Hydrogenation of unsaturated carbonyls

The hydrogenation of unsaturated carbonyl compounds has recently become a research field studied in great detail. The reason is the practical importance of the selective hydrogenation of two different functional groups of these easily accessible compounds. Of the possible products, saturated oxo compounds and unsaturated alcohols are useful synthons in preparative organic chemistry. The increased interest in this field is verified by the appearance of comprehensive works in recent years1,365-367. 

Concept of the synthesis of novel platinum catalysts for the selective hydrogenation of unsaturated carbonyl compounds... 

A new method of synthesis of selective platinum catalysts for the hydrogenation of unsaturated carbonyl compounds is presented. Platinum was deposited on the supports tailored with the monolayer of transition metal oxide. Selectivity of these catalysts strongly depended on the type of inorganic support as well as on the type of transition metal in the monolayer. Catalysts were tested in the hydrogenation of furfural, crotonaldehyde and cinnamaldehyde. Selectivity of the synthesis of the appropriate unsaturated alcohols was enhanced when compared with the reactions performed over classical Pt-metal oxide catalysts. 

In this work, a concept of the modification of platinum catalysts is presented. According to our proposal, the monolayer oxide systems should be the appropriate carriers that can influence the noble metal properties. The monolayer carrier effect depends on its acid-base or electro-nucleophilic properties and the number and capacity of monolayers. This is of great importance in the selective hydrogenation of unsaturated carbonyl compounds. The... 

The results obtained in this work indicate that the selectivity of platinum catalysts for the hydrogenation of unsaturated carbonyl compounds can be markedly improved by doping the carrier with a monolayer of transition metal ion. It is also worth to note that sandwich-type catalysts (ZrC>2/Pt/Zr02/Ti02) are also active and selective in the studied reaction. We believe that on the surface of this system, the end-on adsorption of aldehyde molecule has been forced. 

In this work we have shown that Ziegler type cobalt catalysts are rather selective for the hydrogenation of unsaturated carbonyl compounds into unsaturated alcohols. Nevertheless the selectivity was more important with cinnamaldehyde than with other aldehydes, which showed the eflfect of the aromatic cycle on the stabilization and the adsorption of the conjugated double bond. 

Nevertheless only scare data is available in the recent literature on the application of Group VIII noble metal (M) or rhenium-based mono- and Re-M bimetallic catalysts, in the hydrogenolysis of esters or hydrogenation of acids to alcohols. Recently a few publications, - and patents. have been reported on the transformation of different carbonyl compounds (saturated and unsaturated esters, acids and carboxamides) over rhenium-containing catalysts. In the bimetallic catalysts used for the hydrogenation of carbonyl compounds the rhenium was combined with Pd, or Rh. In the case of catalysts used for the hydrogenation of unsaturated carbonyl compounds the rhenium is usually modified with tin. ... 

Positionalisomeri tion occurs most often duting partial hydrogenation of unsaturated fatty acids it also occurs ia strongly basic or acidic solution and by catalysis with metal hydrides or organometaUic carbonyl complexes. Concentrated sulfuric or 70% perchloric acid treatment of oleic acid at 85°C produces y-stearolactone from a series of double-bond isomerizations, hydration, and dehydration steps (57). 

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. 

Reduction of unsaturated aldehydes seems more influenced by the catalyst than is that of unsaturated ketones, probably because of the less hindered nature of the aldehydic function. A variety of special catalysts, such as unsupported (96), or supported (SJ) platinum-iron-zinc, plalinum-nickel-iron (47), platinum-cobalt (90), nickel-cobalt-iron (42-44), osmium (<55), rhenium heptoxide (74), or iridium-on-carbon (49), have been developed for selective hydrogenation of the carbonyl group in unsaturated aldehydes. None of these catalysts appears to reduce an a,/3-unsaturated ketonic carbonyl selectively. 

Unmodified poly(ethyleneimine) and poly(vinylpyrrolidinone) have also been used as polymeric ligands for complex formation with Rh(in), Pd(II), Ni(II), Pt(II) etc. aqueous solutions of these complexes catalyzed the hydrogenation of olefins, carbonyls, nitriles, aromatics etc. [94]. The products were separated by ultrafiltration while the water-soluble macromolecular catalysts were retained in the hydrogenation reactor. However, it is very likely, that during the preactivation with H2, nanosize metal particles were formed and the polymer-stabilized metal colloids [64,96] acted as catalysts in the hydrogenation of unsaturated substrates. 

Enantioselective hydrogenation of unsaturated ketones giving chiral unsaturated alcohols is achievable by only limited catalysts. With most existing heterogeneous and homogeneous catalysts, saturation occurs at C=C bonds preferentially over C=0 [270]. Thus development of carbonyl-selective and enantioselective hydrogenation is highly desirable. 

Frankel et al. (40) have used dicobalt octacarbonyl as a catalyst for the hydrogenation of unsaturated fats and found it to be active at lower temperatures than iron carbonyl. 

Although the hydrogenation of hydrogen cyanide to methylamine was achieved as early as 1863 (Debus, 1), the history of modern catalytic hydrogenation began in 1897 with the discovery by Paul Sabatier and R. Senderens of the vapor phase hydrogenation of unsaturated compounds over a nickel catalyst (Sabatier and Senderens, 2). Sabatier has said that his interest in the action of nickel was provoked by the newly discovered Mond process for the purification of nickel by the formation and decomposition of nickel carbonyl. The capacity of nickel... 

On the other hand, hydrogenations under mild conditions, in particular those at ordinary temperature and pressure, are advantageous for monitoring the extent of conversion of substrate exactly and thus achieving selective hydrogenation successfully, as in selective hydrogenation of alkynes to alkenes and in selective hydrogenation of the carbon-carbon double bond of unsaturated carbonyl compounds. 

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