Acid catalysis, general epoxide reactions

The interpretation of the observed rate increase of the reaction of epichlorohydrin with iodide ion in the presence of acetic acid was questioned by Long and Paul, who suggested that specific effects of the medium brought about by 4.0 M acetic acid may be responsible for the rate increase rather than general acid catalysis.72 However, the alternative mechanism for acid-catalyzed epoxide hydrolysis proposed by these workers, an A-l mechanism involving a carbocation intermediate, has been ruled... 

General acid catalysis in the hydrolysis of 81 is quite facile. This reaction, as discussed in Section Benzylic epoxides and arene oxides and shown in Scheme 39, involves proton transfer to the epoxide oxygen concerted with epoxide C-O bond breaking to form a carbocation 83. For primary ammonium ions with pKa < 8, only the acid form of the amine is reactive, and carbocation formation is irreversible,... 

The photoaddition of alcohols to epoxides which is catalyzed by Fe203 (Fe3+ and other metal ions) [39], Scheme 14(c) functions similarly. The metal catalysis of this reaction was detected when the solvent and reagent methanol had been distilled from EDTA to remove metal ion traces and the reaction found to proceed orders of magnitude slower in the purified solvent. Photo-electron transfer from the epoxide to Fe(III) as acceptor generates an oxonium cation that is highly susceptible to nucleophilic attack by the alcohol. Since the catalysis by metal ions in this case is quite general, their functioning simply as Lewis acids cannot be excluded. 

In Chapter 7, we briefly mentioned that there were a number of biological methods for resolution of racemic mixtures of chiral compounds. Because enzymes, biological catalysts, are composed of chiral amino acids, they will generally accept only the natural enantiomer of a compound as their substrate. Some enzymes are very specific as to their substrate, but others can be used with whole classes of substrate, even where these are not very close in overall structure to their natural substrates. The simplest form of resolution involves a racemate, where one enantiomer is transformed by the enzyme and the other is not. This is called kinetic resolution, since it depends on the relative rates of enzymatic catalysis of the reactions of one enantiomer over the other. Our first example is shown in Figure 15.22. When the epoxide is opened by the Grignard reagent, only the tra s-product is formed, but this is a racemic mixture. The alcohol is converted to an ester, which is then hydrolyzed enzymatically using a lipase enzyme from Pseudomonas fluorescens. Notice the very mild conditions for... 

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