Aldehydes Using chromium-based oxidants

Chromium-based oxidants, noteworthy for their specificity and ease of use, continue to be popular. Enayatollah Mottaghinejad of Azad University of Iran, Tehran has found (Tetrahedron Lett. 2004,45, 8823) that barium dichromate, easily prepared, smoothly oxidizes alcohols to aldehydes and ketones in refluxing acetonitrile. 

The periodinane (10) may also be prepared from o-iodobenzoic acid by oxidation with potassium bromate and then treatment with acetic anhydride18 (see Expt 6.36 for detailed formulation). It should be noted that the organic derivatives of pentacoordinate iodine(v) are termed periodinanes.18b This compound (the systematic name is l,l,l-triacetoxy-2,l-benzoxiodol-3(3//)-one) has found use as an oxidant of primary alcohols to aldehydes and alicyclic ketones to secondary alcohols it is claimed to have advantages over the chromium-based oxidation reagents. 

Because of the toxicity of chromium-based reagents, other reagents for the oxidation of alcohols have been developed. One of the more common is hypochlorous acid (HOCl). HOCl is unstable, so it is generat in situ (in the reaction mixture) by an acid-base reaction between H" and "OCl (using CH3COOH and NaOCl). Secondary alcohols are oxidized to ketones and primary alcohols are oxidized to aldehydes. Also see the Swem oxidation (Problem 70). 

Vigorous oxidation leads to the formation of a carboxylic acid but a number of meth ods permit us to stop the oxidation at the intermediate aldehyde stage The reagents most commonly used for oxidizing alcohols are based on high oxidation state transition met als particularly chromium(VI)... 

The starting materials of the aldehyde method may be sulfonated. For example. Cl Acid Blue 9 [2650-18-2] Cl Food Blue 2 (Cl 42090), is manufactured by condensing a-(A/-ethylanilino)-y -toluenesulfonic acid with o-sulfobenzaldehyde. The leuco base is oxidized with sodium dichromate to the dye, which is usually isolated as the ammonium salt. In this case, the removal of the excess amine is not necessary. However, this color caimot be used in the food sector because separation of the chromium compounds from the dye is difficult. An alternative method which gives food-grade Cl Acid Blue 9 (14) and dispenses with the use of sodium dichromate employs oxidative electrolysis of the leuco base (49). 

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. 

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