Alkyl halides catalytic hydrogenolysis

While removal of an AT-alkyl substituent is not always a feasible process benzyl groups can be removed by reduction with sodamide or by catalytic hydrogenolysis. If such reductive methods fail, an oxidation with chromium trioxide in acid may be successful. Other groups are not so readily displaced, but a procedure involving transalkylation with benzyl chloride followed by debenzylation can be employed to convert 1-methylimidazole into imidazole (Scheme 137). The reaction is capable of extension and operates because the quaternary salt is in equilibrium with both 1-alkylimidazoles and the alkyl halides. Under conditions in which the more volatile alkyl halide can escape from the system the 1-benzylimidazole builds up. 

Catalytic hydrogenolysis of alkyl halides is one of the classical methods of alkane syntheses. Among other catalysts, Raney Ni has proved to be quite effective for the hydrogenolysis of iodides (equation 39) . A relatively recent discovery is the reductive dehalogenation of alkyl halides under the conditions of ionic hydrogenation. 

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