Chromium complexes, reaction with peroxides

Chromium complexes in general are poor catalysts for the epoxidation of alkenes with TBHP due to the decomposition of the oxygen donor with formation of molecnlar oxygen . Epoxidation reactions with this metal are known with other oxygen transfer agents than peroxides (e.g. iodosylbenzene) and will not be discnssed here. 

III.B.2), complexes with manganese, chromium, as well as second- and third-row transition metal ions (e.g., ruthenium) oxidation reactions with dioxygen alone or with other peroxides (e.g., ferf-butyl-peroxide) the stabilization and spectroscopic characterization of mononuclear superoxo, peroxo, and oxo complexes other catalytic processes (e.g., the iron-catalyzed aziridination), enantioselective reactions with chiral bispidine ligands and the iron oxidation chemistry continues to produce novel and exciting results. 

Reactions of hydroperoxides with oleflns in the presence of a variety of other metal centers have also been investigated. Hydrogen peroxide epoxidizes olefins as well in the presence of oxy compounds of W, Mo, V, Os, Ti, Zr, Th, Nb, Ta, Cr and Ru [411-422]. Although CrOa-oxidation of oleflns has been shown to give epoxides [423-425], chromium complexes such as [Cr(acac)a] are not particularly effective epoxidation catalysts at elevated temperatures [426]. It has recently been shown [427] that OSO4 is an effective catalyst for the hydroxylation of oleflns by tert-butyl hydroperoxide in base equation (268). 

The oxidation of [Ru(NH3)g] + by HaOa is very slow and is probably catalysed by iron impurities in the complex. The reaction shows a zero-order dependence on ruthenium(ii) and extrapolation of data from experiments with added iron(iii) indicates that much of the observed rate for the process is the result of the iron(in) pathway, with little or no contribution from the intrinsic reaction. The corresponding reaction with Og is at least 10 times faster. The mechanism of the catalytic decomposition of H2O2 by chromium(vi) in neutral media has been reported. In dilute hydrogen peroxide solutions (<1.5M), the formation of an intermediate complex [Cr209] has been proposed ... 

Chromium plating from hexavalent baths is carried out with insoluble lead-lead peroxide anodes, since chromium anodes would be insoluble (passive). There are three main anode reactions oxidation of water, reoxidation of Cr ions (or more probably complex polychromate compounds) produced at the cathode and gradual thickening of the PbOj film. The anode current density must balance the reduction and reoxidation of trivalent chromium so that the concentration reaches a steady state. From time to time the PbOj film is removed as it increases electrical resistance. 

This is an alternative method of introducing copper into an o-hydroxyazo dye structure. The azo compound is treated with a copper(II) salt and an oxidant in an aqueous medium at 40-70 °C and pH 4.5-7.0. Sodium peroxide, sodium perborate, hydrogen peroxide or other salts of peroxy acids may be used as oxidants, the function of which is to introduce a second hydroxy group in the o -position [25]. This process is reminiscent of earlier work on Cl Acid Red 14 (5.51 X = H), an o-hydroxyazo dye that will not react with a chromium (III) salt to form a 1 1 complex but will do so by oxidation with an acidified dichromate solution. This oxidation product was later found to be identical with that obtained by conventional reaction of Cl Mordant Black 3 (5.51 X = OH) with a chromium(III) salt [7]. 

An interesting alternative to the use of chromium(VI) oxidants for the conversion of 1 to 2 involves the use of a low-valent iron reagent prepared in situ by the action of hydrogen peroxide on an iron(II) complex of 1 (73). Vinblastine (as the free base) is treated with 2 equiv of perchloric acid in acetonitrile at -20°C. Ferrous perchlorate is then added, followed by the addition of excess 30% hydrogen peroxide. Work-up of the reaction mixture with a saturated solution of ammonium hydroxide gives 2 in yields of 35-50% after chromatography. 

Italogenation catalyst. Chromium carbonyl catalyzes the monohalogenation of cyclohexane by CC14 (78% yield). Other cycloalkanes undergo the same reaction, liioniination can be effected in this way with CBrCl,. Other metal carbonyl complexes arc less active. Cr(CO)f, is actually more efficient than di-/-butyl peroxide. A free indical mechanism is involved. 

Macroporous glycidyl methacrylate-ethylene glycol dimethacrylate (GMA-EGDM) copolymer beads were synthesised and characterised for pore volume and surface area. These reactive copolymers were derivatised with 2-picolyl amine and coordinated with chromium and vanadium ions. The peroxocomplexes of these supported metal complexes were generated by the addition of hydrogen peroxide / tertiary butyl hydroperoxide(tert. -BHP) and shown to catalyse a variety of oxidation reactions. 

The oxidation of alcohols to carbonyl compounds is a fundamental reaction that has synthetic and chemical importance. Using chromium-based catalysts, researchers have developed several catalysts that have impacted alcohol oxidation reactions. Recently, homogeneous catalysts have had problems with catalyst/product separation and suffer from poor catalyst recyclability. Therefore, the quest for a resolution to this problem has led researchers to scaffold salen complexes onto a silica-based material such as MCM-41. Zhou et al. used an ion-exchangeable, layered polysiloxane support to immobili.se their sulfonato-(salen)Cr(m) complex. They reacted benzyl alcohol, cyclo-hexanol and -hexanol with hydrogen peroxide as oxidant in an ionic liquid at 40 °C. Several ionic liquids were investigated [BMImX (BMIm = 1-n-butyl-3-methylimidazolium X =PF6, BF4, NOs")] and compared for each substrate. 

Recent studies involving syntheses of homogeneous catalysts for amoxidation have begun.Chromium and molybdenum imide complexes [M(NBu02 (OSiMe3)2] have been utilized as model compounds. The reaction of such complexes with toluene and benzoyl peroxide affords PhCH = NBu ... 

Cr (02)4 to Cr04. This was confirmed by Peters et al. (1975). The same decompensation is considered hkely to occur in the reaction of sodium chro-mate(Vl)with H2O2. Equation [140] represents Haber-Weiss reaction that superoxide anion and hydrogen peroxide can combine together directly to generate the hydroxyl radical. The rate constant of this reaction is very small in the absence of matals as iron or copper, however. HO may be formed then a chromium(V)-peroxide complex decompses into Cr(Vl) complex and hydroxyl radical as in the Fenton reaction ... 

---