Lipase peracid formation

The lipase method of in situ peracid formation is relatively safe, and the selectivity to epoxide, i.e. the amount of epoxide formed compared with the... 

Figure 3.7 Epoxidation of alkenes by lipase catalysed peracid formation in a two-phase system.

Another example where an enzyme is used to mediate a completely different reaction than it normally does is the lipase-catalyzed formation of peracids by reaction of H202 with a carboxylic acid [43]. As illustrated in Fig. 15 this allows for the one step epoxidation of olefins by in situ formation of RC03H. The carboxylic acid can be used in catalytic amounts providing an overall epoxidation with H202. By a suitable choice of carboxylic acid the reaction can be carried out in a two-phase system. The scope of such novel transformations must be enormous. 

H2O2 yields of epoxide varied from 0 to 91% (57). Formation of dihydroxy acids or estolides did not occur. The lipase was still active after 15 uses. Forty carboxylic acids were used as substrates for the lipase-catalyzed formation of peracids (58). Good agreement between the amount of peracid formed and epoxidation of 1-octene was obtained. Neither a-substituted carboxylic acids nor aromatic acids was converted to peracids. 

Recent studies have attempted to improve the efficiency of epoxidation under milder conditions that minimize the formation of byproducts. Chemo-enzymatic epoxidation uses the immobilized lipase from Candida antartica (Novozym 435) (56) to catalyze conversion of fatty acids to peracids with 60% hydrogen peroxide. The fatty acid is then self-epoxidized in an intermolecular reaction. The lipase is remarkably stable under the reaction conditions and can be recovered and reused 15 times without loss of activity. Competitive lipolysis of triacylglycerols is inhibited by small amounts of fatty acid, allowing the reaction to be carried out on intact oils (57). Rapeseed oil with 5% of rapeseed fatty acids was converted to epoxidized rapeseed oil in 91% yield with no hydroxy byproducts. Linseed oil was epoxidized in 80% yield. Methyl esters are also epoxidized without hydrolysis under these conditions. 

Although strictly speaking a hydrolase, a suitable lipase when combined with H2O2 and a carboxylic acid can carry out oxidation reaction through the formation of a peracid (45). This reaction has been used for polymo modification, e.g., epoxidation of polybutadiene (46a) and oxidation of hydroxyethylcellulose (46b). 

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