Carbon-oxygen bonds benzylic, reduction

In general, the breaking of the carbon—oxygen bond appears difficult to achieve and needs activation by strongly polar groups. For these reasons, alcohol deprotection is carried out cathodically under the form of benzylic[91] and allylic [92] ethers as well as tosylates [91]. Thus, Torii [93] used the 4-nitrobenzyl group to protect alcohols. The deprotection was carried out in two steps (1) reduction of the nitrogroup of the amine and (2) oxidation of the amine at a platinum electrode (yield up to 93%). 

Certain types of functionality can be removed and replaced by hydrogen under catalytic hydrogenation conditions. This is called hydrogenolysis. For example, aromatic halogen substituents are usually removed by reduction over metal catalysts. Aliphatic halides are less reactive but hydrogenolysis is promoted by base. Carbon oxygen bonds at benzyl and allyl positions are cleaved by catalytic hydrogenation. ... 

Finally, side-chain functionalization may be obtained through a PET process. Apart from the several arylations discussed in Sect. 2.1.3, oxygenation of benzyl radicals obtained from deprotonation or carbon-carbon bond cleavage is preparatively useful (the method has been recently reviewed [254]), and other functionalization are possible, notably fluorination of benzyl substrates by irradiation in the presence of titanium dioxide and fluoride ions [255], Furthermore, the modification of a substituent, as a typical example the reduction of a nitro group [18, 27, 256-259] or an azo group [266], is in some cases obtained through a PET process. 

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