Deactivation during aldehyde hydrogenations

Aldol condensation is a potential side reaction that can occur during aldehyde hydrogenation, especially at high or low pH. It leads to tar formation, which can deactivate the catalyst via a fouling mechanism. 

During hydrogenation of aldehydes, especially over platinum oxide, catalyst deactivation occurs. The reasons for this deactivation are not well understood and several theories exist.6... 

The lipase from Serratia marcescens has a high enantioselectivity (E = 135) for the (2R,3S)-(4-methoxyphenyl) glycidic acid methyl ester, which acts as a competitive inhibitor. The formed acid (hydrolyzed (+)-methoxyphenylglycidate) is unstable and decarboxylates to give 4-methoxyphenylacetaldehyde this aldehyde strongly inhibits and deactivates the enzyme. It is removed by transfer to the aqueous phase by formation of a water-soluble adduct with sodium hydrogen sulfite added to the aqueous phase. The bisulfite acts also as a buffer to maintain constant pH during synthesis. 

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