Hydrogen bonding peroxy acetic acid

FIGURE 6.14 A one-step mechanism for epoxida-tion of alkenes by peroxy-acetic acid. In (a) the starting peroxy acid is shown in a conformation in which the proton of the OH group is hydrogen bonded to the oxygen of the C=0 group, (b) The weak O—O bond of the peroxy acid breaks, and both C—O bonds of the epoxide form in the same transition state leading to products (c). 

The effect of the medium on the rates and routes of liquid-phase oxidation reactions was investigated. The rate constants for chain propagation and termination upon dilution of methyl ethyl ketone with a nonpolar solvent—benzene— were shown to be consistent with the Kirkwood equation relating the constants for bimolecular reactions with the dielectric constant of the medium. The effect of solvents capable of forming hydrogen bonds with peroxy radicals appears to be more complicated. The rate constants for chain propagation and termination in aqueous methyl ethyl ketone solutions appear to be lower because of the lower reactivity of solvated R02. .. HOH radicals than of free RO radicals. The routes of oxidation reactions are a function of the competition between two R02 reaction routes. In the presence of water the reaction selectivity markedly increases, and acetic acid becomes the only oxidation product. 

Moderate selectivity has been observed in the epoxidation of the homoallylic alcohol (64 equation 25). In the nonsteroidal conformation of (64), the axial hydroxy is suitably oriented for forming a hydrogen bond with the peroxy acid in the transition state. That the epoxidation of (64 R = OH) is indeed hydroxy-directed is supported by the observation that the epoxidation of the acetate (64 R = OAc) furnishes exclusively the a-epoxide (66 R = OAc). 

Organic peroxy acids, especially at low temperatures, oxidize sulfides to sulfoxides 163, S24], whereas tetrabutylammonium persulfate 209] at room temperature and hydrogen peroxide at higher temperatures yield sulfones 163, 324], However, hydrogen peroxide in acetic anhydride at room temperature yields sulfoxides 166], Under these conditions, double bonds resist epoxidation 163, 166, 324] (equation 553). 

Alkenes are converted to epoxides by oxidation with peroxy acids, and thereby they are protected with regard to certain chemical transformations. Alkaline hydrogen peroxide selectively attacks enone double bonds in the presence of other alkenes. The epoxides can be transformed back to alkenes by reduction-dehydration sequences or using triphenylphosphine, chromous salts, zinc, or sodium iodide and acetic acid. A more advantageous and fairly general method consists, however, of the treatment of epoxides with dimethyl diazomalonate in the presence of catalytic amounts of binuclear rhodium(II) car-boxylate salts. This deoxygenation proceeds under neutral conditions and without isomerization or cy-clopropanation of the liberated alkene (Scheme 97). Furthermore, epoxides can be converted to alkenes with the aid of various metal carbonyl complexes. Thus, they may be nucleophilically opened with... 

Epoxidation of alkenes normally occurs with approach of the peroxy-acid from the less-hindered side of the double bond. However, where there is a polar substituent, particularly in the allylic position, this may influence the direction of attack by the peroxy-acid. Thus, whereas 2-cyclohexenyl acetate gives a mixture consisting predominantly of the trans-epoxide (as expected with attack from the less-hindered side of the double bond), the free alcohol gives almost exclusively the cw-epoxide under the same conditions (5.42). The stereoselectivity and the faster rate of reaction with the hydroxy compound result from hydrogen bonding of the reactants. 

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