Hole generation mechanism, high

Fig. 24 - Hole generation by tunnelling mechanism in high electric field with consecutive electrode reactions...

Kim, Chin, and co-workers have described a highly interesting oxyanion hole mimic that transforms L-amino acids to D-amino acids [97]. The mechanism involves stabilization of the enolate intermediate by an internal hydrogen bond array generated by urea group (Scheme 4.14). In the presence of an external base, such as triethylamine, the receptors readily promote the epimerization of a-amino acids, favoring the D-amino acids due to unfavorable steric interactions in the receptor-L-amino acid complex. These receptors can also be viewed as chiral mimics of pyridoxal phosphate [98]. 

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