Hydrogenation trans-fatty acid reduction

A process with potential practical applicability is the hydrogenation of edible oils. Reduction of multiply unsaturated triglycerides with hydrogen over Ni-based catalysts is frequently used to gain autoxidative stability of edible oils. According to the Polanyi-Horiuti mechanism, multiple 1,2 or 1,4 diadsorption of the fatty acid tail with exclusively c/s-configuration around the double bonds causes cis-trans isomerisation, whilst the number of double bonds is being reduced. The trans-fatty acid chains have adverse effects on the human metabolism and must be minimized. 

A considerable amount of work haa been devoted to the catalytic reduction of fatty acid and fatty alcohol epoxides. Hydrogenation of is- and tran -Q, 1 tl-epoxyooUtbcanoic add, for example, has been reported108t-1887 1188 to give isomeric 10-hydroxyoctadecanoio acids exclusively (Eq. 344). Preferential attack of hydrogen on C< was... 

As an example of a biological hydrogenation, one of the steps in fatty-acid biosynthesis involves the reduction of trans-crotonyl ACP to yield butyryl AGP (Figure 8.7). Note the similarity of this mechanism with the mechanism for biological hydrations that we saw at the beginning of Section 8.4. In both, a carbonyl group next to the double bond is needed and an anion intermediate is involved. 

FIGURE 8.7 Reduction of the carbon-carbon double bond in trans-crotonyl ACP, a step in the biosynthesis of fatty acids. One hydrogen (blue) is delivered from NADPH as a hydride ion, H the other hydrogen (red) is delivered by protonation of the anion intermediate with an acid, HA. As is often the case in biological reactions, the structure of the biochemical reagent, NADPH in this case, is relatively complex considering the apparent simplicity of the transformation itself. 

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