Chromium, powder Subject

Chiral salen chromium and cobalt complexes have been shown by Jacobsen et al. to catalyze an enantioselective cycloaddition reaction of carbonyl compounds with dienes [22]. The cycloaddition reaction of different aldehydes 1 containing aromatic, aliphatic, and conjugated substituents with Danishefsky s diene 2a catalyzed by the chiral salen-chromium(III) complexes 14a,b proceeds in up to 98% yield and with moderate to high ee (Scheme 4.14). It was found that the presence of oven-dried powdered 4 A molecular sieves led to increased yield and enantioselectivity. The lowest ee (62% ee, catalyst 14b) was obtained for hexanal and the highest (93% ee, catalyst 14a) was obtained for cyclohexyl aldehyde. The mechanism of the cycloaddition reaction was investigated in terms of a traditional cycloaddition, or formation of the cycloaddition product via a Mukaiyama aldol-reaction path. In the presence of the chiral salen-chromium(III) catalyst system NMR spectroscopy of the crude reaction mixture of the reaction of benzaldehyde with Danishefsky s diene revealed the exclusive presence of the cycloaddition-pathway product. The Mukaiyama aldol condensation product was prepared independently and subjected to the conditions of the chiral salen-chromium(III)-catalyzed reactions. No detectable cycloaddition product could be observed. These results point towards a [2-i-4]-cydoaddition mechanism. 

The rapid autocatalytic dissolution of aluminium, magnesium or zinc in 9 1 methanol-carbon tetrachloride mixtures is sufficiently vigorous to be rated as potentially hazardous. Dissolution of zinc powder is subject to an induction period of 2 h, which is eliminated by traces of copper(II) chloride, mercury(II) chloride or chromium(III) bromide. 

One of the first applications of electron spin resonance (ESR) spectroscopy to catalysis was in a study of the chromia-alumina system, and during the last five years or so a number of publications have appeared dealing with this subject. The ESR spectra of supported chromia catalysts have been interpreted in terms of various chromium ion configurations or phases, each of which wiU be discussed below. It will be seen that these data substantiate many of the conclusions drawn from the magnetic susceptibility data described above, and, in addition, they provide a deeper insight into the molecular structure of chromia-alumina catalysts than can be obtained from static susceptibility measurements alone. This body of research serves as a very good illustration of the potential usefulness of ESR spectroscopy to the catalytic chemist, particularly when one considers that all of the data to be discussed below were obtained on poorly crystallized, high surface area powders, typical of practical catalysts. 

When combined with oxidizers such as chlorates, bromates, peroxides, persulfates, and chromium trioxide and subjected to impact, percussion, or heating, powdered zinc explodes. Explosion may result when the powder metal is heated with manganese chloride, hydroxylamine, ammonium nitrate (an oxidizer), potassium nitrate (an oxidizer), sulfur, or interhalogen compounds. Zinc bums in fluorine and chlorine (moist), and reacts with incandescence when mixed with carbon disulfide. 

Potassium or sodium-potassium alloy mixed with ammonium nitrate and ammonium sulfate results in explosion (NFPA 1986). Violent reactions may occur when a metal such as aluminum, magnesium, copper, cadmium, zinc, cobalt, nickel, lead, chromium, bismuth, or antimony in powdered form is mixed with fused ammonium nitrate. An explosion may occur when the mixture above is subjected to shock. A mixture with white phosphorus or sulfur explodes by percussion or shock. It explodes when heated with carbon. Mixture with concentrated acetic acid ignites on warming. Many metal salts, especially the chromates, dichromates, and chlorides, can lower the decomposition temperature of ammonium nitrate. For example, presence of 0.1% CaCb, NH4CI, AICI3, or FeCb can cause explosive decomposition at 175°C (347°F). Also, the presence of acid can further catalyze the decomposition of ammonium nitrate in presence of metal sulfides. 

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