Corrolazines: new frontiers in high-valent metalloporphyrinoid stability and reactivity

The activation of dioxygen and its analogues, such as hydrogen peroxide, by metalloporphyrins leads to the generation of high-valent metal-oxo species. This process is critically important to heme-catalyzed reactions, such as for cytochrome P450, and synthetic porphyrin-catalyzed oxidations. We have synthesized a new ring-contracted porphyrinoid system called a corrolazine that is designed to stabilize high oxidation states, including high-valent metal-oxo species. The corrolazine ligand stabilizes manganese(V) terminal oxo and terminal imido complexes for isolation, both of which are only transiently observed with normal porphyrin macrocycles. Examination of both oxygen atom transfer and hydrogen atom abstraction reactions for the Mn(V)-oxo complex has led to a number of mechanistic insights regarding these transformations. The activation of H 2O 2 to give the Mn(V)-oxo complex exhibits some dramatic and unexpected axial ligand effects that call into question the normal role of axial ligands in O-O bond cleavage pathways.     

Reactivity of high-valent iron-oxo species in enzymes and synthetic reagents: a tale of many states

The Account discusses the phenomenon of two-state reactivity (TSR) or multistate reactivity (MSR) in high-valent metal-oxo reagents, projecting its wide-ranging applicability starting from the bare species, through the reagents made by Que, Nam, and collaborators, to the Mn(V)-oxo substituted polyoxometalate, all the way to Compound I species of heme enzymes. The Account shows how the behaviors of all these variegated species derive from a simple set of electronic structure principles. Experimental trends that demonstrate TSR and MSR are discussed. Diagnostic mechanistic probes are proposed for the TSR/MSR scenario, based on kinetic isotope effect, stereochemical studies, and magnetic- and electric-field effects.     

Metal-Oxo Species in P450 Enzymes and Biomimetic Models. Oxo-Hydroxo Tautomerism with Water-Soluble Metalloporphyrins

Oxygenation reactions (hydroxylation, epoxidation, N- or S-oxide formation, etc.) catalyzed by cytochrome P450 enzymes and related biomimetic models involve an electrophilic oxidative species as the active species, namely a high-valent metal-oxo intermediate. Among the different methods to study the oxygenation reactions mediated by high-valent metal-oxo porphyrin complexes, the recent discovery of oxo-hydroxo tautomerism provides a useful tool to investigate the mechanism of O-atom transfer reactions in aqueous media.     

Direct functionalization of M-C (M = Pt(II), Pd(II)) bonds using environmentally benign oxidants, O2 and H2O2

Atom economy and the use of "green" reagents in organic oxidation, including oxidation of hydrocarbons, remain challenges for organic synthesis. Solutions to this problem would lead to a more sustainable economy because of improved access to energy resources such as natural gas. Although natural gas is still abundant, about a third of methane extracted in distant oil fields currently cannot be used as a chemical feedstock because of a dearth of economically and ecologically viable methodologies for partial methane oxidation. Two readily available "atom-economical" "green" oxidants are dioxygen and hydrogen peroxide, but few methodologies have utilized these oxidants effectively in selective organic transformations. Hydrocarbon oxidation and C-H functionalization reactions rely on Pd(II) and Pt(II) complexes. These reagents have practical advantages because they can tolerate moisture and atmospheric oxygen. But this tolerance for atmospheric oxygen also makes it challenging to develop novel organometallic palladium and platinum-catalyzed C-H oxidation reactions utilizing O(2) or H(2)O(2). This Account focuses on these challenges: the development of M-C bond (M = Pt(II), Pd(II)) functionalization and related selective hydrocarbon C-H oxidations with O(2) or H(2)O(2). Reactions discussed in this Account do not involve mediators, since the latter can impart low reaction selectivity and catalyst instability. As an efficient solution to the problem of direct M-C oxidation and functionalization with O(2) and H(2)O(2), this Account introduces the use of facially chelating semilabile ligands such as di(2-pyridyl)methanesulfonate and the hydrated form of di(2-pyridyl)ketone that enable selective and facile M(II)-C(sp(n)) bond functionalization with O(2) (M = Pt, n = 3; M = Pd, n = 3 (benzylic)) or H(2)O(2) (M = Pd, n = 2). The reactions proceed efficiently in protic solvents such as water, methanol, or acetic acid. With the exception of benzylic Pd(II) complexes, the organometallic substrates studied form isolable high-valent Pt(IV) or Pd(IV) intermediates as a result of an oxidant attack at the M(II) atom. The resulting high-valent M(IV) intermediates undergo C-O reductive elimination, leading to products in high yields. Guidelines for the synthesis of products containing other C-X bonds (X = OAc, Cl, Br) while using O(2) or H(2)O(2) as oxidants are also discussed. Although the M(II)-C bond functionalization reactions including high-valent intermediates are well understood, the mechanism for the aerobic functionalization of benzylic Pd(II) complexes will require a more detailed exploration. Importantly, further optimization of the systems suitable for stoichiometric M(II)-C bond functionalization led to the development of catalytic reactions, including selective acetoxylation of benzylic C-H bonds with O(2) as the oxidant and hydroxylation of aromatic C-H bonds with H(2)O(2) in acetic acid solutions. Both reactions proceed efficiently with substrates that contain a directing heteroatom. This Account also describes catalytic methods for ethylene dioxygenation with H(2)O(2) using M(II) complexes supported by facially chelating ligands. Mechanistic studies of these new oxidation reactions point to important ways to improve their substrate scope and to develop "green" CH functionalization chemistry.     

Knowledge of the proposed chemical mechanism of action and cytochrome p450 metabolism of antimalarial trioxanes like artemisinin allows rational design of new antimalarial peroxides

Evidence is reviewed elucidating the mechanism of iron-induced triggering of antimalarial trioxanes. As prodrugs, trioxanes undergo homolytic, inner-sphere, reductive cleavage by ferrous iron to form sequentially oxy radicals, carbon radicals, high-valent iron-oxo species, epoxides, aldehydes, and dicarbonyl compounds. One or more of these reactive intermediates and neutral alkylating agents likely kill the malaria parasites. Several new, orally active antimalarial peroxides have been designed rationally based on this fundamental mechanistic paradigm. Incorporating metabolism-blocking substituents also provides some new, potent, semi-synthetic artemisinin derivatives.     

Epoxidation of cis-cyclooctene with molecular oxygen catalyzed by magnesium oxide supported polytitazane cobalt (III) complexes

Magnesium oxide supported polytitazane cobalt (III) complex is demonstrated as an effective catalyst for the epoxidation of cis-cyclooctene with molecular oxygen at atmospheric pressure in the presence of isobutyraldehyde as the sacrificial reductant. XPS data show that the high-valent cobalt (III) is stabilized by the multidentate nitrogen ligand of polytitazane. The conversion of cis-cyclooctene is as high as 95.2% with 100% selectivity to the cis-cyclooctene oxide at 25°C within 5 h. The catalyst can be used at least nine times without loss of its activity.     

Non-heme Fe(IV)-oxo intermediates

High-valent non-heme iron-oxo intermediates have been proposed for decades as the key intermediates in numerous biological oxidation reactions. In the past three years, the first direct characterization of such intermediates has been provided by studies of several alphaKG-dependent oxygenases that catalyze either hydroxylation or halogenation of their substrates. In each case, the Fe(IV)-oxo intermediate is implicated in cleavage of the aliphatic C-H bond to initiate hydroxylation or halogenation. The observation of non-heme Fe(IV)-oxo intermediates and Fe(II)-containing product(s) complexes with almost identical spectroscopic parameters in the reactions of two distantly related alphaKG-dependent hydroxylases suggests that members of this subfamily follow a conserved mechanism for substrate hydroxylation. In contrast, for the alphaKG-dependent non-heme iron halogenase, CytC3, two distinct Fe(IV) complexes form and decay together, suggesting that they are in rapid equilibrium. The existence of two distinct conformers of the Fe site may be the key factor accounting for the divergence of the halogenase reaction from the more usual hydroxylation pathway after C-H bond cleavage. Distinct transformations catalyzed by other mononuclear non-heme enzymes are likely also to involve initial C-H bond cleavage by Fe(IV)-oxo complexes, followed by diverging reactivities of the resulting Fe(III)-hydroxo/substrate radical intermediates.     

Mononuclear metal-o(2) complexes bearing macrocyclic N-tetramethylated cyclam ligands

Metalloenzymes activate dioxygen to carry out a variety of biological reactions, including the biotransformation of naturally occurring molecules, oxidative metabolism of xenobiotics, and oxidative phosphorylation. The dioxygen activation at the catalytic sites of the enzymes occurs through several steps, such as the binding of O(2) at a reduced metal center, the generation of metal-superoxo and -peroxo species, and the O-O bond cleavage of metal-hydroperoxo complexes to form high-valent metal-oxo oxidants. Because these mononuclear metal-dioxygen (M-O(2)) adducts are implicated as key intermediates in dioxygen activation reactions catalyzed by metalloenzymes, studies of the structural and spectroscopic properties and reactivities of synthetic biomimetic analogues of these species have aided our understanding of their biological chemistry. One particularly versatile class of biomimetic coordination complexes for studying dioxygen activation by metal complexes is M-O(2) complexes bearing the macrocyclic N-tetramethylated cyclam (TMC) ligand. This Account describes the synthesis, structural and spectroscopic characterization, and reactivity studies of M-O(2) complexes bearing tetraazamacrocyclic n-TMC ligands, where M ═ Cr, Mn, Fe, Co, and Ni and n = 12, 13, and 14, based on recent results from our laboratory. We have used various spectroscopic techniques, including resonance Raman and X-ray absorption spectroscopy, and density functional theory (DFT) calculations to characterize several novel metal-O(2) complexes. Notably, X-ray crystal structures had shown that these complexes are end-on metal-superoxo and side-on metal-peroxo species. The metal ions and the ring size of the macrocyclic TMC ligands control the geometric and electronic structures of the metal-O(2) complexes, resulting in the end-on metal-superoxo versus side-on metal-peroxo structures. Reactivity studies performed with the isolated metal-superoxo complexes reveal that they can conduct electrophilic reactions such as oxygen atom transfer and C-H bond activation of organic substrates. The metal-peroxo complexes are active oxidants in nucleophilic reactions, such as aldehyde deformylation. We also demonstrate a complete intermolecular O(2)-transfer from metal(III)-peroxo complexes to a Mn(II) complex. The results presented in this Account show the significance of metal ions and supporting ligands in tuning the geometric and electronic structures and reactivities of the metal-O(2) intermediates that are relevant in biology and in biomimetic reactions.     

High-Valent Iron-Oxo and -Nitrido Complexes: Bonding and Reactivity

Multiple-bonded iron-oxo and -nitrido species have been identified or proposed as key intermediates in a range of important chemical transformations. The reported model complexes feature various coordination geometries and distinct electronic structures, and therefore exhibit diverse reactivity. The present contribution highlights the synergy from both experimental and theoretical standpoints to elucidate their different bonding situations and delineate their common mechanistic features in hydrogen-atom abstraction processes. Our analysis reveals that a radical centered on the abstracting atom E (E=O, N), which is generated via homolysis of covalent Fe−E bonds upon approaching the transition state, is an intrinsic C−H cleaving agent. The iron-oxo species is predicted to be more reactive than its nitride congener, in general, because the O−H bond formed in the H-atom transfer process is often stronger than the corresponding N−H bond.     

Theoretical studies on high-valent manganese porphyrins: Toward a deeper understanding of the energetics,electron distributions,and structural features of the reactive intermediates of enzymatic and synthetic manganese-catalyzed oxidative processes

We present here a relatively comprehensive theoretical study, based on nonlocal density functional theory calculations, of the energetics, electron distributions, and structural features of the low-lying electronic states of various high-valent intermediates of manganese porphyrins. Two classes of molecules have been examined: (a) compounds with the general formula [(P)MnX₂]⁰ (P = porphyrin; X = F, Cl, PF₆) and (b) high-valent manganese-oxo species. For [(P)Mn(PF₆)₂]⁰, the calculations reveal a number of nearly equienergetic quartet and sextet states as the lowest states, consistent with experimental results on a comparable species, [(TMP)Mn(ClO₄)₂]⁰ (TMP = tetramesitylporphyrin). In contrast, [(P)MnCl₂]⁰ and [(P)MnF₂]⁰ have a single well-defined S = 3/2 Mn(IV) ground state, again in agreement with experiment, with the three unpaired spins largely concentrated (>90%) on the manganese atom. Manganese(IV)-oxo porphyrins have an S = 3/2 ground state, with the three unpaired spins distributed approximately 2.3:0.7 between the manganese and oxygen atoms. The metal-to-oxygen spin delocalization, as measured by the oxygen spin population, for Mn^IV = O porphyrins is less than, but still qualitatively similar to, that in analogous iron(IV)-oxo intermediates, indicating that the Mn^IV = O bond is significantly weaker than the Fe^IV = O bond in an analogous molecule. Thus, the optimized metal—oxygen bond distances are 1.654 and 1.674 Å for (P)Fe^IV(O)(Py) and (P)Mn^IV(O)(Py), respectively (Py = pyridine). This is consistent with the experimental observation that Mn^IV = O stretching frequencies are over 10% lower than Fe^IV = O stretching frequencies for analogous compounds. For [(P)Mn(O)(PF₆)]⁰, [(P)Mn(O)(Py)]⁺, and [(P)Mn(O)(F)]⁰, the ground states clearly correspond to a (d_xy)² Mn(V) configuration and the short Mn–O distances of 1.541, 1.546, and 1.561 Å for the three compounds, respectively, reflect the formal triple bond character of the Mn–O interaction. Interestingly, the corresponding Mn(IV)-oxo porphyrin cation radical states are calculated to be a few tenths of an electrovolt higher than the Mn(V) ground states, suggesting that the Mn(IV)-oxo porphyrin cation radicals are not likely to exist as ground-state species.     

Alkene oxidation catalyzed by transition metal substituted Keggin-type heteropolyanions

Alkene oxidations with various oxidants (tert-butyl hydroperoxide, iodosylbenzene and molecular oxygen in the presence of isobutyraldehyde (IBA)) catalyzed by transition metal monosubstituted heteropolyanions, PW₁₁MO ₃₉ ^n– (PW₁₁M; M=Co^II, Mn^II, Cu^II, Ti^IV, Ru^IV, V^V and Nb^V), have been studied. Orders of catalytic activity of PW₁₁M are different for the oxidants studied. Radical chain mechanisms are proposed fort-BuOOH and O₂/IBA. Preliminary coordination of the oxidant to PW₁₁M is not a necessary step of its homolytic activation. Epoxidation with PhIO requires its coordination to the catalyst and most likely includes the formation of active metal-oxo species.     

Double bond oxidation of unsaturated fatty acids

Different oxidizing agents for performing the cleavage oxidation of the double bond of the unsaturated fatty acids are presented, and their economic performance is analyzed. Ozone and sodium hypochlorite are the most commercially efficient oxidants. Laboratory work for the oxidation of oleic acid to azelaic and pelargonic acids using hypochlorite as oxidant is described. The advantages of working in an emulsion system and using RuCl₃ as a catalyst are discussed, and a possible mechanism of the reaction is presented. A flow sheet for an industrial process based on this concept is proposed. A simulation of a plant using this technology is made by a computerized model, and the economic parameters obtained permit us to conclude that the sodium hypochlorite can be an interesting reagent for industrial oxidations of double bonds in fatty acids.     

Visible-light photolysis of corrole-manganese(IV) nitrites to generate corrole-manganese(V)-oxo complexes

Photolysis of highly photo-labile corrole-manganese(IV) nitrites by visible light was studied in three corrole systems with different electronic environments. As observed in all three systems, homolytic cleavage of ON bond of nitrite ligand resulted in one-electron photo-oxidation to generate manganese(V)-oxo corroles, as determined by their distinct UV–vis spectra and kinetic behaviors. The spectral and kinetic results are rationalized by a multiple oxidation model, where the electron-demand Mn^V-oxo species may serve as direct two-electron oxidant for oxygen atom transfer reactions and less electron-demand systems undergo a disproportionation reaction to form a putative manganese(VI)-oxo corrole as the true oxidant.     

High-valent iron(IV)-oxo complexes of heme and non-heme ligands in oxygenation reactions

High-valent iron(IV)-oxo species have been implicated as the key reactive intermediates in the catalytic cycles of dioxygen activation by heme and non-heme iron enzymes. Our understanding of the enzymatic reactions has improved greatly via investigation of spectroscopic and chemical properties of heme and non-heme iron(IV)-oxo complexes. In this Account, reactivities of synthetic iron(IV)-oxo porphyrin pi-cation radicals and mononuclear non-heme iron(IV)-oxo complexes in oxygenation reactions have been discussed as chemical models of cytochrome P450 and non-heme iron enzymes. These results demonstrate how mechanistic developments in biomimetic research can help our understanding of dioxygen activation and oxygen atom transfer reactions in nature.     

Active Species of Nonheme Iron and Manganese-Catalyzed Oxidations

The intense catalytic and spectroscopic studies of the last decade provided important insights into the mechanisms of some nonheme iron-catalyzed oxidations with hydrogen peroxide. For manganese-based analogs, direct spectroscopic data on the structure of the reactive intermediates are scarce; mechanistic proposals are mainly based on catalytic studies and on analogy with iron systems. Herein, these data are summarized and contemporary mechanistic landscape is presented. We have mainly focused on iron and manganese complexes with N ₄-donor aminopyridine ligands, which are one of the most successful catalysts for chemo-, regio- and enantioselective transformations of organic substrates with H₂O₂ and H₂O₂/CH₃COOH as oxidants. The low-spin Fe^III–OOH, Fe^IV = O and Fe^V = O species can be spectroscopically trapped in the catalyst systems studied, low-spin Fe^V = O intermediate being the most likely key oxidizing agent.     

Copper(I)-Dioxygen Adducts and Copper Enzyme Mechanisms

Primary copper(I)-dioxygen (O₂) adducts, cupric-superoxide complexes, have been proposed intermediates in copper-containing dioxygen-activating monooxygenase and oxidase enzymes. Here, mechanisms of C−H activation by reactive copper-(di)oxygen intermediates are discussed, with an emphasis on cupric-superoxide species. Over the past 25 years, many synthetically derived cupric-superoxide model complexes have been reported. Due to the thermal instability of these intermediates, early studies focused on increasing their stability and obtaining physical characterization. More recently, in an effort to gain insight into the possible substrate oxidation step in some copper monooxygenases, several cupric-superoxide complexes have been used as surrogates to probe substrate scope and reaction mechanisms. These cupric superoxides are capable of oxidizing substrates containing weak O−H and C−H bonds. Mechanistic studies for some enzymes and model systems have supported an initial hydrogen-atom abstraction via the cupric-superoxide complex as the first step of substrate oxidation.     

Hypersensitive radical probes and the mechanisms of cytochrome P450-catalyzed hydroxylation reactions

The title probes are precursors to kinetically calibrated, aryl-substituted cyclopropylcarbinyl radicals that rearrange with picosecond lifetimes. Applications in studies of cytochrome P450-catalyzed hydroxylation reactions are reviewed. Initially confusing results regarding lifetimes of radicals in the hydroxylation reactions were resolved when second-generation probes that distinguish between radicals and cations were employed. The results indicate that two electrophilic oxidizing species are involved in P450-catalyzed hydroxylations, an iron-oxo species that inserts oxygen and a hydroperoxo-iron species that inserts OH(+). The cationic rearrangement products are ascribed to reactions of the protonated alcohol products formed from the latter.     

Electron deficient manganese(III) corrole catalyzed oxidation of alkanes and alkylbenzenes at room temperature

At room temperature electron deficient manganese (III) corrole complexes (1–3) were successfully employed as catalysts in the oxidation of alkanes and alkylbenzenes using m-chloroperbenzoic acid (m-CPBA) as the terminal oxidant. Adamantane has been selectively hydroxylated to adamantane 1-ol and 2-ol with higher preference for the tertiary position. Cyclohexane has also been oxidized. The present oxidizing system also oxidizes toluene, ethylbenzene and diphenylmethane. High valent oxomanganese(V) species has been proposed to be the active oxidant. The high-valent oxomanganese(V) corrole undergoes hydrogen atom transfer (HAT) reaction with 2,4,6-tri-t-butylphenol (TTBP) resulting in the formation of oxidized phenoxyl radicals. Kinetic studies have led to the determination of second-order rate constants for the hydrogen atom transfer reactions. The kinetic experiments reveal a first order reaction rate dependence on the concentration of catalyst as well as on that of the oxidant.     

Synthetic applications of nonmetal catalysts for homogeneous oxidations

Nonmetal oxidation catalysts have gained much attention in recent years. The reason for this surge in activity is 2-fold: On one hand, a number of such catalysts has become readily accessible; on the other hand, such catalysts are quite resistant toward self-oxidation and compatible under aerobic and aqueous reaction conditions. In this review, we have focused on five nonmetal catalytic systems which have attained prominence in the oxidation field in view of their efficacy and their potential for future development; stoichiometric cases have been mentioned to provide overview and scope. Such nonmetal oxidation catalysts include the alpha-halo carbonyl compounds 1, ketones 2, imines 3, iminium salts 4, and nitroxyl radicals 5. In combination with a suitable oxygen source (H2O2, KHSO5, NaOCl), these catalysts serve as precursors to the corresponding oxidants, namely, the perhydrates I, dioxiranes II, oxaziridines III, oxaziridinium ions IV, and finally oxoammonium ions V. A few of the salient features about these nonmetal, catalytic systems shall be reiterated in this summary. The first class entails the alpha-halo ketones, which catalyze the oxidation of a variety of organic substrates [figure: see text] by hydrogen peroxide as the oxygen source. The perhydrates I, formed in situ by the addition of hydrogen peroxide to the alpha-halo ketones, are quite strong electrophilic oxidants and expectedly transfer an oxygen atom to diverse nucleophilic acceptors. Thus, alpha-halo ketones have been successfully employed for catalytic epoxidation, heteroatom (S, N) oxidation, and arene oxidation. Although high diastereoselectivities have been achieved by these nonmetal catalysts, no enantioselective epoxidation and sulfoxidation have so far been reported. Consequently, it is anticipated that catalytic oxidations by perhydrates hold promise for further development, especially, and should ways be found to transfer the oxygen atom enantioselectively. The second class, namely, the dioxiranes, has been extensively used during the last two decades as a convenient oxidant in organic synthesis. These powerful and versatile oxidizing agents are readily available from the appropriate ketones by their treatment [figure: see text] with potassium monoperoxysulfate. The oxidations may be performed either under stoichiometric or catalytic conditions; the latter mode of operation is featured in this review. In this case, a variety of structurally diverse ketones have been shown to catalyze the dioxirane-mediated epoxidation of alkenes by monoperoxysulfate as the oxygen source. By employing chiral ketones, highly enantioselective (up to 99% ee) epoxidations have been developed, of which the sugar-based ketones are so far the most effective. Reports on catalytic oxidations by dioxiranes other than epoxidations are scarce; nevertheless, fructose-derived ketones have been successfully employed as catalysts for the enantioselective CH oxidation in vic diols to afford the corresponding optically active alpha-hydroxy ketones. To date, no catalytic asymmetric sulfoxidations by dioxiranes appear to have been documented in the literature, an area of catalytic dioxirane chemistry that merits attention. A third class is the imines; their reaction with hydrogen peroxide or monoperoxysulfate affords oxaziridines. These relatively weak electrophilic oxidants only manage to oxidize electron-rich substrates such as enolates, silyl enol ethers, sulfides, selenides, and amines; however, the epoxidation of alkenes has been achieved with activated oxaziridines produced from perfluorinated imines. Most of the oxidations by in-situ-generated oxaziridines have been performed stoichiometrically, with the exception of sulfoxidations. When chiral imines are used as catalysts, optically active sulfoxides are obtained in good ee values, a catalytic asymmetric oxidation by oxaziridines that merits further exploration. The fourth class is made up by the iminium ions, which with monoperoxysulfate lead to the corresponding oxaziridinium ions, structurally similar to the above oxaziridine oxidants except they possess a much more strongly electrophilic oxygen atom due to the positively charged ammonium functionality. Thus, oxaziridinium ions effectively execute besides sulfoxidation and amine oxidation the epoxidation of alkenes under catalytic conditions. As expected, chiral iminium salts catalyze asymmetric epoxidations; however, only moderate enantioselectivities have been obtained so far. Although asymmetric sulfoxidation has been achieved by using stoichiometric amounts of isolated optically active oxaziridinium salts, iminium-ion-catalyzed asymmetric sulf-oxidations have not been reported to date, which offers attractive opportunities for further work. The fifth and final class of nonmetal catalysts concerns the stable nitroxyl-radical derivatives such as TEMPO, which react with the common oxidizing agents (sodium hypochlorite, monoperoxysulfate, peracids) to generate oxoammonium ions. The latter are strong oxidants that chemoselectively and efficiently perform the CH oxidation in alcohols to produce carbonyl compounds rather than engage in the transfer of their oxygen atom to the substrate. Consequently, oxoammonium ions behave quite distinctly compared to the previous four classes of oxidants in that their catalytic activity entails formally a dehydrogenation, one of the few effective nonmetal-based catalytic transformations of alcohols to carbonyl products. Since less than 1 mol% of nitroxyl radical is required to catalyze the alcohol oxidation by the inexpensive sodium hypochlorite as primary oxidant under mild reaction conditions, this catalytic process holds much promise for future practical applications.     

A revision of the mechanism of o-xylene oxidation to phthalic anhydride with V/Ti/O catalysts, and the role of the promoter Cs

This paper describes the effect of Cs oxide, a promoter of an anatase-supported 7 wt.% V₂O₅ catalyst for the oxidation of o-xylene to phthalic anhydride. The effect of Cs was investigated in relation to the reaction network at 320 °C, i.e., under conditions of incomplete o-xylene conversion. It was found that at this temperature the catalytic performance was affected by a surface-saturation effect. However, Cs had the role of favoring the desorption of intermediates and keeping a cleaner and more oxidizing catalyst surface, so accelerating the conversion of the reactant and the consecutive oxidations occurring upon the reaction intermediates finally leading to phthalic anhydride. This de-saturation derived from the enhancement of the V re-oxidation rate, which is one main effect of Cs in V/Ti/O catalysts. Besides the route of o-xylene oxidation to the main reaction intermediate o-tolualdehyde, a parallel reaction pathway included the direct oxidation of o-xylene to phthalaldehyde; the latter was one intermediate in the formation of phthalide, via intramolecular condensation, and of phthalic acid.