Chromium hydride, decomposition

Catalysts suitable specifically for reduction of carbon-oxygen bonds are based on oxides of copper, zinc and chromium Adkins catalysts). The so-called copper chromite (which is not necessarily a stoichiometric compound) is prepared by thermal decomposition of ammonium chromate and copper nitrate [50]. Its activity and stability is improved if barium nitrate is added before the thermal decomposition [57]. Similarly prepared zinc chromite is suitable for reductions of unsaturated acids and esters to unsaturated alcohols [52]. These catalysts are used specifically for reduction of carbonyl- and carboxyl-containing compounds to alcohols. Aldehydes and ketones are reduced at 150-200° and 100-150 atm, whereas esters and acids require temperatures up to 300° and pressures up to 350 atm. Because such conditions require special equipment and because all reductions achievable with copper chromite catalysts can be accomplished by hydrides and complex hydrides the use of Adkins catalyst in the laboratory is very limited. 

Halogen-substituted arene chromium tricarbonyl complexes undergo nucleophilic substitution by alkoxide ions at a considerably enhanced rate over the free arene 39, 327, 444). The effect of the chromium tricarbonyl moiety on the rate of substitution of the arene is approximately equal to that of a -nitro group (39). Treatment of ethylbenzene chromium tricarbonyl with fcrf-butyl lithium followed by hydrolysis and decomposition of the resulting complexes with Ce(IV) yielded m- and p-ethyl-tcrt-butylbenzene along with some unsubstituted ethylbenzene (55). The reaction represents a novel nucleophilic displacement of a hydride ion and contrasts with the metalation reaction observed with w-butyl lithium (304). 

An alternate route to the 5,7-diene system is by the hydride induced decomposition of A 7-toluenesulfonylhydrazones (113) (31, 41). The corresponding 7-oxo-steroids (114) can be prepared by allylic oxidation of A -sterol esters with chromium trioxide-amine complexes in methylene chloride at room temperature (163). The attractive feature of this method is that the product formed in the toluenesulfonyl hydrazone decomposition is virtually free of the 4,6-diene isomer. 

Attempts to develop an activated cathode for chlorate cells have not yet been successful, and a material for the application faces many constraints. Some important properties for a chlorate cathode are (a) low overpotential for hydrogen evolution, (b) high stability during hydrogen evolution (resistant to the mechanical stress from gas bubbles and no detrimental hydride formation), (c) resistant during shut downs (low corrosion rate at open circuit in chlorate electrolyte), (d) low activity for hypochlorite decomposition, (e) low activity for reduction of hypochlorite and chlorate in the presence and in the absence of the chromium hydroxide film (the latter a step in the search for a chromate-free process), (f) relatively resistant to impurities in the electrolyte, (g) easy to manufacture, (h) easy to install in existing cell concepts, and (i) cost-effective. 

BM (indicates one free electron) for each compound. In addition, all compounds have a similar golden orange color while a change in color is characteristic of transition metals going from one valence state to another. Surely (7-bonded chromium in such a series should exhibit different valence states and not just one unpaired electron, as is noted. The UV absorption by chromium ions is characteristic for different valence states. It is clear that the tentative structures cannot account for the same absorption maxima exhibited by compounds [5-42], [5-44], and [5-46]. Reductive decomposition of [5-46] by lithium aluminum hydride gave 2 moles of biphenyl and no benzene. 

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