Chromium compounds reactions, reagents

Chromate conversion coatings are thin, noncrystalline, adherent surface layers of low solubiHty phosphoms and/or chromium compounds produced by the reaction of suitable reagents with the metal surface (2,3). The two classes of chromate coatings are chromium phosphates (green chromates) and chromium chromates (gold chromates). 

Hydrogen transfer reactions from an alcohol to a ketone (typically acetone) to produce a carbonyl compound (the so-caUed Oppenauer-type oxidation ) can be performed under mild and low-toxicity conditions, and with high selectivity when compared to conventional methods for oxidation using chromium and manganese reagents. While the traditional Oppenauer oxidation using aluminum alkoxide is accompanied by various side reactions, several transition-metal-catalyzed Oppenauer-type oxidations have been reported recently [27-29]. However, most of these are limited to the oxidation of secondary alcohols to ketones. 

Intramolecular Nozaki reaction. The addition of a vinyl-chromium compound to an aldehyde (12, 137 14, 96) is useful for macrocyclization, particularly if the CrCl2 reagent is activated by a catalytic amount of Ni(acac)2. Under these conditions, the vinyl iodide with a w-formyl group (1) cyclizes to (+ )-brefeldin C 12) and the 4-epimer as a 1 4 mixture in 60% yield.1... 

In the process, ethanol is oxidized, and the chromium reagent is reduced. The new chromium compound is a different color, and therefore, the progress of the reaction can be monitored by the color change from reddish-orange to green. This reaction formed the basis for many of the early breath tests that assessed the level of blood alcohol. Breath tests that utilize this reaction are still commercially available. The test consists of a tube, where one end is fitted with a mouthpiece and the other end has a bag. The inside of the tube contains sodium dichromate that is adsorbed onto the surface of an inert solid, such as silica gel. As the user blows air through the tube and fills up the bag, the alcohol in the user s breath reacts with the... 

In equation 1, the Grignard reagent, C H MgBr, plays a dual role as reducing agent and the source of the arene compound (see Grignard reaction). The Cr(CO)g is recovered from an apparent phenyl chromium intermediate by the addition of water (19,20). Other routes to chromium hexacarbonyl are possible, and an excellent summary of chromium carbonyl and derivatives can be found in reference 2. The only access to the less stable Cr(—II) and Cr(—I) oxidation states is by reduction of Cr(CO)g. 

The precipitate is soluble in free mineral acids (even as little as is liberated by reaction in neutral solution), in solutions containing more than 50 per cent of ethanol by volume, in hot water (0.6 mg per 100 mL), and in concentrated ammoniacal solutions of cobalt salts, but is insoluble in dilute ammonia solution, in solutions of ammonium salts, and in dilute acetic (ethanoic) acid-sodium acetate solutions. Large amounts of aqueous ammonia and of cobalt, zinc, or copper retard the precipitation extra reagent must be added, for these elements consume dimethylglyoxime to form various soluble compounds. Better results are obtained in the presence of cobalt, manganese, or zinc by adding sodium or ammonium acetate to precipitate the complex iron(III), aluminium, and chromium(III) must, however, be absent. 

Chromium hexacarbonyl is prepared by the reaction of anhydrous chromi-um(lll) chloride with carbon monoxide in the presence of a Grignard reagent. A 60% product yield may be obtained at the carbon monoxide pressures of 35 to 70 atm. Other chromium salts may be used with carbon monoxide and Grignard reagent in the preparation. The compound may also be obtained by the reaction of a chromium salt with carbon monoxide in the presence of magnesium in ether or sodium in diglyme. 

Each substrate was oxidized to the corresponding carbonyl compounds in good yields. Moreover, the coexisting olefin linkage remained intact upon treatment with the oxodiperoxochromium complex and no epoxy compounds were observed in the reaction mixture. Hexavalent chromium reagents such as anhydrous chromium trioxide and pyridinium... 

The unusual copper nitrate-acetic anhydride reagent reacted with acetyl-acetone itself to form bis-(3-nitro-2,4-pentanediono) copper(II) (V), the same compound prepared by Nyholm (18). A novel elaboration of this reaction was treatment of hydrated chromium (III) nitrate with acetylacetone in acetic anhydride to yield a mixture of mono- and dinitrochromiumacetylacetonate (XI and XII). 

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