Aqueous rhodium(III) hydrides and mononuclear rhodium(II) complexes

In aqueous solutions, as in organic solvents, rhodium hydrides display the chemistry of one of the three limiting forms, i.e. {Rh(I)+ H+}, {Rh(II)+ H.}, and {Rh(III)+ H-}. A number of intermediates and oxidation states have been generated and explored in kinetic and mechanistic studies. Monomeric macrocyclic rhodium(II) complexes, such as L(H2O)Rh2+ (L = L1 = [14]aneN4, or L2 = meso-Me6[14]aneN4) can be generated from the hydride precursors by photochemical means or in reactions with hydrogen atom abstracting agents. These rhodium(II) complexes are oxidized rapidly with alkyl hydroperoxides to give alkylrhodium(III) complexes. Reactions of Rh(II) with organic and inorganic radicals and with molecular oxygen are fast and produce long-lived intermediates, such as alkyl, superoxo and hydroperoxo complexes, all of which display rich and complex chemistry of their own. In alkaline solutions of rhodium hydrides, the existence of Rh(I) complexes is implied by rapid hydrogen exchange between the hydride and solvent water. The acidity of the hydrides is too low, however, to allow the build-up of observable quantities of Rh(I). Deuterium kinetic isotope effects for hydride transfer to a macrocyclic Cr(v) complex are comparable to those for hydrogen atom transfer to various substrates.     

Multi‐faceted Reactivity of Alkyltellurophenols Towards Peroxyl Radicals: Catalytic Antioxidant Versus Thiol‐Depletion Effect

Hydroxyaryl alkyl tellurides are effective antioxidants both in organic solution and aqueous biphasic systems. They react by an unconventional mechanism with ROO^. radicals with rate constants as high as 10⁷ M ^?1 s^?1 at 303 K, outperforming common phenols. The reactions proceed by oxygen atom transfer to tellurium followed by hydrogen atom transfer to the resulting RO^. radical from the phenolic OH. The reaction rates do not reflect the electronic properties of the ring substituents and, because the reactions occur in a solvent cage, quenching is more efficient when the OH and TeR groups have an ortho arrangement. In the presence of thiols, hydroxyaryl alkyl tellurides act as catalytic antioxidants towards both hydroperoxides (mimicking the glutathione peroxidases) and peroxyl radicals. The high efficiency of the quenching of the peroxyl radicals and hydroperoxides could be advantageous under normal cellular conditions, but pro‐oxidative (thiol depletion) when thiol concentrations are low.     

Zeolite-encapsulated vanadium picolinate peroxo complexes active for catalytic hydrocarbon oxidations

Zeolite-encapsulated vanadium (IV) picolinate complexes were prepared by treatment of dehydrated VO(2+)–NaY zeolite with molten picolinic acids. Treatment of the NaY-encapsulated VO(pic)₂ complex with urea hydrogen peroxide adduct in acetonitrile allowed to generate peroxovanadium species. The structure of vanadium peroxo species was studied by UV–vis, Raman and XAFS spectroscopies which suggested the formation of monoperoxo monopicolinate complex which could be active intermediate for various oxidation reactions with the catalysts. To elucidate effect of the encapsulation on catalytic performance, the catalytic properties of the encapsulated complexes were compared with that of corresponding homogeneous catalyst H[VO(O₂)(pic)₂]·H₂O. The novel `ship-in-a-bottle' catalysts retain solution-like activities in aliphatic and aromatic hydrocarbon oxidations as well as in alcohol oxidation. In addition, the encapsulated vanadium picolinate catalysts showed a number of distinct features such as preferable oxidation of smaller substrates in competitive oxidations, increased selectivity of the oxidation of terminal CH_3⁻ group in isomeric octanes and preferable (sometimes exclusive) formation of alkyl hydroperoxides in alkane oxidations. The distinct features were explained in terms of intrazeolitic location of the active complexes that imposed transport discrimination and substrate orientation. On the basis of the experimental data, a possible mechanism was discussed. Stability of the vanadium complexes during the liquid phase oxidations and leaching from the NaY zeolite matrix were also examined.     

Activating a Peroxo Ligand for C−O Bond Formation

Dioxygen activation for effective C?O bond formation in the coordination sphere of a metal is a long‐standing challenge in chemistry for which the design of catalysts for oxygenations is slowed down by the complicated, and sometimes poorly understood, mechanistic panorama. In this context, olefin–peroxide complexes could be valuable models for the study of such reactions. Herein, we showcase the isolation of rare “Ir(cod)(peroxide)” complexes (cod=1,5‐cyclooctadiene) from reactions with oxygen, and then the activation of the peroxide ligand for O?O bond cleavage and C?O bond formation by transfer of a hydrogen atom through proton transfer/electron transfer reactions to give 2‐iradaoxetane complexes and water. 2,4,6‐Trimethylphenol, 1,4‐hydroquinone, and 1,4‐cyclohexadiene were used as hydrogen atom donors. These reactions can be key steps in the oxy‐functionalization of olefins with oxygen, and they constitute a novel mechanistic pathway for iridium, whose full reaction profile is supported by DFT calculations.     

Solvent-free oxidation of alcohols by hydrogen peroxide over chromium Schiff base complexes immobilized on MCM-41

A series of chromium(III) Schiff base complexes immobilized on MCM-41 were prepared and characterized by various physicochemical and spectroscopic methods. The complexes were used for the selective oxidation of alcohols by 30% hydrogen peroxide without any organic solvent, phase transfer catalyst or additive. The immobilized complexes proved to be effective catalysts and generally exhibited much higher catalytic performance than their corresponding homogeneous analogs. The catalytic performance of the immobilized complexes was also found to be closely related to the Schiff base ligands used. Under the optimal reaction conditions, secondary alcohols, cyclic alcohols and benzyl alcohol were prevailingly oxidized to their corresponding ketones or aldehydes.     

Multielectron atom transfer reactions of perchlorate and other substrates catalyzed by rhenium oxazoline and thiazoline complexes: reaction kinetics, mechanisms, and density functional theory calculations

The title complexes, the Re(O)L(2)(Solv)(+) complexes (L = hoz, 2-(2'-hydroxyphenyl)-2-oxazoline(-) or thoz, 2-(2'-hydroxyphenyl)-2-thiazoline(-); Solv = H(2)O or CH(3)CN), are effective catalysts for the following fundamental oxo transfer reaction between closed shell molecules: XO + Y --> X + YO. Among suitable oxygen acceptors (Y's) are organic thioethers and phosphines, and among suitable oxo donors (XO's) are pyridine N-oxide (PyO), t-BuOOH, and inorganic oxyanions. One of the remarkable features of these catalysts is their high kinetic competency in effecting perchlorate reduction by pure atom transfer. Oxo transfer to rhenium(V) proceeds cleanly to afford the cationic dioxorhenium(VII) complex Re(O)(2)L(2)(+) in a two-step mechanism, rapid substrate (XO) coordination to give the precursor adduct cis-Re(V)(O)(OX)L(2)(+) followed by oxygen atom transfer (OAT) as the rate determining step. Electronic variations with PyO derivatives demonstrated that electron-withdrawing substituents accelerate the rate of Re(VII)(O)(2)L(2)(+) formation from the precursor adduct cis-Re(V)(O)(OX)L(2)(+). The activation parameters for OAT with picoline N-oxide and chlorate have been measured; the entropic barrier to oxo transfer is essentially zero. The potential energy surface for the reaction of Re(O)(hoz)(2)(OH(2))(+) with PyO was defined, and all pertinent intermediates and transition states along the reaction pathway were located by density functional theory (DFT) calculations (B3LYP/6-31G). In the second half of the catalytic cycle, Re(O)(2)L(2)(+) reacts with oxygen acceptors (Y's) in second-order reactions with associative transition states. The rate of OAT to substrates spans a remarkable range of 0.1-10(6) L mol(-)(1) s(-)(1), and the substrate reactivity order is Ph(3)P > dialkyl sulfides > alkyl aryl sulfides > Ph(2)S approximately DMSO, which demonstrates electrophilic oxo transfer. Competing deactivation and inhibitory pathways as well as their relevant kinetics are also reported.     

Pincer Thioamide and Pincer Thioimide Palladium Complexes Catalyze Highly Efficient Negishi Coupling of Primary and Secondary Alkyl Zinc Reagents at Room Temperature

Pincer thioamide Pd^II complex 2 was prepared, and its reaction with cyclohexylzinc chloride yielded novel pincer thioimide Pd^II complex 3 besides Pd⁰ species. The structures of complexes 2 and 3 were confirmed by X‐ray analysis. Both complexes are efficient catalysts for Negishi couplings involving primary and secondary alkyl zinc reagents bearing β‐hydrogen atoms. At a concentration of 0.1–0.5 mol % both catalysts readily promoted reactions at room temperature or even at 0 °C. The operational simplicity of these processes, in conjunction with the easy accessibility of both catalysts and substrates, promises synthetic utility of this new methodology. An experiment on a scale of 19.35 g carried out at very low catalyst loading of 2 (turnover number: 6 100 000) highlighted the potential application of the catalytic system. Monoalkyl and dialkyl zinc reagents displayed different reactivities and selectivities in reactions with aryl iodides catalyzed by complexes 2 or 3 , and isomerization in reactions involving acyclic secondary alkyl zinc derivatives was suppressed by using appropriate amounts of dialkyl zinc reagents. Based on preliminary kinetic profiles and reaction evidence, three possible pathways are proposed for the reactions involving acyclic secondary alkyl zinc reagents to rationalize the difference between mono‐alkyl and dialkyl zinc derivatives.     

EPR spin-trapping evidence for the direct,one-electron reduction of tert-butylhydroperoxide to the tert-butoxyl radical by copper(II): paradigm for a previously overlooked reaction in the initiation of lipid peroxidation

Lipid peroxidation is often initiated using Cu(II) ions. It is widely assumed that Cu(II) oxidizes preformed lipid hydroperoxides to peroxyl radicals, which propagate oxidation of the parent fatty acid via hydrogen atom abstraction. However, the oxidation of alkyl hydroperoxides by Cu(II) is thermodynamically unfavorable. An alternative means by which Cu(II) ions could initiate lipid peroxidation is by their one-electron reduction of lipid hydroperoxides to alkoxyl radicals, which would be accompanied by the generation of Cu(III). We have investigated by EPR spectroscopy, in conjunction with the spin trap 5,5-dimethyl-1-pyrroline N-oxide, the reactions of various Cu(II) chelates with tert-butylhydroperoxide. Spectra contained signals from the tert-butoxyl, methyl, and methoxyl radical adducts. In many previous studies, the signal from the methoxyl adduct has been assigned incorrectly to the tert-butylperoxyl adduct, which is now known to be unstable, releasing the tert-butoxyl radical upon decomposition. This either is trapped by 5,5-dimethyl-1-pyrroline N-oxide or undergoes beta-scission to the methyl radical, which either is trapped or reacts with molecular oxygen to give, ultimately, the methoxyl radical adduct. By using metal chelates that are known to be specific in either their oxidation or reduction of tert-butylhydroperoxide (the Cu(II) complex of bathocuproine disulfonic acid and the Fe(II) complex of diethylenetriaminepentaacetic acid, respectively) for comparison, we have been able to deduce, from the relative concentrations of the three radical adducts, that the Cu(II) complexes tested each reduce tert-butylhydroperoxide directly to the tert-butoxyl radical. These findings suggest that a previously overlooked reaction, namely the direct reduction of preformed lipid hydroperoxides to alkoxyl radicals by Cu(II), may be responsible for the initiation of lipid peroxidation by Cu(II) ions.     

Neutral versus charged species in enzyme catalysis. Classical and free energy barriers for oxygen atom transfer from C4a-hydroperoxyflavin to dimethyl sulfide

Theoretical calculations at the B3LYP/6-31+G(d,p) level have been used to study the oxidation of dimethyl sulfide by a series of bicyclic and tricyclic model C4a-flavin hydroperoxides. The intrinsic gas-phase reactivity of tricyclic C4a-hydroperoxyflavin 4 is ca. 10(9) greater than t-BuOOH but is ca. 10(7) less reactive toward the oxidation of dimethyl sulfide than peroxyformic acid. The SN2-like attack of the nucleophile on the distal oxygen of the hydroperoxide and the relative reactivity of the peracid are in excellent agreement with the earlier experimental data of Bruice. The effect of N1 or N5 hydrogen-bonding interactions on the activation barriers for oxygen atom transfer have been examined. Classical energy barriers for oxygen atom transfer from neutral and ion-paired forms of C4a-hydroperoxyflavin to dimethyl sulfide are predicted to differ by a small margin, suggesting that proton distribution exerts a relatively small influence on the reactivity of alkyl hydroperoxides. Isolated N1- and N5-protonated cations exhibit artificially low barriers as a consequence of their location in a high energy region of the potential energy surface domain.     

Hydrogen‐Bonding Catalysis of Alkyl‐Onium Salts

The synthetic utility of alkyl‐onium salt compounds is widely recognized in the field of organic chemistry. Among the wide variety of onium salts, quaternary ammonium, phosphonium, and tertiary sulfonium salts have been the most useful compounds in organic syntheses. These compounds have been very useful reagents in the construction of organic building blocks. In addition, onium salts are known as reliable catalysts, which are used to promote important organic transformations by serving as phase‐transfer and ion‐pair catalysts through the activation of nucleophiles. Although phase‐transfer catalysis is a major direction for onium salt catalysis, hydrogen‐bonding catalysis of alkyl‐onium salts, which is promoted via the activation of electrophiles, has recently become a relevant topic in the field of onium salt chemistry. This Minireview introduces new possibilities and future directions for alkyl‐onium salt chemistry based on its use in hydrogen‐bonding catalysis and on its overall utility.     

Asymmetric reduction of aromatic and hetero cyclic ketones by hydrosilylation and hydrogen transfer in the presence of optically active rhodium catalysts

A study has been made of enantioselective hydrosilylation and reduction, by hydrogen transfer, of prochiral alkyl phenyl ketones or alkyl hetaryl ketones over various optically active catalysts. A total of 14 aromatic and heterocyclic carbinols were synthesized with preparative yields of 54–100%. The most effective catalytic systems were found to be complexes of RhCl₃ and [Rh(cod)Cl]₂ with the known optical inductor (S,S)-i-Pr-Pybox, with which we have obtained for the first time a series of heterocyclic secondary alcohols with an enantioselectivity of 20–63%.Latvian Institute of Organic Synthesis, Riga, LV-1006. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 342–358, March, 1996. Original article submitted December 11, 1995.     

Four-coordinate iron complexes bearing alpha-diimine ligands: efficient catalysts for atom transfer radical polymerisation (ATRP)

Four-coordinate iron(II) complexes bearing alpha-diimine ligands with alkyl substituents are shown to be efficient catalysts for the well-controlled atom transfer radical polymerisation of styrene; catalysts containing aryldiimine ligands support competitive beta-hydrogen chain transfer processes.     

Primary Amines by Transfer Hydrogenative Reductive Amination of Ketones by Using Cyclometalated IrIII Catalysts

Cyclometalated iridium complexes are found to be versatile catalysts for the direct reductive amination (DRA) of carbonyls to give primary amines under transfer‐hydrogenation conditions with ammonium formate as both the nitrogen and hydrogen source. These complexes are easy to synthesise and their ligands can be easily tuned. The activity and chemoselectivity of the catalyst towards primary amines is excellent, with a substrate to catalyst ratio (S/C) of 1000 being feasible. Both aromatic and aliphatic primary amines were obtained in high yields. Moreover, a first example of homogeneously catalysed transfer‐hydrogenative DRA has been realised for β‐keto ethers, leading to the corresponding β‐amino ethers. In addition, non‐natural α‐amino acids could also be obtained in excellent yields with this method.     

Synthesis and oxygen-atom transfer reactions of 3-hydroperoxy-3,4,4,5,5-pentasubstituted-1,2-dioxolanes

A series of 3-hydroperoxy-3,4,4,5,5-pentasubstituted-1,2-dioxolanes 2a-d were synthesized in good yield from the corresponding 3-hydroxy-1,2-dioxolanes by reaction with concentrated hydrogen peroxide in acetonitrile with p-toluenesulfonic acid as catalyst. The 3-hydroperoxy-1,2-dioxolanes were effective oxygen-atom transfer reagents for the oxidation of thioanisole, triethylamine and 2,3-dimethyl-2-butene to the sulfoxide, N-oxide and epoxide, respectively. The reactions occurred under mild conditions and were found to be of the second order overall. The second order rate constants (k₂) were determined for oxidation of thioanisole by 2a-d in deuteriochloroform. For 2a , k₂ values for N-oxidation and epoxidation were also measured. The 3-hydroperoxy-1,2-dioxolanes were found to be less reactive than the structurally similar cyclic α-azohydroperoxides but much more reactive than simple hydroperoxides. The mechanism of oxygen-atom transfer is postulated to occur via nucleophilic attack of the substrate on the terminal oxygen of the hydroperoxide. Intramolecular hydrogen bonding of the hydroperoxy proton to a dioxolane oxygen appears to account for the reaction order in aprotic media.     

Liquid-phase oxidation of alkylaromatic hydrocarbons and their cyclohexyl derivatives to hydroperoxides in the presence of phthalimide catalysts

Regularities of liquid-phase oxidation the alkyl aromatic hydrocarbons and their cyclohexyl derivatives to hydroperoxides at presence of phthalimide catalysts are studied. It is established that N-hydroxylphthalimide increases the speed of oxidation of these hydrocarbons and provides high selectivity of formation of their hydroperoxides.     

Petits cycles-XXXIX: Photooxygenation de vinylcyclopropanes

The reaction of singlet oxygen with vinylcyclopropanes at 0° in acetone gives exclusively hydroperoxides which were reduced by PPh₃ to vinylcyclopropylcarbinols a, b (formed by abstraction of alkyl hydrogen) and cyclopropylidenecarbinols c (formed by abstraction of cyclopropyl hydrogen). The formation of alcohols c is mainly observed for photooxygenation of 1,1-dicyclopropylethylene and α-cyclopropylstyrene derivatives.     

From polyoxometalates to polyoxoperoxometalates and back again; potential applications

This short review is concerned with sustainable chemistry and recent research progress in catalysis systems for the use of aqueous hydrogen peroxide or dioxygen. Some achievements in the development of catalysts for epoxidations and for carbon–carbon bond cleavage are presented. Special emphasis is placed on fully inorganic systems, some with the dimeric moiety [M₂O₂(μ-O₂)₂(O₂)₂], (M = V, Mo, W) which have more scope than those containing organic ligands or supports, however robust. Oxoperoxometalate species with or without assembling ligands can be used for homogeneous, two-phase and phase-transfer catalyses and to prepare mesoporous materials (M-MCM-41, M-SBA-15, etc.) and supported catalysts for effective immobilization of activated metal peroxo complexes. Moreover, the decomposition of molybdenum and vanadium oxoperoxo species in water and phosphoric acid leads to an elegant method for preparing H_3+n[PMo_12−nV_nO₄₀]·aq (n = 2–9) at room temperature, avoiding the tedious synthesis with diethyl ether extraction. Spectrometric, structural and reactivity data on the precursor complexes lead to a more rational approach to catalysis systems and to the discovery of novel classes of precursors and/or catalysts for the selective transfer of oxygen to organic substrates.     

Next‐Generation D2‐Symmetric Chiral Porphyrins for Cobalt(II)‐Based Metalloradical Catalysis: Catalyst Engineering by Distal Bridging

Novel D₂‐symmetric chiral amidoporphyrins with alkyl bridges across two chiral amide units on both sides of the porphyrin plane (designated “HuPhyrin”) have been effectively constructed in a modular fashion to permit variation of the bridge length. The Co^II complexes of HuPhyrin, [Co(HuPhyrin)], represent new‐generation metalloradical catalysts where the metal‐centered d‐radical is situated inside a cavity‐like ligand with a more rigid chiral environment and enhanced hydrogen‐bonding capability. As demonstrated with cyclopropanation and aziridination as model reactions, the bridged [Co(HuPhyrin)] functions notably different from the open catalysts, exhibiting significant enhancement in both reactivity and stereoselectivity. Furthermore, the length of the distal alkyl bridge can have a remarkable influence on the catalytic properties.     

Electrocatalytic Reduction of Dioxygen at Co(II) Complexes Supported Carbon Electrode

Phthalocyanines or porphyrins of cobalt and iron are known as effective catalysts of oxygen reduction. Carbon based electrodes are modified by.the adsorption of these macromolecular compounds, which mediate the electron transfer. Oxygen is reduced to hydrogen peroxide and/or water at these modified electrodes via a two- or four- electron transfer reaction.     

Synthesis and full characterization of molybdenum and antimony corroles and utilization of the latter complexes as very efficient catalysts for highly selective aerobic oxygenation reactions

Two molybdenum and three antimony corroles were isolated and characterized by NMR, EPR, and electrochemistry. The very negative reduction potentials of the (oxo)molybdenum(V) corroles are clearly related to their inactivity as oxygen transfer reagents and the unsuccessful attempts to isolate lower-valent molybdenum corroles. X-ray crystallography of the (oxo)molybdenum(V) corrole 1a and the trans-difluoroantimony(V) corrole 2c, the first of their kind, revealed that their molecular structures represent extreme cases of such complexes: a highly domed corrole with very large out-of-plane metal displacement for 1a (0.73 Angstroms) and a very flat corrole with the metal ion in its center for 2c. All three antimony corroles displayed high activity and selectivity as catalysts for the photoinduced oxidation of thioanisole by molecular oxygen, with superior results obtained in alcoholic solvents with 2c as catalyst. Allylic and tertiary benzylic CH bonds were also oxidized under those conditions, with absolute selectivity to the corresponding hydroperoxides.