Carbocations, benzylic destabilizing groups

The secondary benzylic alcohol l-phenylethan-l,2-diol requires 20 hours of treatment at room temperature to produce a 64% yield of 2-phenylethanol (Eq. 43).137 Under the same conditions, methyl mandelate fails to undergo reduction, presumably because of the greater carbocation-destabilizing effect of a neighboring carboalkoxy compared to a hydroxymethyl group (Eq. 43).137... 

To avoid any loss of benzyl ester protection during acidolytic removal of the benzyloxy-carbonyl and ferf-butoxycarbonyl groups, electron-withdrawing substituents were used to destabilize the intermediate benzyl cation and thus to increase the acid stability. In addition to the very useful 4-nitrobenzyl esters (vide infra), the picolyl ester (see Section 2.2.1.2.2.3) as well as halo-P l or cyano-P°°l substituted benzyl esters have been reported, the latter being rarely used for a-carboxy protection. Conversely, an increase in sensitivity toward acids can be achieved by introduction of electron-releasing substituents, such as methoxy or methyl groups. Addition of scavengers to quench intermediate carbocations and to prevent electrophilic substitutions at sensitive amino acid side chains is beneficial in the deprotection of such esters. 

The electron-withdrawing NO2 group will destabilize the carbocation so the benzylic halide will be less reactive, while the electron-donating OCH3 group will stabilize the carbocation, so the benzylic halide will be more reactive. 

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