Chromium complexes 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. 

Bisbenzene chromium has been found to be a catalyst for the polymerization of ethylene at 200°-250°C. Chromium metal was postulated as the active catalyst in the system 410, 411). The polymerization of ethylene by bisarene chromium(I) salts in the presence of (i-Bu)3Al has also been studied (406). The catalytic activity was found to be a function of both the arene and the anion present. When bisarene chromium complexes are air oxidized in water, hydrogen peroxide is produced ... 

Similarly, a single peroxidic resonance at 458 ppm has been observed by Curci et al. [255] for the diperoxomolybdenum complex 17 although the peroxo oxygens should not be equivalent in a pentagonal bipyramidal structure of this type. The analogous chromium complex gives two resonances at 772 and 820 ppm in a 1 1 intensity ratio. 

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 ... 

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). 

Chromium (ITT) can be analy2ed to a lower limit of 5 x 10 ° M by luminol—hydrogen peroxide without separating from other metals. Ethylenediaminetetraacetic acid (EDTA) is added to deactivate most interferences. Chromium (ITT) itself is deactivated slowly by complexation with EDTA measurement of the sample after Cr(III) deactivation is complete provides a blank which can be subtracted to eliminate interference from such ions as iron(II), inon(III), and cobalt(II), which are not sufficiently deactivated by EDTA (275). 

Wet-Chemical Determinations. Both water-soluble and prepared insoluble samples must be treated to ensure that all the chromium is present as Cr(VI). For water-soluble Cr(III) compounds, the oxidation is easily accompHshed using dilute sodium hydroxide, dilute hydrogen peroxide, and heat. Any excess peroxide can be destroyed by adding a catalyst and boiling the alkaline solution for a short time (101). Appropriate ahquot portions of the samples are acidified and chromium is found by titration either using a standard ferrous solution or a standard thiosulfate solution after addition of potassium iodide to generate an iodine equivalent. The ferrous endpoint is found either potentiometricaHy or by visual indicators, such as ferroin, a complex of iron(II) and o-phenanthroline, and the thiosulfate endpoint is ascertained using starch as an indicator. 

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. 

Adam and Lohray122 have used thianthrene 5-oxide (88) as a mechanistic probe in oxidations with transition metal peroxides. They oxidized 88 with various diperoxo complexes of chromium, molybdenum and tungsten and formulated a plausible mechanism on the basis of the products formed, 89 and 90. 

Bis(azide)—rubber resists, 15 157 Bis-(P-hydroxyethyl) terephthalate, 10 487 Bis(biphenyl) chromium(I) iodide, physical properties, 6 528t Bis(carbamoyl) peroxides, 18 477 Biscarbonato uraniumfVI) complexes, 25 431... 

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. 

---