Chromium iodide, formation

Vinyl halides represent yet another important class of intermediates in the conversion of ketones to alkenes. The most widely applied conditions for the conversion of ketones into vinyl halides are those developed by Barton et a/. ° for the conversion of 3p-acetoxyandrost-5-ene-17-one into 3P-hydroxyandrosta-5,16-diene (Scheme 44). These conditions of vinyl halide formation and subsequent reduction have been useful in a number of steroid systems for the introduction of a A -carbon-carbon double bond and have been shown to be compatible with such functional groups as alcohols, isolated double bonds and acetals. The scope of vinyl iodide formation from hydrazones has been studied by Pross and Stemhell, and recently the original reaction conditions were improved by using sterically hindered guanidine bases rather than triethylamine. Haloalkenes have also been prepared from the corresponding ketones by treatment with iodoform and chromium chloride or with phosphorous penta-halides. ... 

Acyclic a,a-disubstituted tin enolates 6 inevitably form as cis/trans-mixtures. Nevertheless, application of the chromium alkylation protocol with the modified salen complex 7 provides fair enantioselectivity with various alkylating agents R CH2X hke allyl bromide, benzyl bromide, allyl iodide, and ethyl iodoacetate, as outlined in Scheme 5.5. A plausible explanation is based on the assumption of a rapid cis/trans-isomerization of the tin enolates 6 through the C-bound tautomer and the postulate that one of the enolate diastereomers reacts distinctly faster than the other. The role of the additive BugSnOMe, which has a beneficial effect on the enantioselectivity, might be to catalyze the cis/trans-isomerization of the enolate. Several models have been proposed for the mechanisms of the enantioselective enolate alkylation like transmetallation of tin into a chromium enolate, formation of a stannate by iodine transfer from chromium to tin, as well as activation of the alkyl halide by chromium [5]. 

V02+) by chromium(VI) (HCrO ), when carried out in the presence of iodide ions, results in formation of triiodide ions, I3,4 This reaction occurs rather rapidly, whereas both HCrO and vanadium(V) (VOJ, a product) oxidize I- so slowly that these reactions can be ignored. The net reactions with and without I" are... 

The formation of the iodide is exothermic and chromium is deposited on the filament (I2 is automatically re-cycled). Similar reactions may be used for the purification of... 

Imidodiphosphoric acid tetramide, formation of, from phosphoryl triamide, 6 110, llln. Iodination apparatus, for preparation of chromium(III) iodide, 6 128... 

Moissan 3 in 1880 called attention to the fact that the affinities of chromium, manganese, iron, cobalt, and nickel for oxygen and the heats of formation of oxides, chlorides, bromides, iodides, and sulphides decrease as the atomic weights rise. 

Fluoro- and chloro-substituted tricarbonyl chromium arene complexes can be reductively coupled with ketones, in the presence of samarium iodide, to give tertiary alcohols (Scheme 109). A related reaction of a tethered alkoxyimine furnished a tricyclic compound (Scheme 110). Samarium also promotes the formation of a benzyhc ketyl... 

H.OJ and 5 ml of 10% potassium hydroxide (or sodium hydroxide). Boil the solution gently for 1 h to oxidize Cr to C.f + and to get rid of excess peroxide. Add 10 ml of 6 N H,SO to acidify the solution. After standing in the dark for 5 min, add 20 ml of 10% (w/w) potassium iodide. The solution turns to a dark reddish brown owing to the formation of iodine. Titrate the iodine in the solution with 0.1 N standard solution of sodium thiosulphate until the colour of the solution turns to yellow. Add 1-2 ml of starch solution (1% w/w) as an indicator. The colour of the solution turns to dark blue. Continue titration until the colour just disappears. Record the volume of the standard solution of sodium thiosulphate used (V ). The concentration C (g 1 ) of initial chromium sulphate solution can be calculated by the following equation ... 

Iodine forms compounds with all elements except the noble gases, sulfur, and selenium, but only reacts indirectly with carbon, nitrogen, oxygen, and some noble metals. Ferric and cupric salts, and compounds of vanadium, chromium, and manganese in their highest oxidation status are reduced in acid solution by iodide ions, with liberation of free iodine. Iodine reacts with hydrocarbons, but equilibria are often unfavorable because hydrogen iodine is liberated, which may reduce the alkyl iodide to the hydrocarbon with formation of iodine. 

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