidine and benzimidazole moieties.Under the optimum reaction conditions, high to excellent enantioselectivities were achieved when using chalcones and cisdisubstituted olefins as substrates.In contrast, relatively poor facial control was observed in the cases of terminal, cis-disubstituted olefins, and α,β-unsaturated esters. 173It was discovered independently that introduction of electron-donating substituents to the pyridine rings of the aminopyridine ligands led to the enhancement of both the efficiency and enantioselectivities of the Mn-catalyzed epoxidation with hydrogen peroxide as terminal oxidant.In the cases of α,β-unsaturated carbonyl compounds and chromenes as substrates the products were obtained with high enantioselectivities.Furthermore, study of the use of the Mn complex for the diastereoselective epoxidation of unsaturated steroids revealed that the formation of βepoxides was favored when using the Mn catalysts. 174,175In 2013, Gao et al. developed a porphyrin-inspired chiral Mn-amino-oxazoline complex and applied it successfully in the asymmetric epoxidation of a variety of olefins using aqueous hydrogen peroxide as the terminal oxidant.In the cases of trans-stilbene and cyclic olefins such as indene, dihydronaphthalenes, as well as chromenes as substrates, the products were furnished in good to excellent enantiomeric excess.Moreover, the authors also observed an increase in enantioselectivity when the reactions were conducted in the presence of an acidic additive. 176,177New chiral manganese complexes of N4 ligands derived from 2-acetylpyridine were prepared and used as catalysts in the enantioselective epoxidation of olefins, using H2O2 as an oxidant to give epoxides, with excellent conversions (up to 99%) and enantiomeric excess (up to 88%) within 1 h at 0 °C. 178he Mn 1,4,7-trimethyl-1,4,7-triazacyclononane dimers were activated by carboxylate-containing solids for cyclooctene epoxidation/dihydroxylation with H2O2 at 0 °C.Dihydroxylation selectivities are a function of the carboxylate employed as well as the nature of the other groups on the solid cocatalyst surface.SiO2 grafted with propionate groups gave ∼50% cis-diol selectivity, but further modification with alkyl or perfluoroalkyl silanes increased cis-diol selectivity up to ∼60%.Dihydroxylation selectivity is also ∼60% for SiO2 grafted with benzoate or ∼70% for terephthalic acid chemisorbed on TiO2. 179It has been demonstrated that chiral manganese aminopyridine complexes [LMn II (OTf)2] are able to epoxidize olefins with excellent enantioselectivities (up to 99% ee, several examples) with various terminal oxidants (H2O2, alkyl hydroperoxides, peroxyacids, and iodosylarenes).The mechanisms of enantioselective olefin epoxidations on aminopyridine manganese(II) complexes [LMn II (OTf)2] with different oxidants was also considered. 180he catalytic property of phosphomolybdic acid (PMA) functionalized knitting aryl network polymer (KAP) was investigated for the selective oxidation of cyclooctene and other olefins with H2O2 as oxidant.When ethyl acetate was used as reaction medium, the catalyst gave a 78.6% conversion of cyclooctene with nearly 100% selectivity for epoxide.Moreover, this catalyst shows high activity and selectivity for epoxide formation for a variety of olefins, and it can be reused for several times without obvious loss of activity.Also, the PMA/KAP catalytical system can even oxidize the relative inert terminal olefins of 1-hexene and 1-octene to their corresponding epoxides. 181old-nanoparticle-containing mesoporous monoliths were subsequently investigated in flow reactors for the oxidation of cyclohexene to a mixture of oxygenates: cyclohexene oxide (56.7%), 2-cyclohexen-1-one (22.7%) and 2-cyclohexen-1-ol (20.6%).The conversion of cyclohexene was low (2.7%), and in some experiments tert-butyl hydroperoxide was a better oxidant than hydrogen peroxide. 182

Hydroxylation, aminohydroxylation and aminochlorination.
Current methodology for the catalytic cisdihydroxylation of alkenes with preparative significance in organic synthesis is mainly focused on osmium and ruthenium compounds.For the synthesis of the 1,2-diols, cis-1,2-dihydroxylation of alkenes catalyzed by osmium(VIII) tetroxide (OsO4) is a powerful method.However, OsO4 is quite toxic due to its highly volatile and sublimable nature.Thus, the development of alternative catalysts for cis-1,2-dihydroxylation of alkenes has been highly challenging.A simple protocol that uses a nitrogen-based tetradentate ligand, tris(2-pyridylmethyl)amine (tpa), for an osmium center and [Os III (OH)(H2O)(L-N4Me2)](PF6)2 ( L-N4Me2 = N,N'-dimethyl-2,11-diaza [3.3](2,6)pyridinophane) as catalysts and H2O2 as terminal oxidant for cis-1,2-dihydroxylation of alkenes was presented.Unfunctionalized (or aliphatic) alkenes and alkenes/styrenes containing electron-withdrawing groups were selectively oxidized to the corresponding vicinal diols in good to excellent yields (46-99%).For the dihydroxylation of cyclohexene, the catalyst amounts can be reduced to 0.01 mol% to achieve a very high turnover no. of 5500.The active oxidant is hydroperoxide identified as the Os V (O)(OH) species, which is formed via the hydroperoxide adduct, an Os III (OOH) species. 183 For the first time, methyltrioxorhenium (MTO) has been applied as a catalyst for the dihydroxylation of functionalized allenes 30 in the presence of hydrogen peroxide as the oxidant.The regioselectivities turn out to be well controlled, affording β-carbonyl-γ-hydroxyl diphenyl phosphine oxides 31 as the only product (Scheme 18).The axial chirality of optically active allenes can also be nicely transferred to the chirality center of the products.A possible mechanism, proceeding via regioselective epoxidation of the electron-rich carboncarbon double bond, a subsequent intermolecular nucleophilic attack of a water molecule on the in situ formed epoxide via neighboring group participation (NGP), followed by a rearrangement has been proposed as the major reaction pathway. 185ccordingly, the development of alternative catalysts for the 1,2-dihydroxylation of alkenes with hydrogen peroxide catalysed by iron complexes has been recently reported.It has been seen that the most effective non-heme iron catalysts are those possessing nitrogen-based tripodal or linear tetradentate ligands, such that two cis labile sites available for coordination with exogenous ligands become available.In an investigation of the dihydroxylation of alkenes, the iron complexes Fe(bpmen)(OTf)2 (bpmen: bispicolyl-N,Nmethylethylenendiamine) and Fe(tpa)(OTf)2 (tpa: tripicolylamine.OTf: trifluoromethanesulfonate) were applied as the catalysts.A broad set of substrates was oxidized and in most cases the reaction was regio-and stereo-selective.The example shown in Scheme 19 is trans-dihydroxylation of (Z)-jasmone 32 to diol 33. 186ron(III) phthalocyanine complexes were employed for the first time as mild and efficient Lewis acid catalysts in the selective oxidation of cyclohexene to cyclohexane-1,2-diol.It was found that the catalyst FePcOTf shows excellent conversion and moderate selectivity relative to other iron(III) phthalocyanine complexes. 187Simple Fe(NO3)3•9H2O was demonstrated to be able to catalyze the oxidation of monoterpenes by hydrogen peroxide in methanol.In these reactions, α-and β-pinene were preferentially oxidized to allylic ethers (i.e., myrtenol methyl ether). 188For oxidation of cyclohexene with hydrogen peroxide the selectivity for α-hydroxylation is favoured and cyclohex-2-en-1-ol is produced when iron(II) and cobalt(II) phthalocyanines are catalysts. 189t was demonstrated that mesoporous ZrO2-POMs composites based phosphotungstate and Tisubstituted phosphotungstates composites exhibited catalytic activity for the oxidation of cyclohexene with H2O2 as oxidant resulting in mixtures of oxidation products mainly cyclohexen-3-one, cyclohexen-3-ol and cyclohexan-1,2-diol. 190 The direct formation of α-alkoxyalcohols from the corresponding olefin through the oxidation of cyclohexene with hydrogen peroxide is catalyzed by commercially avilable molybdenum catalysts.2-Methoxycyclohexanol was obtained with high yields and 99% selectivity by using as catalyst the [C4mim]4[Mo8O26] complex (C4mim = 1-butyl-3-methylimidazolium). 191xidation of terminal alkenes was smoothly catalyzed by a recyclable and environmentally friendly tungsten catalytic system: [(C 18 H 37 ) 2 N(CH 3 ) 2 ] 3 [PW 4 O 16 ]/H 2 O 2 /formic acid.This new catalytic system is not only capable of catalyzing oxidation of terminal alkenes with a phase-transfer character, but also under solventfree conditions, avoiding the use of chlorinated solvents.Many different kinds of terminal alkenes could be converted to the corresponding 1,2-diols of high purity in high yields.The catalyst could be easily separated and reused after reaction. 192llylic C-H bond oxidation, resulted in the formation of unsaturated α-alcohol or α-ketone, was demonstrated on cyclohexene and α-pinene.Oxidation of α-pinene with hydrogen peroxide catalyzed with VO(OAc)2 gave verbenon as the major product (45.9%selectivity at 82.9% conversion) and formation of campholenic aldehyde, myrtenal, verbenol and epoxypinane as byproducts was also observed. 193The αpinene conversion was close to 25%, verbenone being the major product using M41S type nano-structured material with Cu content ca.7% in the reaction of α-pinene oxidation with H2O2.The reaction has a high practical value because verbenone can provide a starting point for the synthesis of taxol, an important therapeutic agent and as an intermediate for manufacture of pesticides and in the flavour industry.Oxidation of cyclohexene leads to cyclohex-2-en-1-ol and cyclohex-2-en-1-one.The reaction path involves a first stage where the interaction Cu-Me(x) catalyst with H2O2 yields ·OH and ·O2H species.Then ·OH attacks the olefin with formation of allylic oxidation products such as cyclohex-2-en-1-ol and cyclohex-2-en-1-one or verbenol and verbenone.It displays preferential attack on the C-H bond over C=C bond and promotes the allylic oxidation pathway while epoxidation is unfavored. 194 palladium diacetate -catalyzed intramolecular aminohydroxylation (Scheme 20) and aminoacetoxylation of alkenes was developed, in which H2O2 was applied as the sole oxidant.A variety of related products could be successfully obtained with good yields and excellent diastereoselectivities, which directly derived from oxidative cleavage of an alkyl C-Pd bond by H2O2.Facile transformation of these products provided a powerful tool for the synthesis of 2-amino-1,3-diols and 3-hydroxy amino acids as well as β-acetoxylated piperidines.Preliminary mechanistic studies revealed that a main nucleophilic attack by water (SN2 type) at the high-valent Pd center contributes to the final C-O(H) bond formation. 195,196Scheme 20.Pd-catalyzed intramolecular aminohydroxylation of alkenes.
A highly selective Pd-catalyzed oxidative intramolecular chloroamination of unactivated alkenes has been developed by using a environmentally benign catalytic system, in which H2O2 was used as oxidant and CaCl2 as chlorine source.A variety of chlorinated piperidine compounds were obtained with good regio-and diastereoselectivities (Scheme 21).A possible catalytic cycle involves Pd(II)-catalyzed trans-aminopalladation of the alkene by nitrogen nucleophilic attack at the termninal 6-endo carbon then generated Pd(II) intermediate undergoes an oxidation by H2O2.Direct reductive elimination from the Pd(IV) intermediate generates the C-Cl bond. 197he effect of 6-methyl-substituted di(2-pyridyl)methanesulfonate (Me-dpms) and di(2-pyridyl)ketone (dpk) ligands on the selectivity oxidation of ethylene with H2O2 in acetic acid solution has been explored.The results indicated that the palladium complexes supported by dpk and Me-dpms ligands are efficient catalysts for selective oxidation of ethylene with 30% aqueous H2O2 to glycol acetates.Glycol acetates could be obtained in up to 90% yield based on H2O2 and with 95% selectivity under mild conditions (1 atm pressure of ethylene, 20 o C), with 1 mol% catalyst loading.By contrast, the reaction catalyzed by Pd(OAc)2 alone, is inefficient in terms of utilization of H2O2 and is unselective, producing mostly vinyl acetate and acetaldehyde in low yield.The most active catalyst, developed in this work, consists of a mixture of palladium acetate and derived dpk and Me-dpms complexes. 198

Oxidation of styrenes.
The particular case of styrene oxidation is of considerable interest for academic research and also for industry due to the important products that can be obtained, such as styrene oxide, benzaldehyde or phenylacetaldehyde.Like the oxidation of other alkenes, the initial step of polyoxymetalate catalyzed oxidation of the C=C bond in styrene 34 and other vinylbenzenes is formation of an epoxide 35 (Scheme 21).This can be selectively obtained by hydrogen peroxide in the presence of phosphomolybdic acid supported on ionic liquid modified MCM-41 199 or cis-dioxomolybdenum(VI) Schiff base complex. 200Oxidation of styrene with H2O2 in presence of NaHCO3 catalyzed by polystyrene (PS), supported PS-[Mo VI O2(hap-iah)(MeOH)], obtained by the condensation of equimolar amounts of orthohydroxy acetophenone (hap) and indole-3-acetic hydrazide (iah), gave mainly styrene oxide as a major product. 201In a report on the catalytic properties of symmetrical and unsymmetrical imidazolium-containing manganese porphyrins in hydrogen peroxide oxidation of styrene the high conversion of the substrate (80-100%) and high selectivity in forming the epoxide (above 80%) was noted. 148Other reports on the epoxidation of styrene have been discussed in Sections 2.1 28,33 and 3.3.1 143,146,150,163,164,166,167,168 In most cases, the unstable oxirane ring of epoxides immediately undergoes subsequent reactions, and various compounds, shown in Scheme 22 are formed as final products.From a study on the oxidation of styrene catalyzed with polyoxometalates (BW11M, PW11M, SiW11M; M = Mn, Co, Fe), it was concluded that styrene oxide 35 and diol 36 are potential intermediates in the formation of benzaldehyde.Benzaldehyde can be formed by C-C cleavage of the diol (after hydrolysis of the epoxide) and by oxirane ring opening by direct action of H2O2, which results in the formation of formaldehyde and benzaldehyde.Formation of the epoxide is described as the formation of an intermediate peroxide or hydroperoxide complex with the catalyst, in which H2O2 is coordinated to a metal centre of the POM.The oxygen transfer from the bonded peroxide to styrene may occur by different mechanisms, namely, through the formation of a 5-membered cyclic intermediate involving a metal centre (M = W, Fe, Mn, Co).Probably this intermediate could also react with a H2O2 molecule, resulting in the direct formation of benzaldehyde.This third pathway for the C=C cleavage is suggested since, the epoxide and the diol may not be responsible for all the benzaldehyde formed.Phenol was also detected in a very small amount.The formation of phenol 39 is well established in the Baeyer-Villiger oxidation of aromatic aldehyde 37 via the corresponding formate ester 38, which may be hydrolyzed to the corresponding phenol. 202As reported by Hulea and Dimitriu, the formation of benzaldehyde (major product) takes place via two different mechanisms.(1) In the presence H2O2 styrene 34 is oxidized to styrene epoxide 35, which transforms into a hydroxyl-hydroperoxy-ethyl benzene intermediate 40 when H2O2 attacks it as a nucleophile.Cleavage of hydroxyl-hydroperoxystyrene produces both benzaldehyde and formaldehyde.(2) Direct oxidative cleavage of the C=C bond of styrene via a radical mechanism, yields both benzaldehyde 37 and formaldehyde.Benzoic acid 41 is one of three minor products, which is expected to form from benzaldehyde 37 through oxidation.Another minor product is 2-hydroxyacetophenone 42 formed from 1-phenylethane-1,2diol 36, which is believed to be produced through hydrolysis of styrene epoxide 35 making use of the H2O present in aqueous 30% H2O2. 203atalysts incorporating tungsten in a silica matrix, composed of isolated [WO4] 2− species, isolated tungsten species or low oligomeric tungsten oxide species and crystalline WO3 species, were tested in the liquid phase oxidation of styrene using H 2 O 2 as oxidant.Benzaldehyde was obtained as the major product with 100% selectivity. 204Vanadyl(IV)acetylacetonate anchored SBA-15 catalyst, was reported as a versatile, recyclable, and reusable solid catalyst for the oxidative cleavage of C=C bonds of styrene and its derivatives to the corresponding carbonyl compounds at up to 38-95% conversion levels with 63-99% selectivity of cleavage products under mild reaction conditions particularly at atmospheric pressure, room temperature, with aqueous H2O2 oxidant. 205The catalytic performances of spinel Mg-Cu ferrite complex oxides such as Mg0.5Cu0.5Fe2O4were evaluated as catalysts in oxidations of styrene in water.Styrene conversion at 22% and 84% selectivity for benzaldehyde was obtained at 80 °C for a nine hour reaction.The catalyst can be magnetically separated and reused without loss of activity. 206The Ti-phenyl@SiO2 with Si/Ti molar ratio of two showed the best catalytic performance for the oxidation of styrene, with 92% conversion and 99% selectivity towards benzaldehyde. 207A porous carbon layer formed from carbon-coated titania (C@TiO2) exhibited high catalytic activity towards hydrogen peroxide oxidation of styrene to a mixture of benzaldehyde and phenylacetaldehyde. 208 Ferrocene and ferrocenyl-modified periodic mesoporous organosilica (PMO) were employed in the catalytic oxidation of styrene at 55 °C using either hydrogen peroxide or tert-butylhydroperoxide as oxidant.The main reaction product was always benzaldehyde, together with other products including styrene oxide, benzoic acid and 2-hydroxyacetophenone.Using a styrene : H2O2 molar ratio of 1 : 5, the highest PhCHO yields after 24 hours were 65% (85% selectivity) for ferrocene and 34% (83% selectivity) for the modified PMO.The modified PMO could be recovered and reused, albeit with a fall in catalytic activity due to partial metal leaching during the first catalytic run. 214A novel visible light photocatalyst of iron(II) bipyridine complex, encapsulated within an NaY zeolite was successfully used as the catalyst for selective oxygenation of styrene with high turnover (1800) and high benzaldehyde selectivity (86%).This photocatalyst was also effective for the oxidation of other olefins, such as 4-cyanostyrene, 4-methoxystyrene, 3-methylstyrene, 1,1diphenylethylene, cis-stilbene, trans-stilbene, trans-β-methylstyrene, α-methylstyrene, triphenylethylene, 1octene, 2-octene, cyclohexene and cyclooctene, to the corresponding aldehydes, ketones or epoxides. 215The selective styrene oxidation to benzaldehyde was also studied over Cr-ZSM-5 zeolite.The effects of various reaction parameters such as reaction time, catalyst mass, styrene/hydrogen peroxide molar ratio, solvent nature and reaction temperature were evaluated. 216mong oxidovanadium(IV)-exchanged zeolites-Y, the most active catalyst was the zeolite abbreviated as [V IV O(pydx-en)]-Y ].Oxidations of styrene, cyclohexene and methyl phenyl sulfide were investigated using these complexes as catalysts.Under the optimized reaction conditions for [V IV O(pydx-en)]-Y ] a maximum of 85.5% conversion of styrene was obtained in six hours of reaction time.The selectivity of the various products followed the order: benzaldehyde > 1-phenylethane-1,2-diol > benzoic acid > phenyl acetaldehyde.A maximum conversion of 96% was achieved for cyclohexene.The selectivity of the various products was: cyclohex-2-en-1-one > cyclohex-2-en-1-ol > cyclohexane-1,2-diol. 217 Very high selectivity toward cyclohex-2en-1-ol was achieved when Fe-bipyridine complexes encapsulated into Y-type zeolite Na-Y ([Fe(bpy)3] 2+ @Y) were used as the catalysts. 218olymer and non-polymer-grafted dioxidomolybdenum(VI) complex [Mo VI O 2 (sal-iah)(MeOH)] having donor ligand H2sal-iah (derived from salicylaldehyde and indole-3-acetic hydrazide), catalyzes the oxidative bromination by H2O2 of styrene and trans-stilbene.Under the optimized reaction conditions, the oxidative bromination of styrene gave 96% conversion in two hours, where 2-bromo-1-phenylethan-1-ol 48 (45%) and 1phenylethane-1,2-diol 49 (51.2%) are the main products, and 1,2-dibromo-1-phenylethane 47 (2.0%) is the minor product.Oxidative bromination of trans-stilbene gave 96% conversion in two hours, with 2,3diphenyloxirane (trans-stilbene oxide), 1,2-dibromo-1,2-diphenylethane and 2-bromo-1,2-diphenylethanol as the products.
The peroxide [Mo VI O(O2)] 2+ intermediate species formed under reaction conditions, may oxidize the bromide (to Br2, Br3 − and/or HOBr), which may react with styrene to give bromonium ion 46 as intermediate.The attack of nucleophile Br − on the α-carbon of the intermediate gives 1,2-dibromo-1-phenylethane 47 while attack by H2O gives 2-bromo-1-phenylethan-1-ol 48.The nucleophile H2O may further attack at the α-carbon of 2-bromo-1-phenylethan-1-ol to give 1-phenylethane-1,2-diol 49 (Scheme 24).The observed poor selectivity of dibromo product 47 is possibly due to its fast sequential conversion into the mono-bromo derivative 48, which in turn forms the diol 49.The conversion of monobromo derivative alone under the optimized reaction conditions was also tested and the formation of 1-phenylethane-1,2-diol was observed.The presence of acid in the aqueous phase possibly promotes the protonation of the peroxido moiety and the presence of KBr is essential to obtain reasonable amounts of brominated products. 219Sodium bicarbonate-assisted oxidation, by H2O2, of styrene and cyclohexene using polymer grafted dioxidomolybdenum(VI) complex as a catalyst was also reported. 220cheme 24.Proposed mechanism for the formation of three main products (47-49), after the bromonium ion is formed by action of catalytically generated HOBr on styrene.

Oxidative cleavage of alkenes.
Oxidative cleavage of alkenes is one of the most prevalent reactions in organic synthesis.The two main oxidative cleavage pathways can be summarized: conversion of alkenes to vicinal diols and subsequent cleavage with NaIO 4 or other oxidants and ozonolysis, in which the olefin is directly cleaved into functionalized products depending on work-up conditions.Ozonolysis is quite reliable, but has always had major safety concerns.Ozone gas is highly toxic and its generation requires specialized equipment.Examples of alternative reactions to ozonolysis include Noyori's cleavage of olefins to carboxylic acids with hydrogen peroxide and Na2WO4, Ranu's InCl3/t-BuOOH cleavage of alkenes and alkynes to carboxylic acids, and several other transition metal-mediated protocols. 222,223More recently, oxidative cleavage of aryl and alkyl olefins to the corresponding aldehydes, catalyzed by OsO4, employing H2O2 as the terminal oxidant was reported.Aryl olefins were cleaved in good to excellent yields regardless of arene electronics.Alkyl olefins are cleaved in moderate to good yields for di-and tri-substituted alkenes. 224he first Fe-based catalytic system for the oxidative cleavage of unsaturated fatty acids and esters to carboxylic acids use a system comprising [Fe(OTf)2(6-Me-PyTACN)] 50, (6-Me-PyTACN = 1-[(6-methyl-2pyridyl)methyl]-4,7-dimethyl-1,4,7-triazacyclononane, OTf = trifluoromethanesulfonate anion) as the catalyst either with a combination of hydrogen peroxide and NaIO4 or exclusively with NaIO4 as the oxidant, and operates at 0 °C or ambient temperature (Scheme 25).Under these conditions, methyl oleate 51 was converted in a one-pot procedure into 50-55% of both nonanoic and azelaic acid, together with some epoxide and aldehyde intermediates as byproducts.This catalytic system provides an alternative to the industrial ozonolysis of oleic acid and to catalytic Ru-and Os-based systems for the oxidative cleavage of unsaturated fatty acids and esters. 225or the one-pot oxidative cleavage of internal alkenes to aldehydes, the nonheme iron complex [Fe(OTf)2(mix-bpbp)] (bpbp=N,N'-bis(2-picolyl)-2,2'-bipyrrolidine) was used as catalyst. 226A green synthesis of adipic acid, in 82% yield, from cyclohexene oxidized by H2O2-silicotungstic acid under ultrasonication was reported. 227In the same reaction a novel silica-functionalized ammonium tungstate interphase catalyst successfully used. 228A new synthesis strategy for the preparation of metallic silver nanoparticles (AgNPs) supported on tungsten oxide (WO3) nanorods with diameters between 40 and 60 nm in the presence of a cationic surfactant cetyltrimethylammonium bromide (CTAB) was developed.The catalyst was very effective in liquid phase hydrogen peroxide oxidation of cyclohexene to adipic acid.A cyclohexene conversion of >99.9% with an adipic acid selectivity of >94% was achieved with a very high turnover frequency. 229Ligated Pd(II) complexes have been studied for the catalytic oxidation of terminal olefins to their corresponding methyl ketones using hydrogen peroxide as the terminal oxidant. 230A series of quaternary ammonium perrhenate 3hexyl-1-methylimidazolium hydrogen sulfate ([Hmim]HSO4) composite ionic liquids were prepared and used both as catalyst and solvent in oxidation of cyclooctene to suberic acid in the presence of hydrogen peroxide as a green oxidant.The yield of suberic acid under the mild conditions was from good to high. 231-Pyridylmethyl pendant armed ethylene side-bridged cyclam complexes of copper(II), iron(II), and manganese(II) have been prepared, characterized, and screened for oxidation catalysis.Using these complexes and hydrogen peroxide as terminal oxidant 1,4-cyclohexadiene was dehydrogenated to benzene in 71% yield (conversion 86%). 232

Conclusions
In this review an attempt has been made to summarize progress in the exploitation of the metal-containing catalysts for hydrogen peroxide oxidations of C-H, C-C and C=C bonds, made in recent years, based on the literature from 2011 to early 2016.Different compounds and complexes mainly of iron, vanadium, copper, manganese, rhenium, vanadium, molybdenum, titanium, tungsten, palladium, osmium, zirconium, chromium, silver, cobalt and also bismuth and aluminum are indicated as oxygen-transfer agents while 30% aqueous hydrogen peroxide is a stoichiometric oxidant.Most of the described oxidant/catalyst systems are ecologically friendly reagents particularly these using the iron and manganese and vanadium compounds, and complexes or heterogenous systems where catalyst is easily separated from the reaction products and can be reused without loss of the activity.They are convenient for modern organic synthesis and have been used for various groups of organic compounds among them for hydrocarbons.The review covers recent advances in their application for oxidation of C-H, C-C and C=C bonds in alkanes, arenes and nonfunctionalized as well as in functionalized alkenes.The most important reactions are hydroxylation of benzene rings, oxyfunctionalization of the methyl and methylene groups in alkanes, cycloalkanes and alkylarenes toward carbonyl and carboxylic group as well as in epoxidation, α-hydroxylation, 1,2-dihydroxylation and other reactions of alkenes Based on the described reactions, new methodologies for synthesis of alcohols, phenols, aldehydes, ketones, epoxides and carboxylic acids are indicated.Different mechanisms for the oxygen-transfer processes, heterolytic or homolytic, and the stereochemistry of some important reactions are discussed and their scope and limitations are indicated.
It is hoped that this review can serve as a critical overview of the area and the contribution helps in encouraging further research in this field.I would like to apologize to anyone who finds my description of her or his work inadequate or whose work has been omitted.

Scheme 3 .
Scheme 3. Overall radical mechanism of the catalyzed oxidation of alkanes by H2O2 .

4 A
Scheme 12.A possible reaction mechanism for the catalytic oxidation of benzene by FeCl3/mpg-C3N4 hybrids.
Scheme 4. Catalytic cycles for the formation of HOO • and HO • radicals based on μ-oxo vanadium complexes.