Neutral epoxidizing agents

An epoxidizing agent that has found widespread use is dimethyl dioxirane (DMDO). The reagent is generated from acetone and Oxone , a source of potassium peroxomonosulfate (KHSO5) (5.50). Epoxidation with DMDO occurs under mild, neutral conditions, without any nucleophilic component, which is ideal for preparing sensitive epoxides. For example, the enol ether 41 was epoxidized selectively using DMDO (5.51). ... 

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... 

E. E. Stangland, B. Taylor, R. P. Andres, W. N. Delgass, Direct vapor phase propylene epoxidation over deposition-precipitation gold-titania catalysts m the presence of H2/O2 Effects of support, neutralizing agent, and pretreatment,/. Phys. Chem. B 109 (2005) 2321. 

Alkenyloxirans 16 are reactive allylating agents used under neutral conditions. The epoxy ring is opened by Pd(0) to form 7r-allylpalladium with generation of an alkoxide anion, which abstracts a proton from a pronucleophile to produce the a-hydroxy- 7r-allylpalladium 17. Reductive elimination of 17 affords either 1,4-adducts 18 or 1,2-adducts 19 [8,9]. The 1,4-adducts are mainly obtained under usual conditions due to the electronic effect of the epoxide oxygen. The 1,4-adducts 18 are allylic alcohols, and can be used again for the allylation after esterification to yield 19a. 

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