Electron-withdrawing groups oxygen atom transfer

The Darzens reaction is the base-promoted generation of epoxides XIII from aldehydes (or ketones) XI and alkyl halides XII, the latter carrying an electron withdrawing group, for example the carbonyl, nitrile, or sulfonyl, in the a-position (Scheme 6.83) [188, 189]. It is, formally, addition of a carbene to the C=0 double bond (Scheme 6.83, path B) and thus complements oxygen atom transfer to olefins... 

The oxygen atom transfer from a peroxy acid to an alkene is facilitated by electron-donating substituents on the carbon-carbon double bond and electron-withdrawing groups on the peroxy acid.16 Further, conclusions which can be made from the experimental data collected over the years, include ... 

The self-consistent trend in pK values for yeast and plant peroxidases may indicate that the same acid groups are present in all enzymes, in which case electronic withdrawing effects which increase the acidities operate in the order TuPy>TuPT >HRP>CcP. It is suggested that a major difference between the yeast and plant peroxidases and metmyoglobin is that the peroxidases have an acid group appropriately positioned to accept the peroxide proton from one oxygen atom and to transfer it back to the other. ... 

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