Peracids alkene epoxidation

Asymmetric synthesis, chiral auxUiaries, 868 Asynchronous transition states, peracid alkene epoxidation, 50-8... 

Hartree-Fock calculations, peracid alkene epoxidation, 48-50 Hazardous materials commercial codes, 621 emergency response, 746-7 environmental hazards, 747, 751-3 labels, 751-3 NIOSH Pocket Guide, 749 occupational hazards, 747-9 safety issues, 744-9 HDL see High-density lipoprotein Heat of formation see Enthalpy of formation HEHP (1-hydroxyethyl hydroperoxide), 605, 638... 

Simple alkenes, including cyclohexene, react rapidly with electrophiles such as bromine or peroxy-acids (Chapter 20). Bromine gives a product of tram addition, peracids give epoxides by cis addition. Under the same conditions benzene does not react with either reagent. 

Peracids may epoxidize alkenes faster than they take part in Baeyer-Villiger reactions, so unsaturated ketones are not often good substrates for Baeyer-Villiger reactions. The balance is rather delicate. The two factors that matter are how electrophilic is the ketone and how nucleophilic is the alkene You might like to consider why this reaction does work, and why the C=C double bond here is particularly unreactive. 

On the basis of theoretical studies by Bach and co-workers,17 it was found that the nucleophilic 71-bond of the alkene attacks the 0-0 cr-bond in an Sn2 fashion with displacement of a neutral carboxylic acid. There are, however, some mechanistic anomalies. For example, a protonated peracid should be a much more effective oxygen transfer agent over its neutral counterpart, but experiments have shown only modest rate enhancements for acid catalysed epoxidation. Early attempts to effect acid catalysis in alkene epoxidation where relatively weak acids such as benzoic acid were employed proved unsuccessful.18 The picture is further complicated by contradictory data concerning the influence of addition of acids on epoxidation rates.19 Trichloroacetic acid catalyses the rate of epoxidation of stilbene with perbenzoic acid, but retards the rate of a double bond containing an ester constituent such as ethyl crotonate.20 Recent work has shown that a seven-fold increase in the rate of epoxidation of Z-cyclooctene with m-chloroperbenzoic acid is observed upon addition of the catalyst trifluoroacetic acid.21 Kinetic and theoretical studies suggest that the rate increase is due to complexation of the peroxy acid with the undissociated acid catalyst (HA) rather than protonation of the peroxy acid. Ab initio calculations have shown that the free energy of ethylene with peroxy-formic acid is lowered by about 3 kcal mol-1 upon complexation with the catalyst.21... 

Epoxidation. With this peracid alkenes are epoxidized in dichloromethane, often in quantitative yield. The carboxylic acid by-product is very insoluble in the reaction medium and easily removed. 

Hydrogen peroxide can react with carboxylic acid to form peracid when lipase enzyme is used as a catalyst instead of inorganic acid. The peracid will epoxidize alkenes, which results in the regeneration of the carboxylic acid (Scheme 7). The best results were obtained in the nonpolar solvents toluene and hexane or when mixtures of alkene and carboxylic acid were used without solvent (55). Lipases were immobilized on cellulose, polysulfone membranes, or polypropylene beads (56). The lipases were used to convert caprylic acid to its peracid. The peracid was used to convert oleic acid to its epoxide. The best yield of epoxy stearic acid (81%) was obtained with the lipase from Candida antartica. Using a commercial preparation of immobilized C. antartica lipase, various unsaturated carboxylic acids were treated with... 

---