Chromium salts alkyl halides

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. 

It may be noted also that there are a number of reasonably stable lithium alkyl anions, which are made by interaction of lithium alkyls with metal halides. Some of these, notably the copper(i) alkyls, Li[CuR2], are important reagents in organic synthesis (see Section 25-H-2). Some chromium alkyl salts have anions with multiple metal—metal bonds,49 e.g., Li4[Cr2Me8] 4C4HsO. 

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