1,3-Dienes, hydrogenation studies competitive

We found previously (10) that in a natural mixture of mono-, di-, and triunsaturated fatty esters the hydrogenation of monoenes with Fe(CO)s was minor. Therefore, competitive hydrogenation studies were carried out with an equal mixture of methyl oleate and linoleate. Diene hydrogenation in such a mixture was indeed dominant (Figure 2). At O.IM initial concentration of Fe(CO)s the formation of stearate was a minor reaction the diene-Fe(CO)3 complex reached a maximum of 4% and remained constant. On the other hand, at 0.5M Fe(CO)5, stearate formation became a more important reaction diene-Fe(CO)3 reached a maximum of 7% and decreased during the course of hydrogenation. Free conjugated dienes were minor products. 

Absolute rates for the addition of the methyl radical and the trifluoromethyl radical to dienes and a number of smaller alkenes have been collected by Tedder (Table l)3. Comparison of the rate data for the apolai4 methyl radical and the electrophilic trifluoromethyl radical clearly show the electron-rich nature of butadiene in comparison to ethylene or propene. This is also borne out by several studies, in which relative rates have been determined for the reaction of small alkyl radicals with alkenes. An extensive list of relative rates for the reaction of the trifluoromethyl radical has been measured by Pearson and Szwarc5,6. Relative rates have been obtained in these studies by competition with hydrogen... 

Hubaut et has studied the liquid phase hydrogenation of polyunsaturated hydrocarbons and carbonyl compounds over mixed copper-chromium oxides. The selectivity of monohydrogenation was almost 100 % for conjugated dienes but much lower for a,p-unsaturated carbonyls. This was due to the adsorption competition between the unsaturated carbonyls and alcohols as primary products. It was suggested that the hydrogenation site was an octahed-rally coordinated Cu ion with two anionic vacancies, and an attached hydride ion. The Cr ion in the same environment was probably the active site for side reactions (hydrodehydroxylation, nucleophilic substitution, bimolecular elimination). 

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