Reduction homogeneous iron catalyst

Reduction of Unsaturated Compounds with Homogeneous Iron Catalysts... 

An even more effective homogeneous hydrogenation catalyst is the complex [RhClfPPhsfs] which permits rapid reduction of alkenes, alkynes and other unsaturated compounds in benzene solution at 25°C and 1 atm pressure (p. 1134). The Haber process, which uses iron metal catalysts for the direct synthesis of ammonia from nitrogen and hydrogen at high temperatures and pressures, is a further example (p. 421). 

Fig. 17. Picture of the reduction process of a singly promoted iron catalyst, (a) Unreduced large catalyst particle with the promoter distributed homogeneously, (b) Catalyst after short reduction. Aluminum-rich regions appear, (c) Catalyst after further reduction consists of a-Fe and FcA1204 inclusions, (d) Fully reduced catalyst consists of small a-Fe particles with A1203 inclusions. Figure according to Tops e et al. (95).

Reduction of nitro compounds to amines is a synthetically important reaction (98) and is practiced since the birth of modern chemical industry—many aromatic amines are key intermediates in production of dyes and pesticides. However, the stoichiometric reductions using iron or alkali metal hydrogen sulfides or catalytic hydrogenations with heterogeneous catalysts leave room for improvements in selectivity, especially with reference to halonitro-derivatives. There are many homogeneous catalysts such as the rhodium carbonyls in the presence of amines or chelating diamines, or [Rus(CO)i2] in basic amine solutions that are... 

Reduction of unsaturated carbonyl compounds to the saturated carbonyl is achieved readily and in high yield. Over palladium the reduction will come to a near halt except under vigorous conditions (73). If an aryl carbonyl compound, or a vinylogous aryl carbonyl, such as in cinnamaldehyde is employed, some reduction of the carbonyl may occur as well. Carbonyl reduction can be diminished or stopped completely by addition of small amounts of potassium acetate (i5) to palladium catalysts. Other effective inhibitors are ferrous salts, such asferroussulfate, at a level of about one atom of iron per atom of palladium. The ferrous salt can be simply added to the hydrogenation solution (94). Homogeneous catalysts are not very effective in hydrogenation of unsaturated aldehydes because of the tendencies of these catalysts to promote decarbonylation. 

FIGURE 4.3. Redox and chemical homogeneous catalysis of trans-1,2 dibromocyclohexane. a cyclic voltammetry in DMF of the direct electrochemical reduction at a glassy carbon electrode (top), of redox catalysis by fhiorenone (middle), of chemical catalysis by an iron(I) porphyrin, b catalysis rate constant as a function of the standard potential of the catalyst couple aromatic anion radicals, Fe(I), a Fe(0), Co(I), Ni(I) porphyrins. Adapted from Figures 3 and 4 of reference lb, with permission from the American Chemical Society. 

Aniline has also been obtained (93%) from a homogenous reduction with preformed tetraethylammonium hydridotri-iron undecacarbonyl [10], In this reduction, the yield is comparable, or superior, to that obtained when the more reactive iron pentacarbonyl or di-iron nonacarbonyl complex is used in the absence of a phase-transfer catalyst. 

Carbon monoxide reduces aromatic nitro compounds when iron pentacarbonyl is used as catalyst." A direct homogeneous catalytic reduction of nitro derivatives with water under moderate carbon monoxide pressure also occurs when rhodium carbonyl derivatives in aqueous organic bases are used as catalysts (equation 21). Presumably hydridorhodium carbonyl species are the active agents whose preferred formation in aqueous organic base may be analogous to that of iron carbonyl hydrides. 

Apart from raising the pH level, precipitation can be induced in a variety of other ways. Anionic species, for example, can be deposited on the surface of suspended carriers by decreasing the pH level. This procedure has been used for vanadium(V) and Mo(Vl). Oxidation at a pH level where the ions of the lower valency are soluble and the oxidized species insoluble, can also be utilized to precipitate from a homogeneous solution. Iron(lI/lll) and Mn(IlI/rV) are cases in point. Oxidation can be effected by dissolved agents like nitrate, or electro-chemically. Reduction to insoluble ionic compounds has been applied with Cr, Cu, and Mo. Reduction to the metal has also been practised, and appears to work especially well with noble metals. Decomplexing is another possibility e.g. destruction of complexing EDTA by hydrogen peroxide has been employed to produce supported catalysts. 

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